Economic Geology Vol. 69, 1974, pp. 646-656

K-Ar Age Relationsof GranodioriteEmplacement and Tungstenand Mineralizationnear the , Humboldt County, *

MILES L. SILBERMAN,BYRON R. BERGER,AND RANDOLPHA. KOSKI

Abstract

A granodiorite stock intrudes complexly folded and thrust-faulted Paleozoic sedi- mentary rocks in the OsgoodMountains of eastern Humboldt County, Nevada. Within the metamorphicaureole surroundingthe pluton, the sedimentaryrocks are converted to cordierite hornfels and marble; tungsten-bearingtactites developedalong the con- tacts of the granodiorite. Cutting the granodiorite and sedimentary rocks is the Getchell fault, along which the disseminatedgold bodiesof the Getchell mine are localized. K-Ar ages of the granodiorite,andesite porphyry, tungsten-bearing tactites, and altered granodioritefrom the Getchellmine indicate that emplacement,alteration, and mineralizationare all part of a magmatic-thermalepisode which took place ap- proximately 90 m.y. ago.

Introduction (1966) .tentatigelysuggested a similar conclusion THE Getchellgold depositin Humboldt County is basedon his studyof the patternsof distributionof one of a group of disseminatedgold deposits•of heavyminerals and gold within the stock. Nevada. Other ore depositsof this type include Geologic Setting Carlin, Cortez, and Gold Acres (Roberts et al., The Getchellmine lies at the northeasternmargin 1971). Structural and geochemicalsettings are dis- of the OsgoodMountains. The geologyof the Os- tinctive, but the depositsshow some general simi- good Mountainsquadrangle, which coversthe north larities: the presenceof a fractured,permeable zone part of the Osgood Mountains and includes the prior to ore deposition,deposition of silica replacing Getchellmine, was describedby Hobbs (1948) and limestonewith the introductionof gold, the presence Hotz and Willden (1964). The Getchellore de- of carbonaceousmaterial in some of the , the posit was describedby Joralemon(1951). The predominanceof very fine-grainedto submicroscopicgeologic setting is summarizedby Hotz and Willden gold particlesin the ore, and high gold-silverratios (Fig. 1). (Roberts et al., 1971; Wells et al., 1969; Radtke The northern Osgood Mountains consistof com- and Scheiner, 1970; Hausen and Kerr, 1968). plexlyfolded and thrust-faulted Paleoz.oic sedimentary Altered igneous rocks occur within the ore zones rocks intrudedby a Cretaceousgranodiorite stock. of all the disseminatedgold deposits. The impor- The stockhas two lobes,a northern and a southern, tanceof factorscontrolling ore depositionapparently joined by a thin septurn;aeromagnetic data (B. R. variedfrom depositto deposit(Roberts et al., 1971). Berger, unpub. data) substantiatethe hour-glass Although several K-Ar ages have been reported shape. The sedimentaryrocks intrudedare prin- from ore zonesof disseminatedgold depositsat Gold cipally argillites and limestoneof the Prcble forma- Acres (Silberman and McKee, 197'1) and Cortez tion of Cambrianage. Thoughnot apparentin out- (Wells et al., 1971) ages are not yet conclusively crop, the aeromagneticdata suggestthat the stock known for any of thesedeposits, and only spatialre- has steeplimbs dippingoutward on both east and lations have been demonstrated between these ore west flanks. depositsand nearby igneousrocks. Dacite porphyrydikes cut the Paleozoicsedimen- This paperreports several ages on sericiteand K- tary rocks and are probably related geneticallyto feldsparfrom the altered igneousrock at the Getchell the stock. In the northernpart of the OsgoodMoun- ore depositand showshow theseages supportthe tains,andesite porphyry dikes cut both sedimentary geneticassociation of the stockand Au depositsug- rocks and the granodiorite. Andesite dikes cut the gestedby earlier workersand documentedwith geo- sedimentaryrocks at the Getchellmine. They are chemicaldata by Ericksonet al. (1964). Neuerburg highlysheared and hydrothermallyaltered in under- * Publication authorizedby the Director, U.S. Geological ground workings and in the North Pit of the Get- Survey. chell mine. As the dikes follow the trend of the 646 K-,4RAGE RELATIONS AND TUNGSTEN .dND GOLD MINERALIZATION 647

117o20' 117Ol5' 41Ol5' EXPLANATION

Alluvium UNCONFORMITY

Oliwne basalt UNCONFORM ITY

Docile Grono- oo porphyry diorite

Sedimentary rocks

Battle Formation

Gaughs Canyon Formation UNCONFORMITY

Volmy Comus Formation Formation 41ø I0'

Harmony' Farmallon

Prohie Formation

K-AR Mine (MB-35) Sample Iocollly

Thrust foull Foull Dashed where approximately Dashed where approximately located. Sawteeth on /ocafed; dotted where upper p/o/e. concealed; queried where doubtful or t'nferred I Mile I 41o07'18"

FIG.1. Geologicmap of thenorthern part of theOsgood Mountains, modified from Hotz and Willden (1964, pl. 1). fault zone or occupyportions of it, their original tic with the granodiorite.The ageof theseandesitic positionappears to havebeen controlled by an early dikes is critical becauseveins of gold-bearingma- fracturethat later developedinto the Getcheilfault terial cut them at the Getcheil mine. zone. Granodioritedikes also parallel the fault zone. The granodioritestock has a conspicuousmeta- There is somedisagreement over the age of the morphicaureole as muchas 10,000feet wide. The andesiteporphyry dikes. Joralemon(1951) sug- shales of the Preble formation are transformed to gestedthat the dikesare an earlyphase of Tertiary biotite-,andalusite-, and cordierite-bearinghornfels; volcanism;Hobbs (1948) suggestedthat they are a the limestonesto marble, light-coloredcalc-silicate later phaseof the sameLate Cretaceousigneous rock, and dark-coloredtactite composedof garnet, activitythat producedthe granodioritestock; Hotz pyroxene,and other calcium-bearing silicates, along and Willden (1964) suggestedthat they are cogene- with minor scheelite. Thirteen tungsten mines 648 M. L. SILBERMAN,B. R. BERGER,AND R. A. KOSKI

pyrite is the most common. At someplaces, schee- lite is found in calc-silicaterock and altered grano- diorite. At others,scheelite occupies narrow seams and/or small intenselyaltered areas or pocketsin the granodiorite along the contacts. These small, but very rich .ore pocketsconsist of muscovite,hy- drobiotite,small quartz crystals,and scheelite.Schee- lite-bearingquartz veins are found cutting the gold ores at both the Getcheil and Ogee-Pinson mines. A major structuralfeature of the OsgoodMoun- tains is the Getcheil fault along the eastern margin of the range (Fig. 1). Horizontal mullions and slickensideswith superimposedvertical striae show that fault movementwas largely parallel to the strike prior to the block faulting (Hobbs, 1948). Jorale- mon (1951) indicatesthat the latest movementwas vertical. Evidence at the Getchell mine of post- mineralization movement on the Getcheil fault is the displacementof Quaternary alluvium and develop- EXPLANATION ment of slickensides on the surfaces of altered rock.

Rhyohtetuff The Getchell Gold Deposits ":•::'':" ).- The disseminatedgold deposit is localizedalong Ande$•tePOrl•hyry the Getcheilfault. In the southernand central parts of the mine area, granodioriteforms the walls of the GrOnOd•Orite fault zone,whereas farther north the fault cuts argil- lite and limestoneof the Preble formation (Jorale- LlrneslOne mon, 1951; Hotz and Willden, 1964). In the center pit, barren granodioriteforms the hangingwall. In Ar(:jiIhte or h0rnfele the South Extension Pit, gran.odioritein the foot- wall is partly altered (sericiticalteration) and veined by calciteand dolomite; it containssulfides and ap- proximatelyeight ppm gold. Both oxide and sulfide

K-AR $ornple Iocohty gold depositsoccur. The oxide ore, being easily recovered,was exploitedduring early miningopera- 0 500 1000 FEET I I tions. The sulfide ores, which are more extensive,

/ were mined later. Exten$,onPfl' Joralemon(1951, p. 270) reportedthat the gold A A' ore bodiesare sheetlikemasses that lie alongvarious strandsof the Getcheilfault zone (Fig. 2). These massesconsist of shearedand mineralized argillite and limestonecut by quartz and calciteand dolomite veins; within the fault zone, the rocks contain a soft, Section 4 carbon-bearinggouge (gumbo)--minute quartzcrys- FIG. 2. Geology of the Getcheil mine area, modified from tals embeddedin a nearly submi.croscopicinter- Hotz and Willden (1964, P1. 11). growth of quartz and amorphouscarbon. The gumbo formed in the gouge zone by hydrothermal meta- located around the periphery of the stock in the morphismof carbonaceouslimestone and shale ac- tactite zone produceda total of 1,344,150tons of companiedby the introductionof fine-grainedquartz. scheelite-bearingore (Hotz and Willden, 1964). It was a major bearerof oxide gold ore. The sul- The ore is mainlyscheelite-bearing tactitc; powel- fide ores consist of carbonaceous limestone with iron lite is accessoryand metallicsulfides are presentin and arsenic sulfides. minor amounts. Uncommon small local concentra- The arsenic sulfides, realgar and orpiment, are tionsof molybdeniteare observed in tactiteclose to the associatedwith gold in the vicinity of the Getcheil granodioritecontact. Pyrite, chalcopyrite,sphalerite, mine. Accordingto Joralemon(1951), they are the andgalena are erraticallydistributed in the tactite; most abundantmetallic minerals and are primarily K-AR AGE RELATIONS AND TUNGSTEN AND GOLD MINERALIZATION 649 restricted to the areas within the veins that contain itc porphyry as early or middle Tertiary. He sug- notablequantities of gold. .The gold ore containsa gested,on the basis of the early work of Ferguson few thousandthsto a few hundredthspercent WOa; (1929) who assignedages to hydrothermalore de- ' scheeliteand hfibneritehave been identified. Every positsin Nevada, that the Getche11gold depositwas mineral is host for someof the gold. Pyrite, mar- late Tertiary. Becausethe ore bodieswere localized casite, and the carbonaceousgumbo are the most in the fault zone that cut the granodiorite,liotz and important,but goldoccurs sparsely in realgat,arseno- Willden (1964) suggestthat the Getchellmineraliz/•- pyrite, and quartz. The gold is present largely as tion waslater than the emplacementand solidification discrete,microscopic particles ranging from a frac- of the granodioritestock, to whichthey assigna Late tion of a micronto ,•1 mm. Somegold principally Cretaceousage based on a Pb-a age determination outside of the ore shootsmay be present in solid of 69 m.y. (Jaffe et al., 1959, entry no. 77). liotz solutionin pyrite and carbon (Joralemon,1951). and Willden consider the mineralization to be Terti- Gold occursin the sedimentaryrocks of the fault ary. Estimatesof the age of the Getchellgold min- zone and in altered andesitc dike rocks. Wall rocks eralization were thus based on the assumptionthat as far as 400 feet from the ore body contain 0.01 to the andesitc porphyry dikes were considerably 0.08 ounceper ton gold and traceamounts'occur younger than the granodiorite, or on Ferguson's farther out. The boundaryfor the ore depositsis (1929) attempted classificationof preciousmetal economic,not mineralogic(Hotz and Willden, 1964; depositsin Nevada. Joralemon, 1951). Joralemon's (1951) detailed Ericksonet al. (1964) demonstratethe occurrence studiesof the ore depositled him to concludethat of small amounts of tungstenin and over the gold- the richest parts of the ore bodieswere the main bearingore. They also show that arsenicoccurred channelwaysthrough which the mineralizing solu- in amountsto 2,000 ppm in oxidized iron-rich ma- tions passed. These are also the areas of richest terial from the tactitc-tungstenores; the elements concentrationof visible gold. Joralemon(1951, p. As and W are commonto both the tactitc and gold 299) indicatesthat realgar,marcaslte, and economi- deposits. On the basisof their geochemicalstudies, cally important gold concentrationswere deposited Ericksonet al. (1964) believethat the varioustypes in the mainchannelways at nearlythe sametime that of metal depositsare probablygenetically related to solutionsin the otherparts of the veinswere deposit- each other and to the granodioriteintrusive mass ing disseminatedpyrite and submicroscopicgold in and that they formed as successiveand overlapping noncommericalamounts. These solutions spread stages of one period of complex mineralization. out from the main channelways,altering the wall Neuerburg (1966) has studied the distribution of rocks--includinggranodiorite and the andesitcpor- sulfides, gold, and other heavy minerals in the phyry dikes--and depositingsmall amountsof gold. granodiorite. lie concludesthat areasof high-sulfide South of the Getche11mine, gold occursin economic concentrationadjacent to the contact of the grano- and subeconomic amounts in silicified shale and lime- diorite represent points at which late magmatic stonewith no traces of realgar or orpiment and only mineralizingfluids issued from the stock. He demon- minor amountsof arsenopyrite. stratesthat these areas of postulatedfluid exit cor- respondto tungsten depositsin the adjacent lime- Geologic Estimates of the Age of Tungsten and Gold Mineralization stone country rocks. The gold content of the granodiorite is one to two orders of magnitude The tungsten depositsin the tactitc zones sur- greater than that for crustal rocks or for granitic roundingthe granodioritestock are generallythought rocksof similarcomposition. The highestgold con- to be the same age as the stock. Geologicand centrationsin the stockare in large part adjacentto stable isotopedata (unpublished) indicate that the the Getchellfault and to the low-gradegold deposits scheelitewas depositedduring the final stagesof in the Ogee-Pinsonmine severalmiles to the south. silicificationby solutionssweated out of the grano- Neuerburg(1966) suggeststhat the spatialrelation dioriteafter emplacement.liotz and Willden (1964, may indicatea geneticrelation like that postulated p. 86) suggestthat some scheelitewas deposited for the associationof high-sulfideconcentrations and whenthe tactitcwas formed and that scheelitedeposi- tungstendeposits. tion continuedduring the waningstages of the mag- Conversely,Roberts et al. (1971), in reviewing matic episode. the occurrenceof the disseminatedgold depositsin The geologicrelations of the Getche11gold deposit north-central Nevada, suggestthat the mineral as- do not provide closely defined age limits for gold semblageof the gold depositin the Getcheil fault deposition. Ore depositioncould have occurredany zone appearsto indicatea distinctlylower tempera- time after the emplacementof the andesitcporphyry ture than the assemblagein the tactitc-tungsten dikes. Joralemon(195.1) tentatively dated the andes- deposits,concluding that the gold depositsare con- 65O M. L. SILBERMAN, B. R. BERGER, AND R. A. KOSKI

TABLE 1. Chemical Compositionof Altered and Unaltered Granodiorite and Andesite The maj,orelement content of the stockis similar to that of other granodiorites in north-central Oxides MB33B MB34 MB32 MB32A GL3 Nevada. The andesiteporphyry dike is similar in compositionto the granodiorite. The petrography Chemical Analyses of these rocks is described in Hotz and Willden (1964). SiO2 65.9 61.7 64.9 65.2 44.0 A120• 16.8 17.8 16.9 17.0 16.8 Two sericitizedblocks of granodioritethat were Fe•O• 1.5 1.0 0.85 1.2 2.3 mineralizedin the Getchellfault zonehave provided FeO 2.0 3.4 1.5 1.6 0.96 MgO 1.4 2.2 1.6 1.6 2.4 datableminerals from the gold deposit. The min- CaO 4.3 5.2 5.3 4.9 10.4 eralogyand chemistryof one of the samplesstudied Na•O 3.6 3.6 3.5 3.3 0.06 in detail is presentedhere as a basisof comparison K•O 2.8 2.8 3.3 2.7 7.8 H•O 0.72 0.93 0.84 0.74 3.3 for alteration studiesin spatiallyassociated igneous TiO• 0.53 0.70 0.63 0.58 0.62 rocks in other disseminatedgold deposits. P•O5 0.19 0.26 0.26 0.27 0.27 MnO 0.07 0.08 0.06 0.05 0.10 The blockof granodioriteadjacent to the footwall COs -- 0.09 -- -- 10.1 of the Getchell fault in the South Extension Pit (Fig. 2) was sampledfor K-Ar dating and for Total 99.8 99.8 99.6 99.1 99.1 chemicalanalysis (sample GL-3). Hydrothermal Normatire Mineral Composition alteration has resultedin recrystallizationof plagio- (calculated on a volatile-free basis1) claseand partial recrystallizationof orthoclaseto a fine-grainedfelty mass of sericite (<0.01 mm in Q 22.4 14.2 19.2 22.9 13.4 C 0.03 ------8.33 size). Coarsebiotite booksfrom the original grano- or 16.7 16.8 19.8 16.3 46.5 diorite have been recrystallizedto coarsergrained ab 30.8 30.9 30.1 28.5 0.51 an 21.6 24.5 21.0 24.0 -- sericite(approximately 0.1-0.6 mm in size), pyrite, wo -- 0.70 2.39 0.30 and leucoxene. Someoriginal orthoclaseand quartz en 3.53 5.56 4.04 4.06 0.93 fs 1.71 4.48 1.14 1.10 -- TABLE 2. Semiquantitative SpectrographicAnalyses of mt 2.20 1.47 1.25 1.77 1.64 Altered and Unaltered Granodiorite and Andesite hm .... 1.19 il 1.02 1.35 1.21 1.12 1.19 cc .... 18.1 Element mg .... 4.29 (ppm) MB33B MB34 MB32 MB32A GL3

Total 100 100 100 100 97 Ag .... 0.7 As .... 10,000 Au ..... 1 With exception of GL-3. B .... 300 For sample locations, see Table 4. For descriptions, see Ba 1,500 1,500 2,000 1,500 3,000 Silberman and McKee (1971) and text. Methods used are Be ...... those described in U.S. Geol. Survey Bull. 1144-A, supple- Bi ..... mented by atomic absorption; analysts: Paul Elmore, Lowell Cd ..... Artis, G. W. Chloe, Hezekiah Smith, James Kelsey, and J. L. Co 5 10 10 7 7 Glenn. Cr 5 10 10 7 7 Cu 2 7 150 150 7 La -- 50 30 -- • siderablyyounger, possibly early Tertiary. They Mn 700 700 300 200 500 emphasize,however, that the gold deposititself has Mo ------5 -- not been dated. Nb 10 10 10 7 -- Ni -- 5 10 5 • Pb 15 10 10 • -- Geochemistry oœAltered and Pd ..... Unaltered Igneous Rocks Sb ..... Sc 10 15 10 7 ' 10 Chemical analyseswere made and norms calcu- Sr 1,000 1,500 1,000 700 300 V 70 100 70 50 70 lated on rock samplesrecently collected by the writers:two samplesof granodiorite(MB-33B, MB- Y 20 20 20 15 15

34), two samplesof andesiteporphyry from the Zr 100 70 100 100 70 North Pit (MB-32 and 32A), and a sample of altered granodiorite from the South Extension Pit Semiquantiative spectrographic analyses; analyst: Chris (GL-3) (Table 1). The sampleswere alsoanalyzed Heroupolos. Results reported as midpoints of geometric brackets whose boundaries are 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, semiquantitativelyby spectrography(Table 2) and 0.12, etc. The precisionof a reported value is approximately quantitativelyfor some trace elements (Table 3). plus or minus one bracket at 68 percent or two brackets at Normative mineral contentsof the unalteredsamples 95 percent confidence.Detectability limits: Ag 0.7, As 200, Au 15, B 7, Ba 2, Be 1, Bi 7, Cd 50, Co 2, Cr 1, Cu 1, La 30, are plotted on a quartz-plagioclase-orthoclasedia- Mo 2, Nb 7, Ni 1, Pb 7, Sb 100, Sc 2, Sr 5, V 3, W 30, Y 10, Zn gram (Fig. 3). 100, Zr 5. K-AR AGE RELATIONS AND TUNGSTEN AND GOLD M1NERAL1ZATION 651

phenocrystswere not destroyedduring alteration; additional K-feldspar may have crystallizedduring /• , MB32ond * MB338 alteration; however, no indication of this was found •* M• 34 in the thin sections examined. Most of the K20 m z n • [de n, 19641 that was added was fixed as sericite. Calcite and mixtures of calcite and dolomite form veins several centimeters to one-half millimeter in width and are developedpervasively throughout the altered grano- diorite. Sulfides, particularly pyrite, realgar, and orpiment,occur within thesecarbonate veins and are disseminatedthroughout the altered rock. Comparisonof the major elementchemistry of the alteredgranodiorite (GL-3, Table 1) from the South ExtensionPit with unalteredgranodiorite is difficult becausethe sampleis pervasivelyveined with dolo-

mite and calcite. It is clear,however, that potassium (AN+AEI] OR hasbeen added to the rock, and petrographicexamin- Fro. 3. Normafive qua,rtz,plagioclase, orthoclase diagram ationindicates that the additionalpotassium has been for samplesfrom the Osgood Mountains pluton, calculated fixed as sericite. Hornblende,biotite, and plagio- on a volatile-free basis. clase have largely been destroyedor recrystallized to sericite. The Ca and Mg originally presentin examination,only one episodeof sericite formation thesephases is now probablyin the carbonate.Jora- appearsto be present. lemon (1951) suggeststhat someof the calcitein the veins was from solution of limestone beneath the ore Trace Elements deposit. An additional source outside the altered The trace element analyses (Table 3) show the area for magnesium(now fixed in dolomite) may additionof arsenic,gold, mercury, antimony, barium, have been required. and rubidium and the loss of strontium to the altered That sericiticalteration accompanied the gold min- and veinedgranodiorite (sample GL-3) in the South eralizationis demonstratedby sericitizedcordierite Extension Pit. The arsenic is present as realgar crystalsin gold-bearinghornfels. Away from the and orpimentin the calcite-dolomiteveins and dis- gold-bearingzones, the cordieritein this hornfelsis seminatedthroughbut the rock. Ericksonet al. not altered. On the basis of the petrographic (1964) show that arsenic, mercury, and tungsten have the greatest enrichment and are the most TABLE 3. Quantitative Analysesfor SelectedTrace abundantmetals in the zone of gold mineralization. Elements and Potassium Barium also increases in the ore zone. Of these metals,only tungstenwas not detectedin the altered Element MB33B MB34 MB32 GL3 granodiorite. The block of granodioritethat yielded K 2.17% 2.20% 2.50% 6.97% sampleGL-3 constitutespart of a mineralizedpod, Rb 69 ppm 62 ppm 59 ppm 192 ppm characteristicof the spottysulfide mineralization in Sr 602 670 623 182 the South Extension Pit of the Getchell mine. Cu 3.8 9.2 160 NA Pb 6.5 7.5 42 NA Zn 50 43 24 NA Rocks Dated by the K-Ar Method As < 10 < 10 40 5,000 t Au 0.06 0.06 0.06 7.8 • K-Ar ages of two samplesof granodiorite,one Hg 0.03 0.02 0.03 19.5a near the Alpine tungstenmine at the westernmargin Sb < 1 < 1 < 1 10a of the stock and the other near the Valley View Ag 0.2 <0.2 0.4 NA mine at the southeasternmargin of the stock (Fig. • As by modifiedGutzeit method; analyst: E. J. Fenelly. 1), and of one sampleof andesiteporphyry from the 2Au by fire assay-atomicabsorption method; analysts: D. east wall of the North Pit of the Getchellmine (Fig. Goss, R. Rahill. 2), listed on Table 4, were published earlier by s Hg and Sb by instrumental methods, analysts: R. L. Turner, J. B. McHugh. Silbermanand McKee (1971). K, Rb, Sr by X-ray fluorescence;analysts: B. P. Fabbi and Two samplesof altered granodiorite(GL-3 and L. F. Espos. Cu, Pb, Zn by atomic absorptionspectroscopy; analyst: C. L. Burton. As by colorimetric spectroscopy; Section4, Table 4) were collectedfor dating. Per- analyst: H. McBride. Au by atomic absorption; analyst: H. vasive slickensides and kinked muscovite lamellae A. McBride. Hg by instrumental method; analyst: C. A. in replacedbiotite phenocrystsat locality GL-3 indi- Curtis. Sb by colorimetricmethod; analysts: M. S. Rickard, S. I. Hoffman. Ag by atomic absorption;analyst: J. G. Viets. cate that fault movementaffected this locality after NA, not analyzed. alteration. Fine-grained sericite from the ground- 652 M. L. SILBERM`4N, B. R. BERGER, .4ND R. .4. KOSKI

TABLE4. K-Ar Agesof Altered and Unaltered IgneousRocks from the GetcheilMine and Various Tungsten Mines, Northern OsgoodMountains

Apparent Samplenumber Location Rock type Mineral age Reference USGS(M)-MB33BSouth-central sec.6, T. 38 GranodioriteBiotite 89.9-t- 1.8 Silberman•nd N., R. 42 E., near McKee (1971) Alpine tungsten mine USGS(M)-M B34 East-central sec. 29, T. 38 Granodiorite Biotite 92.2 q- 1.8 Silberman and N., R. 42 E., near Hornblende 89.5 q- 1.8 McKee (1971) Va.lley Viewtungsten mine USGS(M)-MB32 SE¬ sec. 29, T. 39 N., R. Andesite Biotite 89.9 q- 1.8 Silberman and 42 E., east wall of north porphyry McKee (1971) pit, Getcheil mine dike KIR-114 NW¬ sec. 17, T. 38 N., R. Sericite-bearing Sericitex 87.6 q- 3.4 This report 42 E., Kirby tungsten seam in mine tactire MAR-103 East-centralsec. 24, T. 38 Sericite-.bearingSericite x 88.4 q- 3.3 This report N., R. 41 E, Marcus seam •n tungsten mine tactire MB55 SE-}sec. 31, T. 39 N., R. Sericite-.bearingSericite • 92.6 q- 2.8 This report 42 E., Mountain King seam •n tungsten mine altered granodiorite Sec. 4 Approx. center, sec.4, T. Altered grano- Sericitex 92.2 q- 2.8 This report 38 N., R. 42 E., Section diorite sill 4 Pit GL-3A SW• sec. 33, T. 39 N., R. Altered, Orthoclase 74.7 q- 2.2 This report 42 E., west wall of mineralized Fine-grained sericite 67.0 q- 2.0 This report South Extension Pit, granodiorite (<0.01 mm) Getchell mine Coarse-grained 80.9 q- 3.3 This report sericite (0.1 to 0.6 ram)

• 2 M• muscovite. massof the GL-3 samplewas obtainedby crushing (Section4, Table4), southof the mainpart of the and grinding a specimenand letting the ground ma- Getchellmine (Fig. 1). The sill intrudesargillite terial settle in a water column. After 30 hours, the of the Preble formationand both the sill and argillite suspensionwas decantedand dried. A grainsize show sericitic alteration. Coarse flakes of muscovite estimate of less than 0.001 mm was calculated on the (up to 2 mm in size) occurin a grussymatrix of basis of Stokes' law. Coarse-grainedmicaceous quartzand K-feldspar. Gold was not detected in the flakes were concentratedfrom another part of the granodiorite,but the surroundingaltered Preble specimenby using heavyliquids. These micaceous argillite was stronglymineralized (B. R. Berger, flakesconsisted of muscovitelamellae, 0.2-0.6 mm in unpub.data, 1972). About36,000 tons of high-grade si•ze,intergrown with leucoxene,pyrite, and some oxidegold ore wasmined from the pit. A sericite calcite. The low K20 contentof the mineralseparate concentratewas obtainedfrom the alteredgranodior- was causedby this intergrowth of potassium-free ite and this sericite,like the GL-3 sample,has the minerals. No attempt was made to regrind the 2Mx muscovitestructure but is not intergrownwith sericite intergrowthsfor purification becauseof the potassium-freeminerals. Mineral separates for sam- possibleloss of argon from deformationof the sam- pie GL-3 wereprepared at the NevadaBureau of ple. An X-ray diffractionanalysis of boththe coarse- Minesand Geology. The mineralseparate from the and fine-grainedsericite showedit to have the 2Mx Section4 Pit samplewas preparedat the U.S. structure of muscovite. GeologicalSurvey. A K-feldspar separate was obtained from the Three samplesfrom sericite-bearingseams were coarse-grainedfraction 'by heavy liquids and a mag- collectednear tactite-granodioritecontacts (Fig. 1 netic separator. X-ray analysis using Wright's and Table 4). SampleMAR-103 at the Marcus (1968) methodconfirmed an orthoclasestructure. mine is from the quartz-calcite-sericitepods above Another sample of sericitizedgranodiorite was a massivetungsten-bearing garnet tactite bed. KIR- collected from an altered sill in the Section 4 Pit 114 at the Kirby mine is from a thin-beddedgarnet- K-AR AGE RELATIONS AND TUNGSTEN AND GOLD MINERALIZATION 653

TABLE 5. K-Ar Analytical Data for Previously Unpublished Age Determinations KaO *Ar4ø(RAD) Ar4ø(RA'D) Apparentage Sample number Mineral (percent) moles/gm X 10-xø Ar4%t•l (m.y.)

G-K114 Sericite 4.82 6.538 0.627 87.6 4- 3.4 4.92 6.403 0.571 G-M 103 'Sericite 8.72 11.52 0.699 88.4 4- 3.3 8.65 11.78 0.786 (M) Sericite 9.20 12.87 0.972 92.6 4- 2.8 MB-55 9.17 (M) Sericite 10.33 14.42 0.979 92.2 4- 2.8 Sec. 4 10.33 (M) Sericite 9.17 9.242 0.859 67.0 4- 2.0 GL-3 ( <0.01 mm) 9.17 Sericite 5.97 , 7.251 0.885 80.9 4- 3.2 (0.2-0.6 mm) 5.67 6.07 6.06 Orthoclase 14.38 16.19 0.959 74.7 4- 2.2 14.40 (M) • Hornblende 0.675 0.9272 0.621 89.5 4- 1.8 MB34 0.695 0.695 0.688 0.710 0.703 0.655 0.657 Biotite 8.70 12.19 0.861 92.2 4- 1.8 8.62 8.80 8.90

t Age of hornblenderevised from that reported in Silberman and McKee (1971) based on new K:O analyses.No change in biotite age. Argon analyseswere made by standard isotopedilution techniques (Dalrymple and Lanphere, 1969). The sampleswere fusedby RF inductionheating and the extracted argon analyzedwith a Neir-type, 6«-inch-radius,60ø-sector mass spectrometer (USGS-(M) samples)or an MS-10 massspectrometer (geochron samples), both operated in the static mode. Analyst: M. L. Silberman ((M) samples).Potassium analysis ((M) samples)by differential flame photometry usinglithium metaboratefusion technique; lithium serving as an internal standard. Analyst: Lois Schlocker. Potassium analysis (G samples) by differential flame photomerry usinga Beckman DU spectrophotometer.Constants used: Xe = 0.585 X 10-10 yr-,1 X• -- 4.72 X 10- 10 yr-,1 4øK/K•ot• -- 1.22 X 10-4 gm/gm. Plus/minus is estimated analytical uncertainty at one standard deviation. (M) analyses performedat U.S. GeologicalSurvey Laboratory. G analysesperformed at Krueger Enterprises,Geochron Laboratories Division, Isotope Laboratory, Cambridge, Mass.

diopside skarn with shaly hornfels breaks. The granodioriteis 90 m.y. (Table 4). The concordance sericiteis intergrownwith quartz alongthe boundary of the biotite and hornblendeages from sampleMB- between a tactite bed and unmineralized marble. 34 strengthensthe validity of the 90-m.y. age. The SampleMB55 wascollected from a cavity within the andesiteporphyry in the North Pit also gave a 90- granodioritealong the contactof granodioritewith m.y. age (sampleMB-3.2, Table 4). On the basisof tactite at the Mountain King mine (Fig. 1), where chemicalsimilarity, proximity, and age, the andesite sericite-bearingseams contain scheelite(Hotz and porphyry is most likely a phase of this magmatic Willden, 1964). episode. The agreementof the K-Ar ages of the The sericitein thesesamples was coarsegrained, granodioriteand andesiteporphyry establishes a firm in flakesup to 1« mm across. Mineral separations upper limit on the age of the ore deposit. and all analytical work on the samplesfrom the The muscovite samples from the tactite zones Marcus and Kirby mines were done by Geochron gave agesof 87.6, 88.4, and 92.6 m.y. These ages Labs, Inc. Mineral separation for the Mountain are broadlythe same(within analyticaluncertainty) King mine samplewas done at the Nevada Bureau as the age of the granodiorite. Both the geologic of Mines and Geology. Analytical data are given relationsand K-Ar ages indicate a genetic relation for previouslyunpublished ages (Ta'ble5). between granodiorite and tungsten-bearingtactite deposits. K-Ar Ages Altered granodioritefrom the Section4 Pit (Table The age of the granodiorkebased on averaging 4), which containedthe coarsestgrained, purest the three agesfrom the two specimensof unaltered muscovite,has an age of 92.2 m.y., the samewithin 654: M. L. SILBERMAN, B. R. BERGER, AND R. A. KOSKI analyticaluncertainty as the age determinedfor un- ment along the Getchell fault. Evidence for this alteredgranodiorite. The SouthExtension Pit sam- movementhas been discussed.Lee et al. (1970) ple (GL-3, Table 4) gave three separate,discordant have shownthat the K-Ar age of a mica near a large ages; all of theseare significantlyyounger than the fault can be reducedby fault movement. age of the pluton. Other interpretationsof the data are possible,in- The 67-m.y. sericitedate of sampleGL-3 (Table cludingthe suggestionthat the 67-m.y. fine-grained 4) must be considereda minimumage becauseof the sericiteage (GL-3) repregentsthe true age of min- very fine grainsizeof this groundmasssericite. Han- eralization and that the .coarse-grainedsericite and sonand Gast (1967) and Hart (1964) showedthat K-feldsparages were partially resetby the thermal the retentivity of argon in micas is dependenton effects of the mineralization. This would require grainsize in a contact metamorphicenvironment. the crystallizationof two generationsof sericite--the Where the grainsizeof the sericiteis very fine, a fine-grainedsericite, during Au mineralization,and relatively large surface area is exposed,probably the coarse-grainedsericite at some earlier time. permitting increasedargon loss at relatively low Petrographicevidence for this is lacking. The fine- temperatures. grainedsericite is foundreplacing plagioclase, and K- There are two probable interpretationsof the feldspar,which in unalteredgranodiorite tends to be orthoclaseage and the coarse-grainedsericite age interstitial to plagioclase,and quartz. The coarser of 75 and 81 m.y., respectively,from sampleGL-3 sericite lamellae along with leucoxene and iron (Table 4). The first is that mineralizationin the oxides are found replacingthe biotite phenocrysts. South Extension Pit area occurred approximately These relationsare exactly the sameas are seenin 81 m.y. ago and that temperaturesin the vicinity epithermaldeposits where K-silicate alterationhas of the ore body remained high enough to prevent affected intermediate volcanic rocks and the process quantitativeretention of argonin the feldsparuntil has occurredduring a single stage. Another sug- about 75 m.y. ago. Studiesby Hart (1964) and gestionis that mineralizationcould have been still Aldrich et al. (1965) have demonstratedthat K- younger (e.g., Tertiary) and all of the GL-3 ages feldsparis less retentiveof argon than micaswhen are hybrid, being reset to varying amounts. These affected by a thermal event. Thus, the coarse- possibilitiescannot be entirely ruled out, however. grained sericiteage of 81 m.y. would more nearly The discordantages occur in a highly deformed reflect the age of mineralizationthan the feldspar block of altered rock, whereasthe K-Ar age of the age. If this is correct,the agesimply that tempera- sericitefrom the undeformedblock of altered grano- tures remainedrelatively high for a periodof at least diorite(Section 4 Pit) showsno apparentargon loss. several million years in the South Extension Pit The physicaleffects of deformationextend down to and that hydrothermalmineralization in the Getchell the size range of individual crystals (kinking of area took place10 m.y. after the depositionof tung- muscovitelamellae) and we suggestthat the defor- sten and the Section4 gold depositswhich are all mation,or perhapsthe frictionalheat generatedby approximately90 m.y. old (Table 4). The large it, causedthe argon loss. The lossvaries in amount thermalmetamorphic halo to the eastof the Getchell relative to already establishedcriteria on mineral mine could indicate a long-term event. However, retentivityof argon and on grainsize(Hanson and detailed studies of the age relations of vein and Gast, 1967; Hart, 1964). Hence, we suggestthat alteration minerals in epithermal ore depositsand the discordantGL-3 agesare bestexplained by post- modern hydrothermalsystems (Silberman et al., mineralizationdeformation. Using our favored in- 1972; Silberman and Ashley, unpub. data, 1972; terpretation,we believethat the tungstenore bodies Silbermanand White, unpub.data) and in the Bing- andthe Getchellgold deposit formed shortly after the ham Canyonporphyry copper deposit (Moore and emplacementof the granodioriteand andesitepor- Lanphere,1971) havedemonstrated that lifetimesof phyryat approximately90 m.y.ago during a hydro- hydrothermalore systemsare of the order of 1 to 3 thermal stage that overlappedand succeededthe m.y. No systemwith a lifetimeof morethan 3 m.y. magmaticactivity. The entire processstarted with has yet beendocumented. We considerit unlikely emplacementof the stockand contactmetamorphism that hydrothermalalteration and ore depositionat of the adjacent Paleozoicsedimentary rocks and Getchelllasted for the 10-m.y. span. continuedwith a hydrothermalstage that produced An alternativeinterpretation, one that the writers the tungstenin the tactites adjacent to the grano- favor, is that the ageof 81 m.y. determinedon coarse diorite. The Getchell gold mineralizationwas a sericite(67 m.y. on fine-grainedsericite) and the last, lower temperaturephase of the samehydro- K-feldsparage of 75 m.y. from the SouthExtension thermalactivity. We view the entire processof Pit (sampleGL-3, Table3) resultedfrom argon loss emplacementof the granodioritepluton, the andesite due to deformationfrom postmineralizationmove- dikes, the tactite-tungstendeposits, and the hydro- K-AR AGE RELATIONS AND TUNGSTEN AND GOLD MINERALIZATION 655 thermal gold depositsas all belongingto the same gest sericiticalteration occurred during ore deposi- thermal episode. Becauseof th'e analyticaluncer- tion. Roberts(Roberts et al., 1971, p. 27) believes taintiesof ages (2 to 3 m.y.), the data do not permit that gold mineralizationat Gold Acres may be re- an estimateof the length of time the entire thermal lated to intrusiverhyolite porphyrydikes of Oligo- systemoperated, but previousand current studies cene age that are associatedwith gold-quartz veins on lifetimesof hydrothermalsystems suggest that 2 at Tenabo, four miles to the northeast. No igneous to 3 m.y. is a reasonableestimate. rockshave beenfound in the vicinity of Gold Acres We wish to stressthat our interpretationdoes not other than the altered sills and quartz monzonite require a genetic relation betweenthe pluton and mentionedabove. The coarse-grainedsericite dated gold deposits. Further geochemicaland isotopic at Gold Acres is similar to that of the Section 4 Pit studiesare necessaryto clarify the geneticrelations sampleat Getcheil. Althoughthe studyhas not been betweenthe differentepisodes of the thermal system. detailedenough to demonstrateconclusively that the [These studiesof primary hydrothermaland altera- Gold Acres mineralizationis 92 to 94 m.y. old, the tion minerals in the igneousrocks as well as in presentdata stronglysuggest this conclusion. the tungstenand gold depositsin the Getcheilarea At Carlin, thin felsic dikes have been altered to an are in progressat the U.S. GeologicalSurvey. assemblageof quartz, kaolinite, and sericite, indica- Experimentalgeochemical studies on the solubilities tive of an argillic alterationassemblage (Meyer and of tungstenand gold under varying conditionsof Hemley, 1967). Sericite has been identifiedin the temperature,pressure, and solutioncomposition are main pit in altered mineralizedlimestones of the in processat StanfordUniversity. Comparisonof RobertsMountains formation adjacent to the dikes the resultsof thesestudies should give new evidence as well as within the dikes themselves(A. S. as to the geneticrelations between the mineral de- Radtke, unpub. data, 1972). Altered dike material positsof the Getchellarea (A. S. Radtke,writ. com- with coarse-grainedsericite was collected'during mun., 1973).] geochemicalsampling (A. S. Radtke, unpub. data, Ages of Mineralization of Other 1972), but the sericiteconcentrate was inadequate for K-Ar dating. Disseminated Gold Deposits The Cortez gold depositmay not be directly dat- Roberts et al. (1971) suggestthat most of the able by the K-Ar method. The gold ore occursin disseminatedgold depositsin north-centralNevada altered,brecciated limestone along the margin of a formed during an early Tertiary intrusive-metallo- rhyoliteporphyry dike which has undergoneargillic genicepoch, one of severaldefined by Silbermanand alteration (Wells et al., 1969). Biotite and sani- McKee (1971). The basisfor this suggestionis not dine from an unalteredpart of the dike are datedat clear from the data presented. 34 and 35 m.y., respectively(Wells et al., 1971). At Carlin, Cortez, Gold Acres, Northumberland, Many quartzporphyry dikes occur in the Cortezarea and Preble, altered igneousrocks, usually felsite and mineralizationof the Cortez gold mine is be- dikesor sills,are presentwithin the mineralizedzones lieved to have been contemporaneouswith, or later (Wrucke and Armbrustmacher,1969; Wells et al., than,emplacement of thesedikes (Wells et al., 1971). 1969; Hausen and Kerr, 1968; A. S. Radtke, oral In summary,Getchell has now beendated at ap- commun.,1972; John Livermore, oral commun.). proximately90 m.y.; Gold Acreshas a probableage In at leastthree of the disseminateddeposits, coarse- of 92 to 94 m.y., Cortez is youngerthan 35 m.y., grained sericite formed in dikes or sills spatially and the age of Carlin is uncertain. Careful study associated with th• ore. of the mineral assemblagesin alteredigneous rocks, At Gold Acres, felsite sills in the ore zone re- which occur in the ore zones of all the disseminated crystallizedto quartz,sericite, and iron oxide (C. T. golddeposits, may revealdatable sericite (2M• mus- Wrucke, unpub.data, 1972), an assemblageof min- covite) and it may be possibleto determinethe age erals correspondingto sericiticalteration descri'bed of mineralizationof most of these deposits. Care by Meyer and Hemley (1967). High gold values mustbe taken,however, in samplingto seethat areas occur in the fractured altered felsite (Wrucke and of postmineralizationdeformation are avoided and Armbrustmacher,1969). Silberman and McKee that the sericitechosen for datingis as coarse-grained (1971) report a K-Ar age of 94 m.y. from coarse- as possible. grainedsericite (2Ma muscovite)from a felsitesill Acknowledgments at Gold Acres. Sericite (2M• muscovite)from a drill coreof alteredquartz monzonite which lies 500 We wishto thankRoger Ashley, Ralph Erickson, feet below the Gold Acres Pit gave a similar age of GeorgeWalker, Richard Marvin, and Arthur Radtke 92 m.y., biotitefrom the samedrill core gave a 99- and an anonymousreviewer for critically reading m.y.age. Silbermanand McKee (1971,p. 25) sug- earlier versions.of this manuscript. Their sugges- 656 M. L. SILBERM,4N,B. R. BERGER,AND R. A. KOSKI tionshave considerably added to the rangeof possible cessory minerals of igneous rocks (1953-1957): U.S. Geol. Survey Bull..1097-B, p. 82. interpretationsof the data presented. The writers, Joralemon, Peter, 1951, The occurrence of gold at the however,bear soleresponsibility for the conclusions Getchell mine, Nevada: Ecoa. G•.oL.,v. 46, p. 267-310. reached. Lee, D. E., Marvin, R. F., Stern, T. W., an6 Peterman, Z. E., 1970,Modification of K-At agesby Tertiary thrusting M. L. S. AND R. A. K. in the Snake Range, White Pine County, Nevada, in U.S. GEOLOGICAL SURVEY GeologySurvey Research1970: U.S. Geol. Sur.veyProf. Paper 700-D, p. D92-D102. 345 MIDDLEFIELD ROAD Meyer, Charles, and Hemley, J. J., 1967, Wall rock altera- MENLO PARK, CALIFORNIA94025 tion, in Barnes, H. L., ed., Geochemistryof hydrothermal B. R. B. ore deposits: New York, Holt, Rinehart and Winston, CONTINENTAL OIL COMPANY p. 166-235. 1085 EAST SECONDSTREET Moore, W. J., and Lanphere, M. A., 1971, The age of por- phyry-type copper mineralization in the Bingham RENO,NEVADA 89502 district, Utah---A refined estimate: EcoN. Gv.oL., v. 66, October11, 1973; January3, 1974 p. 331-334. Neuerburg, G. J., 1966, Distribution of selected accessory minerals in the Osgood Mountains stock, Humboldt REFERENCES County, Nevada: U.S. Geol. Survey Misc. Geol. Inv. Aldrich, L. T., Davis, G. L., and James, H. L., 1965, Ages Map 1-471, scale 1:24,800. of minerals from metamorphic and igneous rocks near Radtke, A. S., and Scheiner,B. J., 1970, Studiesof hydro- Iron Mountain, Michigan: Jour. Petrology, v. 6, pt. 3, thermal gold deposition(I), Car.lin gold deposit,Nevada-- p. 445-472. The role of carbonaceousmaterials in gold deposition: Dalrymple, G. B., and Lanphere, M. A., 1969, Potassium- EcoN. Gv.OL., v. 65, p. 87-102. argon dating--Principles, techniques,and applicationsto Roberts,R. J., Radtke, A. S., and Coats, R. R., 197,1,Gold- geochronology: San Francisco, Calif., W. H. Freeman bearing deposits in north-central Nevada and southwest and Co., 258 p. Idaho, with a sectionon Periods of plutonismin north- Erickson, R. L., Marranzino, A. P., Oda, Uteana, and Janes, central Nevada by M. L. Silberman and E. H. McKee: W. W., 1964, Geochemical exploration near the Getchell EcoN. GEOL.,V. 66, p. 14--33. mine, Humboldt County, Nevada: U.S. Geol. Survey Silberman, M. L., Chesterman,C. W., Kleinhampl, F. J., Bull. 1198-A, p. A1-A26. and Gray, C. H., Jr., 1972, K-At ages of volcanic rocks Ferguson, H. G., 1929, The mining districts of Nevada: and gold-bearing quartz-adularia veins in the Bodie EcoN GEOL.,V. 24, p. 115-148. mining district, Mono County, California: Ecoa. GEOL.. Hanson, G. N., and Cast, P. W., 1967, Kinetic studies in V. '67,p. 597--604. contact metamorphic zones: Geochim. et Cosmochim.Acta, Sil,berman,M. L., and McKee, E. H., 1971, K-Ar ages of v. 31, p. 1119-1153. g,ranitic plutonsin north-central Nevada: Isochron/West, Hart, S. R., 1964, The petrology and isotope-mineralage no. 1, p. 15-32. relations of a contact zone in the Front Range, Colorado: Wells, J. D., Elliott, J. E., and Obradovich,J. D., 1971, Jour. Geology,v. 72, p. 493-525. Age of the igneousrocks associatedwith ore deposits, Hausen, D. M., and Kerr, P. F., 1968, Fine gold occurrence Cortez-Buckhornarea, Nevada, /n GeologicalSurvey Re- at Carlin, Nevada, in Ridge, J. D., ed., Ore deposits in search1971: U.S. Geol. Survey Prof. Paper 750-C, p. the United States, 1933-1967 (Graton-Sales vol.): New C127-C135. York, Am. Inst. Mining Metall. Petroleum Engineers, Wells, J. D., Stoiser, L. R., and Elliott, J. E., 1969, Geology p. 908-940. and geochemistryof the Cortez gol.ddeposit: EcoN. GEOL., Hobbs, S. W., 1948, Geology of the northern part of the V. 64, p. 526--537. Osgood Mountains, Humboldt County, Nevada: unpub. 'Wright,T. L., 1968,X-ray and opticalstudy of alkali Ph.D. thesis,Yale Univ., New Haven, 97 p. feldspar An X-ray method for determining the composi- Hot.z, P. E., and Willden, Ronald, 1964, Geology and mineral tion and structural state from measu,rement of 20 values deposits of the Osgood Mountains quadrangle, Humboldt for three reflections: Am. Mineralogist, v. 53, p. 38-104. County, Nevada: U.S. Geol. Survey Prof. Paper 431, Wrucke, C. T., and Armbrustmacher,T. J., 1969,Structural 128 p. controls of the gold deposit at the open-pit mine, Gold Jaffe, H. W., Gottfried, David, Waring, C. L., and Worth- Acres, Lander County, Nevada labs.]: Geol. Soc. America ing, H. W., 1959, Lead-al.phaage determinationsof ac- Abs. Programs, v. 1, no. 3, p. 75.