Reconstruction of Normal Fault Blocks in the Ann-Mason and
Blue Hill Areas, Yerington District, Lyon County, Western Nevada
Carson A. Richardson* and Eric Seedorff
Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721-0077
ABSTRACT
The Yerington porphyry copper-skarn-iron oxide-copper-gold district is a clasVLFDUHDRIFRQWLQHQWDOH[WHQVLRQKDYLQJEHHQH[WHQGHGPRUHWKDQE\PXOWLSOH
generations of Cenozoic east-dipping normal faults that penetrated a minimum of 8
km depth into the crust, initiated at high angles, and rotated to shallower dips until
being cut by younger sets of faults. This study examines the Cenozoic normal faults
in the vicinity of the Ann-Mason and Blue Hill areas through detailed mapping of
two major faults, logging intervals of drill core containing the fault damage zones,
and constructing fault surface maps, i.e., geologic maps of the proximal footwall and
hanging wall of faults superimposed on structural contour maps of the fault planes.
Six normal faults, representing four geometric sets or temporal generations of faults
numbered from oldest to youngest are described and analyzed, with particular emphasis on the oldest two generations. Fault surface maps constrain the slip vectors
of the faults and their variability along strike. Faults of the latest three generations
strike northerly and dip easterly. Faults of the second generation, which are of middle
0LRFHQHDJHLQFOXGHWKH%OXH+LOOaNPVOLSZLWKLQFUHDVLQJGLVSODFHPHQWWRWKH
QRUWKDQG6LQJDWVHNPVOLSIDXOWV7KHVHVHFRQGJHQHUDWLRQIDXOWVKDYHGDPage zones that persist ~15 m on either side of the fault and have hanging-wall splays
WKDWPHUJHLQWRWKHPDLQIDXOWVXUIDFH7KH¿UVWJHQHUDWLRQIDXOWVDUHUHSUHVHQWHGE\
the 1A fault, which is one of a series of sub-parallel, generally small-offset faults that
presently strike southeast and dip steeply to the southwest. Analysis of drill hole data
indicates that the 1A fault, which might have the most amount of slip of any fault in
the set, has ~230 m of apparent sinistral separation. The incremental untilting of the
three later generations of faults restores the 1A fault to a steeply south-dipping fault
with normal, dip-slip displacement. The 1A fault appears to connect with a fault that
is exposed at the surface west of the area of drill hole constraints and cuts the Weed
+HLJKWVPHPEHURIWKH0LFNH\3DVV7XII0DDQGLVFXWE\WKHPLGGOH0LRFHQH
Singatse fault, bracketing its timing.
The structural contour maps reveal new insights into the subsurface geometry of
VHYHUDOIDXOWV7KH%OXH+LOOIDXOWSUHVHQWO\GLSVaƒVRXWKHDVWDORQJWKHQRUWKZHVWern side of a large mullion in the fault plane. A stepwise reconstruction indicates that
DOOIDXOWVKDGLQLWLDOGLSVRI•ƒZLWKWKH6LQJDWVHLQLWLDWLQJDWƒ7KHVLJQL¿FDQFH
RI WKH ¿UVW JHQHUDWLRQ RI IDXOWV DW<HULQJWRQ WKRXJK SRRUO\ XQGHUVWRRG PD\ KDYH
counterparts in eastern Nevada, where sets of easterly striking normal faults also
formed prior to periods of extreme extension. The restorations demonstrate that mineralization in the Ann-Mason and Blue Hill areas originated in the same dike swarm,
and thus are genetically related, and that another dike swarm to the south passing
near the Casting Copper-Ludwig septum could have porphyry mineralization to the
northwest at depth on one or both sides of the Blue Hill fault.
Key Words: Structural reconstruction, Yerington district, Basin and Range province,
porphyry copper
*E-mail: carichardson@email.arizona.edu
1153
1154
INTRODUCTION
Carson A. Richardson and Eric Seedorff
vantage of dense drill hole data in and around the Ann-Mason
porphyry copper deposit. Entrée Gold has added over 35 km of
Multiple sets of rotated normal faults (also called “domino- exploration drilling since 2010 in the study area. As described
style” or rotational planar normal faults), for which the Yer- below, the Cenozoic faults of the district can be grouped into
ington district is well known, are a manifestation of high-mag- four geometric sets or temporal generations, numbered from
nitude extension in the Basin and Range province (Wernicke oldest to youngest. The geology of the district offers numerDQG%XUFK¿HO'DYLVRQ5RWDWHGQRUPDOIDXOWVDUH ous geologic markers that can be utilized in fault restorations,
essential in not only preserving near-surface crustal architec- including a Tertiary erosion surface above which Oligocene
ture over geologic time, but also in exposing deep subsurface volcanic rocks were deposited, dike swarms and other discrete
features without requiring large topographic relief (Seedorff, phases of a Jurassic batholith, and faults of older generations
1991a; Dilles and Einaudi, 1992; Seedorff et al., 2008). Debate that must connect following each step of a structural restoraon the mechanics and kinematics of crustal extension can be tion. Fault surface maps, i.e., geologic maps of the immediate
furthered through three-dimensional structural reconstructions footwall and hanging wall of each fault overlain on structure
of mineral deposits with their abundance of subsurface data contour maps of the fault planes, are employed as a method
IURP GULOO KROHV DQG FDUHIXO ¿HOG REVHUYDWLRQV IURP PDSSLQJ to provide detailed, three-dimensional control on the geometry
of natural and man-made exposures (Proffett, 1977; Proffett of faults and to assess and portray changes in the magnitude
and Dilles, 1984; Davison, 1989; Stockli et al., 2002; Surpless, and direction of slip along each fault. The results are based on
2012). Beyond that, structural reconstructions suggest where detailed mapping of the traces of two major faults, logging infault-offset blocks of mineralized systems may exist, thus yield- tervals of drill core to describe and to constrain the limits of
ing possible exploration targets (Ransome, 1903; Lowell, 1968; the fault damage zones, and constructing fault surface maps of
Wilkins and Heidrick, 1995; Nickerson et al., 2010).
VHOHFWHGIDXOWV:H¿QGWKDWWKHIDXOWVLQ<HULQJWRQDUHQHLWKHU
The Yerington district is a hybrid/superimposed sys- perfectly planar nor strongly listric in geometry, but instead curtem with porphyry copper, skarn, and iron oxide-copper-gold viplanar. Analysis indicates that the 4th and 3rd generations of
(IOCG) mineralization (Einaudi, 1977; Harris and Einaudi, faults have somewhat greater magnitudes of slip than previous1982; Dilles, 1987; Barton and Johnson, 2000; Dilles et al., ly reported. A fault of the 3rd generation, with a geometry that
2000b) that has been structurally dismembered and rotated is not tightly constrained, may have a different slip direction
60 to 90° by late Cenozoic faults, as indicated by highly tilted than previously inferred. A representative fault is characterized
2OLJRFHQHDVKÀRZWXIIVZKLFKH[WHQGHGWKHGLVWULFWE\PRUH from a largely undescribed fault set, striking at high angles to
than 150% (Geissman et al., 1982a; Proffett and Dilles, 1984). the other fault sets. We show through a structural reconstruction
Reconstructions based on previous mapping and geophysical that the restored position of offset pieces of the Ann-Mason and
work show that the district underwent pre-Oligocene deforma- Blue Hill ore bodies belong to the same dike swarm and indition that tilted it t20° westward prior to Miocene and younger cate where the present locations of offset, buried portions are
faulting that produced even larger degrees of tilting (Geissman possibly located.
et al., 1982b; Proffett and Dilles, 1984), resulting in a total of
about 90° of westward tilting. Recent work continues to add to LOCATION AND HISTORY
the understanding of the structural geology of the area, particularly east and south of the main district (e.g., Surpless, 2010, Location
2012; Schottenfeld, 2012).
In part because of the extraordinary three-dimensional exThe Yerington district is located in west-central Nevada,
posure, the greater Yerington district continues to be a natural near the western margin of the Basin and Range province and
laboratory for investigating hydrothermal processes and the within the Walker Lane dextral shear zone (Proffett, 1977; Hargenesis of porphyry copper deposits (e.g., Dilles et al., 2015) dyman and Oldow, 1991; Surpless, 2012). The district encomand for understanding the origin of IOCG deposits in porphyry passes the Buckskin, Singatse, and Wassuk Ranges and Smith
copper districts (Runyon, 2013; Runyon et al., this volume). In and Mason Valleys (Figures 1, 2). It is a composite porphyry
the last decade, the Yerington district has undergone a revival of copper-IOCG district consisting of the Yerington, Ann-Mason,
exploration and development activities, by Entrée Gold at Ann- Blue Hill, MacArthur, Bear, and Pumpkin Hollow deposits as
Mason and Blue Hill, by Quaterra at McArthur and the Yer- well as many smaller historic workings (Knopf, 1918; Carten,
ington mine, and by Nevada Copper at Pumpkin Hollow. The 1986; Dilles, 1987; Dilles et al., 2000a). The study area is due
associated drill holes from exploration offer additional controls west of the town of Yerington and spans the breadth of the Sinfor structural geologic studies.
gatse Range and northern Smith Valley. Triassic through ReThe purpose of this study is to further the understanding cent sedimentary, intrusive, and extrusive rocks record a series
of the kinematic evolution of normal faults in the greater Ann- of discrete events across the Buckskin, Singatse, and Wassuk
Mason and Blue Hill areas within the Yerington district, build- Ranges that provide multiple stratigraphic and structural marking on previous published geologic information and taking ad- ers to aid in structural reconstruction.
Ann-Mason structural reconstruction
1155
Figure 1. Location Maps. A: Map of southwestern North America, showing the limits of the Basin and Range province and the locations of highly extended locals
PRGL¿HGIURP6HHGRUIID'LFNLQVRQ6HHGRUIIHWDO/RFDOHVDUH<HULQJWRQ5R\VWRQ+DOO5RELQVRQ$MR6LHUULWD3LPD
6DQ0DQXHO.DODPD]RR5D\7HD&XS5LYHUVLGH*OREH0LDPLDQG/HPLWDU0RXQWDLQV%6LPSOL¿HGJHRORJLFPDSRIWKHJUHDWHU<HULQJWRQ
district, western Nevada, with locations of major ranges and basins, mines, deposits, and prospects, and the limits of the Walker Lane Shear Zone in the easternmost
part of the district (adapted from Bingler, 1978, Stewart and Carlson, 1978, Hudson and Oriel, 1979, Proffett and Dilles, 1984, and Dilles and Wright, 1988). The
box in the left-center of the map denotes the location of Figure 2.
District exploration and mining history
Mining in the Yerington district began as late as 1865 with
the Ludwig, Mason Valley, and Bluestone mines in the Singatse
Range producing copper sulfates (Jackson et al., 2012). These,
and many of the small, underground, historic workings (dominantly skarn and carbonate replacement) in the district such as
the McConnell, Homestake, Western Nevada, Nevada-Empire,
Douglas Hill, Casting Copper, Malachite, Blue Jay, and Montana Yerington mines were connected by the Nevada Copper
Belt Railroad built in the early 1900s to the Thompson smelter
near Wabuska, approximately 15 km north of Yerington, which
operated between 1912 and 1928 (Tingley, 1990; Jackson et al.,
2012; Spiller et al., 2012). Anaconda began exploration in the
Yerington district in 1941, initially focusing on the area around
the Nevada-Empire mine located above the center of the pres-
1156
Carson A. Richardson and Eric Seedorff
ent-day Yerington pit. Spurred by U.S. government subsidies
to encourage domestic copper production, Anaconda began
mining in 1951 (Bryan, 2012). Anaconda’s Yerington porphyry
copper mine was active from 1951–1979, and its geologists discovered several more deposits (Ann-Mason, Blue Hill, MacArthur, and Bear), making important contributions to the geologic
literature (e.g., Proffett, 1972, 1977; Proffett and Proffett, 1976;
Einaudi, 1977; Brimhall et al., 2006). Other notable mineralized occurrences in the district include the Pumpkin Hollow deposit, initially discovered by U.S. Steel in 1960, and the Lagomarsino deposit, a faulted piece of the Bear system discovered
by Phelps Dodge (Tingley, 1990; Doebrich et al., 1996). Following the loss of their Chilean and Mexican properties in the
PLGV$QDFRQGDZDVSXUFKDVHGE\WKH$WODQWLF5LFK¿HOG
Company (ARCO), which shut down operations in 1979 and
sold the former Anaconda-held properties in the Yerington district by 1981. Since 1984, the history and control of the district
KDVEHHQIUDJPHQWHGDQGLVEULHÀ\VXPPDUL]HGLQ7DEOHEXW
the most active current operators are Nevada Copper, Entrée
Gold, and Quaterra.
GEOLOGIC FRAMEWORK
Tectonomagmatic and metallogenic setting
The Yerington district and associated mineralization is
)LJXUH6LPSOL¿HGJHRORJLFPDSRIWKH6LQJDWVH5DQJHPRGL¿HGIURP3URIIHWWDQG'LOOHVVKRZLQJWKHWUDFHVDQGVXUIDFHSURMHFWLRQVRIIDXOWVDGGUHVVHG
LQWKLVVWXG\ZLWKWKHWUDFHRIVHFWLRQ66ƍ)LJXUHVKRZQ6HFWLRQOLQHVIURP3URIIHWWDQG'LOOHVDUHQRWHGZLWKWKHVHFWLRQQDPHDQG3'
Ann-Mason structural reconstruction
1157
Table 1. RECENT MINING AND EXPLORATION HISTORY OF THE YERINGTON DISTRICT.
Mining Company
Years of Activity Major Properties
Entrée Gold (US), Inc.
2010-Present
Nevada Copper
2005-Present
Singatse Peak Services, LLC
[Quaterra Resources, Inc.]
2011-Present
Bronco Creek Exploration, Inc.
[Eurasian Minerals, Inc.]
2009–2010
Honey Badger Exploration, Inc.
(formerly Telkwa Gold Corporation)
PacMag Metals Limited
2007–2009
2005–2010
Lincoln Gold Corportation
2004–2005
Giralia Resources NL
MIM (Mount Isa Mines)
Phelps Dodge Corporation
2003
2002–2003
1995
Arizona Metals Company (ARIMETCO)
1989–1999
Cyprus Metals Exploration Corporation
Plexus Resources, Inc.
1989–1998
1984–1989
Activity & Claims of Major Discovery
Key References
Ann-Mason, Blue
Hill, Roulette
Continued drilling, mapping, and geophysical
Jackson et al.,
surveying; Preliminary Economic Assessment
2012
completed in 2012
Pumpkin Hollow
Feasibility study completed in 2012; continued Spiller et al., 2012
drilling, mapping, and geophysical surveying
Yerington Mine,
Completed a Preliminary Economic Assessment
Bryan, 2012;
MacArthur
RQWKH0DF$UWKXUGHSRVLWFRQ¿UPDWLRQGULOOLQJRQ Henderson et al.,
the Yerington mine and Bear deposits
2012
Roulette
Joint drilling project with Entrée Gold leading to
Jackson et al.,
discovery of the Roulette prospect
2012; Eurasian
Minerals, 2014
Ann-Mason, Blue Soil geochemistry anomalies to the northwest of
Jackson et al.,
Hill, Roulette
the Blue Hill deposit
2012
Ann-Mason, Blue Drilling (RC and DD) of the Ann-Mason and Blue
Jackson et al.,
Hill, claims around Hill deposits (6,973 m and 3,438 m, respectively);
2012
Minnesota
drilled a magnetic anomaly around the Minnesota
mine, encountering pyrite-dominated, copper-poor
mineralization.
Blue Hill
Gold exploration program (soil geochemistry and
Jackson et al.,
RC drilling) at Lincoln Flat property; company
2012
declined to earn in due to results of exploration
program
Ann-Mason
No exploration work conducted
Jackson et al.,
2012
Ann-Mason
Geophysical surveying
Jackson et al.,
2012
Blue Hill
Blue Hill deposit drilling
Jackson et al.,
2012
Yerington mine,
SXEW heap leach operation processing “subJackson et al.,
MacArthur, Anngrade” stockpiles from Yerington Mine and
2012
Mason
MacArthur
Pumpkin Hollow Drilling primarily expanded the Pumpkin Hollow’s Spiller et al., 2012
North and South deposits
Pumpkin Hollow
Minimal exploration (two assessment drill holes) Spiller et al., 2012
1%6XEVLGDU\FRPSDQ\¶V³SDUHQWFRQWUROOLQJ´FRPSDQLHVDUHQRWHGLQEUDFNHWVLIKHDYLO\LQWHUWZLQHG
dominated by the Yerington batholith, which formed from
169.4–168.5 Ma (Dilles and Wright, 1988). The Yerington
batholith is a product of Jurassic magmatism in the western
United States that is divided by Barton et al. (2011) into two
episodes of mineralization: an early period (~210–190 Ma) of
weak, sparse porphyry Cu, Zn-Pb-Ag, and IOCG mineralization and a mid-Jurassic (170–155 Ma) period containing all
YROFDQRJHQLFPDVVLYHVXO¿GHDQG:$XV\VWHPVDQGPRVWSRUphyry Cu and IOCG systems.
The dextral transtensional Walker Lane initiated in the
mid-late Cenozoic as a response to the changed plate boundDU\FRQ¿JXUDWLRQDVHDUO\DVa0DDVHYLGHQFHGLQWKH*LOOLV
and Gabbs Valley Ranges, and movement is bracketed between
23.1 and 22.2 Ma at the present day western boundary in the
Wassuk Range (Hardyman and Oldow, 1991; Dilles and Gans,
1995; Surpless, 2012). Starting in the mid-Miocene, intermediate-silicic volcanism, large-magnitude extension, and basaltic
magmatism characterized the western Great Basin from the To-
quima Range in central Nevada to Death Valley, with much of
the intermediate-silicic volcanic rocks deposited in west-draining
paleo-river valleys prior to the development of a drainage divide
within the Sierra Nevada (Figure 1 of Henry and Faulds, 2010).
Fault systems of rotated normal fault sets have been documented
ZLWK•H[WHQVLRQDWWKH+DOO5RELQVRQDQG5R\VWRQGLVtricts, in addition to Yerington (Shaver and McWilliams, 1987;
Seedorff, 1991b; Seedorff et al., 1996). Throughout the Basin
and Range province, two broad structural patterns have been
observed: a “Basin and Range” pattern (i.e., high-angle normal
fault-bounded horsts and grabens) with minimal extension and
rotation, and a pattern of low-angle normal faults and/or rotated
normal faults associated with high-magnitude extension and
rotation (Proffett, 1977; Stewart, 1998; Eckberg et al., 2005).
Mesozoic geology and structure
A succession of mid-Triassic (or older) volcanic and sedi-
1158
Carson A. Richardson and Eric Seedorff
mentary rocks associated with a Triassic continental margin
volcanic arc were faulted and folded synchronous with Jurassic plutonism (Proffett and Dilles, 2008). The Middle Jurassic
is characterized by high-K, calc-alkaline silicic to intermediate
magmatism between 170–165 Ma (Dilles and Wright, 1988;
Barton, 1996). Two suites of volcanic rocks, the Artesia Lake
and the Fulstone Spring Volcanics, are deduced through conWUDVWLQJ VXO¿GH FRQWHQW WR EH WKH UHVSHFWLYH V\QHPSODFHPHQW
and post-emplacement vented products of the 169.4–168.5
Ma Yerington batholith (Lipske and Dilles, 2000; Proffett and
Dilles, 2008). Major normal faults bound a large Mesozoic
graben that contains the Yerington batholith and that may have
formed during rapid magma withdrawal (Dilles and Proffett,
2WKHUIDXOWVWKDWKDYHEHHQLGHQWL¿HGZLWKLQWKH/XGZLJ
septum were active at the time of emplacement and were utiOL]HGIRUÀXLGÀRZDQGGLNHHPSODFHPHQW
The Yerington batholith is a composite pluton with three
distinct, successive phases: the McLeod Hill quartz monzodiorite, the Bear quartz monzonite, and the Luhr Hill granite.
(DFK SKDVH LV VLJQL¿FDQWO\ YROXPHWULFDOO\ VPDOOHU WKDQ WKH
prior one (75, 19, and 6 vol. % of the batholith, respectively),
suggesting the continued evolution and differentiation of the
batholith as it crystallized over time (Dilles, 1987). This is further supported by the increasing SiO2 content (60, 66, 68 wt.
%, respectively) and estimated average depth of emplacement
(<1, 1.5, 2.5–5 km, respectively), which suggests a tendency
toward downward and inward crystallization of the composite pluton. Based on crosscutting and spatial relationships, the
granite porphyry dikes associated with the formation of the
Luhr Hill granite are demonstrably the source of mineralizLQJÀXLGVIRUWKHSRUSK\U\GHSRVLWV'LOOHV'LOOHVDQG
Einaudi, 1992). The ore shells formed at depths of 3–6 km,
and the highest Cu grades are found in the upper 2.5 km of
each of the three major granite porphyry dike swarms (Dilles
and Proffett, 1991). The mineralizing porphyry dikes have an
aplitic groundmass, with 50% phenocrysts (orthoclase-quartzplagioclase-hornblende-biotite). The porphyry dikes presentO\VWULNHa1ƒ:DQGGLSƒ1DQGLQSUHWLOWFRQ¿JXUDWLRQ
would have had a strike of ~N35°W and dipped ~90°. The
porphyry dikes formed at Jurassic paleodepths of ~1 to 5.5
NPFRPPRQO\KDYHSUHWLOWYHUWLFDOH[WHQWVRIa±NP
DQGGLVSOD\ZLGWKVRIPWRP3URIIHWWDQG'LOOHV
1984).
In the Ann-Mason fault block, approximately 42% of the
surface exposure of the batholith is pervasively altered (Dilles
and Einaudi, 1992, p. 1974 and Figure 2B). The alteration
types present are: skarn, which occurs along the fringes of
the dike swarm where carbonate beds were encountered and
form Mg-rich and Ca-rich skarns (Harris and Einaudi, 1982;
Einaudi, 2000); main stage potassic and sodic-calcic alteration, occupying the central part of the dike swarm; and late
stage sodic and sericitic alteration, which overprint some of
the earlier potassic alteration and extend to higher pre-tilt levels of the system.
Cenozoic geology and structure
The Yerington district has been extended more than 150%
by multiple successive sets or generations of east-dipping normal faults that penetrated a minimum of 8 km depth into the
crust, initiated at high angles, and rotated to shallower dips until
being cut by new fault sets (Proffett, 1977; Dilles and Gans,
1995). This is clearly established by the steep westward tilting
RIDSUHFLVHO\GDWHGVHULHVRI2OLJRFHQHDVKÀRZWXIIV±
0DWKDWZHUHFRQ¿QHGWRDSDOHRYDOOH\WKDWÀRZHGHDVWWRZHVW
through the Yerington district prior to the rise of the Sierra Nevada (Dilles and Gans, 1995; Henry and Faulds, 2010; Henry
and John, 2013). These tuffs, originating 200 km east in the
Toquima Range, were deposited onto the existing paleosurface
prior to extension, and their contact with the Jurassic batholith
(and local gravels) largely marks the mid-Tertiary erosional unconformity in the district (Proffett and Proffett, 1976; Garside
et al., 2002). The deposition of volcanic rocks followed ~20° of
westward tilting prior to the onset of Basin and Range faulting
(Geissman et al., 1982b). This Oligocene erosion surface and
the overlying volcanic rocks provide a key structural marker for
the initiation of Basin and Range faulting in the Yerington district. The amount of tilting and extension that occurred during
slip on faults of each generation is further constrained by the
DJHVRILQWHUPHGLDWHWRPD¿FYROFDQLFURFNVWKDWPDQWOHROGHU
faults and tilted strata and that are less rotated than older rocks
(Proffett, 1977; Dilles and Gans, 1995).
Based on the work of Dilles and Gans (1995), faults of the
oldest generation of east-dipping faults, including the Singatse
and Blue Hill faults, were active between ~13.8–12.6 Ma, with
a±ƒRIWLOWLQJa±NPVSDFLQJDQG”NPRIRIIVHWRQLQGLvidual faults. Faults of the second generation, including the May
Queen fault, were active from 11-8 Ma, with incremental tilting
of 30–35°in the Wassuk Range and lesser amounts in the Singatse Range, were irregularly spaced, and had <1 km offset. Faults
of the third generation, including the Sales and Montana-Yerington faults, have been active since ~4 Ma, were associated with
~10° of incremental tilting, were spaced at 1–4 km, and have ~2
km offset. Faults of all sets exhibit local trough shapes that may
be part of larger mullions, whose axes trend roughly east-west,
in which the concave side of the trough faces up and towards the
east, as shown by the mapped traces of faults and structure contour maps constrained by drill hole piercements of faults.
Proffett (1977) demonstrated that high-magnitude extenVLRQLVPDQLIHVWHGPRVWO\E\FORVHO\VSDFHGIDXOWVRIWKH¿UVW
generation that have large amounts of displacement and large
degrees of tilting of hanging-wall rocks, whereas the two
younger generations of faults are associated with wider spacing
of faults and smaller degrees of tilting. The tilting associated
with the normal faulting has essentially laid the district on its
side (Proffett, 1977; Geissman et al., 1982a). This has generated and preserved a vertical cross section, with the Ann-Mason fault block alone displaying a 6-km range in paleodepth
(Dilles and Einaudi, 1992; Dilles et al., 2000a). The genetic
1159
Ann-Mason structural reconstruction
relationships of the faults have not, however, been universally
agreed to and have been a matter of some debate (Proffett,
1977, 2002; Brady et al., 2000, 2002), being part of a broader
discussion about the nature of extensional faulting (e.g., Wernicke, 2009).
sive contact, stratigraphic contact, erosion surface, or older
fault plane).
In order to better understand the dismemberment of the
Ann-Mason deposit, the structural history following offset on
the Singatse fault has to be resolved before addressing where
the offset portion(s) of the Ann-Mason porphyry copper system
METHODS
may be located. This involves the reconstruction of fault surface maps for the May Queen, Montana-Yerington, and Sales
Field work for this study was conducted in June and July faults, which structurally offset the Singatse fault and create
2013 as part of a thesis project sponsored by Entrée Gold (Rich- gaps in the Singatse fault surface (Figure 2). As these younger
ardson, 2014). Detailed, Anaconda-style, outcrop mapping faults are dominantly under cover and were not the subject of
(Einaudi, 1997; Brimhall et al., 2006) at a scale of 1:2,000 fo- WKHRULJLQDO¿HOGZRUNWKHGDWDIRUWKHVHIDXOWVDUHGHULYHGIURP
cused on outcrops along the traces of the Singatse and Blue Hill the map and cross sections of Proffett and Dilles (1984), which
faults. Approximately 670 m of selected drill core were logged are based on years of mapping, drilling, and careful construcat a scale of 1:100, in which lithology, structure, proportions of tion of detailed cross sections. Interpretations of the magnitude,
YDULRXVVXO¿GHVPLQHUDORJ\RIYHLQVDQGDOWHUDWLRQHQYHORSHV slip direction, and initial structural attitudes of the faults in each
and average width and spacing of veins were recorded. This of the fault sets are summarized in Table 2.
core constitutes intervals through the Blue Hill and Singatse
faults, generally 30–100 m intervals of core from the hanging- STRUCTURAL GEOLOGY
wall through the footwall.
Geologic maps of the proximal footwall and hanging wall
7KH IROORZLQJ VHFWLRQ ¿UVW GHVFULEHV WKH FKDUDFWHU RI WKH
surfaces of the faults were constructed by using data from exist- fault zones of the Singatse and Blue Hill faults, based on obing geologic maps, drill hole intercepts, and original mapping servations in drill core. Then each of the faults is described,
IURP¿HOGZRUN7KHVHPDSVDUHWKHQVXSHULPSRVHGRQDVWUXF- grouped according to geometric characteristics (fault sets) and
tural contour map of the fault. Such fault surface maps, also relative ages, from youngest to oldest.
known in the petroleum industry as fault cutoff maps (Tearpock and Bischke, 2003; Groshong, 2006, p. 229; Seedorff et Fault damage zones
al., this volume), can present traditional lithologic and structural data, but can also show overlays of other features such as
The growth and progressive development of faults is comhydrothermal alteration and metal grades (e.g., Keeler, 2010; monly represented by what is recorded along the fault slip surSchottenfeld, 2012). Such maps permit a rigorous, three-di- face. The fault zone, however, extends beyond this “fault core”
mensional reconstruction of the faults by restoring the hanging to include a surrounding volume of rock wherein deformation is
wall view over the footwall view, showing how the magnitude more broadly distributed, known as the damage zone (McGrath
and direction of slip varies across the fault surface. The number and Davison, 1995; Caine et al., 1996; Kim et al., 2004). In the
RIVLJQL¿FDQW¿JXUHVLQVOLSPHDVXUHPHQWVLVNHSWWRWZRHJ Yerington district, kinematic indicators along the slip surfaces
NP UHÀHFWLQJ WKH FRQ¿GHQFH LQ ORFDWLQJ FRQWDFWV DQG are found locally, but commonly faults are deduced by the prespiercing points for structural reconstructions, and is better ence of clay gouge and comminuted rock, commonly juxtaposconstrained than in many deposits due to the density of drill ing Tertiary volcanic rocks against Jurassic intrusive rocks.
data and mapping of surface features. Unique determinations
Selected intervals of drill holes that pierce the Singatse and
of the magnitude and direction of slip require piercing points Blue Hill faults, which currently dip at low-angles, were logged
RQHLWKHUVLGHRIWKHIDXOWSODQHZKLFKFRPPRQO\DUHGH¿QHG to characterize the subsurface expression of the fault wall-damby the intersection of two other geologic contacts (e.g., intru- age zones. The hanging walls of the faults (of which all but one
Table 2. FAULT CHARACTERISTICS.
Fault
Montana-Yerington
Sales
May Queen
Singatse
Blue Hill
1A
Present
Strike
(Quadrant)
Present
Dip
Direction
Present
Dip
Slip
Magnitude
(km)
N15°W
N10°W
N00°W
N05°W
N70°E
S61°E
075°
080°
090°
085°
160°
199°
51°
52°
35°
13°
22°
71°
1.1
1.6
1.4
3.8
2.8
0.2
Slip Direction Direction of
(in modern
Increasing
plan view)
Displacement
E
E
SE
E/ENE
E/ESE
ESE
N
S
N
S(?)
N
—
Restored
Strike
Restored Dip Restored
(Quadrant)
Direction
Dip
N13°W
N09°W
N00°W
N02°W
N22°E
N86°E
073°
081°
090°
088°
112°
176°
60°
62°
60°
73°
70°
56°
1160
Carson A. Richardson and Eric Seedorff
Figure 3. Photographs of drill core. A: A clay-rich gouge with well-rounded clasts of the intrusion in it. B: A 10-cm thick zone of subrounded to well-rounded clasts
in a clay-rich matrix. C: An example illustrating the abundance of synthetic clay shears that are found at depth.
had Oligo-Miocene volcanic rocks) commonly have a brecciated zone that is 1–11 m thick (n = 10, mean = 8.2 m, mode =
3 m), with local ~3 m-thick brecciated zones occurring in the
footwall of the faults (Figure 3a-b). Above the main hangingwall breccia zone and locally in the footwall smaller brecciated zones are observed, commonly ranging from 2–10 cm. In
drill hole EG-AM-11-025, the clasts become more rounded towards the fault contact, but in many drill holes the brecciated
zone consists of more angular fragments, suggesting localized
episodes of brecciation as the fault slipped. Clasts constitute
~30–60% of the breccia, ranging in size from 2 mm to 25 cm.
The clasts are subangular to subrounded (locally rounded),
poorly sorted, and commonly heterolithic. The matrix composition varies between brecciated zones but is dominantly a clayrich matrix surrounded by subangular to subrounded clasts with
OHVVDEXQGDQWEUHFFLDVZKHUHJUDQXODU¿QHJUDLQHGURFNÀRXU
is the matrix. The breccias are termed chaotic breccias followLQJWKHFODVVL¿FDWLRQRI:RRGFRFNDQG0RUW/HVVFRPmonly, strongly comminuted, poorly indurated, clay-rich shear
zones are present in half of the drill holes logged (Figure 3c).
These zones range in thickness from 1–14 m (n = 7, mean = 6
m, mode = 5.5 m). Fabrics developed in comminuted rock in
the damage zone commonly are aligned parallel to the fault
surface, which suggests an important component of motion in
the damage zone parallel to the motion of the fault, although
many small shears also are oriented at high angles to the fault
surface. In the case of the Singatse fault, discrete bands of
clay gouge are locally observed and range from 4–40 cm in
thickness.
Fault set 4
Faults of set 4 are northerly striking, steep, easterly dipping
faults that cut all other major faults in the district. Set 4 includes
the Montana-Yerington and Sales faults.
Montana-Yerington fault
The Montana-Yerington fault (Figure 4) is the rangebounding fault along the eastern extent of the Singatse Range.
The Montana-Yerington fault in map pattern takes the form of
an embayment, representing a down-dip corrugation and the
beginning of a salient or change in orientation on its southern
end. It structurally offsets and creates gaps in the fault surfaces
of older faults, including both the May Queen and Singatse. The
structure contour map of the Montana-Yerington fault in present-day orientation reveals that the fault strikes approximately
N15°W and dips ~51° E (Figure 4), similar to dips measured in
outcrop (Proffett and Dilles, 1984) of 56 and 58° E. The fault
is trough-shaped and planar-curviplanar in geometry. Geologic
maps of the hanging wall and footwall show that the juxtaposiWLRQRIWKHORZHUaNPRIWKHÀRRURUVKRXOGHURIWKH<HULQJWRQ
batholith against the uppermost ~0.5 km of the Jurassic rocks
and lower Tertiary volcanic rocks by the Singatse fault (Figure
4a-b). On the westernmost portions of the geologic maps of the
fault surface, the May Queen fault juxtaposes the Guild Mine
member of the Mickey Pass Tuff in the hanging wall against
the Weed Heights member of the Mickey Pass Tuff through the
Bluestone Mine Tuff and Lincoln Flat Andesite in the footwall
(Figure 4a-b).
Ann-Mason structural reconstruction
Sales fault
The Sales fault is located ~1–2 km east of the Singatse
Range in Mason Valley; along strike to the south it becomes
the range-bounding fault in the Mickey Pass area, as the Sales
fault bends in map pattern and changes from being salient to
recessed in map pattern towards the south. In addition, the
Sales fault splays southward into two closely spaced faults at
the surface before rejoining and apparently being cut off by or
merging with the range-bounding Montana-Yerington (possibly
Bluestone Mine?) fault. It structurally offsets and, therefore,
creates gaps in the fault surfaces of both the May Queen and
Singatse faults. The structure contour map of the Sales fault
reveals that the fault, in its present orientation, strikes ~N10°W
and dips ~52° E (Figure 5), similar to a dip measured in outcrop
(Proffett and Dilles, 1984) of 58° E. The Sales fault is planarcurviplanar in geometry and gently bends to the south, with the
dip direction changing from ENE to ESE. Geologic maps of
the hanging wall and footwall show a large mass of the Luhr
Hill granite that is in fault contact with a thin carapace of Bear
quartz monzonite, due to the Singatse fault (Figure 5a-b). The
Singatse fault is itself cut by the May Queen fault in the plane
of the Sales fault and juxtaposes the above described rocks with
a veneer of the uppermost rocks of the batholith across the Tertiary erosion surface through the volcanic rocks to the Bluestone Mine Tuff. A small fault (Figure 5, black north-trending
OLQHRIXQFHUWDLQDI¿QLW\LVVKRZQLQ3URIIHWWDQG'LOOHV
1161
and is shown as being internal to the volcanic rocks and tipping
out along strike over a relatively short distance.
Fault set 3
7KHRQO\VLJQL¿FDQWIDXOWLQWKHVWXG\DUHDWKDWLVDPHPEHU
of fault set 3 is the May Queen fault, although faults with a
similar orientation occur elsewhere in the district (Proffett and
Dilles, 1984; Schottenfeld, 2012) and to the southeast in the
Wassuk Range (Surpless, 2010).
May Queen fault
The May Queen fault mapped by Proffett and Dilles (1984)
is a moderately dipping fault within and to the east of the Singatse Range. The May Queen fault offsets the Singatse fault
and creates gaps in its surface; in turn, the May Queen fault is
dismembered by the Montana-Yerington and Sales faults into
three smaller fault blocks (Figure 6). The rotation associated
with movement on the Montana-Yerington and Sales faults
results in variations in dips and strikes of the different faultbounded fragments of the May Queen fault. The structurally
lowest fragment is in the hanging wall of the Sales fault, where
the May Queen fault dips ~10° ESE; the fault fragment located
in the footwall of the Sales and the hanging wall of the Montana-Yerington fault dips ~38° NE; the fault fragment located
in the footwall of the Montana-Yerington fault is dipping ~32°
Figure 4. Montana-Yerington fault surface maps (red fault trace in Figure 2). A. Footwall geology layer superimposed with structure contours. B. Hanging-wall
geology layer superimposed with structure contours. Contour interval is 100 m. Section lines from Proffett and Dilles (1984) are noted with the section name and
PD84. UTM coordinates (Zone 11) are in 1000 m intervals. Legend is applicable to Figures 4–7, 10, and 11.
1162
Carson A. Richardson and Eric Seedorff
Figure 5. Sales fault surface maps (orange fault in Figure 2). A. Footwall geology layer superimposed with structure contours. B. Hanging-wall geology layer
superimposed with structure contours. Contour interval is 100 m. Section lines from Proffett and Dilles (1984) are noted with the section name and PD84. UTM
coordinates (Zone 11) are in 1000 m intervals. See Figure 4 for legend.
E (Figure 6a-b). All fragments are scoop- or shovel-shaped and
face eastward (Figure 6a-b). Geologic maps of the hanging wall
and footwall show that the trace of the Singatse fault in the plane
of the May Queen juxtaposes Luhr Hill granite (Jpqm) against
the uppermost portions of the Yerington batholith through the
erosion surface and volcanic stratigraphy to the Bluestone Mine
member and Lincoln Flat Andesite (Figure 6a-b). Another fault
WKDWPD\EHORQJWRWKLVIDXOWVHWVHHGLVFXVVLRQLQ³6LJQL¿FDQFH
of 3rd-generation faults”) was measured as striking N60°E and
dipping 42° SE at the surface. The surface of the fault has slickenlines with a trend of S75°E and plunge ~32°.
Fault set 2
Faults of set 2 are closely spaced, northerly striking, gently
easterly dipping faults that include the Singatse and underlying
Blue Hill faults, as well as others shown on the map and cross
sections of Proffett and Dilles (1984). The faults of set 2 are cut
by faults of sets 4 and 3 and cut faults of set 1. The Singatse and
Blue Hill faults are largely subparallel and thus are assigned to
the same set; nonetheless, the Singatse fault cuts the downdip
portion of the Blue Hill fault.
Singatse fault
The Singatse fault (Figure 7) is a gently dipping, concaveto-the-east fault that in map pattern wraps around much of the
Singatse Range, with the northern and southern portions of
the fault trace occurring near Mason Pass and Mickey Pass,
respectively. To the east, the Singatse fault is structurally dismembered by the May Queen, Montana-Yerington, and Sales
faults into a series of small, buried fault blocks. The geology
of the southwestern portion of Singatse fault block is tightly
constrained through abundant drill data (over 80 km of drilling
at Ann-Mason in the last 55 years). The structure contour map
of the Singatse fault reveals the overall strike of the fault is
~N05°W; it is curviplanar in geometry and dips ~13° E along
the trough of the fault (Figure 7). Geologic maps of the footwall
display the geology in the plane of the existing Singatse fault,
Ann-Mason structural reconstruction
including the porphyry dike swarm (Jqmp) associated with the
Ann-Mason deposit and the position of the 1A fault (described
below under fault set 1). In addition, the geology was continued
into up-dip, eroded portions of the fault surface by projecting
WKHJHRORJ\DORQJFURVVVHFWLRQ66ƍXSWRWKHSURMHFWHGIDXOW
surface (Figure 8). Geologic maps of the hanging wall show a
network of interacting hanging wall splays that repeat portions
of the Oligocene volcanic succession in several places (Figure
7b). Several smaller faults create small gaps in the hanging wall
and footwall of Singatse fault and have previously been interSUHWHGE\3URIIHWWDQG'LOOHVVHFWLRQ$$ƍWREHORQJWR
the 3rd fault set, as they cut the Singatse and are truncated by the
Montana-Yerington fault (Figure 7b). Stratigraphically below
WKHYROFDQLFURFNVLVWKHXSSHU”NPRI<HULQJWRQEDWKROLWK
which is offset by the Montana-Yerington fault. The Singatse
fault is further dismembered by both the May Queen and Sales
faults (Figure 7a-b). The hanging-wall portion of Ann-Mason
dike swarm has been translated into these down-dropped fault
blocks.
Blue Hill fault
The constraints on the nature of the Blue Hill fault are
based on surface mapping, including an exploration trench
1163
(Figure 9a, c-d), and from drilling intercepts at the Blue Hill
prospect, the Roulette target, and one deep hole to the northwest of Ann-Mason just beyond the Singatse fault (Figures 2,
10). At the surface, the Blue Hill fault is expressed as one to
several shears of clay gouge ~8–20 cm in thickness (Figure 9ad). A discrete slip surface with slickenlines was not observed.
The Blue Hill fault is projected further down dip beneath the
Singatse fault, though it has not been intercepted by drill holes
at those depths. A point derived from the intersection of the secWLRQ66ƍ)LJXUHRIWKLVVWXG\ZLWKVHFWLRQ$$ƍRI3URIIHWW
and Dilles (1984) is also used as an additional constraint on the
position of the Blue Hill fault (Figures 8, 10). In the Blue Hill
area, the Blue Hill fault dips ~22°SE and is planar-curviplanar
in geometry. This portion of the fault appears to be the northwestern side of a trough, or bend, in the fault (Figure 10a-b).
Structure contours are projected northeastward until they are
truncated by the Singatse fault. The geologic maps of the hanging wall and footwall of the fault surface show plutonic rocks of
the Yerington batholith (all phases except the Luhr Hill granite)
in the eastern portions, with the Oligo-Miocene volcanic rocks
overlying the Tertiary erosion surface in the western-central
portions. In the hanging wall of the Blue Hill fault, drill holes
intercepted the Guild Mine member of the Mickey Pass Tuff
Figure 6. May Queen fault surface maps (magenta fault in Figure 2). A. Footwall geology layer superimposed with structure contours. Lighter colored areas are
projected geology up into the air. B. Hanging-wall geology layer superimposed with structure contours. Contour interval is 100 m. Section lines from Proffett and
Dilles (1984) are noted with the section name and PD84. UTM coordinates (Zone 11) are in 1000 m intervals. See Figure 4 for legend.
1164
Carson A. Richardson and Eric Seedorff
Figure 7. Singatse fault surface maps (dark blue fault trace in Figure 2). A. Footwall geology layer superimposed with structure contours.
Lighter colored areas are projected geology up into the air. B. Hanging-wall geology layer superimposed with structure contours. Contour
interval is 100 m, with dashed contours projected into the air. Fault gaps from small faults that have been qualitatively closed are shown.
The trace of the Ann-Mason ore body from the footwall is shown as dismembered fragments in the hanging wall. Section lines from Proffett
and Dilles (1984) are noted with the section name and PD84. UTM coordinates (Zone 11) are in 1000 m intervals. See Figure 4 for legend.
Ann-Mason structural reconstruction
1165
)LJXUH6LPSOL¿HGFURVVVHFWLRQ66ƍRIURFNW\SHIURPWKH&DVWLQJ&RSSHU/XGZLJDUHDWKURXJKWKH$QQ0DVRQ
deposit. The section is oriented N20°E, perpendicular to the strike of the granite porphyry dikes, to illustrate the
location of the dike swarms in the subsurface against the Blue Hill fault. Shown on Figs. 2, 7, 9.
beyond the position based on projection of the surface geology
(Figure 10b). Using the architecture of the Singatse fault (see
above) as a template, two hanging-wall splays are interpreted
to be present in the hanging wall of the Blue Hill fault to satisfy
the subsurface data (Figure 10b).
Fault set 1
There are ~15 closely spaced faults of similar orientations
that are intercepted in drill core under the structural cover of the
Singatse fault. The 1A fault is examined because it is relatively
well exposed by drilling and may be the fault with the most
offset in this set, although the amount of offset is smaller than
for faults of the sets described above.
Souviron, 1976, as Jqmp-a dikes) that can be traced from the
southeastern portion to the center of the hanging wall and footwall maps (Fig 11a-b).
INTERPRETATIONS: RECONSTRUCTIONS BASED
ON FAULT SURFACE MAPS
Crosscutting relationships of faults: Fault generations
The faults have been heretofore grouped by their descriptive characteristics, primarily their differences in strike and dip,
into four geometric sets. The shared features between the faults
of the same sets as well as the consistent crosscutting relationships between the various fault sets indicate that each set can be
regarded as temporally distinct generation of faults. Thus, the
1A fault
The 1A fault presently strikes N61°W and dips 71°SW faults of the 4th generation are the youngest, and faults of the
(Figure 11). It is planar in geometry and is truncated up dip by 1st fault generation are the oldest. According to Andersonian
the Singatse fault (dark blue). Drill piercements show that the fault mechanics, normal faults initiate at high angles (58–68°)
1A fault is truncated at an elevation of ~1600 m and has been in the uppermost crust and rotate to lower angles concurrent
drilled down dip to an elevation of ~600 m. Surface geologic with slip (Anderson, 1951; Collettini and Sibson, 2001). The
maps of the fault (Figure 11a-b) mainly reveal a swarm of por- rotated, lower angle faults can become locked when the stress
phyry dikes (Jqmp) crosscutting the McLeod Hill quartz mon- required to produce continued slip along them is greater than
zodiorite (Jgd). Nonetheless, two distinctive geologic markers is required to initiate a new set of high angle faults (Sibson,
are also present: (1) a mass of the Luhr Hill granite (Jpqm) 1977). These locked, dismembered fault plane fragments are
occurs in the structurally deeper, southern portions of hanging then buttressed between the high-angle, newly active faults
wall and footwall of the 1A fault, and (2) a discontinuous, but (Proffett, 1977; Chamberlin, 1983; Gans et al., 1985; Wernicke
FOHDUWUHQGRIDGLVWLQFWLYHW\SHRISRUSK\U\GLNHVGH¿QHGE\ and Axen, 1988; Davis et al., 2004).
1166
Carson A. Richardson and Eric Seedorff
4th-generation faults
Montana-Yerington fault
Unique piercing points present in the hanging wall and footwall that are used to determine the slip magnitude are shown in
Figure 4. Along the southern portion of the fault, east of Mickey
Pass, there is ~0.4 km of slip measured in the plane of the fault
from the cutoff of the contact between the Weed Heights and the
Guild Mine members of the Mickey Pass Tuff by the May Queen
fault (Figure 4). Along the northern portion of the MontanaYerington fault, the cutoff of the contact of the Bluestone Mine
Tuff and underlying Singatse Tuff by the May Queen fault has
a ~1.0 km of slip (Figure 4). The slip direction up the plane of
the fault changes south to north from N45°W to N75°W (Figure
4). The higher magnitude of slip in the northern portion of the
fault is constrained in the arcuate portion or trough of the fault,
whereas the less displaced southern portion of the fault occurs
along a more planar section of the fault (Figure 4). Slip increases
to the north, and consequently, suggests that the fault is tipping
out to the south. If the observed average change in offset of ~0.6
km over ~3.5 km is representative along strike, then the fault tip
would be located ~2.8 km south of Mickey Pass.
Sales fault
Restoring the hanging-wall map over the footwall map
results in a reconstruction with several unique piercing points
from which to measure the slip in the plane of the Sales fault
(Figure 5). Along the southern portion of the fault, the cutoff of
Figure 9. Blue Hill fault surface features. A: A photograph facing southwest in the Blue Hill fault trench area showing the 8–15 cm thick white-grey clay gouge.
B: A photograph facing west-southwest of the Blue Hill fault exposed in a small exploration pit on the southwestern edge of the Blue Hill area. C: A photograph
facing northeast of the Blue Hill fault trench showing the colored clay/sheared zones of the Blue Hill fault zone. Conglomerate clasts are seen in the far right of
photograph. D: Anaconda-style pace-and-compass map of the Blue Hill fault trench. Mineral codes: (Qz) quartz, (F/Fds) feldspar, (Sr) sericite, (Cl) chlorite, (Bt)
biotite, (Hb) hornblende, (Ep) epidote, (Gl Lm) glassy limonite, (Jar) jarosite, (Tm) tourmaline. Minerals noted in brackets are the suggested original mineral being
replaced by the mineral preceding the brackets (e.g., sericite after feldspar is Sr[Fds]).
Ann-Mason structural reconstruction
1167
Figure 10. Blue Hill fault surface maps (light blue fault trace in Figure 2). A. Footwall geology layer superimposed with structure contours. Lighter colored
areas are projected geology up into the air. B. Hanging-wall geology layer superimposed with structure contours. Contour interval is 100 m, with broad dashed
contours projected into the air and narrow dashed contours projected at depth. The projected location of the offset portions of the dike swarms in the footwall
are shown. Section lines from Proffett and Dilles (1984) are noted with the section name and PD84. UTM coordinates (Zone 11) are in 1000 m intervals.
1168
Carson A. Richardson and Eric Seedorff
Figure 11. 1A fault surface maps (purple fault trace in Figure 2). A. Footwall geology layer superimposed with structure contours. B. Hanging-wall geology layer
superimposed with structure contours. Contour interval is 200 m. The dark blue line in the top right of each map is the line of intersection with the Singatse fault,
representing where the 1A fault is truncated by the Singatse fault. C: Footwall alteration layer. D: Hanging-wall alteration layer. E: Footwall copper grade layer. F:
Hanging-wall copper grade layer. UTM coordinates (Zone 11) are in 200 m intervals.
Ann-Mason structural reconstruction
1169
)LJXUH/RZHUKHPLVSKHUHHTXDODUHDVWHUHRJUDSKLFSURMHFWLRQVRIGDWDIURPIDXOWVDQGYROFDQLFFRPSDFWLRQIROLDWLRQLQDVKÀRZWXIIVDUHVKRZQWKURXJKWKH
rotations of the various fault blocks. The dashed lines are the volcanic rocks (compaction foliations dips at 60° = green, 70° = green-yellow, 80° = brown, 90° =
black), solid lines are Montana-Yerington (red) and Sales (orange), solid lines are May Queen (strawberry red = 32° dip of May Queen, magenta = 35° dip of May
Queen, maroon = 38° dip of May Queen), the solid lines are the Blue Hill (light blue) and Singatse (dark blue), and the dash-dot line is the 1A fault (purple). A:
7KHSUHVHQWGD\FRQ¿JXUDWLRQRIWKHIDXOWDQGFRPSDFWLRQIROLDWLRQSODQHV%&RPSDFWLRQIROLDWLRQDQGIDXOWSODQHFRQ¿JXUDWLRQIROORZLQJWKHUHVWRUDWLRQRIWKH
WKJHQHUDWLRQIDXOWVZLWKƒRIWLOWLQJDERXWD16KRUL]RQWDOD[LV&&RPSDFWLRQIROLDWLRQDQGIDXOWSODQHFRQ¿JXUDWLRQIROORZLQJUHVWRUDWLRQRIWKHUGJHQHUDWLRQIDXOWVZLWKƒRIWLOWLQJDERXWD16KRUL]RQWDOD[LV'&RPSDFWLRQIROLDWLRQDQGIDXOWSODQHFRQ¿JXUDWLRQIROORZLQJUHVWRUDWLRQRIWKHUGJHQHUDWLRQIDXOWV
ZLWKƒRIWLOWLQJDERXWD16KRUL]RQWDOD[LV(&RPSDFWLRQIROLDWLRQDQGIDXOWSODQHFRQ¿JXUDWLRQIROORZLQJUHVWRUDWLRQRIWKHUGJHQHUDWLRQIDXOWVZLWKƒRI
WLOWLQJDERXWD16KRUL]RQWDOD[LV)&RPSDFWLRQIROLDWLRQDQGIDXOWSODQHFRQ¿JXUDWLRQIROORZLQJDUHVWRUDWLRQRIWKHQGJHQHUDWLRQIDXOWVZLWK±ƒRIWLOWLQJ
(depending on the amount of tilt assigned to the 3rd generation faults) about N-S horizontal axis to return the Singatse fault to 60°. G: Compaction foliation and
IDXOWSODQHFRQ¿JXUDWLRQIROORZLQJDUHVWRUDWLRQRIWKHQGJHQHUDWLRQIDXOWVZLWK±ƒRIWLOWLQJGHSHQGLQJRQWKHDPRXQWRIWLOWDVVLJQHGWRWKHUGJHQHUDWLRQ
faults) about a N-S axis to return the shallowest dipping volcanic units to horizontal. Stereographic rotations were performed with the program OSXStereonets
(Allmendinger et al., 2012).
1170
Carson A. Richardson and Eric Seedorff
the Tertiary erosion surface by the May Queen fault results in
~1.6 km of slip. Along the northern portion of the Sales fault,
the restoration of the cutoff of the Singatse fault by the May
Queen fault results in ~1.0 km of slip. The higher magnitude of
slip along the southern portion of the fault occurs in a bend in
the plane of the fault, possibly representing a salient in the fault
(Figure 5). This geometry constrains the sense of motion, as the
fault should move down the trough with minimal lateral motion
across an adjacent ridge. The slip direction changes north to
south from S30°W to S60°W. The magnitude of slip increases
to the south, suggesting that the fault is tipping out to the north.
Using the observed average increase of ~0.6 km over ~2.5 km,
the fault tip is predicted to be located ~3.8 km further north
along strike, due east of the MacArthur pit.
that the footwall of the Montana-Yerington has not undergone
any vertical axis rotation.
Overlaying the slip paths on the structure contours of the
fault show that they are moving at acute angles (~20°) to axes of
trough in the fault. Assuming the fault initiated at 60° (Anderson, 1951), this might imply that the fault underwent ~12–18°
of horizontal-axis tilting as a result of differential movement on
the younger, crosscutting faults (Figure 12c-e).
6LJQL¿FDQFHRIUGJHQHUDWLRQIDXOWV
This fault generation is not well exposed in the Singatse
Range compared to the Wassuk Range and is described to offset
the earlier 2nd generation faults “slightly” (Dilles and Gans,
1995, p. 482). The reconstruction based on the fault surface
maps presented here shows that this generation has ~40% great6LJQL¿FDQFHRIWKJHQHUDWLRQIDXOWV
er offset than previously depicted (<1 km of slip derived from
These two faults, representing the youngest normal fault ''ƍ RI 3URIIHWW DQG 'LOOHV 7KLV FRXOG EH GXH WR WKH
system in Yerington, both dip ~50°E, suggesting that they have fact the inferred slip direction is at ~45° to the lines of section
XQGHUJRQH ƒ RI ZHVWZDUG WLOWLQJ VLQFH WKH\ ¿UVW LQLWLDWHG LI used in previous reconstructions and was unable to account for
they initiated at a dip of 60° (Figure 12a-b). This interpretation this in- and out-of-section movement. If this one reconstruction
is consistent with the gentle westward dips of surface exposures were broadly valid, then it would require amending the stateRIEDVDOWLFDQGHVLWHODYDÀRZVQHDUWKH:HVWHUQ1HYDGDPLQH ment by Proffett (1977, p. 255) that most, if not all, faults in
southwest of the town of Mason (Proffett and Dilles, 1984) and <HULQJWRQKDYH³QRVLJQL¿FDQWQRUWKVRXWKFRPSRQHQWRIGLVZLWKVLPLODUGLSVRQWKHWRSRIEHGURFNRQWKHÀRRUVRIPRG- placement.” The May Queen fault can then be characterized as
ern half-grabens, as shown on the cross sections of Proffett and an oblique-slip normal fault with a right-lateral slip component,
Dilles (1984) that are constrained by drilling and geophysical making it analogous with the oblique normal faults mapped in
constraints.
the Wassuk Range by Dilles (1993). Whether the slip direction
The Montana-Yerington and Sales faults exhibit opposing of the May Queen is representative of the entire 3rd generation
directions of increasing displacement, as is typical of adjacent or is an outlier is unclear, and this uncertainty generates an alfaults forming segments of a modern normal fault system (Pea- ternate possibility. If a second fault of this generation were to
cock and Sanderson, 1997). The map pattern (as seen on in have an opposing direction of increasing displacement, as is obthe fault surface maps of the May Queen, Figure 6a-b), which served for the 4th-generation faults, then the net slip direction
shows increasing widths of fault gaps to the north and south and of the 3rd-generation fault system could have been east-west.
an unfaulted zone bridging the area between the two faults, suggests that this is a buried relay ramp underneath the alluvium 2nd-generation faults
of Mason Valley.
Singatse fault
3rd-generation faults
One important geologic marker for determining the slip
on the Singatse fault is the Tertiary erosion surface cut by an
May Queen fault
earlier normal fault (southern circled location, Figure 7). Other
Geologic markers that constrain the magnitude and direc- piercing points are created by the position of the Ann-Mason
tion of slip in the southern portion of the fault surface are the porphyry dike swarm relative to other phases of the batholith
contact of a porphyry dike (Jqmp) and the Border quartz mon- (Figure 7). Several of the smaller faults in the hanging wall
zonite (Jbqm) cut by the Singatse fault (Figure 6); they result in that create gaps have been qualitatively closed, as their lateral
a restoration of ~1.3 km of slip. Markers used in the northern extents are entirely within the fault surface map. The broad
portion of the fault are the contact of the McLeod Hill quartz trough-shaped geometry of the fault helps constrain the restomonzodiorite (Jgd) and the Border phase (Jbqm) against the ration, as slip oblique to fault grooves or corrugations is more
Tertiary erosion surface, which result in a slip of ~1.4 km. Thus GLI¿FXOWWRDFFRPSOLVKDVLWSODFHVDODUJHUVWUDLQRQZDOOURFNV
the increases in displacement to the north on the May Queen is (Needham et al., 1996). Following the closure of the younger
~0.1 km over 2.7 km, or 60 m per km. Following closure of the fault gaps, the restoration of the Singatse fault shows minimal
4th generation fault gaps in the fault surface, the present-day rotation. The up-dip slip direction, measured near the erosion
slip directions, which range from N35°W to N45°W rotate 5° surface, is N85°W; ~3.6 km of slip is inferred along the northnorthward, thus to N30°W to N40°W and steepens the dip of ern portion of the fault block versus ~3.8 km of slip along the
the fault to ~42-48° (Figure 6). This is based on the assumption southern portion of the fault block.
Ann-Mason structural reconstruction
Blue Hill fault
Several piercing points are available to measure the amount
and direction of slip in the plane of the fault (Figure 10). To
the south, the contact of the Bear quartz monzonite (Jqm) and
McLeod Hill quartz monzodiorite (Jgd) in the Roulette area
where drill holes cut the Tertiary erosion surface indicates 2.4
km of slip. To the north, the cutoff of the Bear quartz monzonite (Jqm) and Border quartz monzonite (Jbqm) contact by the
Tertiary erosion surface (Figure 9a, c-d) indicates a slip here
of 2.8 km. This demonstrates that slip on the Blue Hill fault is
increasing to the north. The present-day slip directions change
slightly (~2°) from south (N84°W) to north (N82°W), consistent with a slight change in slip magnitude moving along the
strike of the fault.
6LJQL¿FDQFHRIQGJHQHUDWLRQIDXOWV
This generation of faults, with slip magnitudes over twice
that of the later generations of faults, was responsible for a large
fraction of the total extension in the district. The tilting of the
later two generations of faults equates to 22–28°; adding this
amount to the down-trough dip of the Singatse fault steepens
the fault to ~35–41° prior to the initiation of the 3rd generation
faults. Additional tilting of ~19–25° would place the Singatse
fault back at ~60° based on the assumption that the fault initiated at t60° (Anderson, 1951; Dilles and Gans, 1995; Collettini
and Sibson, 2001). This amounts to a minimum of 47° of westward tilting due to faulting in the Yerington district by returning
all faults to ~60° (Figure 12f). A key structural marker in previous reconstructions in the Yerington district has been the dips
of the volcanic units, with the assumption that they originally
were deposited horizontally, although caution is warranted in
areas such as this where tuffs were deposited in paleovalleys
due to differential compaction (e.g., Henry and Faulds, 2010).
The average dip of volcanic rocks in the district is 60° to 90°
to the west. The fault restorations thus far would take volcanic
rocks dipping 60° to gentle dips of ~13°W. An additional 13° of
tilting associated with the 2nd generation faults would restore
volcanic rocks that dip 60° to horizontal and steepen the dips of
the Singatse and Blue Hill faults to 73° and 69°, respectively,
when they initiated (Figure 12g).
The difference between this restoration and prior work partially arises from the fact that previous reconstructions of the
GLVWULFWKDYHEHHQGRQHDORQJOLQHVVLPLODUWRVHFWLRQ$$ƍRI
3URIIHWWDQG'LOOHV&URVVVHFWLRQ$$ƍUXQVVXESDUDOOHO
to structure contour lines of the Singatse fault along the side
of a trough, giving it a lower apparent dip; as the down-trough
direction of the Singatse fault is at an acute angle to the line of
section, which is nearly due E-W at this location, the restoration would still return the volcanic rocks to horizontal in crossVHFWLRQDO YLHZ DORQJ$$ƍ7KLV LV WKH LPSOLFLW DVVXPSWLRQ LQ
cross-sectional restorations, that the slip direction is parallel to
the section and entirely in the plane of the fault. In the case of
Yerington, this is valid for the faults of the 2nd and 4th generations, less so for faults of the 3rd generation, and does not allow
1171
the faults of the 1st generation to be accounted for in the same
two-dimensional reconstruction.
Although the Blue Hill and Singatse faults have been assigned to the same generation, the relationship between the
Blue Hill and Singatse faults deserves further analysis. In terms
of geometric relationships, the trace of the Blue Hill fault is
truncated by the Singatse fault at the modern surface (Proffett
and Dilles, 1984), the Blue Hill fault presently dips southeast
(Figure 10), and the Singatse dips east (Figure 7). In terms of
kinematics, there is also a difference in slip of ~1 km between
the two faults. Two potential scenarios are proposed: A) that
the Blue Hill fault entirely predates the Singatse fault, and B)
that the two faults initiated more or less concurrently, and as the
fault system evolved the Singatse grew and breached the Blue
Hill fault, thus inactivating it. In scenario A, the Singatse fault,
being younger, would have offset the Blue Hill fault through its
life cycle, suggesting that the continuation of the Blue Hill fault
in the hanging wall of the Singatse fault should be ~3.8 km east
of the present truncation of the Blue Hill fault. In scenario B,
both faults would have similar ~2.6 km slip magnitudes when
the Singatse fault cut and inactivated the Blue Hill fault. This
would then suggest that the continuation of the Blue Hill fault
would be ~1.2 km east of the present truncation of the Blue Hill
fault. To our knowledge, neither of these hypotheses has been
tested in drilling or mapping.
The attitude of the exposed portion of the Blue Hill fault
has hereto been ascribed to being located on the side of a scoop.
The projected, in-the-air, continuation of the structure contours
north of the Blue Hill fault trace must clear all current topography at minimum. The in-the-air fault surface at some point
PLJKWFKDQJHLQÀHFWLRQDORQJDEHQGVXFKWKDWLWPLJKWLQWHUsect the modern surface again. To the north of the Blue Hill
area the Martha Washington fault dips 15° NNE and could be
the hypothesized continuation of the Blue Hill fault plane (Figure 2). The eroded portion of the fault projects over the Blue
Hill deposit as a salient in the fault, and the modern, exposed
fault segments would be embayments in the larger fault. This
hypothesis is consistent with the approximate alignment of the
Tertiary erosion surface on Proffett and Dilles (1984) from the
hanging wall of the Blue Hill fault to the hanging wall of the
Martha Washington fault.
1st-generation faults
1A fault
The geologic markers used for restoring the fault are the
structurally deep contact between the Luhr Hill granite (Jpqm)
and the McLeod Hill quartz monzodiorite (Jgd) and aligning
the distinctively textured porphyry dikes (Jqmp-a). In its present orientation, the 1A fault has ~230 m of apparent left lateral
slip in plan view (Figure 11). The slip runs nearly parallel to
the modern strike of the fault, i.e., restoring the fault N70°W
parallel to structure contours yields true, not apparent, slip
(Figure 11). This amount of slip is additionally supported by
1172
Carson A. Richardson and Eric Seedorff
hanging wall and footwall maps of alteration types and copper grades (Figure 11c-f). As this fault is shown to be truncated the Singatse fault, then this fault must have been rotated passively by the later three generations of faults as they
tilted ~60° westward. Stereographic rotations of the 1A fault
through the tilt history of the younger faults restores the 1A
fault to a strike of N86°E and a dip of ~56°S (Figure 12a-g).
Coupling the offset direction with this rotation, the 1A fault
was initially a down-to-the-south normal fault with a dip-slip
displacement of 230 m.
6LJQL¿FDQFHRIVWJHQHUDWLRQIDXOWV
This generation of faults has little attention in the literature to date. Faults of similar orientations have been known by
previous workers in the district (Souviron, 1976; Figure 3 of
Dilles and Einaudi, 1992) but have received little attention, presumably due to the relatively small offset and their orientation,
which cannot be portrayed easily in east-west reconstructions.
Faults of similar orientations that predate high-magnitude extension have been observed in east-central Nevada (Gans, 1990)
and may represent possible analogs to the 1st-generation faults
in Yerington. Other faults in the Yerington district are observed
to record small amounts of strike-slip offset. These are hypothesized to be genetically related to the 1A fault set. Indeed, a
possible surface continuation of the 1A fault is observed in the
hanging wall of the Blue Hill fault (Figure 10b). This fault has
a similar strike and separation. The lack of observed connection
between the two faults is attributed to the strike of the fault being subparallel to the strike of the Jqmp dikes between the two
areas and has hitherto been unrecognized. Further characterization of these faults and their offsets and faulting history would
be a useful avenue of future work.
IMPLICATIONS FOR MINERAL DEPOSITS AND
EXPLORATION
Each of the porphyry copper deposits in the Yerington district is related to a separate swarm of porphyry dikes, each centered on a cupola of the Luhr Hill granite (Proffett, 1979; Dilles
and Proffett, 1995). Each set of Jurassic dikes was emplaced
as a northwest-striking, subvertical swarm, with a central knot
located over the top of the cupola (Dilles and Proffett, 1995). Porphyry copper mineralization with associated potassic alteration
dominates in the central knot of the dike swarms, whereas IOCG
mineralization and calcic (endoskarn) and other forms of alteraWLRQGRPLQDWHWKHGLVWDO¿QVRIWKHVZDUPV'LOOHVHWDOD
Implications for dismemberment of Ann-Mason porphyry
system
southwestern most portion of the Singatse fault. The ore body
is truncated by the Singatse fault at the surface and by the Blue
Hill fault at depth. Restoration of the Blue Hill fault places the
Ann-Mason dike swarm over the dike swarm at Blue Hill (Figures 7, 10), thus suggesting that the Blue Hill area is the pre-tilt
northwestern lateral continuation of the Ann-Mason porphyry
system. The Ann-Mason ore body lies ~1.5–2 km below the
Tertiary surface, whereas the Blue Hill ore body is ~1 km below
the Tertiary surface in pre-tilt position, placing it at structurally higher levels in the Ann-Mason dike swarm. The portion of
the Ann-Mason ore body that is truncated by the Singatse fault
has been translated 4 km east, where it is further dismembered
and down dropped by younger generations of faults. Thus, the
pre-tilt southeastern lateral continuation of the Ann-Mason porphyry system is buried under the cover of Mason Valley, with a
large fragment of it located near sea level beneath the Yerington
pit (Figure 7b). Although the dike swarms associated with the
Yerington mine and the Ann-Mason deposit each dip ~45°N,
they are vertically separated and thus constitute two distinct
dike swarms and associated hydrothermal systems in reconstructed view (Dilles and Proffett, 1995). The magnitude of slip
on the 1A fault, though modest in comparison to other faults in
WKH GLVWULFWLV VLJQL¿FDQWLQ WKH FRQWH[WRI EORFN PRGHOLQJRI
grades and mine planning.
Implications for dismemberment of Casting Copper dike
swarm and Roulette target
The skarn deposits located in metasedimentary rocks south
of Ann-Mason are related to an additional dike swarm that intruded the Ludwig septum. The dikes have been conduits for
PLQHUDOL]LQJÀXLGVGXULQJWKHLUHPSODFHPHQWDQGFRROLQJKLVtories (Harris and Einaudi, 1982; Dilles and Proffett, 1995; EinDXGL7KHPLQHUDOL]LQJÀXLGVZRXOGKDYHEHHQVRXUFHG
from a central knot of porphyry dikes presumed to occur at
GHSWKIURPZKLFKWKHÀXLGVZRXOGKDYHULVHQDQGPRYHGODWerally until they encountered the carbonate rocks, forming the
copper-bearing garnet-pyroxene skarn deposits at Casting Copper, Ludwig, Douglas Hill, and McConnell. One possible target,
LGHQWL¿HGE\%URQFR&UHHN([SORUDWLRQDQGRSWLRQHGWR(QWUpH
Gold, is what is known as the Yerington West property (Eurasian Minerals, 2014) or Roulette target (Entrée Gold, 2010),
which was initially drilled in 2010. Entrée Gold reported mineralized intercepts, but the potassic core apparently has yet to be
drilled. Figure 8 shows where the Casting Copper dike swarm
might project into the hanging wall and footwall surfaces of the
Blue Hill fault, which might be used for further exploration of
the Roulette target (Figure 2).
DISCUSSION
The Ann-Mason ore body is located within the Ann-Mason
dike swarm (Figure 9 of Seedorff et al., this volume). The deposit is located underneath post-ore structural cover, beneath
Tertiary volcanic rocks that occur in the hanging wall of the
Fault surface maps: Three-dimensional reconstructions
Structural contour maps have been routinely generated
Ann-Mason structural reconstruction
WR GH¿QH WKH WKUHHGLPHQVLRQDO H[WHQWV RI IDXOWV DQG EHGGLQJ
surfaces for a century or more, especially in the petroleum
industry. Most structural reconstructions, however, have been
based on carefully chosen lines of cross section (e.g., Dahlstrom, 1969) and thus are a two-dimensional technique. Fault
surface maps, however, are a method employed to analyze
data in three dimensions (Keeler, 2010; Schottenfeld, 2012;
Seedorff et al., this volume). In the petroleum industry, fault
surface maps are used and are known as fault cutoff maps or
Allan diagrams (Allan, 1989; Tearpock and Bischke, 2003;
Groshong, 2006, p. 229) but are not commonly used in the
minerals industry.
In the Yerington district, two-dimensional structural reconstructions have been a powerful exploration tool, resulting in the discovery of the Ann-Mason deposit (Proffett, 1977;
Proffett and Dilles, 1984; Hunt, 2004). Faults of the second
through fourth generations have similar northerly strikes, allowing for robust reconstructions along east-west cross sections
)LJXUH 3URIIHWW 6HFWLRQ$$ƍ 3URIIHWW DQG 'LOOHV
)DXOWVRIWKH¿UVWIDXOWJHQHUDWLRQVXFKDVWKH$IDXOW
(Figure 11), however, strike at acute angles to the section and
thus cannot be readily restored using the same cross section. In
this study, the fault surface maps that incorporate the abundant
new drill hole data around Ann-Mason were used to constrain
the shapes of several faults in the Yerington district, to identify
geologic markers and piercing points, and to perform stepwise
reconstructions. Movement on the 1A fault has been restored
using lithologic markers, and the resulting offset also is consistent with the patterns of alteration and copper grades. SigQL¿FDQWYDULDWLRQVLQWKHPDJQLWXGHDQGGLUHFWLRQRIVOLSDFURVV
fault surfaces are observed for several faults in the Yerington
district. Reconstructions can be used to aid exploration of the
dismembered hydrothermal systems along both the Ann-Mason
and Casting Copper dike swarms.
The quality of the geologic inputs has a large impact on
the utility and validity of structural reconstructions, whether
GHVLJQHGIRUVFLHQWL¿FRUSUDFWLFDOSXUSRVHV7HFKQLTXHVVXFK
as fault surface maps can be used in almost any situation, but
the advantages are most apparent where three-dimensional constraints are abundant. Perhaps the greatest value in wider use
of fault surface maps would be to better portray the locations
and types of three-dimensional data available, which would
permit readers and other users to have a better understanding of the nature of the constraints and thus to evaluate better
the validity of a proposed reconstruction hypothesis. Readers
and other users also might more easily develop and evaluate
alternative hypotheses and more readily dispense with unviable proposals.
1173
by normal faults (Wilkins and Heidrick, 1995; Seedorff et al.,
2005; Maher, 2008). The surface geology around porphyry deposits then more nearly represents a series of cross-sectional
views of the porphyry systems as the amount of tilting approaches 90°, exposing the vertical and lateral extents of these
paleohydrothermal systems. Examples include: Ajo (Hagstrum
et al., 1987), Globe-Miami (Maher, 2008), Hall (Shaver and
McWilliams, 1987), Kelvin-Riverside-Tea Cup-Grayback
(Nickerson et al., 2010), Robinson (Seedorff et al., 1996; Gans
et al., 2001), and San Manuel-Kalamazoo (Lowell, 1968; Force
et al., 1995), in addition to Yerington. Coupling these exposures
with the abundance of drill data in the areas around these mined
and prospective porphyry deposits has furthered our understanding of the characteristics of different levels of porphyry
systems (Lowell and Guilbert, 1970; Sillitoe, 1973; Seedorff et
al., 2008). In the example of San Manuel-Kalamazoo, an understanding of the alteration patterns of the San Manuel ore body
and how they related to levels in the system lead to the successful discovery of the Kalamazoo deposit that was structurally
offset by the San Manuel fault (Lowell, 1968, 1991).
An understanding of and appreciation for structural dismemberment by normal faults in the Yerington district is what
led to the discovery of the Ann-Mason porphyry deposit (Hunt,
2004). The 0.15% Cu ore shell of the Ann-Mason deposit is
truncated by the Singatse fault east of UTM 304,400E and between 4,317,300N and 4,318,000N (Jackson et al., 2012). The
Singatse fault has translated the offset portion of the ore shell
east, where it has been further dismembered by younger normal faults. A reconstruction presented here indicates that the
continuation of the offset Ann-Mason ore body is currently located at depth (~1.5 km) beneath the Yerington pit. The Casting
Copper dike swarm is related to skarn mineralization, and the
core of the related porphyry system is as yet undiscovered. The
increased control on the subsurface geometry of the Blue Hill
fault provided better constraints on where the dike swarm is
projected and its continuation across fault blocks, which could
be used to guide drilling for ore in the footwall or hanging wall
of the fault.
When exploring porphyry prospects, deposits, and systems, it should be the practice of every geologist to bear in mind
the level in the system that they are examining. Moving across
faults and seeing stark changes in alteration-mineralization zonation is indicative of large movements along the faults, which
may yield exploration targets. Complexly deformed districts
such as Yerington or Robinson could hide offset pieces of the
system at depth and/or under structural cover or post-mineral
cover.
Geometry and growth of normal faults
Porphyry systems dismembered and tilted by normal fault
systems
Throughout the southwestern United States, porphyry
deposits are commonly structurally dismembered and tilted
Normal faults are the time-integrated products of crustal
extension, recording the variety of mechanisms and processes
occurring during fault rupturing and slip. Individual exposures
are commonly segments of large faults, which in turn are part of
1174
Carson A. Richardson and Eric Seedorff
larger fault systems, where several to many fault segments act
in tandem to extend an area (Jackson and White, 1989; dePolo
et al., 1991). Normal faults display a variety of geometries and
morphologic characteristics. They are commonly not perfectly
planar but instead may exhibit an overall concave upward curvature (Proffett, 1977; Jackson and McKenzie, 1983). Many
faults have the same general strike along their traces, but with
local variations as the fault plane curves and then returns to the
overall strike direction again. This curvature in the fault plane
can represent locations where a relay ramp was breached between two overlapping tips of discrete faults in the same system
and took over to continue transferring strain between the faults
as slip continued (Peacock and Sanderson, 1994; Childs et al.,
1995; Walsh et al., 1999). Displacement has a complex relationship to geometry, commonly greatest near the center of the
fault and decreasing to zero at the tips at either end (Peacock
and Sanderson, 1997). The style of normal faulting observed
may fall along a spectrum, from the “domino-style” end member where faults slip and rotate to lower angles where they may
lock and are then cut by higher angle faults due to the stress
differential required for continued slip (Proffett, 1977; Davison,
1989), to the metamorphic core complex end member where
high-angle normal faults may be kinematically linked to movement on a gently dipping, basal master fault (Whitney et al.,
2013; Platt et al., 2015). The question of just how the crust extends, its relationship to fault geometry, and the degree to which
faults are kinematically linked remains a matter of debate and
continued research (Jackson and McKenzie, 1983; Roberts and
Yielding, 1994; Wernicke, 2009).
In the Yerington district, years of geologic mapping and
drilling have provided abundant information regarding the local
strikes and dips of normal faults at the surface and their geometry at depth. The faults are not perfectly planar nor are they
listric—they are instead curviplanar. Along strike the faults exhibit embayments and salients in outcrop that produce troughs
and ridges in the third dimension. The recesses and salients in
the youngest, 4th-generation faults, if rotated to lower angles,
would produce the troughs, or scoops, and ridges, or noses, that
are observed in faults of older generations. The two faults of the
4th generation that were analyzed display opposing directions
of increasing slip, suggesting that a buried relay ramp is present
between them. The Singatse and Blue Hill faults contain hanging wall splays, with no footwall splays observed. This study
also suggests that the Blue Hill and Martha Washington faults
may represent adjacent embayments in a single fault that at the
surface have become separated by truncation by the Singatse
fault and subsequent erosion (Figure 2).
Although the increase in new drilling in the last decade,
including in the greater Ann-Mason area, augments the known
exposures of faults, the fault surface maps presented here underscore the uneven spatial distribution of constraints and the
considerable uncertainties that remain in the geometry of faults
in the district. These uncertainties contribute to the opportunity
for new mineral discoveries in the Yerington district.
FUTURE WORK
Further work in the Yerington district could lead to a better
understanding of the geometry and kinematic history of normal
faulting. In particular, faults of the 3rd generation, including the
May Queen fault, deserve further study to determine whether
WKH\LQGHHGKDYHDVLJQL¿FDQWVWULNHVOLSFRPSRQHQWRIRIIVHW
Fault surface maps of the rest of the 1st-generation faults, exHPSOL¿HGE\WKH$IDXOWPLJKWOHDGWRDEHWWHUXQGHUVWDQGLQJ
RIWKLVJHQHUDWLRQRIIDXOWVDQGLWVJHRG\QDPLFVLJQL¿FDQFHDV
ZHOODVEHWWHUGH¿QLQJWKHJHRORJ\RIWKH$QQ0DVRQGHSRVLW
Additional drilling south of Blue Hill might further constrain
the subsurface geometry of the Blue Hill fault and aid exploration for mineralization that seems to be associated with the
Casting Copper dike swarm. Detailed mapping and exploration
in the hanging wall of the Singatse fault could delineate the
continuation of the Blue Hill fault north of Ann-Mason and de¿QHLWVUHODWLRQVKLSWRWKH0DUWKD:DVKLQJWRQIDXOW)LJXUH
CONCLUSIONS
Faulted and tilted mineralized systems provide rare opportunities to describe the three-dimensional characteristics of
faults. Additionally, the abundant recent drill-hole information
and detailed mapping in the greater Ann-Mason area provide
new constraints in the Yerington district. The data permit more
robust three-dimensional structural reconstructions to quantify
and to test theories and conventional wisdom about continental
extension and the relationship of the structurally dismembered
porphyry systems in the district. Fault surface maps are a tool
to describe along-strike and down-dip variations and to portray
the associated three-dimensional constraints on fault geometry.
The normal faults in the Yerington district do not have
strongly listric geometries, but rather are curviplanar. An analysis of the May Queen fault, a 3rd generation fault, suggests that it
has a greater magnitude of slip than previously reported and may
have a markedly different slip direction than faults of the 4th and
QG JHQHUDWLRQV )RU WKH ¿UVW WLPH D VHULHV RI FORVHO\ VSDFHG
subparallel faults of an earlier (1st) generation are characterized.
The 1A fault, which may be the fault in this generation that has
the most offset, has 230 m of apparent dextral slip. Faults of this
set presently strike southeast and dip steeply to the southwest but
formed as steeply southwest dipping, dip-slip normal faults.
Restoration of movement on the Blue Hill fault places the
Ann-Mason deposit over the Blue Hill deposit; both deposits
are structurally offset pieces of mineralization related to the
Ann-Mason dike swarm. The Casting Copper dike swarm could
host porphyry-style mineralization at depth on one or both sides
of the Blue Hill fault.
ACKNOWLEDGMENTS
7KLV UHVHDUFK ZKLFK FRPSULVHG WKH ¿UVW DXWKRU¶V 06
WKHVLVKDVEHHQ¿QDQFLDOO\DQGVFLHQWL¿FDOO\VXSSRUWHGE\(Q-
Ann-Mason structural reconstruction
trée Gold. Thanks are due to Tom Watkins for providing the
UHVHDUFK RSSRUWXQLW\ DQG RYHUVHHLQJ WKH ¿HOG ZRUN DQG WR
Norm Page, Jon Gant, Ben Hartmon, Matt Cunningham, and
&KDUOLH&DQQRQIRUWKHLUVFLHQWL¿FDVVLVWDQFHGXULQJWKHSURMect. We appreciate discussions with previous workers in the
GLVWULFW SDUWLFXODUO\ -RKQ 'LOOHV DQG 0DUN %DUWRQ )XUWKHU ¿nancial support was provided by a Society of Economic Geologists Foundation-Barrick Gold Corporation Graduate Student
Fellowship to CAR, a scholarship from the Tucson Section of
the Society for Mining, Metallurgy, and Exploration to CAR,
and by Science Foundation Arizona and the Lowell Institute for
Mineral Resources at the University of Arizona. Mark Barton,
Paul Kapp, Lucia Profeta, and Fleetwood Koutz are thanked
for their comments and feedback on evolving versions of this
manuscript. Stereographic analysis was conducted with the
program OSXStereonets. Many thanks go to Simone Runyon,
&DOHE.LQJDQG-DVRQ0L]HUIRULQWURGXFLQJWKH¿UVWDXWKRUWR
many interesting ideas, concepts, and papers, thereby enrichLQJWKLVSURMHFW/DVWO\WKH¿UVWDXWKRURZHVDKHDUWIHOWGHEWRI
gratitude to Aryn Hoge who was never afraid to call him out
when he was straying toward becoming a dilettante and through
many an enlightening and cogent discussion resulting in clearer
and more focused writing and speaking on structural geology
and ore deposits.
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