Economic Geology Vol 90, 1995,pp. 1841-1856
QUARTZ TEXTURES IN EPITHERMAL VEINS, QUEENSLAND--CLASSIFICATION, ORIGIN,
AND IMPLICATION
GuoYI DONG,GREGGMORRISON*, ANDSUBHASH JAIRETH** Department of Earth Sciences, JamesCookUniversity of NorthQueensland, Townsville 4811,Australia Introduction
formation andfluidconditions, andtoexplore therelationship andgoldmineralization on a broad In hydrothermal veins,quartzis a dominant ganguemin- betweenquartztextures scale. A systematic evaluation of three-dimensional distribueralandis typically the onlyphasedeposited throughout the tion of quartz textures and textural assemblages in selected lifeofthehydrothermal system. Therefore, thecharacteristics systems andtherebythe texturalzoningmodel of quartz--itsmorphology, crystalstructure, chemical com- epithermal in anotherpaper. position, andphysicochemical properties--might reflectdif- will be presented Most of the spedmens usedin thisstudywerecollected feringhydrothermal conditions duringveingrowth,including from the late Paleozoie epithermal veinsofnorthQueensland, thosewhichfavorgoldmineralization. Regional tectonicandmetallogenic studies (MorManymoderntechniques are beingusedto characterizeAustralia. mostof theveins veinquartzandtodistinguish mineralized quartzfrombarren rison,1992a,b;Walsheetal., 1995)suggest of a Carboniferous continental quartz,e.g.,fluidinclusions (Roedder,1984;Sherlock etal., formedduringdestruction magmatic are. They are hosted largely in volcano-sedimentary 1993),oxygenisotopes (Rye and Rye, 1974;Matsuhisa et rocksof andesitie to rhyolitiecomposition. Deal., 1985),electron paramagnetic resonance (VanMoortand andvolcanic tailed work on representative deposits (Digweed, 1991; Tare Russell,1987), cathodoluminescence (Nickel, 1978), thermo1995)hasdefineda luminescence(Sankaranetal., 1983; Hochmanetal., 1984), et al., 1992;Bobisetal., 1995;Worsley, late Paleozoie epithermal province in north Queensland with traceelementsanalyses (Anufriyev etal., 1973),massspecsimilarto theTertiaryprovince ofthewestern trometryof thethermally released gas(BarkerandRobinson, manyfeatures (Morrison,1992a,b). All thedeposits included 1984), infrared (Wu and Yu, 1987), and microstructuralfea- UnitedStates in this study are of the adularia-serieite type in the classificatures(Steninaetal., 1989).In general,all thesetechniques havemetwithmixedsuccess, anda fewofthedistinguishingtion of Heald etal. (1987). features formineralized andbarrenquartzhavebeendefined. Classificationof Quartz Textures However,highcost,difficulty withinterpreting data,andlimiA classification of quartztexturesin epithermalveinsis tations in experimental equipment inhibitmostofthesetechdeveloped from a review oftheavailable descriptive literature niquesaseffectiveandeconomical exploration tools. of approximately 400 spedmensand 150 Thereisa fundamental way'to characterize veinquartz,i.e., and observation from morethan 20 epithermaldeposits and the morphology of quartzandits aggregates. Adams(1920) thin sections Thirteenquartztexturesare defined(Fig. 1) on wasthefirstto propose a detaileddescription of thecommon prospects. relations amongindividual microscopic characteristics ofveinquartz.Hispaperhasbeen the basisof mutualgeometrical or crystal aggregates, and/ortheinternalfeatures of the mostvaluable basefor subsequent studies on textures of crystals, grains.Mostof the textures described arereadily veinquartz.Theworkof Spurr(1926),Shaub(1934),Stillwell individual A few,however, canonlybe (1950), Lovering(1972), Boyle(1979), Sanderand Black identifiedin handspecimens. viewed under the microscope. The majority of textural terms (1988),andSaunders (1990),amongothers,alsoconsidered areadoptedfromexisting terminolthe character of quartzin specific environments. Recently, usedin thisclassification wherenecessary. DowlingandMorrison(1990)undertook an investigation of ogywithsomemodification Thisstudydealsonlywithlowquartz(Phillips andGriffen, quartztextures in various typesof hydrothermal veinsin north 1981). Based on the size of individual grains it can be subdiQueensland anddeveloped a general quartztextural classificamicrocrystalline, and eryptotion.Eleventextures weredefinedandgroupedto evaluate videdinto:(macro)crystalline, (BatesandJackson, 1987).Chalcedony refersto four gold-mineralizing environments (i.e., epithermal, por- crystalline quartz,eitherwithfibrous or granular habit phyry,plutohie, andslatebelt),eachwith a distinctquartz eryptoerystalline (Phillips and Griffen, 1981). These terms will be applied in texturalassemblage associated withgoldmineralization. description for somequartztextures. In thelightof thisgeneralsuccess, moredetailedworkon the following quartztexturesin epithermalveins,wherethereis a wide Massive rangeof quartztextures, wascarriedoutin thepresentstudy. Themainthemes ofthispaperareto develop a unifiedclassi- Thisis a general termthatrefersto quartzveinswhichhave ficationof commonquartztextures in epithermal veins,to a moreor lesshomogeneous appearance overwideareasand understand theirpossible origins in termsoftheprocesses of display anabsence ofbanding, shear fractures, orsimilar features. Crustiform * Present address: Klondike Exploration Services, 7 MaryStreet,Townsville, Queensland 4811, Australia. ** Present address:Mineral ResourcesBranch, Bureau of Mineral ResourcesCanberra, A.C.T. 2600, Australia.
0361-0128/95/1759/1841-16 $4.00
The termerustiform is analogous to erustifieation banding described by Adams(1920), Lindgren(1933),and Shaub (1934).Thistextureinvolves successive, narrow(upto a few
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centimeters), andsubparallel bandswhichare distinguishedslightopticaldifferences in maximum extinction positions. This by differences in texture,mineralproportions, and/orcolor. texture isusually welldeveloped onthemargins ofquartz crystals Commonly, bandingis symmetrically developed fromboth witha deareuheclral core(Fig.3b) or aspatches throughout wallsof a fissure (Fig.%). quartzcrystals (Fig.3e).The term"feathery" is adopted from Cockade: Thisis a subtypeof erustiform texture,as de- Adams (1920),anda simfiar texture hasbeenreported bySander scribedpreviously by Taber in Adams(199.0)and Spurt andBlack(1988),whocalledit "plumose." (199.6). In breeeias, concentric erustiform bandssurrounding isolated fragments of wallrocksor earlyveinmaterials pro- Flamboyant duee cockade texture. This texturehasbeen described by Adams(1920) and Colloform
Sanderand Black(1988). The chief characteristic of this tex-
tureistheradialorflamboyant extinction ofindividual quartz Thistermwasfirstproposed by Rogers (1917).In general, crystals with a moreor lessroundedcrystaloutline.Similar wherethe external surface of a mineralor mineralaggregateto the featherytexture,it caneitherbe developed in the rim shows combined spherical, botryoidal, reniform, andmammil- of a quartzcrystalwith a dear euhedralcore(Fig. 3d), or laryforms,it iscalledeolloform. Forsilicaminerals, thistexture throughout the crystal(Fig. 3e). is a characteristic featureof ehaleedonie aggregates in fine rhythmic bands(Fig.2b).Underthe microscope, chalcedonyGhostsphere in eolloform banding oftenhasa microfibrous habitwithsharp This texture commonlyoccurswithin microcrystalline re-entrantanglesbetweenadjacent contacting spheroids. quartzascloudyspheres highlighted by the distribution of impurities (Fig. 3f). Ghost-sphere texture may be regarded Moss as a specialmosstexture,becauseboth textureshavethe Thistexturehasfeatures similarto the "miero-botryoidal samefeature--spherical distribution of impurities withinsilgel structure" described by Adams(1920).In handspeci- icaphases suchasamorphous silica,chalcedony, or quartz. mens,silicaaggregates displaya heterogeneous turbidap- However,if the hostis quartz,ghost-sphere textureis used pearance,similarto mossvegetation (Fig. 2e). Underthe to characterize theinternalfeatureof quartzcrystals. Ghostmicroscope, groupsof spheres (usuallyrangingfrom0.1-1 sphere texturemaygradate tomosaic texturewheretheimpummin diam)arehighlighted bythedistribution ofimpurities ritiesaregradually eliminated andcrystal boundaries become withinaggregates of silicaminerals (Fig.2d).Somespherical interpenetrating. Somequartzcrystals withghost-sphere teximpuritiesalsoshowan internalconcentric or radiating pat- turedisplay radialextinction andtherefore sharethecharactern. Mosstexturemaygradateto eolloform textureif the teristicfeatures of theflamboyant texture. spheres becomeinterconnected. Pseudobladed Comb
Aggregates of quartzor chalcedony maybe arranged in a Combtexturerefersto groupsof parallelor subparallelbladedorplatyform.Threesubtypes aredefinedonthebasis quartzcrystals whichareoriented perpendicular toveinwalls, of the morphology of the aggregate of blades. thusresembling the teeth of a comb(Fig. 2e). Normally Latticebladed:Thistextureis comparable withthe "pseucrystals display a uniformgrainsizeandhaveeuhedral termi- domorphiclameliar,platy, or tabular"quartztexturedenations attheirfreeends.Thistextureisfrequently described scribedby Lindgren(1899),Schrader(1912),and Morgan in the literature,includingAdams(1920), Schieferdecker(1925).It displays a network of intersecting silicablades with (1959),andBoyle(1979). polyhedral cavities partlyfilledwithcombquartzcrystals (Fig. 4a).In thinsections, eachbladeconsists of a seriesofparallel Zonal seams separated by quartzcrystals or crystallites whichhave Zonaltexturedisplays alternating clearand milkyzones grownsymmetrically aboutthe seamsandperpendicular to withinindividualquartzcrystals (Fig. 2f). Milky zonesare them(Fig.4b). usually crowded withfluidor solidinclusions andarealways Ghostbladed:Bladesare identifiedon the polishedsurparallelto crystalgrowthfaces. facesof handspecimens by concentrations of impurities. Commonly blades are dispersed randomly within quartzagMosaic gregates andlackcavities betweentheblades(Fig.4e).Under thebladesaredifferentiated fromthematrix Aggregates of microcrystalline or crystalline quartzcrystals themicroscope, havehighlyirregularandinterpenetrating grainboundariesbydifferences in grainsize,shape, and/oroutlines of impurities (Fig. 4d). The thick silica blades usually have a ragged (Fig. 3a).Inhand specimens, thesample usual!y has avitreous andtightlypackedappearance. Thistextureis equivalent to shapewith a setof parallelpartings. a jigsawtexturewhichis oneof mostcommon microtextures Parallelbladed:Silicabladesare parallelwithina group in jasperold (Lovering,1972)andis alsonotedin someepi- butadjacent groups mayhavedifferentorientations. Theoutthermaldeposits (Saunders, 1990). line of groupsdefinesan overallgranularpatternin hand specimens (Fig.4e).The microscopic featureof theparallel Feathery textureis essentially similarto thatof lattice-bladed texture: a setofparallelseams, separated either Underthemicroscope withcrossed polars, individual quartz eachgroupcomprises crystals display a splintery or feathery appearance seenonlyas byrectangular quartzcrystals (Fig.4f),orbyprismatic crystals
SCIENTIFIC COMMUNICATIONS
Texture
Sketch
of
Texture
Type
Grain Size
Grain Form
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Internal Feature Morphology of Individual of Crystal Crystal Aggregate
References
Primary Growth Textures
Massive
variable anhedralnot applicable
homogenous Smimov (1962) Bates &
Jackson(1987)
Crustiform
variablevariable notapplicable
successive banding
Adams(1920) Shaub(1934) Lindgren(1933) Buchanan(1981)
Cockade
variablevariable not applicable
concentric banding
Adams(1920) Spurt(1926)
free
semi-spherical, Rogers(1917) reniform, Adams(1920) mammillary
Colloform
fibrous not applicable anhedral
Moss
[me
Comb
variableprismaticnotapplicable parallelAdams(1920) orientated Schieferdecker (1959) Boyle (1979)
variable notapplicable spherical
variable prismatic zonal
Adams(1920)
notapplicable Smirnov(1962)
FIe;. 1. Classification of quartztextures.
and/orcrystallites growing perpendicular totheseams. Adams (1920)described a texturecalled"lameliarquartz"whichhas features verysimilarto the parallel-bladed texture.
Saccharoidal
In thistexture, loosely packed vitreous to milky, fine-grained quartzaggregates havetheappearance of sugarin handspeciPseudoacicular mens(Fig.5e).Underthemicroscope, abundant elongated subhedralcrystals, somewithdoubleterminations, arerandomly Thepseudoaeieular texturewasfirstdescribed byLindgren distributed in a matrixof smaller,anhedral grains(Fig.5d). andBancroft(1914)fromthe Republicdistrict,Washington,Locallythereis an alignment of elongate crystals givingthe andwasalsoreportedbyAdams(1920)andSchrader (1923). impression of a crudemeshor network. Thisis the "retiform In handspecimens, aggregates of silicaminerals, commonly structure" described by Lindgren(1901),Adams(1920),and associated with adulariaor its weathering products(serieite "retieulated texture" described by Lovering (1972). or kaolinite),displaya radialaeieularappearance (Fig. 5a). Under the microscope, this is indicatedby lineararrange- Discussion on the texturalclassification mentsof fine-grained, sometimes roughlyrectangular, quartz in sometextbooks (e.g.,Stairnov,1962;Bates eryst'als and/orbyalineardistribution ofadularia oritsweath- Asdiscussed eringproducts (Fig.5b). andJackson, 1987),theterm"texture" isusedforthegeneral
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Texture
Sketch
of
Texture
Type
Grain Size
COMMUNICATIONS
Grain Form
Internal Feature Morphology of Individual of Crystal Crystal
Recrystallization
References
Aggregate
Textures
Mosaic • free
Feathery
anhedralnotapplicable interpenea'atingLovering(1972) Saunders (1990)
variableprismaticplumose
not applicable Adams(1920) Sander and
Black(1988)
Flamboyant
variable round
radial
not applicable Adams(1920) Sander and
Black(1988)
Ghost-sphere
f'me
anhedmlspherical
notapplicableAdams(1920)
free
anhedmlnotapplicable to prismatic
intersecting Lindgren(1899) bladed Schrader (1912) Morgan(1925)
free
anhedmlnotapplicable
Replacement Textures
Ghost-bladed
intersecting bladed
fine
anhedmlnotapplicable parallel to rectangular
Adams(1920)
bladed
P0x'0llel-bladed • Pseudo-acicular
free
anhedralnotapplicable acicular to reckangular
Saccharoidal
free
anhedmlnotapplicable to prismatic
FIG. 1.
Lindgren and Bancroft(1914) Adams(1920) Schrader (1923)
interlocking Lindgren(1901) Adams(1920) Loveting(1972)
(Cont.)
of its mineral aggregates whichdifferfromone physical appearance orcharacter ofa rock,including thesize rangement in shape, size,composition, andtexture. Thisisbest andshape of,andthemutualrelations among, itscomponentanother byerustiform structure. However, thetwoterms minerals. Saeeharoidal andmosaic textures typically belongto represented interchangeably, andsome textures mayparallel thiscategory. Theterm"structure" is generally usedforthe areoftenused features. Forinstance, eolloform andcomb largerfeatures of a rockandisdetermined bythespatial ar- majorstructural
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FIC. 2. QuartztexturesI. a. Crustiform: alternating finebandsconsisting of pinkadularia,microcrystalline quartz, combquartz,andchlorite, developed frombothwallsof a fissure. Centralextended lode,Cracow, Queensland (department catalog no.35322).b. Colloform-crustiform: classic examples of colloform (botryoidal) andcrustiform (alternating) bands
cohsidered characteristic ofepithermal veins. McLaughlin, California (35324). c.Moss: silica aggregates display aheterogeneousturbidappearance, similarto moss vegetation. Pajingo, Queensland (35327).d. Moss:groups of spheres highlighted by the distribution of impurities withinaggregates of sfiicaminerals. Pajingo, Queensland (35327).Plane-polarized light. Scalebar = 0.2 mm.e. Comb:groups of parallelor subparallel quartzcrystals orientedperpendicular to veinwall, resembling theteethof a comb/WhiteHopelode,Cracow,Queensland (35329).f. Zonal:withinindividual quartzcrystals, therearealternating clearandinclusion crowded zones.QuartzHill, Queensland (35330).Metricscales= 1 cm.
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F[c. 3. Quartztextures II. a. Mosaic: aggregates of microcrystalline quartzcrystals withhighlyirregular andinterpenetratinggrainboundaries. Pajingo, Queensland (35331).Crossed polars. b. Feathery1: a featheryappearance in the rims of the crystals witheuhedralcores,seenonlyasslightopticaldifferences in maximum extinction positions. In another position (e.g.,bottom center)thequartzcrystal displays a verysimilar interference colorbetween theeuhedral coreand rims.Pajingo, Queensland (35332). Crossed polars. c.Feathery 2: a feathery appearance seenaspatches throughout quartz crystals. Afarti,Queensland (35333).Crossed polars. d. Flamboyant 1:radialor flamboyant extinction of individual quartz crystals withmoreor lessrounded crystal outline.In thissample, the flamboyant textureiswelldeveloped in therimsof crystalline quartzcrystals witheuhedralcores.Centralextended lode,Cracow,Queensland (35334).Crossed polars.e. Flamboyant 2:flamboyant extinctions seenthroughout thecrystals withrounded surface inbands. Pajingo, northQueensland (35336).Crossed polars. f. Ghostsphere: solidand/orfluidinclusion defined spheres withinmicrocrystalline quartzcrystals. Centralextended lode,Cracow, Queensland (35337).Crossed polars.Scalebars= 0.2 min.
SCIENTIFIC COMMUNICATIONS
ERIC
F•c. 4, Quartztextures III. a. Latticebladed:a network of intersecting sfiicablades withpolyhedral cavitiesi Bimurra, Queensland (35339)1 b. Latticebladed: in thinsection, eachbladeconsists of a series ofparallel seams separated byquartz crystals or crystallites whichhavegrownsymmetrically abouttheseams andperpendicular to them,Bimurra, Queensland (35339)1 Crossed polars. c. Ghostbladed: blades areidentified on thepolished surface of thehandspecimens by the concentration of impurities. Thistexturecommonly occursin crustiform bandsandlacksthe cavities betweenbladesß Woolgar, Queensland (35340). d. Ghost bladed: aggregates ofquartzcrystals withsuperimposed bladed texture identified byoutlines ofimpurities andfinergrainsize.Woolgar, Queensland (35340). Crossed polars. e.Parallel bladed: silica blades areparallelwithineachgroupbutadjacent groups havedifferentorientations. Bimurra, Queensland (35341).f. Parallel bladed: eachgroupiscomposed of a setof parallel-oriented quartzcrystals whichhavemoreor lessrectangular shapesß Bimurra, Queensland (35341).Crossed polars.Scalebars= 0.2 mm,metricbars= 1 cm.
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Fro. 5. Quartztextures IV. a. Pseudoacicular: aggregates of silicaminerals, commonly associated withadulariaor its weathered products (kaolinitc or illitc),display a radialacicular appearance, caused bydifferences in colorand/orreliefin handspecimens. Pajingo, Queensland (35342).b. Pseudoacicular: acicular appearance is indicated underthemicroscope bylineararrangement of fine-grained quartzcrystals andlineardistribution of day minerals. Pajingo, Queensland (35342). Crossed polars. c.Saccharoidal: loosely packed fine-grained quartzaggregate, having a sugary appearance inhandspecimens. Rose's Pridelode,Cracow, Queensland (35343).d. Saccharoidal: underthe microscope, slender subhedral crystals are randomly distributed in a matrixof smaller, anhedral grains. Locallythereis alignment of elongated crystals givingthe impression of a crudemeshtexture.Rose's Pridelode,Cracow, Queensland (35343).Crossed polars.Scalebars= 0.2 ram, metric bars = I cm.
canbe usedas bothtextures andstructures. Manypeople The Possible Originsof QuartzTextures todaypreferto grouptextureandstructure together in regard of quartztextures is alwaysa difficultsubtothegeneral features ofa rockorvein(Lovering, 1972;Craig Interpretation ject, since it requires a substantial knowledge of thesolubiliandVaughan, 1981;Augustithis, 1982;MacKenzie et al.,1982). ties of silica minerals, various kinetic processes suchaspolyThis,concept hasbeenapplied in thepresent study. coagulation, nucleation, crystallization, dissoluSincethecriteriaforthisclassification aredefined byvari- merization, of silicaminerals, mostof which ousparameters (suchasthe morphology of mineralaggre- tion,and'recrystallization particularly in very complex gates,the internalfeatureof anindividual crystal), a certain are still not well understood, hydrothermal systems. The following discussion attempts to specimen couldbe described in severaltexturaltermsby explanations towardthe originsof usingdifferentcriteria.For example, combtexturedescribesprovidesomepossible basedoncomparative observation andliteragroups of quartzcrystals sharing thesameorientation; how- quartztextures, remainat anempiriever,individual crystals in combtexturecouldalsodisplay turereview.Mostof theinterpretations zonaltextureor featherytexture.Crustiform texturerefers calstage. Threemajorclasses are considered froma geneticpoint tothebanded arrangement ofmineralaggregates whichdiffer fromoneotherin textureandcomposition, it naturally in- of view:(1) primarygrowthtextureswhichrepresentthe formedduringcrystal growthor thedeposition eludesmanyothertextures withineachband.The wayto morphologies dealwiththisproblem istonamealltextures observed, sothat of amorphous silica;(2) recrystallization textures whichresult the characteristic of the samplecanbe illustrated entirely. fromtherecrystallization ofchalcedony, orcrystallization and
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rateof growthis perpendicular to the subsequent recrystallization of amorphous silicato quartz; directionof maximum Thisrequires relatively slowchanging condiand(3) replacement textures whichrepresent partialor com- growthsurface. plete pseudomorphs of other mineralsby silicaminerals tionsin an openspaceduringcrystalgrowth. Zonaltextureis confinedto quartzcrystals that growdiwithinveins.Mostofthequartztextures described abovecan be fittedintooneof thesethreecategories. rectlyfromhydrothermal fluid.Thisrequires thehydrothermalfluidto be onlyslightlysaturated withrespectto quartz, Primarygrowthtextures suggesting slowchanging orverymildlyfluctuating conditions In general, anyprocess whichcauses changes in fluidcondi- duringcrystalgrowth(Fournier,1985). tionsmayleadto the formationof simplecrustiform bands. textures Theseprocesses include:cooling, mixingof twofluids,reac- Recrystallization tionsbetween wallrocksandthefluid,andboiling(Buchanan, All silicaminerals exceptquartzare metastable andhave 1981).However,to producecomplex crushform bandswith a tendency to convert to quartzafterdeposition. Thepossible numerous repetitivechanges in mineralcomposition (e.g., waysof formingvariousreerystallization texturesare illusquartz,adularia, sulfides) and/ortextures, thehypothesis "epi- tratedin Figure6. sodicpressure release" (Buchanan, 1981)isa feasible mecha- Closeinspection of a featherytexturerevealsthat some nism.Thedropsin thetotalconfining pressure willallowthe individual domains of extinction resemble small subhedralfluidsto boil,resulting in lossof gases, cooling, pH rises,and euhedral quartzcrystals (seeFig.3b).Thisdoselyresembles precipitation oforeandgangue minerals. Asminerals deposit, epitaxialgrowthof quartz(Rimstidtand Cole, 1983),i.e., the near-surface veinletsbecomefilledby theseminerals, smallquartzcrystals growor accumulate on a largeexisting effectively forminga sealedcapto thefracturesystem. Once quartzcrystal whichactsasa surface favorable fornucleation sealed,the pressure increases andboilingat depthceases. andgrowth(Fig.6-A1).Laterthesesmallcrystals arereerysTectonism, ormorelikelyhydrofracturing, canbreaktheseal- tallizedin approximate crystallographic continuity with the ingcapto allowa second, andlater,episode of boilingand hostquartzcrystal.The originalshapeof smallcrystals is mineralization, andagainsealthe system. In thismanner,a preserved asa slightdifference in extinction whichmaybe repetitively bandedcrustiform textureis formed. inducedby dislocations alongtheboundary of adjacent small The separation of fragments in loosebrecciaby the force crystals duringthe reerystallization. of growing crystals, proposed by Adams(1920),is a feasible Thefinalappearance of a feathery textureis controlled by explanation for thecockade textureobserved in the majority the mutualrelationships betweensmallcrystals andthe host of samples in the presentstudy.Theseshowquartzprisms crystal.For example,if smallcrystals growor accumulate bristlingfromall surfaces of fragments andsharpcontacts on a euhedralquartzcrystal,afterreerystallization, feathery betweenfragmentsand bandedmaterials.Hydrothermalextinction isconfined to thegrainmargins. If thehostquartz brecciation generally precedes the formationof a cockade crystalcontinues to growtogetherwith or afterthe growth texture,asbrecciation allows deposition ofsilicaminerals and or accumulation of smallcrystals, eventually thesesmallcrysotherminerals aroundnewlyformedfragments. talswillbe enclosed bythehostgrain.Afterreerystallization, Colloform and moss textures both have distinct rounded featheryextinction will be developed as patchesor zones forms,although oneexhibits continuous bandsandtheother throughout the grain(Fig. 6-A2),with someindividual doisolatedspheres. Two processes, bothindicative of a silica mainsof extinction evenerossing euhedralgrowthzonesof precursor, wereproposed to explain theformation ofrounded the crystal(seeFig.3e). forms.The firstoneis thatof the precipitation of silicagel The initialcomponents of a flamboyant textureare likely in free space(Rogers,1917;Adams,1920).The controlling to be aggregates of fibrouschalcedony withroundedexternal factorfor thisprocess is considered to be surface tension, a surfaces, whichoriginate fromsilicagel,eitherascoatings on propertyof fluidscausedby intermolecular forcesnearthe earlyformedquartzcrystals or wallrock(Fig.6-C2,referto surface, tendingto reshape all nonspherical surfaces into a colloform texture),or asgroups of spheres (Fig.6-D2, refer spherical, minimumfreeenergyconfiguration (cf.Adamson, to mosstexture).Whentherecrystallized materials followthe 1976).Thesecond process isthatofthesegregation ofimpu- crystallographic orientation of initialnucleiof eachchalceritiesby crystallization fromsilicagel (Adams,1920;Keith donicspheroid or thatof the largecrystaluponwhichthey andPadden,1963,1964a,b; Oehler,1976).The principal arecoated,crystalline or microcrystalline quartzcrystals with requirement for thisprocess is a veryslowrateof impurity radiating extinction, possibly inducedby the dislocation bediffusioncompared with the rate of crystalgrowth,which tweenadjacentchalcedonic fibers,are formed(Fig. 6-C3). typicallyoccursin viscous silicagel with impurities. The Thiscanbe illustrated in a seriesof photographs (Fig. 7). slightlydifferentappearance betweencolloformand moss A ghost-sphere texturecouldbegenerated fromrecrystallitextures maybecaused bythedifferentoccurrences of initial zationof amorphous silicaor chalcedony witha mosstexture nuclei:thoseadheredonwallrockor earlyformedveinrock (Fig.6-D1 andD2), if originalspherically distributed impuriresult in the formationof colloformtexture;whereasthose tiesarepreserved in quartzcrystals dueto theirlowsolubility suspended in silicagelleadto theformation of mosstexture. (Fig. 6-D3). Toformcombtexture, geometrical selection mustproceed A mosaictexturehasbeensuggested as the productof effectively (Grigor'ev, 1961,p. 190).Geometrical selection is recrystallization of massive chalcedony or amorphous silica a typeof competition for spacebetweenadjacentcrystals, (Lovering,1972).A similartextureis commonly foundin whichresultsin the growthof onlythosecrystals wherethe calcitemarbles (Harker,1950;Augustithis, 1985).It ispossi-
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Polymerization and aggregation __--
--
I-
5o.,icaio Possible
silicaprecursors textures
/ /xcz • /• ,,a•xo2 to the qua 3
Feathery
Jigsaw
Flamboyant
Variousquartztextures asindicatedin figure 1
Ghost-sphere
FIG.6. Theinterpreted originofrecrystallization textures. Thedefinition ofpolymerization, aggregation, crystallization, condensation, andsolidification follows to Iler (1979).Various formsof opal:B1,C1, andD1; various formsof chalcedony: B2, C2, andD2; various formsof quartz:A1,A2, B3,C3, andD3.
ble that recrystallized coarsegrainslocallyfollowthe shape featureof calcite(Fig.9c andd). AsnotedbyAdams(1920), oftheoriginal smallgrains, forming highlyirregular andinter- replacement proceeds alongtheseplanesmoreeasilythan penetrating boundaries of thecrystals. alongrhombohedral cleavage planes. Thisselective replacementyieldsa setofparallelstructures withinlattice-orghostReplacement textures bladedpseudomorphs, whicharedefinedunderthe microscope either by different grainsizesof quartz,by different Thepossible processes offorming various replacement texcontents of impurities, or bypreferred orientation of quartz turesaredemonstrated in a flowchart(Fig.8). Notsurprisingly, parallel-bladed texture, whichshows Pseudobladed (latticebladed,ghostbladed,and parallel grains. outlinefor eachgroup,is the productof bladed): Calcite and barite are the most common soluble an overallgranular selective replacement of massive granular calcite. phases thatmaybe replaced by quartzin epithermal veins. As illustrated in Figure 8-A2, quartz crystals starttoreplace In oursample collection, primarybladedbariteusually hasa alongthe outlineof the crystals andpinaspindlelike shapeandis commonly dissolved, leavingspin- bladedcarbonate partings withinthecrystals. Astheprocess goesfurther, dlelikemoldsratherthanbeingreplaced by quartz.In con- coidal thatthereplacement frontis at theboundary betrast,primarybladedcalciteoftendisplays verysimilarmor- it appears tween the replaced mineral (carbonate) and the replacing phologies to thoseof latticeor ghost-bladed pseudomorphs mineral(quartz),andquartzcrystals in theformerlayerkeep (Fig. 9a). Finally,as carbonate is totallyreUnlikebarite,carbonate crystals usually contain numerous growingsimultaneously. in everytwo adjacent layersmerge microscopic inclusions, dominated by ironhydroxides. After moved,quartzcrystals backtobackagainst eachotherand carbonate is replacedby quartz,theseimpuritiesare pre- intoa seam,withcrystals of crystals increasing outward(Fig. 8served dueto theirlowsolubility andusually stilldefineorigi- with the grain-size featureof lattice-bladed nalcrystaloutlines. Occasionally rhombic cleavage tracesof A3).Thisis the typicalmicroscopic texture(seeFig. 4b). originalcarbonate canalsobe preserved (Fig.9b). withbladed Thelamellar parting, whichisparallel tothebasalpinaeoid If the startingmaterialis quartzintergrown of carbonate crystals, is the mostdistinctive morphologicalcarbonate (Fig.8-B1),the replacive quartzcommonly grows
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veloprectangular formscommonly seenin parallelbladed texture(Fig.8-C3,alsoseeFig.4f). Thisispossibly because replacement takesplacemorereadilyalongthelameliarpartingswithincarbonate crystals thantransverse to them(Adams, 1920).
A pseudoacicular textureis formedviathereplacement of calciteby quartzandadulariaalonga setof radialacicular structures withincalcitecrystals (Fig.8-D1),assuggested by Lindgren andBancroft (1914),Adams(1920),andSchrader (1923).Partialreplacement of calciteby quartzalongthis structure is alsovisiblein somesamples (Fig.9e). The presence of radial-acicular structurewithincalcite crystals is somewhat problematic, sincethisstructuredoes notfollowanyconsistent structural featurein calcitecrystals suchasrhombohedral dearageplanes,twinplanes,or basal pinaeoid planes.Asshownby NieldandHeniseh(1969)and Gareia-Ruiz andAmoros (1981),calcitecrystals grownin sfiiea gel are usuallyturbidwith somespedfiebut unusual morphologies suchas radialfibers.Dissolution of turbid calcitecrystals in acidleavesa residueof silicagel which hasthe samestructure asthe originalgrowthmedium.It is indicated from these results that the silica network which
constitutes the gel is incorporated into the growingcalcite crystals moreor lessintact.In thisway,an unusualradialaeieular structure, composed of sfiiea, isformedwithincalcite crystals. Laterreplacement by quartzshouldpreferentially follow this structure where there are sfiiea inclusions for nu-
cleation(Fig.8-D3). A saeeharoidal texturehasbeengenerally interpreted as the productof the replacement of calcite(Lindgren,1901; Adams, 1920;Lovering, 1972).Presumably, diffusion ofsfiieabearingfluidthroughrandomly distributed crystallographic defects, ratherthanalongthelameliar partings (e.g.,forming parallel-bladed texture),withinmassive granularcarbonate is responsible for the initialnucleation of quartzcrystallites randomly dispersed in carbonate crystals (Fig.8-E2).Further diffusion alongtheboundary between carbonate andquartz leadsto the formationof slendersubhedral-euhedral, or even
doublyterminated, quartzcrystals (Fig.9f) whicheventually interlock, formingsaccharoidal texture(Fig.8-E3).In some cases,remnantsof calcite are found within the saccharoidal
texture.Theycouldlaterbe replaced byfinergrainedaggregatesof anhedralquartz,or be dissolved, givinga porous appearance whichis commonly seenin the saccharoidal texture.
Implications of ThisStudy 1. The mostconspicuous quartztexturesin epithermal of a silicagel FIG.7. Anearlystageof recrystallization, formingflamboyant texture.a. veinsare thosewhichrecordthe presence (e.g.,colloform, moss,ghostsphere,flamboyant, Colloform-banded, initialchalcedony coating ondearquartzcrystals (plane- precursor polarized light).b. In theareaclose toclearquartzcrystals, initialchalcedonyandpseudoacicular). To formsilicagel, the fluidneedsto hasrecrystallized withthesameoptical orientation asthehostquartzcrystals be highlysupersaturated with respectto amorphous silica (crossed polars). c.In themaximum extinction position, recrystallized materi(e.g.,below100øC), als showing flamboyant texture(crossed polars).McLaughlin, California (Fournier,1985).At low temperature (35344). Scale bars = 0.2 mm. the solubility of amorphous silicais relatively low(e.g.,364 ppmat 100øC,cf. Fournier,1985,appendix) andthe rateof onexisting quartzcrystals withthesamecrystallographic ori- silicaprecipitation isveryslow(RimstidtandBarnes,1980). entation,andthe originalbladedformis onlydefinedby Therefore, thefluidmayeasilyreachequilibrium withamorconcentrations of impurities (Fig.8-B3,cf. Fig. 4d).After phoussilicaat lowtemperatures. For example, coolingof a replacing massive granular carbonate, quartzcrystals mayde- dilutefluidin equfiibrium withquartzfroma hot reservoir
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LatticeBladed
GhostBladed
ParallelBladed
Pseudo-Acicular
Saccharoidal
FIc..8. Interpretation ofstages intheformation ofvarious replacement quartz textures. Thetoprowrepresents original formsof calcite; the middlerowshows initialstageof replacement of calciteby quartz;the bottomrowshows various quartztextures (indicated in Fig.1) formedaftercomplete replacement of calcite.
fromsilicagel,alongwithothergeological, mineral(e.g.,at 230øCwith silicaconcentration of 388 ppm) may inherited or fluidinclusion evidence of relatively hightemperayielda fluidsupersaturated withrespect to amorphous silica ogical, environat about100øC.Thismayoccurwhenan ascending hydro- tures,is a goodindicatorof boilingin epithermal thermalfluidrisesfastenoughandsilicadoesnotprecipitate ments. 2. The recognition of a carbonate precursor is alsoimduringthe ascent.In fact,colloform andmosstextures have beenobserved in a numberof siliceous sintersin activegeo- portant. In epithermal environments, theprecipitation ofvein ismostlikelydrivenbythelossof CO2duetoboiling, thermalsystems (e.g.,Whiteet al.,1956;Herziget al.,1988; calcite generation ofCO•-ionsfromthedissociFournieret al., 1991).At highertemperatures, the fluidsu- andthesubsequent 1985).In persaturated with respect to amorphous silicashouldhavea ationof HCO• (Henley,1985;ReedandSpycher, calcitemaybe precipitated wherecoolermarginal relativelyhighsilicaconcentration (e.g.,at 220øCthe fluid addition, saturated with respectto amorphous silicacontains about fluids come into contact with hotter rocks due to its retrobut thisprocess is commonly restricted to 1,070ppmsilica,cf. Fournier,1985,appendix), sucha high gradesolubility, andshallow partsof a system (Simmons and silicaconcentration is not easilyattainedsimplyby cooling the margins 1994).The controlof calcitemorphology has from a hot reservoir.However,wherethe fluid undergoes Christenson, of a vastgeological andgeochemical literaboiling,significant coolingdue to adiabatic expansion (de- beenthe subject 1968;Kirovet al., 1972;Folk, 1974; creasing thesolubility of silicaminerals) andthelossofwater ture (e.g.,Bischoff, andRoy,1974;Lahann, 1978;GivenandWikinson tothevaporphase(increasing silicaconcentration in residual McCauley crystal morphology of calciteis suggested solution) canmakethe fluidhighlysupersaturated with re- 1985).In general, mainly bytherateof crystal growth, Ca'2+/ spectto amorphous silica,evenat relatively hightempera- to becontrolled of impurityions tures.For example, assuming a reservoir hasa temperature CO•- ratiosin thefluid,andthepresence •-, butthemechanisms arestill of 300øCandpressure of 200bars,thewaterin equilibrium suchasMg•+, Na+, andSO4 obscure andunverified bydirectevidence. In many withquartzundertheseconditions shouldcontainabout750 somewhat systems, bladedcalciteis commonly reppmSIO2.Thiswill yielda fluidjustsaturated withamor- activegeothermal to theboilingzoneandmaycontain coexisting liquidphoussilicaat about170øC(cf. Fournier,1985,appendix). stricted inclusions (Browne,1978;Keithet al., 1978; According to theenthalpy balance calculation (e.g.,Henley, andvapor-rich 1984),isoenthalpic boilingof a fluidfrom300øto 200øCwill Tulloch, 1982; Simmonsand Christenson,1994). However, it is difficultto conclude that cause25 percentwaterloss.Thusif takingthe steamloss withoutcleargeneticevidence, intoaccount(assuming 30%waterlossasa maximum), the otherformsof veincalcite(e.g.,granular)cannotformin or thatbladedcalciteformsexclusively silicaconcentration in sucha fluid mayreachabout1,070 boilingenvironments, ppm,whichisequivalent tothesolubility of amorphous silica in boilingzones. moresoluble withdecreasing temperature atabout220øC. Therefore, thepresence ofthequartztextures Calcitebecomes
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Fzc.9. Primary calciteandpartialreplacement of calcite. a. Primary-bladed calcite: a network of intersecting bladed calciteseparated by polyhedral cavities, identicalmorphology with lattice-bladed pseudomorphs in Figure4a. Komata mine,Coromandel, NewZealand. b. Rhombic cleavage traces: aftercalciteis replaced by silica,rhombic cleavage traces areoccasionally preserved bythedistribution ofbrownish-colored impurities. Notenocalcite isleftinthissample. Barambah Creek,Queensland. Crossed polars.c. Partialreplacement of bladedcalcite: bladedcalciteis beingreplaced by quartz alongparallelpartings, identicalmorphology with ghost-bladed pseudomorphs in Figure4d. Red Dome,Queensland. Crossed polars.d. Partialreplacement of granular calcite:granular calciteis beingreplaced by fine-grained quartzalong parallelpartings, identical withparallel-bladed pseudomorphs in Figure4f. Yandan, Queensland. Crossed polars. e. Partial replacement of granular calcite: partialreplacement of granular calciteby quartzalongradialacicular structure in the crystal. Standard lode,Cracow,Queensland. Crossed polars.f. Partialreplacement of granular calcite: granular calciteis partlyreplaced byrandomly dispersed slender subhedral-euhedral quartzcrystals. Rose's Pridelode,Cracow, Queensland. Crossed polars.Scalebars= 0.2 min.
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butlesssoluble asthepartialpressure ofCO2decreases (Ellis, regarding the hypothesis of colloidal goldtransport, it is a 1959).Henceboilingmayprecipitate or dissolve calcite,de- mechanism worthyof furtherinvestigation. pendingon the composition of the hydrothermal solution, Conclusions thepressure atwhichboiling isinitiated, thedropintemperaclassification of quartztextures hasbeendetureandpressure andwhetherthe system is openor closed. A descriptive froma reviewoftheliteratureandtheexamination of In gas-rich, opensystems, boilingdramatically reduces the veloped fromepithermal veinsin Queensland. Thirteen partialpressure of CO2.At relatively lowtemperatures, the manysamples lossof C02 will causesignificant enrichment of CaC03in texturaltypeshavebeendefined,mostof themhavebeen genetically andgrouped intothreemajorclasses: the fluiddueto the highsolubility of calcite.Consequently,interpreted (2)reerystallization textures, and boilinginitiatedfromgas-rich andopensystems at relatively (1)primarygrowthtextures, textures. It hasbeenrecognized thatsome low temperatures favorsprecipitation of largeamounts of (3) replacement quartztextures canbeusedasanindicator of boiling calcite.If theseearlyprecipitated calcitecrystals arenotim- specific environments, andthereisapositive correlation mediately segregated fromresidualfluids,theyare readily in epithermal andthe quartztextures indicadissolved andreplaced bysilicaminerals whenthefluidcools betweengoldmineralization tiveof silicagelprecursors. further,formingvarious replacement textures. thegroundwork forfurthersystematic Thus,if a fluidundergoes boilingwhichresults in theloss Thisstudyprovides evaluation of the distribution of quartz textures andtextural of CO2withoutrapidcooling (e.g.,isothermal boiling), or if a assemblages in selected epithermal systems. A textural zoning fluidisheated byhotterrocks, thefluidmaybesupersaturated theverticalposition with respectto calcitebut undersaturated with respectto model,whichcanbe usedto determine system andto predictthe likelylocus quartz.In thiscondition, calcite precipitates aloneandislater withinan epithermal of gold mineralization, will be proposed in another paper. replaced by silicaminerals, forminglattice-bladed, parallelbladed, orsaccharoidal textures--depending onthemorpholAcknowledgments ogyof carbonate precursors andthedevelopment ofcleavages Gold andfractures, as discussed previously. On the otherhand, Thisstudywasa partof projectP247"Epithermal Deposits in Queensland" sponsored by Australian Mineral wherethefluidundergoes boiling whichinduces boththeloss Industries Research Association. The support fromthe staff ofCO2andrapidcooling (e.g.,isoenthalpic orsubisoenthalpic of the Earth Sciences Department at James boiling, cf.ReedandSpycher, 1985),thefluidcouldbesuper- and students Cook University, sponsor companies, and AMIRA is gratesaturated with bothcalciteand quartzor evenamorphous fully acknowledged. Two Economic Geology reviewers are silica. In thiscase,calcite andquartzoramorphous silica presincerely thanked for their constructive comments. cipitate simultaneously, forming ghost-bladed orpseudoacicuREFERENCES lartextures whencalcite islaterreplaced bysilica. 3. Fromthe comparison betweenwell-mineralized vein Adams,S.F.,1920,A microscopic studyof veinquartz:ECONOMIC GEOLsystems (suchasCracow,Pajingo, andMountCoolon)and OGY,v. 15, p. 623-664. 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