THE OCCURRENCE OF GOLD AT THE GETCHELL
MINE, NEVADA. x
PETER JORALEMON.
ABSTRACT.
The Getchellveinsare lenticularreplacement
bodieslying alongarcuate
branches
of a complex
range-front
faultsystem.The faultzonecutsall
rocksof the districtand is tentativelydatedas late Tertiary.
The moreintenselymineralized
portionsof the depositform a shallow
blanketwith rootsthat projectdownwardinto areasof sparsemineralization. Thegoldshoots
arerestricted
to areasOfintense
mineralization.
Native gold and native silver are the only economic
minerals. The
greatbulkof thegolc•occursin minutebutmicroscopically
visibleparticles.
Somegoldmayalsooccurin submicroscopic
particlesandsomemaybe in
solid solutionin pyrite and carbon.
The ore minerals,dissolvedin alkali sulfidesolutions,are believedto
havebeendeposited
•vhenthe sulfide/onconcentration
in the hydrothermal
liquiddecreased,
makingunstablethe doublesulfidesof gold;iron, and
arsenic.
The Getcheildepositis similarin manywaysto the Nevadaquicksilver
depositsand present-dayhot-springdeposits. The Getcheilore occurrence
may representa gradationfrom the commonepithermalgold depositto the
cinnabardeposit. It is therefore placedcloseto the feeblestend of the
epithermalgroup.
INTRODUCTION.
In the past, a numberof important gold mines have producedmillions of
dollarsin gold from ore in which no gold couldbe seen. The Getchellgold
depositis the.mostrecentlydiscovered
of theseminesof "invisible"gold. Becausegoldhadnot beenseenin theore,eitherat Getcheilor at similardeposits,
the problemof the state,or occurrence,of the gold has been subjectto considerablecontroversy. A prime purposeof the presentstudyhas beento ascertainwhetherthe modernmicroscopicand polished-section
techniquesare
capableof sheddingsomelight on the problemsof gold occurrence.
Location and ttistory.--The Getcheil mine is locatedin the northeastern
cornerof the OsgoodRange in east-centralHumboldt County,Nevada (Fig.
1). It lies 26 milesnorth of the town of Golcondaand some15 milesnorthwest of Red House, the nearestrailroad station.
The gold depositis in the old Potosi mining district but was itself not
discovered
until 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,mininghadnearlyexhausted
the oxidizedore
and it becameapparentthat milling practicesin use were not suitablefor
xUndertaken
in partasa Samuel
Franklin
Emmons
Memorial
Fellowship
award.
267
268
PETER
]ORALEMON.
treating the sulfideore. Mining operationswere halted and an intensive
metallurgicalresearchprogramwas begun. In 1947,a pilot mill was started
treating100tonsa day. By'thesummerof 1948a satisfactory
flow sheethad
beendevelopedand construction
beganon a 1,500-tonmill. Production,at
presentlimitedto 1,000tonsa day, will be increaseduponthe completionof
the mill.
Acknowledgments.--The
writer is indebtedto the staff of the Getcheil
mine,andparticularlyto Mr. R. A. Hardy, for financialassistance
andwholeheartedcooperation.G. S. Koch and R. S. Creelyably assistedin the two
summers of field work.
114ø
......
---RAILROAD
H I OHWAY
•..,..o,
•oo
i
FIG. 1. Index map of Nevadashowinglocationof Getchellmine.
The laboratoryportionof this studyhas beencarriedon with the aid
of a SamuelFranklin EmmonsMemorial Fellowship.
The facultyof theDepartment
of Geology
at HarvardUniversityhasmade
valuablecriticismand givenmuch'assistance
in the laboratorywork. ProfessorL. C. Gratonin particularhas donatedgenerously
of his time and
experience.All polishedsections
werepreparedby Mr. CharlesFletcherat
theI-IarvardLaboratoryof MiningGeology.
OCCURRENCE OF GOLD .4T GETCHELL MINE, NEV.4D.4.
GENERAL
269
GEOLOGY.
The Osgood;Rangeis apparentlya typicalGreat BasinRange. Its steep
flanksaretoppedby rollinguplandsin thecenterof therangeandit is bounded
on at leastone sideby normalfaultsthat are responsible
for the uplift of the
range.
The mountainsare composed
of sediments
of Paleozoicagethat havebeen
cut by a seriesof faultsinto isolatedstructuralblocks(16).2 The attitude
of the rocksvarieswidelyfrom blockto block. The sediments
are intruded
by a large stockof Mesozoicgranodioriteand a seriesof related dacite
porphyrydikes,andby smallTertiary andesiteporphyrydikes. Basaltflows
anda smallbodyof rhyolitetuff areproducts
of Tertiaryvolcanism.Irregular
patchesof the tuff nearthe minehavebeensilicifiedand closelyresemblethe
opalitefoundin certainNevadaquicksilverdeposits(3).
•"
. •.
•,
""
'•'""C;?•
•-:½i':*
..,
•':'•.•
.:-- .;....
•'
..:
Fro. 2. Getchell Mine, Nevada. The scars on •e hill behind •e camp mark the
footwalls of ½e open
DescriptiveGeologyoI the GetchellMine.•The beddedrocksof the mine
area are interbeddedlim•ston• and carbonaceous
argillit• (Fig. 5). Th• most
stri•ng feature of the sedimentsis the extremel•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 sediments
are 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
apparentlylocalizedin and n•ar this fault zone. A roughly conicalpipe of
alteredrhyoliteporphyrytuff li•s to the eastof this fault.
• Numbers in parenthesesrefer to •ibliograpby at end of paper.
270
PETER JORALEMON.
Structure.--The principalstructureof 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 northerlydirectionand is composed
of 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
mullionand drag-foldstructuresto be a rift fault, alongwhich the hangingwall block movedan unknowndistancewith respectto the footwall block
(Fig. 6). Later normalmovement,indicatedby steeplypitchingslickensides
engravedon the mullion faces,took place along this footwall strand. The
steeper,hanging-wallbrancheswere formed at that time and occur above
flexuresin the footwallfaultwherethe dip flattens,asshownin Figure4.
ORE DEPOSITS.
GeneralFeatures.--The goldore bodiesare sheet-likemassesthat lie along
the variousstrandsof the Getchellfault zone. They extendat least7,000feet
horizontallyand 800 feet down the dip, and vary in width from a few feet
to morethan 200, averagingabout40 feet wide. The vein pattern,extremely
complexat the surface,becomessimplerwith depthand, deeperthan 800 feet
belowthe surface,all the branchveinsapparentlyjoin to form a dinglepersistentstructure,illustratedin Figure 4.
Gan#ueMineraIs.--The veinsconsistof shearedand mineralizedargillite
and limestonecut by quartz and calciteveins and containinga soft, plastic
gumbothat has replacedthe sediments. The gumbois the principalgoldbearer. Barite,gypsum,fluorite,andchabaziteare minorganguemineralsand
occurin smallisolatedpocketswithin the veins. The shapeof the moreheavily
mineralizedportionsof the veins is that of an irregular, relatively shallow
blanketwith three root-likeprojectionsthat extenddownwardinto the region
of sparsemineralization,as illustratedin the longitudinalprojection,Figure 3.
The three "roots" of the ore body appearto mark the original major channelways alongwhich the hydrothermalfluid passedin greatestabundance.
There are striking differencesin the mineralizationwithin as contrasted
with outsidethesemajor channelways.Within theseroots,now markedby
high-gradegold ore, depositionhasbeenlargelyby non-selective
replacement.
In theseareas,all rocktypesandearlyganguemineralshavebeenreplacedwith
equaleaseby the ganguemineralsand the sulfidesof the later phasesof the
ore-formingperiod. The larger part of the material depositedhere is apparently magmaticallyderived,and the processof depositionhas beenone of
wholesalereplacement. In the sparselymineralizedstretchesbetweenthe
ore shoots,replacementhas been limited and extremely selective,and all
materialexceptsmallamountsof auriferouspyrite appearsto havebeenlocally
derived. In these outlying ar•easof the veins, mineralizationinvolved the
local reworkingof rock mineralswith only minor introductionof material
OCCURRENCEOF GOLD AT GETCHELL MINE, NEVADA.
271
272
PETER J'OR.,'ILEMON.
5300 + -t +Cd
SECTION
SECTION
B'B'
SECTION
G G'
•9oo
5800
57oo
5600
5500
5400
5•tO0
•
Limestone
•
Unminerolizud
fou$l
•'1'• Rhyolitu
tuff
Centoct
:i:...:-•Andesite
porphyry-•--'] Argillitu •
•
Gold
vein --"• Grodotiono$
coatoct
f'G[•-•
ß
Gronodior
itu
4•
•
8• ' I• Feet
FIG. 4. Geologiccross-section
throughGetcheilore body.
OCCURRENCEOF GOLD AT GETCHELL MINE, NEVADA.
273
foreign to the rocks. Thus. calcite veins are restricted to beds of limestone
and quartz veinsand lensesoccuronly 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 submicroscopic
intergrowth of quartz and amorphous
carbon
(Fi•s.9, 11,12,14). In places
veinsofgumbo
several
feetwideand
parallelto the surroundingsedimentaryrocksend abruptlyagainstunsheared
limestoneor argillite. That it is of hydrothermaloriginis indicatedby the fact
that in thin sectionit appearsactuallyto replacethe wall rock, as shownin
Figures8, 9, and 10. It was apparentlyformedin two stages. First was the
replacementof wall rock by fine-grainedquartz to form a porousaggregateof
subhedralquartz crystals in narrow bedding veins. tn the secondstage,
carbon,probablyderivedfrom underlyingargillite, was introducedinto the
porousquartz veins and was depositedinterstitially to the quartz. Rather
surprisingly,thoseportionsof the veins that containedthe most gumboand
wouldordinarilybe considered
to be but slightlypermeable,remainedthe most
permeablesections'of the veins and the later hydrothermalsolutionswere
there concentrated.
MetalliferousMinerals.--The depositionof importantamountsof gangue
mineralshadapparentlystoppedby thetimeof maximumdeposition
of sulfides.
'Pyrite and minor pyrrhotite are the earliest sulfidesand in the ore shoots
theyoccurin irregularporousmasses,
as shownin Figure 26. .In the outlying
segmentsof the veinspyrite has a tendencyto assumeeuhedralforms. Replacementhasbeenthe importantmechanism
of depositionof all later sulfides
within the ore shoots(Fig. 12) whereasfracture-fillingis predominantin the
sparselymineralizedzones(Fig. 13).
Arsenopyriteis presentin minuteeuhedralcrystalsand is almostentirely
restricted'
to theportionsof the veinsoccurringin the andesiteporphyry.
Marcasiteconstitutesabout 5 percentof the iron sulfidesin the central
channelways
but is rare or non-existentin the outlyingareas. It occursas
minutelathsandas irregularshellsor rimsencirclingearlierpyritegrains.
Arsenic sulfidesare the most abundantof the metalliferousmineralsand,
unlikepyrite, are entirelyrestrictedto the areaswithin the veinsthat now contain noteworthyquantitiesof gold. Orpiment is apparentlyearlier than
realgarand is foundonly in isolatedpockets,characteristically
occurringin
low-gradesectionsof the veins. In these pockets,orpimentconstitutesas
muchas 40 percentof the mineralizedmaterialand is veinedand coatedby
realgar. Realgar occursthroughoutthe commercialore bodiesin 'amounts
rangingfrom 1 to 10 percent. It showssucha consistent-relation
to gold
that the presence
of realgaris usuallya strongindicationof goodvalues.
Stibnite,like orpiment,occursin restrictedpocketsin whichit is present
in two unusualforms. Stibnite occurringin narrow, pipe-likebodiesis
presentin a seriesof clustersof hair-likecrystalsso minuteas to suggesta
softblackvelvetcoatingon the rocks. The molybdenum
mineralilsemannite
.is doselyassociated
with this stibnite. In one smallarea the fracturesin the
rock are coatedwith metastibnite,
a red paint or stainwhichhasbeenshown
by X-ray photographsto be finely dispersedstibnite. Similar forms of
274
PETER JORALEMON.
stibnitehavebeenfoundin the sinterand the hot springmudsat Steamboat,
Nevada (4).
Cinnabaris presentas a thin coatingon calciteand chabazitecrystalsin
a narrow
and discontinuous branch vein.
While
it does not occur in com-
Fro. 5. Interbeddedlimestoneandargillite from Getchellmine area. The vein
material has replacedportionsof this formation,preservingthe thinly laminated
structure.
Fro. 6.
Fault surface that forms the foot-wall
strand of the Getchell vein
system. The nearly horizontalmullion structureis strong evidencein favor of
horizontalfault 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 carbonaceous
gumboand
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 beenreplacedby coarsequartz grains. In the processof this replace-
ment 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,itsverypresence
suggests
a possible
geneticrelationbetween
Getchellandthe cinnabardeposits
of Nevada.
Magnetiteis apparently
the latestof the metalliferous
mineralsassociated
with gold,and is the onemostclosely'
relatedto gold. It occursonly in
microscopic
particlesand hasbeenseenonlyin the gumboand andesite. It
is plainlynot a detritalmineral. It is so closelyassociated
with gold that
any polishedsections
containinga clusterof magnetitegrainshavealsosome
goldcloseat hand,generallywithina millimeter(Figs. 22, 23).
PRECIOUS
METALS.
Distribution.
Economic
amountsof goldare entirelyrestrictedto theveins•withinwhich
the richer ore shoots are loci of more intense mineralization.
The richer
goldoreis in theshapeof a relativelyshallow,discontinuous
blanketwiththree
root-likedownwardprojections,as illustratedin the projectionin Figure'3.
The cause of localization of the ore shoots is discussed in a later section.
The
portionsof the veinsbetweenthe rich shoots,and.the wall rock for several
hundred feet on either side of the veins contain everywherea small amount
of gold,rangingfrom 0.01 to 0.08 ouncesper ton. The commercialgoldore
hasbeendeveloped
overa strikelengthof 7,200feetandto a depthof asmuch
as 800 feet. At this depth,rich gold ore is still present.
Visible Gold.
Becauseof the crumbly,non-cohesive
natureof the gumbo,it wasnecessary
in polishingspecimensof the ore for microscopicinvestigationto developa
techniquewhich would allow the polishingof even the smallestparticlesof
gold.. Mr. CharlesFletcherof the Laboratoryof Mining Geologyat Harvard
Universitypreparedthe polishedsectionsand his patienceand skill haveaided
immeasurablyin this investigation.
In the early stagesof the microscopicstudy,extremelyfew gold particles
were to be seen,and it becameapparentthat the polishingmethodsin use
werenot satisfactory
for thisproblem. The polishingtreatmentfinallyadopted
represents
the resultsof progressive
trials and improvements.In this process,
thesampleto bepolishedis first impregnated
with a liquidbakeliteresinoidand
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 severalstagesof polishingon the Graton-
Vanderwiltpolishing
machine.The multipleimpregnation
is necessary
becausea singleimpregnationpenetratesonly a short distanceinto the section.
By usingthis methodof polishingand by usinga microscope
with a Zeiss2millimeterobjectivelens,N.A. 140, mineralgrainshavingan area of as little
as 0.1 squaremicronsare visibleand recognizable.
The propertiesof gold are so diagnosticas to make it an ideal subject
for sucha detailedstudy. Its brilliant yellowcolormakesit easilyrecognized
in largerparticlesbut whenthe grain sizedecreases
to 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 shatteredmineralin 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 showsthe
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 becomeslessnoticeableand other features must be used
for identificationof minuteparticles. The extremesoftnessof gold prevents
the achievement
of a perfectpolishon smallgrains,but every minutegrain of
goldhasa characteristic
undulant,"egg-shell"texture (Figs. 24, 25, 26). Gold
is exceededin reflectivityonly by native silver,platinum,and copper,and the
egg-shelltexture may be usedfor positiveidentificationof all particlesto a
limit of microscopic
visibilityat a magnificationof X 2,500.
The describedpolishingtechniqueis probablynot perfect; indeed it is
reasonablycertain that somegrains that would have been abovethe limit of
visibility, had they remainedin place and beenpolished,were torn from the
polishedsurface. The proportionof grainsthus lost undoubtedlywould increaseas the grains becamesmaller. Furthermore,somegrains of visible
size may not have been seen. Even with the most careful polishing,the
extremesoftness
of goldmakesinevitableits depression
belowthe impregnated
surroundings. Hence there is an obliquebandaroundevery 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 relativeimportanceof this relief shadow
increaseswith decreasingsizeof gold grain. If the diameterlimit of visibility
of a grainof goldideallypolishedis "x," andthe width of the obliqueshadow '
in actualpolishing
is"y,"thenanimperfectly
polished
g.rainmusthavea diameter of x + 2y to be visible. All grainssmallerthan this would appearonly
as tiny blackpits.
Finally it is certain that not all grains possessing
a true diameter above
the ideallimit of visibilitythat were actuallypresentat the planeof polishwere
seen. This planemusthavetouchedmany grains,largeand small,sofar from
their equatorof maximumdiameterthat 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
graphshowsthe enormousincreasein the abundance
of goldparticleswith de-
creasing
grainsize. It alsoshowsthat the greatbulkof the goldis in the
rare and widely scatteredlarger particleshaving an exposedarea of more
than 1,000 squaremicrons.
All gold particlesseenin the Getchellore are roughlylenticularin crosssection. To get a better idea of the three dimensionalshapeof the particles,
the lengthand width of individualgrainsvisibleon the polishedsectionwere
measuredand, after further polishingaway a measuredthicknessfrom the section, the dimensionsof the samegrainswere remeasured;this procedurewas
repeated,stepby step,at about2-microndepthintervals,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
dimensions
of individualgrainsand the numberof grainsvisible on every
polishedsurface,it is possibleto estimatethe minimumgold contentof the
polishedspecimen
andto compareit with the assayvalueof the specimen
from
whichthe sectionwas prepared.
!
OCCURRENCEOF GOLD AT GETCHELL MINE, NEVADA.
279
The averagepolishedsectionmay be regardedas a rectangularblockwith
dimensions2 x 1 x 1 centimeters. Each blockweighsabout2.7 gramsand
336,000suchblockswouldbe requiredto makea ton of ore. Gold with some
silverpresenthas a specificgravity of about17.3. One cubiccentimeterof
gold weighs0.556 Troy ounces,and ore containing0.5 ouncesof gold per
ton must contain0.9 cubiccentimetersof gold. If the gold occurredin cubes
one centimeteron an edge,an enormousnumberof polishedsectionsmight
haveto bepreparedbeforethis goldwouldbe encountered.If the assumption
is made,however,that goldis presentin grainshavingan averagedimensionof
1 x 1x • micronsand is sodistributedthroughthe ore that eachmassthe size
of a polishedsectionholdsa representative
amount,thenoneton of ore containing 0.5 ouncesof goldwouldcontain2 x 10x-ø
grainsof gold. Every polished
ß sectionblockwould containan averageof 5.94 x 10ø gold particles. Because
of the abundance
of non-opaque
mineralsin the Getchellore, it is possibleto see
not only mineralsactuallycut by the polishedsurfacebut alsomineralslying
asmuchasonemicronbeneaththe surface. T.huseachpolished
sectiori
maybe
visualizedas beingdividedinto a seriesof flat sheetsonemicronthick. Every
sheetwould contain1,045particlesof gold. There are many assumptions
in
thesecalculations,
but the resultsgive at leasta rough idea of the numberof
goldparticlesonemightexpectto seeif all goldoccurredin visiblegrainsof the
sizespecified. Increasingsizeof grainsgreatlyreducesthe number.
The investigationof particlesizeand numberhas shownthat there is little
correlationbetweenthe assayvalue of the sampleand the calculatedvalue of
the polishedsection. As an example,five polishedsectionswere prepared
•rom an ore samplewhichwas shownby assayto contain1.06 ouncesof gold
per ton. In oneof thesesections,fewer than 20 gold particlesof micronsize
or largerwereseen,whileanothercontained
morethan800particlesof the same
or larger size. The other three varied widely but fell within theselimits.
The conclusion
mustbe 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 gumbowhichis generallyan excellentindicatorof rich ore.
Mostof thevisiblegoldis intimatelyassociated
withthefine-grained
quartzcarbon matrix of the gumbo. Many of the particlespartially rim larger
quartz grains. Someof the gold is rather uniformlydistributedthroughout
the gumboin smallisolatedparticles. This is slightlymoreabundantin those
bands in the ore that contain more carbonaceous
material. These particles
average4 squaremicronsin exposedarea, somewhatlarger than the average
of all visiblegrains. In additionto the uniformlybut sparselydistributed
particles,goldgrainsalsooccurin clustersof roughlylenticularoutline(Figs.
27, 28). Thesegold-richpodsare composed
of abundantgold grainsthat
averageone half of a squaremicron in exposedarea. They are commonly
so abundantthat under low power magnificationthe entire area of the lens
hasa paleyellowcolor. Yet withinthe lensesgoldoccupies
lessthan 1 percent
of the area. Like the individualgrains mentionedabove,this gold occursin
280
PETER
JORALEMON.
the carbonaceous
matrix, and quartz grains within the gold pods standout
as barren islands.
The lenses are as much as 100 microns wide and are com-
monlyseveralmillimeterslong. Theseaggregates
of goldparticleshavebeen
seenonlywithin the gumboandare commonlyelongateparallelto the structure
of the gumbo. Even within the gumbo,theselensesare rare and the presence
of oneor two in a samplewill givean erratichigh assay.
Gold in thesepodsis intimatelyassociated
with magnetiteand carbonaceous
matter. Magnetiteis slightlymoreabundantthan goldand, like gold,is very
looselyembedded
in the matrix and is easilyremovedby rubbing. During
theimpregnation
of the polishedsections,
bothmagnetiteandgoldfrom someof
theselenseswere pulled out from the carbonaceous
matrix by the inrush of
resinoldafter evacuationand were carried along open cracksin the section,
finally forming inclusionsin veinlets of resinold. This magnetiteseldom.
showsa euhedralformandgenerallyoccursin irregularlyroundedmasses
and
angularfragments. It is largelyrestrictedto the gumbo. Rarely,bothgold
and magnetiteare rimmedby a mamillaryhalo of carbon(Fig. 23). There
is thusa strongsuggestion
that therehasbeena redistributionof smallamounts
of carbonduringor after the main periodof gold and magnetitedeposition.
Magnetiteoccursnotonlyin the richgoldlensesbut alsodisseminated
through
thegumboin smallisolatedgrains. No magnetite
hasbeenseenoutsideof the
ore bodies.
In addition to the occurrencesdiscussedabove, gold is found rarely as
inclusions
in the sulfides. All of the major sulfidescontainsomevisiblegold,
but pyrite and marcasiteare the principalhosts. Lessthan I percentof the
totalnumberof goldparticlesseenoccursin sulfides. It maybethatthe larger
goldparticleswere deposited
almostentirelyin the gangue,but thereis some
evidenceto suggestthat suchis not the case. The gold particlesoccurringin
the sulfidesare far morelooselyheld than that in the gangue. The.particles
in the sulfidesthat were not wrestedfrom the surfaceduring grinding and
polishingare invariablyremovedwhen 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 marcasitecontainsomevisiblegold,but the more
porousvarietiesaremorecommonly
hostsfor themetal(Figs.25,26). Where
gold particlesoccuras inclusionsin subhedralcrys.
tals of pyrite they are
generally
in theouterpartsof thecrystals. Nonehasbeenseenin thecoresof
thesecrystals.
Summaryof Occurrence
of VisibleGold.--Visiblegold occurssparsely
disseminated
througheveryrocktypein theveinsandis particularly
abundant
in the gumbo. Golddistribution
is quitesimilarto that of 'marcasite
and
realgar,and is more restrictedthan pyrite. The only mineralsthat are
strikinglyassociated
with goldare carbonand magnetite.Laboratorytests
showa strongdirectrelationbetween
carbon
andgold,andthegoldcontent
of carbonconcentrates
increases
asthepurityof theconcentrate
is increased.
The sizeof theparticlesof visiblegoldrangesfroma fractionof a rrlicron
to nearlyonemillimeter;
thesmaller
grainsarefar moreabundant
thanthe
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
281
larger. The weight-distribution
curveshownby thebrokenlinein Figure17
shows
thatthelargerpartof themassof goldisfromthelessabundant,
larger
particles.
The moreporousformsof the sulfidescontainmoregoldthan the solid
euhedralcrystals. There is apparentlya strong,directcorrelationbetween
the amountof surfaceexposedin the host minerals,that is, the porosity,
andtheamountof contained
invisiblegold.
"Invisible"
Gold.
It is probablethatnotall thegoldoccursin visiblegrains. The "invisible"
goldmay occurin'any of threeforms. First, it may occurin the nativestate
in particles
belowthelimitofmicroscopic
visibility. Second,
it maybepresent
in a compound
suchasthetelluride,likewisesubmicroscopic
or elseundetected.
Finally, it may occurin solidsolutionwith the sulfidesor adsorbedonto the
surfacesof carbonand the sulfides. This may be visualizedas a two- or
three-dimensional
solution,dependingon whethergold was adsorbedonto
thehoststructureafter or duringthe growthof the latter. An adsorbedfilm of
goldon a pre-formed
mineraldiffersfrom goldin atomicdispersion
through
the host mineral structureonly in that it is a planar rather than a threedimensional feature.
The frequencydistributionchart in Figure 28 has beenpreparedon a
logarithmic
scale. Thischartshowsthegreatrangein sizebetween
thegold
atomand the smallestvisibleparticle.
Goldin SolidSolution
in SullSdes.--According
topresent
theory,twomajor
types of solid solutionoccur in nature. First is substitutionalsolid solution.
In this,ionsof the minor(guestor solute)component
occupythe positionin
the crystallatticethat wouldbe normalfor ionsof the dominant(hostor
solvent)component.Depending
on whetherthe radiusof the guestis larger
or smallerthan that of the host,therewill be an expansionor contractionof
the resultingcrystalstructure. As the differencebetweenthe radii of the host
and foreignions decreases,
the degreeof possiblesolid solutionincreases.
Thegrowthof sucha solid.
solution
is caused
byadsorption
of foreignionsonto
thegrowingsurface
of thehost. At thesurfaceof a crystalat anystageof its
growth there is an unsaturatedfield of force, sincethe outermostions at this
stagehavenotenoughneighbors
to satisfytheirnormalcoordination.During
growth,ionsin.solutionmakecoordination
by attachment
to the thin surfaceof
thecrystal. A foreignionin thesolution
mayrespond
to thisfieldof forceand
beaddedto thegrowingcrystal,playingtheroleof proxyfor a dominant
ionin
thecrystal,
provided
thattheradiiof thecorresponding
ionsor atomsin questiondonotvaryby morethanabout15percentof thesizeof thesmaller. The
covalentradiusof the iron ionsin pyriteis 1.23A ø andthe octahedralcovalent
radiusofgoldis 1.40A ø (25,p. 170). Sothecovalent
radiusofthegoldionis
within 15 percentof that of iron, and goldshouldbe ableto enterinto substitutionalsolidsolutionin pyrite.
The secondtype of solid solutionis known as interstitialsolid solution..
In this, the.soluteatomsare believedto be dispersed
in the structureof the
282
PETER
JORALEMON.
solventatomsor ions. The soluteor foreignatomsare introducedin addition
to, ratherthan in placeof, the hostor solventions. This type of solidsolution
is rare andgenerallyoccurswherethe foreignion is distinctlysmallerthanthe
host.
Synthetic•turiferousPyrite.--Maslenitsky(22) addedgold chlorideand
colloidalgold in the courseof synthesisof pyrite. He was ableto controlthe
amountof goldcontained
in the pyriteup to a maximumamountof about9.7
..- • .•.•..
•, •' •,•.
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 sedimentary structure has been preserved. Cross nicols, X 40.
Fro. 12. Photomicrographof polishedsectionof 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
..
14
FIG. 13. Thin sectionof realgar and vein stuff. The realgar appearsblack in
the forked vein that is rimmedby thin layersof quartz crystals. The black euhedral
crystalsoutsidethe vein are of pyrite. The vein cutsa fine-grainedquartz-sericite
intergrowth that is an unreplacedfragment of argillite. Such material is not a
conspicuousgold-carrier. Crossnicols, X 40.
Fxc. 14. Photomicrograph
of polishedsectionof realgarin ore gumbo. The
realgar(light gray) surrounds
islands
of subhedral
quartzcrystals,
havingreplaced
the fine-grainedquartz-carbon
matrix of thesegrains. X 310.
ouncesper ton. Investigation
of the auriferouspyriteunder X 1,200magnificationrevealed
nogoldandthegoldmayhavebeenin solidsolutionin the
pyrite. The methodof polishing
usedin the preparation
of thesepolished
sections
is notknown.In manycases
specimens
thatrevealno goldwhen
284
PETER
JOR.4LEMON.
polished
on a clothlap maybe shownto containabundant
visiblegoldafter
repolishingon a lead lap.
Kuranti (18) prepareda synthetic
auriferous
pyritein whichgoldwasinvisibleevenunderthe highestmagnification
possible.Again the methodof
polishing
may haveobscured
somevisiblegold. The goldwas'shownby
spectrographic
analysisto be distributeduniformlythroughoutthe pyrite,and
X-ray picturesof the pyriteshowedthat the pyritelatticeconstantvariedwith
increasing
goldcontent,reachinga constant
valueat 2,000gramsof goldper
tonof pyrite. The evidence
mayindicatethatsolidsolutionof goldin pyriteis
possible.Unfortunately
theoriginalJapanese
publication
describing
thiswork
is not available.
Natural
Auriferous
Pyrite.--Itisprobable
thatgoldin solidsolution
in
pyritemaybe synthesized.A naturallyoccuringexampleof suchsolidsolution hasnot beenpreviouslydescribed,
althoughmanywritershave attacked
the problem.
R. E. Head (15) suggested
that at leastsomegoldin particlesof micronic
diameterin pyriteoccursin tiny leavesor flakescoatedwith a magnetic
iron
mineral,lying betweencrystallographic
planesin the pyrite. Sucha feature
mightsuggestexsolution
of gold. Edwards(6, p. 130) statesthat suchfine
goldparticlesrarelyshowthe orderedarrangement
in the pyritethat wouldbe
expectedif this were so.
Haycock(14) prepareda frequency-distribution
chart of sizesof gold
particlesin 50 Canadianores,and foundthat the greatestnumberof gra.ins
were well within the rangeof microscopic
visibility. Over 75 percentof all
thevisiblegoldfallswithinthe microscopic
ratherthanthe megascopic
range.
By projectingthiscurveintothe rangeof submicroscopic
sizes,Haycockconcludesthat goldof colloidalsizeshasonlyslightimportance
and goldin solid
solutionin sulfidesmaybe of evenlesseconomic
value. He pointsout that
untilthenaturalexistence
of goldin solidsolutionin pyriteor othersulfides
is
provedto be significant,
it is morein accordwith probabilityto stressthe importanceof normalgoldparticlesof submicroscopic
size.
Auger (2) studiedpyritefromvariousgolddeposits
in Canada. He made
spectrographic
analyses
of thepyriteandfoundthatthespectralgoldlineswere
inconsistent,
even within one deposit. He concluded
that gold, whenever
present,is unevenlydistributedin the pyrite crystalsand appearsin the
spectrum
onlywhenthe analyzedsamplecontains
smallparticlesof gold.
Augerfelt that this evidencestronglysuggests
that golddoesnot takepart in
the structureof the pyrite crystalas solidsolution,and that gold is not precipitatedat the sametime as the pyrite. His study includedonly natural
pyriteandhis conclusions
do not constitutea denialof the possibilityof goldin
solid solutionin pyrite. It might be well to mentionthat gold is relatively
insensitiveto spectrographic
analysis,as the lower limit of sensitivityis about
0.001 percent.
Pyrite and arsenopyritefrom the Dolphin East Lode, Fiji (28), were
found to containas much as 33 ouncesof gold per ton, althoughonly 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,enlargedto visiblesizeby theheating,wereformedby the coalescence
of submicroscopic
gold. Stillwell interpretstheseresultsas an indicationof
the presenceof gold in limited solidsolutionin the sulfides. Heating of the
gold-bearing
pyrite, however,wouldtend to preserve,ratherthan terminate,
any stateof solidsolution. It is perhapsmore probablethat this Fiji gold is
presentin independent
particleslargerthan the goldunit cell. The coalescence
of smallerparticlesrepresentsa changeto a form havinga lower, and thus
morestable,surfaceenergy,if the originalgoldis in colloidal-sized.
particles.
Back-reflection
X-ray photographs
of pyritefromAmador,California,taken
at Harvard,showthat the structureof someof the pyriteis distorted,having
a cubeedgeof 5.477kx, while otherof the pyrite hasthe normalundistorted
structurewith A0 equalling5.405kx. Spectrographic
analyses
indicatethat
the distortedmaterialcontainsabout5 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
differentialetchpattern,theauriferous
pyritebeingetchedmoredeeplythanthe
lower gradematerial. This evidencestronglysuggests
that at leastsomeof
the Amadorgoldis in solidsolutionwith the pyrite.
Similar etchingor tarnishingwas noted at the Simmer and Jack Mine
on the Rand someyearsago (12). At that deposit,and at adjacentmines,
it wasfoundthat oneof the dependable
signsof better-than-average
goldvalues
in theReefwasthe rapidityof tarnishof the pyriteat thoseplaces. The pyrite
of rich stretches
of the conglomerate
woulddevelopstrongirridescence
on a
freshlyblastedfacewithina day or two. This differentialtarnishmay have
beenbroughtaboutby the presence
of goldin solidsolution
in som.e
of the
pyrite.
Auriferous Pyrite in GetchellOre.--Pyrite from the ore shootsin the
Getchellminecontainssomesmallparticlesof visiblegold,commonly
located
near the edgesof the pyrite grainsor near open spaceswithin the grains.
Furthergoldhas.apparently
beenlostduringpolishing. It is probable,
however,that the goldof visiblesizesis not abundantenoughto accountfor the
total gold value of the pyrite. No visiblegold has beenseenin polished
sections
of pyriticconcentrates
from the ore, althoughthey containabout0.5
ouncesof gold'per ton.
Samplesof the sulfideconcentrates
wereheatedto 600ø C in the presence
of sulfurvaporin a sealedtube. Each polishedsectionpreparedfrom this
materialreveals
from10 to 20 particles
of goldaveraging
0.3 microns
in
diameter. All particlesoccurat the boundaries
of pyrite grains(Fig. 24).
Heatingof the pyriteapparently
broughtabouttwo relatedprocesses.First,
it causedthe submicroscopic
goldto migratetowardthe marginof the host
grains,thusriddingthe bulk of the pyriteof its gold. Second,the heating
caused'
the coalescence
of the goldpresentto form, in someinstances,
visible
grains. This heatingprocess
cannotbeassumed
to makeall the grainsvisible;
probablyonly the richerpyrite grainscontained
enoughgoldto yieldvisible
particles.This ,experiment
suggests
thatthepyritemaynotcontainuniform
286
PETER
JOR.4LEMON.
16
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 polishedsectionof ore in andesiteporphyry. The
white euhedralcrystal is arsenopyriteand the irregular light gray massis magnetite.
Magnetite apparentlyhas replacedboth the gangue minerals and arsenopyrite.
X 425.
OCCURRENCEOF GOLD ,4T GETCHELL MINE, NEVADA.
287
amountsof gold,although,as previouslystated,all pyrite containssomegold.
Grain sizeof the pyrite is apparentlynot an importantfactorin determining
the amountof containedgold; largegrainsshowno moretendencyto contain
visiblegoldthan do the smallerparticles. This experimentprovesthe existenceof submicroscopic
goldin the pyrite,but it doesnot proveor evensuggest
whetherthegoldoccursin atomicor ionicdivisionor in particleswith dimensionsexceedingthoseof the gold unit cell.
A comparison
between
auriferous
pyriteandartificiallyprepared
activated
charcoalcontaininggold may seemextreme,but a brief description
of the
charcoal
and its effectson goldmay shedsomelight on the problemof auriferouspyrite. Metallurgistshavelongbeenawareof the fact that charcoalis
able to adsorbgold selectively
from a complexsolution. This propertyis
utilizedat the Getchellminewhereactivatedcharcoalis usedto removegold
from cyanidesolutions.The charcoal
probablyadsorbs
goldin the form of a
doublecyanide.with sodiumor calcium.
GrossandScott (13), largelyon the basisof chemicaltests,foundevidence
suggestingthat the gold adsorbedon the activatedcharcoalwas not in the
metallicstate. However,polishedsectionsof treated charcoalcontaining
nearly200ounces
of goldpertonwereprepared
at theHarvardLaboratoryof
MiningGeology,and in these,abundant
metallicgoldwasfoundpresentin
smallbut visibleparticles(Fig. 27). It is possible
that the •.dsorbed
gold
cyanide
salt,beingextremely
unstable,
mayhavebrokendownto metalliC
gold
afterstanding
for somemonths. Sucha breakdown,
however,wouldprobably
form extremelyminuteparticlesand it is apparentthat the goldseenin the
polishedsections
of the charcoalis too coarse-grained
to havebeenformedin
thisway. Sosomeotherfactormayhavebroughtaboutthecoalescence
to visible
size. Whateverthisforcemaybe,it is strongenough
to overcome
theadsorptiveattraction
of thecharcoal
for thegold,astheadsorbed.
ionsappearto have
beenpulledfromthecarbonsurface
to formlargerparticles. It is alsopossible
thatthefirstgoldadsorbed
on thecharcoal
actedlaterasnucleifor thegrowth
of metallicgoldparticles. The first layeror film of goldatomsis assumed
to
havebeendeposited
by adsorption,
whilefurthergrowthmayhavebeenaccomplished
by normalforcesof crystallization.
Like the visiblegoldin the sulfides,thesegoldparticlesare extremely
looselyheldon the surfaceof the charcoal
andbut few canescaperemoval
duringpolishing.The goldparticlesin the charcoal
andpyritealikeoccur
eitheralongthe marginsof the hostgrainsor bordering
minuteopenspaces
withinthegrains. In thecaseof thecharcoal,
theamountof goldthat canbe
adsorbed
varieswiththeexposed
surface
of thecharcoal,
andmostgoldoccurs
in the moreporousmaterial.
It maybe merelycoincidence
that the pyriteandmarcasite
in the richest
oreareporous
andhaveirregularsurfaces,
whereas
thepyritethatoccurs
away
fromtheoreshoots
is generally
moresolidandmoreregularin outline. But
the factthatvisiblegoldassociated
with sulfides
generally
occursin the ir-
regular,
porous
formsofthesulfides
suggests
thatthisporosity
mayhavebeen
importantin localizing
the gold. Laboratorytestshaveshownthat a sub-
288
PETER ]ORALEMON.
stantialpart of the gold occurringwith the sulfidesis associated
with the fine-
grainedportionsof the latter. A largegrainhaslessexposedsurfacethanan
equivalentvolumeof small grains, and hencecan adsorbless material. An
euhedralcrystallikewisehas lesssurfacearea and thereforelessadsorptive
capacitythan an irregularshapeof the samevolume.
The mechanism.
of adsorptionis hereproposedto explainthe precipitation
of goldon pyrite,othersulfides,and,to a largerextent,onthe carbonin the ore.
This process
differsfromthatof solidsolutionin thatthelatterinvolvesadsorption of the foreignionsduringthe growthof the host. If the bulk of the gold
associated
with pyrite representsa solid solutionof gold in the sulfide,then
gold would have been introducedinto the ore at the sametime as the sulfides,
andtheamount
ofgoldshould
ber.oughly
proportional
totheamount
ofsulfides.
Most of the gold values, however, are restricted to narrow bands whereas
the sulfidesare widespread
throughoutthe ore bodies. Thus, it is apparent
that the solutionsfrom whichthe bulk of the gold was depositedtravelledalong
morerestrictedchannelways
than did the solutionsthat deposi[ed
the sulfides.
It is not meantto denythe possibleexistenceof goldin a three-dimensional
solidsolutionin the sulfides,but rather to emphasizethat any suchsolidsolution canbe of only minor importanceat Getcheil. Indeed,it may well'be that
a very smallamountof gold is in solidsolutionwith the pyrite.
Growth]Plechanism
of GoldParticles.--If a hydrothermalsolutionis unsaturatedwith gold,neithernativegoldnor any goldmineralsmay 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 solutiondoesnot exceedthe limit of
solubilityat a givenplace,all the goldmay occurin solidsolutionin the pyrite.
If the ratio is higher, the pyrite will still containgold in solid solutionand
will alsocontainlarger gold particlesas randominclusions. Where the ratio
is muchhigher, theseinclusionswill attain visible size.
In any deposit,the physico-chemical
conditions,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,
concentrated
solutionsto form discreteparticlesas well as to occurin solid
solutionin pyrite. In the outlying,low intensityzones,wheredepositionwas
from dilutesolutions,the only golddeposited
wouldbe that in solidsolution
in the sulfides.
In the Getcheildeposit,the entire zonecontainingthe ore bodiesmay be
visualizedasan enormous
blockof submarginal
gold-bearing
rockwith smaller,
superimposed
bodiesof richerore, as shownin the projectionin Figure 3.
The economically
worthlesssections
of the veinsand the wall rock contain
a rather uniformamountof gold, varyingfrom 0.01 to 0.08 ouncesper ton.
Theselowvaluespersistasfar intothehanging-andfoot-wallcountryasdoes
thepyrite. Pyriteconcentrates
fromthemine,regardless
of thesection
of the
veinsfromwhichthepyritewastaken,contain
a minimum
of about0.5ounces
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
289
ff
_
- •
Grain
area
FIGURE 17. FREQUENCY DISTRIBUTION OF
in square
VISIBLE
microns
GOLD GRAINS, GETCHELL
MINE
Fro. 17. Frequencydistributionof visible gold grains, Getchell mine, basedon
measurementsof 2,639 grains.
of gold per ton. The ratio of concentration
of pyrite in the concentrates
is
about 1:50. Similarly,the ratio of concentration
of gold from 0.01 to 0.5
ounces
pertonis about1:50. Sothelow-gradegoldvariesdirectlyandclosely
with the pyrite.
It is to bepresumedthat the peripheral,submarginal
goldandpyrite which.
290
PETER
JOR.4LEMON.
havethe sameextensivedistributionin the wall rock and low-gradeportionsof
the veinsare more or lesscontemporaneous,
productsof depositionfrom the
low-intensityphaseof mineralization. During the growthof the pyrite, and
probablyalsoof themarcasite,
it is possible
that goldwasbeingadsorbed
onto
the sulfidestructurein extremelysmallamounts. This actionmay havetaken
placeoutsideaswell aswithinthe orebody.
The extentof solidsolutionof goldin pyritemustbelimitedandis probably
dependenton the temperatureof formation. In a low-intensitydepositsuch
as Getcheil,the minimumsolubilityof gold is probablyof the order of 0.5
ouncesof gold per ton of pyrite. In parts of the ore body where the temperatureof formationof pyritewassomewhat
higherthanin the areasfringing
the ore bodies,the solubilitymight be higher.
In the outer,low-gradezonesat Getcheil,the gold-pyriteratio in the solu-
tionswasprobablylow, and muchof the goldappearsto havebeendeposited
in solid solutionin pyrite. In the central channelwayswhere more intense
physico-chemical
conditionsexisted,the ratio and limit of solubilitywere
higherand somegoldoccursin visibleparticles.
Van Aubel (29) has concludedthat the particle size of native gold is
commonlya functionof the modeof formationof the associated
ore minerals;
when gold is contemporaneous
with the enclosingmineralsit is extremely
fine-grained;whenit 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 distancetravelledfrom the main channels,were available
and the growthof individualgoldparticleswas limited. In the ore shoots
where gold depositionfrom more concentrated
solutionscontinueduntil the
endof the periodof mineralization,
the goldparticlescontinuedto grow,and
many achievedmicroscopically
visiblesize.
The foregoingdiscussion
is admittedlytheoretical. It dealslargelywith
that gold,or a part of that gold,whichis submicroscopic
andis intimatelyassociatedwith the sulfides. Suchgoldconstitutes
only a smallproportionof
the total that occurs in the richer ore shoots. The writer feels that the evi-
dencefavoringthe theo!;yof goldin limitedsolidsolutionis strongenough
to justify consideringit.
ConclusionsRegarding Gold Occurrence.
Goldat Getcheiloccursin particlesrangingin sizefrom slightlylargerthan
thegoldunitcelltonearlyonemillimeter
in diameter,
andprobably
alsoin atomic
dispersion.Submicroscopic
goldisapparently
morewidespread
thanthevisible
metal,occurringthroughout
the ore body. Microscopically
visiblegoldhasa
veryrestricted
occurrence
andis foundonlyin thericheroreshoots,
in which
placesit accounts
for thegreatbulkof the totalgold. The massof low-grade
rock far outweigh,s
that of the ore shoots,and the actual numberof submicroscopic
particlesis probablyfar greaterthanthat of visiblegrains.
Discussion
hasbeenpresented
whichsuggests
that the widelydistributed
but low-gradegoldmaybe mainlyin solidsolutionin the pyrite,whereas
the
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
291
gold in the ore shootsis surelyin discreteparticlesof the native metal much
larger than the gold unit cell. In the very rich portionsof the veins, part
of the goldis in still larger particles.
The informationgatheredby measuringvisiblegold grainsand presented
in Figure 17 may shedsomelight on the questionof the characterof submicroscopic
gold. Thesedata havebeenusedin the compilationof the chart
in Figure 28. In order to showthe full range in sizes,a logarithmicscale
has beenusedon both the ordinateand abcissa. Since the numberof grains
or particles measuredmake up a representativesample of all the visible
goldparticlesretainedduringthe polishing,the curvegivesan approximation
of sizesthroughoutthe ore body.
The curve in the visible range may probablybe projectedat least some
distanceinto the microscopic
field with somedegreeof reliability. The critical
questionis whetherthe curvewill drop to zero at or beforereachingthe area
of the gold unit cell following the samegeneralpattern as Haycock'scurve
(14), or will continueupwardalongthe projectionfrom the knownportionof
the curve, as indicatedin Figure 28.
The submarginalgold valuesare widely scatteredthrough the ore body,
and the mass of submarginalmineralizedrock far outweighsthat of ore.
One ton of rockcontaining0.01 ouncesof goldcontains2.76 x 102 goldatoms,
and if presentin truly atomicdispersion,
this enormousfigurewouldrepresent
the numberof pointsat which gold occursin one ton of low-graderock. If
the goldin the richerore werecontainedwhollyin spheresof onemicronradius,
oneton of orecarrying0.55 ouncesof goldwouldcontain2.38 x 10• particles.
Sincethe rock containingsubmarginalgold valuesis so muchmore abundant
than is the ore, it is probablethat the goldin atomicor ionicdispersionis more
commonthan are particlesof greater than unit cell dimensions. If, as has
beensuggested,adsorptionhas beenan active factor in gold deposition,then
pyrite and carbonsurfaceswill containan adsorbedfilm of gold ions. In the
abundant,lower-gradeportionsof the ore bodywherethe growthof goldwas
limited, large particleswould not be formed, but much of the gold in the
adsorbedfilms may have actedas nucleifor limited further gold deposition.
The numberof particlesslightlylarger than the unit cell would be enormous.
In the smallerore shoots,much of the gold grew to visiblesize. Granting
this, the goldof atomicsizeis mostabundant,the submicroscopic
particlesare
the .nextmostcommon,and microscopically
visiblegrainsare leastabundant
but representan overwhelmingproportionof the total massof gold within
the ore bodies.
Thereforethe curvein Figure 28 mightbe expectedto rise alonga more
or less straight line through the submicroscopic
field toward the unit cell
area. If the curveas indicatedon the chart is roughlycorrect,then the frequencydistributionof goldparticles•houldbe in harmonywith the proposed
processof golddeposition.
In the marginalphaseof mineralization,
gold atomswere adsorbedinto
the pyrite structureduringthe growthof the latter mineral,and the solutions
containedtoo little gold to allow the growthof metallicgold particles. In
292
PETER
JOR.4LEMON.
the major channelways
the earlier mineralswere beingdisplacedupwardand
outwardby the mineralsof the moreintensephase. In thesechannelways
the
solutionswere hotterand the gold contentwas suchas to allow the growthof
nativegold. Here the gold was adsorbedonto the surfacesof the carbon,
pyrite,and marcasiteto form nucleifor the growthof larger particles. Becausethe gold concentration
in solutionwashigherin the centralchannelways
than in the outlyingareasof the vein and wall rock, the gold formedin this
moreintensephasewouldhavemoreopportunity
for continued
growth. Hence
the greatbulk of economic
amountsof goldis concentrated
in thesechannelways. The ratio of visibleto submicroscopic
gold is far greater in these
channelways
than in the low-gradeportionsof the ore body,and the bulk of
the weightof goldis in coarseparticles.
The particlesizeof gold is thereforea functionof the lengthof time of
growthandthe concentration
of goldin solution. The solutions
in the outer
portionsof the orebodycontained
onlyvery smallamountsof goldandthere
wasnoopportunity
for growthof largeparticlesin thoseplaces. The presence
of goldparticlesof colloidalsizedoesnot necessarily
implydeposition
from a
colloidal
suspension.The chartin Figure28 intimatesthatthe colloidal-sized
particlesrepresent
onlythe dimension
attainedat a givenstagein the normal
growthof particles. Everylargerparticle,beingcrystalline,
musthavegrown
from particlesinitiallyof the unit celldimension.
CauseoI Localization
oI Gold.
Structural Features.--The localizationof the three deep "roots" of the
rich ore, as shownin verticalprojectionin Figure 3, suggests
that theseroots
lie alongtherespective
axesof originalchannelways
throughwhicha greater
amountof gold-bearing
hydrothermal
fluid passed.Solutionsrisingalong
thesechannels
spreadoutintoa far widerareaon reaching
thezoneof intense
shattering
nearthesurface. The threecentralchannelways,
onein thenorth
end,onenearthecenter,andonenearthesouthendof theorebodies,
remained
as themajorchannelways
throughout
the periodof golddeposition.Sincea
largervolumeof gold-bearing
solutions
passed
throughthesechannels
than
throughcorresponding
volumes
of rockin the outlyingregionof shattering,
moregoldper ton wasdeposited
withinthe channelways.
The goldin the
extensive,
low-gradeportionsof theorebodiesis moreor lesscontemporaneous
with that in the rich ore shoots,althoughdepositionin the shoootsmay have
continued
longerthanin themoredistantveinmaterial. Theserichershoots
coincidewith the zonesof extensivedevelopment
of gumbo,suggesting
that
evenbeforegolddeposition
began,theseareasweremostpermeable
to the
mineralizing
solutions.The gumbo,beingunconsolidated,
was morepermeable,and thus morefavorableto transportof the gold-bearing
solutions
thanwastheenclosing
rock. Theconcentration
of carbon
in thegumbo
made
it a betterprecipitantthanthe argilliteor limestone.
In theGetchell
ore,a smallproportion
of thetotalmassof goldoccurs
in
thewidespread
pyrite,probably
because
it wasdeposited
asa constituent
part
of thathost. The bulkof thegold,however,is irregularlydistributed
in every
OCCURRENCE
OF GOLD
AT GETCHELL
MINE,
NEV,4D,'t.
293
--
18
20
.'-.
21
FIG. 18. Photomicrographof polishedsectionof heavy mediaconcentrate. The
large white grains are of native gold and the mottledlight gray mineral is realgar.
These are the largest gold grains ever seen in Getchell ore, and the only grains
[oundthat are megascopicallyvisible.' X 70.
Fig. 19. Photomicrograph
of polishedsectionof ore gumbo. The large white
grain is native gold; a part of this grain is submerged
beneaththe quartz gangue
and appearsas an indistinctlylighted area. The irregular mottled surface of the
goldgrain showsthe "egg-shell"textureso characteristic
of minutegold grains.
X 3,225.
Fro. 20. Photomicrograph
of polished
sectionof goldore,showinga relatively
large gold grain that has replacedthe fine-grainedquartz-carbonmatrix to the
gumbo,moldingitself aroundthe larger subhedralquartzgrains. X 3,225.
Fro. 21. Photomicrograph
of polishedsectionof goldore, showingisolated
gold grain. The egg-shelltexture is apparent. X 3,225.
mineral
in theorebodyandtherefore
cannot
havebeenselectively
precipitated
byanyof them. Thegoldwasdeposited
in or onthesevarious
minerals
apparentlybecausethey had somefeaturesin common.
Favorabilityof Hosts.--Sincenearlyeverymineralin the ore hasacted
asa hostor platform
forthegold,thecause
of localization
of goldis probably
a factorcommon
to all minerals.Thiscannotbecrystalstructure
or chemical
294
PETER
]OR,'ILEMON.
composition. The concentrationof gold in the more porousmineralsin the
ore suggests
that thisfactormay be the amountof surfaceexposedto the goldbearingsolutions. The mechanismof adsorptiondiscussedin the sectionor.
submicroscopic
goldmay applyto the depositionof gold in or on everymineral.
Pyrite (Fig. 26), marcasite,and carbonoccurin irregular porousmasseswith
abundantopenspaces,whereasrealgar,whichis more abundantthan the other
threeminerals,characteristically
occursin densemasseswith little or no open
spaces. As wouldbe expected,realgaris the leastimportantgoldbearer.
During the period of mineralization,gold was constantlybeing carried in
solution. In the earlierphasethe solutionsweretoo diluteto be saturatedwith
any gold compound,but other minerals, carbon, pyrite, and marcasitein
particular,adsorbedsmallamountsof this metal. Becausecarbonand marcasitehad the greatestsurface,they adsorbedthe greatestamountsof gold.
Gold remainedin solutionafter the depositionof the sulfideswas completed,
and the processof adsorptioncontinuedto act, forming atomic-thinlayers of
gold on the surfacesof mineralsaround which the solutionspassed. In the
later stageof deposition,native gold was depositedin the more permeableparts
of the ore shootswhere the solutionswere more intenseand the gold concentrationin solutionwas higher. Adsorptionwas no longer quantitatively
important.
The metallicbondmay be visualizedas a cloudor atmosphereof electrons
interstitialto the metalatomsandholdingthemin positionin the crystalstructure. The presenceof this electroncloudalsobringsaboutan 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 ionsmay act as a nucleusof
growth of larger particlesof the metal. So in the importantstagesof deposition in the Getchellore, the predominantprocesswas one of normal metal
growthon nucleithat hadbeenformedpreviouslyby adsorption. The physicochemicalconditionsat that time were suchthat gold, no longer stablein solution, was precipitatedon earlier formedgold particles. The possiblechanges
in theseconditionsare discussed
in the ch&pteron genesis.
Native
Silver.
A mineral has beenseenin many polishedsectionsof Getchellore which,
on accountof its physicalproperties,is taken to be native silver. It is white,
isotropic,and has extremelyhigh reflectivity,considerablyhigher than that of
gold; it is very soft, havingthe samemottled,egg-shelltexture that is seen
in gold. The only mineralthat fits this descriptionis nativesilver; conclusive
chemical
testshavenotbeenpossible,
however,because
theparticlespresentare
toosmall. It occursin sizesanalogous
to thoseof gold,althoughmegascopically
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,averagesabout 10:1. Earlier studiesof the
ore suggested
that the silverprobablyoccursin electrum,a naturalgold-silver
OCCURRENCE OF GOLD ,:IT GETCHELL MINE, NEVADA.
295
alloy. Only a few particleshave beenseenin the ore, however,and mostof
the silver occurs as the native metal.
The silver is probablya hypogenemineral. As the associationof native
silverand native gold in hydrothermaldepositsis rare, the evidencefavoring
a hypogeneorigin of the silver must be critically examined. (1) In general
the amountof silver in the ore variesinverselywith the amountof gold. Extremely rich portionsof the ore body containlesssilver 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 specksassociatedwith the gold are carbonparticles. X 2,275.
FxG.23. Photomicrograph
of polishedsectionof goldore, showinga gold-rich
lens. All white grainsare gold. The gray angularfragmentsare magnetiteand
the small,irregularlyroundedgray particlesare carbon. The goldparticleon the
left is rimmedby a discontinuous
haloof carbonaceous
matter,as is the magnetite,
bottom center. X 2,275.
296
PETER
JORALEMON.
silver. If silverwere a supergene
mineral,derivedfrom hypogeneelectrum,
it shouldbe mostabundantin the ore richestin gold.
(2) Native silverpersistswith no largedecrease
in amountto the deepest
levelsof the mine. There is no shallowblanketof richer than averagesilver
ore, as would be expectedif the silver were supergene.
(3) Silver particlesshowthe samevariationsin grain size as the gold
particles. If silverweresupergene,
formedundertotallydifferentconditions
fromthosein whichgoldwasformed,it wouldbeunlikelyto occurin grainsof
approximately
the samesizeas the gold particles. Had silver beenderived
from electrumby meteoricleaching,the residualgold would be expectedto
occurin porousmasses,but the goldparticlesare solid.
(4) Like gold,silveroccursin thecarbonaceous
matrixof thegumboandin
sulfides,particularlythe porouspyrite-marcasiteintergrowths. It is loosely
held in the host mineralsand is easilyremovedby rubbing. Unlike gold,
silver particlesdo not occur in lenticularaggregates. Otherwisethe habit
of silverclosely•resembles
that of thewidespread
but 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. Goldin mostlow-intensitydepositscontainsa considerable
amount
of altoyedsilver. At the Getchellmine,however,eventhe goldand silverthat,
on the basisof similar distribution,appearto be more or lesscontemporaneous
occuras separateminerals,althoughthe two may commonlybe seenin the same
field of view on onepolishedsection.
A similar paradox has been noted in the occurrenceof gold and silver
precipitatedfrom the cyanidesolutionin the millingprocess. Gold and silver,
bothcarriedin solutionin the cyanide,are pr•ecipitated
as the separatemetals
and not as the gold-silveralloy.
Similarlyin the supergene
zonesof other Nevadapreciousmetaldeposits,
gold.and silver occur as separateminerals. Thus, gold solutions,whether
naturallyoccurringin the earth or artificiallyin a cyanidetank, apparentlyare
unableto form a gold-silveralloy. But in most epithermalgold depositsin
Nevada,electrumis present. It is possible
that with decreasing
temperatureof
solution,the tendencyfor the formationof electrumdecreases. If this is so,
the temperatureof ore solutionsin the Getchelldepositmust have beenlower
thanthoseof otherpreciousmetaldepositsof Nevada.
Elsewherein the world,hypogene
nativesilveris foundin four maintypes
of deposits:(a) associated
with sulfides,silvermineralszeolites,barite,fluorite,
and quartz (Kongsberg,Norway); (b) occurringwith nickel and cobalt
sulfides
andarsenides
in a calcite
gangue(Cobalt,Ontario);(c) withiaraninite
andnickel-cobalt
minerals(Great Bear Lake); and (d) with nativecopper
andzeolites(KeweenawPeninsula)(24, p. 98). No description
of hypogene
silverin minoramountsin any otherdepositdominantlyof goldhasbeenfound
by the writer.
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
GENESIS.
297
•
Sequenceoi Deposition.
Ore depositionis a gradationalprocess. It is not one in which the widespreaddepositionof one mineral stopsbeforeanothermineral beginsto be
deposited. Any mineralwill be depositedwhen the physico-chemical
conditions are such as to make its constituents unstable in solution.
It would be
folly to assumethat, at any one time during deposition,.
similar conditions
existedthroughoutthe entire area of mineralization. So a mineral that was
depositedlate in the sequencein one place may actually be earlier than a
mineraldepositedearly in the sequence
in anotherspot. Any discussion
of
exact sequence,therefore,must be limited to a small locality within the ore
body,wheresimilarphysico-chemigal
conditionsexistedat any giventime.
In general,the sequenceof deposition(graphicallypresentedin Fig. 29)
of what is now the economicore at Getcheil, was as follows. Early, lowintensitysolutions,beingdilute, were able to do no.more than locallyrework
therockconstituents
andintroduce
smallquantities
of 9)•ritecarryingminor
gold. Coarse-grained
quartzandcalciteandsmallam011.
ntsof 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
thoseof severalothers,sothesemineralsmaybe considered
as more or lesscontemporaneous.In any one placein the ore body, gangue
mineralsderivedfrom the nearbyand underlyingrockswere depositedearly
in the sequence
andmetallicmineralsforeignto the countryrockweremainly
deposited
somewhat
later. Mostof thegold,togetherwith silverandmagnetite,
is apparentlythe latestof the metallicmineralsdeposited.
In the outlying,submarginal
portionsof the ore bo•y,•the sequence
was
apparentlysimilar to that in the now economicsections,exceptthat the later
marcasite-realgar-gold
perioddidnotappearthere. Deposifi6n'in
thesefring.ing areasof the main ore bodymay be considered
as beingmoreor lesscontemporaneous
with the economically
importantphaseof deposition
in the central
channelways. The early, gold-poorstageof depositionin thesechannelways
precededany deposition
in the outlyingareas.
'The distributionof economic
amountsof gold is quite similarto that of
realgar. There are apparentlytwo modesof occurrenceof'visiblegold: (1)
it is sparselydistributedthroughoutthe ore bodies,in gumbo,argillite,and
limestone
alike. This form of goldwasdeposited
earlyin the periodof formation of the realgarandthesewidelyscatteredgoldparticlesoccurin all ore that
containsrealgat,althoughtheyrarelyoccurwithinrealgargrains. Goldof this
stageprobablybeganits growthby adsorptionon carbon,porouspyrite, and in
realgar only where favorableconditionsexisted. Silver and.the rare electrum
wereprobablymoreor lesscontemporaneous
with thisearlier,low-gradephase
of goldmineralization
astheyhavethesamegeneraldistribution. The precious
298
PETER
JORALEMON.
metalsdepositedat this time are not to.be confusedwith the non-commercial
amountsof gold believedto be in solidsolutionin the earliestpyrite. To the
contrary,gold of this period constitutesmost of the gold in the ore bodies
exceptthe extremelylocaland rich shoots. (Gold of this periodis illustrated
in Figs. 18, 19, 20, 21.)
24
,
.c
Fxc. 24. Photomicrographof polishedsectionof pyritic concentrate,heated to
600ø C, showinggold 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 polishedsectionof pyritic ore. The small gold
grain occursin a poroussectionof the pyrite-marcasiteintergrowth. X 2,875.
(2) Toward the end of the period of depositionof marcasiteand realgat,
the mostintenseore solutionspassedthroughthe few mostpermeablechannels
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,lensesof goldand magnetite(Figs. 22, 23).
Minuteamounts
of carbonweredeposited
at thistime,rim.ming
occasional
particlesof gold.
Summaryof Sequence.--The
changes
in thenatureof deposition
with time
werebroughtaboutlargelyby changingconditionsof deposition. The changes
were gradationaland the mineralscharacteristic
of the early stagewere being
depositedin the outlyingareasat the sametime that the late stageminerals
were forming in the ore shoots. At the earlieststageof depositionat the
known depthhorizon,pyrite containinglow-gradegold 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-character
increased,pyrite and lean gold
were depositedfarther and farther out, whereasthe mineralsfrom solutionsof
higherintensitywere depositedtoward the centersof mineralizationby replacementor re-solutionof thosemineralsthat hadbeenstableduringthe earlierand
feeblerstagesthen and,there existing. Realgar, marcasite,and the economically important gold, which latter was becomingricher with time, were depositedin the main channelwaysat aboutthe sametime that the solutionsin the
outlyingportionsof theveinsand wall rockwere depositingdisseminated
pyrite
and included.
submicroscopic
gold in non-commercial
amounts.
Nature of theHydrothermalFluid.
The hydrothermal fluid containedwater, arsenic, iron, and sulfur, with
minor amountsof copper,gold, silver, molybdenum,mercury, barium, and
fluorine. It alsocontained
silica,lime,andcarbon,whichwereprobablylargely
of localderivation. This fluid was hot, probablyat a temperatureof between
oneand two hundreddegreesCentigrade. At depthit was probablyalkaline,
althoughthereis no directevidenceof this. The underlyingrockscontainfar
greaterquantities
of alkalinethanacidelements,
so,evenif,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
hasbeenplacedon auric chlorideas the form in whichgold
is transported (21). However, chlorides,if present at all, are rare constituentsof an ore body,especially
as comparedto sulfides;and eventhough
mostchlorinecompounds
arehighlysoluble
andwouldnotbeprecipitated,
there
is rarely, in any district, tangibleindicationthat chlorinein whatever state of
combinationhas passedthroughthe channelwaysin the quantitiesrequired
if the metalswere broughtto the placeof depositionas chlorides. It seems
far morereasonable
to assumethat the goldis transportedas a solublesulfide.
Smith (27) suggested
that 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 groupof metalswhosesulfidesare highly solublein dilute
aqueous
solutions
of alkalisulfides
a.troomtemperature.
With goldin this•
300
P•ET•ER .IO R,4L•EM ON.
group are mercury,bismuth,antimony,arsenic,and tellurium. Silver is the
mostsolubleof the group of metalswhosesulfidesare solublein aqueoussolutions of alkali sulfidesat high temperatures. This may explain the close
spatialrelation 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,cannotyet be regarded
as settledwith finality,and musthopefullywait for additionalexperimentation
and ideas. But severalconsiderations
make the thesis of transportationas
doublesulfidesor sulfidecomplexesworthy of considerationfor the caseat
Getcheil.
Precipitationof Gold.-•Alkali sulfideseasilyform doublesaltswith certain
metallic
sulfides.
If the sulfide concentration
of sodium thio-metallic
salts
is lowered sufficiently,simple metal sulfidesare precipitated. For gold, a
reductionin sulfidecontentin solutioncausesseparationas the metal rather
than as sulfide,sincegold sulfideis unstableat temperaturesgreater than 40ø
C (27).
If sucha processmay accountfor the depositionof mineralsin the Getchell
deposit,then there shouldbe someevidenceof decreasingsulfideion content
in solution. Orpiment,with a highsulfurcontent,is earlierthanrealgar,which
containslesssulfur. This may be an indicationof decreasing
sulfideconcentration. Pyrrhotite, pyrite, and marcasiteare early and magnetiteappearsto
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 formedand that with a decreasein sulfideconcentrationpyrite disappearsand pyrrhotite is deposited. With a further
decrease
in sulfur,pyrrhotite,magnetite,andhematiteare formed. This shift
fromthedisulfides
throughthenear-monosulfides
to the oxideswith a decrease
in sulfideion concentrationis thus entirely compatiblewith the observedsequenceat Getchelland suggestsa similar decreasein sulfideconcentration
in the solutionsas mineral depositionproceeded.
Changesin Characterof SolutionsDuring Deposition.--Thepresenceof
early calciteand pyrite and later marcasiteand suchsulfatesas barite and
possibly
gypsumsuggests
a gradualchangefromalkalineto neutralandfinally
to acid solutions. The presenceof marcasiteespeciallyindicatesa changeto
acid solutions(1). This gradualchangeis not uncommonin near-surface
deposits
andisparticularlycommon
in fumaroles
or hot springs. It essentially
represents
a changeof sulfideto sulfateion. This maybeexplainedby Allen's
reaction,in whichfree sulfur reactswith water at certaintemperaturesto form
the sulfideion and sulfateradical,as follows:4S + 4H20: 3H,o + H.oO4.
If H2S is removedfrom the system,the reactionwill movetowardthe right,
bringingabouta concentration
of sulfuricacidwhichmayconverttheinitially
alkaline solution to acid.
On enteringintoshattered,
permeable
rocksuchas the Getcheilfault zone,
the ore solutions
undergolossof pressurewhichfacilitatesthe lossof a gas
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
301
phase,suchas H.•S, from the liquid phase. With this selectiveevaporation,
the sulfurcontentof the remainingsolutionswill decreaseas they changefrom
alkalineto acidcomposition. With decreasing
sulfurcontentand accompanying acidification,pyrite will alter to marcasite,realgarwill be formedin placeof
orpiment,and gold will be precipitated. With further decreasein sulfur, the
iron sulfideswill ceasedepositingand magnetitewill beginto form. In those
environmentswhere sulfuric acid is abundantlydeveloped,hypogenesulfate
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 hydrothermalsolutionsdid
not passthrough all portionsof the veins at equal rates, with the consequences
discussedbelow. The ore-carryingsolutionswere probablysomewhathotter
FIG. 26. Photomicrographof polishedsectionof 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. Photomicrograph
of polishedsectionof gold-richactivatedcharcoal
usedin the recoveryof the gold in the milling process. White grain is gold,on rim
of a charcoalcell. This charcoalis usedbecauseof its high adsorptivepower.
Compareporosityand surfaceto that of the pyrite in Figure 26.
thanthewall rockand,duringpart of theperiodof mineralization
at least,were
actuallyheatingthe rocksthroughwhichthey passed. As the rock became
hotterthe solutions
couldlosean increasin.
gly smallproportion
of their heat.
Thus,at anyonepointin the vein,thetemperatures
of the wall rockandof the
passing
solutions
wererisingandthegeothermal
gradientwastherebecoming
steeper. This increase
in temperature
wasnotuniformthroughout
the veins,
butwasmoremarkedin thelociof greaterpermeability
throughwhichgreater
volumesof the heat-carrying
solutions
passed. This process
mustnot be misinterpreted
to meanthatanygivenportionof themovingsolution
wasactually
becoming
hotterwithtime;it wassimplycoolingat an increasingly
smallrate,
andtherefore
wasfollowed
byportions
of solutions
progressively
hotter.
302
PETER JORALEMON.
Sowithintheveinstherewerehot,permeable
lociseparated
by lesspermeable
andcooler
places.Theseparation
fromthesolution
of H2Sin thegas
phase
wouldbefavored
bya sufficiently
hightemperature
tokeepupitsproducß
tionby theAllenreactioncitedabove,andby a pressure
so10wasnotto retain
Lower limit of
pic visibilif
/
/
/
rlimit
of
coloidal
porticles-/
Lower
limit
ofcoloidal
/
z
Gold
unit
cell
Gold ot_Olla.
__
Fro.
28.
Increasin•l frequencyof oCCurrence
--
Frequency
distribution
of goldgrains,showingprobable
extension
in submicroscopic
range. A logarithmicscaleis usedon the abcissa.
theH2Sinsolution.Suchratherspecial
combination
oftemperature
andpressurewouldbe mostprobablyalongthe morepermeable
andhencehotteraxes
of flow.
Thustheoresolutions
probably
remained
alkaline
in thelesspermeable
portions
of the¾ei•swhiletheywerebecoming
neutralor somewhat
acidin
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
303
thehotterore shoots. Pyrite wasdeposited
throughouttheveinsin the earliest,
alkalineperiodof deposition,and only in the lower-grade,lesspermeableand
coolersectionsduring later stagesof mineralization. Realgar,marcasite,and
the richer occurrences
of gold were restrictedto the hotter parts of the veins
where the solutionswere neutral or acid. The gold, realgar, and magnetite
were depositedfrom hotter solutionsthan thosethat formed the pyrite and
the conventionalgangueminerals.
In thepasttherehasbeena tendencyonthe part of geologists
to assumethat
the terms "low-temperaturemineral" and "late mineral" are synonymous.
Graton (11 ) hasquestioned
the validity of this reasoning,and recentwork at
Butte has indicatedthat late mineralswere actuallyformed at higher temperatureand underconditionsof higherintensitythan the early minerals(26).
Further evidencein supportof suchrelationsof 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 respectivebedsbecausethey were
al•ready
considerably
depleted
in intensity,andcouldonlytransportdissolved
materiala few inches. Thoseportionsof the later solutionsthat were ableto
remainwithin the more permeableloci in the veins,benefittingby the heating
already accomplished
there, were able to do a more "intense"kind of work.
Ganguemineralsin thesefavored portionswere no longer so localized,and
quartz replacedlimestone,argillite, and andesitewith about equal ease. In
place of the early, coarse-grained
quartz 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 probablyrepresentdepositionfrom mor• concentratedand
hence more intense solutions.
Age of Mineralization.
The Getchelloreswere depositedat sometime subsequent
to the intrusion
ßof the andesiteporphyry,whichis tentativelydatedas early or middleTertiary.
A period of silicificationto form the opalite occurredafter depositionof the
rhyolite tuff. The strongestmineralizationin the Getcheilore bodiesoccurs
towardthe northendof the mineralizedzonenearestto the deeppipeof rhyolite
tuff. It is possiblethat the oresare later than the rhyolite and representa
late phaseof that periodof igneousactivity.
The Getchellore bodyis similarin manywaysto the gold-realgardeposits
at Manhattan, Nevada, about 150 miles to the south. At Manhattan, both
earlyandesiteandlate rhyolitedikesoccurnearthe ore body. In the description of this deposit,Ferguson(8) suggests
that the goldmineralization
may
be post-rhyolite. He points out that at Round Mountain, 50 miles north of
Manhattan,goldorecontaining
realgaroccursin a rhyoliteplugandis therefore laterthanthe rhyolite(7), andthat thesetwo deposits
may belongto a
post-rhyolite
periodof mineralization.The closeparallelismin the character
304
PETER JORALEMON.
of the Getcheiland Manhattan depositssuggeststhat they may belongto the
samegeneralperiodof mineralization.
In his description
of the miningcampsof Nevada,Ferguson(9) showsthat
theTertiary preciousmetaldeposits
in that areamaybe dividedintotwo groups
of widely differingcharacter. In the older groupthe depositsare generally
morepersistent
laterallyandvertically. They all containmanganiferous
calcite
and havea ratio of silverto goldgreaterthan 1:1. In the othergroup,gold
is generallypresentin larger amountsthan silver and the structuresare less
persistent.Gold.field,
typeexampleof thisgroup,has,agold-silver
ratioof 7:3.
Realgaris notfoundin mostof thegold-richdeposits. In thosein whichit
is found, Getchelland Manhattan,the bullion reportsindicatethe great preponderanceof gold over silver, the gold-silverratio varying from 10:1 at
Getchellto 17:1 at Manhattan. It is possiblethat this-type of ore deposit,
characterized
by a very highgold-silverratio andthe presence
of realgar,may
representa third, still later groupof depositsformedin late Plioceneor early
Pleistocenetime. Thesedepositshave 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 closelyrelatedspatiallyto realgarand carbon.
In each,lead,zinc,andcoppersulfides
are rare or nonexistent.Bothcontain
relativelylarge replacement
bodiesthat are more importantthan the single
narrow fissure veins.
Ferguson(8, p. 106) suggests
thatthehighgold-silver
ratiomaybe dueto
thepresence
of arsenicminerals. He proposes
the theorythat hypogene
solutions rich in arsenicand free from lead, zinc, and coppertend to precipitate
gold without any importantmixture of silver. There appearsto be no
chemicalbasisfor sucha reactionand the commonoccurrenceof gold-poor
proustite
deposits
wouldseemto disprove
thistheory. Butler(5) gavea more
plausible
explanation,
suggesting
thatthepresence
of carbonmaybeimportant
inbringing
abouta highgold-silver
ratio,ascarbon
isknowntoprecipitate
gold
selectively
froma gold-silver
solution.Carbonis undoubtedly
importantin
precipitating
largeramounts
ofgold,buttheremaybea simpler
explanation
of
the high gold-silverratio.
Hydrothermal
solutions,
on rising,maybe expected
to changegradually
fromalkalineto somedegreeof acidity. As a consequence,
goldwill tend
tobedeposited
assoonassufficient
sulfide
ionsareremoved,
butsilversulfides
andsulfosalts,
beingsoluble
in acidsolutions,
will no longerbe deposited
but
will be sweptalongthe channelways.
The sulfuricacidwill graduallybe
lessened
in thecourse
ofthisjourney,partlythroughtheprecipitation
of sulfates
andpartlybyreaction
withsericite
andfeldspars
toformclayminerals.When
andwheretheaciditybecomes
sufficiently
lowered,
thesilver-sulfur
compounds
couldagainbeprecipitated
to forma shallow
silverzonecapping
thegold.
It is possible
thatthesolutions
mightreachthesurface
beforeadequate.
reductionof the acid had .occurred.In sucha case,the resultantdeposit
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
305
wouldconsistonlyof 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.
EARLY
I
LATE
INGFIEASIN•;INTENSITY
LARGELY
OF
LOGAL
DERIVATION
I LARGELY
OF
MAGMATIG
DERIVATION
Deposited
from
olkolin•
solutIQIt$
I Deposited
from
neutral
solutions
J Deposited
from
ocid
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 changes
in character
of thesedeposits
maybeexplained
by the factthat
decreasing
agehaspermittedan increasing
escape
fromerosiveremovalof the
shallowest
portionsof the initialdeposits.
The similarities
between
the Getchell
andManhattan
deposits
havebeen
discussed
previously.Thereare,however,certaindiscrepancies
between
the
306
PETER
JOR.4LEMON.
two deposits. The presenceof adulariain someminesat Manhattansuggests
that it may representa slightlymore intensephaseof mineralization. Native
silveras a hypogenemineralis 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 theGetchelldepositthat setsit apartfromthetypicalepithermal
golddepositsis the nearlycompletelack of certaincommonepithermalstructures suchas "colloform"banding,crustification,and comb structure. This
suggeststhat the Getchelldepositmay be intermediatein characterbetweenthe
typicalepithermalpreciousmetaland the quicksilverdeposits. Many of the
quicksilverveinsof Nevada showa closerelationshipto both the gold-silver
andstibnite-realgar
typesof deposits(20, p. 473). The depositsat Steamboat
Hot Springs,'locateda few miles southof Reno, containsomecinnabarand
showmanystrikingsimilaritiesto the Getchelldeposit. At Steamboatas well
as Getcheil,Tertiary igneousactivity is represented
by early andesiteand late
pumiceousrhyolite. The rhyolite at Steamboatis Pleistoceneor recent,a fact
that may suggesta Quaternaryagefor the Getchellrhyolitetuff. Silica in the
form of opal,chalcedony,
and fine-grainedquartzis abundantin eachdeposit.
Accordingto Brannocket al (4) the Steamboatwaterscontaina mudwhichis
largelycomposed
of tiny particlesof opalor a silicagel. This mudapparently
closelyresemblesthe Getchellgumbo,and its other featuresstrengthenthe
resemblance.Minute acicularcrystalsof stibniteoccurin the mud and have
alsobeenfound floatingas a thin scumon the surfaceof the watersof the
spring. These fine-grainedaggregatesof stibnite needlescloselyresemble
the unusualfelt-like stibnitecrystalsfrom the Getchellore body. Pyrite and
realgarhavebeenidentifiedin the Steamboatmud,and the presenceof an iron
oxide,possiblymagnetite,is suggested.The moststrikingfeaturecommonto
the mud and the gumbois the presenceof gold. Two of the mostimportant
wells at Steamboatcontainedmud with between0.3 and 0.4 ouncesof gold per
ton. The ratio of goldto silverin thesemudsis 1:3. Copperis presentin the
muds,as in the Getchellgumbo,in extremelysmallamounts.
The siliceoussinter at Steamboatcontainspyrite, arsenopyrite,stibnite,
and somecinnabar. The sinteris stainedin placeswith a red-brownantimony
ochre, metastibnite; ilsemannitehas 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
deposits
lie in a relativelynarrow,northerlytrendingbelt that cutsthe west-
centralpart of thestate. The chiefgolddeposits,
on the otherhand,showno
regularityof distribution,
butarewidelyspaced
throughout
theentirestate.
The Great Basinis composed
of a seriesof northerly-trending
structures,
bothfoldsandfaults. Even the recentfaults(10) trendin a northerlydirection.
Most of the cinnabardepositsare relativelyyoung,and all are probably
Plioceneor later. It is possible
that a majorlateTertiarybelt of faultingis
OCCURRENCEOF GOLD AT GETCHELL MINE, NEVADA.
307
now markedby the larger cinnabardepositsshownin Figure 30. Sucha belt
would act as a locusfor the cinnabar-depositing
solutions.
Nolan (23, p. 152) has describeda geanticlinethat dividedthe Paleozoic
geosyncline,
persistingfrom late Devonianto early Permiantime. The axis
of this geanticlineis shownin Figure 30. It may be merelycoincidence
that
mostof the larger cinnabardepositsand the Getchellore bodieslie alongor
near this axis, as the periodof geanticlinaldeformationwas over by Permian
o } .
o
ß
Getcheil
mine
.
•
mom
o
o o/
ASteamboat
/ lB
ß• Springs
•
/
•
•
ßManhattan
District
ß
---x• Probable
axisof late
•
Paleozoic
geanticline •
ß
_ :_o,,
_
Major
quicksilver
deposits
0
FIGURE 50
MAP OF NEVADA SHOWING LOCATION OF
MAJOR GOLD AND QUICKSILVER
DEPOSITS
time whereasthe goldand cinnabardeposition
was late Tertiaryor early
Quaternary. It is nevertheless
possible
thatthe geanticlinal
axiswasandhas
continued
to bea zoneof weakness
alongwhichlater,deep-seated
faultshave
formedfrom time to time. Along suchfaults,mineralizingsolutionscould
riseto deposit.
cinnabaror gold.
The Getchellmine is locatedwithin this cinnabarbelt, near its eastern
margin. The'Manhattan
deposits
lie slightlyeastofthisbelt. It mayagainbe
308
PIETER JOR.4LIEMON.
coincidence
that thesetwo gold occurrences
lie near or within the belt, but their
mineralogicand textural featuressuggesta geneticrelation to cinnabar de-
posits. It is probable
thattheGetcheil'
andManhattangolddeposits
represent
a gradationbetweenthe earliergoldoressuchas Goldfieldand the morerecent
cinnabardeposits.
A periodof mineralization
may thusbe considered
to havebegunin middle
Tertiary time with the deposition
of silver-goldore bodies,graduallychanging
in characterto form the deposits
in whichgoldslightlyexceeded
silver. Somewhat later the arsenicalGetcheil-Manhattan
type was formed,with gold far
outweighingsilver,and with cinnabarpresentin smallamounts. The Getchell
and Manhattandepositsapparentlymay be regardedas a link towardbridging
the gap betweenthe preciousmetal and the mercurydeposits. In the latest
important stageof this period of mineralization,the cinnabarore bodieswere
formed. The final dyingstageof mineralizationis represented
by Recenthot
spring deposits. Many of theseare similar in characterto the cinnabarde-
posits,but a few,suchasthe Golconda
tungsten-manganese
blanket(17), ar•
completelydifferent. The hot spring depositspresumablyrepresenta less
intensephaseof the sameperiod of mineralizationthat formed the cinnabar
vein merely becausethe former are shallower. The variation in intensity
would includechangesin temperature,pressure,and particularlyin concentration. ' Regardingthepossibilityof a protractedepochof Tertiary mineralization,
Lindgren said, "This whole processof metallization,beginningat the earliest
Cretaceous,closingat the end of the Tertiary, and feebly continuingtoday, is
actuallyone continuousoperationof separationof volatile constituentsfrom
the magma. . ." (19, p. 179).
Depth of Formation.
The Getchellore bodieswereformedundera relativelyshallowcover. The
largerugs in the ore and the intenselyshatterednatureof the ore zoneare the
bestevidencefavoringdepositionin a shallowenvironment. The plastic,unconsolidated
characterof the gumbolikewisesuggestsa near-surfaceorigin.
As relativelythin outliersof rhyolitetuff still remainnear the ore zone,it is
probablethat erosionhas not removedmore than a few hundredfeet of rock
sincethe depositionof the rhyolite.
The apparentlycloseaffiliationbetweenthe Getchellmineralizationand
thecinnabar
deposlts
suggests
thattheywereformedundersomewhat
similar
conditions. Baileystatedthattheopalitetypeof cinnabardepositwasprobably
formedat depthof about500feetbeneaththe surface.(3, p. 20). The present
topsof the Getcheilore bodieswereprobablydeposited
withina thousand
feet
of the then-existingsurface.
Genetic Classification.
The Getcheilore bodiesare clearlyepithermalas outlinedby Lindgren.
(20). They constitute
a particularand self-characteristic
example,a narrow
subdivisionof the broaderepithermalclass. The Getchelldepositbelongs
OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA.
309
amongthosethat representthe least intenseportion oœthe epithermalclass.
Graton,in proposingthe telethermaldepth-zone(11, pp. 547-551), has suggestedthat the epithermaldepositsdo not representthe very lowest intensity
end membersoœthe hydrothermalfamily. He points out that certain lowintensitystibnite-realgarand cinnabardepositsmay be a gradationalform between typical epithermaldepositsthat are characterizedby relatively rapid
depositionand telescoping
becauseof steepthermalgradients,and still œeebler
telethermaldeposits
formedby leisurelydeposition
in a coolerenvironment.
Despiteits manyunusualfeatures,the Getcheildepositmustbe considered
as epithermalon the basisof the telescopednature of mineralization. It has
beensuggested
that this depositmay representa gradationbetweenthe typical
epithermalpreciousmetalandthecinnabardeposit. The increasing
differences
from the progressively
olderandsomewhatdeeper-lyingpreciousmetaldeposits
andthe growingevidences
of similaritywith the cinnabardepositsprogressively
closerto Getcheilin youthand shallowness,
placethe Getcheildepositcloseto
thefeeblestendof the epithermalgroup. It may well be that hot-springsinters
were formed at the surfaceonly a few hundred feet abovethe presentvein
outcrops.
PARK CITY, UTAH,
Nov. 3, 1950.
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