Research Paper
GEOSPHERE
GEOSPHERE; v. 12, no. 1
doi:10.1130/GES01249.1
8 figures; 2 tables
Structural reconstruction and age of an extensionally
faulted porphyry molybdenum system at Spruce Mountain,
Elko County, Nevada
James R. Pape1,*, Eric Seedorff 1, Tyler C. Baril2, and Tommy B. Thompson2
Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA
Ralph J. Roberts Center for Research in Economic Geology, Mail Stop 169, University of Nevada, Reno, Reno, Nevada 89557-0232, USA
1
2
CORRESPONDENCE: pape.j22@gmail.com
CITATION: Pape, J.R., Seedorff, E., Baril, T.C., and
Thompson, T.B., 2016, Structural reconstruction and
age of an extensionally faulted porphyry molyb­
denum system at Spruce Mountain, Elko County,
­Nevada: Geosphere, v. 12, no. 1, p. 237–263, doi:​10​
.1130​/GES01249.1.
Received 10 August 2015
Revision received 24 September 2015
Accepted 17 November 2015
Published online 23 December 2015
ABSTRACT
This study integrates a cross-sectional restoration of normal faults and porphyry-related hydrothermal alteration zones with six new U-Pb zircon dates to
constrain the ages of mineralization and large-magnitude Cenozoic extension at
Spruce Mountain, northeastern Nevada (USA). Paleozoic sedimentary rocks and
sparse rhyolitic intrusive rocks host a porphyry molybdenum deposit dated as
ca. 38 Ma that contains associated skarn, carbonate replacements, fissure veins,
and jasperoid. At least six crosscutting sets of normal faults with variable dip directions (east, west, and north) and angles (<15° to >60°) are identified at Spruce
Mountain, reflecting overprinting phases of extension. Based on the restored
cross section, normal faulting at Spruce Mountain resulted in ~6.9 km (120%)
of total extension and ~35° of net eastward tilting. The first four fault sets, late
Eocene (ca. 38 Ma) or older, collectively accommodate most of the total extension (~5.4 km). Later faults probably were active in the Miocene and Quaternary.
All restored faults had initial dips of 45° or greater, and the restored preextensional structure of Spruce Mountain consists of west-vergent folds and gentle
westward dips of Paleozoic rocks. Spruce Mountain is classified as a rhyo­litic
porphyry Mo or Climax-type deposit, with extensive skarn. Eocene rhyolite
porphyry dikes associated with porphyry Mo mineralization locally intrude the
early generations of normal faults at Spruce Mountain. However, many normal
faults also postdate molybdenum mineralization and have partially dismembered the system. Cross-sectional restoration of the postmineralization faulting
suggests that the original deposit had an ~6 km long by ~3 km deep elliptical
pattern with carbonate replacement deposits surrounding skarn.
INTRODUCTION
Cenozoic extension has played an important role in the formation, preservation, and exposure of ore deposits in the Basin and Range province (western
United States and Mexico; Dreier, 1984; Spencer and Welty, 1986; Seedorff,
For permission to copy, contact Copyright
Permissions, GSA, or editing@geosociety.org.
*Present address: ExxonMobil Exploration Company, 22777 Springwoods Village Parkway,
Spring, Texas 77389, USA
1991a). However, postmineralization extension can also complicate the exploration and development of mineralized systems by dismembering and tilting ore deposits, thus distorting their original structural configurations (e.g.,
Proffett, 1977; Wilkins and Heidrick, 1995; Rahl et al., 2002; Begbie et al., 2007;
Stavast et al., 2008). When applied to structurally dismembered mineralized
systems, cross-sectional structural reconstructions can be used to predict
where fault-offset fragments of the system are located and to delineate potential exploration targets (e.g., Lowell, 1968; Seedorff, 1991b; Dilles and Proffett,
1995). In addition, structural restorations can lead to a better understanding of
the preextensional geology of a region and provide insights into mechanisms
of crustal extension, particularly when they are integrated with absolute age
constraints (e.g., Proffett, 1977; Gans and Miller, 1983; Dilles and Gans, 1995;
Surpless, 2012).
This study focuses on the Spruce Mountain district of northeastern Nevada,
a topographically rugged area with >1 km of local relief and complex structure
produced by faults with opposing senses of displacement (Hope, 1972). Spruce
Mountain is ~30 km east of the Ruby Mountains–East Humboldt Range metamorphic core complex and a few kilometers from the southwestern tip of the
adjacent Pequop Mountains (Fig. 1). The Spruce Mountain mining district covers the summit and northern flank of Spruce Mountain and extends northward
onto Spruce Mountain Ridge. The district contains porphyry molybdenum and
associated skarn, carbonate replacements, fissure veins, and jasperoid and
has been explored for Carlin-type gold mineralization (LaPointe et al., 1991;
Wolverson, 2010; E.M. Struhsacker, 2014, personal commun.). The Spruce
Mountain district is ~40 km south-southwest of the Pequop mining district,
where Carlin-type gold deposits were discovered recently (Bedell et al., 2010;
Felder et al., 2011; Smith et al., 2013).
This region has undergone significant amounts of both Mesozoic crustal
shortening and subsequent Cenozoic extension (e.g., Camilleri and Chamberlain, 1997), but there is ongoing debate over the total amount of extension in
eastern Nevada and the relative importance of late Eocene versus middle Miocene extension (e.g., Seedorff, 1991a; Miller et al., 1999; Sullivan and Snoke,
2007; Henry, 2008; Colgan et al., 2010).
We present a detailed, map-based structural analysis and cross-sectional
reconstruction of Spruce Mountain coupled with new U-Pb zircon dates of
© 2015 Geological Society of America
GEOSPHERE | Volume 12 | Number 1
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
237
Research Paper
116° 00’ W
115° 30’
114° 30’
115° 00’
Wells
US 93
by
Va
ll
AR
ey
Ru
Idaho
Ru
lt
Wendover
Pequop
Syncline
US 93A
Utah
Hu
Be
Maverick Springs
Range
reek
rr y C
Che ange
R
Se
r
vie
nt
Va ing
lle ton
y
RM
Delcer
Buttes
ne
ici
ed ge
M Ran
Antler
Belt
Nevada
SR
Area of
Fig. 3
by
40° 30’ N
Mo
un
tai
ns
Emigrant Pass
Goshute-Toan
o Range
Carlin
Pequop Mount
ains
be
Ad
o
mboldt
East Hu
Range
nd
Tre
rlin
Ca
Elko
Spruce
Mount
ain
Wood
Hills
Ra
41° 00’
ng
e
I-80
40 km
Figure 1. Regional location map. Location of the Peqoup syncline, outline of Figure 3, and Carlin trend are shown. Outcrops of pre-Cenozoic rocks are outlined in gray. Inset: Index map of Nevada,
Utah, and southern Idaho showing location of the regional map (gray box) relative to Ruby Mountains–East Humboldt (RM), Albion–Raft River–Grouse Creek (AR), and Snake Range (SR) metamorphic core complexes (black). The eastern limits of the Sevier fold and thrust belt and allochthonous western facies rocks of the Antler orogenic belt are also shown (modified from Howard, 2003).
i­gneous dikes that provide constraints on (1) the age of extension and porphyry molybdenum mineralization, and (2) the original geometry of mineralized zones prior to subsequent extensional deformation. We conclude that at
least 6 individual sets of crosscutting normal faults accommodated ~6.9 km
(120%) of extension at Spruce Mountain, and that the first 4 fault sets, which
collectively accommodate 5.4 km of extension, were active at or before ca.
38 Ma. Although the oldest normal faults currently dip at <15°, based on the
structural reconstruction, all faults had initial dips of ~45° or greater. We also
discuss implications of this cross-sectional reconstruction for the timing and
style of extension in the northeastern Great Basin and its importance to mineralization processes and mineral exploration at Spruce Mountain.
GEOSPHERE | Volume 12 | Number 1
GEOLOGIC SETTING
Spruce Mountain is part of a belt of Precambrian to Mesozoic continental
margin strata and Mesozoic to Cenozoic igneous rocks situated to the east
of contractional allochthons emplaced during the mid-Paleozoic to mid-Meso­
zoic (Figs. 1 and 2). It is significant that this belt of rocks is also within a zone
of meta­morphic core complexes that characterizes the eastern fringe of the
hinterland of the Mesozoic Sevier fold and thrust belt (Fig. 1; Dickinson, 2011;
Cashman et al., 2011). Cordilleran metamorphic core complexes, which are
exposures of formerly deeply buried mid-crustal and lower crustal metamorphic-plutonic rocks, are the iconic landforms of extended crust (Crittenden
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
238
Research Paper
Cenozoic Stratigraphy
Quaternary alluvial deposits
Oligocene to Miocene volcanic and
sedimentary rocks including
Humboldt Fm.
Eocene Volcanic Rocks
Triassic to Ordovician
Aged Sedimentary rocks
2
Triassic
Proterozoic-Mesozoic Stratigraphy
1
Thaynes Formation
6
7
Ord. Sil. Dev.
5
Miss.
Penn.
4
8
9
10
Cambrian
Stratigraphic Thickness (km)
3
Permian
Park City Group
Proterozoic
13
Pequop Formation
Rib Hill Formation
Riepe Spring Formation
Ely Limestone
Diamond Peak Formation
Chainman Formation
Joana Limestone
Guilmette Formation
Simonson Dolomite
Laketown Dolomite and
Lone Mountain Dolomite
Eureka Quartzite
Pogonip Group
Windfall Formation
Dunderberg Shale
Hamburg Formation
Secret Canyon Formation
El Dorado Formation
Pioche Shale
Prospect Mountain Quartzite
11
12
Loray Formation
McCoy Creek Group
INTRUSIVE ROCKS AND U-Pb GEOCHRONOLOGY
14
Figure 2. Generalized stratigraphic column for the Spruce Mountain area. Based on data
from Thorman (1970), Hope (1972), Stewart (1980), Coats (1987), and Swenson (1991).
GEOSPHERE | Volume 12 | Number 1
et al., 1980; ­Sullivan and Snoke, 2007). Spruce Mountain is located in the middle of three core complexes, Ruby Mountains–East Humboldt Range, Albion–
Raft River–Grouse Creek, and Snake Range (Fig. 1).
In proximity of Spruce Mountain, crustal thickening events, perhaps Late
Jurassic to Late Cretaceous in age, are recorded by thrust faults and regional
metamorphic rocks exposed in the Ruby Mountains–East Humboldt Range to
the northwest, in the Wood Hills to the north, and in the Pequop Mountains to
the northeast (Thorman, 1970; Howard, 1980; Snoke and Miller, 1988; Hudec,
1992; Camilleri and Chamberlain, 1997). Thermobarometry and thermochronology data from the East Humboldt Range and the Wood Hills suggest that
a decompression event may have occurred there as early as the Late Cretaceous, which may reflect the onset of crustal extension in northeastern Nevada
(Hodges et al., 1992; McGrew and Snee, 1994; McGrew et al., 2000; Druschke
et al., 2009; Hallett and Spear, 2014). However, work has also underscored the
importance of middle Miocene extension in the region (e.g., Miller et al., 1999;
Colgan and Henry, 2009).
Spruce Mountain exposes Ordovician to Permian miogeoclinal sedimentary rocks (Fig. 2) in numerous fault-bounded blocks (Fig. 3A). Lower Paleozoic
rocks exposed at the deepest structural levels may have reached greenschist
facies metamorphic grade (Camilleri and Chamberlain, 1997). Throughout
most of the district, the Paleozoic rocks dip to the east at moderate to steep
angles, but in some fault blocks in the northern part of the district, Paleozoic
rocks dip consistently westward (Fig. 3A), similar to the nearest exposures of
Paleozoic rocks 10 km west of the district (Hope, 1972).
No Mesozoic sedimentary rocks crop out at Spruce Mountain, but ~1.0–
1.2 km of lower Triassic marine shale and limestone are exposed ~10 km east
of Spruce Mountain within the core of the Pequop syncline (Fraser et al., 1986;
Swenson, 1991). Cenozoic volcanic and sedimentary rocks unconformably
overlie Paleozoic rocks at Spruce Mountain. A southeast-dipping Cenozoic
­dacite tuff crops out near Cole Creek on the eastern side of the district and
overlies Permian and upper Pennsylvanian rocks (Fig. 3A). Sedimentary rocks
that crop out in the southwestern corner of the district (Fig. 3A) dip gently to
moderately to the east-northeast (Harlow, 1956; Hope, 1972) and have been
assigned to the Miocene Humboldt Formation (Harlow, 1956; Coats, 1987).
Although intrusive rocks constitute only a small fraction of the surface exposures at Spruce Mountain, a variety of intrusive rocks have been observed
in the district (Fig. 3A; Table 1). Prior to this study, there were no radiometric
ages on intrusive rocks from Spruce Mountain. However, in nearby ranges,
most igneous rocks range from Jurassic through late Eocene in age (e.g.,
Wright and Snoke, 1993; Henry, 2008; Bedell et al., 2010).
The mapped distribution of intrusive rocks at Spruce Mountain is shown
in Figure 3A. Six samples that represent many of the igneous rocks known at
Spruce Mountain were selected for U-Pb zircon geochronology (Baril, 2013)
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
239
Research Paper
e
22
Mdpc
SMIg47
38
20
Tqfp
rth
SMIg45
27
Qa
No
Pea
k
DSdu
25
12
MDgu
lt
DSdu
t fa
ult
lt
au
tf
Op
47
40° 30′ 30″N
76
Spring
fault
49
Prh
Pp
k fau
Sout
ce
78
42
Qa
Pp
49
Pp
Map Units
Quaternary deposits
Ppc
Permian Park City Group
Th
Tertiary Humboldt
Formation
Pp
Permian Pequop Formation
Tertiary dacite tuff
Prh
Permian Rib Hill Formation
Trp
Tertiary rhyolite porphyry
P rs
Pennsylvanian-Permian
Riepe Spring Limestone
e
Pennsylvanian Ely Limestone
bx
Iron-stained quartz breccia
Mdpc
Mississippian Chainman/Diamond
Peak, undivided
Jf
Jurassic felsic dikes
MDgu
Devonian-Mississppian Guilmette/Pilot-Shale/Joanna Limestone, undivided
DSdu
Silurian-Devonian Dolomite,
undivided
Op
GEOSPHERE | Volume 12 | Number 1
40
50
31
25
25
20
27
Qa
Prh
20
25
Qa
25
39 Pp
P rs
Ppc
34
Th
20
10
25
Pp
17
18
0 km
36
Tt
Tertiary quartz-feldspar
porphyry
36
Pp
P rs
32
Prh
Map Symbols
Qa
Tqfp
47
55
60
65
e
34
25
56
e
57
45
69
58
h Pea
36
30
25
33
e
Prh
P rs
46
42
e
25
50
59
lt
50
29
46
34
25
Spru
e
Pp
45
35
28
35
33
Prh
20
42
P rs
Mdpc
46
e
40
Eas
W
es
36
South Peak
of Spruce Mountain
35
Pp
41
fault
Coyote
45
50
27
Prh
45
35
15
30
32
e
25
35
33
25
Tt
64
33
48
Qa
P rs
Tt
25
47
Qa
50
e
Mdpc
e
80
k
ee
Cr
le
Co
35
Qa
Trp
35
North Peak
of Spruce Mountain
fau
25
53
34
20
P rs 25
38
SMIg51
SMIg49
N
Pp
le
fau
lt
bx
Banner
Hill
50
nn
Mine Shaft fault
18
SMIg44
Trp
ct fJf
e
sp
Pro
e
79
32
t
aul
Pp
12
lt
7
Prh
e
Ba
MDgu
40° 33′ 00″
e
Jf
30
SMIg46
e
39
25
23
33
18
24
40
West fa
ult
27
114° 45′ 30″
25
ault
27
P rs
Pp
Mdpc
53
114° 48′ 00″
Qa
er
Hil
lf
31
Peak
f
P rs
Sa
dd
114° 50′ 30″
au
114° 53′ 00″ W
North
A
Strike and dip of
bedding
Contact
Figure 3 (on this and following page). Geologic maps of the Spruce Mountain area.
(A) Map shows lithologic units, extensional faults color coded by relative ages,
locations of samples for U-Pb dates with
ages, and line of reconstructed cross section (see text). Map compiled and modi­
fied from Hope (1972), Fraser et al. (1986),
and Swenson (1991), incorporating information from Harlow (1956), Coats (1987),
and this study. Revisions in local areas
based on footnotes in proof on map of
Hope (1972), mapping by Saunders and
Bresnahan (2010), and our field mapping
and observations.
2 km
Fault Sets
(Fault traces dashed and queried where
relative age relationships are uncertain)
Youngest
High-angle normal fault
(ball and bar on
downthrown side)
Low-angle normal fault
(hachures on hanging wall )
6
5
4
High-angle transfer fault
(Mesozoic?)
2
1
3
U-Pb sample location
Oldest
Ordovician Pogonip Group
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
240
Research Paper
114° 50′ 30″
114° 48′ 00″
2
9
t
nn
er
Hil
ult
dle
aul
Ba
North Peak
of Spruce Mountain
Sa
d
Pea
kf
Nor
th
rth
ult
Figure 3 (continued ). (B) Map showing
simplified distribution of selected alteration-mineralization features and numbered locations of selected historical
mines. Extensional faults color coded
by relative ages, intrusive rocks, and the
line of the reconstructed cross section
are shown for reference. Sericitic alteration (not shown) occurs only locally in
porphyry dikes. Map compiled from Hope
(1972), LaPointe et al. (1991), American
CuMo Mining Corporation (2014), and our
observations.
au
tf
Peak
f
ault
W
es
fault
Coyote
South Peak
of Spruce Mountain
t fa
No
lt
1
Pea
k fa
4
5
l fa
ul t
8
Eas
Mine Shaft fault
40° 33′ 00″
3
7
t
aul
ct f
lt
6
spe
Pro
114° 45′ 30″
fau
114° 53′ 00″ W
West fa
ult
B
40° 30′ 30″N
Spring
South
Spru
ce
fault
0 km
Map Units
Trp
Tqfp
Jf
Tertiary rhyolite porphyry
Iron-stained quartz breccia
Tertiary quartz-feldspar
porphyry
Quartz-molydenite veins at depth
Jurassic felsic dikes
Skarn +/- hornfels with copper
Jasperoid
Carbonate replacement
deposits with Pb-Zn-Ag-Cu
GEOSPHERE | Volume 12 | Number 1
Fault Sets
Map Symbols
Alteration Zones
High-angle normal fault
(ball and bar on
downthrown side)
Low-angle normal fault
(hachures on hanging wall )
High-angle transfer fault
(Mesozoic?)
Historical mine site
1
2
3
4
5
6
7
8
9
Ada H Mine
Black Forest Mine
Kille Mine
Monarch Mine
Standard Mine
Banner Hill Mine
Keystone Mine
Paramount Mine
Spence Mine
2 km
(Fault traces dashed and queried where
relative age relationships are uncertain)
Youngest
6
5
4
3
2
1
Oldest
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
241
Research Paper
TABLE 1. SUMMARY OF U-Pb AGES OF IGNEOUS ROCKS IN THE SPRUCE MOUNTAIN DISTRICT
Sample
UTM
easting*
UTM
northing*
Rock type
(phenocryst content, vol%)
SMIg44
679937
4492126
Porphyritic rhyolite (5)
SMIg45
680682
4490741
Rhyolite porphyry (30)
SMIg46
683420
4493380
Dacite (none)
SMIg47
684690
4491810
Porphyritic dacite (15)
SMIg49
685957
4492716
Coarse-grained rhyolite
porphyry (30)
SMIg51
686910
4492715
Rhyolite porphyry (15)
Local stratigraphic
hosts of intrusion
Pennsylvanian Ely
Limestone
Pennsylvanian Ely
Limestone
Pennsylvanian–Permian
Riepe Spring Limestone
Mississippian Chainman
Shale
Devonian Guilmette Formation
and Pennsylvanian Ely
Limestone
Pennsylvanian Ely
Limestone
Alteration, mineralization
Sericite and kaolinite after
feldspars
Sericite and kaolinite after
feldspars
Sericite after plagioclase
Propylitic alteration (chlorite,
calcite, sericite, kaolinite)
Quartz ± molybdenite veinlets,
potassic alteration, minor
chalcopyrite and pyrite
Potassic alteration and weak
argillization; elsewhere can
contain quartz veins
Age
(Ma)
2σ
(Ma)
37.7
0.5
37.3
0.4
155
4
156
2
39.1
0.6
37.5
0.4
*Universal Transverse Mercator North American Datum (NAD27) datum, UTM zone 11N.
and are described in Appendix 1. Igneous rocks with a fine-grained groundmass are assigned volcanic rock names (whether extrusive or shallow intrusive), whereas coarser grained porphyritic and equigranular rocks are given
plutonic rock names. Hypabyssal rocks may have either a volcanic or plutonic
name, depending on grain size. Sample locations are listed in Table 1 and plotted in Figure 3A. Table 1 also lists the stratigraphic hosts of each sample; this
bears on the possible depth of emplacement of the intrusions, as well as the
alteration-mineralization characteristics of each sample, which are relevant to
evaluating the age of alteration mineralization. U-Pb dates were obtained from
microanalysis of zircons by the laser ablation–inductively coupled plasma–
mass spectrometry (LA-ICP-MS) method at Boise State University (Idaho).
Ana­lytical methods and reporting procedures are described in Appendix 2, and
results are summarized in Table 1.
Late Jurassic Dacite
A sample from a small dike of Late Jurassic porphyritic dacite that crops
out on the North Peak of Spruce Mountain (SMIg47) is dated as 156 ± 2 Ma
(Table 1). In addition, a sample from a small exposure of Late Jurassic flowbanded dacite (SMIg46), dated as 155 ± 4 Ma (Table 1), crops out ~1 km north
of the Monarch mine (Fig. 3B).
Late Eocene Rhyolite
An east-west–trending dike or band of dikes of late Eocene rhyolite porphyry and quartz-rich breccias cuts across the northern side of Spruce Mountain, crossing the range at a pass near the Kille mine (Figs. 3A and 3B; Hill, 1916;
Hope, 1972). Although poorly exposed, surface and dump samples indicate
GEOSPHERE | Volume 12 | Number 1
the presence of several varieties of rhyolite porphyry (72%–74% SiO2 ) dikes,
including altered porphyry dikes with ~25–30 vol% phenocrysts of quartz,
K-feldspar, plagioclase, and minor biotite set in an aplitic groundmass and
less altered varieties with ~10–15 vol% phenocrysts and an aplitic groundmass
(E. Seedorff, personal data; Baril, 2013). A sample of coarse-grained rhyo­lite
porphyry with ~30 vol% phenocrysts was collected near the Black Forest mine
in the north-central part of the district (SMIg49). This sample is potassically
altered and cut by quartz veins with minor molybdenite (Figs. 3A, 3B), and has
a U-Pb zircon age of 39.1 ± 0.6 Ma (Table 1). Dates (40Ar/ 39A) on K-feldspar and
biotite from the same sample, SMIg49, are similar to the U-Pb age, indicating
rapid cooling in the late Eocene (C.D. Henry, 2013, written commun.). A sample
of rhyolite porphyry with ~15 vol% phenocrysts (SMIg51) that was collected
farther east of the Black Forest mine has a U-Pb zircon age of 37.5 ± 0.4 Ma
(Table 1). Samples from the vicinity of the Standard mine on the western side
of the district (Figs. 3A, 3B) yield U-Pb zircon ages of 37.7 ± 0.5 Ma on a flowbanded porphyritic rhyolite dike (SMIg44) and 37.3 ± 0.4 Ma on a rhyolite porphyry with ~30 vol% phenocrysts (Table 1).
Undated Intrusive Rocks
In addition to the intrusive rocks dated as part of this study, several other
occurrences of undated igneous rocks have been reported at Spruce Mountain. A lamprophyre dike of unknown age was observed in the underground
workings of the Black Forest mine (Fig. 3B) by Schrader (1931), and an undated
stock of hornblende diorite intrudes Permian strata near the northern end of
Spruce Mountain Ridge 19 km beyond the northern boundary shown in Figure
3. Small, altered dikes also crop out on Banner Hill (Fig. 3), some of which
contain hydrothermal chlorite and epidote (Hill, 1916).
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
242
Research Paper
MINERAL DEPOSITS AND THE AGE OF
PORPHYRY Mo MINERALIZATION
The Spruce Mountain district contains a porphyry molybdenum deposit
asso­ciated with rhyolite poryphyry intrusions and may also host Carlin-type
gold mineralization. Our geochronologic data and mapping indicate that the
porphyry moldybdenum deposit formed in the late Eocene, ca. 38 Ma, whereas
the age of Carlin-type mineralization in the district remains undetermined.
Skarn and carbonate replacement deposits were first discovered in the
Spruce Mountain district in 1869, and there was at least small production from
these ores for most years between 1869 and 1961 (Hill, 1916; Schrader, 1931;
Smith, 1976; LaPointe et al., 1991). The district historically has been mined for
lead and silver, with subordinate zinc, copper, and gold, from small underground mines, including the Ada H, Banner Hill, Black Forest, Keystone, Kille,
Monarch, Paramount, and Standard (Fig. 3B). The principal hypogene ore minerals are argentiferous galena, tetrahedrite, sphalerite, chalcopyrite, p
­ yrite,
arseno­pyrite, and minor bornite (Schrader, 1931). Scheelite occurs at the Atlan­
tic prospect, which is a few hundred meters west of the Black Forest mine
(Stager and Tingley, 1988). Supergene phases include cerussite, anglesite,
malachite, and chrysocolla along with lesser amounts of wulfenite, calamine,
and smithsonite. Oxidation extends to depths of 70 m or more, and most of the
production was from supergene-enriched carbonate ores that contain higher
silver contents than sulfide ores (LaPointe et al., 1991).
A map of alteration-mineralization products is shown in Figure 3B, which
complements the companion geologic map (Fig. 3A). As shown on these
maps, a belt of skarn, quartz-rich breccias, and local hornfels is discontinuously exposed at the surface along the east-west–trending band of silicic
porphyry dikes, but hornfels is absent and skarn is mostly supplanted by
jasperoid and carbonate replacement and fissure vein deposits in calcareous rocks at the eastern end of the band (Hope, 1972; LaPointe et al., 1991).
At the surface, the porphyry dikes are variably altered but have rare quartzmolyb­denite veinlets with associated potassic alteration and locally exhibit
quartz-sericite-pyrite alteration. In contrast, the skarns consist principally of
garnet, pyroxene, actinolite, and fluorite (Granger et al., 1957; E. Seedorff, personal data; LaPointe et al., 1991; American CuMo Mining Corporation, 2014).
Skarn is localized around the porphyry dikes. Copper and local tungsten are
mostly restricted to skarn; zinc grades generally decrease toward porphyry
dikes (Schrader, 1931). Lead and silver grades are highest in fissure fillings,
commonly in marble or limestone (LaPointe et al., 1991). Both skarn and altered igneous rocks are enriched in tin, tungsten, and fluorine (E. Seedorff,
personal data; Tingley, 1981a, 1981b).
From the 1950s through the early 1980s, the district was explored for porphyry mineralization, and at the end of this period a resource of ~80 Mt of lowgrade molybdenum-copper mineralized rock (average grade not reported) was
delineated in the southwestern part of the district by a joint venture of Freeport
Exploration and AMAX (LaPointe et al., 1991). Complete results are not publicly
available, although published cross sections showing selected results suggest
GEOSPHERE | Volume 12 | Number 1
that a well-mineralized area occurs near the Kille mine (American CuMo Mining Corporation, 2014) in the footwall of a steeply dipping normal fault (probably the West fault; Fig. 3B) and in the footwall of the gently dipping North Peak
fault. In recent years, this part of the district has been controlled by Mosquito
Gold Mines Limited, now known as American CuMo Mining Corporation; this
company reports molybdenum intercepts (copper grades not reported) from 5
of the historic drill holes that are 105–366 m in length (American CuMo Mining Corporation, 2014). The average grades in each of the 5 intercepts range
from 0.05% to 0.17% MoS2 (~0.03%–0.1% Mo). A length-weighted average of
these intercepts, which might be a rough estimate of the average grade of
the resource, is 0.089% MoS2 (~0.053% Mo). Molybdenum is preferentially localized in porphyry dikes, whereas copper is concentrated in adjacent skarns
(American CuMo Mining Corporation, 2014) and appears to be best developed
in Pennsylvanian Ely Limestone. Lead, zinc, and silver preferentially occur in
carbonate replacement deposits and fissure veins in limestones and other calcareous and dolomitic rocks, especially of Pennsylvanian–early Permian age.
Descriptions of underground exposures (e.g., Schrader, 1931) indicate that
porphyry dikes at least locally intruded along normal faults, and a porphyry
dike is mapped as intruding along the North Peak fault (Fig. 3A). Likewise,
porphyry-related skarn mineralization at least locally occurs adjacent to dikes
and along faults (Schrader, 1931), and brecciated quartz veins occur within the
Prospect fault (Figs. 3A and 3B).
There are several textural varieties of porphyritic rhyolite and rhyolite porphyry at Spruce Mountain. A phenocryst-rich (~30 vol%) rhyolite porphyry that
is dated as 39.1 ± 0.6 Ma contains quartz veins and molybdenite (though the
veins and molybdenum in the dated intrusion could be related to a younger
intrusion). An intrusion with 15 vol% phenocrysts dated as 37.5 ± 0.4 Ma locally
contains quartz veins, and intrusions as young as 37.3 ± 0.4 Ma with 30 vol%
phenocrysts underwent strong hypogene sericite alteration. Analogies with
porphyry molybdenum deposits elsewhere in the world (e.g., Wallace et al.,
1968; Carten et al., 1988) suggest that Spruce Mountain may have several, possibly spatially separated, mineralizing intrusions.
Although some crosscutting relationships between faults and rhyolite intrusions are known (Table 2), other relationships remain to be determined. The
spatial and genetic relationship between various rhyolite porphyry dikes and
porphyry-related mineralization also is only weakly constrained (see Table 2).
Therefore, it is not possible to determine whether porphyry-related mineralization was genetically related to intrusions as old as 39.1 ± 0.6 Ma, to intrusions
as young as 37.3 ± 0.4 Ma, or to multiple intrusions of different ages. Consequently, we regard porphyry molybdenum mineralization at Spruce Mountain
as having formed in the late Eocene, ca. 38 Ma.
In the northwestern part of the Spruce Mountain district, six holes drilled
by Santa Fe Mining in the mid-1980s intersected significant gold-bearing intervals, and Renaissance Gold has explored the western side of the district for
Carlin-type gold mineralization in the Pilot Shale and for gold skarn mineralization (Wolverson, 2010). The U-Pb dates place no constraints on the age of
Carlin-type gold occurrences that are reported in the district.
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
243
Research Paper
TABLE 2. FAULTS AND FAULT SETS AT SPRUCE MOUNTAIN
Exposed crosscutting
relationships with
faults of older sets
Faults cut and offset faults
Some faults locally mantled
of sets 4 and 1; no spatial
by Quaternary alluvium, but
overlap with faults of set 5
faults on western range front
active in Quaternary
Faults cut and offset faults
Faults mantled by Quaternary
of sets 4 and 2; no spatial
alluvium
overlap with faults of set 6
Faults cut and offset faults
Intruded by late Eocene (ca. 38 Ma)
of sets 3, 2, and 1
rhyolite porphyry dikes; Miocene
sedimentary rocks mantle faults
of set 4 and seem to have fanning
upward dips from ~30° to 10°E
Faults seemingly cut and
Uncertain
offset faults of set 2
Faults seemingly cut and
Tertiary tuff mantles faults of set 2;
offset faults of set 1
dips ~35°SE
N.A. (oldest set)
Intruded by late Eocene (ca. 38 Ma)
rhyolite porphyry
Fault set
6 (youngest)
West fault
Northwest to
northeast
Northwest and
southwest, steep
to moderate
Eastward
5
Unnamed faults only
Northeast
Southeast, steep
Westward
4
Banner Hill, East,
Mine Shaft faults
North
East, moderate
to steep
Westward
3
Unnamed faults only
East-west
North, steep
Southward
2
Saddle, Coyote, and
Prospect faults
North Peak, South Peak,
Spruce Spring faults
South to
southwest
Variable; North Peak
fault strikes
northeast
West to northwest,
moderate to steep
Variable and shallow;
North Peak fault dips
13°–17° northwest
Eastward
1 (oldest)
Strike direction
Present-day
dip of faults
Expected direction
of concurrent tilting
of fault blocks
Examples of
named faults
Variable; eastward
for North Peak fault
Relationship to Cenozoic
stratigraphic and
intrusive units
Notes
Restricted to western
part of area
Restricted to
southeastern-most
part of area
Spruce Spring fault
cuts and offsets
South Peak fault
Note: N.A.–not applicable.
PLAN-VIEW ANALYSIS OF FAULTING AT SPRUCE MOUNTAIN
As part of a larger study (Pape, 2010), a map-based, plan-view analysis of
extensional faulting at Spruce Mountain was done to help constrain the relative timing relationships among crosscutting normal faults and to determine
the present-day geometries of low-angle faults in the district (Figs. 3 and 4).
The surface geologic map of the Spruce Mountain quadrangle of Hope (1972)
is the primary resource for this analysis, and additional data are incorporated
from other sources, including our new field mapping and checks of key geologic relationships. The names of faults follow those of previous workers
where possible; certain previously unnamed faults have been given names
here for ease of reference.
Crosscutting Fault Relationships
Numerous crosscutting high- and low-angle normal faults have accommodated extensional strain at Spruce Mountain. These crosscutting faults are
grouped into six sets of similar relative age and orientation, and are numbered
1 through 6, from oldest to youngest (Table 2; Fig. 3). In some cases, particularly in the southeastern part of the study area where numerous faults intersect, the crosscutting relationships among faults are uncertain. In these cases,
faults have been assigned to a fault set, but the fault traces in Figure 3 are
dashed and queried to reflect this uncertainty.
GEOSPHERE | Volume 12 | Number 1
Faults from the two relatively youngest sets, fault sets 5 and 6 (Table 2),
do not intersect each other within the study area, and thus their relative ages
cannot be firmly established. Faults of set 6, which crop out on the western
side of the study area, include the present-day range-bounding faults and the
relatively large offset West fault. Faults in this set have northwest to northeast
strikes and dip at high to moderate angles to the northwest and southwest.
Northeast-striking, southeast-dipping, high-angle normal faults of small offset
in the southeasternmost part of the study area are assigned to set 5. Faults of
both of these sets cut faults grouped into the next oldest set, set 4 (Table 2).
Fault set 4 consists of a group of generally north-striking, east-dipping, moderate- to high-angle normal faults that crop out throughout the study area.
Fault set 3 consists of nearly east-west–striking, north-dipping high-angle
faults that traverse the central portion of the study area. Fault set 2 consists
of generally south- to southwest-striking, west- to northwest-dipping, high- to
moderate-angle normal faults that crop out throughout the study area. Certain
west-dipping faults grouped within fault set 2 may, in detail, cut each other.
This is especially true in the southeastern part of the study area, where numerous faults intersect and crosscutting relationships are difficult to determine.
Despite this complexity, these faults have similar orientations and display
simi­lar crosscutting relationships relative to faults from other fault sets; therefore, for simplicity, they have been grouped into fault set 2.
The oldest fault set includes all low-angle normal faults that are present
within the study area. Where they crop out, these low-angle faults are always
cut by high- to moderate-angle faults. However, like faults assigned to fault set
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
244
Research Paper
e
to the axis of the syncline during Mesozoic folding. Given the probable compressional origin of these faults and the fact that they are not intersected by the
restored cross section, they are not analyzed further here.
e
e
t
aul
ct f
spe
Pro
0
880
Mdpc
Mine Shaft fault
Structure Contour Maps of Low-Angle Faults
9200
bx
9600
Mdpc
0
1000
rth
Qa
Qa
Pea
k
fau
lt
Op
DSdu
e
Mdpc
t
00
88
7200
8400
8000
7600
9600
00
92
South Peak faul
9200
8800
e
Prh Pp
k
Pea
Eas
g
rin
Sp
ce lt
ru fau 9600
Sp
W
es
tf
au
l
t
DSdu
rth
No
t fa
ult
No
lt
fau
Qa
Pp
Pp
1km
Figure 4. Structure contours on gently dipping Spruce Spring, South Peak, and North Peak faults
constructed from intersections of mapped fault traces with topography. Contours are elevations
in feet at 200 ft (~60.9 m) intervals; contours are dashed where projected beyond the fault plane
intersection with the land surface. Map unit outlines and abbreviations correspond to those of
Figure 3.
2, individual low-angle faults within fault set 1 in some cases cut and offset
other faults grouped in the same set. Collectively, these low-angle faults are
the oldest extensional structures that crop out at Spruce Mountain (Table 2)
and have, therefore, been included within the same fault set.
Although not strictly included as part of the structural analysis presented
here, a final set of small-offset, high-angle faults crops out in the northeasternmost portion of Figure 3. These faults are part of a family of transfer faults that
occur as subsidiary structures to the Pequop syncline and appear to incorporate a large component of strike-slip offset (Fraser et al., 1986; Swenson, 1991).
These faults likely accommodated localized differential strains perpendicular
GEOSPHERE | Volume 12 | Number 1
Three major low-angle faults (the North Peak, South Peak, and Spruce
Spring faults) that are assigned to fault set 1 (Table 1) crop out on the peak of
Spruce Mountain (Fig. 3). Structure contour maps of these faults were generated from the intersection of their mapped traces with topography to better constrain their dips and three-dimensional geometries (Fig. 4). All three
low-angle faults appear to have relatively planar geometries where they crop
out at the surface (Fig. 4). Dips of faults calculated from these structure contour
maps show that the Spruce Spring fault dips 15°–18° to the southwest, the
South Peak fault dips south ~20°, and the North Peak fault dips northwest at
13°–17°. A subsidiary low-angle fault was originally mapped by Hope (1972)
structurally above the North Peak fault on the North Peak of Spruce Mountain
that juxtaposed the Ely Limestone with the Diamond Peak Formation and/or
Chainman Shale (undivided). Field observations documented in Pape (2010),
however, provide no evidence for this subsidiary fault. Instead, the contact between the Diamond Peak Formation and overlying Ely Limestone is interpreted
to be a depositional contact.
The direction of slip on these low-angle faults has not been directly determined. Strata in the upper and lower plates of these faults dip fairly uniformly
eastward, and stratigraphic section is always missing between the hanging
walls and footwalls of the faults, which place younger strata on older strata. A
westward, normal-sense, hanging-wall transport direction can explain these
observations without requiring exceptionally large amounts of slip on these
faults (Hope, 1972).
Mapped relationships among these faults (Fig. 3) indicate that the Spruce
Spring fault is structurally above and truncates the South Peak fault (Hope,
1972). The relationship between the North Peak fault and the Spruce Spring
and South Peak faults is less certain. The North Peak fault may be truncated by
the northward projections of the Spruce Spring and South Peak faults (Hope,
1972); alternatively, the South Peak fault and North Peak fault may be the
along-strike continuation of the same fault.
GEOLOGIC CROSS SECTION
Stratigraphic and Structural Constraints
As part of our structural analysis of the Spruce Mountain district, we constructed a northwest-southeast–trending cross section through the Spruce
Mountain area showing our interpretation of the subsurface distribution of
faults and stratigraphic units (Fig. 5A) as well as an overlay schematically showing zones of alteration (Fig. 5B). Thicknesses of units are based primarily on
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
245
Research Paper
A
North Peak of
Spruce Mountain
WNW
ESE
3
Dg
lt
lt
fau
OCl
eu
Ds
DSd
faul
t
el
Coy
ote
fau
OCl
P rs
Mdcp
Dg
Hill
lt
OCl
eu
el
dle
eu
Sad
Pro
sp
W ect
fau
es
tf
lt
au
lt
eu
Prh Pp
North Peak fault
Mdcp
Dg
3 km
Pp
Permian Pequop
Formation
Mdcp
Mississippian Chainman/Diamond
Peak, undivided
Prh
Permian Rib Hill
Formation
MDgu
Devonian-Mississppian Guilmette/Pilot-Shale/Joanna Limestone, undivided
P rs
Pennsylvanian
Ely Limestone
P rs
Dg
P rs
Pp
ner
fau
0
OCl
Prh
P rs
Ban
Dg
ft
ha
OCl
DSd
OCl
S
ne
1
e
Dg
Mdcp
Prh
Mdcp
ult
DSd
Mi
Elevation (km)
el
Ds
Ds
DSd
Mdcp
t fa
Eas
2
North Peak fault
el
eu
Pennsylvanian-Permian
Riepe Spring Limestone
eu
Ely Limestone
Upper Member
el
Ely Limestone
Lower Member
DSdu
Silurian-Devonian
Dolomite, undivided
Ds
Devonian Simonson Dolomite
DSd
Silurian Devonian Simonson
Dolomite
OCl
Ordovician Pogonip Group
Fault Sets
(Faults of sets 5 and 3 do not project into
section)
Youngest
6
4
2
1
Oldest
Figure 5 (on this and following page). Geologic cross section across the Spruce Mountain area along line of section shown in Figure 3, with no vertical exaggeration. (A) Cross section showing litho­
logic units and extensional faults color coded by relative ages, with projected locations of measured bedding attitudes shown near the surface. To aid in constraining the fault restoration, certain
lithologic units have been subdivided relative to the associated map (Fig 3B); where this is the case, unit correlations are shown in the legend of the cross section. Note that the igneous rocks are
omitted because of poor three-dimensional control (see text).
measurements reported by Hope (1972). Faults belonging to sets 6, 4, 2, and 1
project into the cross section (Fig. 5). Publicly available exploration drilling data
in the Spruce Mountain district are sparsely distributed and limited to relatively
shallow depths (typically <0.5 km). Therefore, it not possible to meaningfully
show the distribution of igneous rocks on the cross section of Figure 5A.
Within the Spruce Mountain district, attitudes of bedding typically do not
change substantially between the footwall and hanging-wall blocks across
faults, suggesting that these faults are planar or curviplanar in the subsurface.
There are few direct measurements of fault dips in the Spruce Mountain area
because of poor exposure of fault surfaces in the district. Hope (1972) reported
that the East fault and West fault dip ~45° to the east and west, respectively,
near where these faults intersect the cross section (Fig. 5). In addition, the
GEOSPHERE | Volume 12 | Number 1
North Peak fault projects into the cross section with an apparent northwest dip
of ~5° (Fig. 5). Otherwise, dips on faults are based on relationships between
topography and the surface traces of faults.
It is uncertain how the Spruce Spring and South Peak faults may project
into the cross section (Figs. 3 and 4). Hope (1972) interpreted both of these
faults as steepening northward and truncating the North Peak fault south of
the cross section. Alternatively, the South Peak fault could represent the alongstrike continuation of the North Peak fault if the projection of this fault is bowed
over the peak of Spruce Mountain. It is also possible that the Spruce Spring
fault may project into the cross section at high structural and stratigraphic
­levels. However, given the uncertainties in its three-dimensional geometry, the
Spruce Spring fault was not projected into the section of Figure 5.
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
246
Research Paper
B
North Peak of
Spruce Mountain
WNW
ESE
Hill
t
Coy
ote
faul
Pro
sp
W ect
fau
es
tf
lt
au
lt
ner
lt
fau
lt
fau
0
Ban
ft
ha
S
ne
1
Sad
ult
dle
fau
t fa
Eas
lt
North Peak fault
2
Mi
Elevation (km)
3
3 km
Alteration Zones
Figure 5 (continued ). (B) Overlay showing distribution of
selected alteration-mineralization features, with extensional
faults shown for reference from A, color coded by relative age.
By comparison with the associated map (Fig. 3B), note that
the line of section departs somewhat from the overall trend of
the known distribution of alteration mineralization; in those
portions of the section, alteration-mineralization features are
structurally projected into the cross section. Given the limited information about crosscutting relationships between
individual faults, alteration-mineralization features, and intrusions and their subsurface distribution, this overlay should be
regarded as a preliminary, largely speculative interpretation.
Iron-stained quartz breccia
Fault Sets
(Faults of sets 5 and 3 do not project into
section)
Youngest
Quartz-molybdenite veins at depth
Skarn +/- hornfels with copper
Jasperoid
Carbonate replacement deposits with Pb-Zn-Ag-Cu
6
4
2
1
Oldest
Present-Day Distribution of Structures
The cross section shown in Figure 5 represents the interpretation of the
present-day structure of the Spruce Mountain area that best fits available data.
A critical aspect of the subsurface structure is the geometry of the low-angle
North Peak fault, which is cut and downdropped on either side of the peak of
Spruce Mountain by the East and West faults (Fig. 5). In Figure 5, the North
Peak fault is inferred to maintain a shallow west-dipping orientation in the subsurface and to root westward in the apparent hanging-wall transport direction.
The eastern continuation of the North Peak fault is interpreted to project above
the present-day land surface in the eastern part of the cross section, where it is
cut by the small-offset Coyote fault, and to project above the southern Pequop
Range east of Spruce Mountain (Figs. 3 and 5).
This interpretation differs substantially from the previous structural interpretation along the same line of section by Hope (1972), who depicted the
North Peak fault as abruptly changing orientations from a subhorizontal dip
GEOSPHERE | Volume 12 | Number 1
on the peak of Spruce Mountain to dipping ~40° E in the eastern portion of the
cross section after being downdropped by the East fault. Hope’s (1972) structural interpretation results in the North Peak fault projecting beneath the southern Pequop Range, where no surface exposures of major extensional faults are
present (Fraser et al., 1986; Swenson, 1991), and, therefore, requires the entire
southern Pequop Range to be underlain by the North Peak fault.
STRUCTURAL RECONSTRUCTION
Methodology
A stepwise cross-sectional restoration of extensional faulting illustrates
the Cenozoic structural evolution of the Spruce Mountain area (Fig. 6). The
primary geologic markers employed for restoring fault offsets are Ordovician
through Permian sedimentary rocks that are assumed to maintain relatively
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
247
WNW
~12.6 km
ESE
Fault Sets
(Faults of sets 5 and 3 do not project into
section) Youngest
248
A
6
4
2
1
Present-day
structure
Oldest
B
Minimum relative
paleosurface elevation
allowed at each reconstruction step
(ignores paleotopography)
Fault set 6
restored
C
0 km
0 km
4 km
Pp
Fault set 4
restored
Prh
Permian Limestone and
Siltstone
P rs
eu
D
Pennsylvanian
Limestone
el
Mdcp
Missippian Sandstone, Siltstone,
Conglomerate and Shale
MDgu
Fault set 2
restored
Ds
Devonian and Silurian
Limestone and Dolomite
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
4 km
DSd
OCl
E
Ordovician - Cambrian,
undivided
~5.7 km
Figure 6. Stepwise structural reconstruction of the central Spruce Mountain area, based on cross section of Figure 5. (A) Present-day structure and pin lines along cross section,
with faults and key stratigraphic horizons projected above the topographic profile. Horizons projected above present-day topography are shown with 50% transparency. (B–E)
Sequential restoration of crosscutting normal faults of sets 6, 4, 2, and 1 (faults of sets 3 and 5 not present in cross section). The dashed gray horizontal line shown in each
restoration panel represents the minimum relative paleosurface elevation allowable at each restoration step (ignoring local topography). The magnitude of fault block rotation
applied to each panel relative to immediately preceding panel is as follows: (B) 22°WNW (C) 33°ESE (D) 20°WNW (E) 27°WNW.
GEOSPHERE | Volume 12 | Number 1
Research Paper
Fault set 1
restored
(pre-extension)
Research Paper
constant gross thicknesses within the study area. No attempt was made in the
reconstructed cross section to differentiate stratigraphic units older than the
Pogonip Group, which is the oldest marker employed in the reconstruction,
and igneous units are omitted for reasons cited herein.
Roughly simultaneous movement on subparallel faults is expected from
geometrical considerations (e.g., Davison, 1989), and certain active fault systems in the Basin and Range province behave in that manner. For example,
major steeply east dipping faults at Rainbow Mountain, Dixie Valley, and Fairview Peak, western Nevada, exhibited coordinated movement during a series
of earthquakes in 1954 that accommodated westward tilting of the intervening basins and ranges. In addition, concurrent movement was observed on
the west-dipping, antithetic Westgate and Gold King faults immediately east
of that fault system (e.g., Slemmons and Bell, 1987; Hodgkinson et al., 1996;
Caskey et al., 1996). Detailed studies of areas with multiple sets of Cenozoic
domino-style faults are also consistent with coordinated movement on major
subparallel normal faults and synthetic splays (e.g., Proffett, 1977; Gans and
Miller, 1983).
For Spruce Mountain, we utilize a stepwise approach to restoration, restoring movement on all faults of a given set concurrently, beginning with the
youngest set (set 6), then working backward in time, set by set (Fig. 6). The
movement on any given fault is assumed to become locked when the stress
required to produce continued slip along them is greater than required to initiate a new set of high-angle faults. Once a fault is cut and offset by a fault of a
younger set, the older fault also is buttressed against any renewed movement.
Given the nature of the geologic complexity coupled with limited three-dimensional exposure at Spruce Mountain, however, it is not possible to preclude the
possibility that some faults with opposing dips that are members of adjacent
fault sets might have been active concurrently. The offsets on faults in the
cross section (Fig. 5A), which belong to sets 6, 4, 2, and 1, are restored in a
stepwise fashion from youngest to oldest (Fig. 6). The cross section is restored
as a series of rigid fault blocks, ignoring internal deformation (e.g., folding)
within individual fault blocks. These assumptions can lead to space problems
where fault blocks are differentially tilted. However, these space problems are
small relative to the uncertainties of subsurface fault dips and are assumed to
be accommodated by slight curvature of faults and/or internal deformation
within major fault blocks.
Evidence for Tilting
Several lines of evidence suggest that extensional faulting was accompanied by fault block tilting, ultimately resulting in a net eastward tilting of fault
blocks in the Spruce Mountain area. (1) Outcrops of (Miocene?) sedimentary
rocks within the southern portion of the study area display generally eastward
dips between 15° and 30°. (2) The trace of the basal contact of the undated
(likely either Eocene or Miocene) Cenozoic dacite tuff relative to topography
(Fig. 3) suggests that this tuff dips gently to moderately eastward in the central
GEOSPHERE | Volume 12 | Number 1
Spruce Mountain district. Moreover, although no strike and dip measurements
directly constrain the bedding orientation of this tuff in the immediate study
area, a single dip measurement of 35°SE was obtained on a small outcrop of
the tuff ~3 km beyond the northern edge of the area in Figure 3 (Hope, 1972),
and is consistent with the observation that the tuff unit as a whole appears to
have been tilted eastward. (3) Paleozoic strata within the Spruce Mountain area
dominantly dip eastward, which is consistent with eastward extensional tilting
if these strata were horizontal or gently dipping prior to Cenozoic extension.
This is a reasonable assumption if the preextensional structure in the Spruce
Mountain area was characterized by broad open folding of a style similar to the
adjacent Pequop syncline immediately to the east. (4) East-dipping Paleozoic
strata in the footwalls and hanging walls of the normal faults that currently
dip at low angles generally display moderate to high (>40°) cutoff angles with
these faults where they are exposed at the surface, suggesting that these faults
initiated at high to moderate angles. (5) The inferred westward hanging-wall
transport directions of the currently low-angle faults is consistent with eastward fault block tilting if these faults initiated as higher angle, west-dipping
normal faults.
Given this evidence, rotation consistent with the sense of slip of each restored fault set is applied to the stepwise cross-sectional restoration (Fig. 6).
Although the magnitude of tilting associated with each fault set is uncertain,
there are some loose constraints on the overall history of tilting (Table 2).
The undated southeastward-dipping Cenozoic tuff mantles faults of sets 1
and 2, implying that most or all of the southeast-directed tilting of this unit
resulted from movement on faults of younger sets. In addition, the sedimentary rocks in the southeastern part of the map area, probably belonging
to the middle Miocene Humboldt Formation, lap across faults of set 4 and
appear to have fanning-upward dips from 30° to 15°E (Table 2). This suggests that fault blocks in the southern part of the district have undergone
a net eastward tilting of as much as 30° since the mid-Miocene. Given the
relative age constraints, and considering that both the Cenozoic dacite tuff
and sedimentary rocks display similar eastward dips in the central Spruce
Mountain district, it is likely that most of the eastward tilting of these rocks
occurred in association with slip on west-dipping faults grouped with fault
set 6 (Figs. 3 and 6). Because cutoff-angle relationships suggest that the
currently low-angle faults in the Spruce Mountain area most likely initiated
at high to moderate angles, the final reconstructed panel (Fig. 6E) is shown
tilted 36° west relative to the present-day cross section. This restores the
North Peak fault, which currently dips northwest at 13°–17° (Fig. 4), to an initial northwestward dip of ~45°–50° (Fig. 6E). Although the Spruce Mountain
area contains faults of varying polarity over time (Table 2), the net eastward
tilting of ~35° suggests that movement on faults of the three sets of downto-the-west faults, of the first, second, and sixth generations of normal
faults, dominated the overall tilting history.
Further constraint on the approximate history of fault block tilting associated with each fault set comes from the probable amount of differential burial
and/or exhumation of the Paleozoic marker strata at each time step in the
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restoration. Apatite fission track and (U-Th)/He analyses from probable Jurassic granite plutons that intrude Permian strata at Delcer Buttes (~25 km
southwest of Spruce Mountain) are consistent with these strata having remained at low temperatures (<60 °C) throughout the Cenozoic (Colgan et al.,
2010). Given the proximity of the Spruce Mountain district to Delcer Buttes
(Fig. 1) as well as the presence of probable Paleogene rocks that unconformably overlie Permian and Pennsylvanian rocks in the Spruce Mountain district,
it is reasonable to assume that Permian strata were not deeply buried (i.e.,
no greater than 3 km, and probably less) at Spruce Mountain at the onset of
Cenozoic extension. These Permian rocks most likely remained at relatively
shallow depths throughout active Cenozoic extension as well. Because rocks
at or below the present-day land surface must have also been at or below the
paleosurface throughout the deformational history of Spruce Mountain, the
cross-sectional restoration provides constraint on the deepest possible paleosurface elevation relative to the restored Paleozoic strata at each time step.
This paleosurface constraint is depicted in Figures 6B–6E as a straight, dashed
line (ignoring any paleotopographic relief). At each restoration step, tilting
magnitudes were applied to the section that maintain consistency with probable shallow burial depths of Permian and younger strata at Spruce Mountain
during the Cenozoic. The restoration shown in Figure 6 does not require more
than 2 km of burial for Permian marker rocks at any given step in the restoration (although greater burial depths are still permissible within the available
constraints) and is also consistent with other tilting constraints based on dips
of rocks in the district.
Restored Preextensional Structure of the Spruce Mountain Area
The magnitude of extension restored in the cross section, which is the difference in final and initial distances between reference lines of 12.6 and 5.7 km,
respectively, is ~6.9 km, which is equivalent to 120% extension (Fig. 6). Although the total amount of extension across the section is large, it was distributed among numerous faults. The largest amount of slip on any one fault in
the restored section, the Banner Hill fault, is 1.9 km; however, the North Peak
fault, West fault, Mine Shaft fault, and Saddle fault all have offsets >1 km. Some
low-angle faults mapped in the area, such as the Spruce Spring fault, were not
projected into the reconstructed cross section; if, alternatively, they are present
in the section, then this would increase the estimate of extension across the
Spruce Mountain area, such that the estimates of the amount of extension
based on Figure 6 may be closer to a minimum value.
Note that reconstruction of Figure 6 implies that some beds rotated
through horizontal as a result of tilting associated with multiple generations
of normal faults; for example, Paleozoic rocks on the eastern side of the map
area that currently dip 20°–30°SE (Figs. 3 and 6A) would have dipped ~10°–15°
northwest prior to faulting (Fig. 6E). As a result, the attitudes of these Paleozoic
rocks in the restored section imply west-vergent folding and gentle westward
dips (~15°). The overall westward dip of beds in the restored section is inter-
GEOSPHERE | Volume 12 | Number 1
preted to reflect that the Spruce Mountain district was involved in large-scale,
open folding of a character similar to the Pequop syncline immediately east of
Spruce Mountain. Spruce Mountain may have been on the western limb of an
anticline that connected to the Pequop syncline prior to being dismembered
by extensional faulting and may have connected to another upright syncline
further west. The kilometer-scale, west-vergent anticline shown in the restored
cross section (Fig. 6E) is consistent with the scale and character of west-vergent folding and thrust faulting that occurred in the southern Pequop Mountains during development of the Pequop syncline (Swenson, 1991) and with
the notion that the Spruce Mountain area was similarly deformed during the
same contractional deformational event.
Regional studies of Mesozoic deformation and metamorphism in the
­Pequop Mountains–Wood Hills–East Humboldt Range region to the north of
the Spruce Mountain area and in the Ruby Mountains to the west have demonstrated that significant crustal thickening must have occurred in northeastern
Nevada during the Mesozoic (e.g., Thorman, 1970; Snoke and Miller, 1988;
Camilleri and Chamberlain, 1997). Metamorphic mineral assemblages and
sheared fabrics locally present in impure limestones of the Pogonip Group
on the southern side of Spruce Mountain have been interpreted to indicate
that these rocks reached lower greenschist facies metamorphic conditions
during the Mesozoic and that significant structural thickening occurred in the
Spruce Mountain area during the Mesozoic (Camilleri and Chamberlain, 1997).
However, no thrust faults have been mapped locally in the Spruce Mountain
district, and the reconstructed cross section (Fig. 6) suggests that significant
structural duplication of stratigraphic section between Ordovician and lower
Permian strata has not occurred at Spruce Mountain. Thus, any significant
structural thickening and overthrusting at Spruce Mountain during the Mesozoic was likely limited either to deeper or, possibly, shallower structural and
stratigraphic levels than are currently exposed at the surface.
Constraints on Age of Extensional Faulting at Spruce Mountain
Several observations constrain the timing of faulting at Spruce Mountain.
First, all normal faults in the Spruce Mountain district deform Paleozoic rocks
and dismember older Mesozoic contractional structures and, given regional
considerations regarding the onset of extension in the Great Basin (e.g., Wernicke et al., 1987; Seedorff, 1991a; Dickinson, 2006), are most likely all Cenozoic
structures, although ages as old as Late Cretaceous are possible. Eocene rhyolite porphyry dikes (ca. 38 Ma; Table 1) crop out throughout the central portion
of the Spruce Mountain district (Fig. 3). These dikes cut or intrude the North
Peak fault (a member of fault set 1) and locally intrude the Mine Shaft fault (a
member of fault set 4) (Fig. 3; D. Pace, 2013, personal commun.). Thus, both of
these faults, and by analogy other faults of these sets, were active during or
prior to the late Eocene. Sedimentary rocks that crop out in the southern part
of the study area, which probably correlate with the middle Miocene Humboldt Formation (Harlow, 1956; Coats, 1987), dip eastward and appear to dis-
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play fanning-eastward dips, indicating that at least some extensional faulting
within the Spruce Mountain district occurred during the middle to late Miocene. Based on relative age constraints, some of the faults that are grouped in
sets 5 and 6 were probably active in the middle to late Miocene. The ages of
faults that were active during the Quaternary have been determined mostly on
the basis of photogeologic evidence (e.g., Dohrenwend et al., 1991, 1996). As
shown by dePolo (2008), several normal faults mapped on the eastern sides
of Spruce Mountain Ridge and Spruce Mountain ruptured in the Quaternary
(i.e., in the past 1.8 m.y.), and the latest slip on normal faults mapped along
the western sides of Spruce Mountain Ridge and Spruce Mountain probably
is even more recent (latest Quaternary, i.e., <130 k.y.). Notably, several faults
grouped in fault set 6 appear to have been active during the Quaternary, including the relatively large offset West fault. Given the current absolute and
relative age constraints at Spruce Mountain, it is not possible to determine
rigorously which faults grouped in sets 5 and 6 initiated during the probable
middle to late Miocene extensional event and which faults initiated later. It is
possible (and even likely) that some faults included in these relatively youngest fault sets initiated during the middle to late Miocene and subsequently
were active more recently, whereas other faults also grouped in these sets
may have initiated much later.
Restoration of Magmatic-Hydrothermal Features at Spruce Mountain
The reconstruction in Figure 6 and crosscutting relationships can be utilized to infer the original geometry of mineralized zones at Spruce Mountain.
Because porphyry dikes intrude faults of sets 1 and 4, and quartz veins appear
to have formed along the Prospect fault of set 2, it appears that at least some,
if not all, of the hydrothermal alteration features formed after movement on
faults of sets 1, 2, and 3, and perhaps during movement on faults of set 4.
Thus, the hydrothermal system might have been subjected to extension and
associated tilting related to movement on faults of sets 5 and 6 and possibly
some portion of the movement on faults of set 4. Consequently, the alteration
zones were probably formed after the time represented in the restoration by
Figure 6C, but before that in Figure 6B. Figure 7 shows the restored distribution
of interpreted hydrothermal features (Figs. 3B and 5B) prior to movement on
faults of sets 4 through 6.
Given the large geologic uncertainties, the distribution of features is plausibly consistent with a relatively upright hydrothermal system that developed as
faults of set 4 began to move. Based on this reconstruction, the original two-dimensional geometry of mineralized zones was a ~6 km long by ~3 km deep elliptical pattern with carbonate replacement deposits surrounding skarn. In addition, given our assumptions regarding the timing of mineralization relative
to extensional faulting, the stepwise reconstruction of Figure 6 indicates that
the hydrothermal system underwent ~10° of westward net tilting and ~2.7 km
of extension associated with postmineralization deformation by fault sets 4
through 6.
GEOSPHERE | Volume 12 | Number 1
DISCUSSION
Comparison and Classification of Mineral Deposits
at Spruce Mountain
The presence of rhyolite and granite porphyry dikes, quartz-molybdenite
veins, and the geochemical enrichment in Sn-W-F suggest that the mineral
deposits at Spruce Mountain, except for the Carlin-type prospect, are various
manifestations of a porphyry molybdenum deposit. The limited subsurface
information available suggests that the proximal part of the system contains
quartz-molybdenite veins and is relatively copper free, in contrast to porphyry
molybdenum deposits of the quartz monzonitic-granitic porphyry Mo-Cu subclass such as the Hall (Nevada Moly) and Buckingham deposits in Nevada
(Seedorff et al., 2005). The best copper grades at Spruce Mountain seem to
be somewhat more distal, located in skarns that are developed fairly close to
contacts with rhyolite and granite porphyry.
The best analogues for Spruce Mountain are the Mount Hope deposit in
Eureka County 145 km to the southwest (Westra and Riedell, 1996) and several
molybdenum prospects in White Pine County to the south (Seedorff, 1991a).
Spruce Mountain and Mount Hope are assigned to the rhyolitic porphyry Mo
category (of Seedorff et al., 2005), which includes end-member deposits such
as Climax and Henderson (Colorado), but also deposits that have less evolved
igneous compositions (e.g., lower SiO2, Rb, and Nb and higher Sr and Zr contents) and metals with higher overall Cu:Mo ratios, such as Mount Hope. The
latter deposits were referred to as transitional deposits by Westra and Keith
(1981, Table 2 therein) and Carten et al. (1993, Table II therein). In the classification of Ludington and Plumlee (2009), the Spruce Mountain deposit would be
regarded as a Climax-type deposit.
Spruce Mountain may be the best example of a rhyolitic porphyry Mo or
Climax-type porphyry molybdenum deposit with extensively developed skarn.
The overall copper content of Spruce Mountain and certain prospects in White
Pine County such as the Ellison district (Johnson, 1983) may be somewhat
higher than other rhyolitic porphyry molybdenum deposits where carbonate
rocks are largely to entirely absent, such as Climax, Henderson, Mount Hope,
and Pine Grove (Utah). Carbonate wall rocks probably promoted efficient precipitation of copper in skarn at Spruce Mountain and the occurrences in White
Pine County compared to other rhyolitic porphyry Mo deposits.
Significance of Ages of Igneous Rocks and Ore Deposits
at Spruce Mountain
With some notable exceptions, including a large body of work in the Ruby
Mountains–East Humboldt metamorphic core complex (e.g., Hodges et al.,
1992; Wright and Snoke, 1993; Brooks et al., 1995; Henry, 2008; Bedell et al.,
2010), there are few radiometric ages in this part of northeastern Nevada,
where exposures of igneous rocks generally are sparse. U-Pb ages reported
here for igneous rocks at Spruce Mountain are Late Jurassic and late Eocene.
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Alteration Zones
Fault Sets
Youngest
Iron-stained quartz breccia
2
1
Quartz-molybdenite veins at depth
Skarn +/– hornfels with copper
Oldest
Jasperoid
Carbonate replacement deposits with Pb-Zn-Ag-Cu
0
Figure 7. Interpretive preliminary reconstructed cross section of selected
alteration-mineralization features at Spruce Mountain, showing how
present-day distribution of features from Figure 5B might be distributed
prior to movement of faults of set 4 in geologic reconstruction of Figure 6.
3 km
The Jurassic rocks at Spruce Mountain occur as small dikes that are dated
as 155–156 Ma. Large Jurassic intrusions occur in the Dolly Varden Mountains, Delcer Buttes, Ruby Mountains, and Snake Range (Miller et al., 1988;
Miller and Hoisch, 1995; Barton et al., 2011), and small dikes of Jurassic age
are widespread but sparsely present in other parts of northeastern Nevada,
such as in the Pequop Mountains (Bedell et al., 2010; Camilleri, 2010; Wyld and
Wright, 2015). At some localities such as Spruce Mountain, the Jurassic rocks
GEOSPHERE | Volume 12 | Number 1
are barren, whereas they are at least variably mineralized at the present levels
of exposure at other sites, such as the Delcer district (LaPointe et al., 1991) and
the Victoria skarn breccia pipe in the Dolly Varden district (Atkinson et al., 1982;
Zamudio and Atkinson, 1995).
Dated rhyolite porphyries at Spruce Mountain range in age from ca. 39
to ca. 37 Ma (Table 1) and have ages similar to those of granite and rhyolite
porphyry bodies in nearby parts of eastern Nevada (Stewart and Carlson, 1976;
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Research Paper
Seedorff, 1991a). The Spruce Mountain porphyry molybdenum deposit, dated
here as ca. 38 Ma, has an age similar to that of the Mount Hope porphyry
molybdenum deposit in Eureka County (ca. 37 Ma; Westra and Riedell, 1996).
Although ore deposits have formed in the Great Basin over many intervals of geologic time, especially numerous epithermal deposits in the Miocene
(e.g., John, 2001), the late Eocene was an important period of ore formation
in the Great Basin, including Carlin-type gold deposits, porphyry copper and
related skarn, replacement, vein and distal disseminated deposits, porphyry
molybdenum, and a few adularia-sericite–type or low sulfidation–type epithermal silver-gold deposits (e.g., Seedorff, 1991a).
Structural Restoration Uncertainties and Constraints
Section balancing and restoration techniques were originally developed in
contractional settings (Dahlstrom, 1969) and have since been widely applied
in the structural analysis of fold and thrust belts (e.g., Hossack, 1979; Mitra,
1992; Wilkerson and Dicken, 2001). Cross-sectional reconstructions are less
commonly attempted in extensional environments but have been increasingly
applied in settings of continental extension, particularly in the Basin and Range
province (e.g., Proffett, 1977; Wernicke and Axen, 1988; Smith et al., 1991; Seedorff, 1991a; McGrew, 1993; Brady et al., 2000; Colgan et al., 2008, 2010; Nickerson et al., 2010). As these extensional restorations become more commonly
applied, it is important to acknowledge key uncertainties that are inherent to
cross-section reconstructions, particularly in areas of polyphase deformation
such as northeastern Nevada.
A common uncertainty in cross-section restorations, and one of the most
important uncertainties in the Spruce Mountain restoration presented here, is
the slip direction of the individual restored faults and the possibility of fault
movement out of the plane of the restored cross section. No reliable piercing
points or slickenline data are available from within the Spruce Mountain district to fully constrain the kinematic histories of each fault, and as a simplifying
assumption for the cross-sectional restoration, all restored faults in Figure 6
are assumed to have moved in a pure dip-slip sense, parallel to the line of section. Although this is probably a reasonable approximation of the major fault
slip directions given that the line of section was chosen to extend orthogonal
to the strike orientations of the major mapped normal faults, this simplified
kinematic history is unlikely to be true in detail. This simplifying assumption
of pure dip-slip motion along the line of the restored cross section effectively
adds uncertainty to the estimates of offset on individual faults, as well as the
extension estimates at each step of the cross-section restoration.
In addition, uncertainties regarding present-day fault dips and the magnitude of tilting applied to each step of the restored cross section have a direct
impact on extension estimates at each step of the restoration. Of these two
variables, the net tilting magnitude uncertainty is the most easily quantified.
For example, we restore ~35° of net eastward tilting between the present day
and reconstructed cross sections (Figs. 6A, 6E). Although this tilting magnitude
satisfies all available geologic constraints, it is likely close to a minimum esti­
GEOSPHERE | Volume 12 | Number 1
mate of the overall net tilting associated with extensional faulting at Spruce
Mountain. As much as 10° of additional east-directed tilting could reasonably
be restored in the final reconstructed cross section (Fig. 6E) without requiring
excessive burial depths of Permian sedimentary rocks. Restoring an additional
10° of eastward tilting in Figure 6E would increase the overall extension estimate by 0.8 km to 7.7 km. Moreover, the tilting uncertainties of the intermediate
restoration steps are more poorly constrained than the overall net tilting history, and based on burial and/or exhumation and fault geometry constraints,
we estimate a tilting uncertainty of ±10° for each intermediate step. Although it
is a straightforward exercise to quantify the impact of fault dip uncertainty on
extension estimates for an individual curviplanar normal fault or even a single
set of kinematically linked normal faults (e.g., Wernicke and Burchfiel, 1982),
for an intensely faulted area with multiple overprinting, crosscutting fault sets,
such as Spruce Mountain, this exercise becomes more challenging. In part this
is because, in a complexly faulted area like Spruce Mountain, the crosscutting
fault relationships themselves provide considerable constraint on the range of
structurally viable fault dips and geometries in the subsurface, and individual
fault geometries frequently cannot be varied independently and still result in
viable retrodeformable cross sections. Thus, in structurally complex areas like
Spruce Mountain, accurate quantification of the impact of fault dip uncertainty
on extension estimates would necessitate the construction of multiple alternative cross-section reconstructions.
The polyphase deformational history of northeastern Nevada further
complicates the structural restoration of the Spruce Mountain district (Figs. 6
and 7). Specifically, the primary structural markers used for restoring fault offsets are Paleozoic passive margin strata, but these strata had an earlier history
of contractional deformation during the Mesozoic and possibly in the Paleozoic
(Snyder et al., 1991; Carpenter et al., 1993; Camilleri and Chamberlain, 1997;
Howard, 2003; Trexler et al., 2003, 2004; Dickinson, 2006; Long, 2012, 2015;
Rhys et al., 2015). At Spruce Mountain, Cenozoic extension has dissected and
obscured the earlier contractional structures. Hence, there are uncertainties in
both the nature of the extensional deformation, as well as the configuration
of the preextensional structure. Consequently, an iterative approach is used
in this study to produce a viable cross-sectional restoration, wherein the pre­
exten­sional model is progressively modified to produce a restored extensional
structural geometry that satisfies all of the available geologic constraints.
This process results in a degree of internal circularity within the retrodeformable cross section because the extensional structure and the preextensional
structural model are not mutually independent, i.e., uncertainties regarding
the structure of both the extended and restored states of the cross sections
effec­tively increase the number of potentially valid retrodeformable cross-sectional models.
Additional uncertainties are also introduced when attempting to reconstruct mineralized systems. Reconstruction of the geometry of magmatichydro­thermal systems requires constraints on the ages of various generations
of faults relative to the ages of intrusion and associated alteration mineralization, but the geometric constraints applicable to preextensional hydrothermal
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Research Paper
systems, such as Yerington (Proffett, 1977; Richardson and Seedorff, 2015) or
Teacup (Nickerson et al., 2010), are more straightforward than for synextensional hydrothermal systems such as Spruce Mountain. The observation that
a given intrusive rock intrudes a fault requires that the intrusion postdate at
least the initial movement on the fault; if the intrusion (e.g., a dike) is sheared
or brecciated along the fault, then the same fault may have continued moving
after the dike intruded the fault zone. Thus, the amount of postemplacement
deformation of the dike associated with movement on the fault may range
from 0% to nearly 100% of the total deformation associated with movement on
a given fault, and this may introduce considerable uncertainty regarding the
amount of postemplacement extension. Analogous reasoning applies to the
relative ages and amounts of postmineral extension of hydrothermal systems
that may be related to intrusions.
Despite these uncertainties, every step in a viable reconstruction must satisfy certain firm constraints: they cannot imply erosion of units that are still
present in the bedrock today, nor can bedrock units exposed at the surface
today be buried to depths during any stage in the structural evolution that are
inconsistent with their present-day textures and thermochronologic characteristics. Moreover, the restoration must result in reasonable fault geometries
and imply plausible fault kinematics. The reconstruction presented here meets
these tests, although there are uncertainties in the assignments of the amount
of extension to any generation of faults. The most robust reconstructions are
possible from locales where there are abundant, widely distributed, pre-ore,
syn-ore, and post-ore strata and intrusions that create compelling crosscutting
relationships and that contain radiometrically datable units.
Distribution, Polarity, Amount, and Style of
Large‑Magnitude Extension
Cenozoic upper crustal extensional strain was generally directed east-west
to northwest-southeast within northeastern Nevada and is distributed hetero­
geneously, with areas of high extensional strain separated by relatively low
strain areas (Seedorff, 1991a; Colgan and Henry, 2009). This heterogeneous
distribution of upper crustal extensional strain is a characteristic feature of
extension in the Great Basin (e.g., Gans and Miller, 1983). Combined with a
relatively uniform crustal thickness of 30–35 km across the eastern Great Basin
(e.g., Allmendinger et al., 1987), this heterogeneity requires large-scale middle
to lower crustal flow of rocks from domains that have undergone little ­upper
crustal extension into regions of large upper crustal extension (e.g., Gans,
1987; Smith et al., 1991; MacCready et al., 1997).
Consistent with these regional observations, normal faulting and large
extensional strain was directed east-west to northwest-southeast at Spruce
Mountain and is more intense at Spruce Mountain than in the nearby southern Pequop Range, which is an essentially unextended structural block that
preserves Mesozoic compressional structures (Swenson, 1991; Camilleri and
Chamberlain, 1997). Our reconstruction of Spruce Mountain indicates that the
5.7 km distance between reference lines in the restored cross section (Fig. 6E)
GEOSPHERE | Volume 12 | Number 1
was extended by 6.9 km, resulting in a present-day length of ~12.6 km (Fig.
6A). This is equivalent to 120% extension between the fully restored and extensionally deformed states. Spruce Mountain, however, differs somewhat from
other nearby highly extended areas in northeastern Nevada in that the large
extensional strain in the central Spruce Mountain district is distributed among
numerous, closely spaced, small to moderate offset (<2 km) faults in multiple,
superimposed sets of faults. By contrast, extension in nearby southern Ruby
Mountains appears to have been accommodated primarily by large offset (13–
22 km) on a single west-dipping fault that produced eastward tilting (Colgan
et al., 2010). Although Colgan et al. (2010) did not specify the amount of extension in the southern Ruby Mountains and vicinity, our choice of reference lines
in their reconstruction (Colgan et al., 2010, fig. 11 therein) implies only ~45%
extension, albeit over a present-day width of ~19.3 km. The relative amount of
extension at Spruce Mountain is roughly three times that of the southern Ruby
Mountains, although the absolute amount of extension at Spruce Mountain
(~6.9 km) is somewhat smaller than that in southern Ruby Mountains (~8.6 km).
Notably, there is at least one fault within the southern part of the Spruce
Mountain district, the Spruce Spring fault, which appears to have much larger
offset than any other mapped faults in the district. This currently low-angle
fault (which is grouped as part of fault set 1) places Permian on Ordovician
rocks in the southern part of the Spruce Mountain district (Fig. 3). Assuming a
previously unbroken stratigraphic thickness of ~4.5 km for the Ordovician to
Permian sequence (Fig. 2), a ~50° initial westward dip for the Spruce Spring
fault, and a gentle westward preextensional dip of 15° of the Paleozoic rocks
at Spruce Mountain (Fig. 6), a minimum ~8 km of offset would be required on
the Spruce Spring fault to place Permian on Ordovician strata. However, the
Spruce Spring fault cuts off the South Peak fault, so the stratigraphic section
was not unbroken; therefore, the 8 km estimate may considerably overestimate the amount of slip on the Spruce Spring fault.
The more distributed overall style of brittle extension at Spruce Mountain
nonetheless contrasts with the style of extension observed in the southern
Ruby Mountains. Precisely why extensional strains are in some cases distributed among numerous, closely spaced faults and elsewhere concentrated on
a few large-offset faults is not well understood. Pervasive and distributed extensional faulting of bidirectional polarity can occur in transfer zones between
major normal fault systems (e.g., Faulds and Varga, 1998). Geophysical models also suggest that the style of extension may be strongly affected by the
thermal structure of the lithosphere and that normal faults that undergo large
amounts of hydrothermal cooling during extension may have a tendency to
accumulate greater amounts of offset (Lavier and Buck, 2002).
Initial Dips of Normal Faults
The angle of initiation of normal faults in continental extensional tectonics is a persistent controversy (e.g., Proffett, 1977; Wernicke, 1981; Gans et al.,
1985; Roberts and Yielding, 1994; Wong and Gans, 2008), and structural reconstruction is an important method for constraining initial dips of faults. The
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Research Paper
reconstruction of Spruce Mountain presented here as well as a published reconstruction of the southern Ruby Mountains (Colgan et al., 2010) indicate that
the near-surface dips of major normal faults in this region generally initiated at
moderate to high angles (i.e., >45° and commonly ~60°). These faults rotated to
lower angles as they moved and, in many cases, subsequently were passively
rotated to still lower angles (or higher angles) by movement and associated
tilting by crosscutting younger faults. Although uncertainties remain in these
reconstructions, the evidence nonetheless suggests that the normal faults in
this region, including all of the faults in the Spruce Mountain area, initiated
with moderate to steep dips at least through the seismogenic upper crust.
Structural Complexity and Changing Polarity of Faulting
Many extended regions contain more than one generation of normal faults,
each of which constituted a normal fault system that was active for a finite
period of time. For example, the Yerington district, western Nevada (­Proffett,
1977), the Royston district, west-central Nevada (Seedorff, 1991b), and the
Hunter district, east-central Nevada (Gans, 1982) all are characterized by several generations of faults with northward strikes and down-to-the-east offsets. As maps and cross sections of the Yerington district reveal, the resulting
geol­ogy is complex (Proffett and Dilles, 1984; Richardson and Seedorff, 2015).
The structure of such extended regions is nonetheless structurally relatively
­simple, in the sense that most of the faults with significant displacement have
similar strikes and senses of displacement.
The geology of the Spruce Mountain area is also complex (Figs. 3 and 5),
but in contrast to many areas within the Basin and Range province such as
Yerington, the polarity of faulting changed over time: three sets of down‑tothe-west faults that produced large amounts of tilting, two sets of down-to‑theeast faults that produced small amounts of westward tilting, and one set of
down-to-the-north faults with small amounts of southward tilting (Table 2).
Periods of incremental eastward tilting were interspersed with periods of incremental westward tilting, but time-integration of crustal extension resulted
in ~35 ° of net eastward tilting of the district (Fig. 6).
Areas with superimposed sets of faults with variable strike directions or
differing senses of displacement produce less net tilting of strata for comparable amounts of extension (Axen, 1986). Few reconstructions (e.g., Shaver and
McWilliams, 1987) have been previously attempted in regions that have faults
of opposing polarities, such as Spruce Mountain, where the structural geometries are especially challenging to reconstruct.
Timing of Upper Crustal Extension
Extension across the Basin and Range province is time transgressive (e.g.,
Wernicke et al., 1987; Seedorff, 1991a; Dickinson, 2002). Within the Great Basin,
extension initiated at least as early as the Eocene in the north-central Great
Basin, and the most intense extensional deformation in the past 5–10 m.y.
may have been in the southwestern Great Basin in the Death Valley–Beatty
GEOSPHERE | Volume 12 | Number 1
region. Sound field, geochronologic, and thermochronologic evidence has
shown that there was an important middle Miocene extensional event at many
sites around the Basin and Range (e.g., Miller et al., 1999; Stockli et al., 2002;
­Surpless et al., 2002; Colgan and Henry, 2009). However, not only has there
been a spatial migration in the initiation of extension in a given area, but some
areas are extended again following a hiatus in normal faulting (e.g., Seedorff,
1991a; Seedorff and Richardson, 2014). Even in places such as the greater
Yering­ton district where most of the extension is mid-Miocene or younger,
extension was partitioned temporally between three intervals: a period of extreme extension ca. 15–13 Ma and two less significant periods ca. 11–8 Ma and
ca. 4 Ma to present (Dilles and Gans, 1995; Stockli et al., 2002; Surpless, 2012).
At other locales, such as the Robinson district in east-central Nevada, many
fault generations may have been active within only a few million years or less,
without a significant hiatus (Gans et al., 2001).
Given the available age constraints, at least three periods of extension
within the Spruce Mountain district can be recognized: an important phase of
extension during or before the late Eocene, another period that likely occurred
in the middle to late(?) Miocene, and ongoing Quaternary extension. The earliest period of extension occurred after Mesozoic contraction and before and/or
broadly concurrent with intrusion of rhyolite porphyry dikes in the district ca.
38 Ma. This first period of extension coincides with movement on the first four
normal fault sets in the Spruce Mountain district (Table 2) and may have been
a single continuous event or may have been composed of multiple, discrete
extensional episodes separated by periods of relative inactivity. Based on the
structural reconstruction (Fig. 6), this early extensional period produced the
largest amount of extension at Spruce Mountain (5.4 km). A later period, likely
of middle to late Miocene age, produced the east-directed tilting of sedimentary rocks in the southeastern part of the district when faults grouped in sets
5 and 6 were active. The most recent period of extension occurred with the
latest movement on faults grouped in set 6, which includes range-front fault
scarps mapped along the western sides of Spruce Mountain Ridge and Spruce
Mountain that offset Quaternary deposits (dePolo, 2008).
In spite of the regional importance of middle Miocene extension in northeastern Nevada, other periods of extension cannot be overlooked. Extension at
ca. 38 Ma or earlier appears to have resulted in considerably more extensional
strain at Spruce Mountain than later episodes of extension. Late Eocene extension may have contributed more extensional strain than the mid-Miocene
event in other parts of the Great Basin, including areas of east-central Nevada
that are south of Spruce Mountain, such as the northern Egan Range and the
Robinson district (Gans and Miller, 1983; Gans et al., 1989, 2001). Even in areas
where the magnitude of middle Miocene extension overwhelms the importance of older extension, the earlier events nonetheless require geo­dynamic
explanation. Moreover, late Eocene extension, regardless of magnitude,
certainly played a role in localizing ore formation in many locales (e.g., late
­Eocene ores at Bingham, Utah, Redmond and Einaudi, 2010; at Cove-McCoy,
Nevada, Johnston et al., 2008; at Battle Mountain, Nevada, Keeler and Seedorff, 2007; and at many sites in White Pine County, Nevada, Seedorff, 1991a).
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
255
Research Paper
Implications of Crustal Extension and Structural Restorations
for Mineral Exploration
Preexisting mineralized systems can be dismembered and tilted by normal faults (e.g., Proffett, 1977; Wilkins and Heidrick, 1995; Rahl et al., 2002;
Begbie et al., 2007; Stavast et al., 2008), thereby aiding scientific understanding of the systems by creating greater exposures of their vertical and lateral
extents (e.g., Dilles and Einaudi, 1992; Seedorff et al., 2005, 2008). Mineralized
systems may also provide additional geologic markers for reconstructions,
such as hydrothermal alteration or metal zoning patterns, which may aid
structural reconstructions, especially where there are large areas with uniform rock types (Maher, 2008; Nickerson et al., 2010; Houston and Dilles, 2013;
Seedorff et al., 2015). Mineralized systems or fault-bound fragments of them,
especially those in the footwall blocks of large-displacement normal faults,
may be uplifted and eroded during extension, yet in other cases, ore may
be downdropped, buried, and preserved under postmineral rocks and may
constitute an exploration target (Maher, 2008; Richardson and Seedorff, 2015).
Numerical modeling indicates that crustal extension at the scale of an orogenic belt enhances the preservation of mineral deposits formed at relatively
shallow levels of the crust, such as porphyry and epithermal systems; i.e.,
it postpones their erosion (Barton, 1996). Crustal extension also creates permeability and drives fluid circulation in the brittle upper crust (Ilchik and Barton, 1997), favoring the formation of certain types of deposits during periods
of crustal extension, including epithermal silver-gold deposits, Carlin-type
gold deposits, iron oxide–copper–gold deposits including detachment-style
Fe-Au-(Cu) deposits), and certain classes of porphyry systems (Dreier, 1984;
Spencer and Welty, 1986; Seedorff, 1991a; Tosdal and Richards, 2001; John,
2001; Barton et al., 2011; Coolbaugh et al., 2011). The geometric constraints
on determining the amount of deformation of synextensional hydrothermal
systems such as Spruce Mountain, however, are less straightforward than for
preextensional deposits (see preceding).
Although the precise amount of deformation of the porphyry molybdenum
system at Spruce Mountain system is not tightly constrained, it is clear that the
Spruce Mountain system has been structurally dismembered, with deeper levels generally exposed on the western side and shallower levels on the eastern
side of the district. Abrupt increases in grade with depth occur beneath some
post-ore faults, and some of the highest molybdenum grades intersected to
date are in the footwall of a steeply dipping post-ore normal fault. Thus, several of the generations of faults postdate ore formation, and the net tilting of
the highly segmented hydrothermal system is ~10° west. It is highly unlikely,
therefore, that the mineralizing plutons are entirely upright or fully intact. Early
generations of normal faults are locally intruded by porphyry dikes and host
tabular bodies of quartz-rich breccia. Mineralized skarn is spatially associated
with the margins of porphyry dikes and occurs in certain premineral to synmineral normal faults.
The structural and magmatic setting of Spruce Mountain may be analogous to the Hunter district in White Pine County, where a rhyolitic center
GEOSPHERE | Volume 12 | Number 1
was emplaced at the onset of extension in that area during the late Eocene.
Hydrothermal activity accompanied emplacement of porphyry dikes along
normal faults, and faults continued to slip after crystallization of the dikes. In
the Hunter district, most, if not all, normal fault–related tilting postdated intrusion of the dikes and extrusion of coeval volcanic rocks (Gans, 1982; Gans
and Miller, 1983). Both the Hunter and Spruce Mountain districts are synextensional porphyry systems. Occurrences of Carlin-style mineralization at Spruce
Mountain may belong to a genetically unrelated, earlier or later, system that
partially overlaps the porphyry molybdenum system spatially. Spatial superposition of genetically unrelated systems is common in the Great Basin (e.g.,
Keeler and Seedorff, 2007) and is especially common for Carlin-type gold systems (e.g., Seedorff, 1991a; Seedorff and Barton, 2004; Hastings, 2008; Lubben
et al., 2012; Hoge et al., 2015).
The early sets of faults at Spruce Mountain are prospective because they
could have influenced the locations of porphyry intrusive centers, and
they were conduits for hydrothermal fluid flow that produced skarn and carbonate replacement mineralization. In contrast, later generations of faults have
dismembered the system and contributed to its post-ore tilting; hence, detailed structural restorations may be used to predict where mineralization is in
a given block, where it is displaced by a post-ore fault, and thus where it might
be discovered in nearby fault blocks. Results of future exploration activities will
permit refinement or revision of this two-dimensional structural interpretation.
With greater three-dimensional exposure (e.g., access to drilling results), an
attempt at a three-dimensional structural restoration might be justified.
CONCLUSIONS
Complexly faulted Ordovician through Permian miogeoclinal sedimentary
rocks, Cenozoic volcanic and sedimentary rocks, and sparse intrusive rocks are
exposed at Spruce Mountain. Skarn, carbonate replacements, fissure veins,
and jasperoid that were exploited by small underground mines in the district,
principally for lead and silver, originally were the distal expressions of a porphyry molybdenum system at depth, although the system is now tilted and
dismembered. The porphyry system, here dated by U-Pb zircon as late Eocene
(ca. 38 Ma), is related to sparsely exposed rhyolite porphyry and granite porphyry intrusions. Spruce Mountain is a rhyolitic porphyry Mo or Climax-type
porphyry molybdenum deposit with extensive associated skarn. The district
also contains a Carlin-type gold prospect of unknown age.
Based on a plan-view analysis of geologic maps and a reconstructed cross
section, at least six crosscutting sets of normal faults have been delineated
with contrasting polarities of faulting. From oldest to youngest, these sets
have senses of displacement of (1) down to the west, (2) down to the west,
(3) down to the north, (4) down to the east, (5) down to the east, and (6) down
to the west. The present-day fault network reflects the overprinting of multiple
phases of extension, ranging in age from late Eocene or older to Quaternary, in
which periods of incremental eastward tilting were interspersed with periods
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
256
Research Paper
of westward tilting. Based on age constraints from six new U-Pb age dates
of igneous dikes within the district, some of which cut and intrude the oldest
fault sets, the earliest phase of extension occurred at or before ca. 38 Ma. Early
extension was accommodated by the oldest four fault sets, all of which are
late Eocene or older in age, and resulted in the largest extensional strain in
the district (5.4 km). Probable middle to late Miocene sedimentary rocks exposed in the southeastern part of the district were tilted during a later phase
of normal faulting, which (along with minor additional extension during the
Quaternary) resulted in ~1.5 km of additional extension in the central part of
the district. Thus, we estimate that Cenozoic normal faulting accommodated
a total of 6.9 km of extension (120%) across the central portion of the Spruce
Mountain district. Although the oldest normal faults that crop out at Spruce
Mountain currently dip <15°, based on the structural reconstruction, all faults
in the reconstructed section had initial dips of ~45° or greater. The recognition
of large-magnitude late Eocene or older extension at Spruce Mountain is important in that Paleogene extension in this region of northeastern Nevada has
previously been regarded by many as relatively insignificant compared with
later phases of extension during the middle Miocene. Our restorations and
age dates at Spruce Mountain show that Eocene extension was at least locally
significant in northeastern Nevada.
Within the central Spruce Mountain district, large extensional strains have
been distributed among numerous normal faults. Stepwise, fault by fault restoration of the normal faults at Spruce Mountain yields a restored section that
implies west-vergent folding and gentle westward dips of miogeoclinal strata.
This geometry is interpreted to reflect that the Spruce Mountain area was on
the western limb of an anticline that connected to the Pequop syncline to the
east prior to being dismembered by extensional faulting. The structural reconstruction indicates that the entire extensional history of the district produced
net eastward tilting of ~35°, which suggests that movement on faults of the
three sets of down-to-the-west faults of the first, second, and sixth generations
dominated the tilting history of the area.
The earliest four sets of normal faults at Spruce Mountain are pre-ore or
syn-ore faults relative to hydrothermal alteration-mineralization related to the
porphyry molybdenum system. Faults of these sets are prospective as they
probably were conduits for hydrothermal fluid flow that produced skarn and
carbonate replacement mineralization. However, the Spruce Mountain system
has also been structurally dismembered by younger sets of normal faults. Net
tilting of the hydrothermal system by postmineral faults is probably ~10° westward, and deeper levels of the system are generally exposed on the western
side of the district. Cross-sectional restoration of the postmineralization extensional faulting and associated tilting suggests that the original deposit had a
~6 km long by ~3 km deep elliptical pattern in 2 dimensions, with carbonate
replacement deposits surrounding skarn. Spruce Mountain joins Bingham,
Utah, Cove-McCoy, Nevada, several deposits in the Battle Mountain district,
Nevada, and many sites in White Pine County, Nevada, as mineral deposits
where late Eocene extension, regardless of magnitude, played a role in localizing ore formation.
GEOSPHERE | Volume 12 | Number 1
ACKNOWLEDGMENTS
This paper is a collaboration that incorporates results from two independently funded M.S.
thesis projects. Pape’s study and associated field work at the Lowell Institute of Mineral Resources
at the University of Arizona was financially supported by an Arthur A. Meyerhoff Memorial Grant
from the American Association of Petroleum Geologists Foundation, a Hugh E. McKinstry Student
Research Award from the Society of Economic Geologists, and Science Foundation Arizona. Baril’s
study was supported by Renaissance Gold Inc. through the Ralph J. Roberts Center for Research
in Economic Geology at the University of Nevada, Reno, with radiometric dating conducted at the
Isotope Geology Laboratory at Boise State University. We gratefully acknowledge all sources of
financial support, and analytical support from the staff at the Isotope Geology Laboratory at Boise
State University (Idaho).
We thank Roy Johnson, Clay Postlethwaite, Phil Nickerson, and Chris Henry for helpful discussions and critical comments, and Mark Pecha for assistance with interpretation of the U-Pb
results. We also thank William Pape, who assisted in field mapping; Mark Barton, Joe Colgan, Pete
DeCelles, Dave John, Allen McGrew, Julie Rowland, and an anonymous reviewer for comments
on earlier versions of the manuscript; Eric Struhsacker of Renaissance Gold Inc. for leads regarding recent exploration at Spruce Mountain; and Phil Gans for discussions of the geology of Spruce
Mountain with Seedorff decades ago. We also acknowledge a helpful review of an earlier version
of the manuscript by Dan Pace of Renaissance Gold Inc. that led to the collaboration.
APPENDIX 1. DESCRIPTIONS OF SAMPLES DATED
BY U‑Pb ZIRCON METHOD
SMIg44
SMIg44 is an Eocene porphyritic rhyolite dike ~1.1 km northwest of the Standard mine; it is
50 cm wide and strikes north-south. The texture is porphyritic with 5% phenocrysts and distinct
flow banding along strike. The phenocryst assemblage includes 50% plagioclase 0.1–1 mm, 25%
resorbed quartz 1–4 mm, 20% potassium feldspar 1–2 mm, and 5% biotite 0.1–0.5 mm. Sericite and
kaolinite have partially to completely replaced the feldspars.
SMIg45
SMIg45 is an Eocene rhyolite porphyry intrusion ~0.6 km southeast of the Standard mine
and 0.4 km west of the Ada H mine, and occurs as a circular body of float ~100 m wide. The rock
is white to light gray with a porphyritic texture and shows pervasive alteration effects, including
bleached and clay-altered feldspars and abundant iron oxides that may represent oxidized pyrite.
Phenocrysts represent 30% of the rock and consist of 40% partially resorbed quartz 1–3 mm, 40%
potassium feldspar 2–10 mm, and 20% plagioclase 1–2 mm. The matrix is a very fine grained
mixture of quartz and feldspars. Sericite and kaolinite have completely replaced the feldspars.
SMIg46
SMIg46 is a Jurassic dacite ~1.5 km northwest of the Kille mine and is a light gray competent rock that forms large outcrops and exhibits distinct flow banding. The texture is fine-grained
equigranular. The rock is composed of 70% plagioclase 1–2 mm and 30% quartz 0.5–2 mm; mafic
minerals have been destroyed. Sericite alteration of plagioclase is pervasive. Calcite locally overprints sericite alteration.
SMIg47
SMIg47 is a Jurassic porphyritic dacite ~1.2 km south of the Kille mine that occurs in an elongate body of pale green rock 200 m long by 100 m wide. The texture is porphyritic with 15% pheno­
crysts in a fine-grained matrix. The phenocryst assemblage includes 60% plagioclase 0.5–2 mm,
20% quartz 0.5–2 mm, and 20% biotite 0.5–1 mm. The groundmass is composed of plagio­clase,
bio­tite, and quartz. This rock has been strongly propylitized, with complete alteration of plagio­
clase and biotite to chlorite and calcite. Plagioclase phenocrysts also display minor s­ ericite and
kaolinite alteration.
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
257
46
44
0.0066
0 .0 0 6 8
42
40
0.0062
0 .0 0 6 4
0 .0 0 6 0
42
38
2 0 6 P b /2 3 8 U
0.0058
38
36
0.0054
0 .0 0 5 6
34
34
0 .0 0 5 2
0.0050
0 .0 0 4 8
0 .0 2 5
0 .0 3 5
0 .0 4 5
0 .0 5 5
30
0.0046
0.00
0 .0 6 5
2 0 7 Pb/ 2 3 5 U
32
0.02
0.04
0.06
207 Pb/ 235 U
data -poin t error ellipses are 2 σ
Sample SMIg46
0 .5
0.030
2600
2200
0 .4
2 0 6 Pb/ 2 3 8 U
2 0 6 P b /2 3 8 U
1000
0.22
160
0.024
150
140
0.022
2
4
6
8
10
2 0 7 Pb/ 2 3 5 U
12
14
130
0.020
0.12
0.14
0.16
0.18
Sample SMIg51
48
207 Pb/ 235 U
50
Sample SMIg49
0.0075
0 .0 0 7 4
46
0 .0 0 7 0
0.0065
0.24
0.26
data-poin t error ellipses are 2σ
44
42
2 0 6 Pb / 2 3 8 U
0.20
0.026
data-poin t error ellipses are 2σ
0 .0 0 6 6
2 0 6 Pb / 2 3 8 U
Research Paper
d ata-p o int erro r ellip ses are 2σ
170
1400
0
190
180
1800
0 .1
0 .0
0.10
Sample SMIg47
0.028
0 .3
0 .2
0.08
38
0.0055
40
0 .0 0 6 2
0 .0 0 5 8
34
36
0 .0 0 5 4
30
0.0045
0.02
0.03
0.04
0.05
207 Pb/ 235 U
0.06
0.07
0.08
0 .0 0 5 0
0 .0 2 5
0 .0 3 5
Figure A1. Concordia diagrams for U-Pb dates. See Appendix 2 for interpretations.
0 .0 4 5
0 .0 5 5
207 Pb/ 235 U
0 .0 6 5
0 .0 7 5
GEOSPHERE | Volume 12 | Number 1
2 0 6 Pb / 2 3 8 U
Sample SMIg45
258
Sample SMIg44
0 .0 0 7 2
data-poin t error ellipses are 2σ
0.0070
Pape et al. | Structural reconstruction, Spruce Mountain, Nevada
data-poin t error ellipses are 2 σ
Research Paper
SMIg49
SMIg49 is an Eocene coarse-grained rhyolite porphyry ~0.5 km southeast of the Black Forest
mine; it is light gray and contains 30% phenocrysts set in a fine-grained matrix of quartz and
feldspar. Phenocrysts are 20% resorbed quartz 1–6 mm, 50% potassium feldspar 3–9 mm, 20%
plagioclase 1–3 mm, and 10% biotite 0.5–1.5 mm. Very minor disseminated pyrite and chalcopyrite
are present. Quartz veins 1–20 mm wide are common, some of which contain fine-grained molybdenite. The rock has been potassically altered with pervasive K-feldspar.
SMIg51
The second rhyolite porphyry unit occurs along a roughly linear trend across the entire study
area that may be a large dike; the dated rock was collected on the south side of Banner Hill ~1.3 km
east of the Black Forest mine. Exposures are commonly either masses of float or highly weathered rock exposed by prospect pits. The rock is bright white with a porphyritic texture. Parallel
quartz veins 1–3 mm wide ~10/m are present in outcrops on the western edge of the study area.
Phenocrysts make up 15% of the rock and consist of 60% quartz 1–3 mm, 20% potassium feldspar
1–2 mm, and 20% plagioclase 1–2 mm. Alteration is typically weak argillization, although one outcrop on the eastern edge of the study area contains potassium feldspar flooding, 1–2 mm fluorite
veins, and minor copper oxides.
APPENDIX 2. LASER ABLATION–INDUCTIVELY COUPLED
PLASMA–MASS SPECTROMETRY U-Pb ZIRCON
GEOCHRONOLOGIC METHODS
Sample Preparation
Zircon grains were separated from rocks using standard techniques and mounted in epoxy
and polished until the centers of the grains were exposed. Cathodoluminescence (CL) images
were obtained with a JEOL JSM-1300 scanning electron microscope and Gatan MiniCL. CL images
guided the placement sites for microanalysis. U-Pb dates were obtained from spot analyses by
laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) at the Isotope Geology
Laboratory at Boise State University (Idaho).
Analytical Methods and Reporting Procedures
U-Pb isotope systematics and trace element compositions were analyzed by LA-ICP-MS using
a ThermoElectron X-Series II quadrupole ICP-MS and New Wave Research UP-213 Nd:YAG UV (213
nm) laser ablation system. In-house analytical protocols, standard materials, and data reduction
software were used for simultaneous acquisition and real-time calibration of U-Pb dates and a
suite of high field strength elements (HFSE) and rare earth elements (REE) using the high sensitivity and unique properties of the interface (Xs cones), extraction lens, and quadrupole analyzer of
the X-Series II. Zircons were ablated with a laser diameter of 25 mm using fluence and pulse rates
of 5 J/cm2 and 10 Hz, respectively, during a 30 s analysis (15 s gas blank, 45 s ablation) that excavated a pit ~25 µm deep. Ablated material was carried by a 1.2 L/min He gas stream to the nebulizer
flow of the plasma. Dwell times were 5 ms for Si and Zr, 100 ms for 49Ti and 207Pb, 40 ms for 238U,
232
Th, 202Hg, 204Pb, 206Pb and 208Pb isotopes, and 10 ms for all other HFSEs and REEs. Background
count rates for each analyte were obtained prior to each spot analysis and subtracted from the raw
count rate for each analyte.
For U-Pb dates, instrumental fractionation of the background-subtracted 206Pb/ 238U and
207
Pb/ 206Pb ratios was corrected, and dates were calibrated with respect to interspersed measurements of the Plešovice zircon standard (Sláma et al., 2008). Signals at mass 204 were indistinguishable from zero following subtraction of mercury backgrounds measured during the gas blank
(<1000 cps 202Hg), and thus dates are reported without common Pb correction. Radiogenic isotope
ratio and age error propagation for each spot includes uncertainty contributions from counting
statistics and background subtraction. For concentration calculations, background-subtracted
count rates for each analyte were internally normalized to 29Si and calibrated with respect to NIST
SRM-610 and SRM-612 glasses as the primary standards.
GEOSPHERE | Volume 12 | Number 1
Errors on weighted mean 206Pb/ 238U dates given in the following are presented at 2s as follows:
weighted mean date ± x/y, where x is the internal error and y is the error including the uncertainty on the standard calibration, propagated in quadrature. Weighted mean calculations were
performed using Isoplot 3.0 (Ludwig, 2003). A standard calibration uncertainty of 1.0% (2s) is used
because that is the average of these experiments. In Table 1 errors on single dates do not include
uncertainties on the standard calibration.
We experimented with other schemes to reject certain individual spot analyses. Although
these routines in some cases resulted in slightly different dates, all dates were well within the
uncertainties reported here.
Zircon standard AUSZ2 was measured as an unknown during the experiment as a quality
control standard; 23 analyses of AUSZ2 yielded a weighted mean 206Pb/ 238U date of 38 ± 0.5 Ma,
in agreement with the chemical abrasion–thermal ionization mass spectrometry weighted mean
date of 38.86 ± 0.01 Ma (2s internal error, J. Crowley, 2012, personal commun.).
Results
Analytical results are summarized in Table 1; Figure A1 shows concordia diagrams for the six
analyzed samples.
Sample SMIg44 has an interpreted age of crystallization of 37.7 ± 0.5 Ma and exhibits minor
normal discordance, reflecting several spot analyses that exhibit lead loss.
Sample SMIg45 has an interpreted age of crystallization of 37.3 ± 0.4 Ma and exhibits normal
and reverse discordance, probably reflecting lead loss for some of the analyzed spots.
Sample SMIg46 has an interpreted Jurassic age of crystallization of 155 ± 4 Ma and has several spot analyses that exhibit lead loss, but it also shows the greatest evidence of inheritance,
likely with several different Precambrian age cores with ages of ca. 1.1 Ga and ca. 2.7 Ga.
Sample SMIg47 has an interpreted Jurassic age of crystallization of 156 ± 2 Ma and exhibits
minor discordance, reflecting several spot analyses that exhibit lead loss.
Sample SMIg49 has an interpreted age of crystallization of 39.1 ± 0.6 Ma and exhibits normal
and reverse discordance, reflecting lead loss for some of the analyzed spots.
Sample SMIg51 has an interpreted age of crystallization of 37.5 ± 0.4 Ma and exhibits discordance that likely results from lead loss for some of the analyzed spots.
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