Carbonate Replacement - Intrusion Related

8/8/2019 Carbonate Replacement - Intrusion Related

1/56

NEVADA BUREAU O F MINES AND GEOLOGY

BULLETIN 110

INTRUSION-RELATED,POLYMETALLIC CARBONATEREPLACEMENT DEPO SITSIN THE EUREKA DISTRICT,EUREKA COUNTY, NEVADA

Peter G . Vikre

Detailed descriptions of Cretaceous intrusion-re la ted , ska rn and po lymeta l l i c ca rbona tereplacement deposits in the Eureka district ,Eureka County, Nevada.


2/56

University and Community College System of Nevada1998

Board of RegentsJill Derby, Chair

Mark Alden Madison Graves I1

Shelley Berkley David L. PhillipsThalia Dondero Nancy PriceJam es Eardley Howard RosenbergDorothy Gallagher Tom Wiesner

Richard Jarvis, Chanc ellor

University of Nevada, Ren oJosep h N. Crowley, President

Mackay School of MinesJan e Long, Dean

Nevada Bureau of Mines and GeologyJona than G. Price, Dir ect ofita te Geologist

Scientific Research StaffEconomic Geology

Steph en B. Castor, Research GeologistJohn W. Erwin, Geophysicist (Emeritus)Liang-Chi Hsu, Research Mineralogist (Em eritus)Daphne D. La Pointe, Research GeologistKeith Papke, Industrial Minerals Geologist (Emeritus)

Jos eph V. Tingley, Research G eologistEngineering Geology

John W. Bell, Research Engineering GeologistCraig M. dePolo, Research GeologistAlan R. Ramelli, Rese arch G eologist

Environmental Geology and HydrogeologyDonald C. Helm, Adjunct Research ScientistP. Kyle House, Research GeologistPaul J. Lechler, Chief C hemist/Geochem istJames G. Rigby, Research GeologistLisa Shevenell, Research Hyd rogeo logist

Geologic MappingHarold F. Bonh am, Jr., Research Geologist (Emeritus)Jim Faulds, Research GeologistLarry J. Garside, Research GeologistChristopher D. Henry, Research Geologist

Edit ing: Dick Me euwigType se tt i ng: J ac k Hu r shGraphic s : Kr is P izar ro

Research and Administrative Support StaffAdministration and Publication Sales

Terri M. Garside, Management AssistantCheryl Steed, Ma nagem ent AssistantCharlotte Stock, Program Assistant

Analytical Laboratory, Sample Curation, and GeologicInformation

David Davis, Geologic Information SpecialistPaul J. Lechler, Chief C hemist/Geochem ist

Mario Desilets, Chemist an d Quality Assuran ce OfficerBret Pecoraro, Laboratory Assistant

Cartography, Publication Support, GeographicInformation Systems, and DatabasesRon Hess, Information System s Specialist/GIS SupervisorGary Johnso n, Information S ystem s Specialist

Janis Klimowicz, CartographerDick Meeuwig, EditorSusan 1. Tingley, Publication Manager/Senior Cartog rapher

Kris R. Pizarro, Cartographic SuperuisorRobert Chaney, Cartograp herJack Hursh, Jr., Cartograph er

First edit ion, f irst prin t ing, 1998, 500 c o p i e sPr in ted by: Be ar Indus tr ies, Sparks , Nevada

For sale by th e Nevada Bureau of Mines and Geology, Mail Sto p 1 7 8 , University of Nevada, R eno, NV 8 95 57 -0 08 8(702)784-6691x 2 e-mail: info@nbmg. nr. edu


3/56

NEVADA BUREAU O F MINES AN D GEOLOGY

BULLETIN 11 0

INTRUSION-REUTED,POLYMETALLIC CARBONATEREPLACEMENT DEPOSITSIN THE EUREKA DISTRICT,EUREKA COUNTY, NEVADA

Peter G . Vikre

MACKAY SCHOOL OF MINES

UNIVERSITYOF NEVADA

RENO


4/56

Abstract 4

Introduction 5

Geology 5

Structure 12Alteration 15

Alteration of Cretaceousintrusive rocks 15Contact zone alteration 16

Pyroxene+garnet skarn 16Hydrous skarn 18

Paragenesis of iron minerals 19Mineralogical correlation of skarn and replacement deposits 19Conditions of skarn formation 19

Alteration associated with replacem ent deposits 20Stratigraphic and structural controls of replacem ent deposits 20

Replacement and vein deposits 21Replacement deposits 2 1

Form s of replacement deposits 21Oxidized replacement deposits 22Sulfi de mineral paragene sis 22Vein depo sits 23

Sphalerite textures and compositions 23Mo notonically zoned sphalerite 26Finely banded sphalerite 26

Arseno pyrite com positions 27Metal zoning 27

Fluid physical characteristics, comp ositions, and comp onent sources 31Fluid inclusion microthermometry 31

Veins in granodiorite, hydrous skam , and marble 31Replacem ent deposits 35Quartz porphyry 38Veins south of Ru by Hill 38

Fluid isotope comp ositions 38Ruby Hill structure 39Isotope exchange haloes 39Or e and alteration fluids 42

Sulfur isotopes 44Temperatures 44Provenance 44

Lead isotopes 45

Formation of skam s and replacement deposits 45Eureka district 45Regional comparison 50

Acknowledgments 50

References 50

Appendix: Comp ositions of Eldorado Dolomite, Hamburg Dolomite, and the Ruby Hill granodiorite 52


5/56

Geog raphic and generalized structure map of the Eureka district showing locations mentioned in the text and areas offigures 2 and 6 6

Geolog ic structure, alteration, and magnetism of the area south of Ruby Hill10

Geologic and alteration sections (A-A', B-B') in the vicinity of Ruby Hill12

Section through the Granite Tunnel, Ruby Hill 14Compositions of garnet and pyroxene from contact zone alteration 16

Vein, skarn, and replacement d eposits at the north end of Prospect M ountain18

Geolog ic rib map of the 2250 level, Fad Shaft 21

Minor elements in pyrite 23

Section A-A' showing sphalerite compositions and stratigraphy, and compositional traverses across finely bandedsphalerite 30

Oxygen isotope depletion and minor element distribution in dolomite wall rocks enclosing sulfide replacement north of RHill 32

Part of section A-A' through Ruby Hill showing metal ratios in replacement deposits 34

Pb/Zn vs. Ag/Au in replacement deposits associated with granodiorite and quartz porphyry 35

Part of section A-A' through Ruby Hill showing oxygen isotope depletion haloes 42

Stable isotope plots of fluids and dolomites 43

Lead isotope ratio plots for sulfide minerals and Cretaceous feldspars 45

Cretaceous and present section (corresponding to A-A') through Ruby H ill 49

Lead isotope and lead grades of carbonate replacement deposits in the North American cordillera 5 0

Pre-Tertiary stratigraphy in the Eureka district 7

Radioisotopic ages of igneous and alteration minerals in the Eureka district9

Compositions of calc-silicate minerals in contact zone alteration 17

Sulfide mineral compositions 24

Mino r elements in pyrite, galena, and sphalerite 26

Sphalerite stratigraphy and composition 28

Fluid inclusion microthermometric d ata 36

Oxygen isotope analyses of dolomites in diamond drill holes 40Stable isotope compositions of fluid inclusions, and silicate, sulfide, and carbonate minerals 41

Sulfur isotope compositions of sulfide minerals 46

Sulfur isotope compositions of disseminated pyrite in Late Proterozoic-Early Cambrian rocks 48

Lead isotope compositions of sulfide minerals and feldspars 48


6/56

ABSTRACT

Th e Eureka district, Eureka County, Nev ada, includes carbonatereplacement and minor vein deposits in Middle and LateCambrian Eldorado Dolomite and Hamburg Dolomite fromwhich significant amo unts of lead, silver, gold, and zinc w ererecovered, and bulk-mineab le, low-grade gold deposits in LateCambrian and Ear ly Ordovic ian l imes tones . The lower

Paleozoic section at Eureka has been strongly deformed an ddismembered by regional folding, thrust faults, and normalfaults, both prior to and after mineralization, and intruded byLate Cretaceous granodiorite, the Ruby Hill stock, and byquartz porphyry. T he radioisotopic ages of th e intrusions areanalytically indistinguishable at -107 Ma, and both intrusionsare spatially an d temporally related to replacement deposits.Proximal to the contact with the Ruby H ill granodiorite stock,dolom ite and limestone are altered to an early skarn assemb lagedom ina ted by d io ps i d ic pyrox ene (Di ,, ,, Hd, ,, , J o,,,, ,grossularitic and andraditic garnets (Gr,, Al,, (Sp+Py+Uv),;Gr24 ( S ~ + P ~ + U v ) 2 ;r43.5 S ~ ,P~+U v)0.5) , ndsubordinate magneti te; distal ly, dolomite, l imestone, and

c a l c a r e o u s s h a l e a r e m a r b l e i z e d a n d h o r n f e l s e d .Pyroxene+garnet skarn is partly replaced by hydrous skarnconsisting of quartz, pyrrhotite, pyrite, amphibole, chlorite,serpentine, dolomite, calci te, and other hydrated si l icateminerals. Oth er intrusion-related alteration includes three veinsets and selvages in granodiorite and skarn that are composedof qu artz, microcline, sericite, and su lfide minerals. P arageneticrelations among magnetite, pyrrhotite, pyrite, and sphaleritein bo th ska rns and r ep l acem en t depos i t s i nd i ca t e loca lpermutations in the stabilities of Fe-0-S minerals within anoverall trend of progressive sulfidation, and suggest nearlysynchronous formation of pyroxene+garnet skarn, hydrousskam, veins in granodiorite and hydrous skam, and replacement

deposits following intrusion of the Ru by Hill granodiorite stock.Quartz porphyry, pervasively altered to quartz, sericite, andpyrite, is associated with no obvious contact alteration butcontains irregular sh ear and breccia zones filled with lead andzinc sulfides, pyrite, sericite, and quartz.

Carbonate replacement depo sits, which comprise the bulkof historical production, con sist of lenses, pods, and pipes ofpyrite, galena, sphalerite, several other sulfide and sulfosaltminerals, dolomite, subo rdinate calcite, and uncommon q uartzand barite. Sulfide minerals are nearly completely oxidized todepths exceeding -250 m (>80 0 feet). Metals in the importantoxidized replacement deposits on Ruby Hill are enrichedseveral times more than equivalent sulfide replacement deposits

down-faulted and preserved at depth north of Rub y Hill. Thesedeep sulfide replacement deposits are separated from the RubyHill stock by thrust and normal fau lts with hundreds of metersor more of displacement. The distribution of skarn assemb lagesand oxygen isotope depletion in dolomite indicates that thedisplaceme nt by both fault sets largely if not entirely postdatesintrusion of granodiorite, and that net displacement movedreplacement deposits southw ard an d apically, relative to thegranodiorite stock, from their original sites. Mo st replacementdeposits are confined to the low est dolomites in the Paleozoic

section but, other than semi-pervasive fracturing of dolomitestructural control of replacement de posits is subtle, contactswith enclosing dolomite are abrupt, and megasco pic wall-rockalteration is absent. Individual sulfide replacement masses areenveloped by shells of ore metals enrichment and oxygenisotope depletion that are up to several times the dimensio nsof the sulfide masses, markedly enlarging explo ration targetsize. In sulfide replacement deposits pyri te has replaced

hydrothermal dolomite and pyrrhoti te, remnants of whichremain as inclusions. Pyrite has been part ly replaced byspha l e r i t e , ga l ena , and o the r su l f i de mine ra l s , bu t t hepa ragenes i s o f mine ra l s younge r t ha t py r i t e i s l a rge lyindistinguishable. Silver in sulfide replacement deposits (andin productive quartz veins) occurs in various lead-antimo ny-a r s e n i c - c o p p e r - b i s m u t h s u l f o s a l t m i n e r a l s , a n d g o l d ,unobserved in sulfide replacement deposits, mostly reports withpyrite in metallurgical tests. All su lfide minerals ex cept galenaare finely fractured and cemented with calcite, which uponslight weathering renders sulfide masses extremely friable.Productive vein deposits in the district are composed of q uartzand sulfide and sulfosalt minerals, some of which are comm onto hydrous skarn and sulfide replacement d eposits.

Two sphaler i te tex tura l types , monotonica l ly zonedsphaleri te and finely banded sphaleri te, and variat ions inPbIZn and AgIAu in sulfide replacement dep osits, are spatiallyrelated to granodiorite and to quartz porphyry, indicating thatmetal deposition w as associated with the emp lacement of bothintrusions. Monotonically zoned sphalerite, which is spatiallyrelated to granodiorite, occurs in both hydrous skarn and insulfide replaceme nt deposits both north and south of Ruby Hill.Spha l e r i t e t ex tu re s and compos i t i ons , com mon su l f i demineralogy (pyrrhotite, pyrite, sphalerite, and galena) amonghydrous ska m, hydrous skarn v eins, and sulfide replacementdeposits, and paragenetically early pyrrhotite inclusions insulfide replacement dep osits further su pport the interpretationthat hydrous skarn and replacement depo sits are coeval withthe granodiorite intrusion.

S k a r n m i n e r a l a s s e m b l a g e s a n d f l u i d i n c l u s i o ncompositions (XCO,-0.07 5) indicate that pyroxene+garnetskarn formed above 470C, and hydrous skarn a t lowert empera tu re s , m in ima l ly 420C . Min imum en t r apmen ttemperature (pressure-corrected) and salinity ranges for fluidinclusions in hyd rous skarn (-340 to 385OC, NaCI,,= 6 to 13wt.%) and sulfide replacement deposit m inerals in the vicinityof Ruby Hill (-360 to 39SC, NaCI, < 3.5 to 10.4 wt.%),determined by microthermo metry, are broadly similar totemperatures derived from arsenopyrite+sphalerite+pyrite(+pyrrhotite) equilibria (averaging -320 and 3 lSC for hydrousskarn and sulfide replacement dep osits, respectively; maximumfor hydrous skarn is -385C) and, in part, to sulfur isotopefractionation temperature modes (350 and 270C for sulfidereplacement deposits). Th e differences between skarn mineralequi l ibr ia tempera tures and f lu id inc lus ion ent rapmenttemperatures, -25 to80C, are perhaps a result of insufficientpressure corrections. Pressures of- to 1.6 kilobars, determinedfrom H,O+CO,+NaCl fluid systematics and hornblend e AITgeobarometry, indicate that granodiorite emplacem ent andformation of skam s and sulfide replacement deposits occurred


7/56

at minimum dep ths of 3.8 to 6.1 km (2.3 to 3.7 miles). Hydrogenand oxygen isotope compositions of fluid inclusion watersextracted from skarn and sulfide replacement depo sit mineralsare compatible with Cretaceous meteoric water, mixed withincrements of magmatic water, that has exchanged varyingamounts of oxygen with carbonate wall rocks.

Paragenetic and stable isotope relat ionships betweenpyroxen e+garnet skarn, hydrou s skarn minerals, hydrothermal

dolom ite, and sulfide minerals in su lfide replacement depo sits,and Eldorado Dolom ite enclosing sulfide replacement depositsindicate that hydrothermal dolomite initially crystallized indistal replacement deposit sites from Cretaceo us meteoric waterthat partially exchanged oxygen with Cambrian dolomites.Economic quant i t ies of base and prec ious meta l su l f ideminerals, which replaced hydrothermal dolom ite and Cambriandolomites in the same distal sites, pyroxene+garnet skarn, andhydrous skarn minerals were deposited from mixtures ofmagmatic and partially exchanged meteoric water. Comparedto hydrous skarn water, sulfide replacement water, which alsocreated the metal enrichment and oxygen isotope depletionshells enclosing sulfide replacement deposits , has similar

deuterium compositions but lighter (more exchanged) oxygenisotope compositions. Differences in the degree of oxygenisotope exchange am ong w aters that deposited hy drothermaldolomite, sulfide replacement deposit minerals, and hydrousskarn minera ls may ref lec t , in addi t ion to meteor ic andmagm atic water sources, contrasts in dolomite permeabilities.

Su l fu r i so tope compos i t i ons o f py r i t e , ga l ena , andsphalerite in veins in granodiorite, hydro us skarn and veins inhydrous skarn, and in replacement deposits are consistentlyheavy, averaging -14%0, and lead isotope compositions ofsulfides and igneou s feldspar are uniform and radiogenic.Apermissive source of most sulfur and lead at Eureka is subjacentLate Proterozoic-Early Cambrian siliciclastic rocks which

contain disseminated pyrite with similarly heavy sulfur andradiogenic lead isotope compositions. Small components ofisotopically lighter sulfur and less radiogen ic lead are required,and both were presum ably derived from older Proterozoic rocksor magmas .

Th e Eureka mining district lies immediately south and w est ofthe town of Eu reka, Eureka Coun ty, Nevada (fig. 1). It includesbase and precious metal deposits in lower Paleozoic carbonaterocks that have been complexly deformed and displaced by

several tectonic ev ents, and altered by two intrusions. Recordedproduction is approximately 285,000 tonnes (3 13,000 tons) oflead, 6,360 tonnes (7,000 tons) of zinc, 91 0 tonnes (1,000 tons)of copper, 53.6 tonnes (1.65 million ounces) of gold and 1,266tonnes (3 9 million oun ces) of silver from -1.7 million tonnesof ore during the period 1866-1964 (Nolan, 19 62; Nolan andHun t, 1968). These totals are understated ow ing to incompleterecords prior to 19 01 when m uch of the production took place,and mos t zinc was never recovered. The major m etal depositsare classified as limestone replacement by Nolan (1962), andwill be referred to as replacement depo sits herein. Quartz vein,

and in recent years, disseminated gold deposits have also beenmined in the district.

The district measures about 22 km (13 miles) in a north-south direction and is 1.7 to 3.3km (1 to 2 miles) wide (fig.I) .Most replacement ore production occurred at Ruby Hill and areserve of several millions of tons of sulfide replacement existsin a block down -dropped by faulting north of Ruby Hill (Lov e1966). Smaller replacemen t deposits are situated north of R uby

Hill on Mineral Point and south of Ruby Hill on the north endof Prospect Mountain. Disseminated gold at the Windfall andRatto Canyon deposits was mined in the 1970s and 1980s. In1993, Homestake Mining Company announced the discoveryof a significant dissemin ated gold resource, the West Archim edesdeposit, east of Mineral Point (Dilles and others, 1996).

The spatial relat ions of metal deposits to intrusions,alteration of igneous and carbon ate rocks, elevated temperaturesof ore formation, metal zoning, and isotopic compositions ofminerals and fluids indicate that igneous rocks are the heatsource for the replacement and quartz vein deposits. This paperfocuses on the stratigraphy, structure, alteration, and mineraldeposits near Ruby Hill, where mine workings and drill holes

provide sufficient exposures to reconstruct the tectonic andhydrothermal history of the district. Current interest in thedistrict derives from the high grades of base metal o res, thesignificant amounts of gold and silver in those ores, and thedisseminated gold depos its found in several parts of the district

Early geologic examinations (King, 187 8; Hague, 1883;1892; Curtis, 1884; Walcott, 1884) are largely su pplanted bythe work of Nolan and others (1956), Nolan (1 962), and Nolanand Hunt (1968); Nolan's (1962) geolog ic map of the districtremains unsurpassed in scope and accuracy. Publicationsconcerned with mining and exploration (Vanderburg, 1938;Binyon, 1946; Sharp, 19 47; Johnso n, 1958; Miesch and No lan,1958), and drill ho le cores, logs, and map s in th e possession of

the Ruby Hi l l Mining Company and Homestake MiningCompany were essent ia l to th is s tudy. Surface mapping,underground mapping, and drill hole logging, which providedgeological and structural data on the form and distribution ofaltered and mineralized rocks, were supplemented by severalanaly t ic methods tha t he lped def ine minera l zoning andevo lu t i on o f hyd ro the rma l even t s a t Eu reka . E l ec t ronmicroprobe analyses, X-ray diffractometry, fluid inclusionmicrothermometry, analyses of both stable and radiogenicisotopes, and major and minor elem ent analyses were used todetermine the processes that altered rocks a nd form ed ore.

More than 2,100 m (>7,000 feet) of Cambrian rocks underliethe Eureka district, although, because of deformation, in noplace is the complete Camb rian section intact. Camb rian rockscompr i s e , f rom o ldes t t o younges t , P rospec t Moun ta inQ u a r t z i t e , P i o c h e S h a l e , E l d o r a d o D o l o m i t e , G e d d e sL i m e s t o n e , S e c r et C a n y o n S h a l e , H a m b u rg D o l o m i t e ,Dun derberg Shale, and the Windfall Formation (table1 ; Nolanand others, 1956). The Cambrian siliciclastic and carbonaterocks are overlain by Ordovician through Early Cretaceous


8/56

Figure 1. Map of the Eureka district, Eureka County, Nevada, showing generalized pre-Tertia rystratigraphy, major structural elements (after Nolan, 1962; Nolan and others, 1971, 1974), geographicpoints referred to in the text, and the areas of figures 2 and 6.

6


9/56

Table 1. Pre-Tertiary stratigraphic section of the Eureka district (modified slightly from Nolan and Hunt, 1968).

Cretaceous quartz porphyry-

sills and dikesgranodiorite

-intrusive stock south of Ruby Hill--

ntrusive con tact -

Cretaceous Newark Canyon Formation 2002 freshwater conglomerate, sandstone, grit, shale, and limestone

unconformity

Permian Carbon Ridge Formation 1,000f thin-bedded sandy and silty limestone; some includedsandstone and dark shale.

unconformity-Ely Limestone absent -ate Mississippian Diamond Peak Formation 0-300 conglomerate, limestone and sandstone

Chainman Shale 500f exposed black shale with thin interbedded sandstone

break in section

Middle and Devils Gate Limestone 500k exposed thick-bedded imestone, locally dolomitized.

Late Devonian

Break in section-Nevada, Lone Mountain, and Roberts Mountains Formations absent

Late Ordovician Hanson Creek Formation 300k exposed dark-gray to black dolomite

unconformity?

Middle to Eureka Quartzite 300Late(?) Ordovician

thick-bedded vitreous quartzite

unconformity--

arly and Middle Ordovician Pogonip Group 1,600-1,830 chiefly cherty thick-bedded limestone at top and bottom;thinner bedded shaly limestone in middle

Late Cambrian Bullwhacker Member 400 thin-bedded sandy limestone

Windfall

---ormation Catlin Member 250 interbedded massive limestone, some cherty, and thin sandy

limestone

Dunderberg Shale 265 fissile brown shale with interbedded thin nodular limestone

Middle and Late Cambrian Hamburg Dolomite 1,000 massively bedded dolomite; some limestone at base

Clarks Spring Member 425-450 thin-bedded platy and silty limestone,with yellow or redargillaceous partings

Secret Canyon Shale--

ower Shale Member220-225 fissile shale at surface; green siltstone underground

Middle Cambrian Geddes Limestone 330 dark-blue o black limestone; beds -8-30 cm thick;some black chert --ldorado Dolomite 2,500k massive gray to dark dolomite; some limestone at or near base

Early Cambrian Pioche Shale 400-500 micaceous khaki-colored shale; some interbedded sandstoneand limestone--

Prospect Mountain 1,700k fractured gray quartzite weathering pink or brown; a few thin

Quartzite (base not exposed) interbeds of shale


10/56

sedimentary strata, Oligocene volcanic rocks, and Quaternarycolluvium. Middle and Late Cambrian E ldorado Dolomite andHamburg Dolomite are the important host rocks for thecarbonate replacement and Windfall gold deposits. Lesseramounts of replacement ore and some gold deposits occur incarbonate rocks of the Late Cambrian W indfall Formation andEarly and Middle Ordovician Pogonip Group.

Eldorado Dolomite, the host rock for the replacement

deposits in and north of Ruby Hill, includes two varieties ofdolomite and minor rem nants of fine-grained and well-beddedlimestone. On e variety of dolomite is blue-gray, massive, andthick bedded. The seco nd variety is lighter gray, coarser grained,d isplays l i t t le texture , and predominant ly encloses thereplacement deposits. Both varieties of dolomite are finelyfractured, and both are interpreted to have been recrystallized,in part hydrothermally, from the fine-grained and well-beddedlimestone (Wheeler and Lemmon, 1939; Nolan and others,1956; Nolan and H unt, 1968). The Hamburg Dolomite is similarin appearance to Eldorado Dolom ite and is also finely fracturedin the vicinity of replacement deposits . Some HamburgDolomite is also thought to be a product of hydrothermal

alteration (Nolan and others, 1 956). Ham burg Dolomite ismarbleized and altered topyroxene+ garnet skarn and hydrousskarn south of Ruby Hill and on Mineral Hill, and containsreplacemen t deposits in the vicinity of Prospect Mountain andon Mineral Point (fig. 1). Major oxide com positions of EldoradoDolomi t e and Hamburg Do lomi t e a r e c lo se t o t ha t o fstoichiometric dolomite (Appendix).

Several igneous rocks intrude the Paleoz oic section. A bodyof Cretaceous granodiorite (classified after LeMaitre, 1989;quartz diorite of N olan, 1962; granodiorite porphyry of Lan glois,1971), known as the Ruby Hill stock, is exposed on the southslope of Ruby H ill and on the north end of Mineral H ill (table 2,fig. 2A). Although granod iorite is in close proximity to the large

replacem ent dep osits of Ruby Hill, it is separated from them byseveral postmineralization faults. North of Ruby Hill towardMineral Po int (fig. 1 ) an irregular mass of qua rtz porphyry, alsoCretaceous in age (table 2), crops out near a second gro up ofsmaller replacement deposits on Mineral Point.

Although poorly exp osed at the surface, granodiorite wasencountered in many drill holes. Drill holes, coupled with thedistribution of contact alteration and configu ration of total fieldmagnetic contours south of Ruby Hill (figs. 2A and 2B),indicate that granodiorite has lateral dimensions of hundredsto thousands of meters. The minimum vertical dimension,known from drill holes, exceeds 670 m (fig. 3), giving theintrusion stock- like proportions. Granodiorite con sists of

approximately 45 to 50% plagioclase (andesine), 24 to 27%quartz, 15% K-feldspar, 10% biotite and chlorite, and accessoryhornblende, magn etite, epidote, sphen e, apatite, and titaniumoxide (Langlois, 1971 Appen dix). Locally it has subporphyriticto glomeroporphyritic texture with hypidiomorphic 1- to 5-mm phenocrysts of quartz, plagioclase, bioti te, and rarehornblende set in a slightly finer-grained quartz, plagioclaseand K-feldspar matrix. K-Ar ages of biotite phenocrysts ingranodiorite average 100.6 Ma (table 2). An40Ar/39A r iotitephenocryst age is 106.3 c0.8 Ma (table 2). An 4"Ar/'9Ar age ofbiotite+feldspar (granodiorite) endoskarn is 105.0f0.5 Ma

(biotite), and an 40Ar/39Ar ge of a quartz+m uscovite vein inhydrous skarn is 107.9 M.5 Ma (muscovite, table 2).

Quartz porphyry intrudes Late Cambrian DunderbergShale and Windfall Formation limestone s in the vicinity of theBullwacker, Holly, and Bowman Mines on Mineral Point(fig. 1). In drill holes quartz porphyry also intrudes Ham burgDolomite, Windfall Formation, and Ordovician P ogonip Grou procks (table 1) north and west of the Fad S haft (fig. 3). Based

on surface exposures and drill hole intercepts, quartz porphyrymasses have s i l l - l ike a t t i tudes and dimensions . Latera ldimensions of quartz porphyry sills are up to hundreds o f meters(Nolan, 1962), and vertical dimensions are tens of m eters.

Quartz porphyry is composed of up to 10% rounded quartzphenocrysts, 3 to 5 mm in dimension, and 20 to 30% relictfeldspar phenocrysts, 1 to 3 mm in dimension, that have beencompletely replaced by quartz, sericite, kaolinite, calcite, andsul f ide minera ls . Severa l percent chlor i t ized b io t i te (?)phenoc rys t s occu r i n co re f rom some d r i l l ho l e s . Thephenocry sts are set in a fine-grained to mic rocry stalline matrixof quartz, sericite, kaolinite, calcite, and sulfide minerals. Inplaces kaolinite forms a significant proportion of the rock

(Langlois, 1971). Sulfide minerals are predom inantly pyrite,but in the vicinity of mine workings and in drill core nearreplacement sulfide deposits, sphalerite and galena , in additionto pyrite, are disseminated in the matrix and occur in thinbreccia and fault zones. Coarser-grained sericite (muscovite)occurs with sulfide minerals in some breccia and fau lt zones,and in selvages to those zones. No unaltered quartz porphyryhas been observed in the Eureka district, and quartz pheno crystsare the only remaining major primary mineral. Chemicalanalyses of least altered samples show less than 63% silica(Langlois , 1971) , sugge s t ing an in termedia te to maf iccomposition for the original rock. Quartz porphyry K-Armuscovite ages average 100.3 Ma , and two 40ArPy Ar uscovite

ages average 107.5 Ma (table 2).The ages determined for each intrusion by both methodsare analytically ind istinguishable (table 2), but it is not clearwhy an -6 Ma age difference for the same rocks resulted fromthe two dating techniques. If the mo re precise 40Ar/19Ar gesare accepted, then granodiorite and quartz porphyry are both-107 Ma.

Rem nants of a variety of Oligocene eruptive roc ks, rangingin composition from rhyo lite to basalt, are scattered througho utthe district, and dikes related to extrusive rocks have beenencountered in mine workings and drill holes (EUD 91- 13, fig.2A , table 2), and are locally exposed. Mo st eruptive rocks areunaltered but porphyritic dikes altered to clay minerals and

alunite occur in the W indfall (R ustler Pit) and Ratto CanyonM i n e s ( f i g . l ) , a n d a t t h e H a m b u rg M i n e ( c a r b o n a t ereplacement deposit). The se altered dikes texturally resembleradioisotopically dated Oligocene volcanic rocks (Blake andothers, 1975), and two altered d ikes penetrated in drill holeshave Oligocene ages (731-890, Shadow Cany on, and EDT93-35, Zulu Canyon, fig. 2A, table 2). The altered hornblende-feldspar porphyry dike in Zulu Canyon is unusual in that thephenocrysts and matrix magne tite contain abundant pyrrhotiteinclusions. Altered and presumed Oligocene dikes in twodisseminated gold deposits, coupled with common structural


11/56

Table 2. Radioisotopic ages of Eureka district intrusive rocks and alteration minerals. Rock and mineral identificationsare as given by the source; muscovite is considered compositionally equivalent to coarser-grained sericite. Some drillhole (DDH) locations are shown on figures 2A and 3.

Loc ation Percent K-Ar Age(Ma)Sample no. Rock type (DDH no.-ft .) Mineral K,O% ' 4 0 ~ r ollgm ' 4 0 ~ r 4 0 ~ r l ? g ~ rMa ) Source

quartzdiorite

Rogers Tunneldump

Ruby Hill stock

igneous 8.31biotite

igneousbiotiteigneous 6.76biotite

quartzmonzoniteporphyry

DDH RH-713,1388-98

granodiorite DDH 7131180

igneous 8.63biotite

granodiorite DDH 7061384.5

vein selvage 9.75muscovite

granodiorite(biotite+feldsparendoskarn)

DDH 7201952

igneous(?) 4.87biotite

hydrous skarrn

(quartz+musco-vite vein)

DDH 718

794

alteration

muscovite

porphyriticdike

hydrothermal(?)muscovite

granodioriteporphyry

DDH 713-1705 vein selvage(?) 10.79microcline

granodioriteporphyry

DDH 713-41 1 vein selvage (?) 6.73sericite

granodiorite DDH 713-1565 vein selvage 13.80K-feldspar

DDH 729 alteration 5.60muscovite+quartz

quartzporphyry

DDH 729 alteration 6.67muscovite

quartzporphyry

quartzporphyry

BullwackerDump

alteration 8.36muscovite

(alteration) 7.72sericite (%K )

andesiteporphyryBullwacker Sill

MineralPoint(?)

quartz-feldsparporphyry dike

Locan Shaft igneousbiotite

altered felsicdike

DDH 731Shadow Canyon

igneousbiotite

hornblende-feldspar dike

Zulu Canyon igneous 0.990hornblende

* 4 0 ~ rr ad io ge ni c 4 0 ~ r

1 Marvin and Cole (1978)2 Armstrong (1970)3 Silberman and McKee (1971)4 This paper; analyses by E.H. McK ee5 Langlois (1971)6 This paper, 40ArP9Ar ge (Ma) plateau ages and total gas ages, analyses by

New Mexico Geochronological Research Laboratory (Socorro, NM).7 M.L. Silberman


12/56

DDH 728

DDH 79-1.000 FEET

DDH 725

300 METERS

Fault, showing dip; dottedGarnet, pyroxene, mt, hydrous where concealedskarn (qtz, amph, chlorite, mica,serpentine, feldspar, po, py) I . A .

Dolomite, calcite, hornfels; mino r 'Ontact

< Strike and dip of beddingy Strike and dip of foliation

A Breccia+ kaolinite; minor po,

Figure 2 A . Map of lithologies, structures, and contact zone alteration assemblages surrounding the Ruby Hill stock. Also shownare drill holes (DDH) from which subsurface data were obtained, geographic points referred o in the text, and lines of sections (figs.3, 9A, 11, and 13). Lithologic designations are mostly those used by Nolan (1962): Kgd = granodiorite, Cpm = Prospect MountainQuartzite, Ce = Eldorado Dolomite, C g = Geddes Limestone, Csc = Secret Canyon Shale, C h = Hamburg Dolomite, qtz = quartz,amph = amphibole, py = pyrite, mt = magnetite, mo = molybdenite. Location of the area of figure 2A i s shown on figure 1.


13/56

contours

X.

-,.-- - - -

Figure 28. Total field magnetic intensity (Teslas) contoured over the area of figure 2 A (data from A S A R C O files).

11


14/56

----

I12.1002,100 ,/-- Total field

/ /12,3002,300 \magnetic intensity

4 12,500 /( / \ 12,500- 4 4 -

\12,700

\ - - - - - - - - 12,9002,900

Figure 3. Sections A-A' and B-B' (on facing page) in the vicinity of the Ruby Hill stock, showing lithologies,contact zone alteration assem bla ges , structures, replacement deposits, drill holes, and mine workings fromwhich subsurface data were obtained, and total field magnetic intensity profiles (Teslas) . Filled (black) sha pe s

in Ruby Hill and beaded on drill holes are oxide and sulfide replacement deposits, respectively. Se e figure 2Afor locations of sections , most lithologic designations, and descript ions of contact zone alteration asse mbla ges.Other lithologic designations: Kgp = quartz porphyry, Op = Pogonip Group, Tv = Tertiary volcanic rocks.

controls of disseminated gold deposits, several gold-mineralized faults , and altered dikes (described below) suggestthat disseminated gold deposits and other gold mineralizedstructures are Oligocene.

Rocks in the Eureka district have been folded and faultedseveral times and the portion of the district near Ruby Hill isespecially structurally complex. Prior to the Cretaceous,Paleozoic rocks were folded, tilted, overturned, and displacedby at least three low-angle thrust faults (Nolan, 1962). Fartherto the south other thrust faults, including the Hoosac fault, havelaterally displaced Paleozoic rocks (Nolan, 1962; Nolan andHunt, 1968; Nolan and others, 1971, 1974; fig. 1). Intrusion ofgranodiori te in the Cretaceous further deformed the Paleozoicsection. Basin-and-range normal faulting has segmented thedistrict and Paleozoic rocks to the east into a series of

alternating, north-trending horsts and grabens; nearly all of theEureka district is confined to the westernmost horst (figs. 1

and 2A). The group of normal faults (Cave Canyon, Sharp,Roberts Tunnel, and Spring Valley faults) along the west sideof the Eureka district horst trends northerly and separates earlyCambrian rocks from Devonian Devils Gate Limestone to thewest, indicating a displacement, barring tectonicforeshortening, of more than 750 m (>2,500 feet). Theeasternmost normal fault, the Jackson fault (and a branch, theZulu Canyon fault), has even greater apparent displacement,based on stratigraphy. The Jackson fault t rends northerly andjuxtaposes Eldorado Dolomite and Hamburg Dolomite on thewest with Ordovician Pogonip Group carbonate rocks to theeast, indicating a total displacement on the order of 1,500 m(-5,000 feet). However, because of tectonic foreshortening,total displacement is probably less. These normal faults definethe boundaries of the horst of Paleozoic rocks which containsRuby Hill and nearly all metal deposits in the district (figs. 1and 2A), and these faults, at least in part, coincide with Tertiary


15/56

BWEST

Elevation(feet)

11,900

ye- - -+-------

Total field .\\0 magnetic intensity '\

12,100 0 \\

00 /

\

U)\

4 \

i 2.300\

'\I- \

12,500'\ '\

.

12,700

MINERALHILL ZULU

CANYON

-11,900

-12,100

-12,300

-12,500

-12,700

BEAST

Elevation(meters)

basin-and-range tectonism. The w esternmost group of normalfaults appear to postdate ore, but parts of the Jackson fault(between the Phoenix S haft and upper Austin Canyon, fig. 2A)and Roberts Tunnel fault (fig. I) are gold-mineralized.

Cam brian rocks in the vicinity of Ruby Hill can be dividedinto two structural domains, one north of the Ruby Hillgranodiorite stock, and on e south of the Ruby Hill granodioritestock (fig. 2A). In the north structural domain the normalstratigraphic section, includin g the Prospect Mou ntain Quartzitethrough the Windfall Formation, has been displaced laterallyalong two parallel thrust faults, the Buckeye and Champion,and vertically by the northwest-trending Ruby Hill, Martin,and Office normal faults (Nolan, 1962; fig.3). The northdomain is bounded on the east by the Jackson normal fault andon the west by the Cave Cany on and S harp faults.

South of the Ruby Hill granodiorite stock, overturnedCambrian rocks define the south domain. Hamburg D olomite,Secret Canyon S hale, Ged des Limestone, Eldorado Dolomite,and Prospect Mou ntain Q uartzite comprise a north-plunging

antiform, cored by Ham burg D olom ite of Mineral H ill, that isbisected on the east limb by the north-south Zu lu Cany on faultand on the west limb by the Robe rts Tunnel fault (Nolan, 1962;fig. 2A). The south structural domain is also bounded on theeast and west by the Jackson and S harp-Cave Can yon faults,respectively. A separate sheet of Prospect M ountain Quartzitehas been displaced over the antiform along the low-angleBuckeye thrust fault which continues northward and dipsbeneath Ruby Hill (figs. 2A and3). Th e Buckeye thrust faultand the Ruby Hill s tock largely obscure the relat ionshipbetween the two structural domains. Th e Champion thru st faultseparates Prospect Mou ntain Quartzite and Eldorado Do lomitein Ruby Hill. Both thrust faults are exposed in the GraniteTunnel (fig. 4).

Th e sequence of structural even ts in the vicinity of RubyHill can be determined by the distribution of hydrothermallyaltered and mineralized rocks. Contact zone ska rn assemblagesthat replace Hamburg Dolomite adjacent to the Ruby Hillgranodiorite stock formed in overturned rocks of the south


16/56

I Ranae of unaltered e. 1,p6sp ect Mountain I

Range of unaltered cpm ,Prospect Mountain

StopeF

quartz, minor kaolinite,muscovite

100 FEET

Sample site30 METERS

Figure 4. Section looking west through the Granite Tunnel in Ruby Hill, showing the Buckeye and Champion thrust faults,contact zone alteration assemblages, and oxygen isotope depletion associated with the Ruby Hill stock and replacement depositsin Ruby Hill . The location of Granite Tunnel is shown on figure 2A and isotope data are from table 8. The ranges of unexchangedor less extensively exchanged pm and e (rectangles) are derived from samples on Prospect Mountain.

domain antiform. Thus, south domain rocks were overturnedand thrust-faulted into position prior to intrusion of thegranodiorite, and later offset by the high-angle Zulu Canyonand Roberts Tunnel faults. Subsequently, the north domain ofnormally sequenced strata was thrust southward over the southdomain antiform and Ruby Hill stock along the Buckeye,Champion, and subparallel thrust faults exposed in the GraniteTunnel (fig. 4) . Two lobes of the Buckeye thrust sheet,composed of Prospect Mountain Quartzite and Pioche Shale,now flank the Ruby Hill stock to the east and west, partlycovering the overturned south domain antiform of Mineral Hilland the northern traces of the Zulu Canyon and Roberts Tunnelfaults (fig. 2A).

Some movement along the Buckeye thrust fault alsopredates intrusion of the Ruby Hill stock as both ProspectMountain Quartzite and Pioche Shale in the hanging wallexposed on Ruby Hill and in the Granite Tunnel are intenselybrecciated and altered to sericite and clay minerals (fig. 4).Quartz-sulfide veins that cut both the Ruby Hill stock andcontact zone skarns also occur in hanging-wall quartzite of theBuckeye thrust fault. However, gouge in and planarity of the

Buckeye thrust fault suggests an increment ofpostmineralization displacement. The Champion thrust faultapparently experienced post-Cretaceous movement as itseparates unaltered Eldorado Dolomite from altered quartziteand shale of the Buckeye thrust fault hanging wall. The absenceof calc-silicate alteration of Eldorado Dolomite coupled withoxygen isotope analyses of footwall and hanging-wall rocksin the Granite Tunnel (discussed in a later section) permit theinterpretat ion that most, if not all movement on the Championthrust fault postdated intrusion of the granodiorite.

The direction of movement along the postmineralizationChampion thrust fault can be surmised from characteristics ofaltered rocks. Drill holes north of Ruby Hill show that the northstructural domain extends at least several miles to the north andis displaced only by northwest-trending normal faul ts, such asthe Ruby Hill, Martin, and Office faults, which successivelydown-drop Cambrian strata and replacement deposits to the northas much as 1,000 m (-3,300 feet; fig. 3). Thermal and mineralzoning in skam and replacement deposits both north and southof the Ruby Hill stock indicate that the replacement depositsformed north of Ruby Hill in north domain rocks that have since


17/56

been displaced southward, perhaps hund reds, but probably notthousands of meters, along the Ch ampion thrust fault.

Init ial displacement on the Jackson fault apparentlyoccurred after intrusion of the Ruby Hill stock because skarnand replacement deposits are cut off by the fault and noevidence of alteration or mineralization has been found bydril l ing east of the fault . Gold occurs in a si l icif ied andferruginous section of north-trending Jackson fault between

the Old Jackson Shaft and prospects at the head of AustinCanyon (fig. 2A). This gold may be associated with the golddeposits which are localized by north-south faults at theWindfall and Ratto Canyon Mines 5 and 10km (3 and 6 miles)south of Rub y Hill, respectively (fig. 1). Altered, north-strikingporphyritic d ikes that texturally resemble Oligocene eruptiverocks in the district (Blake and others, 1975) occur in theWindfall and Ratto Canyon Mines, and altered, porphyriticdikes were intersected in drill holes in Zulu and Austin Cany ons(f ig . 2A) . The d ikes in the dr i l l holes have Ol igoceneradioisotopic ages (table 2) suggesting a genetic relationshipbetween disseminated gold deposits and certain Oligoceneeruptive rocks. The north-south Ro berts Tunnel fault (figs. 1

and ZA), a high-angle fault parallel to the horst-boundingJackson and Sharp faults, may also have been mineralized withgold in the Oligocene.

ALTERATION

Alteration o f Cam brian carbon ate and siliciclastic rocks, andCretaceous intrusive rocks near Ruby Hill and on Mineral Pointcan be spatially divided into: (1) alteration of Cretaceousintrusive rocks, (2) alteration of the contact zone betweenCretaceous intrusive and Cambrian carbon ate and siliciclasticrocks, an d , (3) alteration associated with replacement deposits.

Alteration of granodiorite at Ruby Hill is largely confined toveins and vein selv ages in the intrusion but also includes minordevelopment of endoskarn evident in diamond dri l l holes(DDH) south of the Ruby Hill stock. Some veins similar tothose in the intrusion also cu t adjacent altered sedimentary rocksof the contact zone. Contact zone alteration of Cambriancarbon ate and siliciclastic rocks is restricted to an annular zonebordering granodiorite, but carbonate rocks are extensivelyrecrystallized to marble and minor hornfels farther from thestock. Replacement deposits are distal to but not contiguouswith contact zone alteration. They consist of replacement ofdolomite by hydrothermal dolom ite, minor calcite, and sulfideminerals (now entirely to slightly oxidized) with no obvious

wall-rock alteration, but m easurable element exchange withwall-rock d olomite.

Alteration of Cretaceous Intrusive Rocks

Although g ranodiorite crops out only in two small exposuressouth of Ruby Hill, a much larger subsurface extent is indicatedby the distribution of alteration m inerals, by the configurationof magnetic con tours, and by pen etrations of the intrusion innine rotary and diamond drill holes (figs. 2 and 3). Several of

these drill holes cut thick sections of contact z one alterationand granodiorite below surface oxidation and the followingdescriptions are based largely on drill core.

Vein-related alteration of granodiorite is the product of threedistinct vein types: (1) quartz-microcline-pyrite veins, (2) quartz-sericite-pyrite veins, and (3) carbonate veins. The first typeconsists of quartz, microcline, and pyrite veins, with subo rdinateamounts of calcite, sericite and moly bdenite, that are enclosed

by 2- to 8-cm-thick inner selvages of pink microcline, sericite,unaltered biotite, chlorite, pyrite, and molybdenite. Th ese veinsare up to 1 0 cm thick and their abundance increases with depth.Th e inner selvage microcline replaces primary quartz, feldspars,and mos t b io t i t e , and has l a rge ly des t royed o r ig ina lsubporphyritic texture, producing a xenomo rphic texture. Smallamounts of calcite, epidote, and clinozoisite locally replaceplagioclase, and some biotite is partially altered to chlorite,calcite, epidote, magnetite, sphene, and pyrite in the innerselvages. Outer selvages are light gray to green and compo sedof sericitized feldspars, chloritized biotite, relict quartz, andpyrite. Within outer selvages the original subporph yritic textureof the rock has largely been retained.

The second ve in type , cons is t ing of quar tz , ser ic i te(muscovite) , pyri te, and rare molybdenite, is f lanked byselvages of these minerals which replace primary feldspa rs andbiotite up to 3 0 cm from the vein. The selvages are divisibleinto an inner zone of pale green, coarse grained sericite, quartz,pyrite, and molybdenite in which original rock texture has beentotally destroyed, and an outer zone of part ly serici t izedplagioclase and quartz. Primary feldspars and biotite mayremain in the o uter zone. Q uartz-sericite-pyrite veins are usuallyat most a few centimeters wide but locally attain a thickness ofapproximately 1 0cm. Su lfide minerals locally make u p severalpercent of both quartz-microcline-pyrite andquartz-sericite-pyrite veins.

Sericite-dominated veins (type 2) cut qu artz-microcline-pyrite veins (type l) , and sericite completely replaces selvagemicrocline w here the two vein types are superimposed. Largevolumes of granodiorite consist of coalescing selvages of thetwo vein types. Unaltered granodiori te is locally presentbetween widely spaced veins. Quartz, pyrite, and mo lybdeniteveins, apparently coeval with either quartz-microcline-pyrite(type 1) or quartz-sericite-pyrite (type 2) veins in granodiorite,cut skarn in the contact zone and probably correlate withsulfidation of skarn as described below. S om e of these veinsextend into Prospect Mou ntain Quartzite and Pio che Shale onthe north side of Ruby Hill (fig.4), although their abundancein the siliciclastic rocks is much lower that in the intrusion.

Primary biotite from unaltered granodiorite, type 2 vein selvagesericite (muscovite), granodiorite endoskarn biotite, and skarnmuscovite provided the Cretaceous radioisotopic ages ofgranodiorite (table 2).

Calcite and dolomite veins, the third vein ty pe, cut bothtype 1 and type 2 veins, and carbonate minerals locally pervadeolder vein selvages, partly replacing microcline. Carbonateveins diminish in frequency with depth.

Quartz porphyry is pervasively altered to qu artz, sericite,kaolinite, calcite, and pyrite, as described above. Th e contactsin drill holes are invariably sheared and n o alteration is obviou s


18/56

in adjacent carbonate rocks. Marble and hornfels are locallydeveloped along quartz porphyry contacts on Mineral Pointaccording to Langlo is (197 1). Matrix an d vein selvage sericite(muscovite) provided the Cretaceous radioisotopic ages ofquartz porphyry (table 2).

Contact Zone Alteration

Contac t zone a l te ra t ion cons is t s of two proximal skarnassemblages, pyroxene+garnet skarn and hydrous skarn, anddistal marble with lesser amou nts of hornfels. The proximala l t e ra t i on a s semb lages r e su lt ed f rom me ta soma t ism o fHamburg Dolom ite along the contact with granodiorite southof Ruby Hill. Cambrian siliciclastic rocks near the granodioritestock were also altered but, b ecause of protolith compositions,no calc-silicate minerals formed in them. Distal contact zonealteration consists of extensive v olumes of m arble and minoramounts of hornfels that formed from Cambrian carbonaterocks and shale (Hamb urg Dolomite, Geddes Limestone, andSecret Canyon Shale) south of Ruby Hill. The distribution ofb o t h s k a r n a s s e m b l a g e s , m a r b l e , a n d h o r n f e l s v a r i e sconsiderably in lateral extent and intensity, and is stronglycontrolled by litholog y as well as distance from the stock (figs.2A and 3). Skarn assemblages south of Ruby Hill are distributedannularly around intrusion exposures or subsurface apophyses(encountered in drill ho les), and range in thickness from a fewtens to more than 300 m (> 1,000 feet). Skarn assemblages onthe north and west side s of the Ruby Hill stock, where they aretruncated by the Buck eye thrust fault, are more contiguous thanon the south side of the stock. Marble and hornfels extend nearly1 km (-3,300 feet) south of the Ruby Hill stock. The followingdescription of contact zone alteration is based on both drillcore and su rface exposures.

Pyroxene+garnet skarn

P y r o x e n e + g a r n e t s k a r n c o n s i s t s o f m a s s i v e t o p a r t i a lreplacemen t of carbonate rocks proxim al to the Ruby Hill stockby pyroxenes, garnets, and lesser amounts of magneti te.Pyroxene+garnet skarn that completely replaces HamburgDolom ite adjacent to the north and west sides of the Ruby Hillstoc k cons ists of diopsid ic pyro xen e (average Di,,,, Hd,,, Jo,,,),grossularitic garnet (average Gr,, Al,, (Sp+ Py+ Uv) ,), andsubordinate augite and clinoenstatite (figs. 2A, 3, and5; table3). These minerals reflect the addition of iron , aluminum, andsilica to Hamburg Dolomite, which originally consisted ofsubequal amounts of ca lc ium and magnes ium carbonate(Appendix), during intrusion of granodiorite. On the north andwest sides of the stock, magnetite, pyrrhotite, and pyrite arecomm only intergrown with diopside and grossularite. Subequ alamou nts of magnetite and pyrrhotite also occu r locally in nearlymassive pods that measure up to several meters in dimension.These pods, which are absent south of the stock, are largelyresponsible for the strong and convoluted magnetic gradientson the north and west sides of the stock (figs. 2B and 3), andemph asize the lateral asy mmetry of contact zone alteration.Magnetite and pyrrhotite are much less abundant south of theRuby H ill stock exposure.

(A ) Almandite

(720-1 9398; 1943;1946, Zulu Canyon)

120 analyses)

Charter Tunnel) (DDH 730-61)v v v \/

Andradite Grossularite

(B) Wollastonite

Augite

\@ (DDH 71 8.794 Phoenix ShaftJPigeonite

V Clinoenstatite V \L/ V Clinoferrosillite

EnStatiteIEDT 92-06D. Prospect Mountain Tunnel dump) Ferrosillite

Figure 5. (A) Garnet compositions, expres sed a s mole percente n d m e m b e r g r o s s u l a r it e , a l m a n d i n e , a n d a n d r a d i t e .(B) Pyroxene composit ions, expressed a s mole percent endmem ber wollastonite, ensta tite, and ferrosillite, determined bym i c r o p r o b e, in c o n t a c t z o n e a l t e ra t i o n a s s e m b l a g e s .Representative compositions a re given on table3.

In two areas about 500 to 700 m (-1,650 to 2,300 feet)s o u t h w e s t a n d s o u t h e a s t o f t h e R u b y H i l l s t o c k ,p y r o x e n e + g a rn e t s k a rn c o m p l e t e l y r e p l ac e s H a m b u rgDolomite, Geddes Limestone, and Eldorado Dolom ite (fig. 2A),and similar to pyroxene+garnet skarn north of the Ruby Hillstock, bedding in all three formations is entirely obliterated.These skarn exposures are characterized by diopside andandraditic garnet (avera ge Gr,, Ad,, (Sp+Py +Uv), and Gr,,,,Ad,, Sp , (Py+Uv),,) (figs. 2A , 3, and 5; table 3), and bothoccurrences relate to the relatively shallow subsurface presence


19/56

Table 3. Representative oxide compositi ons (in weight percent oxide) and average end member comp ositi ons (in molepercent) of pyroxenes, garnets, an d amphibole from diam ond drill holes in skarn near the Ruby Hill granodiorit e stock, o nMineral Hill, and on Prospect Mountain. Analyses are by microp robe (T. Solberg, Virginia Tech. Univ.; P Hansley andT. Cookro, Petrographic Consultants, Inc.). DDH (diamond drill hole) 720 is n Zulu Canyon, southeast o f Ruby Hill; 718 i sa the Phoenix Shaft; 730 is o n Mineral Hill; EDT92-06D4 is from the Prospect Mountain Tunnel dump (figs. 2A a nd 3).

A. Representative oxid e comp osit ions

PYROXENES GARNETS

Mineral Diopside Augite Clinoenstatite Grossularite Andradite Grossularite- AmphiboleAndradite

Location 720-1 939AT4 71 8-988c EDT92-06D4 720-1 943~1 730-61 n 730-61 cc 730-91 3-4

SiO,Ti O2

A1203FeO

trio,MnONiO

MgOCaONa20

K2O

TOTAL 100.92 98.70 95.28 97.09 100.44 99.88 99.09

B. End member comp ositi ons (based on 6 oxygens for pyroxene and 12 oxygens for garnets)

Cations

Averaae percent end member

diopside 75.1hedenbergite 24.3johannsenite 0.6

numberanalysesaveraged 37

andraditegrossularite 72almandine 27pyrope+spessartine 1

+uvarovite


20/56

of granodiorite (fig. 3). With increasing distance from the stockmargin, pyroxene+garnet metasomatism of Hamburg Dolomiteand Geddes Limestone grades into marble, and marble gradesinto unaltered dolomite and limestone. Metasomatism andmetamorphism of Secret Canyon Shale, however, extends muchfarther south than marbleization of adjacent Hamburg Dolomiteand Geddes Limestone. One area of skarn and hornfels in SecretCanyon Shale west of Mineral Hill is separated by hundreds

of meters of marble from the sub-annular zones ofpyroxene+garnet skarn that are nearly contiguous with the RubyHill stock (fig. 2A). Distal metamorphic assemblages in SecretCanyon Shale both west and east of Mineral Hill includerecrystallized dolomite and calcite layered with anorthite,quartz, and minor diopside, minerals which reflect thealternating silty limestone and argillaceous shale layers whichcompose the thin-bedded Secret Canyon Shale (Nolan, 1962).However, because of the overall close spatial association tothe Ruby Hill stock, metasomatized and metamorphosed SecretCanyon Shale are considered contemporaneous withpyroxene+garnet skarn.

Large volumes of Hamburg Dolomite, Geddes Limestone,

and the silty limestone layers of the Secret Canyon Shale southof the Ruby Hill stock are recrystallized to marble (fig. 2A).Marble consists predominantly of pure, coarse-grained (up to0.5 cm), white dolomite, although calcitic marble is locallyabundant. Within marbleized Hamburg Dolomite nearest the

u

south side of the stock, irregular pods and lenses ofpyroxene+garnet skarn (with maximum dimensions of tens ofmeters), are largely replaced by hydrous skarn. The close spatialassociation of marble with pyroxene+garnet skam, and distal,generally annular distribution of marble about the Ruby Hillstock suggests that pyroxene+garnet skarn and marble arecontemporaneous.

Because of subtle mineralogical changes, the distributionand paragenesis of contact alteration of Pioche Shale andProspect Mountain Quartzite were not determined in detail.Minerals in Pioche Shale that may be contemporaneous withRuby Hill stock skarns are sericite, kaolinite, and iron oxides(figs. 2A and 4). Several thousand meters south of the RubyHill stock Pioche Shale is composed of quartz, albite,phlogopite, and minor kaolinite and calcite, a modalcomposition closer to its original mineralogy (Nolan, 1962).Other than sericite and kaolinite in shaly beds near the top ofthe unit, no changes in original mineralogy of the ProspectMountain Quartzite are discernible near the stock (figs. 2Aand 4). However, 1 to 3% pyrite, and lesser amounts ofmagnetite and pyrrhotite (observed in drill core) have beenadded to Prospect Mountain Quartzite for up to hundreds ofmeters south of Ruby Hill, and iron oxides, resulting fromweathering of these hypogene minerals, color much of thequartzite south of Ruby Hill red-orange. Pyrite, at least, appearsto have been widely introduced in the relatively chemicallyinert quartzite as a result of intrusion of the Ruby Hill stock.

(Prospect Mountain Tunnel, Ruby Hill Tunnel, Eureka Tunnel)and from dumps near the top of Prospect Mountain (Wabash,Silver Connor) consist of clinoenstatite, magnetite, serpentine,pyrrhotite, quartz, pyrite, and minor amounts of chalcopyriteand molybdenite that replace Hamburg Dolomite (figs. 1 and6, table 3). These skarn occurrences and the distribution ofmagnetic contours relative to outcropping and drilledgranodiorite to the north (figs. 2A and 3) indicate the subsurface

presence of granodiorite beneath Prospect Mountain.

Hydrous skarn

Hydrous skarn consists of quartz, hydrated silicates, and sulfideminerals that both replace pyroxene+garnet skarn and marble,and occur in irregular veins that cut pyroxene+garnet skarn.Hydrous skarn that replaces pyroxene+garnet skarn developedin both carbonate rocks (Hamburg Dolomite, GeddesLimestone) and in silty limestone-shale (Secret Canyon Shale)near the Ruby Hill stock consists of, in addition to quartz, oneor more generations of amphibole (magnesio-hornblende andtremolite), chlorite (pycnochlorite, diabardite, clinochlore),pyrrhotite, pyrite, serpentine (antigorite), molybdenite, andmuscovite (fig. 2A; table 3). In pyroxene+garnet skarn theseminerals entirely replace pyroxene and garnet, rim pyroxeneand garnet, and mimic crystal boundaries and cleavages of both

Replacement sulfides on dum ps 2000 FEETA Calc-silicates on dum ps

CuOx-AsOx-Sb0x veinsh-2Pl-d

0 300 600 METERS

~ l t h o u g h o skarn, marble, or granodiorite are exposed Figure 6. Map showing the location of sulfide replacementon the surface, minor amounts of skarn are found on mine deposits, quartz veins, and calc-silicate minerals on mine dumpsdumps from workings in the northern part of Prospect on the north end of Prospect Mountain. Location of the ar ea ofMountain. Dump samples of skarn from the Diamond Mine figure 6 is shown on figure 1


21/56

minerals. Hydrous skarn minerals that formed more distal tothe Ruby Hill stock in Secret Canyon Shale near Mineral Hill(fig. 2A) include chlorite, tremolite, and kaolinite. Hydrousskarn minerals reflect the addition, or redistribution of water,sulfur, and iron to pyroxene+garnet skarn, Hamburg Dolomite,Geddes Limestone, and Secret Canyon Shale.

Because of several common minerals and distribution, thethree vein types that occur in granodiorite and veins that cut

pyroxene+garnet skarn, most abundant on the north side ofthe Ruby Hill stock, are considered to be coeval with hydrousskarn. Although no individual veins were traced fromgranodiorite to skarn, veins in pyroxene+garnet skarn, up to15 cm thick, consist of quartz+pyritebolybdenite, the samemineralogy that comprises type 1 veins in granodiorite. Seamsand lenses of molybdenite with minor amounts of quartz alsocut pyroxene+garnet skarn. Microcline and sericite, fairlyabundant in veins in granodiorite, are virtually absent in veinsin skarn, although minor amounts of muscovite in one sampleof hydrous skarn were locally abundant enough to yield aradioisotopic age (sample 718-794, 107.W0.6 Ma, table 2).Other type 1 quartz veins that cut pyroxene+garnet skarn

contain epidote and ripidolite.The lenses of pyroxene+garnet skarn in marbleized

Hamburg Dolomite south of the Ruby Hill stock are largelyreplaced by hydrous skarn assemblages consisting of quartz,biotite, chlorite, and sulfide minerals, and veins of calcite,chlorite, epidote, tremolite, and serpentine (fig. 2A). Other thinzones of calc-silicate minerals, micas, and iron sulfides arescattered irregularly in marble.

Paragenesis of iron minerals

The relationships among magnetite and sulfide minerals inpyroxene+garnet skarn, in hydrous skarn, and in replacement

deposits, are complex, and differ on the north and south sidesof the Ruby Hill stock. In general, the textural relations amongmagnetite, pyrrhotite, and pyrite on the north side of the stockare indicative of progressive sulfidation of magnetite, althoughlocal perturbations in magnetite and pyrrhotite stabilities, andin pyrrhotite and pyrite stabilities, are evident. In someoccurrences magnetite coexists in apparent textural equilibriumwith pyrrhotite, while in most occurrences pyrrhotite clearlyhas either replaced magnetite completely or partially alongcrystal margins and internal fractures. Other examples ofreplacement are evident where pyrrhotite and rare laths ofmolybdenite partially replace diopsidefmagnetite, preservingdiopside crystal forms and cleavages.

In most occurrences where pyrrhotite coexists with pyriteon the north side of the stock, pyrite and minor amounts ofmolybdenite, chalcopyrite, sphalerite, quartz, and calcitereplace pyrrhotite. Thus, most, if not all contact zone pyritecan be assigned to hydrous skarn mineral assemblages.Replacement of pyrrhotite by pyrite commences along internalfractures and progresses to a fine-grained porous aggregate ofpyrite grains which results from volume reduction during thenearly isochemical replacement (Murowchick, 1992). Notuncommonly, however, pyrrhotite and pyrite contain inclusions

and thin veins of each other, in addition to remnant inclusionsof magnetite. This mutual veining and replacement suggeststhat some sulfidation of magnetite took place nearthe isobarically divariant stability point of mag-netite+pyrrhotite+pyrite.

On the south side of the stock where magnetite is muchless abundant, pyrrhotite and pyrite occur disseminated inmarble and in lenses of hydrous skarn minerals in marble.

Pyrrhotite in hydrous skarn lenses ranges from massive todisseminated, and is intergrown with calcite of several grainsizes in irregular layers and bands. In these occurrencespyrrhotite is invariably replaced by pyrite and minor amountsof sphalerite, chalcopyrite, barite, and other rare sulfideminerals (galena, stibnite, tennantite, seligmannite, andboulangerite; table 4). Thus, both pyrrhotite and pyrite southof the stock occur in hydrous skarn assemblages.

Mineralogical correlation of skarn and replacementdeposits

In sulfide replacement deposits north of Ruby Hill, wherepyrite, sphalerite, and galena are the dominant sulfide minerals,inclusions of pyrrhotite in pyrite and sphalerite are notuncommon, suggesting that pyrrhotite was the initial sulfidemineral deposited in replacement sites. All sulfide mineralsassociated with sphalerite in hydrous skarn, including severalof the rare sulfide minerals in hydrous skarn south of the RubyHill stock (e.g., seligmannite and boulangerite), occur invarying abundances in both replacement deposits and in veindeposits south of the Ruby Hill stock (Mineral Hill and ProspectMountain (table 4; figs. 1 and 6)). The composition ofsphalerite, another unifying factor among hydrous skarn andreplacement deposits is discussed below. All of theserelationships indicate that pyrrhotite was stable during both

pyroxene+garnet and hydrous skarn formation, and during theinitial stage of distal sulfide replacement of dolomite. Althoughmore than one generation of pyrrhotite may be present, theformation of pyroxene+garnet skarn, hydrous skam, and sulfidereplacement deposits appear to be nearly synchronous, and thetransition from pyroxene+garnet to hydrous skarn toreplacement of carbonate rocks by sulfide minerals is largelymarked by increased sulfur activity.

Conditions of skarn formation

Based on experimental data, the equilibrium temperatures ofcalc-silicate mineral assemblages commonly found in skarnscan be determined if the amount of CO, (expressed as molefraction CO,, or XCO,) in fluid present du ring mineralprecipitation is known (e.g. Einaudi and others, 1981, figs. 5and 6). At Ruby Hill, XCO, measured in fluid inclusions inveins in granodiorite and in hydrous skarn minerals averagesabout 0.075 (discussed in a later section). At this CO, abundanceand 1 to 2 kilobars total pressure, tremolite and antigorite ofhydrous skarn formed at temperatures greater than about 420C.If XCO, = 0.075 is valid for earlier pyroxene+garnet skarn,then garnet formed at or above -470C.


22/56

Alteration Associated with Replacement Deposits component of gold ore in the West Archimedes deposit (-3 km

Alteration of dolomite adjacent to sultide replacement depositsnorth of the Ruby Hill stock is not visually obvious. Thecontacts of sulfide replacement deposits, regardless of form,with enclosing dolomite are abrupt as is characteristic of mostcarbonate replacement deposits (Titley, 1993). Aggregates ofmassive sulfide or oxide minerals formed from sulfide mineralsduring weathering virtually terminate on a grain boundary scalewithin dolomite, and adjacent carbonate crystals show noevidence of metasomatism or recrystallization. However, minorelement enrichment and oxygen isotope depletion in dolomitewall rocks (discussed in subsequent sections) constituteanalytically measurable wall-rock alteration.

Minor amounts of hydrothermal dolomite and subordinatecalcite are internal to replacement deposits. Both dolomite andcalcite are generally white and relatively coarse-grained, andcontain abundant primary fluid inclusions in growth planes.The carbonate minerals are intergrown with metal sulfide andoxide minerals or occur in homogeneous masses measuringcentimeters to tens of centimeters in dimension. No other non-sulfide minerals are present in sulfide replacement deposits asonly dolomite and calcite were stable, or incompletely replaced,during sulfide mineralization. The release of CO, duringcarbonate dissolution and sulfide mineral replacement, coupledwith relatively low temperatures, suppressed the precipitationof wollastonite and other calc-silicate minerals in Eurekadistrict replacement deposits. Calc-silicate minerals are alsoabsent or uncommon in other sulfide replacement deposits incarbonate rocks (e.g. Megaw and others, 1988; Thompson andArehart, 1990; Titley, 1993).

Within the replacement deposits thin (micrometers wide)veins and films of calcite cement sulfide minerals along grainboundaries and internal fractures. Replacement depositsencountered north of the Ruby Hill fault were extremely friableand flowed in drill holes and on the Fad Shaft 2250 (foot) level(Binyon, 1946; Love, 1966) . Microscopic examination ofnumerous samples from the 2250 level shows that the friabilityresults from dissolution of the intergranular calcite. Calcitedissolution is probably effected by modern groundwater thatis slightly acidic from incipient sulfide oxidation, as the sulfidereplacement deposits are saturated with groundwater under highpressure (an unresolved deterrent to mining; Love, 1966).Pyrite, because of its brittleness relative to galena and sphalerite,is nearly always comminuted and cemented by calcite.Sphalerite and galena are not nearly as extensively cementedby calcite, and some masses of these sulfides remain competent.Coupled with the tendency of galena to deform and not fracture.the low solubility of galena in the surface weatheringenvironment explains its presence on mine dumps and in theoxidized ores of Ruby Hill when all other sulfides arecompletely oxidized.

Fine-grained silica, or jasperoid, commonly marginal orinternal to carbonate replacement deposits elsewhere in theAmerican cordillera (Titley, 1993), is absent at Ruby Hill andan extremely rare component of Prospect Mountainreplacement deposits. However, silica alteration occurs withgold mineralization north of Ruby Hill. Jasperoid is a

north of ~ u b ~ - ~ i l l ,jg. 1; Dilles and others, 1996). About 1km north of Ruby Hill on the north side of Adams Hill irregularmasses of fine-grained quartz that replace Hamburg Dolomitenear the contact with overlying Dunderberg Shale were minedfor gold and silver, with lesser amounts of lead (Silver Lick,Bowman, Cyanide, Wales, and Helen Mines). Relative metalabundances in the Adams Hill ores differ markedly from those

in Ruby Hill, Mineral Point and Prospect Mountain replacementdeposits. Ore in them commonly contained >1 ozlton (34 gltonne) gold, tens of odton silver, and only small amounts oflead. Nolan (1962) attributed the precious metal mineralizationin this silica to zoning based on a north-south progression ofincreasing precious metal and decreasing lead content of oreson Mineral Point. Since no quartz occurred with replacementores in the district, the age, paragenetic position, andsignificance of Adams Hill ores are not known. Examinationof fluid inclusions in quartz north ofAdams Hill reveals liquid1vapor indicative of low temperature homogenization.

Stratigraphic and Structural Controls ofReplacement DepositsAll significant replacement deposits in the Eureka district occurwithin two Cambrian carbonate rock formations, EldoradoDolomite and Hamburg Dolomite, and the most importantdeposits are in Eldorado Dolomite. Confinement of carbonatereplacement deposits to a small number of stratigraphic unitswithin thick sedimentary rock sections is common in otherdistricts in the North American cordillera, and large volumesof apparently similar carbonate rocks are not mineralized(Megaw and others, 1988; Titley, 1993). A regional control ofreplacement deposits in Eldorado Dolomite may be thepresence of relatively impermeable, overlying Secret CanyonShale. Secret Canyon Shale may have constituted an aquacludeas evidenced by the water damming ability of the shaleencountered in sinking the Fad Shaft (Love, 1966). For reasonsthat are unclear, most replacement deposits near Ruby Hill aresituated in the lower part of the Eldorado Dolomite. Distinctivelithofacies within Eldorado Dolomite that were preferentiallyreplaced by sulfide minerals have not been identified, and anylocal stratigraphic control of sulfide mineral replacement isextremely subtle. Both dolomites are positioned at the bottomof a thick sequence of lower Paleozoic carbonate rocks, andmay have been preferential sites for replacement because oftheir proximity to potential, subjacent source rocks (discussedin a later section).

A primary structural control on the distribution ofreplacement deposits i n the Eureka district was thought to bezones of fractures in otherwise massive Eldorado and HamburgDolomites (Nolan, 1962), although fractured dolomite extendsfar beyond clusters of replacement deposits at Ruby Hill andon Prospect Mountain. Replacement sultide deposits in thedown-dropped blocks north of the Ruby Hill fault are usuallysituated within zones of fractured dolomite, some of whichcontain disseminated pyrite. Faults cutting or tangential toreplacement deposits that could have served as fluid conduits


23/56

have not been recognized. Permeability for fluid flow wasapparently provided by crushed zones and networks of thinfractures that pervade much of the Eldorado Dolomite, andpossibly enhanced by high temperatures during mineralization(Maxwell and Verall, 1953; Hanson, 1995).

REPLACEMENT AND VEIN DEPOSITS

Metals produced in the Eureka district were mined fromreplacement deposits, quartz veins, and disseminated golddeposits. Most production came from oxidized replacementdeposits on Ruby Hill. Lesser production was derived fromreplacement deposits, also largely oxidized, on Mineral Pointand Prospect Mountain (fig. I), from quartz veins south ofRuby Hill (e.g., Grant Mine, fig. 1) and on Prospect Mountain(e.g., Dead Broke Mine, fig. 6), and from low gradedisseminated gold deposits several kilometers south and northof Ruby Hill (Windfall, Ratto Canyon, and West ArchimedesMines, fig. 1). Forms of replacement deposits, and theirmineralogy and parageneses were determined from field

examination and study of specimens from drill core and minedumps. Sulfide minerals preserved in replacement depositsnorth of Ruby Hill, and in replacement and vein deposits northand south of Ruby Hill were studied in detail in order todetermine pre-weathering paragenetic and pre-tectonic spatialrelationships between replacement sulfide minerals, skarnmineral assemblages, and igneous rocks.

Replacement DepositsThe replacement ores of Ruby Hill were oxidized to the lowestmining levels, about 250 m (-800 feet) below the surface.Replacement ores on Mineral Point and on Prospect Mountainwere also highly oxidized, and only small amounts ofunoxidized sulfide minerals were mined at the three locations.

In Ruby Hill and on Prospect Mountain much ore was recoveredfrom the floors of caves where oxidation of sulfide mineralsand removal of sulfur and zinc residually enriched lead, silver,and gold in the ores. The historically mined grades of 0.5 to2 ozlton (17 to 68 gltonne) gold, tens of ozlton (hundreds ofgltonne) silver, and tens of percent lead (Curtis, 1884;Vandenburg, 1938; Nolan, 1962) were undoubtedly derivedfrom oxidation of sulfide deposits with grades as much as

4 times lower, as evidenced by the tenor of sulfide replacementdeposits encountered by drilling north of Ruby Hill (fig. 3). A

resourceof 3.1 million tons at 3.7% lead, 8.3% zinc, 0.16ozlton (5.4 gltonne) gold, and 5.6 oz Iton (190 gltonne) silveris estimated to exist about 600 to 800 m (-2,000 to 2,600 feet)below the surface in the down-dropped block north of RubyHill (Love, 1966).

Forms of replacement deposits

Replacement deposits enclosed by dolomite in and north ofRuby Hill occur as coherent but inhomogeneous massesof oxide and/or sulfide minerals mixed with lesser amounts ofhydrothermal dolomite and subordinate calcite (described

above), and inclusions of unreplaced wall-rock dolomite.Geometries and distribution of replacement deposits correspondto no predictable pattern, recognizable structural control, orfavorable lithofacies, although all are confined to the EldoradoDolomite within tens to a few hundred meters above theChampion thrust fault. On a local scale both the oxidizeddeposits in Ruby Hill and the deep sulfide deposits north ofRuby Hill are extremely irregular in shape, and collectivelyform an anastomosing series of sub-tabular to semi-horizontalbodies with large lateral dimensions, that are discreet or looselyconnected by pipes and chimneys (Curtis, 1884; Love, 1966;fig. 7). In deep sulfide deposits pure aggregates of sulfideminerals are generally no more than a few to a few tens ofcentimeters in dimension, but masses consisting of 50 to 75%sulfide minerals and 25 to 50% hydrothermal dolomitefcalci te

West Wall, 2250 Level, Fad Shaft

dolomite and calcitewith irregular pods of dolomite with disseminated

dolomite +calcite

cemented with white calcite

227 gpt (6.67 opt) Au0 50 FEET

5.44% Pb10.82% Zn

-15 METERS

Figure 7. Rib map of the 2250 (foot) level (now flooded) from the Fad Shaft, showing the distribution of sulfide replacement ofEldorado Dolomite n the down-dropped block north of Ruby Hill. Section is modified from a map by R.A. Lillibridge, dated 9130165 and provided by the Ruby Hill Mining Company. Designation of some wall rock as limestone may reflect remnant l imestone inEldorado Dolomite that occurs throughout the district (e.g., Nolan, 1962). Location of the Fad Shaft is shown in figures 1 and 2A.gpt = grams per tonne; opt = troy ounces per short ton.


24/56

and dolomite wall-rock fragments have dimensions of metersto tens of meters. Within Ruby Hill, oxidized replacementdeposits have a vertical range in Eldorado Dolomite of >250 m(>800 feet), and north of Ruby Hill sulfide replacement depositshave a known vertical range of - 400 m (-1,320 feet, fig. 3).

At the north end of Prospect Mountain (fig. 6) replacementdeposits occur entirely in Hamburg Dolomite as masses of ironoxides, carbonate minerals, and rare quartz in pipes, tabular

zones, and small pods. Tabular replacement deposits aregenerally less than 2 m wide, dip at high angles, and mayextend tens of meters laterally and below the surface. Pipesand pods have surface expressions of as little as 1 m inmaximum dimension, but may extend to depths of hundredsof meters. Most of them plunge 60" or more. Much of the oremined consisted of cave fill cemented by iron oxides similarto that exposed in a high wall at the Metamoros Shaft (fig. 6).Prospect Mountain replacement deposits generally have greaterheightlwidth aspects, occur over a larger vertical range, andare smaller than the replacement deposits in and north of RubyHill. These contrasting shapes, stratigraphic positions, and sizesprobably reflect subtle differences in the mechanical controls

of replacement between the two dolomite host rocks, as wellas proximity to granodiorite. On the Wabash, Colorado, andProspect Mountain Tunnel dumps on Prospect Mountain (fig.6) calc-silicate-sulfide assemblages indicate that somereplacement pipes, or at least the mine workings exploitingthem, are close to the Ruby Hill granodiorite stock. Thedistribution of magnetic contours around and south of the RubyHill stock also shows that the granodiorite extends beneath thenorthern part of Prospect Mountain.

Oxidized replacement deposits

Lead, zinc, gold, and silver values in oxidized replacementores of Ruby Hill, Mineral Point, and Prospect Mountain occurin cerrusite, anglesite, and plumbojarosite, and in lesseramounts of mimetite, bindheimite, hemimorphite, andsmithsonite (Nolan, 1962; Nolan and Hunt, 1968). Theseminerals are mixed with limonite, goethite, hematite, dolomite,calcite, aragonite, and copper oxides, and small amounts ofbarite, wulfenite, and unreplaced wall-rock dolomite. Allmetallic oxide minerals formed from weathering of sulfideminerals, as remnant nodes of galena, pyrite, and sphaleriteenclosed by iron, lead, zinc, and arsenic oxides exist on minedumps. No silver minerals or gold have been recognized inoxidized replacement deposits.

Sulfide mineral paragenesis

Sulfide replacement deposits north of Ruby Hill, on ProspectMountain, and on Mineral Point consist mainly of subequalamounts of pyrite, sphalerite, and galena, with subordinateamounts of hydrothermal dolomite, calcite, arsenopyrite,tennantite, pyrrhotite, quartz, and chalcopyrite (fig. 7). Locally,relatively pure pods of pyrite, galena, and sphalerite withdimensions of tens of centimeters exist within sulfidereplacement masses north of Ruby Hill and in ProspectMountain. Grain size of pyrite, sphalerite, and galena andhydrothermal dolomite ranges from - to 4 mm in Ruby Hill

deposits; sulfide aggregates in quartz porphyry tend to beslightly coarser grained.

In sulfide replacement deposits north of Ruby Hill pyrite,intergrown with small amounts of arsenopyrite, is partlyreplaced by or intergrown with sphalerite and galena. Thesesulfide minerals replace both hydrothermal dolomitefcalciteand Eldorado Dolomite that encloses sulfide masses. On amicroscopic scale, pyrite contains inclusions of sphalerite,

chalcopyrite, and pyrrhotite. Sphalerite contains inclusions ofchalcopyrite, pyrite, tennantite, and rare pyrrhotite, which issometimes elongated and entrained along cleavages. Tennantitealso fills fractures in and is intergrown with sphalerite. Rareinclusions of seligmannite (CuPbAsS,, table 4) occur in galena.Galena locally encloses pyrite and sphalerite aggregates andcements fractured pyrite and sphalerite.

Pyrite, arsenopyrite, and sphalerite in the replacementdeposits north of Ruby Hill are invariably finely fractured andcemented with calcite. Calcite-filled cracks in these mineralsare up to tens of micrometers wide and may be as dense as tensof cracks per square centimeter. Locally, finely comminutedpyrite and sphalerite are entrained in calcite matrix, further

illustrating that the precipitation of late, fracture-filling calcitefollowed a sulfide deformational event. Galena is entirelyunfractured, perhaps a result of plastic instead of brittledeformation, but there is no evidence of annealed or strainedgalena crystals. Whereas other sulfides in drill holes (Binyon,1946; Love, 1966) and on the Fad Shaft stockpile (derived fromthe 2250 level) are finely comminuted because of dissolutionof late calcite cement, galena masses are generally competent.

Pieces of a post-sulfide breccia consisting of subroundedclasts of sulfides and subangular dolomite+pyrite clasts, mostly


25/56

quartz porphyry-related sulfide replacement deposits and insulfide replacement deposits in dolomite near Ruby Hill aresimilar, although galena is more abundant in replacementdeposits in and near quartz porphyry, and sphalerites in thetwo grou ps of deposits differ in texture and co mposition.

Galena and sphalerite are the main sources of lead andzinc, respectively, in all replace men t deposits. Silver occurs insmall amou nts in tennan tite, galena, acanthite, and miargyrite

(table 4). G old, unob served microscopically, occu rs mostly inpyrite, based on metallurgical tests. In o rder to determine theabundan ce of gold and other minor elements in sulfide mineralsfrom replacement deposits, as well as in other hydrothermalsulfide minerals (pyrite in veins in granodiorite, pyrite inhydrous skarn, andpyrite in disseminated gold deposits) gold,s i l ve r, a r s en i c , an t imony, mercu ry, t i n , and b i smu thconcentrations were determined by fire assay and atomicabsorption spectrometry( U S ; ig . 8, table 5). The major sulfidereplacement minerals (pyrite, sphalerite, and galena) containmode rate to high conce ntrations of all mino r elements analyzed ,with pyrite containing the largest amoun ts of gold , and pyriteand galena containing the largest amounts of silver. Pyrite in

veins in granodiorite and in hydrous skarn contains very lowconcentrations of all mino r elements analyzed, with the possible

E b ( l o3 P P ~ )0 i ( l o 3 pprn)

Veins Hydrous Sulfidein Kgd skarn replacement

deposits

Figure 8. Gold, si lver, arsenic, antimony, and bismuthconcentrations in pyrite from veins in granodiorite, hydrousskarn, and sulfide replacement deposits.

exception of arsenic. Because of small sample size only a fewanalytic data we re obtained for pyrite in one d isseminated golddeposit (Ratto Canyon). Gold at Ra tto Canyon apparently occursin sites other than pyrite, which contain ed -0.07 pp m gold (0.07gltonne), as the sam ple from which p yrite was ex tracted assayedmore than 1 oz goldlton (>3 4 g goldltonne).

Vein deposits

Veins on Prospect Mountain, on Mineral Hill, and in ZuluCanyon (figs. 1 ,3 , and 6) consist predominantly of quartz andsubordinate amounts of calcite and sulfide minerals, range inwidth from a few centimeters to- 1 m, and h ave variable strikesand dips. Most veins were prospected and m inor lead and silverproduction is recorded from the Grant, Dead Broke, G ordon,Lord Byron, Eureka Tunnel, and a few other vein m ines (fig.6; Nolan, 1962). Veins are completely oxidized near the surfacebut sulfide minerals in du mp sam ples include stibnite, galena,sphalerite, pyrite, tetrahedrite, tennantite, jameson ite, zinkenite,boulangerite, and several phase s with atomic prop ortions thatcorrespond to no known minerals ( table 4). Microscopicelectrum and anatase were observed in vein samples from theGrant Mine and E ureka Tunnel dumps, although the neithermine has recorded gold production (Nolan, 1962). Galena isthe source of most vein lead production, whereas silver isd i s t r ib u t e d a m o n g g a l e n a , t e t r a h e d r i t e, j a m e s o n i t e ,g a l e n o b i s m u t i te , a n d b o u l a n g e r i t e ( t a b l e 4 ) . S t i b n i tepredominates and pyrite is uncommon in veins of the Grant,Dugo ut, and Gordon Mines, and in the veins in Zulu and SecretCanyon s (figs. 1 and 2A). AEu reka Tunnel vein contains brecciaclasts of marble, tremolite, and magnetite, supporting thepresence of contact zone alteration and granodiorite underlyingProspect Mountain in the vicinity of the mine workings (fig.6). Based on fluid inclusion microthermom etric measurementsand other evidence presented below, veins on and north ofProspect Mountain may have been formed during hydrothermalevent(s) separate from those that accompanied intrusion ofgranodiorite and qu artz porphyry.

In most veins anh edral sulfide minerals fill interstices ofsubhedral quartz crystals. In other veins agg regates of sulfidesare intergrown w ith fine-grained q uartz, and so me veins includeboth textures. In several mines qu artz+stibnite+ calcite v einsup to 1 m wide are cut by thin (several centimeters wide),coarse-grained, quartz+galena veins, which also contain mino rtennantite and pyrite. Th e thin qu artz+ga lena veins also occurindependently, and galena in them fills irregular openingsamong quartz subhedrons in vein centers. Within individualveins no paragenesis is evident other than the tendency of

sulfide minerals to cluster toward v ein centers.

Sphalerite Textures and Compositions

Sph alerite is widely distributed in the Eureka district and occursin most hydrothermal mineral assemblages: hydrous skarn,veins in hydrous skarn an d marble, veins in granodiorite, sulfidereplacement depo sits, shear and fault zones in q uartz porphyry,and quartz veins in dolomite. The textures and compositionsof the various occurrences o f sphalerite in the vicinity of Ru by


26/56

Table 4. Compositions n weight percent of sphalerite, arsenopyrite, pyrrhotite, and other sulfide minerals in Eureka district DDH core and mine samples. Analysesare by microprobe (T. Solberg, Virginia Tech. Univ.). Some drill hole (DDH) locations are shown on figures 2A and 3.

Sample No. of Stoichi ometry andno. analyses Locatio n Fe As Co Zn Cd Mn Ni Cu Ag Sb Pb Bi S Se Sum DrObable phase

DDH, Fad Shaft

DDH. Zulu CanyonDDH. Phoenix ShaftDDH. Mineral Hill

DDH, Zulu Canyon

DDH, Z

Carbonate Replacement - Intrusion Related

Documents