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Gold.Rich Rim Formation on Electrum Grains in Placers

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Gold.Rich Rim Formation on Electrum Grains in Placers Canadion Minerqlogist Vol. 28, pp. i0148 0990) GOLD.RICHRIM FORMATIONON ELECTRUMGRAINS IN PLACERS JOHN C. GROEN, JAMES R. CRAIG ANDJ. DONALD RIMSTIDT Departmentof GeologicalSciences, Virginia Polytechnic Inslitute and State University, Blacksburg,Virginia 24061 -0420, U.S.A. ABsrRAcr ce m€canismene peut expliquerla pr€sencede gradients abruptsen composition.Une comparaison de l'efficacit6 Grains of placer gold from several localities in the relativede complexationde 49 ligandsdiff€rents montre southeasternUnited Statesexhibit a high-purity gold rim. queCN-, OH-, NH3,Cf, I-, Br- etHS- (en ordre d€crois- The individual rims on the various grains rangefrom ( I sant)pourraient transporter I'or dansun tel mitieu.Un pro- to 60 rm in thicknessand have silver contentsof 3.3 to 0 cessusd'autoelecfio-affinage des grains d'6lectrum dans un wt.9o (averageof 0.9 wt.Yo), eventhough the core com- gravierserart impliqu6 dans I'origine de cesbordures, et positionsrange from 45.1to 2.8 wt.qo silver.This gold rim- agiraitavec un ph€nombnede dissolutionet pr€cipitation ming most likely is responsiblefor the commonly cited cases (cimentation)pour produireles liser6s. of gold from placer deposits assayingat higher values of finenessthan the gold in the correspondingsource lode. (Traduit par la Rddaction) A gold-rich rim apparently forms by precipitation of gold from the surrounding solution, becausesimple leaching of Mots-clds;or (Au), argent(Ag)t enrichissementsecondaire silver from the electrum surface is an ineffective mechan- enor, Au(CN);, 6lectrum,bordures d'or, transfertde ism for the enrichmentof gold. Diffusion of silver to the I'or, 6lectro-affinage,complexes aqueux' thermodyna- surface of a placer gold grain to expose it to oxidizing miquedes complexes aurifbres. meteoricwaters, and thus createa diffusion-enhancedleach- ing process,proceeds far too slowly to producethe observed natural rim thicknesses;furthermore, this mechanismfails INTRoDUcTIoN to produce the sharp gradients in concentration observed in natural grains. Comparison of the complexation effica- Gold, sincebefore recordedhistory, hasbeen the cies of 49 different ligands indicates CN-, OH-, NH3, Cf, most prized of mineral possessions.Because of its I-, Br, and HS- to be the ligands most capable(in decreas- greatvalue, economicconcentrations of gold canbe gold ing order) oftransporting in ordinary stream environ- physically small in size and can contain as little as placer grains is ments. Self-electrorefining of electrum a ppm gold likely processof forming gold-rich rims and probably oper- l-3 so dispersedthat the deposits are atesin tandem with dissolution-precipitation (cementation) difficult to locate and recognize.Consequently, most to producethe observedphenomena. of the greatgold rushesof history, and indeedmany recentdiscoveries, resulted not from initial location Keywords:gold (Au), silver (Ag), secondarygold enrich- of primary gold-bearingdeposits, but rather from ment, Au(CN)2-, electrum, gold rims, gold transport, the detection and recovery of placer gold, which was electrorefining, aqueouscomplexes, gold-species ther- concentratedin streamsafter erosion of the source modynamics. lode deposits.In many areas,only the placer deposits have proven to be economicalto exploil. In 1497, SoN,rMeRs Ulrich Rulein von Kalbe statedthat "The gold gener- ated in river sand is the purest and most exalted kind plusieurs le des Etats-Unis,les A endroitsdans sud-est becauseits matter is most thoroughly refined by the pafticules d'6lectrum des graviers aurifdres montrent une bordure en or trbs pur. Ces bordures peuvent atteindre 60 flow and counterflow ofthe water and also because pm en epaisseur,et contiennententle 3.3 et 090 en poids of the characteristicsof the location where suchgold d'Ag (en moyentre,0.990), malgr6 le fait que le noyau de is found, that is, the orientation of the river in which cesparticules en contientde 45,I d,2.80/0. La pr€senced'un such placer gold is made" [translation by Sisco& tel liser6 d'or serait responsablede la teneur accrue en or Smith (1949)1.Since at leastas early as von Kalbe's desparticules d'6lectrum desgtraviers aurifbres, compar6es time, reports haveindicated that the gold in placer i leurs sourcesmin6ralis6es. La bordure se forme par pr6- deposits is generally purer than that in the lode cipitation de I'or d partir des solutions environnantes; un deposits from which the placerswere derived (Lind- particules simple lessivage de I'argent de la surface des gren l9ll, Boericke1936, Emmons 1937,Mackay inefficacecomme m6canisme d'enrichisse- d'6lectrum est Matn menl, La diffusion d'atomc d'Ag vers la surfaced'un grain 1944,Fisher 1945,Koshman & Yugay 1912, d'or dansun sable,pour les mettre en contact avecI'eau 1984).The purity of gold is expressedin lerms of m6t€orique oxyg6n6e,et ainsi crder un m6canismede lessi- fineness,which is the concentrationof gold in an vagequi d6pendde la diffusion, proc€debeaucoup trop alloy (in wt.9o) divided by the concentrationsof, gold lentementpour expliquer les dpaisseursobservdes. De plus, and silver, the quantity then multiplied by 1000. 207 208 THE CANADIAN MINERALOGIST United Statesand evaluatesthe processesthat may contribute to the formation of the gold-rich rim found on many of these grains. Inasmuch as the processesactive in this study areaare representative of thoseoperating in most other localities,the con- clusionsof this study are believedto have general applicability. The samplesexamined in this study were obtained from severallocalities in the southeasternUnited States(Fig. 1), an areawith a moist, temperatecli- mate and oak-hickory-predominantforests. Exten- sive weatheringhas occurred in the area owing to prolonged subaerial exposure and a current annual precipitation of 100-200cm. Theseenvironmental conditions subject the gold grains to substantial chemicalattack, with local variationsmanifested in differing water and gold chemistries,and the extent of human influences.All streamsfrom which sam- ples for this study were collectedhave streambeds that contain mud to pebble- or cobble-sizedsedi- ment, and generally abundant organic matter (hence a relatively low Eh) in the finer sedimentfractions. Gold grains from Brush Creek in southwestVirginia receivedthe most attention. CnanacreRrsrrcs oF THEGRATNS oF PLACERGoLD The morphology of the grains is influenced by Frc. l Samplelocalities of the gold from this investiga- numerousfactors, including characterofthe origi tion (fromnorth to south):SM4 Manassas quadrangle, nal lode particles, stream energetics,nature of the Virginia;JMB Storckquadrangle, Virginia; JMA Chan- streamchannel material, time spent in the stream, cellorsvillequadrangle, Virginia; BC BrushCreek, Vir- distanceof transport, and chemistryof the stream ginia; SM2 CabamnCounty, North Carolina;SMI water. More than 300gold grainsfrom Brush Creek MontgomeryCounty, North Carolina;NC85 Lilesville were closely examined to identify relationships quadrangle,North Carolina;SM3 White County, betweenmorphology and someof theseparameters. Georgia. The grains were divided into the following five general morphological gtoups: irregular (-4la/0, sphericalto semispherical( - 3390), wafer-shaped Grains of lode and placergold are very rarely pure. (width-to-thicknessratio s5) (-]4s/o), flake-shaped They invariably contain significant amounts of sil- (width-to-thicknessratio >5) (-ll9o), and cylin- ver, and occasionally minor amounts of copper drical (-l9o). The grains recoveredfrom Brush (Stumpfl & Clark 1965), palladium elanison & Creek during this study rangein sizefrom lessthan Fuller 1987),selenium (Dilabio et al. 1985),mer- 0.01 mm to 2 mm (maximum dimension);weights cury (Nysten 1986),and 45 or more other elements range from lessthan 0.01 to nearly 10 mg. including:Fe, Pb, Pt, Bi, Cd, Ni, Te, As, Sb, Rh, The surficial featuresof many of the gold grains and Al @rasmus et ol. 1987). Gold that contains wereinvestigated under reflectedlight and by scan- more than 2090 silver is generally referred to as elec- ning electronmicroscopy. Althouefr the surficial tex- trum, but there is considerablevariation in this tures representa wide and probably continuousspec- arbitrarily set compositional definition. In this study, trum, they have beenbroadly classifiedinto the six "electrum" will be usedonly as a qualitative term typesshown and describedin Figure 2. The general to emphasizethe fact that at least minor silver is relationshipsbetween grain morphologiesand sur- presentin the alloy. Craig & Rimstidt (1985)com- face texturesare quantified in Figure 3. The most piled analytical data for nearly 6500 samples of prevalent correlation exists between irregular grain natural gold, and found that virtually all the sam- morphologiesand smooth, unpitted surfaces.Many ples fall in the range of 500 to 1000fine, with the of the other relationshipsare not as universal,but mode at approximately 920 fine. a general diagonal trend of compatible morpholo- The presentstudy examinesthe nature of placer gies and surface textures can be seenfrom the upper gold grainsfrom somelocalities in the southeistern left to the lower right of Figure 3, where a good COLD.RICH RIMS ON ELECTRUM IN PLACERS 209 ".{:'* Frc. 2. SEM phoromicrographs of the six most readily identifiable textures observed in this study. (a) Euhedral iso- metric electrum crystals on solid, fairly flat substrates of electrum. (b) Well-rounded, srnooth surfaces with little if any evidenceof chemical attack. (c) Smoothly rounded grains with pitted surfaces.(d)
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