The Occurrence of Gold at the Getchell Mine, Nevada

Home , Getchell Mine, Potosi Mining District, Stibnite

THE OCCURRENCE OF GOLD AT THE GETCHELL MINE, NEVADA. x

PETER JORALEMON.

ABSTRACT. The Getchellveins are lenticularreplacement bodies lying alongarcuate branchesof a complexrange-front fault system. The faultzone cuts all rocksof the districtand is tentativelydated as late Tertiary. The moreintensely mineralized portions of the depositform a shallow blanketwith rootsthat projectdownward into areasof sparsemineralization. The goldshoots are restricted to areasOf intense mineralization. Native gold and native silver are the only economicminerals. The greatbulk of thegolc• occurs in minutebut microscopically visible particles. Somegold may also occur in submicroscopicparticles and some may be in solid solutionin pyrite and carbon. The ore minerals,dissolved in alkali sulfidesolutions, are believedto havebeen deposited •vhen the sulfide/onconcentration in the hydrothermal liquiddecreased, making unstable the doublesulfides of gold;iron, and arsenic. The Getcheildeposit is similarin manyways to the Nevadaquicksilver depositsand present-dayhot-spring deposits. The Getcheilore occurrence may representa gradationfrom the commonepithermal gold depositto the cinnabar deposit. It is therefore placedclose to the feeblestend of the epithermalgroup. INTRODUCTION. In the past, a number of important gold mines have producedmillions of dollarsin gold from ore in which no gold couldbe seen. The Getchellgold depositis the.most recently discovered of thesemines of "invisible"gold. Be- causegold had not beenseen in the ore,either at Getcheilor at similardeposits, the problemof the state, or occurrence,of the gold has been subjectto con- siderablecontroversy. A prime purposeof the presentstudy has beento as- certain whether the modernmicroscopic and polished-sectiontechniques are capableof sheddingsome light on the problemsof gold occurrence. Location and ttistory.--The Getcheil mine is located in the northeastern cornerof the OsgoodRange in east-centralHumboldt County,Nevada (Fig. 1). It lies 26 milesnorth of the town of Golcondaand some15 milesnorth- west of Red House, the nearestrailroad station. The gold depositis in the old Potosi mining district but was itself not discovereduntil 1934. Between1934 and 1945, the depositwas extensively exploredby both surfaceand undergroundworkings. A 1,000-tonmill was erected,and in 1944 and 1945, the Getchellmine had becomethe largestgold producerin Nevada. In 1945,mining had nearlyexhausted the oxidizedore and it becameapparent that milling practicesin use were not suitablefor xUndertaken in part as a SamuelFranklin Emmons Memorial Fellowship award.

267 268 PETER ]ORALEMON. treating the sulfideore. Mining operationswere halted and an intensive metallurgicalresearch program was begun. In 1947, a pilot mill was started treating100 tons a day. By'thesummer of 1948a satisfactoryflow sheethad beendeveloped and constructionbegan on a 1,500-tonmill. Production,at presentlimited to 1,000 tonsa day, will be increasedupon the completionof the mill. Acknowledgments.--Thewriter is indebtedto the staff of the Getcheil mine,and particularlyto Mr. R. A. Hardy, for financialassistance and whole- heartedcooperation. G. S. Koch and R. S. Creelyably assistedin the two summers of field work.

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FIG. 1. Index map of Nevada showinglocation of Getchellmine.

The laboratoryportion of this studyhas beencarried on with the aid of a SamuelFranklin EmmonsMemorial Fellowship. The facultyof theDepartment of Geologyat HarvardUniversity has made valuablecriticism and given much'assistance in the laboratorywork. Pro- fessorL. C. Graton in particularhas donatedgenerously of his time and experience.All polishedsections were preparedby Mr. CharlesFletcher at theI-Iarvard Laboratory of MiningGeology. OCCURRENCE OF GOLD .4T GETCHELL MINE, NEV.4D.4. 269

GENERAL GEOLOGY. The Osgood;Range is apparentlya typicalGreat BasinRange. Its steep flanksare toppedby rollinguplands in thecenter of the rangeand it is bounded on at leastone sideby normalfaults that are responsiblefor the uplift of the range. The mountainsare composedof sedimentsof Paleozoicage that havebeen cut by a seriesof faultsinto isolatedstructural blocks (16). 2 The attitude of the rocksvaries widely from blockto block. The sedimentsare intruded by a large stock of Mesozoicgranodiorite and a seriesof related dacite porphyrydikes, and by smallTertiary andesiteporphyry dikes. Basaltflows anda smallbody of rhyolitetuff are productsof Tertiaryvolcanism. Irregular patchesof the tuff nearthe minehave been silicified and closelyresemble the opalitefound in certainNevada quicksilver deposits (3).

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Fro. 2. Getchell Mine, Nevada. The scars on •e hill behind •e camp mark the footwalls of ½e open

DescriptiveGeology oI the GetchellMine.•The beddedrocks of the mine area are interbeddedlim•ston• and carbonaceousargillit• (Fig. 5). Th• most stri•ng feature of the sedimentsis the extreme l•nticularity of the lim•ston• beds, as shown in Figure 3. Limestone formations s•v•ral hundred feet thick may drastically thin or pinch out completelyin a strike length of less than 1,•0 feet. The sedimentsare intrudedby the larg• granodioritestock, the noaheastern lobe of which is cut off by the fault zon• along which the gold ore bodies occur. Later andesiteporphyry dikes, trending more or less northerly, are apparentlylocalized in and n•ar this fault zone. A roughly conicalpipe of alteredrhyolite porphyry tuff li•s to the eastof this fault. • Numbers in parenthesesrefer to •ibliograpby at end of paper. 270 PETER JORALEMON.

Structure.--The principalstructure of the sedimentsis a fold whoseaxis plungesat about 45ø northeast. Along the southernlimb of the fold, the sedimentsstrike northerly and dip moderatelyto the east, and to the north they are isoclinallyfolded and dip to the northeast. This structurehas been importantin localizingthe Getchellore bodies. The Getchell fault zone is the dominant structural feature of the mine area. It trendsin a northerlydirection and is composedof a persistent,footwall strand dipping moderatelyto the east, with steeper,arcuate hanging-wall branches(Figs. 3, 4). The footwall strand is shown by well-developed mullion and drag-foldstructures to be a rift fault, alongwhich the hanging- wall block movedan unknowndistance with respectto the footwall block (Fig. 6). Later normalmovement, indicated by steeplypitching slickensides engravedon the mullion faces, took place along this footwall strand. The steeper,hanging-wall branches were formed at that time and occur above flexuresin the footwallfault wherethe dip flattens,as shownin Figure 4.

ORE DEPOSITS. GeneralFeatures.--The gold ore bodiesare sheet-likemasses that lie along the variousstrands of the Getchellfault zone. They extendat least7,000 feet horizontallyand 800 feet down the dip, and vary in width from a few feet to more than 200, averagingabout 40 feet wide. The vein pattern,extremely complexat the surface,becomes simpler with depthand, deeperthan 800 feet belowthe surface,all the branchveins apparently join to form a dingleper- sistentstructure, illustrated in Figure 4. Gan#ueMineraIs.--The veins consistof shearedand mineralizedargillite and limestonecut by quartz and calciteveins and containinga soft, plastic gumbothat has replacedthe sediments. The gumbo is the principal gold- bearer. Barite, gypsum,fluorite, and chabaziteare minorgangue minerals and occurin smallisolated pockets within the veins. The shapeof the moreheavily mineralizedportions of the veins is that of an irregular, relatively shallow blanketwith three root-like projectionsthat extenddownward into the region of sparsemineralization, as illustratedin the longitudinalprojection, Figure 3. The three "roots" of the ore body appearto mark the original major channelways along which the hydrothermalfluid passedin greatestabundance. There are striking differencesin the mineralizationwithin as contrasted with outsidethese major channelways. Within theseroots, now markedby high-gradegold ore, depositionhas beenlargely by non-selectivereplacement. In theseareas, all rocktypes and early gangueminerals have been replaced with equalease by the gangueminerals and the sulfidesof the later phasesof the ore-formingperiod. The larger part of the material depositedhere is apparently magmaticallyderived, and the processof depositionhas been one of wholesalereplacement. In the sparselymineralized stretches between the ore shoots,replacement has been limited and extremely selective,and all materialexcept small amounts of auriferouspyrite appearsto havebeen locally derived. In these outlying ar•easof the veins, mineralizationinvolved the local reworkingof rock mineralswith only minor introductionof material OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 271 272 PETER J'OR.,'ILEMON.

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5600

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•'1'• Rhyolitutuff • Limestone• Unminerolizudfou$l :i:...:-•Andesite porphyry -•--'] Argillitu • Centoct f'G[•-•ß Gronodioritu • Goldvein --"• Grodotiono$coatoct 4• • 8• ' I• Feet

FIG. 4. Geologiccross-section through Getcheil ore body. OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 273 foreign to the rocks. Thus. calcite veins are restricted to beds of limestone and quartz veinsand lensesoccur only in argillite. The gumbo,the importantgold-bearer, is unusualin that while it appears to be a fault gouge, it actually consistsof minute subhedralquartz crystals embeddedin a nearly submicroscopicintergrowth of quartz and amorphous carbon(Fi•s. 9, 11,12, 14). In placesveins of gumbo several feet wide and parallelto the surroundingsedimentary rocks end abruptlyagainst unsheared limestoneor argillite. That it is of hydrothermalorigin is indicatedby the fact that in thin sectionit appearsactually to replacethe wall rock, as shownin Figures8, 9, and 10. It was apparentlyformed in two stages. First was the replacementof wall rock by fine-grainedquartz to form a porousaggregate of subhedralquartz crystals in narrow bedding veins. tn the secondstage, carbon,probably derived from underlyingargillite, was introducedinto the porous quartz veins and was depositedinterstitially to the quartz. Rather surprisingly,those portions of the veins that containedthe most gumboand wouldordinarily be consideredto be but slightlypermeable, remained the most permeablesections' of the veins and the later hydrothermalsolutions were there concentrated. MetalliferousMinerals.--The depositionof importantamounts of gangue mineralshad apparently stopped by the time of maximumdeposition of sulfides. 'Pyrite and minor pyrrhotite are the earliest sulfidesand in the ore shoots theyoccur in irregularporous masses, as shownin Figure 26. .In the outlying segmentsof the veinspyrite has a tendencyto assumeeuhedral forms. Re- placementhas beenthe importantmechanism of depositionof all later sulfides within the ore shoots(Fig. 12) whereasfracture-filling is predominantin the sparselymineralized zones (Fig. 13). Arsenopyriteis presentin minuteeuhedral crystals and is almostentirely restricted'to the portionsof the veinsoccurring in the andesiteporphyry. Marcasite constitutesabout 5 percentof the iron sulfidesin the central channelwaysbut is rare or non-existentin the outlyingareas. It occursas minutelaths and as irregularshells or rims encirclingearlier pyrite grains. Arsenic sulfidesare the most abundantof the metalliferousminerals and, unlikepyrite, are entirelyrestricted to the areaswithin the veinsthat now contain noteworthyquantities of gold. Orpiment is apparentlyearlier than realgarand is foundonly in isolatedpockets, characteristically occurring in low-gradesections of the veins. In these pockets,orpiment constitutesas muchas 40 percentof the mineralizedmaterial and is veinedand coatedby realgar. Realgar occursthroughout the commercialore bodiesin 'amounts rangingfrom 1 to 10 percent. It showssuch a consistent-relationto gold that the presenceof realgaris usuallya strongindication of goodvalues. Stibnite,like orpiment,occurs in restrictedpockets in whichit is present in two unusualforms. Stibnite occurringin narrow, pipe-like bodiesis presentin a seriesof clustersof hair-like crystalsso minuteas to suggesta softblack velvet coating on the rocks. The molybdenummineral ilsemannite .is doselyassociated with this stibnite. In one smallarea the fracturesin the rock are coatedwith metastibnite,a red paint or stainwhich has beenshown by X-ray photographsto be finely dispersedstibnite. Similar forms of 274 PETER JORALEMON.

stibnitehave beenfound in the sinterand the hot springmuds at Steamboat, Nevada (4). Cinnabaris presentas a thin coatingon calciteand chabazitecrystals in a narrow and discontinuous branch vein. While it does not occur in com-

Fro. 5. Interbeddedlimestone and argillite from Getchellmine area. The vein material has replacedportions of this formation, preservingthe thinly laminated structure. Fro. 6. Fault surface that forms the foot-wall strand of the Getchell vein system. The nearly horizontal mullion structureis strong evidencein favor of horizontal fault displacementalong this surface. OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 275

FIG. 7 Typical vein material. Realgar veins, appearingas white bands,preserve the sedimentarystructure. The black bandsare of carbonaceousgumbo and unreplaced linestone and argillite. A minor pre-mineral normal fault cuts and warps the bedding. FIG. 8. Photomicrographof thin sectionof vein material, showing incipient silicificationof argillite. The fine-grainedargillite shown in the lower half of the picturehas beenreplaced by coarsequartz grains. In the processof this replacement the carbonhas beenlargely removed. The texture of the replacingquartz grains is similar to that of the quartz in the rich ore gumbo. Crossnicols, X 50. 276 PETER JORALEMON. mercialquanti. ties, its verypresence suggests a possible genetic relation between Getchelland the cinnabardeposits of Nevada. Magnetiteis apparentlythe latestof the metalliferousminerals associated with gold,and is the one mostclosely' related to gold. It occursonly in microscopicparticles and hasbeen seen only in the gumboand andesite. It is plainlynot a detritalmineral. It is so closelyassociated with gold that any polishedsections containing a clusterof magnetitegrains have also some goldclose at hand,generally within a millimeter(Figs. 22, 23).

PRECIOUS METALS.

Distribution. Economicamounts of goldare entirelyrestricted to the veins•within which the richer ore shoots are loci of more intense mineralization. The richer goldore is in theshape of a relativelyshallow, discontinuous blanket with three root-likedownward projections, as illustratedin the projectionin Figure'3. The cause of localization of the ore shoots is discussed in a later section. The portionsof the veinsbetween the rich shoots,and. the wall rock for several hundred feet on either side of the veins contain everywherea small amount of gold,ranging from 0.01 to 0.08 ouncesper ton. The commercialgold ore hasbeen developed over a strikelength of 7,200feet and to a depthof as much as 800 feet. At this depth,rich gold ore is still present.

Visible Gold. Becauseof the crumbly,non-cohesive nature of the gumbo,it was necessary in polishingspecimens of the ore for microscopicinvestigation to developa techniquewhich would allow the polishingof even the smallestparticles of gold.. Mr. CharlesFletcher of the Laboratoryof Mining Geologyat Harvard Universityprepared the polishedsections and his patienceand skill haveaided immeasurablyin this investigation. In the early stagesof the microscopicstudy, extremelyfew gold particles were to be seen,and it becameapparent that the polishingmethods in use werenot satisfactoryfor this problem. The polishingtreatment finally adopted representsthe resultsof progressivetrials and improvements.In this process, the sampleto bepolished is first impregnatedwith a liquidbakelite resinoid and then baked. It is next sawedand is again impregnated. Following this it is placedin a bakelitebriquette, and is groundto a smoothsurface. After a final impregnation,it is readyfor the severalstages of polishingon the Graton- Vanderwiltpolishing machine. The multipleimpregnation is necessarybe- causea singleimpregnation penetrates only a short distanceinto the section. By usingthis methodof polishingand by usinga microscopewith a Zeiss2- millimeterobjective lens, N.A. 140, mineralgrains having an area of as little as 0.1 squaremicrons are visibleand recognizable. The propertiesof gold are so diagnosticas to make it an ideal subject for sucha detailedstudy. Its brilliant yellowcolor makes it easilyrecognized in larger particlesbut whenthe grain size decreasesto a diameterof lessthan OCCURRENCEOF GOLD AT GETCHELL MINE, NEVADA. 277

FIG. 9. Photomicrographof this sectionof vein material in limestone. The white, coarse-grainedcalcite is replacedby the carbon-rich ore gumbo that appears black,and by realgar,the shatteredmineral in the lower left. Crossnicols, X 50. Fig. 10. Photomicrographof thin section of coarse calcite partially replaced by high-grade ore gumbo. The calcite in the irregularly mottled grains shows the same extinction over the entire sectionand representsisolated remnants of a once much coarsergrain. Quartz occursin the small clear grains and.the black material is carbon. The texture of the replacing gumbo is typical of the richer material. Cross nicols, X 48. 278 PETER IOR.4LEMON. one micron, the color becomesless noticeableand other features must be used for identificationof minuteparticles. The extremesoftness of gold prevents the achievementof a perfectpolish on smallgrains, but every minutegrain of goldhas a characteristicundulant, "egg-shell" texture (Figs. 24, 25, 26). Gold is exceededin reflectivityonly by native silver,platinum, and copper,and the egg-shelltexture may be used for positiveidentification of all particlesto a limit of microscopicvisibility at a magnificationof X 2,500. The describedpolishing technique is probablynot perfect; indeed it is reasonablycertain that somegrains that would have been abovethe limit of visibility, had they remainedin place and been polished,were torn from the polishedsurface. The proportionof grainsthus lost undoubtedlywould in- creaseas the grains becamesmaller. Furthermore, some grains of visible size may not have been seen. Even with the most careful polishing,the extremesoftness of goldmakes inevitable its depressionbelow the impregnated surroundings. Hence there is an obliqueband aroundevery gold grain that givesno reflectionup the tube of the microscope. The width of this band is a functionof the relative softnessof gold to its surroundingsand thus tends to be independentof grain size; but the relativeimportance of this relief shadow increaseswith decreasingsize of gold grain. If the diameterlimit of visibility of a grain of goldideally polished is "x," and the width of the obliqueshadow ' in actualpolishing is "y,"then an imperfectly polished g.rain must have a diameter of x + 2y to be visible. All grains smallerthan this would appearonly as tiny blackpits. Finally it is certain that not all grains possessinga true diameter above the ideallimit of visibilitythat were actuallypresent at the planeof polishwere seen. This planemust have touched many grains,large and small,so far from their equatorof maximumdiameter that the interceptat the planeof polishdid not attain practicalvisibility. In this investigation,more than 2,600 gold particleswere identifiedand measured.The resultsof thesemeasurements are givenin Figure 17. The graphshows the enormousincrease in the abundanceof goldparticles with de- creasinggrain size. It alsoshows that the greatbulk of the goldis in the rare and widely scatteredlarger particleshaving an exposedarea of more than 1,000 squaremicrons. All gold particlesseen in the Getchellore are roughlylenticular in cross- section. To get a better idea of the three dimensionalshape of the particles, the length and width of individualgrains visibleon the polishedsection were measuredand, after further polishingaway a measuredthickness from the section, the dimensionsof the samegrains were remeasured;this procedurewas repeated,step by step, at about 2-microndepth intervals,until one after another of the grains disappeared.It is apparentthat the gold particlesare lenticular in three as well as two dimensions. Knowing the approximate dimensionsof individualgrains and the numberof grains visible on every polishedsurface, it is possibleto estimatethe minimumgold contentof the polishedspecimen and to compareit with the assayvalue of the specimenfrom whichthe sectionwas prepared.

! OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 279

The averagepolished section may be regardedas a rectangularblock with dimensions2 x 1 x 1 centimeters. Each blockweighs about 2.7 gramsand 336,000such blocks would be requiredto makea ton of ore. Gold with some silver presenthas a specificgravity of about 17.3. One cubiccentimeter of gold weighs0.556 Troy ounces,and ore containing0.5 ouncesof gold per ton must contain0.9 cubiccentimeters of gold. If the gold occurredin cubes one centimeteron an edge, an enormousnumber of polishedsections might haveto be preparedbefore this goldwould be encountered.If the assumption is made,however, that goldis presentin grainshaving an averagedimension of 1 x 1 x • micronsand is sodistributed through the ore that eachmass the size of a polishedsection holds a representativeamount, then one ton of ore containing 0.5 ouncesof goldwould contain 2 x 10x-ø grains of gold. Every polished ß sectionblock would containan averageof 5.94 x 10ø gold particles. Because of the abundanceof non-opaqueminerals in the Getchellore, it is possibleto see not only mineralsactually cut by the polishedsurface but alsominerals lying asmuch as one micron beneath the surface. T.huseach polished sectiori may be visualizedas beingdivided into a seriesof flat sheetsone micron thick. Every sheetwould contain1,045 particlesof gold. There are many assumptionsin thesecalculations, but the resultsgive at least a rough idea of the numberof goldparticles one might expect to seeif all goldoccurred in visiblegrains of the sizespecified. Increasingsize of grainsgreatly reduces the number. The investigationof particlesize and numberhas shownthat there is little correlationbetween the assayvalue of the sampleand the calculatedvalue of the polishedsection. As an example,five polishedsections were prepared •rom an ore samplewhich was shownby assayto contain1.06 ouncesof gold per ton. In one of thesesections, fewer than 20 gold particlesof micronsize or largerwere seen, while another contained more than 800 particlesof the same or larger size. The other three varied widely but fell within theselimits. The conclusionmust be reachedthat the goldis not regularlydistributed, even in a smallsample, but ratherthat it is stronglyconcentrated in certainbands and lenses about one centimeter wide. The five sections mentioned above were all preparedfrom gumbo, so this spotty gold distributionis apparent even within the gumbowhich is generallyan excellentindicator of rich ore. Mostof thevisible gold is intimatelyassociated with thefine-grained quartz- carbon matrix of the gumbo. Many of the particles partially rim larger quartz grains. Someof the gold is rather uniformlydistributed throughout the gumboin smallisolated particles. This is slightlymore abundant in those bands in the ore that contain more carbonaceousmaterial. These particles average4 squaremicrons in exposedarea, somewhatlarger than the average of all visiblegrains. In additionto the uniformlybut sparselydistributed particles,gold grains also occur in clustersof roughlylenticular outline (Figs. 27, 28). These gold-richpods are composedof abundantgold grainsthat averageone half of a squaremicron in exposedarea. They are commonly so abundantthat under low power magnificationthe entire area of the lens hasa paleyellow color. Yet within the lensesgold occupies less than 1 percent of the area. Like the individual grains mentionedabove, this gold occursin 280 PETER JORALEMON. the carbonaceousmatrix, and quartz grains within the gold pods stand out as barren islands. The lenses are as much as 100 microns wide and are com- monlyseveral millimeters long. Theseaggregates of goldparticles have been seenonly within the gumboand are commonlyelongate parallel to the structure of the gumbo. Even within the gumbo,these lenses are rare and the presence of oneor two in a samplewill givean erratichigh assay. Gold in thesepods is intimatelyassociated with magnetiteand carbonaceous matter. Magnetiteis slightlymore abundantthan gold and, like gold, is very looselyembedded in the matrix and is easilyremoved by rubbing. During the impregnationof the polishedsections, both magnetite and gold from someof theselenses were pulled out from the carbonaceousmatrix by the inrush of resinoldafter evacuationand were carried along open cracksin the section, finally forming inclusionsin veinlets of resinold. This magnetite seldom. showsa euhedralform andgenerally occurs in irregularlyrounded masses and angularfragments. It is largelyrestricted to the gumbo. Rarely,both gold and magnetiteare rimmedby a mamillaryhalo of carbon(Fig. 23). There is thusa strongsuggestion that therehas been a redistributionof smallamounts of carbonduring or after the main periodof gold and magnetitedeposition. Magnetiteoccurs not only in the richgold lenses but alsodisseminated through thegumbo in smallisolated grains. No magnetitehas been seen outside of the ore bodies. In addition to the occurrencesdiscussed above, gold is found rarely as inclusionsin the sulfides. All of the major sulfidescontain some visible gold, but pyrite and marcasiteare the principalhosts. Lessthan I percentof the totalnumber of goldparticles seen occurs in sulfides. It maybe that the larger goldparticles were depositedalmost entirely in the gangue,but thereis some evidenceto suggestthat suchis not the case. The gold particlesoccurring in the sulfidesare far more looselyheld than that in the gangue. The.particles in the sulfidesthat were not wrestedfrom the surfaceduring grinding and polishingare invariablyremoved when the immersionoil is wipedfrom the section. So it is probablethat the sulfidescontain. far more gold of visible sizethan canbe seenin polishedsections. All formsof pyrite and marcasitecontain some visible gold, but the more porousvarieties are more commonly hosts for themetal (Figs. 25, 26). Where gold particlesoccur as inclusionsin subhedralcrys. tals of pyrite they are generallyin theouter parts of thecrystals. Nonehas been seen in thecores of thesecrystals. Summaryof Occurrenceof VisibleGold.--Visible gold occurssparsely disseminatedthrough every rock type in theveins and is particularlyabundant in the gumbo. Golddistribution is quitesimilar to that of 'marcasiteand realgar,and is more restrictedthan pyrite. The only mineralsthat are strikinglyassociated with gold are carbonand magnetite. Laboratorytests showa strongdirect relation between carbon and gold, and the gold content of carbonconcentrates increases as the purity of the concentrateis increased. The sizeof the particlesof visiblegold ranges from a fractionof a rrlicron to nearlyone millimeter; the smaller grains are far moreabundant than the OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 281 larger. The weight-distributioncurve shown by thebroken line in Figure17 showsthat the larger part of themass of goldis fromthe less abundant, larger particles. The moreporous forms of the sulfidescontain more gold than the solid euhedralcrystals. There is apparentlya strong,direct correlationbetween the amountof surfaceexposed in the host minerals,that is, the porosity, andthe amountof containedinvisible gold.

"Invisible" Gold. It is probablethat not all the goldoccurs in visiblegrains. The "invisible" goldmay occurin' any of threeforms. First, it may occurin the nativestate in particlesbelow the limit of microscopicvisibility. Second,it maybe present in a compoundsuch as the telluride,likewise submicroscopic or else undetected. Finally, it may occurin solid solutionwith the sulfidesor adsorbedonto the surfacesof carbonand the sulfides. This may be visualizedas a two- or three-dimensionalsolution, depending on whethergold was adsorbedonto the hoststructure after or duringthe growthof the latter. An adsorbedfilm of goldon a pre-formedmineral differs from goldin atomicdispersion through the host mineral structureonly in that it is a planar rather than a three- dimensional feature. The frequencydistribution chart in Figure 28 has beenprepared on a logarithmicscale. This chartshows the greatrange in sizebetween the gold atomand the smallestvisible particle. Goldin SolidSolution in SullSdes.--Accordingto present theory, two major types of solid solutionoccur in nature. First is substitutionalsolid solution. In this,ions of the minor (guestor solute)component occupy the positionin the crystallattice that wouldbe normalfor ionsof the dominant(host or solvent)component. Depending on whetherthe radiusof the guestis larger or smallerthan that of the host,there will be an expansionor contractionof the resultingcrystal structure. As the differencebetween the radii of the host and foreign ions decreases,the degreeof possiblesolid solutionincreases. The growthof sucha solid.solution is causedby adsorptionof foreignions onto thegrowing surface of thehost. At the surfaceof a crystalat any stageof its growth there is an unsaturatedfield of force, sincethe outermostions at this stagehave not enoughneighbors to satisfytheir normal coordination. During growth,ions in. solution make coordination by attachmentto the thin surfaceof thecrystal. A foreignion in thesolution may respond to thisfield of forceand beadded to thegrowing crystal, playing the role of proxyfor a dominantion in thecrystal, provided that the radii of thecorresponding ions or atomsin ques- tion donot vary by morethan about 15 percentof the sizeof the smaller. The covalentradius of the iron ionsin pyriteis 1.23A ø and the octahedralcovalent radiusof gold is 1.40A ø (25,p. 170). Sothe covalent radius of thegold ion is within 15 percentof that of iron, and goldshould be ableto enterinto substitutionalsolid solution in pyrite. The secondtype of solid solutionis known as interstitial solid solution.. In this, the.solute atoms are believedto be dispersedin the structureof the 282 PETER JORALEMON. solventatoms or ions. The soluteor foreignatoms are introducedin addition to, ratherthan in placeof, the hostor solventions. This type of solidsolution is rare and generallyoccurs where the foreignion is distinctlysmaller than the host. Synthetic•turiferous Pyrite.--Maslenitsky (22) addedgold chlorideand colloidalgold in the courseof synthesisof pyrite. He was able to controlthe amountof goldcontained in the pyriteup to a maximumamount of about9.7

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Fro. 11. Photomicrograph of high-grade gumbo. The nearly uniform size of the quartz grains is characteristic of •e richer material. All quartz grains are embeddedin carbon. Although the host rock has been entirely replaced, •e sedi- mentary structure has been preserved. Cross nicols, X 40. Fro. 12. Photomicrographof polishedsection of ore gumbo. Pyrite (white) and realgat (light gray) have replacedthe fine-grained quartz-carbon intergrowth interstitial to the larger quartz grains, leaving the latter as residual islands. This ganguetexture is typical of all high-grade portionsof the ore. X 45. OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 283

FIG. 13. Thin sectionof realgar and vein stuff. The realgar appearsblack in the forked vein that is rimmedby thin layers of quartz crystals. The black euhedral crystalsoutside the vein are of pyrite. The vein cuts a fine-grainedquartz-sericite intergrowth that is an unreplacedfragment of argillite. Such material is not a conspicuousgold-carrier. Cross nicols, X 40. Fxc. 14. Photomicrographof polishedsection of realgar in ore gumbo. The realgar(light gray) surroundsislands of subhedralquartz crystals, having replaced the fine-grainedquartz-carbon matrix of thesegrains. X 310. ouncesper ton. Investigationof the auriferouspyrite under X 1,200mag- nificationrevealed no goldand the gold may have been in solidsolution in the pyrite. The methodof polishingused in the preparationof thesepolished sectionsis notknown. In manycases specimens that reveal no goldwhen 284 PETER JOR.4LEMON.

polishedon a clothlap maybe shownto containabundant visible gold after repolishingon a lead lap. Kuranti (18) prepareda syntheticauriferous pyrite in whichgold was in- visibleeven under the highestmagnification possible. Again the methodof polishingmay haveobscured some visible gold. The goldwas 'shown by spectrographicanalysis to be distributeduniformly throughout the pyrite,and X-ray picturesof the pyriteshowed that the pyritelattice constant varied with increasinggold content, reaching a constantvalue at 2,000grams of goldper tonof pyrite. The evidencemay indicate that solidsolution of goldin pyriteis possible.Unfortunately the original Japanese publication describing this work is not available. NaturalAuriferous Pyrite.--It is probable that gold in solidsolution in pyrite may be synthesized.A naturallyoccuring example of suchsolid solution has not beenpreviously described, although many writers have attacked the problem. R. E. Head (15) suggestedthat at leastsome gold in particlesof micronic diameterin pyriteoccurs in tiny leavesor flakescoated with a magneticiron mineral,lying betweencrystallographic planes in the pyrite. Sucha feature mightsuggest exsolution of gold. Edwards(6, p. 130) statesthat suchfine goldparticles rarely show the orderedarrangement in the pyritethat wouldbe expectedif this were so. Haycock(14) prepareda frequency-distributionchart of sizesof gold particlesin 50 Canadianores, and foundthat the greatestnumber of gra.ins were well within the rangeof microscopicvisibility. Over 75 percentof all the visiblegold falls within the microscopicrather than the megascopicrange. By projectingthis curve into the rangeof submicroscopicsizes, Haycock con- cludesthat goldof colloidalsizes has only slightimportance and goldin solid solutionin sulfidesmay be of evenless economic value. He pointsout that untilthe naturalexistence of goldin solidsolution in pyriteor othersulfides is provedto be significant,it is morein accordwith probabilityto stressthe im- portanceof normalgold particles of submicroscopicsize. Auger (2) studiedpyrite from various gold deposits in Canada. He made spectrographicanalyses of thepyrite and found that the spectral gold lines were inconsistent,even within one deposit. He concludedthat gold, whenever present,is unevenlydistributed in the pyrite crystalsand appearsin the spectrumonly whenthe analyzedsample contains small particles of gold. Auger felt that this evidencestrongly suggests that golddoes not take part in the structureof the pyrite crystalas solidsolution, and that gold is not pre- cipitatedat the sametime as the pyrite. His study includedonly natural pyriteand his conclusionsdo not constitutea denialof the possibilityof goldin solid solutionin pyrite. It might be well to mentionthat gold is relatively insensitiveto spectrographicanalysis, as the lower limit of sensitivityis about 0.001 percent. Pyrite and arsenopyritefrom the Dolphin East Lode, Fiji (28), were found to contain as much as 33 ouncesof gold per ton, although only a few particlescould be seen microscopically. The sulfideswere heated to 600ø OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 285

C and, on cooling,were again examinedmicroscopically. Abundant gold particles,enlarged to visiblesize by the heating,were formed by the coalescence of submicroscopicgold. Stillwell interpretsthese results as an indicationof the presenceof gold in limited solid solutionin the sulfides. Heating of the gold-bearingpyrite, however,would tend to preserve,rather than terminate, any stateof solidsolution. It is perhapsmore probablethat this Fiji gold is presentin independentparticles larger than the goldunit cell. The coalescence of smallerparticles represents a changeto a form having a lower, and thus more stable,surface energy, if the originalgold is in colloidal-sized.particles. Back-reflectionX-ray photographsof pyritefrom Amador,California, taken at Harvard,show that the structureof someof the pyriteis distorted,having a cubeedge of 5.477kx, while otherof the pyrite hasthe normalundistorted structurewith A0 equalling5.405 kx. Spectrographicanalyses indicate that the distortedmaterial contains about 5 ouncesof goldper ton, approximately ten timesas muchas is containedin the normalmaterial. While both types of pyrite appearto be the samein polishedsection, nitric acid bringsout a differentialetch pattern, the auriferouspyrite being etched more deeply than the lower gradematerial. This evidencestrongly suggests that at leastsome of the Amadorgold is in solidsolution with the pyrite. Similar etchingor tarnishingwas noted at the Simmer and Jack Mine on the Rand someyears ago (12). At that deposit,and at adjacentmines, it wasfound that oneof the dependablesigns of better-than-averagegold values in the Reefwas the rapidityof tarnishof the pyriteat thoseplaces. The pyrite of rich stretchesof the conglomeratewould developstrong irridescence on a freshlyblasted face within a day or two. This differentialtarnish may have beenbrought about by the presenceof goldin solidsolution in som.eof the pyrite. Auriferous Pyrite in Getchell Ore.--Pyrite from the ore shootsin the Getchellmine contains some small particles of visiblegold, commonly located near the edgesof the pyrite grains or near open spaceswithin the grains. Further goldhas.apparently been lost duringpolishing. It is probable,however, that the gold of visiblesizes is not abundantenough to accountfor the total gold value of the pyrite. No visiblegold has beenseen in polished sectionsof pyriticconcentrates from the ore, althoughthey containabout 0.5 ouncesof gold'per ton. Samplesof the sulfideconcentrates were heated to 600ø C in the presence of sulfurvapor in a sealedtube. Each polishedsection prepared from this materialreveals from 10 to 20 particlesof goldaveraging 0.3 micronsin diameter. All particlesoccur at the boundariesof pyrite grains(Fig. 24). Heatingof the pyriteapparently brought about two relatedprocesses. First, it causedthe submicroscopicgold to migratetoward the marginof the host grains,thus riddingthe bulk of the pyrite of its gold. Second,the heating caused'the coalescenceof the goldpresent to form, in someinstances, visible grains. This heatingprocess cannot be assumedto makeall the grainsvisible; probablyonly the richerpyrite grainscontained enough gold to yield visible particles.This ,experimentsuggests that the pyritemay not contain uniform 286 PETER JOR.4LEMON.

F•. 15. Hand specimenof typical gold ore. The light material is realgar, occurring as bedding veins in limestone. Irregular patches of gumbo have also replacedthe limestone. About half natural size. F•. 16. Photomicrographof polishedsection of ore in andesiteporphyry. The white euhedralcrystal is arsenopyriteand the irregular light gray massis magnetite. Magnetite apparentlyhas replacedboth the gangue minerals and arsenopyrite. X 425. OCCURRENCE OF GOLD ,4T GETCHELL MINE, NEVADA. 287 amountsof gold,although, as previouslystated, all pyrite containssome gold. Grain sizeof the pyrite is apparentlynot an importantfactor in determining the amountof containedgold; large grainsshow no more tendencyto contain visiblegold than do the smallerparticles. This experimentproves the exist- enceof submicroscopicgold in the pyrite,but it doesnot proveor evensuggest whetherthe goldoccurs in atomicor ionicdivision or in particleswith dimen- sionsexceeding those of the gold unit cell. A comparisonbetween auriferous pyrite and artificially prepared activated charcoalcontaining gold may seemextreme, but a brief descriptionof the charcoaland its effectson goldmay shedsome light on the problemof auriferouspyrite. Metallurgistshave long beenaware of the fact that charcoalis able to adsorbgold selectivelyfrom a complexsolution. This propertyis utilizedat the Getchellmine where activated charcoal is usedto removegold from cyanidesolutions. The charcoalprobably adsorbs gold in the form of a doublecyanide. with sodiumor calcium. Grossand Scott (13), largelyon the basisof chemicaltests, found evidence suggestingthat the gold adsorbedon the activatedcharcoal was not in the metallicstate. However, polishedsections of treated charcoalcontaining nearly200 ounces of goldper ton were prepared at the HarvardLaboratory of Mining Geology,and in these,abundant metallic gold was found present in smallbut visibleparticles (Fig. 27). It is possiblethat the •.dsorbedgold cyanidesalt, being extremely unstable, may have broken down to metalliCgold afterstanding for somemonths. Sucha breakdown,however, would probably form extremelyminute particles and it is apparentthat the goldseen in the polishedsections of the charcoalis too coarse-grainedto havebeen formed in thisway. So someother factor may have brought about the coalescence to visible size. Whateverthis force may be, it is strongenough to overcomethe adsorp- tiveattraction of thecharcoal for thegold, as theadsorbed. ions appear to have beenpulled from the carbon surface to formlarger particles. It is alsopossible that thefirst goldadsorbed on the charcoalacted later as nucleifor the growth of metallicgold particles. The first layeror film of goldatoms is assumedto havebeen deposited by adsorption,while further growth may havebeen ac- complishedby normalforces of crystallization. Like the visiblegold in the sulfides,these gold particlesare extremely looselyheld on the surfaceof the charcoaland but few canescape removal duringpolishing. The goldparticles in the charcoaland pyritealike occur eitheralong the marginsof the hostgrains or borderingminute open spaces withinthe grains. In thecase of the charcoal,the amountof goldthat canbe adsorbedvaries with the exposed surface of thecharcoal, and most gold occurs in the moreporous material. It maybe merelycoincidence that the pyriteand marcasitein the richest oreare porous and have irregular surfaces, whereas the pyrite that occurs away fromthe ore shoots is generallymore solid and more regular in outline. But the factthat visiblegold associated with sulfidesgenerally occurs in the irregular,porous forms of thesulfides suggests that this porosity may have been importantin localizingthe gold. Laboratorytests have shown that a sub- 288 PETER ]ORALEMON. stantialpart of the gold occurringwith the sulfidesis associatedwith the fine- grainedportions of the latter. A largegrain has less exposed surface than an equivalentvolume of small grains, and hencecan adsorbless material. An euhedralcrystal likewise has lesssurface area and thereforeless adsorptive capacitythan an irregularshape of the samevolume. The mechanism.of adsorptionis hereproposed to explainthe precipitation of goldon pyrite,other sulfides, and, to a larger extent,on the carbonin the ore. This processdiffers from that of solidsolution in thatthe latterinvolves adsorption of the foreignions during the growthof the host. If the bulk of the gold associatedwith pyrite representsa solid solutionof gold in the sulfide,then gold would have been introducedinto the ore at the same time as the sulfides, andthe amount of gold should be r.oughly proportional to theamount of sulfides. Most of the gold values, however, are restricted to narrow bands whereas the sulfidesare widespreadthroughout the ore bodies. Thus, it is apparent that the solutionsfrom which the bulk of the gold was depositedtravelled along morerestricted channelways than did the solutionsthat deposi[edthe sulfides. It is not meantto denythe possibleexistence of gold in a three-dimensional solidsolution in the sulfides,but rather to emphasizethat any suchsolid solution can be of only minor importanceat Getcheil. Indeed, it may well'be that a very smallamount of gold is in solidsolution with the pyrite. Growth]Plechanism of GoldParticles.--If a hydrothermalsolution is un- saturatedwith gold,neither native gold nor any gold mineralsmay be formed. As was previouslymentioned, however, the gold, unable to form its own 'mineral,may proxy for iron ions in a growing pyrite crystal. Such a proxy would bring about a solid solutionof gold in the host crystal. Under these conditions,if the ratio of gold.to iron in solutiondoes not exceedthe limit of solubilityat a givenplace, all the goldmay occurin solidsolution in the pyrite. If the ratio is higher, the pyrite will still containgold in solid solutionand will also containlarger gold particlesas randominclusions. Where the ratio is much higher, theseinclusions will attain visible size. In any deposit,the physico-chemicalconditions, which may be called intensity conditions,may be expectedto vary from place to place within the locusof deposition. Thus, a high-intensityzone might be fringed with areas of lower intensity. In the former area, gold would be depositedfrom hot, concentratedsolutions to form discreteparticles as well as to occurin solid solutionin pyrite. In the outlying,low intensityzones, where depositionwas from dilute solutions,the only gold depositedwould be that in solidsolution in the sulfides. In the Getcheildeposit, the entire zone containingthe ore bodiesmay be visualizedas an enormousblock of submarginalgold-bearing rock with smaller, superimposedbodies of richer ore, as shownin the projectionin Figure 3. The economicallyworthless sections of the veinsand the wall rock contain a rather uniformamount of gold, varying from 0.01 to 0.08 ouncesper ton. Theselow valuespersist as far intothe hanging-and foot-wall country as does thepyrite. Pyriteconcentrates from the mine, regardless of thesection of the veinsfrom which the pyrite was taken, contain a minimumof about0.5 ounces OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 289

ff - _ - • Grain area in square microns

FIGURE 17. FREQUENCY DISTRIBUTION OF VISIBLE GOLD GRAINS, GETCHELL MINE

Fro. 17. Frequency distributionof visible gold grains, Getchell mine, basedon measurementsof 2,639 grains. of gold per ton. The ratio of concentrationof pyrite in the concentratesis about 1:50. Similarly,the ratio of concentrationof gold from 0.01 to 0.5 ouncesper ton is about1:50. Sothe low-gradegold varies directly and closely with the pyrite. It is to be presumedthat the peripheral,submarginal gold and pyrite which. 290 PETER JOR.4LEMON. havethe sameextensive distribution in the wall rock and low-gradeportions of the veinsare more or lesscontemporaneous, products of depositionfrom the low-intensityphase of mineralization. During the growthof the pyrite, and probablyalso of the marcasite,it is possiblethat goldwas being adsorbed onto the sulfidestructure in extremelysmall amounts. This actionmay havetaken placeoutside as well aswithin the ore body. The extentof solidsolution of goldin pyritemust be limitedand is probably dependenton the temperatureof formation. In a low-intensitydeposit such as Getcheil,the minimumsolubility of gold is probablyof the order of 0.5 ouncesof gold per ton of pyrite. In parts of the ore body where the temperatureof formationof pyritewas somewhat higher than in the areasfringing the ore bodies,the solubilitymight be higher. In the outer,low-grade zones at Getcheil,the gold-pyriteratio in the solu- tionswas probablylow, and muchof the goldappears to have beendeposited in solid solutionin pyrite. In the central channelwayswhere more intense physico-chemicalconditions existed, the ratio and limit of solubilitywere higherand somegold occurs in visibleparticles. Van Aubel (29) has concludedthat the particle size of native gold is commonlya functionof the modeof formationof the associatedore minerals; when gold is contemporaneouswith the enclosingminerals it is extremely fine-grained;when it is of later formationit tendsto occurin coarsergrains. In the Getchellores, the peripheralgold is in generalsubmicroscopic because only dilute,gold-bearing solutions, exhausted of muchof their solutein the courseof the long distancetravelled from the main channels,were available and the growthof individualgold particleswas limited. In the ore shoots where gold depositionfrom more concentratedsolutions continued until the endof the periodof mineralization,the goldparticles continued to grow, and many achievedmicroscopically visible size. The foregoingdiscussion is admittedlytheoretical. It dealslargely with that gold,or a part of that gold,which is submicroscopicand is intimatelyas- sociatedwith the sulfides. Suchgold constitutesonly a smallproportion of the total that occurs in the richer ore shoots. The writer feels that the evidencefavoring the theo!;yof gold in limitedsolid solution is strongenough to justify consideringit.

ConclusionsRegarding Gold Occurrence. Goldat Getcheiloccurs in particlesranging in sizefrom slightlylarger than thegold unit cell to nearly one millimeter in diameter,and probably also in atomic dispersion.Submicroscopic gold is apparentlymore widespread than the visible metal,occurring throughout the ore body. Microscopicallyvisible gold has a veryrestricted occurrence and is foundonly in thericher ore shoots, in which placesit accountsfor the greatbulk of the totalgold. The massof low-grade rock far outweigh,sthat of the ore shoots,and the actual numberof submicroscopicparticles is probablyfar greaterthan that of visiblegrains. Discussionhas been presented which suggests that the widelydistributed but low-gradegold may be mainlyin solidsolution in the pyrite,whereas the OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 291 gold in the ore shootsis surely in discreteparticles of the native metal much larger than the gold unit cell. In the very rich portionsof the veins, part of the gold is in still larger particles. The informationgathered by measuringvisible gold grainsand presented in Figure 17 may shed somelight on the questionof the characterof submicroscopicgold. These data have beenused in the compilationof the chart in Figure 28. In order to show the full range in sizes,a logarithmicscale has beenused on both the ordinateand abcissa. Since the number of grains or particles measuredmake up a representativesample of all the visible gold particlesretained during the polishing,the curve givesan approximation of sizesthroughout the ore body. The curve in the visible range may probablybe projectedat least some distanceinto the microscopicfield with somedegree of reliability. The critical questionis whetherthe curve will drop to zero at or beforereaching the area of the gold unit cell following the same general pattern as Haycock'scurve (14), or will continueupward along the projectionfrom the knownportion of the curve, as indicatedin Figure 28. The submarginalgold values are widely scatteredthrough the ore body, and the mass of submarginalmineralized rock far outweighsthat of ore. One ton of rockcontaining 0.01 ouncesof goldcontains 2.76 x 102 goldatoms, and if presentin truly atomicdispersion, this enormousfigure would represent the numberof pointsat which gold occursin one ton of low-graderock. If the goldin the richerore were containedwholly in spheresof onemicron radius, oneton of ore carrying0.55 ouncesof goldwould contain 2.38 x 10• particles. Sincethe rock containingsubmarginal gold valuesis so much more abundant than is the ore, it is probablethat the gold in atomicor ionicdispersion is more commonthan are particlesof greater than unit cell dimensions. If, as has been suggested,adsorption has been an active factor in gold deposition,then pyrite and carbonsurfaces will containan adsorbedfilm of gold ions. In the abundant,lower-grade portions of the ore bodywhere the growthof goldwas limited, large particleswould not be formed, but much of the gold in the adsorbedfilms may have acted as nuclei for limited further gold deposition. The numberof particlesslightly larger than the unit cell would be enormous. In the smallerore shoots,much of the gold grew to visible size. Granting this, the gold of atomicsize is mostabundant, the submicroscopicparticles are the .nextmost common,and microscopicallyvisible grains are least abundant but representan overwhelmingproportion of the total massof gold within the ore bodies. Thereforethe curvein Figure 28 might be expectedto rise alonga more or less straight line through the submicroscopicfield toward the unit cell area. If the curveas indicatedon the chart is roughlycorrect, then the frequencydistribution of goldparticles •hould be in harmonywith the proposed processof golddeposition. In the marginalphase of mineralization,gold atomswere adsorbedinto the pyrite structureduring the growthof the latter mineral,and the solutions containedtoo little gold to allow the growth of metallicgold particles. In 292 PETER JOR.4LEMON. the major channelwaysthe earlier mineralswere beingdisplaced upward and outwardby the mineralsof the moreintense phase. In thesechannelways the solutionswere hotterand the gold contentwas suchas to allow the growth of native gold. Here the gold was adsorbedonto the surfacesof the carbon, pyrite,and marcasiteto form nucleifor the growthof larger particles. Be- causethe gold concentrationin solutionwas higherin the centralchannelways than in the outlyingareas of the vein and wall rock, the gold formedin this moreintense phase would have more opportunity for continuedgrowth. Hence the greatbulk of economicamounts of gold is concentratedin thesechannelways. The ratio of visibleto submicroscopicgold is far greater in these channelwaysthan in the low-gradeportions of the ore body,and the bulk of the weightof goldis in coarseparticles. The particlesize of gold is thereforea functionof the lengthof time of growthand the concentrationof goldin solution. The solutionsin the outer portionsof the ore bodycontained only very smallamounts of goldand there wasno opportunityfor growthof largeparticles in thoseplaces. The presence of goldparticles of colloidalsize does not necessarilyimply depositionfrom a colloidalsuspension. The chartin Figure28 intimatesthat the colloidal-sized particlesrepresent only the dimensionattained at a givenstage in the normal growthof particles. Everylarger particle, being crystalline, must have grown from particlesinitially of the unit celldimension.

Causeo I LocalizationoI Gold. Structural Features.--The localizationof the three deep "roots" of the rich ore, as shownin verticalprojection in Figure 3, suggeststhat theseroots lie alongthe respectiveaxes of originalchannelways through which a greater amountof gold-bearinghydrothermal fluid passed.Solutions rising along thesechannels spread out into a far widerarea on reachingthe zone of intense shatteringnear the surface. The threecentral channelways, one in the north end,one near the center, and one near the south end of theore bodies, remained as the majorchannelways throughout the periodof golddeposition. Since a largervolume of gold-bearingsolutions passed through these channels than throughcorresponding volumes of rockin the outlyingregion of shattering, moregold per ton wasdeposited within the channelways.The goldin the extensive,low-grade portions of the orebodies is moreor lesscontemporaneous with that in the rich ore shoots,although deposition in the shoootsmay have continuedlonger than in the moredistant vein material. Thesericher shoots coincidewith the zonesof extensivedevelopment of gumbo,suggesting that evenbefore gold deposition began, these areas were most permeable to the mineralizingsolutions. The gumbo,being unconsolidated, was more permeable,and thus morefavorable to transportof the gold-bearingsolutions thanwas the enclosing rock. Theconcentration of carbon in thegumbo made it a betterprecipitant than the argilliteor limestone. In the Getchellore, a smallproportion of the totalmass of goldoccurs in thewidespread pyrite, probably because it wasdeposited as a constituentpart of thathost. The bulkof the gold,however, is irregularlydistributed in every OCCURRENCE OF GOLD AT GETCHELL MINE, NEV,4D,'t. 293

20 .'-. 21 FIG. 18. Photomicrographof polishedsection of heavy media concentrate. The large white grains are of native gold and the mottled light gray mineral is realgar. These are the largest gold grains ever seen in Getchell ore, and the only grains [ound that are megascopicallyvisible.' X 70. Fig. 19. Photomicrographof polishedsection of ore gumbo. The large white grain is native gold; a part of this grain is submergedbeneath the quartz gangue and appearsas an indistinctlylighted area. The irregular mottled surface of the gold grain showsthe "egg-shell"texture so characteristicof minutegold grains. X 3,225. Fro. 20. Photomicrographof polishedsection of goldore, showing a relatively large gold grain that has replacedthe fine-grainedquartz-carbon matrix to the gumbo,molding itself aroundthe larger subhedralquartz grains. X 3,225. Fro. 21. Photomicrographof polishedsection of gold ore, showingisolated gold grain. The egg-shelltexture is apparent. X 3,225. mineralin theore body and therefore cannot have been selectively precipitated by anyof them. Thegold was deposited in or onthese various minerals apparently becausethey had somefeatures in common. Favorabilityof Hosts.--Sincenearly every mineral in the ore has acted asa hostor platformfor thegold, the cause of localizationof gold is probably a factorcommon to all minerals. This cannotbe crystalstructure or chemical 294 PETER ]OR,'ILEMON.

composition. The concentrationof gold in the more porousminerals in the ore suggeststhat this factormay be the amountof surfaceexposed to the gold- bearing solutions. The mechanismof adsorptiondiscussed in the sectionor. submicroscopicgold may applyto the depositionof gold in or on every mineral. Pyrite (Fig. 26), marcasite,and carbonoccur in irregular porousmasses with abundantopen spaces, whereas realgar, which is more abundantthan the other threeminerals, characteristically occurs in densemasses with little or no open spaces. As wouldbe expected,realgar is the leastimportant gold bearer. During the period of mineralization,gold was constantlybeing carried in solution. In the earlierphase the solutionswere too diluteto be saturatedwith any gold compound,but other minerals, carbon, pyrite, and marcasite in particular,adsorbed small amountsof this metal. Becausecarbon and mar- casitehad the greatestsurface, they adsorbedthe greatestamounts of gold. Gold remainedin solutionafter the depositionof the sulfideswas completed, and the processof adsorptioncontinued to act, forming atomic-thin layers of gold on the surfacesof minerals around which the solutionspassed. In the later stageof deposition,native gold was depositedin the more permeableparts of the ore shootswhere the solutionswere more intense and the gold concentrationin solutionwas higher. Adsorptionwas no longer quantitatively important. The metallicbond may be visualizedas a cloudor atmosphereof electrons interstitialto the metalatoms and holdingthem in positionin the crystalstructure. The presenceof this electroncloud also brings about an attractiveforce for metal ions in a solutionaround the growing crystal. An adsorbedfilm of metal ions contains interstitial electrons which will attract similar cations in solution. In this way, the thin, adsorbedfilm of ions may act as a nucleusof growth of larger particlesof the metal. So in the importantstages of deposition in the Getchellore, the predominantprocess was one of normal metal growthon nucleithat had beenformed previously by adsorption. The physico- chemicalconditions at that time were suchthat gold, no longer stablein solution, was precipitatedon earlier formedgold particles. The possiblechanges in theseconditions are discussedin the ch&pteron genesis.

Native Silver. A mineral has beenseen in many polishedsections of Getchellore which, on accountof its physicalproperties, is taken to be native silver. It is white, isotropic,and has extremelyhigh reflectivity,considerably higher than that of gold; it is very soft, having the samemottled, egg-shell texture that is seen in gold. The only mineralthat fits this descriptionis native silver; conclusive chemicaltests have not beenpossible, however, because the particlespresent are too small. It occursin sizesanalogous to thoseof gold,although megascopically visible silver has not been found. The ratio of gold to silver in bullion assaysvaries from 2:1 to 134:1 and, for the entire bullionproduction, averages about 10:1. Earlier studiesof the ore suggestedthat the silverprobably occurs in electrum,a natural gold-silver OCCURRENCE OF GOLD ,:IT GETCHELL MINE, NEVADA. 295 alloy. Only a few particleshave beenseen in the ore, however,and most of the silver occurs as the native metal. The silver is probablya hypogenemineral. As the associationof native silver and native gold in hydrothermaldeposits is rare, the evidencefavoring a hypogeneorigin of the silver must be critically examined. (1) In general the amountof silver in the ore variesinversely with the amountof gold. Ex- tremely rich portionsof the ore body containless silver than the portionsof averagegrade. There is no zonal relation other than this betweengold and

FiG. 22. Photomicrographof gold-rich carbonveinlet. The small white particlesare native gold, and the large, smoothgrains are of magnetite. The indistinct gray specksassociated with the gold are carbonparticles. X 2,275. FxG.23. Photomicrographof polishedsection of gold ore, showinga gold-rich lens. All white grains are gold. The gray angular fragmentsare magnetiteand the small,irregularly rounded gray particlesare carbon. The gold particleon the left is rimmedby a discontinuoushalo of carbonaceousmatter, as is the magnetite, bottom center. X 2,275. 296 PETER JORALEMON. silver. If silver were a supergenemineral, derived from hypogeneelectrum, it shouldbe mostabundant in the ore richestin gold. (2) Native silverpersists with no largedecrease in amountto the deepest levelsof the mine. There is no shallowblanket of richer than averagesilver ore, as would be expectedif the silver were supergene. (3) Silver particlesshow the samevariations in grain size as the gold particles. If silverwere supergene,formed under totally differentconditions fromthose in whichgold was formed, it wouldbe unlikelyto occurin grainsof approximatelythe samesize as the gold particles. Had silver beenderived from electrumby meteoricleaching, the residualgold would be expectedto occurin porousmasses, but the goldparticles are solid. (4) Like gold,silver occurs in thecarbonaceous matrix of thegumbo and in sulfides,particularly the porouspyrite-marcasite intergrowths. It is loosely held in the host mineralsand is easilyremoved by rubbing. Unlike gold, silver particlesdo not occur in lenticularaggregates. Otherwise the habit of silverclosely •resembles that of the widespreadbut sparselydisseminated gold particles,and the periodsof depositionof silver and this gold were probably overlapping. Why silver did not more commonlyunite with gold to form electrumis not clear. Gold in mostlow-intensity deposits contains a considerableamount of altoyedsilver. At the Getchellmine, however,even the gold and silver that, on the basisof similar distribution,appear to be more or lesscontemporaneous occuras separateminerals, although the two may commonlybe seenin the same field of view on one polishedsection. A similar paradox has been noted in the occurrenceof gold and silver precipitatedfrom the cyanidesolution in the milling process. Gold and silver, bothcarried in solutionin the cyanide,are pr•ecipitatedas the separatemetals and not as the gold-silveralloy. Similarlyin the supergenezones of other Nevadaprecious metal deposits, gold.and silver occur as separateminerals. Thus, gold solutions,whether naturallyoccurring in the earth or artificiallyin a cyanidetank, apparentlyare unableto form a gold-silveralloy. But in most epithermalgold depositsin Nevada,electrum is present. It is possiblethat with decreasingtemperature of solution,the tendencyfor the formationof electrumdecreases. If this is so, the temperatureof ore solutionsin the Getchelldeposit must have beenlower thanthose of otherprecious metal deposits of Nevada. Elsewherein the world,hypogene native silver is foundin four main types of deposits:(a) associatedwith sulfides,silver minerals zeolites, barite, fluorite, and quartz (Kongsberg, Norway); (b) occurringwith nickel and cobalt sulfidesand arsenides in a calcitegangue (Cobalt, Ontario); (c) withiaraninite and nickel-cobaltminerals (Great Bear Lake); and (d) with nativecopper andzeolites (Keweenaw Peninsula) (24, p. 98). No descriptionof hypogene silverin minoramounts in any otherdeposit dominantly of goldhas been found by the writer. OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 297

GENESIS. •

Sequenceo i Deposition. Ore depositionis a gradationalprocess. It is not one in which the wide- spreaddeposition of one mineral stopsbefore another mineral beginsto be deposited. Any mineral will be depositedwhen the physico-chemicalconditions are such as to make its constituents unstable in solution. It would be folly to assumethat, at any one time during deposition,.similar conditions existedthroughout the entire area of mineralization. So a mineral that was depositedlate in the sequencein one place may actually be earlier than a mineral depositedearly in the sequencein anotherspot. Any discussionof exact sequence,therefore, must be limited to a small locality within the ore body,where similar physico-chemigalconditions existed at any giventime. In general,the sequenceof deposition(graphically presented in Fig. 29) of what is now the economicore at Getcheil, was as follows. Early, low- intensitysolutions, being dilute, were able to do no.more than locally rework therock constituents and introduce small quantities of 9)•ritecarrying minor gold. Coarse-grainedquartz and calciteand smallam011. nts of pyriteare the early minerals. Somewhatlater, fine-grainedquartz and carbon,probably derived from underlyingrocks, were introduced,and were followedby the major sulfides,pyrite, pyrrhotite,arsenopyrite, and chalcopyrite. In the still later, economicallyimportant stage, marcasite,realgar, orpiment, stibnite, magnetite,gold, and silver were deposited. The period of depositionof each mineraloverlapped those of severalothers, so these minerals may be considered as more or lesscontemporaneous. In any one place in the ore body, gangue mineralsderived from the nearbyand underlyingrocks were depositedearly in the sequenceand metallic minerals foreign to the countryrock were mainly depositedsomewhat later. Mostof the gold,together with silverand magnetite, is apparentlythe latestof the metallicminerals deposited. In the outlying,submarginal portions of the ore bo•y,•the sequencewas apparentlysimilar to that in the now economicsections, except that the later marcasite-realgar-goldperiod did not appear there. Deposifi6n'inthese fring- .ing areasof the main ore bodymay be consideredas beingmore or lesscon- temporaneouswith the economicallyimportant phase of depositionin the central channelways. The early, gold-poorstage of depositionin thesechannelways precededany depositionin the outlyingareas. 'The distributionof economicamounts of gold is quite similarto that of realgar. There are apparentlytwo modesof occurrenceof'visible gold: (1) it is sparselydistributed throughout the ore bodies,in gumbo,argillite, and limestonealike. This form of goldwas deposited early in the periodof formation of the realgarand thesewidely scattered gold particles occur in all ore that containsrealgat, although they rarely occur within realgargrains. Goldof this stageprobably began its growthby adsorptionon carbon,porous pyrite, and in realgar only where favorableconditions existed. Silver and.the rare electrum wereprobably more or lesscontemporaneous with this earlier,low-grade phase of goldmineralization as they have the samegeneral distribution. The precious 298 PETER JORALEMON. metalsdeposited at this time are not to.be confusedwith the non-commercial amountsof gold believedto be in solidsolution in the earliestpyrite. To the contrary,gold of this period constitutesmost of the gold in the ore bodies exceptthe extremelylocal and rich shoots. (Gold of this periodis illustrated in Figs. 18, 19, 20, 21.)

24 ,

Fxc. 24. Photomicrographof polishedsection of pyritic concentrate,heated to 600ø C, showing gold inclusionin pyrite-marcasiteintergrowth. Finely dispersed gold was apparentlydriven to the opening in the pyrite by the heating. Note the differencein reflectivity betweengold and pyrite. X 2,875. Fxc. 25. Photomicrographof polishedsection of pyritic ore. The small gold grain occursin a poroussection of the pyrite-marcasiteintergrowth. X 2,875.

(2) Toward the end of the period of depositionof marcasiteand realgat, the mostintense ore solutionspassed through the few most permeablechannels in the ore body which lay within.the zonesof abundantgumbo. A marked changein conditionsof depositionmust have occurredto allow the formation OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 299 of extremelyrich, thoughsmall, lenses of goldand magnetite(Figs. 22, 23). Minuteamounts of carbonwere deposited at thistime, rim.ming occasional particlesof gold. Summaryof Sequence.--Thechanges in thenature of depositionwith time werebrought about largely by changingconditions of deposition. The changes were gradationaland the mineralscharacteristic of the early stagewere being depositedin the outlyingareas at the sametime that the late stageminerals were forming in the ore shoots. At the earlieststage of depositionat the known depthhorizon, pyrite containinglow-grade gold was deposited,with somequartz, calcite,sericite, and chlorite,near the major channelways.As the flow of hot solutions continued, more and, more heat was carried into the zonesof mineralization,the intensity-characterincreased, pyrite and lean gold were depositedfarther and farther out, whereasthe mineralsfrom solutionsof higherintensity were depositedtoward the centersof mineralizationby replacement or re-solutionof thoseminerals that had beenstable during the earlier and feebler stagesthen and,there existing. Realgar, marcasite,and the economi- cally important gold, which latter was becomingricher with time, were depositedin the main channelwaysat aboutthe sametime that the solutionsin the outlyingportions of the veinsand wall rockwere depositingdisseminated pyrite and included.submicroscopic gold in non-commercialamounts.

Nature of the HydrothermalFluid. The hydrothermal fluid containedwater, arsenic, iron, and sulfur, with minor amountsof copper,gold, silver, molybdenum,mercury, barium, and fluorine. It alsocontained silica, lime, and carbon, which were probably largely of localderivation. This fluid was hot, probablyat a temperatureof between oneand two hundreddegrees Centigrade. At depthit was probablyalkaline, althoughthere is no directevidence of this. The underlyingrocks contain far greaterquantities of alkalinethan acid elements, so, even if, the solutionwere thoughtto have been acid on leaving its source,it presumablywould soon become alkaline. Transportationof Gold.--Solublecompounds of goldare not abundant. In the past,emphasis has been placed on auric chlorideas the form in whichgold is transported (21). However, chlorides,if present at all, are rare con- stituentsof an ore body,especially as comparedto sulfides;and eventhough mostchlorine compounds are highly soluble and would not be precipitated, there is rarely, in any district, tangibleindication that chlorinein whatever state of combinationhas passedthrough the channelwaysin the quantitiesrequired if the metalswere broughtto the placeof depositionas chlorides. It seems far morereasonable to assumethat the goldis transportedas a solublesulfide. Smith (27) suggestedthat gold may be dissolvedin alkali sulfidesolutions with the formation of alkali thioaurites,the general formula for which is 2Na2S'Au2S. Gold is far more soluble in alkali sulfide solutions than are most metals. It belongsin the group of metalswhose sulfides are highly solublein dilute aqueoussolutions of alkalisulfides a.t room temperature. With goldin this• 300 P•ET•ER .IO R,4L•EM ON. group are mercury,bismuth, antimony, arsenic, and tellurium. Silver is the most solubleof the group of metalswhose sulfides are solublein aqueoussolutions of alkali sulfidesat high temperatures. This may explain the close spatial relation betweenrealgar, gold, and silver, at the Getchellmine. It may alsohelp to explain the sequencethere shown,in which cinnabar,stibnite, realgar, gold, and silver are presentas late minerals. The questionof transportationof gold, however,cannot yet be regarded as settledwith finality,and musthopefully wait for additionalexperimentation and ideas. But several considerationsmake the thesis of transportationas doublesulfides or sulfidecomplexes worthy of considerationfor the caseat Getcheil. Precipitationof Gold.-•Alkali sulfideseasily form doublesalts with certain metallic sulfides. If the sulfide concentration of sodium thio-metallic salts is lowered sufficiently,simple metal sulfidesare precipitated. For gold, a reductionin sulfidecontent in solutioncauses separation as the metal rather than as sulfide,since gold sulfideis unstableat temperaturesgreater than 40ø C (27). If sucha processmay accountfor the depositionof mineralsin the Getchell deposit,then there shouldbe someevidence of decreasingsulfide ion content in solution. Orpiment,with a highsulfur content, is earlierthan realgar,which containsless sulfur. This may be an indicationof decreasingsulfide concentration. Pyrrhotite, pyrite, and marcasiteare early and magnetiteappears to be late. Laboratory experimentswere carried out by the writer in which variousforms of iron sulfideswere synthesizedfrom solutionsat temperatures of from 400 ø to 600 ø C. The results indicate that with an excess of sulfide ions,pyrite and marcasiteare formed and that with a decreasein sulfidecon- centrationpyrite disappearsand pyrrhotite is deposited. With a further decreasein sulfur,pyrrhotite, magnetite, and hematiteare formed. This shift fromthe disulfidesthrough the near-monosulfidesto the oxideswith a decrease in sulfideion concentrationis thus entirely compatiblewith the observedse- quenceat Getchelland suggestsa similar decreasein sulfideconcentration in the solutionsas mineral depositionproceeded. Changesin Characterof SolutionsDuring Deposition.--Thepresence of early calciteand pyrite and later marcasiteand suchsulfates as barite and possiblygypsum suggests a gradualchange from alkalineto neutraland finally to acid solutions. The presenceof marcasiteespecially indicates a changeto acid solutions(1). This gradualchange is not uncommonin near-surface depositsand is particularlycommon in fumarolesor hot springs. It essentially representsa changeof sulfideto sulfateion. This maybe explainedby Allen's reaction,in whichfree sulfur reactswith water at certaintemperatures to form the sulfideion and sulfateradical, as follows: 4S + 4H20: 3H,o + H.oO4. If H2S is removedfrom the system,the reactionwill movetoward the right, bringingabout a concentrationof sulfuric acid which may convert the initially alkaline solution to acid. On enteringinto shattered,permeable rock such as the Getcheilfault zone, the ore solutionsundergo loss of pressurewhich facilitates the lossof a gas OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 301 phase,such as H.•S, from the liquid phase. With this selectiveevaporation, the sulfurcontent of the remainingsolutions will decreaseas they changefrom alkalineto acid composition. With decreasingsulfur contentand accompany- ing acidification,pyrite will alter to marcasite,realgar will be formedin placeof orpiment,and gold will be precipitated. With further decreasein sulfur, the iron sulfideswill ceasedepositing and magnetitewill beginto form. In those environmentswhere sulfuric acid is abundantlydeveloped, hypogene sulfate minerals may be deposited. If such a processhas taken place at Getcheil,one might questionwhy marcasiteand realgarare restrictedto certain zonesor shootswithin the veins. This is believedrelated to the possibilitythat the hydrothermalsolutions did not passthrough all portions of the veins at equal rates, with the consequences discussedbelow. The ore-carrying solutionswere probablysomewhat hotter

FIG. 26. Photomicrographof polishedsection of pyrite. The extreme porosity was not noted under lower magnification. Some black openings are filled with quartz; many are vacant. The white grain in the center is gold. X 4,000. Fx•. 27. Photomicrographof polishedsection of gold-rich activatedcharcoal usedin the recoveryof the gold in the milling process. White grain is gold,on rim of a charcoalcell. This charcoalis usedbecause of its high adsorptivepower. Compareporosity and surfaceto that of the pyrite in Figure 26. thanthe wall rockand, during part of the periodof mineralizationat least,were actuallyheating the rocksthrough which they passed. As the rock became hotterthe solutionscould lose an increasin.gly smallproportion of their heat. Thus,at any onepoint in the vein,the temperaturesof the wall rockand of the passingsolutions were rising and the geothermal gradient was there becoming steeper. This increasein temperaturewas not uniformthroughout the veins, butwas more marked in theloci of greaterpermeability through which greater volumesof the heat-carryingsolutions passed. This processmust not be mis- interpretedto meanthat any given portion of themoving solution was actually becominghotter with time; it wassimply cooling at an increasinglysmall rate, andtherefore was followed by portionsof solutionsprogressively hotter. 302 PETER JORALEMON.

Sowithin the veins there were hot, permeable loci separated by lessper-

meableand cooler places. ß The separation from the solution of H2Sin thegas phasewould be favored by a sufficientlyhigh temperature to keep up its produc- tionby theAllen reaction cited above, and by a pressureso 10w as notto retain

Lower limit of pic visibilif / / / rlimit ofcoloidal porticles-- / Lowerlimitof coloidal / z Gold unit cell Gold ot_Olla.

Increasin•l frequencyof oCCurrence __ -- Fro. 28. Frequencydistribution of goldgrains, showing probable extension in sub- microscopicrange. A logarithmicscale is usedon the abcissa. theH2S in solution.Such rather special combination oftemperature and pres- surewould be mostprobably along the morepermeable and hence hotter axes of flow. Thusthe ore solutions probably remained alkaline in theless permeable portionsof the¾ei•s while they were becoming neutral or somewhatacid in OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 303

the hotterore shoots. Pyrite wasdeposited throughout the veinsin the earliest, alkalineperiod of deposition,and only in the lower-grade,less permeable and coolersections during later stagesof mineralization. Realgar, marcasite,and the richer occurrencesof gold were restrictedto the hotter parts of the veins where the solutionswere neutral or acid. The gold, realgar, and magnetite were depositedfrom hotter solutionsthan those that formed the pyrite and the conventionalgangue minerals. In the pastthere has been a tendencyon the part of geologiststo assumethat the terms "low-temperaturemineral" and "late mineral" are synonymous. Graton (11 ) has questionedthe validity of this reasoning,and recentwork at Butte has indicatedthat late mineralswere actually formed at higher temperatureand under conditionsof higher intensitythan the early minerals(26). Further evidencein supportof suchrelations of temperatureand sequence is found in the Getchelldeposit. The early solutionswere apparentlydilute and undersaturatedwith the ore minerals. These solutionswere able only to redistributethe rock mineralswithin their respectivebeds because they were al•readyconsiderably depleted in intensity,and couldonly transport dissolved materiala few inches. Thoseportions of the later solutionsthat were able to remainwithin the more permeableloci in the veins,benefitting by the heating already accomplishedthere, were able to do a more "intense"kind of work. Gangueminerals in thesefavored portionswere no longer so localized,and quartz replacedlimestone, argillite, and andesitewith about equal ease. In place of the early, coarse-grainedquartz that was depositedin a leisurely mannerfrom dilute, alkalinesolutions, the fine-grainedquartz was formedby more rapid growth from relatively concentratedand probablyacid solutions. The earlier, euhedralpyrite gave way to the later, irregular pyrite-marcasite intergrowthsthat probablyrepresent deposition from mor• concentratedand hence more intense solutions.

Age of Mineralization. The Getchellores were depositedat sometime subsequentto the intrusion ßof the andesiteporphyry, which is tentativelydated as early or middleTertiary. A period of silicificationto form the opalite occurredafter depositionof the rhyolite tuff. The strongestmineralization in the Getcheilore bodiesoccurs towardthe northend of the mineralizedzone nearest to the deeppipe of rhyolite tuff. It is possiblethat the ores are later than the rhyolite and representa late phaseof that periodof igneousactivity. The Getchellore bodyis similarin manyways to the gold-realgardeposits at Manhattan, Nevada, about 150 miles to the south. At Manhattan, both earlyandesite and late rhyolitedikes occur near the ore body. In the descrip- tion of this deposit,Ferguson (8) suggeststhat the gold mineralizationmay be post-rhyolite. He points out that at Round Mountain, 50 miles north of Manhattan,gold ore containingrealgar occurs in a rhyoliteplug and is therefore later than the rhyolite(7), and that thesetwo depositsmay belongto a post-rhyoliteperiod of mineralization. The closeparallelism in the character 304 PETER JORALEMON. of the Getcheiland Manhattan depositssuggests that they may belongto the samegeneral period of mineralization. In his descriptionof the miningcamps of Nevada,Ferguson (9) showsthat the Tertiary preciousmetal deposits in that areamay be dividedinto two groups of widely differingcharacter. In the older group the depositsare generally morepersistent laterally and vertically. They all containmanganiferous calcite and havea ratio of silverto goldgreater than 1:1. In the other group,gold is generallypresent in larger amountsthan silver and the structuresare less persistent. Gold.field,type example of thisgroup, has,a gold-silver ratio of 7:3. Realgaris not foundin mostof the gold-richdeposits. In thosein whichit is found, Getchelland Manhattan,the bullion reportsindicate the great pre- ponderanceof gold over silver, the gold-silverratio varying from 10:1 at Getchellto 17:1 at Manhattan. It is possiblethat this-type of ore deposit, characterizedby a very high gold-silverratio and the presenceof realgar,may representa third, still later groupof depositsformed in late Plioceneor early Pleistocenetime. These depositshave many featuresin commonthat are not sharedby other late Tertiary gold ores. Both contain realgar, orpiment, cinnabar,carbon, stibnite, fluorite, and pyrite. Art unusualgreen calciteis foundin each. Silver sulfidesor sulfosaltsare lacking. At both depositsthe goldis very fine-grainedand is closelyrelated spatially to realgarand carbon. In each,lead, zinc, and copper sulfides are rare or nonexistent.Both contain relativelylarge replacementbodies that are more importantthan the single narrow fissure veins. Ferguson(8, p. 106) suggeststhat the highgold-silver ratio may be dueto the presenceof arsenicminerals. He proposesthe theorythat hypogenesolu- tions rich in arsenicand free from lead, zinc, and coppertend to precipitate gold without any importantmixture of silver. There appearsto be no chemicalbasis for such a reactionand the commonoccurrence of gold-poor proustitedeposits would seem to disprovethis theory. Butler(5) gavea more plausibleexplanation, suggesting that the presence of carbonmay be important in bringingabout a highgold-silver ratio, as carbon is known to precipitategold selectivelyfrom a gold-silversolution. Carbon is undoubtedlyimportant in precipitatinglarger amounts of gold,but there may be a simplerexplanation of the high gold-silverratio. Hydrothermalsolutions, on rising,may be expectedto changegradually from alkalineto somedegree of acidity. As a consequence,gold will tend tobe deposited assoon as sufficient sulfide ions are removed, but silver sulfides andsulfosalts, being soluble in acidsolutions, will no longerbe depositedbut will be sweptalong the channelways.The sulfuricacid will graduallybe lessenedin thecourse of thisjourney, partly through the precipitation of sulfates andpartly by reaction with sericite and feldspars to formclay minerals. When andwhere the acidity becomes sufficiently lowered, the silver-sulfur compounds couldagain be precipitated to form a shallowsilver zone capping the gold. It is possiblethat the solutionsmight reach the surfacebefore adequate. reductionof the acid had .occurred.In sucha case,the resultantdeposit OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 305 wouldconsist only of a goldzone. Getcheil,Manhattan, and Goldfieldmay be examples. There are gradationsin the characterof the depositsof from middleto late Tertiary age (i.e. Tonopah,Goldfield, Manhattan, Getcheil) and it is probable that the entire late Tertiary period was an epochof continualmineralization.

EARLYINGFIEASIN•;INTENSITY I LATE LARGELYOFLOGAL DERIVATION I LARGELY OFMAGMATIG DERIVATION Depositedfromolkolin• solutIQIt$ I Deposited fromneutral solutions J Deposited fromocid solutions

Quartz

ß Carbon

•riclte ------

Chlorlte I ------

Pyrrhotiln ..,,•. --

__ Pyri re -- - _

Ari4.nopyriln - • -- ---

Ghol½opyrire- -- .•11•,,-- --

Orpiment Realgot- --•••B•-- -- MarcQsiln ----i•1•--

Gold

Silver --

Scheelfle --

Ghobnzlfe

B•rile

Gypsum

Fluorite

Stlbnltn

Cinnibor

Magnetite

Fro. 29. Sequenceof depositionof Getchellore minerals.

The changesin characterof thesedeposits may be explainedby the factthat decreasingage has permitted an increasingescape from erosiveremoval of the shallowestportions of the initialdeposits. The similaritiesbetween the Getchelland Manhattan deposits have been discussedpreviously. There are, however,certain discrepancies between the 306 PETER JOR.4LEMON. two deposits. The presenceof adulariain somemines at Manhattansuggests that it may representa slightlymore intensephase of mineralization. Native silveras a hypogenemineral is not reportedfrom Manhattanand, if the presenceof native silver togetherwith native gold is an indicationof lower intensity, the lack of silver at Manhattan corroboratesthe other evidencefavoring higher intensityfor that camp. A featureof the Getchelldeposit that setsit apartfrom thetypical epithermal gold depositsis the nearly completelack of certaincommon epithermal struc- tures suchas "colloform"banding, crustification, and comb structure. This suggeststhat the Getchelldeposit may be intermediatein characterbetween the typicalepithermal precious metal and the quicksilverdeposits. Many of the quicksilverveins of Nevada showa closerelationship to both the gold-silver and stibnite-realgartypes of deposits(20, p. 473). The depositsat Steamboat Hot Springs,'located a few miles southof Reno, containsome cinnabar and showmany strikingsimilarities to the Getchelldeposit. At Steamboatas well as Getcheil,Tertiary igneousactivity is representedby early andesiteand late pumiceousrhyolite. The rhyolite at Steamboatis Pleistoceneor recent,a fact that may suggesta Quaternaryage for the Getchellrhyolite tuff. Silica in the form of opal,chalcedony, and fine-grainedquartz is abundantin eachdeposit. Accordingto Brannocket al (4) the Steamboatwaters contain a mudwhich is largelycomposed of tiny particlesof opalor a silicagel. This mudapparently closelyresembles the Getchellgumbo, and its other featuresstrengthen the resemblance.Minute acicularcrystals of stibniteoccur in the mud and have alsobeen found floatingas a thin scumon the surfaceof the waters of the spring. These fine-grainedaggregates of stibnite needlesclosely resemble the unusualfelt-like stibnitecrystals from the Getchellore body. Pyrite and realgarhave been identified in the Steamboatmud, and the presenceof an iron oxide,possibly magnetite, is suggested. The moststriking feature common to the mud and the gumbois the presenceof gold. Two of the most important wells at Steamboatcontained mud with between0.3 and 0.4 ouncesof gold per ton. The ratio of goldto silverin thesemuds is 1:3. Copperis presentin the muds,as in the Getchellgumbo, in extremelysmall amounts. The siliceoussinter at Steamboatcontains pyrite, arsenopyrite,stibnite, and somecinnabar. The sinteris stainedin placeswith a red-brownantimony ochre, metastibnite; ilsemannite has also been found. These features strongly suggesta very close genetic relation between Getchelland SteamboatSprings. Figure 30 showsthe locationof the-major depositsof Nevada in which either cinnabaror gold is the economicmineral. The importantcinnabar depositslie in a relativelynarrow, northerly trending belt that cutsthe west- centralpart of the state. The chiefgold deposits, on the otherhand, show no regularityof distribution,but are widelyspaced throughout the entirestate. The Great Basinis composedof a seriesof northerly-trendingstructures, bothfolds and faults. Even the recentfaults (10) trendin a northerlydirection. Most of the cinnabardeposits are relativelyyoung, and all are probably Plioceneor later. It is possiblethat a majorlate Tertiarybelt of faultingis OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 307 now markedby the larger cinnabardeposits shown in Figure 30. Such a belt would act as a locusfor the cinnabar-depositingsolutions. Nolan (23, p. 152) has describeda geanticlinethat dividedthe Paleozoic geosyncline,persisting from late Devonianto early Permian time. The axis of this geanticlineis shownin Figure 30. It may be merelycoincidence that mostof the larger cinnabardeposits and the Getchellore bodieslie alongor near this axis, as the periodof geanticlinaldeformation was over by Permian

o o ß }mom . Getcheil mine

. o • o o/ ASteamboat/ ß• Springs lB

• / ßManhattan • • District ß

---x• Probableaxis of late • Paleozoicgeanticline • ß_ :_o,, _ 0 Majorquicksilver deposits

FIGURE 50 MAP OF NEVADA SHOWING LOCATION OF MAJOR GOLD AND QUICKSILVER DEPOSITS time whereasthe gold and cinnabardeposition was late Tertiary or early Quaternary. It is neverthelesspossible that the geanticlinalaxis was and has continuedto be a zoneof weaknessalong which later, deep-seated faults have formedfrom time to time. Along suchfaults, mineralizing solutions could riseto deposit.cinnabar or gold. The Getchellmine is located within this cinnabarbelt, near its eastern margin. The 'Manhattandeposits lie slightlyeast of thisbelt. It mayagain be 308 PIETER JOR.4LIEMON. coincidencethat thesetwo gold occurrenceslie near or within the belt, but their mineralogicand textural featuressuggest a geneticrelation to cinnabar deposits. It is probablethat the Getcheil' and Manhattan gold deposits represent a gradationbetween the earliergold ores such as Goldfieldand the more recent cinnabardeposits. A periodof mineralizationmay thusbe consideredto havebegun in middle Tertiary time with the depositionof silver-goldore bodies,gradually changing in characterto form the depositsin whichgold slightly exceeded silver. Some- what later the arsenicalGetcheil-Manhattan type was formed,with gold far outweighingsilver, and with cinnabarpresent in small amounts. The Getchell and Manhattandeposits apparently may be regardedas a link toward bridging the gap betweenthe preciousmetal and the mercury deposits. In the latest important stageof this period of mineralization,the cinnabarore bodieswere formed. The final dying stageof mineralizationis representedby Recenthot spring deposits. Many of theseare similar in characterto the cinnabardeposits,but a few, suchas the Golcondatungsten-manganese blanket (17), ar• completelydifferent. The hot spring depositspresumably represent a less intensephase of the sameperiod of mineralizationthat formed the cinnabar vein merely becausethe former are shallower. The variation in intensity would includechanges in temperature,pressure, and particularlyin concentration. ' Regardingthe possibility of a protractedepoch of Tertiary mineralization, Lindgren said, "This whole processof metallization,beginning at the earliest Cretaceous,closing at the end of the Tertiary, and feebly continuingtoday, is actually one continuousoperation of separationof volatile constituentsfrom the magma . . ." (19, p. 179).

Depth of Formation. The Getchellore bodieswere formedunder a relativelyshallow cover. The largerugs in the ore and the intenselyshattered nature of the ore zoneare the bestevidence favoring deposition in a shallowenvironment. The plastic,un- consolidatedcharacter of the gumbolikewise suggests a near-surfaceorigin. As relativelythin outliersof rhyolite tuff still remain near the ore zone,it is probablethat erosionhas not removedmore than a few hundredfeet of rock sincethe depositionof the rhyolite. The apparentlyclose affiliation between the Getchellmineralization and thecinnabar deposlts suggests that they were formed under somewhat similar conditions. Baileystated that the opalitetype of cinnabardeposit was probably formedat depthof about500 feet beneaththe surface.(3, p. 20). The present topsof the Getcheilore bodieswere probably deposited within a thousandfeet of the then-existingsurface.

Genetic Classification. The Getcheilore bodiesare clearlyepithermal as outlinedby Lindgren. (20). They constitutea particularand self-characteristicexample, a narrow subdivisionof the broader epithermalclass. The Getchelldeposit belongs OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 309 amongthose that representthe least intenseportion oœthe epithermalclass. Graton,in proposingthe telethermaldepth-zone (11, pp. 547-551), has suggestedthat the epithermaldeposits do not representthe very lowest intensity end membersoœ the hydrothermalfamily. He points out that certain low- intensitystibnite-realgar and cinnabardeposits may be a gradationalform between typical epithermaldeposits that are characterizedby relatively rapid depositionand telescopingbecause of steepthermal gradients, and still œeebler telethermaldeposits formed by leisurelydeposition in a coolerenvironment. Despiteits many unusualfeatures, the Getcheildeposit must be considered as epithermalon the basisof the telescopednature of mineralization. It has beensuggested that this depositmay representa gradationbetween the typical epithermalprecious metal and the cinnabardeposit. The increasingdifferences from the progressivelyolder and somewhatdeeper-lying precious metal deposits andthe growingevidences of similaritywith the cinnabardeposits progressively closerto Getcheilin youthand shallowness,place the Getcheildeposit close to the feeblestend of the epithermalgroup. It may well be that hot-springsinters were formed at the surfaceonly a few hundred feet above the presentvein outcrops. PARK CITY, UTAH, Nov. 3, 1950. BIBLIOGRAPHY. 1. Allen, E. T., Crenshaw, J. L., and Metwin, H. E., Effect of temperature and acidity in formation of marcasite and wurtzite; a contribution to the genesis of unstable forms: Am. Jour. Sci., 4th Set., vol. 38, pp. 393-431, 1914. 2. Auger, P. E., Zoning and district variations of the minor elementsin pyrite of Canadian gold deposits: Ecoa. GzoI.., vol. 36, pp. 401-423, 1941. 3. Bailey, E. H., and Phoenix, D. A., Quicksilver deposits in Nevada: Nevada Univ. Bull., vol. 38, no. 5, 1944. 4. Brannock, W. W., Fix, P. F., Gianella, V. P., and White, D. E., Preliminary geochemical results at Steamboat Springs, Nevada: Am. Geophys. Union Trans., vol. 29, no. 2, 1948. 5. Butler, B. S., Loughlin, G. F., and Keikes, V. C., The ore deposits of Utah: U.S. Geol. Survey Prof. Paper 111, pp. 391-395, 1920. 6. Edwards, A. B., Textures of the Ore Minerals, Australasian Inst. Min. Metallurgy, Mel- bourne, 1947. 7. Ferguson, H. G., The Round Mountain district, Nevada: U.S. Geol. Survey Bull. 725, pp. 383-406, 1921. 8. Ferguson, H. G., Geology and ore deposits of the Manhattan district, Nevada: U.S. Geol. Survey Bull. 723, 1924. 9. Ferguson, H. G., The mining districts of Nevada: Ecom Gzoi.., vol. 24, pp. 115-148, 1929. 10. Gianella, V. P., The earthquake of December 20, 1932, at Cedar Mountain, Nevada, and its bearing on the genesis of Basin Range structure: Jour. Geology, vol. 42, pp. 1-22, 1934. 11. Graton, L. C., The depth zones in ore deposition: Ecom Gwor.., -vol. 28, pp. 513-555, 1933. 12. Graton, L. C., Personal communication, 1950. 13. Gross, J., and Scott, J. W., Precipitation of gold and silver from cyanide solution on charcoal: U.S. Bur. Mines, T. P. 378, 1927. 14. Haycock, M. H., The role of the microscopein the study of gold ores: Canadian Inst. Min. Metallurgy, vol. 40, pp. 405-414, 1937. 15. Head, R. E., Form and occurrence of gold in pyrite from a metallurgical standpoint-- Coated gold: U.S. Bur. Mines, Rept. Inv. 3275, pp. 39-46, 1935. 16. Hobbs, S. W., Geology of the northern part of the Osgood Mountains, Humboldt County, Nevada, Ph.D. Thesis, Sterling Library, Yale University, 1948. 17. Kerr, P. F., Tungsten-bearing manganese deposits at Golconda: Geol. Soc. America Bull., vol. 51, pp. 1359-1390, 1940. 18. Kuranti, G., Synthetic study of gold-containing pyrite: Suiyokwai-Si, vol. 10, pp. 419-424, 1941 (abstracted in Chem. Abstracts, vol. 35, p. 3563, 1941). 310 PETER J'OR.dLEMON.

19. Lindgren, W., Differentiation and ore depositionin the Cordilleran Region in Ore Deposits of the Western States, pp. 152-180, Am. Inst. Min. Met. Eng., New York, 1933. 20. Lindgren, W., Mineral Deposits, McGraw-Hill Book Co., New York, 1933. 21. Lindgren, W., Succession of minerals and temperatures of formation in ore deposits of magmatic affiliation: Am. Inst. Min. Met. Eng. Trans., vol. 126, pp. 356-375, 1937. 22. Maslenitsky, I., On some cases of formation of disperse gold segregations in iron sulfides: Acad. sei. U.R.S.S. Comptes rendus (Doklady), vol. 45, no. 9, pp. 385-388, 1944. 23. Nolan, T. B., The Basin and Range Province in Utah, Nevada, and California: U.S. Geol. Survey Prof. Paper 197-D, 1943. 24. Palache, C., Betman, H., and Frondel, C., The system of mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University 1837-1892, 7th ed., John Wiley & Sons, New York, 1944. 25. Pauling, L., The Nature of the Chemical Bond, 1939. 26. Sales, R. H., and Meyer, C., Wall rock alteration at Butte, Montana: Am. Inst. Min. Met. Eng., Tech. Pub. 2400, 1948. 27. Smith, F. G., The alkali sulfide theory of gold deposition: EcoN. G•ov.., vol. 38, pp. 561-589, 1943. 28. Stillwell, F. L., and Edwards, A. B., An occurrence of sub-microscopicgold in the Dolphin East Lode, Fiji: Australasian Inst. Min. Metallurgy, vol. 141, pp. 31-46, 1946. 29. Van Aubel, R., Sur l'importanee dans les minerals d'or du calibre des partJoules: Annales des mines, 13e sirie, Tome 16, pp. 155-161, Livraison de 1939.