Nature, age, and genesis of quartz-sulfide-precious-metal vein systems in the Virginia City Mining
District, Madison County, Montana
by Marshall Morris Cole
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Marshall Morris Cole (1983)
Abstract:
In the Virginia City mining district, pre-Belt gneisses and the Late Cretaceous granitic Browns Gulch
stock host numerous quartz vein systems. Hypogene mineralization is chiefly gold- and silver-bearing
base-metal sulfides.
The U.S. Grant 3-level vein system, N40°-50°E; 35°-50&deg,NW, 0.3 to 5.0 m wide, is contained in a
shear zone exhibiting about 10 m of syn-ore right-lateral movement. The vein system is composed of
elongate quartz lenses, quartz stringers, tabular quartz bodies, and variable amounts of crushed and
altered gneiss. Altered wall rock gneiss exhibits early potassic alteration (microcline and possibly
quartz) and a subsequent propylitic assemblage (carbonate, pyrite, quartz, chlorite, other
phyllosilicates, and a zeolite?). Post-alteration mineralization occurs as pyrite, followed by variable
amounts of contemporaneous galena, sphalerite, chalcopyrite, sparse tetrahedrite, and rare specular (?)
hematite. Additionally, some (latest) sphalerite replaces pyrite, galena, and chalcopyrite. Observable
gold is very rare. Quartz deposition is pre-, syn-, and post-sulfide mineralization.
The Virginia City district is one of several districts in the Tobacco Root precious-metal mining region.
The region is cored by the 77-72 m.y. old quartz monzonite Tobacco Root batholith. A regional zoning
is present with respect to the batholith in the form of low silver-to-gold and high copper-to-silver ratios
near the batholith, and high silver-to-gold and low copper-to-silver ratios far from the batholith
(Virginia City district).
It is proposed that ores of the district are of Latest Cretaceous to Early Tertiary age (70-60 m.y.B.P.),
based on the occurrence of deposits in the Late Cretaceous Browns Gulch stock, the crosscutting of the
El Fleeda 4-level vein system by a 51 m.y. old andesite plug, and the regional zoning with respect to
the batholith.
An epithermal precious-metal genesis model has been applied to the ores of the district. A geothermal
convection cell powered by heat from Late Cretaceous plutonism, produced large-scale regional
circulation of hydrothermal fluids at shallow crustal levels. These fluids collected (remobilized ?),
transported, and deposited the ore constituents. NATURE, AGE, AND GENESIS OF QUARTZ-SULFIDE-PRECIOUS-METAL
VEIN SYSTEMS IN THE VIRGINIA CITY MINING DISTRICT,
MADISON COUNTY, MONTANA
by
Marshall Morris Cole
A thesis submitted in, p a r tia l f u lfillm e n t
o f the requirements fo r the degree
of
Master o f Science
in
'
Earth Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
December, 1983
;
main lib.
ii
C k755
C o p . Si
APPROVAL
of a thesis submitted by
Marshall Morris Cole
This thesis has been read by each member of the thesis committee
and has been found to be s a tis fa c to ry regarding content, English usage,
form at, c ita tio n s , bibliographic s ty le , and consistency, and is ready
fo r submission to the College o f Graduate Studies.
£{ 'C
/ Z- Y ) -
r
Chairperson, Graduate Committee
Date
Approved fo r the Major Department
m
<S-
3
Head, Major Department
Date
Approved fo r the College o f Graduate Studies
'll
Date
&K--
IW I
Graduate Dean
Ili
STATEMENT OF PERMISSION TO USE
In presenting th is thesis in p a r tia l fu lfillm e n t o f the requirements
fo r a m aster's degree a t Montana State U n iv e rs ity , I agree th a t the
Library shall make i t a v a ila b le to borrowers under rules o f the L ib rary .
B rie f quotations from th is thesis are allowable without special per­
m ission, provided th a t accurate acknowledgment of source is made.
Permission fo r extensive quotation from or reproduction of th is
thesis may be granted by my major professor, or in h is /h e r absence,
by the D irecto r o f L ib ra rie s when, in the opinion of e ith e r , the pro­
posed use o f the m aterial is fo r scholarly purposes.
Any copying or.
use of the m aterial in th is thesis fo r fin a n c ia l gain shall not be
allowed without my w ritte n permission.
Signature
Date
« 2 /, / f ^ - 3
V
ACKNOWLEDGMENTS
This thesis was. funded in part by:
The Montana Bureau of Mines
and Geology; R and D M inerals, Missoula, Montana; Dr. David R. Lageson,
professor a t Montana State U n iv e rsity; the R esearch-C reativity Program,
Montana State U n iv e rsity; The S t. Lawrence Mining Company, V irg in ia
C ity , Montana.
Great appreciation is expressed towards these people
fo r th e ir generous contrib ution s.
The follow ing in d ivid u als have donated th e ir s k i l l s , knowledge,
and support in the compilation of th is th esis:
Drs. Robert A. Chadwick,
David W. Mogk, and David R. Lageson, professors a t Montana State
U n iv e rs ity ; Clyde Boyer, g e o lo g ist, V irg in ia C ity , Montana; Thomas.
A. Callmeyer, f ie ld a s sistan t and graduate student, Montana State
U n iv e rs ity ,
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vi
TABLE OF. CONTENTS
Page
1.
LIST OF TABLES.............................................................. .................... ..
2.
LIST OF FIGURES .........................................
ix
3.
ABSTRACT....................................................................................................
xi
4.
INTRODUCTION...........................................................................................•
Location, Access, andPhysiography ........................................
Purpose and Method of Study. , .
................. . . . . .
Previous Study ..................................................................
v iii
I
I
3
3
5. . MINING HISTORY.....................................................
4
6.
REGIONAL GEOLOGIC SETTING ...................................................... .... .
5
7.
GENERAL GEOLOGY OF THE VIRGINIACITYMINING DISTRICT. . .
8
8.
9.
10.
Rock T y p e s .......................................................................................
S tructure. . '................................ ..................................................
8
Tl
MINING GEOLOGY.......................................................................................
13
Regional S etting : The Tobacco Root Precious-Metal
Mining R e g io n ........................ ..................................................
Mining Geology of the V irg in ia C ity D is tr ic t . . . . . . .
13
16
MINING GEOLOGY OF SELECTED MINES OF THE VIRGINIA CITY
DISTRICT ..........................................’ ............................. .... . . .
20
U.S. Grant Mine.................................................................. ....
. El Fleeda M in e ......................................................................
Black Rock Mine. . . . . . . .
i. . . ................................
Easton-Pacific G ro u p ........................ ..........................................
Prospect Mine. ..................................
S t. Lawrence Mine............................ .... .........................................
Fork Mine............................................. ..............................................
20
36
38
38
40
40
42
GEOLOGIC HISTORY OF THEVIRGINIA CITY DISTRICT........................
44
V ii
TABLE OF CONTENTS— Continued
Page
11.
CONCLUSIONS: AGE AND GENESIS OF QUARTZ-SULFIDEPRECIOUS-METAL VEIN SYSTEMS IN THE VIRGINIA CITY
MINING DISTRICT..................... . . . ■............................. ....
46
Previous In te rp re ta tio n s ..........................................................
Author's In te rp re ta tio n . . .. .....................................................
Age o f Ore Deposits......................................................................
Genesis o f Ores in the V irg in ia C ity Mining D is t r ic t .
46
47
48
51
12.
REFERENCES CITED. ...............................................................................
59
13.
APPENDICES. ......................................................
63
Appendix A
Radiometric Dates (K-Ar Technique).............................
Appendix B
General Data on Some Tobacco Root PreciousMetal Deposits . . .. ....................................
Appendix C
Ore and Gangue Minerals o f Tobacco Root Baseand Precious-Metal Deposits............................. .... .
Appendix D
Lode Production Figures fo r Tobacco Root
Mining D is tric ts (1901 ^ l935) . . . .....................
Appendix E
Lode Production Figures fo r Selected Mines
o f the Tobacco Root Region (1901-1935) . . . .
Appendix F
Lode Production Figures fo r Selected Mines
o f the V irg in ia C ity D is t r ic t (1901-1935). . .
65
67
70
72
74
76
v iii
LIST.OF TABLES
Table
1.
2.
Some o f the quartz vein systems o f the V irg in ia C ity
D is t r i c t .................................................. ................................ .... . . .
Page
.
Paragenetic sequence o f hypogene a lte ra tio n and ore
m in e ra liza tio n in the U.S. Grant 3 -le v e l. ..................................25
18
ix
LIST OF FIGURES
Figure
1.
2.
3.
Page
Mining d is tr ic ts and physiography o f the Tobacco Root
precious-metal mining region. . . . . .
.....................................
2
General geology o f the Tobacco Root precious-metal
mining region .............................................. .........................................
6
Regional zoning o f s ilv e r-to -g o ld and c o p p e r-to -s ilv e r
ra tio s in the Tobacco Root precious-metal mining region . .
17
4.
Cross-section o f U.S. Grant 3 -le v e l vein s tru c tu re .................22
5.
View looking up-dip o f vein stru ctu re on southeast wall
o f U.S. Grant 3 -le v e l d r i f t ................................................................
23
6.
Unaltered gneiss from Browns Gulch......................................................27
7.
S lig h tly a lte re d wall rock from the U.S. Grant 3 - le v e l,
R. P. # 8 .5 ...................................................................................28
8.
Strongly a lte re d wallrock from the U.S. Grant 3 - le v e l,
R. P. # 8 .5 .
................................. \
..............................................29
9.
Strongly p ro p y litiz e d wallrock from the U.S. Grant
3 -le v e l, southwest o f R .P ;#8.5. . ..................................................
30
A ty p ic a l sample of q u a rtz -s u lfid e ore and adjacent
w allrock from the U.S. Grant 3 - le v e l............................. .... . . .
31
10.
11.
Ore sample from the U.S. Grant 3 - le v e l......................................... 33
12.
Ore sample from the U.S.
Grant 3 - le v e l.....................................
.
33
13.
Ore sample from the U.S.
Grant 3 - le v e l. .................................
.
34
14.
Ore sample from the U.S. Grant 3 - le v e l............................. .... . .
34
15.
Ore sample from the U.S. Grant 3 -le y e l. ..................................... 35
16.
Ore sample from the U.S.
17.
Cross-section o f the vein stru ctu re in the S t.
Lawrence 1 5 0 -le v e l.................................................................................. 41
____________________________________________ /S
Grant 3 - le v e l.....................................
.
35
X
LIST OF FIGURES— Continued
Figure
Page
18.
Depositional model o f epithermal precious-m etals..................... 52
19.
Epithermal precious-metal deposits o f Montana .........................
58
xi
ABSTRACT
In the V irg in ia C ity mining d i s t r i c t , p re-B elt gneisses and the
Late Cretaceous g ra n itic Browns Gulch stock host numerous quartz vein
systems. Hypogene m in e ra liza tio n is c h ie fly gold- and s ilv e r-b e a rin g
base-metal s u lfid e s .
The U.S. Grant 3 -le v e l vein system, N40o-50°E; 35o-50°NW, 0.3
to 5 .0 m wide, is contained in a shear zone e x h ib itin g about 10 m of
syn-ore r ig h t-la t e r a l movement. The vein system is composed of
elongate quartz lenses, quartz s trin g e rs , ta b u lar quartz bodies, and
v a ria b le amounts o f crushed and a lte re d gneiss. A ltered wall rock
gneiss e x h ib its e a rly potassic a lte r a tio n (m icrocline and possibly
qu artz) arid a subsequent propyli t i c assemblage (carbonate, p y r ite ,
q u artz, c h lo r ite , other p h y llo s ilic a te s , and a z e o lite ? ). Posta lte r a tio n m in e ra liza tio n occurs as p y r ite , followed by v a riab le
amounts o f contemporaneous galena, s p h a le rite , c h a lc o p yrite , sparse
te tra h e d rite , and rare specular (?) hem atite. A d d itio n a lly , some
( la t e s t ) s p h a le rite replaces p y r ite , galena, and chalcop yrite.
Observable gold is very ra re . Quartz deposition is p re -, syn-, and
p o s t-s u lfid e m in e ra liz a tio n .
The V irg in ia C ity d i s t r ic t is one o f several d is t r ic t s in the
Tobacco Root precious-metal mining region. The region is cored by the
77-72 m.y. old quartz monzonite Tobacco Root b a th o lith . A regional
zoning is present with respect to the b a th o lith in the form o f low
s ilv e r -to -g o ld and high c o p p e r-to -s ilv e r ra tio s near the b a th o lith ,
and high s ilv e r-to -g o ld and low c o p p e r-to -s ilv e r ra tio s fa r.fro m the
b a th o lith (V irg in ia C ity d i s t r i c t ) .
I t is proposed th a t ores o f the d i s t r ic t are o f Latest Cretaceous
to Early T e rtia ry age (70-60 m .y .B .P .), based on the occurrence of
deposits in the Late Cretaceous Browns Gulch stock, the crosscutting
o f the El Fleeda 4 -le v e l vein system by a 51 m.y. old andesite plug, .
and the regional zoning with respect to the b a th o lith .
An epithermal precious-metal genesis model has been applied to
the ores o f the d i s t r i c t . A geothermal convection c e ll powered by
heat from Late Cretaceous plutonism, produced la rg e -s ca le regional
c irc u la tio n , o f hydrothermal flu id s a t shallow crustal le v e ls . These
flu id s co llected (rem obilized ? ), transported, and deposited the ore
constitu en ts.
I
INTRODUCTION
Location, Access, and Physiography
The V irg in ia C ity mining d i s t r ic t , located southwest of V irg in ia
C ity , Madison County, Montana, is the southernmost d i s t r ic t in the
Tobacco Root precious-metal mining region (F ig . I ) .
I t may be sub­
divided in to the Fairweather (located a t V irg in ia C ity ), Highland
(3 km south of V irg in ia C ity ), Summit (10 km south of V irg in ia C ity ),
Nevada (a t Nevada C ity ), and Browns Gulch d is t r ic t s .
The d is t r ic t extends from Alder Gulch. on the north and e a st, to
west of Browns Gulch, and to the flanks of the Gravelly Range on the
south (P late I ) .
I t covers an area of approximately 90 square km.
The V irg in ia C ity area is approachable via Montana Highway 287.
Numerous d i r t and gravel roads provide access w ithin the d i s t r ic t .
Winter access is d i f f i c u l t in the south part of the d is t r ic t but is
r e la tiv e ly easy in the north h a lf.
V irg in ia C ity , elevation 1767 m, s its in a topographic low between
two major mountain ranges.
The Tobacco Root Mountains (summit eleva­
tions greater than 3000 m) are located to the north, and the Gravelly
Range (summit elevations greater than 2900 m) to the south (F ig . I ) .
West of V irg in ia C ity is the Ruby River V a lle y ,e le v a tio n about 1525 m;
to the east is the Madison River V a lle y , elevation about 1500 m.
The major drainages of the d i s t r ic t , Alder and Browns Creeks
(P late I ) , head in the southern portion of the d is t r ic t where they
2
;
./.I
d is tr ic t
TOBACCO
ROOT
d is t r ic t
MOUNTAINS
ZRUBY
RANGE
V ir g in ia
IO K M
C it y
/.
V lrg ln ls \
IC Ity
d is tr ic t
■ \t
MONTANA
F ig u r e s
1 ,2 . 3
M a d le o n C o .
Figure I .
GRAVELLY/
RANGE
Mining d is tr ic ts and physiography of the Tobacco Root
precious-metal mining region. Patterned areas are Late
T e rtia ry and Quaternary unconsolidated or poorly consolidated
sediments. Blank areas are older sedimentary, igneous, or
metamorphic rocks.
>
3
are deeply incised in the fo o th ills o f the Gravelly Range.
Alder
Creek flows north to V irg in ia C ity where i t turns and flows northwest
fo r 16 km u n til i t jo in s the Ruby R iver.
Browns Creek flows north
u n til i t jo in s Alder Creek a t Nevada C ity (3 .2 km northwest o f V irg in ia
C ity ).
Purpose and Method o f Study
The major problem addressed in th is thesis is what processes are
responsible fo r ore genesis in the minipg d is tr ic t?
Precambrian meta-
morphism and possible igneous a c t iv it y , along with Laramide plutonism
and volcanism, play important roles in the geologic h is to ry of the
d is tr ic t.
Hydrothermal a c t iv it y re la te d to e ith e r group o f processes
is capable of producing ore bodies.
A d e ta ile d study involving above- and below-ground mapping, ore
and wall rock microscopy, and K-Ar radiom etric datin g, were done in an
attempt to determine the processes responsible fo r ore genesis.
Previous Study
Previous work in the mining d i s t r ic t is minimal.
Winchell (1914),
Ta n s le y , Schafer, and Hart (1 9 33 ), and Lorain (1937) b r ie f ly described
the general geology and various mines o f the Tobacco Root region and
the V irg in ia C ity d i s t r ic t .
The southern part of the d i s t r ic t was
mapped by Hadley (1969) in the 15 minute Varney quadrangle.
A prelim ­
inary bedrock map o f the northern p a rt o f the d is t r ic t was published by
Weir (1982).
V ita lia n o and Cordua (1979) mapped the southern Tobacco
Root Mountains, located north o f the mining d i s t r ic t .
4
MINING HISTORY.
Placer gold was discovered in Alder Gulch in 1863 approximately
0 .4 km south o f the present s ite o f V irg in ia C ity .
W ithin one y e ar,
auriferous and argentiferous quartz lodes were discovered and developed
Most o f the lodes were in secondarily enriched oxidized ores.
In the
la te 19th century, these ores were exhausted and lower grade primary
■
ores were encountered. As a re s u lt many o f the mines were abandoned.
Mines th a t continued to operate had contained r e la t iv e ly high-grade
hypogene ores.
Mining in the 20th century has been sporadic and
never a t the scale achieved in the la te 19th century.
;
Production fig u res from Lorain (1937) in dicate the to ta l value
o f lode gold from the e n tire Tobacco Root precious-metal mining region
is.ab out 17 m illio n d o lla rs , including 2 .5 m illio n d o lla rs from the
V irg in ia C ity d i s t r i c t .
However, th is is minor when compared to
50 m illio n d o lla rs o f placer gold froni Alder Gulch (L o ra in , 1937).
5
REGIONAL GEOLOGIC SETTING
. The Tobacco Root Mountains, a large northwest-plunging domal
u p l i f t o f m u ltip le deformed and metamorphosed p re-B elt rocks (M ueller
and Cordua, 1.976), is a major geologic featu re in the region of the
V irg in ia C ity d is t r ic t (F ig . 2 ).
The mining d is t r ic t is located in
the southernmost part o f th is p re-B elt te rra in .
The Tobacco Root Mountains are cored by the Late Cretaceous
(72-77 m .y .B .P .; V ita lia n o and others, 1980) quartz monzonite Tobacco
Root b a th o lith (F ig . 2 ).
km area.
This pluton crops out w ithin a 310 square
I t is thought to be g e n e tic a lly related to the Boulder
b a th o lith (Smith, 1970) located 22 km to the northwest (F ig . 2 ).
This in te rp re ta tio n is supported by low S r ^ / S r ^ values (in d ic a tiv e
of a low er-crust or upper-mantle magma o rig in ; Krauskopf, 1979) fo r the
Boulder (Doe and others, 1968) and Tobacco Root ( V it a liano and others,
1980) b a th o lith s .
Scattered throughout the p re -B e lt rocks are numerous smaller
p iutons s im ila r in age and composition to the Tobacco Root batho lith
( V it a liano and Cordua, 1979).
Numerous base- and precious-metal
deposits are associated with these igneous bodies.
Proterozoic, Paleozoic, and Mesozoic sedimentary rocks crop out
on the north and west flanks of the Tobacco Root p re -B e lt te rra in
(F ig . 2 ).
Paleozoic, Mesozoic, and Early T e rtia ry sedimentary rocks
crop out in the G ravelly Range.
Cretaceous and T e rtia ry volcanic rocks
6
B oulder
batholH h
PCm
T o b a cco
batholH h
PCm
TQa
Figure 2.
General geology of the Tobacco Root precious-metal mining
region (modified from Ross and others, 1955). PCm are
p re -B e lt metamorphic rocks; PPM are P roterozoic, Paleozoic,
Mesozoic, and Early T e rtia ry sedimentary rocks; KTi are
Late Cretaceous plutonic rocks; KTv are Cretaceous and
T e rtia ry volcanic rocks; TQa are Late T e rtia ry and
Quaternary sediments.
7
are scattered about the region, the V irg in ia C ity b asalt f ie ld being
the la rg e s t.
Unconsoliated and poorly consoliated T e rtia ry and
Quaternary sediments are abundant in stream and r iv e r v a lle y s .
\
8
GENERAL GEOLOGY OF THE
VIRGINIA CITY MINING DISTRICT
Rock Types
The mining d is t r ic t is almost e n tir e ly underlain by an assemblage
o f p re -B e lt gneisses (P la te I ) .
These rocks are thought to be the
products of three metamorphic events (M ueller and Corduas 1976).
The
e a r lie s t event was probably o f g ra n u lite facies and the sparse occur­
ence of diopside in the gneisses is thought to be a r e l i c t of th a t
'
event.
--
:
A subsequent 2700 m.y.B.P. dynamothermal am phibolite-facies
metamorphism formed the dominant mineral assemblage in these rocks.
The th ird metamorphism was a 1600 m .y.B.P. thermal event th a t reset
mineral ages (Wooden and others, 1978) in the V irg in ia C ity d i s t r ic t .
The dominant lith o lo g y in the mining d is t r ic t is q u a r tz -.
ofeldspathic gneiss containing q u artz, K -feldspar, p e r th ite , and
plagioclase with lesser amounts of hornblende, b io t it e , kyan ite,
epidote, garnet, and diopside.
In outqrop the gneiss is lig h t -
colored and commonly ex h ib its f o lia t io n in the form o f cm- to
m -thick banding.
Migmatite does occur but is r e la t iv e ly minor.
Less common are amphibolites with abundant hornblende (or other
amphiboles), plag io clase, and b io t it e with lesser amounts o f quartz,
fe ld s p a r, epidote and garnet.
massive to strongly fo lia te d .
In outcrop they are dark colored and
9
Rare g a rn e t-ric h units are present in e ith e r of the previously
mentioned lith o lo g ie s .
Porphyroblasts ra re ly a tta in a diameter of
8 cm, with most less than 5 cm.
Several small bodies (0 .0 1 -0 .5 sq. km) o f metamorphosed u ltram afic
rock crop out in the d i s t r ic t .
These rocks are strongly serpentinized
and e x h ib it a white to dark green colo r in outcrop.
Tan or brown meta-dolomite crops out discontinuously on the west side of Browns Gulch and in upper Alder Gulch (P la te I ) .
Thickness
varies from 15 cm to g reater than 20 m but is commonly less than I m.
Minor amounts o f impure q u a rtz ite may occur adjacent to the. meta­
dolomite.
Probably the meta-dolomite was a continuous la y e r o f r e la tiv e ly
uniform thickness but was deformed and attenuated by Precambrian
orogenesis.
The remnants of th is marker u n it were used to recognize
deformation in the metamorphic rocks.
Excluding the meta-dolomite, thicknesses of the previously
described rocks are unknown.
S tra tig ra p h ic order is also unknown
because metamorphism transposed a ll primary structures and te xtu res.
U nfoliated tab u lar pegmatites, commonly 3 or 4 m th ic k , crop
out exten sively w ithin the Precambrian rocks (P late I ) .
They have
sharp, discordant boundaries, and usually contain q u a rtz, s e ric itiz e d
fe ld s p a rs , and b io t it e or muscovite.
or west-northwest.
Most pegmatites trend northwest
K-Ar radiom etric dating indicates an age of 1572
m.y.B.P. ± 51 m.y. fo r b io t it e from a pegmatite in the U.S. Grant
mine (Appendix I ) .
10
The M ississippian Madison Limestone is exposed in upper Alder
Gulch (P la te I ) .
The limestone is in th ru s t contact with the under­
lyin g rocks.
An ore-bearing Late Cretaceous g ra n itic stock is exposed in
upper Browns Gulch (P la te I ) .
This rock (the Browns Gulch stock)
is assumed to be an o u tlie r o f, or g e n e tic a lly re la te d to , the
Tobacco Root b a th o lith located 35 km to the north (Tansley and others,
1933).
Composition is K -feldspar, q u a rtz, plag ioclase, and b io t it e ,
with poorly developed graphic te x tu re .
A marginal pegmatite phase
o f s im ila r lith o lo g y is well developed.
In a d d itio n , the stock is
pervasively fa u lte d and fra c tu re d .
>
Numerous Early T e rtia ry (?) q u artz-sulfide-precious-m etal vein
systems are present in the d i s t r i c t .
Poorly m ineralized vein systems
are more re s is ta n t to weathering and tend to crop out u n like most
highly m ineralized vein systems.
A ll vein systems cut pegmatite
dikes and f o lia tio n o f metamorphic rocks.
P ost-m ineralizatio n Eocene and Oligocene volcanics, dominantly
basalt (Chadwick, 1981), unconformably p v e rlie Early T e rtia ry gravels
or Precambrian rocks along the east fla n k o f the d i s t r i c t .
Two
volcanic plugs w ith in the Precambrian te rr a in cross-cut q u a rtz -s u lfid e
precious-metal vein systems.
Marvin and Dobson (1979) in d icate a
whole rock K-Ar age o f 51.1 m.y.B.P. ± I .2 m.y. fo r the andesite plug
in the El Fleeda mine (P lates 2 , 4; Appendix I ) .
Late T e rtia ry and
Quaternary sediments, including placer deposits, occur in Browns
Gulch and Alder Gulch.
11
Structure
Two episodes o f folding were recognized in the metamorphic
bedrock o f the d i s t r i c t .
The f i r s t event, contemporaneous with
amphibolite facies metamorphism, produced tig h t is o c lin a l folds with
a x ia l plane f o lia t io n .
The second event c o a x ia lly (?) refolded th is
f o lia tio n in to a n o rth e a s t-s trik in g , southeast-vergent, open antiform .
This a n ti form is the dominant stru ctu re in the d i s t r ic t (P late I ) .
The limbs of the fo ld are defined by a series of meta-dolomite outcrops,
and a ttitu d e s o f f o lia t io n in adjacent gneisses, in upper Alder Gulch
and west o f Browns Gulch.
The fra c tu re system w ith in the antiform may have had a strong
control on the emplacement o f pegmatites and quartz vein systems.
Very few of these features were observed in the adjacent synform to
the west.
A m ultitude o f fra c tu re trends is present in the metamorphic
rocks.
The trends o f the vein systems, pegmatites, shear zones,
fa u lts , jo in t p a ttern s, and some stream valleys depict many o f these
patterns (P la te I ) .
The more common trends are N45°E, N45°W, N55°-
70°W, N6&°E, N80o-90°W, N-S, and N25°W.
Evidence o f fa u ltin g a t the surface is observable a t several
lo ca tio n s .
Three northwest-trending fa u lts o ffs e t meta-dolbmite on
the northwest limb of the antiform .
Several northeast-trending fa u lts
o ffs e t the Madison Limestone and the underlying Precambrian rocks.
In
a d d itio n , the limestone is in th ru s t contact with the metamorphic rocks.
12
Faulting is abundant in underground workings although displacement
is freq u en tly less than I m.
A major normal f a u lt in the Cornucopia
mine o ffs e ts the U.S. Grant-Cornucopia group from the El Fleeda-Black
Rock group (P la te 2 ) .
The f a u lt is comprised of a zone o f fa u ltin g
about 2 m wide with abundant slickensides on clay-covered surfaces.
The presence o f shear zones is evident both above and below
ground.
A n o rthw est-striking shear in the northwest limb o f the
antiform e x h ib its r ig h t -la t e r a l o ffs e t in meta-dolom ite.
This shear
contains several blocks o f gneiss and pegmatite "flo a tin g " in
re c ry s ta lliz e d " tr a v e r tin e -lik e " , banded c a lc ite .
,
--V
The nqrthwest'I
'
trending vein system of the Prospect mine appears to be in a shear
zone (L o ra in , 1937).
The vein system of the U.S. Grant mine occupies
a n o rth e a s t-s trik in g shear zone e x h ib itin g r ig h t -la t e r a l o ffs e t.
Other vein systems occupying northeast-trending shear zones occur in
the El Fleeda, Black Rock, and Fork mines.
13
MINING GEOLOGY
■
Regional S e ttin g :
The Tobacco Root Precious-Metal Mining Region
The Tobacco Root precious-metal mining region is comprised of the
V irg in ia C ity , Pony, N o rris , Sheridan, Tidal Wave, and Renpva d is tr ic ts
(F ig . 2 ).
Although mines in the region are small and numerous, there
is a strong s im ila r ity in the character o f the ore deposits (Lorain,
1937).
Most deposits in the region are quartz vein systems, composed o f .
one or more vein s tru c tu re s , occupying fractu res in Precambrian
metamorphics or Laramide g ra n itic intru&ives (see Appendix 2 fo r
d e t a ils ) .
Although there is no sharp lin e of demarcation, vein
stru ctu re morphology varies from tab u lar quartz bodies in clean-cut
tension fractu res to quartz lenses and stringers in strongly crushed
and sheared fractu res (Lorain , 1937).
A few ore bodies occur as disseminations and replacements in
fractu red and a lte re d walI rocks (Appendix 2 ).
Examples are the
Strawn mine (Tidal Wave d i s t r i c t ) , the Broadguage-Tamarack group
(Sheridan d i s t r i c t ) , the Mayflower mine (Renova d i s t r i c t ) , the Boss
Tweed-Clipper group (Pony d i s t r i c t ) , and the A tla n tic and P a c ific
mine (Pony d i s t r ic t ; Tarisley and others, 1933; Lorain, 1937).
Hypogene ore minerals common to a ll Tobacco Root preciousmetal mining d is t r ic t s are gold, p y r ite , galena, s p h a le rite , and
14
chalc o p y rite .
Other minerals in the ore deposits are lis te d in
Appendix 3.
Almost a ll near-surface portions o f Tobacco Root ore bodies are
enriched by oxidation (L o ra in , 1937). . The depth and in te n s ity of
enrichment are v a riab le but the same changes are evident in a ll
enriched ores.
These oxidized ores may contain secondary s ilv e r ,
lead, copper, z in c , and iron minerals a t the expense o f primary
s u lfid e s .
Base- and precious-metal production fig ures o f Tobacco Root
d is tr ic ts and mines are presented in Appendices 4 and 5 , re sp ec tiv e ly .
The follow ing factors in d icate why these fig ures are in part
nonrepresentative o f true production and ore grade:
1)
production records were not kept p rio r t o . 1900 when mining
a c it v it y reached it s peak;
2)
base-metal production was exclusively a by-product of
precious-metal production, and base-metal content was recorded only
when s u ffic ie n t q u an tity was present to pay fo r th e ir refinem ent;
3)
s e le c tiv e mining methods were practiced (freq u e n tly in
sm aller mines) so only the ric h e s t ore shoots were mined;
4)
some of the ores were m illed p rio r to smelting whereas .
others were not;
5)
production from many (e s p e c ia lly sm aller) mines was from
)
shallow, oxidized ores th a t were r e la t iv e ly enriched compared to
hypogene ores a t depth.
• 1
15
According to Lorain (1937) a nearly true measure o f the average
gold content of run-of-the-m ine ore may be obtained by examining
the production records o f mines th a t m illed large tonnages o f ore
mined by non-selectiVe methods ( i . e . Easton-Pacific group. Mammoth
mine, and the Boss Tweed-Clipper group).
Inspection o f these mine
records suggests a uniform gold grade in the Tobacco Root ores
(excluding the unusually rich Mayflower mine in the Renova d i s t r i c t ) .
Gold content of most major ore shoots varies from 0 .2 5 -0 .3 3 ounces
per ton of ore (L o ra in , 1937), although s ilv e r , le a d , z in c , and copper
content vary considerably.
Regardless o f incomplete and somewhat inaccurate production
records, s ilv e r-to -g o ld and c o p p e r-to -s iIv e r ra tio s appear to have an
inverse re la tio n s h ip .
The Pony d i s t r ic t (e s p e c ia lly the Mammoth
mine) is the locus o f copper production, has the highest copper-tosi Iv e r r a t io , and the lowest s ilv e r-to -g o ld r a tio .
At the opposite
end of the spectrum is the V irg in ia C ity d i s t r ic t .
This d is t r ic t
(e s p e c ia lly the E aston-Pacific group.) is the locus of s ilv e r production,
has the highest s ilv e r-to -g o ld r a t io , and the lowest copper-to-siTver
r a tio .
Other Tobacco Root d is tr ic ts appear to have interm ediate
s ilv e r-to -g o ld and c o p p e r-to -s ilv e r r a tio s .
The abundance o f lead
and/or zinc in the Sheridan, Tidal Wave, and Renova d is tr ic ts may in
part be due to zoning, however, the geochemical properties of
( Precambrian or Paleozoic) carbonate rocks may be a major fa c to r in
lo c a liz a tio n o f the lead- and zinc-bearing ores.
Based on th is data, a rough regional zoning o f s ilv e r-to -g o ld
and c o p p e r-to -s ilv e r ra tio s with respect to the Tobacco Root b a th o lith
16
is evident (F ig . 3; Lorain, 1937).
Highest c o p p e r-to -s ilv e r and lowest
s ilv e r-to -g o ld ra tio s occur in and near the main exposure of the
b a th o lith (Pony and Norris? d i s t r ic t s ) .
Lowest c o p p e r-tp -s ilv e r
and highest s ilv e r-to -g o ld ra tio s occur fu rth e s t from the b a th o lith
(V irg in ia C ity d i s t r i c t ) .
Interm ediate ra tio s occur in d is tr ic ts a t
moderate distances from the b a th o lith .
Mining Geology o f the V irg in ia C ity D is t r ic t
The precious-metal deposits of the V irg in ia C ity d is t r ic t
e x h ib it a strong homogeneity in character and morphology.
The
deposits occur as quartz vein systems occupying fractu res in
Precambrian metamorphic rocks, except the Easton-Pacific group which
occurs in the Brown Gulch stock.
(Appendix 2; P late I ) .
These
fractu res are the primary (s tru c tu ra l) control of ore deposition.
Lode mines in the d is t r ic t may be roughly grouped in to northeastand n o rthw est-striking vein systems (Table I ; note the dominance of
the n o rth e a s t-s trik in g vein systems), although a few s trik e northsouth or east-w est.
Vein systems freq u en tly transect fo lia t io n of
metamorphic host rocks, however, some are subparallel ( i . e . the U.S.
Grant mine:
the s trik e o f the vein system is approximately p a ra lle l
to the s trik e o f f o lia t io n , but the dips d i f f e r ) .
P re-m in eralizatio n hydrothermal a lte ra tio n is evident in the
d is tr ic t.
Although microscopic observation is lim ite d , s im ila r
features are observable throughout in the form of an e a rly potassic
a lte ra tio n (m icrocline and possibly quartz) cut by a la t e r propyli t i c
assemblage (carbonate, p y r ite , q u artz, ± c h lo r ite ).
17
RENOVA
4 6 4 . 1 .4 0
T ID A L
' 4 . 7 0 .1 0 6
T obeece
Aoet
^ S H E R ID A N
V IR O If
cm
1 6 . 36.1
Figure 3.
Regional zoning of s i l v e r-to -g o ld and c o p p e r-to -s ilv e r ra tio s
in the Tobacco Root precious-metal mining region. Numbers
shown are s i l v e r-to -g o ld and c o p p e r-to -s ilv e r r a tio s ,
re s p e c tiv e ly , fo r each d i s t r ic t . Ratios are calculated from
to ta l ounces of s ilv e r , to ta l ounces of gold, and to ta l
pounds of copper fo r the period 1901-1935 (data from Lorain,
1937). The c o p p e r-to -s ilv e r r a tio of Norris (0 .4 8 ) is low
possibly because greater than 70% of reported production is
from oxidized ore. Data fo r primary s u lfid e ore production
from the Norris d is t r ic t is a v a ila b le only from the Boaz
mine (Appendix 5 ) , with a s i l v e r-to -g o ld r a tio of 0.97 and
a c o p p e r-to -s ilv e r r a tio of 3 .9 6 . The Boaz copper-tos ilv e r r a tio is congruent with the regional zoning, unlike
the fig u re fo r the e n tire d i s t r ic t . Dashed pattern is
Late Cretaceous plutonic rock; dotted pattern is Late
T e rtia ry and Quaternary sediment; blank area is Precambrian through Early T e rtia ry metamorphic, sedimentary,
or volcanic rock.
18
Table I .
Some o f the quartz vein systems o f the V irg in ia C ity D is t r ic t .
NORTHEAST-STRIKING
NORTHWEST-STRIKING
U.S. Grant:
N40o-50°E; 35o-50°NW
Prospect:
Cornucopia:
N45°E; 45°NW
E aston-Pacific:
El Fleeda:
N40°-50qE; 30°-50°NW
Black Rock:
N50°W; 70°NE
'
N65°E; 50°NW
N70°E; 50°NW
S ilv e r B e ll:
Fork:
N55°W; 70°NE
N50°E; 20o-30°NW
S t. Lawrence:
Alameda:
Mapleton:
N45°W; 75°NE
NSO0E; 45°NW
N35°E; 20°SE
Kearsage:
N23°E; GS0NW
M a rie tta :
N45°E; 40°SE
Hypogene m in e ra liza tio n consists c h ie fly of ubiquitous quartz
and p y r ite , and v a ria b le amounts o f galena, s p h a le rite , chalcop yrite,
and gold.
Numerous other minerals alsq occur (Appendix 3 ) .
Secondary
s u r fic ia l enrichment (by oxidation) is important in the formation of
high-grade ores in the Winnetka, E aston-P acific, Prospect, Mapleton,
Kearsage, Oro Cache, El Fleeda, Alameda, and other mines (Tansley
and others, 1933).
From 1901-1935, production was reported from more than 60
properties (L o ra in , 1937).
During the peak o f mining a c t iv it y (in
the la te 19th century) i t was lik e ly th a t a s ig n ific a n tly greater
number o f mines were operating.
19
The vast m a jo rity o f underground workings a r e caved and
inaccessible.
At present, no mines are producing, although several
are accessible and in operating condition.
Most underground work conducted by the author was in the U.S.
Grant 3 -leveT .
Other mines examined in varying d e ta il were the El
Fleeda, S t. Lawrence, Cornucopia, Black Rock, and Alameda.
Numerous
surface exposures and mine dumps were also observed including the
Prospect, Fork, and E aston-P acific.
In a d d itio n , some information
about these mines was a ttain ed by examining a v aila b le lit e r a t u r e and
consulting with local mining geologists.
20
MINING GEOLOGY
OF
.
SELECTED. MINES OF THE VIRGINIA CITY DISTRICT
.
U.S. Grant Mine
The U.S. Grant mine is located approximately 1.0 km south of
V irg in ia C ity on the west fla n k of Alder Gulch (P la te I ) .
is composed o f 3 le v e ls .
and p a r t ia lly caved.
The mine
The upper I - and 2 le v e ls are abandoned
The lower 3 -le v e l is r e la tiv e ly accessible and
is the focus of underground study.
The U.S. Grant has been one of the most consistent producers in
the d i s t r i c t , though never a very large one (L o rain , 1937; Appendix 6)
The mine (in the 3 - le v e l) has been worked as recently as January, 1981
The 3 -le v e l is composed o f a 65 meter-long a d it in brecciated
b a s a lt, a 665 meter-long d r i f t in Precambrian gneiss, and numerous
stopes and raises (P la te 3 ) .
Elevation o f the po rtal, is approximately
1885 m.
'
Composition o f (u naltered) wall rock gneiss is dominantly
hornblende-bio t i t e - p la g io c lase-q uartz-ep id o te-g arn et, w ith Iesser
amounts of a q u artz-fe ld s p a r-p e rth ite -p la g io c la s e -h q rn b le n d e -b io t it e kyanite-garnet assemblage.
Numerous pegmatites and several a p lite
dikes tran sect wall rock fo lia t io n and are subsequently crosscut by
the vein system (and the re la te d a lte ra tio n h a lo ).
Pegmatite and
a p lite lith o lo g y is q u a rtz -( s e r i c i t i zed) fe ld s p a rs -b io tite .
L
21
A ltered wall rocks contain v a riab le amounts o f hydrothermal and
r e l i c t metamorphic m inerals.
The r e l i c t mineral s u ite contains
s e r ic itiz e d fe ld s p a rs , b i o t it e , q u artz, and hornblende (? ).
Hydrothermal minerals are m icroclin e, q u artz, carbonate, p y r ite ,
c h lo r ite , other p h y llo s ilicates, and ra re ly a z e o lite (? ).
The 3 -le v e l vein system s trik e s N40o-50°E and dips SS0-SO0NW
(P la te 3 ).
The vein system is composed of two s lig h tly overlapping
vein s tru c tu re s .
Vein stru ctu re #1 (V S -I) is present from reference
point (R .P .) #2-#5 (P la te 3 , Sheet I ) beyond which i t pinches out.
Adjacent to and northwest o f the pinch-out is vein stru ctu re #2
(V S -2).
Unlike V S -I, VS-2 is not continuous, hence the secondary
labels VS-2a, VS-2b, e tc .
This d is c o n tin u ity is probably due to
post-ore fa u ltin g o f a continuous vein stru ctu re and may not imply
the presence o f more than one in divid ual s tru c tu re .
Vein stru ctu re thicknesses vary from 0 .3 to 5.0 m, but average
0 .6 to 1.6 m (F ig s . 4 & 5; Plate 3 ). Each stru ctu re is composed of
elongate quartz lenses, ta b u la r quartz bodies, quartz s trin g e rs , and
v a riab le amounts of crushed and a lte re d gneiss.
The quartz contains
few c a vity f i l l i n g s because syn-ore deformation in h ib its th e ir
development.
The e n tire vein system is contained in a shear zone.
Each
vein stru ctu re appears to occur in a single shear of the shear zone.
Shear fra c tu rin g is the primary (s tru c tu ra l) control of ore deposition,
and occurs in several major episodes.
22
Figure 4.
Cross-section of U.S. Grant 3 -le v e l vein stru ctu re (V S -2).
Dip approximately 50° northwest; thickness about 1.3 m.
Located southwest of R .P.#6, Plate 3, Sheet I .
Vein
stru ctu re composed of an elongate quartz le n s , quartz
s trin g e rs , and crushed and a lte re d wall rock ("X" p a tte rn ).
Some wall rock (dot p attern ) and pegmatite feldspar (lin e
p attern ) fragments in quartz lens. S ulfide m ineralizatio n
(black shading) occurs as small seams and pockets in quartz.
Wavy lin e pattern depicts wall rock not contained in vein
s tru c tu re .
23
Figure 5.
View looking up-dip of vein structure on southeast wall
of U.S. Grant 3 -le v e l d r i f t . Located northeast of R.P.#9,
Plate 3, Sheet I . Vein structure composed of an elongate
quartz lens (blank area with th in -lin e d fra c tu re s ) contain­
ing s u lfid e seams (black shading), wall rock fragments
(dot p a tte rn ), and sparse pegmatite feldspar (lin e p a tte rn ).
Wallrock (wavy lin e p attern ) is crushed and a lte re d , and
contains several quartz s trin g e rs . Thickness o f vein
structure is 0.8 to 1.0 m.
24
Most of the vein system (including adjacent wall rocks) was
fractu red and crusted by syn-ore, b r i t t l e , c a ta c la s tic shear.
Right-
la te r a l o ffs e t between hanging wall (HW) and footw alI (FW) has been
documented by observing displacement o f HW pegmatite and a p lite dikes
from corresponding FW portions (P la te 3 ) .
Sparse pegmatit i c feldspar
was observed in the vein stru ctu re between o ffs e t HW and FW pegma­
t it e s (P la te 3; Fig. 5 ).
The magnitude o f wall rock deformation and la te r a l o ffs e t varies
somewhat in the 3 - le v e l.
At lo catio n I (R .P .# 4 .5 ; in the Only
terminus exposed o f e ith e r shear in the 3 - le v e l; see P late 3) w a llrocks are not crushed or displaced.
Lateral o ffs e t in uncrushed or
s lig h tly crushed wall rocks occurs a t locations I I
o ffs e t), I I I
(R .P .#4; 8 .0 m
(R .P .# 8 .5 ; 8 .3 m o f f s e t ) , and IV (R .P .#18.5; 10.3 m
'
o ffs e t).
Lateral o ffs e t in strongly crushed wall rocks is present a t
locations V (R .P .#3; 12.0 m o ffs e t) and VI (R .P .#5; o ffs e t unknown).
At lo catio n V II (R .P .#6) pegmatite blocks flo a tin g in crushed w a llrock suggest extreme deformation and preclude any determination of
the magnitude o f la te r a l movement.
A two-stage a lte r a tio n event is proposed by the author based on
petrographic study of a lte re d walI rocks.
Potassic a lte ra tio n
consisting o f m icrocline and possibly quartz is in part cut by a
subsequent propyli t i c assemblage containing carbonate, p y r ite , qu artz,
c h lo rite and other p h y llo s ilic a te s , and ra re ly a z e o lite (? ) .
depicts the age relatio n sh ip s o f these m inerals.
Table 2
25
Table 2.
MINERAL
QUARTZ
MICROCLINE
CARBONATE
CHLORITE
and other
p h y llo s ilicates
ZEOLITE
PYRITE
GALENA
CHALCOPYRITE
TETRAHEDRITE
HEMATITE
SPHALERITE
Paragenetic sequence of hypogene a lte ra tio n and ore
m in e ra liza tio n in the U.S. Grant 3 - le v e l. Major episodes
of syn-ore shear are also shown.
WALLROCK ALTERATION
ORE MINERALIZATION
_ U _________________ Ll_
_JJ ______________ LI______________
Il
Il
Il
I
Il
I
I
I
I
li_
Il
Il
Il
Il
Il
Il
IL_________________ LI
I
I
I
_u
I
_LI
I
—
_L
I
I
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
Il
—
Il
Il
—
................
I
I
CO
3Z
m
CO
ZE
m
70
Z
O
O
^y O
Z
CO
=T
m
70
Z
O
26
V ariable amounts o f m icrocline and possibly quartz (Figs. 6-8)
occur as random, unoriented replacements o f p re -e x is tin g gneiss.
This
conclusion is based on the follow ing observations:
1)
Metamorphic rocks not in s p a tia l proxim ity to the 3 -le v el
vein system (or other vein systems) contain only s e r ic itiz e d fe ld ­
spars but never n o n -s e ric itic m icrocline (F ig . 6 ) .
M icrocline occurs
(in v a riab le amounts) only in wallrocks adjacent to and w ith in the
3 -le v e l vein system (Figs. 7 & 8 ).
Therefore, m icrocline is post-
sen' c itiz a tio n and is lik e ly a product o f younger hydrothermal
a c t iv it y .
2)
T rip le -p o in t grain-boundary angles o f approximately 120°
between m icrocline and some quartz grains (F ig . 7) in d ic a te te x tu ra l
equilib rium (Stanton, 1972, p. 234).
K-Ar radiom etric analysis was performed on the m icrocline by
Geochron Labs.
The sample contained a small amount o f r e l i c t p e rth ite
and K-feld sp ar (F ig . 8 ) .
An age of 311 m .y.B.P. ± 11 m.y. was
a ttain ed (Appendix I ) suggesting an admixture of Precambrian and
Laramide fe ld s p a r.
A la t e r pervasive propyli t i c a lte r a tio n assemblage occurs as
m icroveinlets crosscutting, and as a m atrix surrounding m icrocline,
q u artz, and r e l i c t minerals (Figs. 8 & 9 ).
The shape and dimensions o f the a lte ra tio n halo are not known.
The halo is probably more or less ta b u la r ( lik e the vein system).
is possibly no g reater than 5 m wide on e ith e r side of the vein
system and - in many places is less than I m.
It
27
M in e ra liza tio n in the 3 -le v e l is almost e n tir e ly hypogene, unlike
the upper two le vels where oxidized ore is more abundant.
Major ore
shoots (which appear to be controlled by shear fra c tu rin g ) occur as
th in s u lfid e -ric h seams and pockets along the hanging wall or footw all
contact (Figs. 5 & 10).
Ore shoots may be continuous fo r tens of meters
along s tr ik e , however, most are less.
Petrographic study of polished-surface ore samples followed the
c r it e r ia o f Ramdohr (1969) in the id e n tific a tio n of minerals and
in te rp re ta tio n of mineral te xtu res.
Figure 6.
Unaltered gneiss from Browns Gulch.
a e rlc ltlc
K -te id a p a r
q u artz
e p ld o te
b lo tlte
a e rlc ltlc
p la g lo c la z e
Jjjj
h o rn b le n d e
I
28
Figure 7.
S lig h tly a lte re d wall rock from the U.S. Grant 3 - le v e l,
R .P .# 8 .5 . Note the addition of m icrocline and possibly
q u artz. Some b io t it e e x h ib its in c ip ie n t propyli t i c
a lte r a tio n to c h lo r ite , other phylI o s i I i cates, and p y rite .
sericitlc
K -feldsp ar
quartz
•< *r
• s -
microcline
•
p yrite
biotite
ch lo rite /o th e r
p h y Iloaillcates
after
b io tite
/ /
A
,y
y ' /
y
Figure 8.
Strongly a lte re d wall rock from the U.S. Grant 3 -le v e l,
R .P .# 8 .5 . This sketch ex h ib its a well-developed potassic
assemblage cut by a propyli t i c assemblage of carbonate,
p y r ite , and quartz. P ro p y litize d b io tite contains c h lo r ite ,
other p h y llo s ilic a te s , and p y rite . K-Ar age o f the feldspar
population in th is sample is 311 m.y.B.P. ± 11 m.y.
30
Figure 9.
Strongly p ro p y litiz e d wall rock from the U.S. Grant 3 -le v e l,
southwest of R. P.# 8 .5 . Fibrous mineral in lower h a lf of
sketch may be a z e o lite . Quartz from vein stru ctu re is
present in upper r ig h t.
aerlcltlc
K -Ie ld s p a r
quartz
carbonate
••y r
•• •
microcline
•
pyrite
zeo lite(? )
31
Figure 10.
A ty p ic a l sample of q u a rtz -s u lfid e ore and adjacent walI rock from the U.S. Grant 3 -le v e l. Wal I rock is crushed
and a lte r e d , and contains quartz s trin g e rs . Quartzs u lfid e ore has concentration of sulfides (black shading)
along wall rock contact, and contains fragments of w a llrock ( lin e p a tte rn ). Thin branching lin es are fra c tu re s .
32
Hypogene ore m in e ra liza tio n consists o f abundant p y r ite ,
moderate amounts o f s p h a le rite and galena, minor chalcopyrite and
te tra h e d rite , and rare specular (?) hem atite.
yellow ) native gold is very ra re .
Observed (b rig h t-
Microscopic flakes o f gold occur
in lim o nite (400x, o il imm.) and possibly in s p h a le rite (140x, o il
imm.) .
Gangue is c h ie fly quartz with minor feld sp ar.
The ore minerals in the 3 -le v e l e x h ib it r e la tiv e ly simple age
re la tio n s h ip s .
p y r ite .
Early quartz (F ig . 11) is followed by quartz and
Fracture f i l l i n g s in p y rite contain quartz (F ig . 12 ); q u artz,
galena, c h a lc o p yrite , and s p h a le rite (F ig . 13); or q u a rtz, galena,
c h a lc o p yrite , s p h a le rite , te tra h e d rite , and hematite (F ig . 14).
S ph alerite appears in part to replace p y rite and galena (F ig . 14),
or p y r ite , galena, and chalcopyrite (F ig s .15& 1 6 ). P ost-sphalerite
quartz occurs as m icroveinlets cu ttin g s p h a le rite and other sulfides
(F ig . 16).
These relatio n sh ip s are depicted in Table 2.
Secondary minerals in the 3 -le v e l ores include m alachite,
chrysocolla, c o v e ili t e , chalc o c ite , manganese oxide, (red ) hematite,
and lim o n ite .
R eflection microscopy o f ore samples ex h ib its
chalcopyrite rimmed by chalco cite which is surrounded by cove11it e .
Post-ore fa u ltin g (in the 3 -le v e l) is abundant though
displacements are usually less than I m and freq uently less than 0.3 m
(P la te 3 ).
Most o f these fa u lts crosscut the vein system ,. hence
post-ore in te rn a l deformation of the vein system is minimal.
The U.S. Grant mine is adjacent and g e n e tic a lly re la te d to the
Cornucopia mine (P la te 2 ) .
The vein systems of both mines occur in
a major n o rth e a s t-s trik in g shear zone or group of overlapping shear
33
Figure 11.
Ore sample from the U.S. Grant 3 - le v e l. Early quartz
("Q") is surrounded by la t e r quartz (w hite) and p y rite
(b la c k ).
Figure 12.
Ore sample from the U.S. Grant 3 -le v e l.
fractu res f i l l e d by quartz.
C2
O
pyrite
P y rite with
34
Figure 13.
Ore sample from the U.S. Grant 3 -le v e l. P y rite with
frac tu re s f i l l e d by q u artz, galena, s p h a le rite , and
c h a lco p yrite.
Figure 14.
Ore sample from the U.S. Grant 3 - le v e l. P y rite with
fractu res f i l l e d by q u artz, galena, s p h a le rite ,
c h a lc o p yrite , te tra h e d rite , and hem atite. S p h ale rite,
in p a rt, appears to replace p y rite and galena.
p y rlle
galena
s p h a le rite
c h a lc o p y rite
te tra h e d rite
h em a tite
35
0 2 5 mm
Figure 15.
Ore sample from the U.S. Grant 3 - le v e l. Sphalerite is
replacing galena, p y r ite , and ch alcop yrite.
Figure 16.
Ore sample from the U.S. Grant 3 -le v e l. S phalerite is
replacing p y r ite , galena, and chalcop yrite. Post­
s p h a le rite quartz cuts across s p h a le rite and p y rite .
▲
36
zones.
This zone (o r group of zones) also contains the El
Fleeda-Homestake-Black Rock group.
The U.S. Grant-Cornucopia group
is o ffs e t from the El Fleeda-Homestake-Black Rock group by a recent
normal f a u lt (N25°W, 810NE; Boyer9 personal comm., 1981; Plate 2 ).
This f a u lt is observable a t the southwest end of the Cornucopia
I - le v e l.
El Fleeda Mine
The El Fleeda mine, located approximately 1.5 km south of
V irg in ia C ity (P la te I ) , is composed o f fiv e levels and an in clined
s h a ft.
A ll workings except the 4 -le v e l are caved and inaccessible.
The 4 -le v e l was worked as re cen tly as the 1960's.
I t consists
o f a 105 meter-long a d it , a d r i f t composed of two branches with a to ta l
length of 120 m, several stopes and ra is e s , and an in clin ed shaft
(P la te 4 ) .
Portal elevatio n is approximately 2000 m.
Composition o f unaltered wall rock gneisses is dominantly
hornblende-bio t i te - q u a r t z - f e ldspar with a lesser amount of quartzfe ld s p a r -b io tite gneiss.
f o lia t io n .
Numerous g ra n itic pegmatites cut wall rock
The pegmatites are in turn cut by the vein system.
A ltered wall rocks e x h ib it potassic a lte ra tio n in the form of
abundant m icrocline and possibly qu artz.
to c h lo rite and rare p y r ite .
B io tite is in p art alte red
M icroveinlets of fin e -g ra in e d mosaic
quartz and hematized p y rite crosscut a ll previously mentioned
m inerals.
Carbonate is not present, however, th is is probably due
to lim ite d sampling.
37
The 4 -le v e l vein system ( lik e th a t of the U.S. Grant 3 - le v e l)
occupies a shear zone.
Average s trik e and dip o f the system is
N4Q°-50°E and 30o-50°NW (P la te 4 ).
The vein system is composed o f two vein structures th a t jo in
along s trik e to the northeast.
is about 2 m.
Maximum thickness of e ith e r structure
Each vein stru ctu re is composed o f quartz lenses,
quartz s trin g e rs , and tab u lar quartz bodies in crushed and alte red
wall rock.
The quartz contains few open-space f i l l i n g s because syn-ore
deformation p a rtly precludes th e ir development.
Considerable post-ore
(including recent) fa u ltin g occurs w ith in and adjacent to the vein
system.
As a r e s u lt, footw all and hanging wall boundaries are
disguised and d i f f i c u l t to lo ca te .
Syn-ore, b r i t t l e c a ta c la s tic shear displaced hanging wall and
footw all portions o f w all rock pegmatites, in addition to destroying
th e ir ta b u la r form.
Subsequent post-ore fa u ltin g has fu rth e r
displaced the pegmatites and surrounding wall rocks.
As a r e s u lt,
analysis o f pegmatite locations yield ed ambiguous data fo r magnitude
and d ire c tio n o f shear.
Hypogene m in e ra liza tio n in the 4 -le v e l contains auriferous and
argentiferous s u lfid e s , dominantly p y rite with lesser amounts of
galena, in a gangue of quartz.
B right-yellow gold occurs as flakes
(1 -2 mm across) in q u artz, but is qu ite ra re .
oxid ized , e s p e cia lly where
system.
ore.
Much of the ore is
post-ore fa u ltin g is present in the vein
In the v ic in it y of pegmatites, feldspar may be found in the
38
A post-ore andesite plug crosscuts the vein system (P la te 4 ).
Whole rock K-Ar radiom etric dating indicates a 51.1 m .y.B.P. ±
1.2 m.y. age fo r the plug (Marvin and Dobson, 1979; Appendix I ) .
This plug appears to separate the El Fleeda vein system from the
Homestake-Black Rock vein system (P la te 2 ).
Black Rock Mine
The Black Rock mine is located 2.5 km southwest o f V irg in ia
C ity a t an elevation of 2090 m (P la te I ) .
Most of the workings are
caved; however, access to one d r i f t level is possible via a 20
meter-long v e n tila tio n ra is e .
Approximately 80 m of d r i f t are
observable.
Wallrock lith o lo g y is h o rn b le n d e-b io tite -q u artz-fe ld s p a r gneiss.
!
Pegmatites are lacking although they are abundant a t the surface
(P lates 1 , 2 ) .
The vein system, a ttitu d e N50°E; 20o-30°NW, is contained in a
shear zone.
I t consists of a single vein structure composed of
tab u lar quartz bodies 0.7 to 1.7 m th ic k (L o rain , 1937) and quartz
stringers in fractu red gneiss.
M in e ra liza tio n occurs as precious-m etal-bearing s u lfid e s ,
dominantly p y r ite .
Gangue is e n tir e ly quartz.
Easton-Pacific Group
The Easton-Pacific group, elevation approximately 2300 m, is
located 7.5 km south o f Nevada C ity in the headwaters of Browns Gulch
39
(P la te I ) .
A ll underground workings are inaccessible due to recent
open-pit operation on the claim .
According to Winchell (1914),
these mines are on the same vein system w ithin the Browns Gulch
stock.
This group has yield ed the la rg e s t tonnage of any property in the
V irg in ia C ity d is t r ic t (L o rain , 1937; Appendix 6 ) .
the Easton has been the major producer.
Within the group,
The Easton claim was
developed southeastward along the vein system, whereas the P a c ific
claim was developed towards the northwest.
The Easton-Pacific vein system s trik e s N55°W and dips 70°NE.
In the Easton mine the vein system is composed of two vein structures
th a t jo in towards the southeast ( WinchelI , 1914). Thickness of the
vein structures varies from 0.5 to 2.0 or 2.6 m ( W inchell, 1914).
D eta ils of P a c ific vein system morphology are lacking in the a v aila b le
lit e r a t u r e .
M in e ra liza tio n varies s lig h tly amongst the two mines.
In the
P a c ific mine antimonial s ilv e r s u lfid es and native gold and s ilv e r
occur in quartz and iron oxides ( W inchell, 1914).
In the Easton
mine the ore contains a rg e n tite , auriferous p y r ite , native s ilv e r ,
te tra h e d rite , native gold, s p h a le rite , and s tib n ite (W in chell, 1914).
Gangue is c h ie fly quartz and fe ld s p a r.
The s ilv e r-to -g o ld r a tio is
higher in the Easton than in the P a c ific , and the r a tio increases
with depth in both mines (Tansley. and others, 1933).
40
Prospect Mine
The Prospect Mine, elevation 1890 m, is located 1.5 km west of
V irg in ia C ity (P la te I ) .
A ll workings are dangerous and inaccessible.
The vein system, a ttitu d e . N45°W; 75°NE, is in quartzofeldspathic
gneiss.
I t is l i t t l e disturbed by post-ore fa u ltin g (Tansley and
others, 1933).
Unlike other productive vein systems in the d i s t r ic t ,
i t is re a d ily traced a t the surface over the f u l l length o f the
Prospect claim .
The vein system appears to be composed o f a single vein
s tru c tu re .
I t contains quartz lenses up to 2.3 m th ic k in sheared
gneiss (L o ra in , 1937).
The p rin cip al hypogene m in e ra liza tio n consists o f galena,
c h a lc o p yrite , and p y r ite , with minor s p h a le rite and n ative gold.
Petrographic study indicates 25% of observable microscopic gold is
contained in q u artz, with the remainder occurring in s u lfid e s , mainly
galena
(Tansley and others, 1933).
Although p y rite is not an
important c a rr ie r of observable gold, i t is assumed th a t p y rite
contains gold as submicroscopic p a rtic le s and/or as a la t t ic e
c o n stitu en t.
S t. Lawrence Mine
The S t. Lawrence mine, elevation approximately 2000 m, is
located about 4 .0 km southwest of Nevada C ity (P la te I ) .
to the mine is via a 50° in clin ed s h a ft.
Entrance
The follow ing data is
from observations by the author in the 1 5 0 -le v e l.
41
The S t. Lawrence vein system, a ttitu d e N65°E; 50°NW, is contained
in quartzofeldspathic gneiss.
I t is composed of two p a ra lle l vein
structures th a t merge toward the northeast.
Maximum distance between
the two structures is about 20 m (Boyer, personal comm. 1982).
The vein structures vary in thickness from 0.3 to 2.3 m.
They
are composed of ta b u lar quartz bodies, exceeding I m in thickness,
and quartz strin g ers in fractured and a lte re d wall rock (F ig . 17).
tabular quartz
qu artz stringer
3 0 cm
Figure 17.
Cross-section of the vein stru ctu re in the S t. Lawrence
1 5 0 -le v e l. Sulfides (black) occur as pockets and blebs.
42
A ltered wall rocks (in and adjacent to the vein system) contain
a pervasive potassic a lte ra tio n assemblage consisting o f m icrocline
and possibly quartz, -Subsequent p ro p y litiz a tio n occurs as massive
dissemination-patches and m icroveinlets o f carbonate and hematized
p y r ite .
No c h lo rite is present probably because the o rig in a l gneiss
lacks b io t it e .
Primary m in e ra liza tio n occurs as blebs and large pockets of
p y rite with some galena and minor chalcop yrite.
Ore from the 150-
level is only s lig h tly oxidized.
Each vein stru ctu re hosts a recent f a u lt .
Both fa u lts e x h ib it
approximately 60 m of le f t - l a t e r a l o ffs e t (Boyer, 1982, personal comm.).
As a r e s u lt, portions o f the vein structures are fra c tu re d , crushed,
and granulated.
Fork Mine
The Fork mine is located in Browns Gulch about 5 km south of
Nevada C ity (P late I ) .
Elevation of the d r i f t is about 1950 m.
The vein system s trik e s N35°E and dips 20°SE (L o ra in , 1937).
Observations of mine dump samples in d icate the vein system contains
much a lte re d and crushed gneissic wall rock, and possibly some pegmatitic
fe ld s p a r.
Microscopic examination o f a single wall rock sample
indicates intense p ro p y litiz a tio n in the form of massive carbonate
and p y r ite , with abundant b io t it e ghosts containing c h lo rite and
p y r ite .
According to Lorain (1 9 37 ), a 0 .7 m eter-thick "quartz vein"
is contained in a "crushed and sheared zone in gneiss".
Ore samples
43
from the dump contain v a ria b le q u a n titie s o f p y rite and galena
in a gangue o f quartz.
44
GEOLOGIC HISTORY OF THE VIRGINIA CITY DISTRICT
Based on data from the southern Tobacco Root Mountains
(Cordua, 1973; M ueller and Cordua, 1976; Burger, 1969; Wooden
and others, 1978), data from maps o f the mining d is t r ic t (W eir, 1982;
Hadley, 1969), and f ie ld relationsh ips observed by the author, the
follow ing geologic h is to ry has been proposed.
Deposition o f p ro to !ith s and subsequent metamorphism p rio r to and
a t 2700 m.y.B.P. formed the p re -B e lt rocks in the mining d i s t r ic t .
Deformation accompanying the 2700 m .y.B.P. metamorphism yielded
tig h t is o c lin a l folds with a xial plane f o lia t io n .
Sometime a fte r th is
deformation but p rio r to 1600 m.y. ago, coaxial (?) re fo ld in g o f the
p re -e x is tin g f o lia t io n produced the n o rth e a s t-s trik in g antiform of
the d i s t r i c t .
About 1600 m .y .B .P ., a thermal event reset mineral
ages, and pegmatites of the d i s t r ic t were formed.
During the Paleozoic and the Mesozoic Eras, sedimentary rocks
were probably deposited in the mining d i s t r i c t , however, th is is
not c e rta in .
The Madison Limestone was observed in upper Alder
Gulch, but was in th ru s t contact with the underlying Precambrian
rocks.
From Late Cretaceous to Mid Oligocene tim e, Laramide orogenic
processes operated in the d is t r ic t and the surrounding region.;
B r i t t l e , non-penetrative, low-temperature deformation in Precambrian
c r y s ta llin e rocks probably created new f a u lt s , fra c tu re s , and shears.
45
and reactivated many p re -e x is tin g weaknesses.
s tra ta were th ru s t onto Precambrian rocks.
Paleozoic and Mesozoic
Plutonism, volcanism,
and hydrothermal a c t iv it y were widespread throughout the area.
The
Browns Gulch stock and marginal pegmatite phase were in je c te d ,
followed by the emplacement o f q u artz-sulfide-precious-m etal ore
bodies.
Subsequent erosion and deposition o f stream gravels was
followed by the extrusion o f voluminous q u an titie s o f Eocene and
Oligocene volcanics.
During Late T e rtia ry and Pleistocene tim e, erosion removed most
of the volcanics, the underlying g ravels, most o f the Paleozoic and
a ll Mesozoic s tr a ta , and some o f the Precambrian rocks.
Sediments
containing placer gold were deposited in the la rg e r stream v a lle y s .
46
CONCLUSIONS: AGE AND GENESIS OF QUARTZ-SULFIDEPRECIOUS-METAL VEIN SYSTEMS IN THE VIRGINIA CITY MINING DISTRICT
Previous In te rp re ta tio n s
The genesis of q u artz-sulfide-precious-m etal vein systems in
the V irg in ia C ity d i s t r ic t is not obvious.
Previous w rite rs express
ideas influenced mainly by the Lindgren magmatic-hydrothermal theory.
Winchell (1914) stated the occurrence o f E aston-Pacific ores
in the Browns Gulch stock was not o f chance but of genetic r e la tio n ­
ship.
He acknowledged th a t vein systems hosted by Precambrian
gneisses contained p y ra rg y rite , s y lvan ite or c a la v e rite , nagyagite,
S tib n ite 1 and te tra h e d rite .
This prompted him to say th a t vein
system constituents "may have traveled a considerable distance from
some parent igneous mass."
However he then stated "the country rock
o f the ores is much broken and a lte re d , and such conditions
f a c i l i t a t e and bear witness to a c tiv e c irc u la tio n of meteoric
waters.
I f such waters penetrated to s u ffic ie n t d e p th ....th e y might
dissolve the m aterials o f the ore deposits from the country rock on
th e ir downward and la te r a l journey and redeposit them in fis s u r e s ..."
In a d d itio n , he indicated the vein systems were d e fin ite ly older
than, and unrelated to . T e rtia ry volcanics in the d i s t r ic t .
Tansley and others (T933) also thought the occurrence of the
Easton-Pacific group in the Browns Gulch stock suggested a genetic
re la tio n s h ip .
Subsequently, they stated i t was probable th a t "the
47
Precambrian rocks o f the region were underlain by monzonitic rocks
which were responsible fo r much of the. fis s u rin g and m in eralizatio n
o f the area."
Lorain (1937) implied an o rig in (o f precious-metal ores in
a ll Tobacco Root d is t r ic t s ) re la te d to Laramide plutonism by his
discussion o f metal zoning with respect to the Tobacco Root bath o lith
A d d itio n a lly he stated the Easton-Pacific group was located in the
Browns Gulch stock, and the M a rie tta , High-Up, and other mines
were closely associated with i t (P la te I ) .
A ll other mines of the
d i s t r ic t are w ith in several kilometers o f the in tru s iv e , with most
of them between the main outcrop o f the b a th o lith and the stock.
Weir (1982) mapped the bedrock o f the north h a lf o f the mining
d i s t r i c t , including outcrop patterns o f quartz vein systems.
He
stated with uncertainty th a t the ore bodies were T e rtia ry in age
though d e fin te ly older than the T e rtia ry volcanics.
Author's In te rp re ta tio n
The follow ing in te rp re ta tio n o f age and genesis of qu artzsulfide-precious-m etal ores in the V irg in ia C ity mining d is t r ic t
is based on petrographic study and K-Ar radiom etric dating of altered
wall rocks in the U.S. Grant 3 - le v e l, information obtained via aboveand below-ground mapping, and data from a v a ila b le li t e r a t u r e .
I t is
assumed th a t the strong s im ila r ity in character and morphology of .
a ll mines in the d is t r ic t allows in te rp re ta tio n s o f data obtained
from a single mine to be applicable to a l l .
48
Age o f Ore Deposits
I t is proposed th a t the age o f emplacement o f ores (in th e ir
present form) in the mining d is t r ic t is between Latest Cretaceous
and Early T e rtia ry time (70-60 m .y .B .P .) .
This in te rp re ta tio n is
supported by several types of evidence.
The occurrence o f the Easton-Pacific group in the Late
Cretaceous Browns Gulch stock indicates the ores bodies (o f the
d i s t r ic t ) must be younger than Late Cretaceous.
I t is reasonable
to assume they are no younger than Early T e rtia ry because an andesite
plug, with an age of 51.1 m.y.B.P. ± 1.2 m.y. (Marvin and Dobson,
1979), cuts the El Fleeda 4 -le v e l vein system (Plates 2 , 4 ) .
A ltered wall rocks from the U.S. Grant 3 -le v e l contain variab le
amounts o f fre s h , n o n -s e ric itiz e d m icro clin e, and, s e r ic itiz e d
Precambrian feldspar (Figs. 7 &. 8 ) .
M icroclin e , because i t lacks
s e r ic it e , is p o s t-s e r ic itiz a tio n .
The youngest known age of any
s e r ic it ic feldspar is 1572 m.y.B.P. ± 51 m.y. from the "Powder
Mag" pegmatite.
Therefore mi croclin e is post-1572 m .y.B.P.
This
age in te rp re ta tio n is also supported by the crosscutting of a ll
pegmatites in the 3 -le v e l by the m icrocline-bearing a lte r a tio n halo.
Based on the geologic h isto ry of the mining d i s t r ic t , the only post1572 m .y.B.P. process capable of forming m icrocline is hydrothermal
potassic a lte ra tio n re la te d to Laramide igneous a c t iv it y .
However,
a t present i t cannot be determined i f s e r ic itiz a tio n is a product of
1600 m .y.B.P. retrograde metamorphism, pre-micrpcline Laramide
hydrothermal a lte r a tio n , or post-pegmatite weathering.
49
An attempt was made by the author to determine the exact age
o f ore m in e ra liza tio n in the U.S. Grant 3 -le v e l vein system.
I t was
assumed th a t a lte ra tio n o f wall rock was approximately synchronous
with ore m in e ra liz a tio n .
This allowed the use of hydrothermal micro-
c lin e fo r determination o f ore body age.
bearing a lte re d wall rock (F ig . 8
is a petrographic sketch o f the
sample) was analyzed by Geochron Labs.
dated by K-Ar technique.
A sample of m icrocline-
The feldspar was removed and
An age of 311 m .y.B.P. ± 11 m.y. was
a tta in e d .
This age appears contradictory to the proposed Latest Cretaceous
to Early T e rtia ry age.
However, careful microscopic examination of
the age-date sample reveals the wall rock feldspar population contains
approximately 85% hydrothermal m icrocline and 15% Precambrian
(v a ria b ly s e r ic itiz e d ) feldspar (F ig .
8 ).
Therefore, th is apparently
contradictory age (and the occurrence of excess Ar
40
) is due to
contamination ( Dalrymple and Lanphere, 1969; Schaeffer and Zahringer,
1966) of the Latest Cretaceous to Early T e rtia ry m icrocline sample
by r e l i c t (1600 m .y.B .P .) fe ld sp ar.
Consequently th is date is a
composite age of m icrocline and r e l i c t feld sp ar.
The previous in te rp re ta tio n by the author appears p lau sib le .
A d d itio n a lly , the a lte rn a tiv e explanation of a Precambrian age
presents problems.
I f the ore bodies are not the products of Latest
Cretaceous to Early T e rtia ry hydrothermal a c tiv ity but instead are
the products of a Precambrian metamorphic re la ted process, one
would expect a maximum age o f ore deposition and wall rock a lte ra tio n
o f about 1572 m.y.
To achieve a 311 m.y. age from 1572 m.y. old
50
m icroclin e, 80% of the. A r ^ must escape the m icrocline la t t ic e .
Data
fo r loss o f Ar4® from m icrocline suggest a maximum o f 20% (Schaeffer
and Zahringer, 1966) or 20-30% (DalrympTe and Lanphere, 1969).
Consequently i t seems u n lik e ly the apparent age of 311 m.y. is
modified from a real age o f 1572 m.y.
I t must also be noted th at
b io t it e from the "Powder Mag" pegmatite in the U.S. Grant 3 -le v e l
(P la te 3 , Sheet I , R.P. #6)' has a K-Ar date of .1572 m.y.B.P. ±
51 m.y.
B io tite is known to re ta in Ar40 b e tte r than m icrocline,
and considering the 1600 m .y.B.P. age fo r the la s t metamorphic
event in the d i s t r i c t , th is b io t it e sample ex h ib its complete
re te n tio n .
Why ( i f m icrocline and b io t it e are synchronous) would
the m icrocline lose an extreme amount of Ar
to ta l retention?
40
while b io t it e exhibits
The most lik e ly explanation is th a t m icrocline is
not 1572 m.y. old but is o f Latest Cretaceous to Early T e rtia ry age.
According to Lorain (1937), a rough regional zoning o f copperto -s ilv e r and s i l v e r-to -g o ld ra tio s occurs with respect to the
Tobacco Root b a th o lith (F ig . 3 ) .
This regional zoning appears to
encompass a ll Tobacco Root mining d is t r ic t s , and th erefo re suggests
a re la tio n s h ip between Tobacco Root precious-metal deposition and.
the Tobacco Root b a th o lith .
Johns (1 9 61 ),
Tansley and others (1933),
and Winchell (1914) report on various d is tr ic ts in the Tobacco Root
region.
They also suggest a possible genetic re la tio n s h ip between
the b a th o lith and precious-metal m in e ra liz a tio n .
I f (most or a l l )
Tobacco Root precious-metal deposits are of s im ila r (b a th o lith -r e la te d )
genesis, they are lik e ly o f s im ila r age.
The occurrence of some
deposits as f r a c t u r e - f illin g s in the Tobacco Root b a th o lith (77-72
51
m .y .B .P .; Appendix 2) and re la te d rocks, and the unquestionable
pre-51 m .y.B.P. age o f the ores in the V irg in ia C ity d i s t r i c t ,
strongly supports a Latest Cretaceous to Early T e rtia ry age fo r
emplacement o f (most or a l l ) Tobacco Root precious-metal ores.
Genesis .of Ores in the V irg in ia C ity Mining D is t r ic t
In conjunction with the proposed Latest Cretaceous to Early
T e rtia ry age o f V irg in ia C ity precious-metal deposits, the follow ing
in te rp re ta tio n o f genesis is suggested.
Precious-metal deposits of the V irg in ia C ity d i s t r ic t (and
the Tobacco Root precious-metal mining region) appear to be the
products a t le a s t in p a rt o f (Tobacco Root) b a th o lith -re la te d
hydrothermal a c t iv it y .
A d d itio n a lly , Precambrian metamorphism may be
responsible fo r an i n i t i a l p a rtia l concentration of ore m etals,
though no evidence is a v a ila b le a t present.
A model depicting genesis of epithermal precidus-metal deposits
( Eimon and A nctiI , 1981; Buchanan, 1981) appears s u ita b le , with some
m o d ific atio n , fo r the V irg in ia C ity deposits (F ig . 1 8 ). I t must be
noted th a t "epithermal" re fle c ts a genetic-class (Buchanan, 1981)
and not a temperature-class of Lindgren (193.3).
The follow ing is a presentation o f the basic concepts o f th is
model.
For more d e ta il re fe r to Buchanan (1981) or Eimon and Anctil
(1981).
The model encompasses a geothermal convection c e ll
(driven by
an igneous heat source; T aylo r, 1973) th a t c irc u la te s a large .
volume o f hydrothermal f lu id s .
The water in these flu id s (as
in
rx>
Figure 18
Depositional model of epithermal precious-metals
Modified from Buchanan (1981)
53
documented in some deposits) is dominantly or e n tir e ly meteoric
(T a y lo r, 1973).
Episodic b o ilin g o f the flu id s in the upper, near­
surface portion o f the convection c e ll is responsible fo r metal
deposition (F ig . 1 8 ).
Precious-metals are deposited a t and above the
b o ilin g le v e l, whereas base-metals are deposited a t and below i t .
Reported temperatures of precious-metal deposition vary from 200o-300°C
(Buchanan^ 1981) to IOO0-SOO0C ( Eimon and A n c til, 1981).
In a d d itio n , numerous p re-m in e raliza tio n fractu res (freq uently
te n s io n al) are required to tra n s p o rt, and accommodate deposition from,
hydrothermal flu id s .
Host rocks are dominantly volcanic or sedimentary
Widespread propyli t i c a lte ra tio n ( c h lo r ite , p y r ite , montmoriI Io n ite ,
carbonate, and i l l i t e ) encloses e r r a t ic a lly present inner a lte ra tio n
assemblages (a d u la ric , p h y llic , a rg il l i e , a l u n it ic , and s i l i c i c ;
Fig. 18).
Also, most recognized epithermal deposits are o f T e rtia ry
age (Buchanan, 1981; Eimon and A n c til, 1981).
Based on th is model, the follow ing genesis has been proposed fo r
the V irg in ia C ity deposits.
A geothermal convection c e ll produced
la rg e -s c a le , regional c irc u la tio n of hydrothermal flu id s in the upper­
most portion o f the c ru s t.
Heat from Laramide plutonism (Tobacco
Root b a th o lith and re la te d rocks) powered the convection c e ll.
Ore
constituents were co llected (in p art remobilized ?) and transported
by hydrothermal flu id s .
Numerous fra c tu re s , mostly w ith in the
n o rth e a s t-s trik in g antiform of the d i s t r i c t , were invaded by the
ore-bearing s o lu tio n s ." With tim e, fractu res th a t exhibited proper
physiochemical environments fo r ore deposition became the locus of
m in e ra liza tio n .
54
At present i t is not known i f b o ilin g o f the ore-bearing flu id s
is the mechanism responsible fo r precious-m etal.deposition in the
V irg in ia C ity d i s t r ic t .
Additional data ( i . e . f lu id inclusion
studies) is needed to make a d e fin itiv e statement.
The source o f the ore constituents is not known, however, based
on current lit e r a t u r e , several statements may be made.
Water in
hydrothermal flu id s is lik e ly in part or e n tir e ly meteoric (T aylo r,
1973).
According
o f the rock
to Boyle (1979) and T illin g and others (1973), any
types in the d is t r ic t are adequate sources o f gold.
Therefore, Precambrian metamorphics and Laramide in tru sive s are
possible source rocks.
Nearby Laramide igneous rocks are possible
sources fo r s u lfu r (Whitney and Stormer, 1983).
In the fu tu re , oxygen,
s u lfu r, and lead isotope studies may provide valuable clues about the
source of some ore constituents.
The V irg in ia C ity d i s t r ic t deviates from the model in several
ways.
Host
In the U.S.
rocks of the d i s t r ic t are n e ith er sedimentary or volcanic.
Grant 3 -le v e l and the S t. Lawrence 1 5 0 -le v e l,
p ro p y litiz a -
tio n is present but appears to be re s tric te d to a narrow irre g u la r
band on e ith e r side of both vein systems.
In the U.S. Grant 3 - le v e l,
the S t. Lawrence 1 5 0 -le v e l, and the El Fleeda 4 - le v e l, m icrocline (and
not a d u la ria ) is the feldspar present in the accompanying a lte ra tio n
assemblage.
A d d itio n a lly , most of the ore-bearing fractu res in the
d i s t r ic t are shear zones and not tension fra c tu re s .
The occurrence o f gold and s ilv e r with abundant base-metals
suggests th a t d i s t r ic t m in e ra liza tio n is from the deeper portion of
the model in the zone o f overlap between base- and precious-metal
55
deposition.
According to the model (F ig . 1 8 ), width o f th is overlap
is about 50 m, however, most mines in the d is t r ic t span a considerably
greater v e rtic a l distance.
The question arises:
why is there an
apparent lack o f major v e rtic a l change in hypbgene m in e ra liza tio n in
the d is tr ic t?
An answer may be th a t the location o f the mechanism
responsible fo r precious-metal deposition cannot remain constant in
space and time (Buchanan, 1981).
V e rtic a l change o f the location of
th is mechanism can explain large v e rtic a l in te rv a ls o f mixed baseand precious-metal m in e ra liza tio n found in the V irg in ia C ity d is t r ic t
and other epithermal deposits (Buchanan, 1981).
A d d itio n a lly , genesis
of these ores in the deep le v e ls of the model may explain the lim ite d
extent of p ro p y litiz a tio n (F ig . 18).
Another point to consider about d is t r ic t m in e ra liza tio n is the
(form er) occurrence o f ric h Alder Gulch placer deposits.
These
deposits yield ed tw enty-fold more gold than the combined production of
a ll lode mines in the d i s t r ic t .
Undoubtedly, the source lodes fo r
these deposits were o f considerably higher grade than t h e ir presently
exposed and eroded ro o ts .
The regional zoning (o f copper) with respect to the b a th o lith
appears to f i t the epithermal model.
Tobacco Root d is t r ic t s with
high c o p p e r-to -s ilv e r ra tio s occur nearest the b a th o lith and are
possibly in d ic a tiv e o f m in e ra liza tio n from the deepest portion of the
model.
Those d is t r ic t s fu rth e r from the b a th o lith e x h ib it m ineraliza­
tio n possibly in d ic a tiv e of less-deep portions of the model, based on
moderate c o p p e r-to -s ilv e r ra tio s fo r these d is t r ic t s .
The V irg in ia
C ity d i s t r i c t , with the lowest c o p p e r-to -s ilv e r r a tio is possibly the
56
shallowest level of epithermal m in e ra liza tio n in the Tobacco Root
region, though s t i l l deep when compared to base-m etal-free epithermal
s ilv e r-g o ld deposits.
The m in e ra liza tio n in the V irg in ia C ity d is t r ic t is s im ila r to
some o f the epithermal deposits o f Eimon and A nctil (1981) and
Buchanan (1981) including Comstock (Nevada), Finlandia (P eru ), and
Guanajuato (Mexico).
This s im ila r ity helps support the author's
in te rp re ta tio n of ore genesis.
The follow ing is a comparison of hypogene m in e ra liza tio n in
the U.S. Grant and the Comstock Lode.
Throughout the Comstock Lode,
dominant ore minerals (in order o f t h e ir abundance) are chalcop yrite,
s p h a le rite , galena, a rg e n tite , and p y r ite , with v a ria b le amounts of
ruby s ilv e rs and (pale yellow ) gold in a gangue of (dominantly)
quartz ( B astin, 1922).
In the U.S. Grant 3 -le v e l dominant ore
minerals (in order of abundance) are p y r ite , s p h a le rite , and galena,
with minor amounts o f chalcopyrite and te tra h e d rite , and rare hematite
and (b rig h t-y e llo w ) gold in a quartz gangue (Table 2 ) .
Microscopic
study of ores from the Comstock Lode suggest contemporaneous deposi­
tio n of the ore constituents during a single m in e ra liza tio n event,
however, in some ores p y rite is e a r lie r than, other m e ta llic minerals
( B astin, 1922).
M in e ra liza tio n in the U.S. Grant 3 -le v e l is the
product o f a two-stage event where quartz and (auriferous ?) p y rite
are followed by q u artz, s p h a le rite , galena, c h a lco p yrite, te tra h e d rite ,
hem atite, and gold (? ).
This comparison c le a rly depicts the strong
s im ila r ity in ore mineralogy and sequence o f deposition between the
Comstock Lode and the U.S. Grant 3 - le v e l.
57
Furthermore, Eimon and A nctil (1981) c ite (not in th e ir te x t but
in a fig u re o f epithermal g o ld -s ilv e r deposits in Montana) V irg in ia
C ity , N o rris , Mineral H ill
(p a rt o f the Pony d i s t r ic t ) and other
g o ld -s ilv e r deposits in the Tobacco Root precious-metal mining region
as examples o f epithermal precious-metal deposits (F ig . 1 9 ).
o FLATHEAD
o LI TTLE ROCKIES
© MAR YS VI LLE
B A S , $ ©ELKHORN
e RADERSBURG
MINERAL HILL
NORRIS
©VI RGINIA CI TY
Iadlson County
Figure 19.
Epithermal precious-metal deposits of Montana,
Modified from Eimon and Anctil (1981).
59
REFERENCES CITED
60
REFERENCES CITED
B astin, E .S ., 1922, Bonanza ores o f the Comstock Lode, V irg in ia
C ity , Nevada: United States Geological Survey B u lle tin 735,
p. 41-64.
Boyle, R.W., 1979, The geochemistry o f gold and it s deposits:
Geological Survey o f Canada B u lle tin 280, 584 p.
Buchanan, L .J ., 1981, Precious-metal deposits associated with volcanic
environments in the southwest, _in Dickinson, W.R., and Payne,
W.D., (e d s .), Relations of tectonics to ore deposits in the
southern C o rd ille ra : Arizona Geological Society D igest, V o l.
14, p. 237-262.
Burger, R.H. 3rd, 1969, S tructural evolution of the southwestern
Tobacco Root Mountains, Montana: Geolocial Society o f America
B u lle tin , V o l. 80, p. 1329-1342.
Chadwick, R .A ., 1981, Chronology and s tru ctu ral settin g of volcanism
in southwestern and central Montana: Montana Geological
Society 1981 Field Conference Guidebook, p. 301-310.
Cordua, W.S., 1973, Precambrian geology of the southern Tobacco Root
Mountains (a b s tra c t): D issertation A bstracts, V o l. 34,
p. 3305B.
Dalrymple, G .B ., and Lanphere, M .A., 1969, Potassium-argon dating:
San Francisco, W.H. Freeman and Company, 258 p.
Doe, B .R ., T i l l i n g , R . I . , Hedge, C .E ., and Klepper, M .R ., 1968,
Lead and strontium isotope studies of the Boulder b a th o lith ,
southwestern Montana: Economic Geology, V o l. 63, p. 884-906.
Eimon, P . I . , and A n c t il, R .J ., 1981, Epithermal precious-metal deposits
Northwest Mining Association Convention, December, 1981, Spokane,
Washington, 17 p.
Hadley, J .B ., 1969, Geologic map of the Varney quadrangle, Madison
County, Montana: United States Geological Survey Map GQ-814,
I :62,500 scale.
Johns, W.M., 1961, Geology and ore deposits of the southern Tidal
Wave mining d i s t r i c t , Madison County, Montana: Montana Bureau of
Mines and Geology B u lle tin 24, 53 p.
Krauskopf, K .B ., 1979, Introduction to geochemistry (2nd e d .):
York, McGraw-Hill, 617 p.
New
61
Lindgrens W., 1933., Mineral deposits (4th ed„):
H i l l , 930 p.
New York, McGraw’
Lorain, S .H ., 1937, Gold lode mines of the Tobacco Root Mountains,
Madison County, Montana: United States Bureau of Mines Information
C irc u la r no. 6972, 74 p.
Marvin, R .F ., and Dobson, S.W ., 1979, Radiometric ages: Compilation
B, United States Geological Survey: Isochron/West, no. 26,
p. 3-32.
M u e ller, P .A ., and Cordua, W.S., 1976, Rb-Sr whole rock age of
gneisses from the Horse Creek, area, Tobacco Root Mountains,
Montana: Isochron/West, no. 16, p. 33-36.
Ramdohr, P ., 1969, The ore minerals and t h e ir intergrowths:
Pergamon Press, 1175 p.
Oxford,
Ross, C .P ., Andrews, D. A ., and W itkind, I . J . (com pilers), 1955,
Geologic map o f Montana: United States Geological Survey,
1:500,000 scale.
Schaeffer, O .A ., and Zahringer, J. (com pilers), 1966, Potassiumargon dating: New York, S pringer-V erlag, 234 p.
Smith, J .L . , 1970, Petrology, mineralogy and chemistry o f the Tobacco
Root b a th o lith , Madison County, Montana (a b s tra c t): Disser­
ta tio n A bstract, V o l. 31, p. 5429B.
Stanton, R .L ., 1972, Ore petrology:
New York, McGraw-Hill, 713 p.
Tansley, W., Schafer, P .A ., and H art, L .H ., 1933, A geological
reconaissance o f the Tobacco Root Mountains, Madison County,
Montana: Montana Bureau of Mines and Geology Memoir 9 , 55 p.
T a y lo r, H .P ., 1973, 0 ^ / 0 ^ evidence fo r meteoric-hydrothermal
a lte ra tio n and ore deposition in the Tonopah, Comstock Lode,
, and G o ldfield mining d is t r ic t s , Nevada: Economic Geology,
V o l. 68, p. 747-764.
T i lli n g , R .J ., G o ttfrie d , D ., and Rowe, J . J . , 1973, Gold abundance
in igneous rocks: Bearing on gold m in e ra liza tio n : Economic
Geology, V o l. 68, p. 168-186.
V i t a l iano, C .J ., Kish, S ., and Towel I , D .G ., 1980, Potassium-Argon
dates and strontium isotopic values fo r rocks o f the Tobacco
Root b a th o lith , southwestern Montana: Isochrbn/West, no. 28,
p. 13-15.
62
V i t a l Iano9 C.J . , and Cordua, W .S., (com pilers), 1979, Geologic map
of the southern Tobacco Root Mountains, Madison County, Montana
Geological Society of America map and chart series'MC-31.,
1:62,500 scale.
W eir, K ., 1982, Maps showing geology and outcrops o f p art o f the
V irg in ia C ity and Alder quadrangles, Madison County, Montana:
United States Geological Survey map MF-1490, 1:12,000 and
I :4750 scales.
Whitney, J .A ., and Stormer, J.C . d r . , 1983, Igneous su lfid e s in the
Fish Canyon T u ff and the ro le of s u lfu r in c a lc a lk a lin e magma:
Geology, V o l. 11, no. 2, p. 99-102.
W inchell, A .N ., 1914, Mining d is tr ic ts of the D illo n quadrangle,
Montana, and adjacent areas: United States Geological Survey
B u lle tin 574.
Wooden, J .L . , V i t a l iano, C .J ., Koehler, S.W., and Ragland, P .C .,
1978, The la te Precambrian mafic dikes of the southern Tobacco
Root Mountains, Montana: geochemistry, Rb-Sr geochronology
and re la tio n s h ip to B elt tecto nics: Canadian Journal of Earth
Sciences, Vol. 15, no. 4 , p. 467-479.
APPENDICES
64
APPENDIX A
RADIOMETRIC DATES (K-Ar TECHNIQUE)■
SAMPLE
K40, ppm
***
B io tite from
unaltered
Powder Mag
pegmatite,
3 -le v e l U. S.
Grant Mine
***
M icrocline
(with minor
amount of
Precambrian
feldspar and
p e rth ite )
from alte red
wall rock, 3level U.S.
Grant Mine
**
Andesite
plug, El .
Fleeda Mine
(whole rock)
*Ar
40
9.507
Ar
40*
, ppm
1.366
Ar40* /K 40
0.1437
CONSTANTS
Xe, - 4.72x10"10/y r
AGE (m .y.)
1572 ± 51
X e = 0.585x 10- l 0 /y r
K40ZK = 1.22xlO-4 g.Zg.
13.797
0.2729
0.01978
X b = 4.72x10-10Zyr
311 ± 11 .
X e = 0.585x10-10Zyr
K40ZK = 1.22xlO-4 g.Zg.
2.567
0.0079
0.00307
refers to radiogenic argon
**Data from Ma fv in and Dob son (1979)
***D ata from Geochron Lab, Cambridge, Massachusetts
X 6 = 4.692x10-10Zyr
X e = 0.581 x l O-10Zyr .
K40ZK = 1.167xlO-4 g.Zg.
51.1 ± 1.2
66
APPENDIX B
GENERAL DATA ON SOME TOBACCO
ROOT PRECIOUS-METAL DEPOSITS
67
DISTRICT
PONY
NORRIS
MINE
ATTITUDE
HOST
DEPOSIT MORPHOLOGY
Mammoth
Strawber­
ry/Key­
stone
Boss TweedC lipper
Garnet
E-W;60S
E-W;5060N
PCgn
PCgn
QVS;LNS
QVS;TB
I
I
N60W;
40SW
N60E;
40-60SE
E-W;
Steep
N40E;
30NW
NE;
Steep
Varies
PCgn
Rplcm-diss
I
QM
QM-K
PCgn
QM
QVS;TB
Rplcmdiss
QVS; STR
I
I
QM
QVS;TB
I
QM
QVS;LNS
1,2
QM
PCgn
PCgh
PCgn
QVS;TB
QVS
QVS
QVS;LNS,STR
QM
QM
QVS;LNS
QVS;TB
I
I
I
I
I
I
I
PCgn
QVS;LNS,STR
I
PCgn
QVS;TB
I
PCgn-K
AS
PCgn
PCgn
PCgn
QVS;TB
PCgn
QVS;LNS,TB
PCgn
QVS;TB
PCgn
PCgn
PCgn
QVS;LNS
QVS
QVS;TB
QM
QVS;TB
PCgn
PCgn
QVS;TB,LNS
QVS;LNS,TB
A tla n tic
& P a c ific
Galena
Rosebud
Revenue
Group
Bull Moose
Boaz
Josephine
Montana
Boy
Madisonian
Norwegian
Washing­
ton subd is t r ic t
VIRGINIA
CITY
NS;25W
N40W;70NE
N40W;50NE
N25-40E;
35NW
N45E;.40NW
N20,40E;
v e rt.
Betty Mae
N45E;
45-60NW
New Deal/
N60-70W;
Heater
35-40SW
M issouriF la tMcKee
lyin g
Prospect
N45W;75NE
Alameda
N70E;50NW
U.S.Grant
N40-50E
35-50NW
El Fleeda
N40-50E
30-50NW
S t. Law­
N65E;
rence
50NW
Winnetka
N76E;50S
S ilv e r Bell N50E;45NW
Black Rock . N50E;
20-30NW
EastonN55W;
P a c ific
70NE
Fork
N35E;20SE
M arietta
N45E;
35-45SE
QVS;LNS
QVS;TB
QVS;LNS,STR1TB
REF.
1,2
■ 1,2
I
I
th is
paper
th is
paper
th is
paper
I
3
th is
paper
I
I
I
68
DISTRICT
MINE '
ATTITUDE
HOST.
DEPOSIT MORPHOLOGY
REF.
SHERIDAN
Lake Shore/
Gladstone
A g ita to r
Broadguag&r
Tamarack.
Red Pine
N30E;
80NW
N10E-50W
N30E;45NW
E-W;35NW
N20E;50NW
PCgn
QVS
1,2
PCm
PCm
QVS; LNS
Rplcm-diss
I
I
QVS;LNS
I
Lucky :
S trik e
Fairview
Highridge
E-W;
45-60N
PCm-K
PCgn
PCgn?
QVS
I
PCm
PCgn-K
SH
PCgn.
P1STR1Rplcm
QVS
I
I
QVS
I
I
PZLS
QVS
I
E-W;
20-25N
N35E
40-50NW
Varies
PZLS
Rpl cm
I
QM
QVS; TB
I
PZLS
PZLS
Rplcm
P,diss
I
1,2
TIDAL
WAVE
N30E;
70-80SE
N30E;70SE
Corncracker
Mountain
View
Hawkeye
RENOVA
Pete and
Joe
Strawn
Mayflower
KEY
D
2)
3)
4)
5)
6)
7)
PCgn = Precambrian gneiss
PCm = Precambrian marble
PZLS = Paleozoic Limestone
SH = Paleozoic Shale
QM = Late Cretaceous in tru s iv e rocks
AS = Andesite s i l l
-K = deposit occurs in contact between the two rock types
indicated
•
Deposit Morphology:
References:
I
2
3
I)
2)
3)
4)
5)
6)
7)
QVS = quartz vein system /structure
LNS = quartz lens (es)
STR = quartz stringers
TB = ta b u la r quartz bodies
Rplcm = replacement deposit
diss = disseminated deposit
P = pipe-shape deposit
= Lorain (1937)
= Tansley and others (1933)
= Boyer (personal comm.)
69
APPENDIX C
ORE AND GANGUE MINERALS OF TOBACCO ROOT
BASE- AND PRECIOUS-METAL DEPOSITS
70
MINERAL
DISTRICT
VIR,. CITY
A rgentite
iArsenop y rite
A zurite
B arite
Bornite
C alc ite
Cerargyr ite
Cerussite
Chalcanth ite
Chalcocite
Chal copyr ite
C h lo rite
Chrysoc o lla
Copper
Cuprite
Dolomite
F lo u rite
Galena
Gold
Hematite
Huebnerite .
Limonite
Magnetite
Malachite
Melaconite
M e la n te rite
M icrocline
Molybden­
it e
Nagyagite
P yrargyrite
P y rite
Pyrolusite
Quartz
S id e rite
S ilv e r
S phalerite
S tib n ite
Sylvanite
Tetrah ed rite
Modified from
PONY
X
SHERDN.
TDLWV.
NORRIS.
X
X
X
X
X
X
RENOVA
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
'
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X .
X
X
X
X
X
X
X
X
X
X
.
X.
X
X
X
X
X
%
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Winchell (1914)
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
.
71
APPENDIX D
LODE PRODUCTION FIGURES FOR TOBACCO ROOT
MINING DISTRICTS (1901-1935)
SILVER
GOLD
DISTRICT
TONNAGE.
405,775
PONY
VIRGINIA 162,501
CITY
66,633
SHERIDAN
47,376
TIDAL
WAVE
56,200
NORRIS*
>70%
o f pro­
duction
from
oxidized
ore
15,227
RENOVA
REGIONAL
753,712
TOTAL
T t l .oz
oz/ton
T t l .oz
oz/ton
Cu/Ag
Ag/Au
Cu (lb )
Pb (lb )
Zn (lb )
116,274
67,719
0.29
0.42
193,209
I ,107,064
0.48
6.81
6.79
0.05
1.66
16.34
1,311,137
55,449
25,840
30,432
0.39
0.64
184,457
143,000
2.77
3.02
0.82
1.05
7.14
4.70
151,661
150,109
1,445,503
2,801,820
294,452
21,873
54,716
0.97
124,345
2.21
0.48
**
2.27
60,162
154,368
NR ■
16,674
1.10
75,628
4.97
1.40
4.54
106,202
1,005,792
311,655
' 0.41
I ,699,003
2.25
1.08
5.45
1,834,720
5,878,101
.
292,600
178,018
NR
NR
NR
316,325
Tonnage of each d i s t r ic t is a sum (o f v a riab le proportion) of mining and d ire c t shipping (usually
oxidized) ores.
NR = no production reported
.
*Includes Washington s u b -d is tric t
**This Cu/Ag is lower than expected probably because >70% o f d is t r ic t production is from oxidized
ores th a t do not express primary metal content. The Boaz mine production is exclusively from
primary ore and Cu/Ag = 3 .96; Ag/Au = 0 .9 7 . Both ra tio s are congruent with the regional zoning
p a tte rn .
Data from Lorain (1937)
73
APPENDIX E
LODE PRODUCTION FIGURES FOR SELECTED
MINES OF THE TOBACCO ROOT REGION (1901-1935)
MINE
TONNAGE
Mammoth
(PONY)
(1905-1935)
Boss
Tweed-Cl ipper
(PONY)
(1904-1935)
Easton-Pacific
214,149
MILL
PRMY . .
171,415
MILL PRMY
(v.c.)
(1902-1935)
Revenue Group
(NORRIS)
(1901-1935)
Missouri-McKee
(NORRIS-Wash.
s u b d str.)
(1905-1935)
BroadgaugeTamarack
(SHRDN)
(1908-1935)
LakeshoreGladstone
(SHRDN)
(1910-1929)
Boaz
(NORRIS)
(1902-1935)
70,049
MILL PRMY
OXDZD
26,095
MILL
OXDZD
14,340
MILL
OXDZD
Ag oz
Au/ton
Ag/ton
53,377
113,938
0.25
0.53
44,223
49,748
0.26
0.29
22,091
759,148
0.32
10.84
15,842
16,878
0.61
0.64
1.07
NR
16,664
41,958
1 .16
2.92
2.52
NR
Au oz
Cu lb .
Cu/Ag
2.13
1,008,111
8.84
1.12
57,090
1.14
Ag/Au
34.4
NR
•"-J
4^
8,976
MILL
OXDZD
5,055
380
0.56
0.04
0.07
4,317
MILL
PRMY
2,436
7,288
0.56
1.68
3.0
1,959
MILL
PRMY
3,752
3,641
1 .92
1.85
0.97
NR
'
.
7,154
0.98
14,425
3.96
From Lorain (1937)
MILL = production from m illin g (low-grade) ores
OXDZD = production from oxidized ores (grade not im plied)
Numbers in parentheses show years of production
NR = no production reported
PRMY = production from primary
(hypogene) s u lfid e ore
APPENDIX F
LODE PRODUCTION FIGURES FOR SELECTED MINES
OF THE VIRGINIA CITY DISTRICT (1901-1935)
76
MINE
EastonP a c ific
(19021935)
U.S.
Grant
(19081926)
Alameda
(19061915)
Winnetka
(1909- '
1932)
M arietta
(1936)
DISTRICT
TOTAL
TONNAGE
70,049
MILL
PRMY/OXDZD
1,764
SHIP
PRMY .
. ?
SHIP
OXDZD
3,322
SHIP
OXDZD
MILL 811
PRMY
I ,0001,300
tons/
month
(1936)
MILL PRMY
162,501
Au oz
Au/tbn
Ag oz ..
Ag/Au
22,091
0.32
759,148
34.4
834
0.50
46,393
55.6
?
1.64
3,205
0.96
236
0.27
?
(3 3 .9 /to n )
4,747
?
?
1.49
?
?
0.27
67,719
20.6
0.42
.
( 1 .11/to n )
4.11
1 ,107,064
16.34
From Lorain (1937)
MILL = m illin g ores (low-grade)
SHIP = shipping ores (high-grade)
OXDZD = oxidized ores (no grade im p lie d ), production dominantly from
PRMY = primary hypogene (s u lfid e ) ores, production dominantly from
Nevada
City
V i r g i n i a Ci t y
BProspect
Location
shown
of
in
sample
figure
6
U . S x-JsG r a n t
H AIameda
V
y
St.
/ B Cornucopi
Lawrence
Black
Rock
Bell
Fork/
E a s t o nK
Vs W 1P a c i f i c
V K e a r s age
Plate
BEDROCK MAP OF THE VIRGINIA CITY MINING DISTRICT
I
Scale
_____________ O N E
1:24,000
ONE
Madison Coun t y , Montana
MILE
KILOMETER
Based
Vitaliano
Contour
interval
=
400
on
and
field
others
work
(I 979),
by
M.
Cole
Hadley
and
( 1 96 9),
data
and
from
Weir
Tansley
( 1 98 2),
and
others
( 1933).
feet
LEGEND
/
Tertiary
volcanic
Includes
some
Tertiary
volcanic
rocks
on
Quaternary
Early
Tertiary
gravels
in
stream
lower
Alder
gravels.
cont act
Gulch.
contact,
approximately
located
plug
fault
Late
Cretaceous
granitic
Browns
Gul ch
fault,
Stock
approximately
normal
Mississippian
Mission
Canyon
Limestone
thrust
fault,
fault,
inferred
r:
t-------
Cambrian
through
Mississippian
sedimentary
located
approximately
teeth
on
overturned
located,
upthrown
antiform
ball
on
downthrown
block
axis,
arrows
show
dip
rocks
shear zone
Precambrian
quartz-feldspar-mica
pegmatite
strike
and
dip of
vert i cal f ol i ati on
P recambrian m e t a - d o l o m i t e . Dots
probable stratig raphic continuity.
represent
Precambrian metamorphic rocks. Dominantly q u a r tz -fe ld s p a r
g n e i s s wi t h some h o r n b l e n d e - b i o t i t e gne i s s . Al so mi nor g a rn e t i f e r o u s g n e i s s , q u a r t z i t e , and m e t a - u l t r a m a f i c i n t r u s i v e s .
shaft
portal
mine, caved
stream
paved
road
foliation
block
(compositional
layering)
direction
of
limbs
/ y i 5 Cf
/L' 4)
r,' r j
P LA T E I
G EO LO G Y IN V I C I N I T Y
OF THE U.S. GRANT MINE
V i r gi n i a Ci ty Mining D i s t r i c t ,
Madison Co un t y, M on t an a
PLATE
a
base
Based
on
map
from
and
field
R
work
&
data
2
D
by
M .
Minerals,
from
Weir
Cole,
Virginia
City,
( 1982)
\
LEGEND
<7 <3
q
<1
Tertiary
volcanic
flows
T ertiary
volcanic
plug
on
Early
T ertiary
gravels
a
Tv
Precambrian
pegm atite
Cornucopia
Ti
Precambrian
m e ta-dolom ite
Precam brian
gneiss,
strike
and
vertical
contact
J---
contact
dip
of
dominantly
foliation
q ua rtzo felds pa thic
(compositional
layering)
foliation
with
dip
with
dip,
approxim ately
located
fault,
ball
on
block
SI
X jlX
normal
ap p ro xim a te
ve rtica l
downthrown
surface
projection
outcrop
of
mine
of
vein
system
with
dip
workings
portal
a
shaft
st ream
Contour
Interval
SCALE
20 0
=
200
feet
1:4 75 0
meters
I____________________I
4 X T
sh
P LA T E 2
1
2
3
4
5
6
7
8
9
PLAN MAP
OF
THE
U.S. GRANT MINE
NO. 3 LEVEL
LOCATION
SHOWN
OF
IN
SAMPLE
FIGURE
9
VIRGINIA CITY MINING DISTRICT
MADISON COUNTY, MONTANA
PLATE 3
S H E E T 1 OF 2
VS-2c:
elong
qtz
qtz
2 .5 -5 .0
G NEISS:
X /7 Z V
U N A L T E R E D OR S L I G H T L Y A L T E R E D , N O T
CRUSHED. H
B g n r H O R N B L E N D E - PPLLAA G I O HBgn:
STRIKE
AND
DI P
OF
FOLIATION
In
strs,
(COMPOSITIONAL
60
cm
crushed
cm
thick,
thick,
and
several
altered
occasional
2 .5 -5 .0
W R ,
suit
cm
several
pockets.
LAYERING)
CLA S E -B lO TlTE
HORIZONTAL
X X
X A X X
G N E I S S :
ALTERED
AND
FOLIATION
CRUSHED
X X % XX
BEARING
vj
V
'I V J
AND
PLUNGE
OF
LINEATION
(SMALL
ISOCLINAL
FOLD
AXES)
Xxx-XX
X- X - X - X
G N E ISS :
ALTERED,
BASALT
BRECCIA:
CRUSHED,
AND
OXIDIZED
FOLIATION
PORTAL
qtz:
v
v
V
V
C O N TA IN S
TERTIARY
VOLCANIC
PEGMATITE
I n:
I ns:
strs:
DIKE
elong:
suit:
APLITE
<
«
% RANDOMLY
F F F F
QUARTZ
LENS
LENSES
STRINGERS
ELONGATE
SULFIDE
ORIENTED
PEGMATITE
VEIN
T i-
ST:
PEGMATITE
FELDSPAR
STRUCTURE
p y r i t e
)
IN
VEIN
BLOCKS
STORE
R S : RAISE
STRUCTURE
WITH
etc.:
LOCATIONS
INFERRED
VEIN
WITH
~L L 1 V a o l IJ~ L I T H O L O G I C
STRUCTURE
FEATURES
ARE
WITH
1 CM
= 2.4
I-------- 1
DIP
CONTACT
REFERRED
TO
IN T E X T
DI P
BASE
PLANAR
d o m i n a n t l y
SCALE:
FAULT
ALL
(
WALLROCK
I , TL, H I ,
Tu
MINERALS
DIKE
WR:
# *
LINEATION
AND
PRECAM BRIAN
METAMORPHIC
CLASTS
1 CM TO
1 M A C R O S S , ALL S T R O N G L Y
O XIDIZED
V
WITH
AT
METERS
I---------------------- 1 6 . 0
METERS
COURTESY
OF
R & D MINERALS,
GEOLOGY
DI P
PROJECTED
MAP
WAIST
LEVEL
M
2.4
MODIFIED
BY
M.
VIRGINIA
COLE
CITY,
thick
suit
MONTANA
seams
c 6 7 < r_ f
17
PLAN MAP
OF
THE
U.S. GRANT MINE
NO. 3 LEVEL
VIRGINIA CITY MINING DISTRICT
!
MADISON COUNTY, MONTANA
P LAT E 3
S H E E T 2 OF 2
V S- 2 e :
t b Ir
qtz
90
cm
thick
some suit pocket s.
V S - 2 e : numerous
abundant
crushed
qtz
sirs <
suit
pockets,
and
altered
5
cm
thick
WR.
G
GNEISS:
U N A L T E R E D OR S L IG H T L Y
ALTERED, NOT
CRUSHED.
H B g n :H O R N B L E N D E -P L A G IO C L A S E -B IO T IT E .
H B G g n :H O R N B L E N D E -P L A G IO C L A S E -G A R N E T -B IO T IT E
V S - 2e:
elong
qtz
several
suit
Ins
suit
pockets,
and
strs
2 .5 -1 0
s e a m s j s. 5
crushed
cm
and
cm
thick,
altered
E
N
qtz:
I n:
D
QUARTZ
LENS
thick,
sparse
WR.
I ns:
%X %
X&X*
GNEISS:
ALTERED
AND
CRUSHED
strs:
V P
PEGMATITE
elong:
ORIENTED
PEGMATITE
F F F
FELDSPAR
PEGMATITE
IN
VEIN
BLOCKS
suit:
WR:
STRUCTURE
ST:
VEIN
STRINGERS
t b Ir : TABULAR
DIKE
RANDOMLY
LENSES
STRUCTURE
WITH
ELONGATE
SULFIDE
MINERALS
(
d o m i n a n t l y
p y r i t e
)
W ALLROCK
STORE
DIP
R S : RAISE
INFERRED
VEIN
STRUCTURE
.-Mfc
FAULT
T 75
WITH
SHEETS
VERTICAL
STRIKE
FAULT
AND
LINE
FOLIATION
MATCH
I , TL, m ,
DIP
DIP
WITH
PLANAR
LOCATIONS
SCALE:
OF
FOLIATION
BEARING
(SM ALL
ALL
etc.:
FEATURES
AND
IS O C LIN A L
ARE
(C O M P O S IT IO N A L
PLUNGE
FOLD
OF
I
REFERRED
CM = 2 . 4
LAYERING)
LINEATION
J
TO
IN T E X T
M
2.4
METERS
6.0
METERS
AXES)
PROJECTED
AT
WAIST
LEVEL
BASE
MAP
COURTESY
OF
R & D MINERALS,
GEOLOGY
MODIFIED
BY
VIRGINIA
M . COLE
CITY,
MONTANA
/L/ 3 7ST
f k 7 $ ^
PLAN
OF
MAP
THE
EL FLEEDA MINE
Portal
fault
with
dip;
dashed
where
inferred
S C A L E
1 : 2 4 0
vertical fault
6 met er s
contact
with
dip
ALL
PLANAR
INCLINED
Base
map
courtesy
Geology
/
/
/
' 75
I
I
z
FEATURES
WORKINGS
of R & D M i n e r a l s , V i r g i n i a
m o d i f i e d by M . C o l e
ARE
City
PROJECTED
AT
WAIST
DEPICTED
BY
DASHED
LEVEL
LINES
I
/
ST
/
MONTANA STATE UNIVERSITY LIBRARIES