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 samples 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) Smoothly rounded surfaces with large pits containing branching-coral type features. (e) Hackly, friable-looking texture similar to that of foun- dry slag orweathered, gritty sandstone.(f) Inegular, lobate or bulbous, "stromatolite-like" texture with occasionally stepped but usually smooth individual lobe surfaces. correlation exists between flake-shaped grains and grain shape,and distanceof transport for the gold lobate to hackly or grainy surface-textures. grains from Brush Creek. Near the headwatersof An apparentrelationship existsbetween grain size, the creek,where the lode depositis located,irregu- 2t0 THE CANADIAN MINERALOGIST

\% of the mor- GRAINMORPHOLOGIES \ Rholoslcal sroup \ "/o of \ lrregular Spherlcal Cyllndrlcal Waler Flake -33o/o) the surface \ ('41o/ol ( ('1olol (-14o/ol ('1 1o/ol texture group \

7O-BOVI sufficlenl Srnootl unpitted (-36%)

't 5-250/o 60-7O"/o a Smmtt UI pitted E ('32o/ol 3 g0-400/" F x Pitted with 90'4Oo/o 15-25o/o lJl branching- F clral type IIJ features C) (-16%) 50-60%

IL Hackly tr to f gralny 'l a (-10%) O-2Oo/" 3O-4Oo/o 4 5'55o/"

lrregular lobate (' 7"/"1 25-55% x Frc. 3, Correlation diagram indicating the predominanceof relationships betweenmorphologies of electrum grains and surfacetextures, The value in the upper-rightcorner of eachbox indicatesthe percentageof grains(with the particular morphology given at the top of the column), which possessthe surfacetexture listed to the left. Similarly, the value in the lower left corner of eachbox indicates the percentageof grains (with the particular surface texture given to the left), which possessthe morphology listed at the top of the column. larly shapedgrains up to I mm are not unconmon, gold color, and, judged by the color of the surface, and the grains averageabout 0.2 mm in maximum would seemto contain over 90 per ct. of gold. This dimension.Downstream 19 km, the averagesize of color, however,is confined to a thin, externalfilm. the gold grainsis lessthan 0.1 mm. Grain morphol- When this is removedby the file or knife the color ogy evolvesin a predictabletrend from the original is quite white. It would seemthat the silverhad been irregular form through to semisphericaland wafer- removed from the surfaceby solution in somenatur- like, ultimately yielding flake shapes. ally formed solvent, and the gold thus was concen- Compositional inhomogeneitywithin the placer trated in the external film. When particles are gold grainsat Brush Creekhas been known sinceas obtained from the interior of quartz in the veins, they early as 1882,when Fontainenoted: "The gold con- often show the white color." tains about 32 per ct. of silver. Most of it has a rich Additional note of the rims on grainsfrom Brush GOLD-RICH RIMS ON ELECTRUM IN PLACERS 2ll

FIc. 4. Back-scatteredelectron imagesof two placer gold grains from Brush Creek, Virginia. (a) Grain number BC 2-6, showingpartially completeformation of a gold-rich rim (briebtest, high-electron-yieldareas), with preferential development in and around embaymentsand pits on the surface. (b) Grain number BC 2-15, showing a much more highly developedgold-rich rim with many voids (obate texture).

Creekwas made by Solberg& Craig (1981);we have the morphologiesof the gold grains. Firstly, polished subsequentlyfound that most of the grainsof placer sectionsof gold in vein quartz show no signs of gold from Brush Creekexhibit somedegree of gold- developmentof a gold-rich rim @riscoll et al. 1990), rich rim development,as illustrated in Figure 4. implying that the rim forms after liberation from the Electron-probemicroanalysis of 29 randomly chosen host rock. Furthermore, irregularly shapedgrains of and representativegrains from Brush Creek (Table placer gold (relatively new to the stream environ- 1) indicatesa compositionalrange of core electrum menl) exhibit little or no developmentof a gold-rich from 549to 849 fine. This rangeis quite large rela- rim. Flat, well-worn, flake-shapedgrains, on the tive to those of other localities consideredin this other hand, exhibit the most extensivedevelopment study (Table 1), and other southeasternU.S. locali- of a gold-rich rim. Spherical, semispherical,and ties (Craig et al. 1980).Microprobe analysisof the wafer-shapedgrain morphologiesshow intermedi- gold-rich rim on grains from all the samplelocali- ate degreesof rim development. ties showsthat 95Voof the rims have compositions The surface featuresobserved by scanningelec- of >985 fine. The remaining 590 of the rims range tron microscopyalso show a generalrelationship to between967 and 985 fine (Table l). It should be rim development.There is an increasingprogression noted that some of the optically distinct rims that in the degreeof rim developmentcorresponding to analyzeat finenesseslower than about 995 may actu- the following respectivesurface textures: smooth, ally be purer, becausethe analyticalvolume of exci- smooth pitted, pitted with branching features,irregu- tation may overlap or penetratelower-fineness core lar hackly, and lobate (Figs. 2b-0. The "euhedral electrum.When viewedin crosssection, these rims crystalson gold substrate" texturewas observedon are not uniform in thickness,but vary from < I to one grain only, #SM105364;these crystals and their 60 pm. The boundary betweenthe individual cores underlying substrate both have fineness values and rims generallyis sharp, and is especiallystrik- between 807 and 820. This compositional ing in reflectedlight if the surfacehas beenlightly homogeneitysuggests that the crystalsare most likely etchedusing a potassiumcyanide plus ammonium residual lode-gold crystals, as all indications suggest persulfatesolution. Grains with an incompleterim that gold remobilized in the weatheringenvironment tend to havegold-rich "pockets" whereinthe enrich- in this study arearesults in virfually pure gold precipi- ment areasoccur in small embaymentson the grain tates. surfaces(Fig. 4a). In other cases,lhe rim is so fully Considerationof theseobservations permits the pervasivethat it completelyencloses the core (Fig. recognition of a sequentialtrend @ig. 5) that relates 4b). Thesepervasive rims commonly exhibit a "swiss formation of the gold-rich rim to the morphology, cheese"texture, in which numeroussmall rounded surface texture, and distance of transport of the voids are found in the gold-rich rim. placergold grain. Most of the observedmorpholo- A few distinct relationshipswere observedbetween giesand surfacetextures can be roughly correlated the characterand prevalenceof gold-rich rims and into this trend of increasingdevelopment of a rim 2t2 THE CANADIAN MINERALOGIST

TABLE 1. CIIEIdICAL AND PEYSICAL DATA with time spent in the stream. The above relation- CONCERNING PLACER, ELPSTR,UM GRAINS ship suggeststhat the physical characteristicsof placer grains can give clues about the potential Grain lilass shape€) Fineness presenceand nature of a gold-rich rim without going Number(l) (mn)O) (ng) Core(4) Rim(4) through lengthy samplepreparations. The contact betweenthe gold-rich rim and the BCI-l(t 0.r8 0.04 s 7n:r298.' 98995(3) BC l-2 0r5 0.06 w 5rG596(4) -re90(4) Iower-finenesscore in the grains from Brush Creek BCI-3 030 omI 6550) ryr0) is very sharp and well defined, facilitating immedi- BC?r 4.09 S .4u4r4(4t 9r&9r9e) aterecognition of the rim, evenwhere it may be thin .4y2-9CaQ' BC2-2 lr0 9.n w iT?-JtgQ) and irregular (Fig. 4a). The rim-core boundary in BCz-3 1.30 4J3 I r0--yr8(3) .sn 0) BC24 1.40 I2L F e-"667(3.) 9fi-r88e) grains from most other localities is equally sharp, BCts 0.65 Nrr. (6) w qr7rc@, 9fl!9rrc2) though generallyless apparent oq/ing to the higher BC2-6 050 Nrt s s53(l) 996(l) averagefineness of their core electrum.The composi- BCz-? 0.95 NJIA F 6n:6[/.c4 9y2(l) BC2-t 0.60 NIt|. I 55&562@) 90(l) tional characteristicsof two representativerims as BC2-9 055 N}L W 6500) sr-.st 8) determined by electron-microprobetraverses are BC2-10 05) Nt. s 6t3-6,/.@.) 9r5(l) shown in Figure 6. Theseiraverses were especially BC2-ll 055 NIrt F 930) 9769nC2' useful in the quantification of the steepcomposi- BC2-r2 0.60 NiA I 66t 0) 9eL%9C2) BCz-r3 055 NJ'{. It 59r(l) s? o) tional gradientsin such rimmed grains. BC2-14 055 Nt{. s 5490) 9rr(r) The compositionsof coexistingrim-core pairs in BC2-15 o.sr Nn'[ F e,5{3,9Q) -9rr94(3) this study, as well as from other sources(data on BC2-16 0.60 NJ\A I 6E5(l) 988(l) BC2-r7 050 NJ'l. W 54(l) e9l(1) 681grains from 32 localities)are presented in Figure BC2-r8 0.?0 NIA S 55s0) 99l(l) 7. The averagecore compositionis roughly equiva- BC2-19 0.A) NLL F 56ao) 9rro) lent to that given by Craig & Rimstidt (1985)for gold BCz-z, 0.65 Ntd. t 78r 0) e5(l) grains from over world. This BCz-26 0.60 NJtrL I lt4 (l) 9930) all the similarity, and BCz-n o.{t Nt. w 6150) W (r) the fact that all of thesegrains have a gold-rich rim, BC 1l 0.?0 0.E4 S 6010) I't (r) suggestthat there is no strong relationshipbetween BC&l 050 026 W 7130) 4)4-gt Q) composition potentialfor rim BCtl 0.90 0.65 F t49(l) gel(r) the of the coreand the BC7-l 020 0.m I n6|J, 9890) development.The plotted rim-core compositional NC85L86I(7) 1.r5 NJ,t F n4(r) 9n$, pairs from Brush Creek(solid circlesin Fig. 7) illus- NCE5IJ62 1.45 NiA F 94r@) 930) trate the unusualcomposition of this gold. The scat- NC85L8G3 Llo NJA I -953-955(2) ND.(o NC85L86.4 0J0 Ntr. s 9v,-,958Q) ND. ter of rim compositionsshown in Figure7 also indi- NC85L86-5 lr5 N.T'. F 8660) 995O) catesthat there is no real relationship betweenthe NCt5L86J6 0.70 NJr[. S 8o(l) w2(r) purity of the gold-rich rim and the composition of NC85L8G7 LzA Nt[. w 994(l) ND. NC85L868 0.90 NJ't, I e45O) ee(l) the underlying electrum core. NCEsLE&9 t.m Nt. s E69O) 991_99r(2) Mineral inclusions observedin the gold grains NC85L&ir0 Lls NIA F 966(l) ND. from Brush Creek are primarily presentin the rim. NC85L86ll 0.65 Nta s sl 0) #-s7(3, The occasionalmineral inclusionsfound within core NCE5L86I2 0:IS NI,t. I cn-&ge) ND. NC85r.J6l3 0.85 NI[. I e6lo) 998o) electrumtend to be considerablylarger than those NC85L8GI4 0.65 NJrr. S 9U-943@) 99(r' in the rims. The majority of inclusions,both in the NC85IJ&15 0.?0 Nra s 9D@' 980) rims and in the cores,appear to be complexsilicates NC85L86I6 0,55 Nra s 9n 8' r{J. (e.9.; amphibolesand pyroxenes),as usually NC85L8G17 0,65 Nra w e4?0) 96 O) they NCttLEGlE 0.60 N}', I 947(r) l{D. contain substantialamounts of Si, Al, Fe, and Ti NC'&ilJ619 0.45 Ni,. I 83c$oe) ND. + K (9) subordinate and Ca, as determinedby energy- JMA I X35 &58 R 957-959(3) ND dispersionX-ray (EDX) analysis.Roughly 20Voof JMA2 435 19.90 R %1.9!9|.J) ND. JMA3 3.10 n5! I tt44-946cj) 965(l) the inclusionsin the rim are believedto be iron oxides JMA4 5.05 rt39 I ctog,ro, 990) JMB l o0) 3.15 2I2L I p$uz,(4' -!fi-9 '.8t3) JMB 5 1.95 t4.92 rr e390) s7--99t2t s[\4c-126 0l) 3.95 88.45 S lme) un(l) (4) iatrsinpareorbeseshdhatcthcnrmbsd@lytcsmdoto s[\,r67994 02) 5.45 231.68 W r6!8drcj) 9rr-996€) denerninetlrvalue crogc sboun, od'-' irrlicrlisvrhlt rtbkt s\{96982o3) 82A &25 r 961@) 9t2-lmg3) luve malyical onr dcriating fro lm ry >4%. $vt 939&r (la) O.tO 0.9J C m9tr2(3) ND. (0 gC"-groi* = plaw gold, Bnrsh Creek, VA. s[\49739&2 255 1.96 C uG8r8€) NJr. (O \.lt'=notmeund. sMt39&3 3.q) 3,& C &r(t) ,.qc2-93@) o) te-g*it"- paleplrcgold,Lilesvillcqd.,NC. sN,t105364(15) 0.75 352 T rr{20(o ND. (E) 'X.O." - nc rtacaed. (9) 'nr{.l'-gtatr - plag gol4 CheeIwtlb qurd-, vA. Cnim witi "Jlll" prefix wcrcgaerously dcccd by Mr. L R. OA .fMg"-gaiu - placer gold, Sbrct qud., VA. (hsfursrrudngwnh.Slf (ll) I\dcClordof Rpderlc&shr8,VA" rnm Sft,t C-126 = Flag gold (?) frm urloom fotity b NC. l@nfi@ thcunited Srd€sNstiorll uamof Nsnml lilsrysnd (u) S[\,t 6?994 - gpld fro rbc Etdmalo Mno, ftmgmcry Ca, NC thrr ,SI\ll. -. arc identified with LANMNH ^Fnpb mnbcrr follosirg tbo ls well wom ed rorded oggscing oeu tmspon in iu hnnry. Q) vatu aasnined by nm of ofr'cal c*innb. (13) SM 969S2 - plag gold, Silvq Cree&,Cabqnrc Co. NC t', $apeabhuddm: S

gvrY-a rrvr. \ {€$#ewM Electrum / \ core-l

Frc. 5. Schematic illustration depicting the chemical and mechanical alterations experiencedby grains of placer gold as they are transported downitream. The figure in each circle is representativeof a sample's appearancein polished section. The foliowing characteristics are typical for each grain's particular stage of development: (1) unrimmed gold grain in mineraliied quartz vein, (2) ir;;gdarly shapedgrain of placer gold with little or no evidenceof gold- ich rimming, (3) spherical to wafer-shaped grain of placer gold vrith spotty to intense development of gold-rich rimming, (4) flake-shaped grain of placer gold with thick, very well-developed gold-rich rim.

1m0 tm0

Gold contonl 89, 80 8&t tr zF 860 tc 79 gfiEHdr a3fr !T z r L----=,".**l|l 9a0 549 2 t & Sl|vsrconbm 54e = EB A o SllYorcontonl kn 313 km 313

J0 0 J0 05101520 25 0 5 10 15 2A DISTANCElllTo GRAIN (pm) DISTANCEINTO GRAIN (pm) Fro. 6. Illustration of the compositionsof placer gold grains. (a) Back-scatteredelectron (BSE) imageof a portion of the polished section of grain number BC 5-l along with a plot of the changein major-element chemistry determined by;lectron-microprobe analysis along the traverse path indicated by the black-on-white horizontal line on the BSE image.O) BSE imageof a portion of the polishedsection of grain number BC 2-7 along with a plot of the change in riajor-element c[emistry determined by electron-microprobe analysis along the traverse path indicated by the black-on-white horizontal line on the BSE image. 214 THE CANADIAN MINERALOGIST

E a E E E li"t-t -qs oo "t"..."?:' i" :i 'f - Fil i a'l"q'"Ii l"?"fl:"'*oo g sI o Ett Ea Ed -c & Eo Bo".io;(b o o E oo o "i: o! o gE c co o 940 o o Og o c oo c S c ooo g, gD o oo o $ t, o oo o c o CO sO g5m c$ tl oooo o 3 oo E co Esso

o Llne ot equal rlm and core tlnenoss

Oo o clust&smtthto8f E Daboror{hlgm o . Thlgstudy, Brus| C-r€okdaia A Thlssludy,other localltls 7i80r- 500 050 700 750 800 850 900 950 1000 Gore Flneness Flc. 7. Scatterplot of rim versw core compositionsof placer electrumgrains with gold-rich rims from Brush Creek (filled circles),other localitiesin this study (filled triangles),Desborough (1970) (open squares), and Giusti & Smith (1984)(open circles).

and hydroxides, as iron is the only major element oxidizing, Au-bearingstream water as it encounters indicatedby EDX analysis.Placer gold grainsfrom more reducing conditions (cementation),and (3) a the other localities commonly have more mineral self-electrorefiningprocess where the electrum at the inclusionsthan at Brush Creek, but usually show grain-solution interface dissolves, and the gold similar inclusion mineralogy and size distributions immediatelyprecipitates back onto the surfaceof the with respectto rim and core. Inclusionsin the core placer grain. electrum of all the grains from this study seemto be primary, relict phases,as describedby Driscoll Preferential dissolution of silver et al, (1990),whereas those in the rim probably have been injected into the ductile, high-purity gold by The most commonly invoked mechanismfor the mechanical working, or have beenincorporated dur- development of gold-rich rims is a process of process ing a of chemicalgrowth. preferential dissolution of silver from the Au-Ag alloy. The relative solubilitiesof Au and Ag in low- D$CUSSIoN temperaturesolutions are discussedby Mann (1983, 1984)and Xue & Osseo-Asare(1985). These studies Introduction indicatethat a model basedon the preferential dissolution of silveris chemicallyreasonable; however, Several explanations have been proposed to such a model fails to explain how silver atoms from account for gold-rich rimming, which givesrise to deeperthan the outer few Engstrdmsofthe grain can gold grainsin placerdeposits having higher average comeinto contactwith the solution. One of the few finenessthan the grains in their parent lode deposits. researchersto addressthis problem was Desborough The three principal modelsof rim formation consi- (1970),who suggested:"Oxidation of silverfrom the deredhere are: (l) preferentialdissolution of silver metal to the ion reducesits size and affords the from placer grains during weathering and transpon, neededmobility for removal from its site in the alloy, (2) precipitation of gold onto placer grains from and it is thus placedin solution. The porosity result- GOLD-RICH RIMS ON ELECTRUM IN PLACERS 2t5 ing from continued silver removal provides a new 943, respectively).These calculations are basedon interfacein the gold-silver alloy for a repetition of a model of non-steady-statediffusion in a semi- the process." This mechanism,however, is proba- infinite mediumpresented in Darken & Gurry (1953, bly ineffectual,as Fontana (1986) commented on an pp. 441-445).In accordancewith this model's req- analogoussystem wherein the "dezincification" of uisite boundary-conditions,the instantaneousand brass results in a copper-rich coating: "A strong constantconcentration of silver at the surfaceof an argument against [zinc being dissolved, leaving electrumgrain is taken as zero, and the initial con- vacant sitesin the brasslattice structurel is that dezin- centration at the semi-infinite distanceremains fixed cification to appreciabledepths would be impossi at the values stated above. The tracer diffusion ble or extremelyslow becauseof difficulty of diffu- coefficients of silver in Au66Auoa,AurrAu2r and sion of solution and ions through a labyrinth of small AueeAul6al 25"C were extrapolatedfrom the data vacant sites". In addition, during the commercial of Malaro et al. (1963)as 1.36 x 10-32,3.39 x refining of electrum,sufficient porosity for selective 10-32and 1.05x 10-31cm2ls, respectively.The more leachingof silver from electrum is only developed recentdiffusion-coefficient data of Cook & Hilliard through a processknown as "inquartation". Inquar- (1969) were not used even though they may be tation involvesthe dilution of molten natural elec- slightly more accurate,as they do not changeour trum with two to four timesits weight of silver. The conclusions,and also cannot be easily adjusted to needfor inquartation is explainedby Bowdish (1983): any desiredcomposition of electrum, as can those "When alloyed with lessthan about three times its of Mallard et al. (1963). own weight of silver, gold will also protect the sil- Diffusion profiles (Fig. 8) calculatedfor reasona- ver from attackby nitric acidsolutions." Placergold ble periodsof geologictime showthat silverdiffuses with a core furenessas hich as 972has been observed to much too slowly in suchdlloys tq depletesilver from developa distinct rim of 998 fineness(Table l, Fig. more than about the outer 2-3 A of the gold grain. 7); such a small difference in silver content would In fact, the number of years necessaryto produce afford very little increased porosity along which a a gold rim roughly equal in thicknessto natural rims leachingsolution could encroach.Finally, grainswith (-8 pm) (Fig. 8b) is between1017 and l0r8 years high silver contentsin the core electrum do not in (about one million times the age of the universe). generaldevelop a thicker or more prolific rim. Furthermore,this mechanismis incapableof yield- Another mechanismfor the removal of silver from ing the extremelysharp rim-core contact observed the core of an electrumgrain is that of silver diffu- in natural grains. sion from the interior of the grain to the surface, Thus, the only conceivablemeans by which selec- whereit can dissolve.This was testedby modeling tive silver dissolution could produce a significant diffusion profiles formed at 25oC in grains of the gold-rich rim would involve repeated cycles of following compositions: Au66Aga6,Au-rrAg25 and 0ngstrdmJevel leaching of silver followed by AuegAgle(equivalent to finenessesof 733,846 and mechanicaldeformation (leadingto re-exposureof a. eo 733 b. 733 S' s5 772 s 772 o g E30 810 810 o AE at g2s t 846 846 @ frg l! 2 880 880 z i20 G= IIJ 912 =F 912 = H1sz rri IL 943 o 943 8r0 I rD5 972 rD 972 0 I OOO 1000 0 4 I 12 16 0 4 I 12 16 DISTANCEINTO GRAIN (A) DISTANCElNTo GRAIN (Pm)

Frc. 8. Observed natural gradients of silver and calculated diffusion profiles of silver in gold. (a) Calculated diffusion profiles of silver in Au75Ag25for the geologicallyreasonable time periodsof l, 10, 50, 300, 1@0,and 3000Ma Oeavy black lines 1-6, respectively)and the diffusion profiles of silver in Au60.Aga0and Au96Ag1safter 300 Ma (fine lines7 and 8, respectively).(b) Observednatural gradient in silver concentrationin grain number BC 5-1, as determinedby electron-microprobeanalyses along a traverse(heaviest black line, #l); calculateddiffusion- profiles of silver in Au75Ag25for time periods of 1014,1015, 1016, 1017, and 1018years, necessaryto simulatethe natural gradientin Ag conceritration(heavy black lines 2-6, respectively);and, for comparisonof the effect of composition, the diffusion profiles of silver in Au66Agaaand Aue6Aglsafter 1016years (fine lines7 and 8, respectively). 216 THE CANADIAN MINERALOGIST

TABLE 2. DISSOLUTION REACNIONS FOR METALLICGOLD

LIGAND AG'q316yO) REACTION logK Sc(2) Ref.(3) TRIVALENT GOLDCOMPLEIGS

NOLIGAI{I}S

N.A"(4) 433.5 Au(s)+ 0.7502a ltt+ = ds3+ a t.lil{p -11.47 NJ|" FL

ONEIJGAI\D

ct- 239.3 Au(s) + 0.7502 1!tI* a Q[- = trgQl2f + l.5H2O {.45 u.E3 BL oH- 184.9 Au(s) + 0.7502 + 2H+ = tuOF2+ + 05H2O -9.{I NJ{" BL

TWOI,IGAIII'S

cl- 1OJ Au(s)+ 0.7502+ 3H+ + 2Cl- = AuCl2++ l.5H2O 6.10 Lg BL SO42- [email protected] Au(s)+ 0.7502+ 3H+ + 2SOa2-= Au(SOa)z-+ l.5Hz0 -537 8.38 MFL oH- {3.6 Au(s)+0J5O2 +H+ +05H20 = Au(OtDz+ :1.{I N.A" BL

TEREELIGAI.IDS

ct- -83.3 Au(s)+ 0.7502+ 3H+ + 3Cl- = AuCtg(rq)+ l.5H2O 10.08 0.43 BL -148.4 oH-,cl- Au(s)+0.7502 +2H*+2Cl- = A@HCl2(sd+0.tH2O \95 t.22 AL oH-,FA2- *-(t Au(s)+0.7502+H+ +05H2O+FA2-= A[(O[I)ZFA- 1.33 -1.94 KBL oH-,cl- -2t7.3 Au(s)+0.7502 +H+ +05HZO+Cl- = Au(OHhgl(aq) :354 L93 AL oH- -2C4.4 Au(s)+ 0.?5OZ+ l5H2O = Ar(OI{h(sd -10.51 N-A L trIOURLIGAIYI}S

cN- t7L4 Au(s)+0.7502 +3H++4CN-= Au(CX\O4-+ l.5H2O @.53 -t4.54 JL NH3 -11.92 Au(s)+ 0.7502+ 3H* + 4NH3= Ag(NH3)a3+.r 1.511rp q.98 -9.15 rL r -45.0 Au(s)+ 0.75O2+ 3H* + 4I- = AuI4-+ l5H2O 36.2t 42t HL scN- 561.6 Au(s)+0.7502 +3H*+,$CN-=Au(SCN)4- + l5H2O 31.05 4.92 L Br -161.3 Au(s)+ 0.?502+ 3H+ + 4Br = AtEr4- + l5H2O 20.92 -238 L ct- -23s.1 Au(!) + 0.7502+ 3H+ + 4Cl- = AuCt4-+ 15H2O '3,A {5E L oH-, cl- -293.6 Au(s)+0.?5OZ +2H* +,cl- = AuOHC!- + 05H2O 5.38 0.m AL OH-,Br -333.7 Au(s)+0.7502 +H+ +0-5H20+2Br =Au(OI!ZBr2- 3.42 -2.4 cL oH-, cl- -349.7 Au(s)+ 0.7502+ H+ + 05H20 + 2Cl- = Au(OH)zClz- -335 1.36 AL oH-,cl- ,403.7 Au(s)+ 0.7502+ l5H2O + Cl- = Au(OE)rCl- -1L42 5.81 AL oH- 455.4 Au(s)+ 0.7502+25H2O = Au(Qt{)a-a t{+ -21.91 N.A" L OH-.Br- -251.9 Au(s)+ 0.75C)2+ 15H2O+ Br- = Au(OH)gBr -34.23 n.6 cL

FTYELIGAI\DS

scN- 654.5 Au(s)+ 0.7502+ 3H+ + 5SCN-= Au(SCII)S2-+ ISHZO 31.0t -3.93 L oH- {16.5 Au(s)+0.7502 +35H2O = Au(OI{)*'+Ztt+ a5.24 NA" L

SIxLIGAT{I}S scN- 747.O Au(s)+ 0.7502+ 3H* + 6SCN-= Au(SChf63-a 151119 31.05 a28 L oH- :163.2 Au(s)+0.7502 +45H2O = A{0I{)63- +3H+ -51.08 N.A" L GOLD.RICH RIMS ON ELECTRUM IN PLACERS 2I7

TABLE 2(contrd). DISSOLUTIONREACTIONS FOR METALLICGOLD

LIGAND Acof(cnx)(r) REACTION logK SC(2) Ref.(3) MONOVALEM GOLD COMPLEXES

NOIIGAIIIIS :1.t7 N/4"(4) 163.6 Au(s)+025021t{* = 6rr* 4-05H2O N.A"

ONEIIGAI{D -2L22 S2- 21,6 Au(s)+ 02502+ [IS- = AuS- + 05H2O t9.82 MEL -l9:n HS- 35.6 Au(s)+ 02502 + H+ + IIS- = AuIIS(aq)+ 0.5Hp n.n GL sq2- -3933 Au(s)+ 025021 l{* 1 $Q32-= AuSq- + 05Hp 5.16 :156 BL -5.65 SZOI2- 418.4 Au(s)+ 0.2502+ H+ + SZOI2-= AuSZO:-+ 05HZO 3.25 BL cl- 5.0 Au(s)+ 02502 + H+ + Cl- = ArC\eq) + 05H2O -238 4.02 BL oH- -54.4 .tr(s) +0.2502+ 0.5H2O= Au(OHXss) -10.52 N.A" BL

TWOLIGANIIS -17.1 cN- 2S5.8 Au(s)+ 0.2502+ H* + 2Cl{- = Ar(CN)2- + 0.5H2O 31.82 I L -15.16 CS(NH/2 -----(6) Au(s)+02502+H++2CS(NHdz=AulCS(NHz)elz++OSHZO Tl-91 HL HS- LSJ Au(s)+ 02502+ H* +2IIS- = Au(HS)2-+ 05H2O 22'n -1268 GL -ll.0l Sq2- -9623 Au(s)+0.2502+ H+ +2SOg2-=Au(SOrlf- +0.5H2O 19.61 BL -10.65 SzQ2- -1030.2 Ar(s) + 0.2502+ r$ + 2SZOr2-= Au(S2O3)i3-+ 05H2O 18.90 J L :1.19 NH3 13 Au(s)+0.25O2+H*+2NH3=Au(NH3b+1g5H2O ll.yl IL -7'08 I- 47.6 Au(s)+ 0.2502+H* + 2I- = AuIt +0.5H2O 1176 HL SCN- 251.9 Au(s)+0.25O2+H++2SCN-= An(SCND- +05H2O 9'E4 4'12 L -3'81 Br -115.0 Au(s)+0.2502+ H+ + 2Br= AuBr2-+O.5H2O 5'21 L -2'19 Cl- -151.1 Au(s)+ 0.2502+ H* + 2Cl-= AuClt + 05H2O l'9E L OH-,Br -lgg.g Au(s)+ 0.2502+ 0.5H2O+ Br = AuOlIBr -3'25 -5'15 DL -3'05 OH-, Cl- -2152 Au(s)+ 0.2502+ 0.5H2O+ Cl- = AuOHCI- -5'35 DL OH- -275.4 Au(s)+ 0.5O2 + 1.5H2O= Au(OH)Z-+ H* -13'34 N'A" DL

DI4OIJ)OOMPLEXES

HS-.52- 2L.O 2Au(s)+ 0502 + H* + 3HS-= AuZGIS}9- + HZO 45.& -15.72 MEL -17.13 52- 116.8 2AuG)+ 0JO2+ 2HS-= Au2S22-+H2O 2I.35 GL

(2) 5"=5fimgrhofcomple,x=thelogarirbmoftheactivityoftheligmdinequili-baiumwiththecorespmding aqueousgold-complex solutim (P(O/ = 0.2 atm. pH = 6), yielding 0.1-ppb of gold' (3) References:A - Baes& Mesmer1974 B - Baranova& Ryzhenko1981, C - Bjemm 19/1, D'IUPAC 1982' E-Johnsonetal. 1978,F-Irtimer1952,c-Renders&Seward 1989,H-Scbmid1985, I-SkibsFd& Bjernrm1974,J-*ibsted&Bjemrm l9n,K-"VarihaletaL l9t4,L-WagmanetaL 1982,M-Webster 1986. (4) N.a" = not applicable. (5) FA = Fulvic acid - stoichiometry unlno*r, therefore AGgundeteminable. (6) No AGf availablefcthe coinplexorfree ligand. fresh electrum). This mechanism is, however, in no Cementqtion way supported by the textures of the observed surfaces. Despite its chemical inertness, gold is transported 2t8 THE CANADIAN MINERALOGIST in significant quantitiesas aqueouscomplexes at high concentrations of gold associatedwith recently temperaturesto form hydrothermal deposits.At low depositediron hydroxides (Lesure 1971). temperatures,gold is much lesssoluble, yet can be At least two detailed accounts suggestthat there complexedby a host of ligands (Table 2). Depend- has beensecondary enrichment of gold in deposits ing on the concentrations of such ligands, and in the southeasternUnited States. Lesure (1971) ambient conditions of Eh and pH, severalof the reported on secondaryenrichment at the Calhoun ligandsappe€u to be capableoftransporting signifi- mine in Lumpkin County, Georgia, where significant amounts of gold in the supergeneenvironment. cant concentrationsofgold (2.9 ppm) occur in fresh The following observationshave beenoffered as evi- limonite coatingsof the walls of an adit. Cook (1987) dence for low-temperature aqueous transport of compiled a seriesof historical accountsof gold mines gold: (l) placer gold grains with smooth, well-formed of Georgia and Alabama, which document large crystalfaces (Warren 1982,Crug&Callahan 1990), accumulationsof coarsegold in quartz veinsjust at (2) high-purity gold crystalsand dendriteson iron the water table. Thesesame veins commonly show oxidesin laterites(Mann 1984,Freyssinel et al. L9g9), a significant decreasein particle size and grade of (3) gold impregnationsof originally carbonaceous gold below the water table. matter (Wilson 1984),and (4) secondarilyenriched The calculatedequilibrium solubility of gold in

a. 1,970 = g o 1e7 A E --t- tr tr o -10.0 15.7 I tr]b g 6 Au(oH); g tr o o -11.0 1.97 (, cC' """"ubT o 0 o C) -12n / 0.197 = 3 Au(OH)n -reo B Au(OH)r' 0.0197

'14$ ptl

b. SEAWATER MINEDRAINAGE € 197,000 g g ;l o : I 9,700 E € i .- .-i c 1,970 3 I j'j -!l -9 tr 6 t' 'l 197 -9 c i,, I i-. a6 o -10 t () 19.7 tr E 'il 'i.'; o 0 .11 : 3' C) 1.97 c t o :' o -12 n : n i,.0.197 = ct) s ,( o -13 rt'l FO n S - ao o @ 0.0197 NO b6N- ts € oo ooo; oo o .14 3 GOLD-RICH RIMS ON ELECTRUM IN PLACERS 2t9

c. GROTJND SURFACEWATER -8 WATER 197,000 = 6 -7 19,700 6 l= E l.ro d € .' ' '--.1I 1,970 3 gc lrli. -9 lsz 6 t[ ittil., E l't'i-ilfflil.; |E c o -10 ""';tile.z o to E tr " (,o o ll,l " l:9 -11 1.97 C o 'l[i rrt o :t C' -'12 0.197 5 ED o I 0.0197 .13 oa . No o F @ @N Fo @N o la@ F @ @ oo tso @o o loo o o o oo oo

Frc. 9. Gold concentration plots. (a) Log-O.Zl Au concentration (molality and ppt) verszspH for gold-hydroxide complexes gold and free gold ions twhere]D(dj : in the absenceof other ligands. The predominant univalent species (indicatedby the upper t*o UoiOcontreiting straight-linesegments) fro_g.l-ow to high pH are Aur- and Au(OH);, with the concentrationof the neutral AuOA' complexconstant a1 lQ-r1'426.The predominanttrivalent gold species(indicated by the lower four, bold, connecting,straightJine segments) from low to high pH include:Au(OH)2+, Au(OH)i, eu(Oila, and Au(OHft. Plotted circles representmeasured natural gold concentrationsof stream waters' wtrereasiriangtes representgrou.tdiuaters, and squaresiepresent mine waters; open symbolsare for-data Yt-tichincluded specific.on.Jrrtrutioo-pffiairs, whereassolid symboli and their associatedrange bars are for data which specified only t*g"r of pH and ioncentration. (b) Log Au concentration (molality and ppt) versustheyear of d€termination for seawiter and mine drainage; dates'along bottom axis indicate the first, last, and several intermediate dates for the correspondingAu-concentration determinations. Symbols used to plot concentrationdata are as follows: small open circle: singleconcentration value, Iarge solid circle: averageconcentration'.lalue for.a.number of analyses' upper xj fimit of deteclion,1("x" with solid lower quadrant): sample(s)with no gold detected,X ("x" with solid quadrant): sample(si$ith a trace of gold detected,solid bar: range of reported gold concenlrations,-dashedbar: .'*g. *ittt no lowei'limit specified,numerals above bars, open circles,and sotid circles:number of individual ana- lysei within the set. Each set of concentrations generally representsanalyses from a particular locality, except for severalof the seawaterdata sets, and some gfouped setsof individual analyses.(c) Log Au concentration (molality and ppt) versasthe year of determination for suiface water and groundwater. Conventions in part (c) are the same probably of improved as foipart O). Reportedvalues of gold concentrationhave declinedover time, as aresult analyd;al te;hniques and the greateicare taken in preconcentration stepsto fully filter the solutions to remove fine particulate mattei and colloidil gold. It should be noted that virtually all of the measuremeutsshown on this figure weremade on samplesfrom regiins wheregold depositsoccur, or wheregold wasbeing exploredfor and expected to occur (exceptofcou.se, for ieawater samples).Data for Figures9a-c weretaken from the following sourcesand tabulationswithin them: Borovitskiy e/ a/. 1966,Brooks et al. 1981,Chernyayev et al. 1969,Crocket 1978'Goleva et al. 1970,Gosling et al. lg7l,Hallet al. 1986,Hamilton et al. 1983,Jones 1970,Kaspar et al. l91,2,,Koideet a/. 1988.Lomonosov et d/. t985, McHugh 1988,Turekian 1968,Turner & Ikramuddin 1982,Volkov & Shakhbazova t975.

pure water as a function of pH is shownby the bold therefore,may or may not be important in account- lines in Figure 9a, constructedfor P(O) = 0.2 atm. ing for the total dissolvedcontents of gold in natural At the near-neutral pH values found in most stream systems,depending upon the relative influences and waters, the equilibrium concentrationof gold ions contributionsof the other gold-complexingligands. and hydroxy complexeswould be expectedto range Table 2 lists the transporting efficacy of a wide from -0.04 ppb at pH 4.5 to - 12ppb at pH 7.0. variety of naturally occurring gold-complexing These values are considerably higher than the ones ligands. The complexingstrength (Ss) of the vari- reported in the literatwe for various fypes of natural ous gold complexesis defined as the logarithm of solutions(plotted points and rangesin Fig. 9), prob- the activity of the ligand in equilibrium with metal- ably as a result of slow kinetics of dissolution or, lic gold and a correspondinggold-complexed solu- more likely, insufficient interaction of solution and tion [pH : 6.0, P(Ot : 0.2 atm.] vielding0.1 ppb gold-bearing sediment. Gold-hydroxy complexes, gold. Note that an 56 value refersto the concentra- 220 THE CANADIAN MINERALOCIST

tion of a free ligand remainingafter gold complexa- natural waters to produce significant transport of tion; it doesnot refer to the total concentrationof gold. The averageconcentration of eachligand form- the ligand. The smaller the value of Ss, therefore, ing a complex with an S" value from Table 2 of the more effectivethe ligand is at complexinggold. -1.65 or more was estimatedfor streamwaters and A concentrationof dissolvedgold of 0.1 ppb was slow-movingsediment solutions from literature data. chosen for the Sq parameter, as it representsthe Thesedata werethen usedin combination with the middle to upper range of recent determinationsof equilibrium constantsgiven in Table 2 to calculate the dissolvedgold content ofvarious natural environ- the expectedconcentrations of the gold complexes ments (Figs. 9a-c). This concentrationis assumed in equilibrium with elementalgold (Table 3). The to be a reasonableestimate of the minimum value concentrationsof gold complexesgiven in Table 3 at which sufficient transport occurs to yield the were calculatedusing both mass-actionand mass- products commonly ascribedto secondaryenrich- balanceconsiderations. The pH value of 6.0 usedin ment of gold (Lesurel97l). calculationsfor both streamwater and sedimentsolu- .,yard- The value of Sq can thus be used as a tion (Table 3) was estimatedfrom the extensivecom- stick" to eliminatefrom considerationall comDlex- pilation of field measurementsby Baas Beckingel ing agentsthat could in no way be capableof produc- ol. (1960).The activity of dissolvedoxygen used for ing significantsupergene transport of gold. Ligands the stream water in calculations in Table 3 is that with valuesof Sq greaterthan -1.65 (: 100 x the for air-water equilibrium. The value of a(O) in averageconcentration ofthe most abundantnatural organic-matter-richsediment solutions was estimated ligand, chlorine) are eliminated from further cou- to be 2.51x 10-6e(Barnes 1979),by assumingthat siderationbecause such ligand concentrationsare vir- the solution exists at the H2S-SO?-boundary at DS tually never observedin typical, fresh surface waters = 0.001 m and 25oC. Stream sedimentstend to or shallow groundwaters,where gold grains with developa sharp redox boundary where HrS exists gold-rich rims are frequently found. Gold-hydroxy just below the streambed - streamwater interface, complexes were chosen for further consideration whereasSO!- predominatesin the stream water. if they are capableof yielding concentrationsgreater The CN- concentration used for stream waters in than I x L}-tt m (1.97 ppt) at normal valuesof pH Table 3 (3.84x l0-8 m or 1.0 pg/L) was estimated for streams,since the 56 parameterdoes not apply from the valuesgiven by Krutz (1981),who meas- to these complexes. Because solutions in these ured the.yearlyvariation in cyanidecontent of the environments are normally quite dilute, all activity Wehebach Brook, which drains a wooded and coefficientswere assumedto equal one. Severalof pasturedarea in Germany.I3utz (1981)also stated the ligandslisted in Table 2 are metastablein solu- that "The sedimentfraction with a particle diameter tions with an Eh buffered by equilibrium with lessthan I mm contains about 0.2 me/kg of total P(O, = 0.2 atm, but sincetheir metastablepersis- cyanide, only a small amount of it (0.04 mglkg) tence is not quantitatively known, they have been being easily set free." Accordingly, the CN- con- ,,S"', assumedto be completelystable for the con- centrationused in Table 3 for streamsediments was sideration.The choicesofpH and Eh are consistent 40 p.e/L (1.54x 10-6m). with valuescommonly found in fresh surface waters Various studies indicate a wide variety of natural @aasBecking et al,1960). As can be seenfrom Table sourcesof cyanide,including certain plants (Lakin 2, 28 of the gold complexeshave 56 values that et al.1974,Hughes 1981, Leduc 1981,'Warren 1982), indicate they are at least strong enoughto warrant and algae (Venneslandet al. l98l). Seigler(1981) a more thorough evaluation. The only ther- describedsome of the natural sourcesof cyanogenic modynamic data available for complex organic glycosidesand cyanolipids, which upon hydrolysis ligands that might dissolvegold pertain to fulvic acid liberate hydrogen cyanide; they include apricot, (Table2), and althoughthe conceptofgold dissolu- peach,and almond trees,as well as other important tion by micro-organisms and complex organic food plants, ferns, gymnosperms, monoco- moleculeshas beensubstantiated by various sources tyledonousand dicotyledonousangiosperms, fungi, (Lakin et al. 1974, Mineyev 1976), sparse ther- bacteria,and severalinsects. Altogether, cyanideis modynamic data preventa further quantitative evalu- produced by more than 2050 plant species.Krutz ation of them. Colloidal and very fine particulate (1981)noted that man-madesources of cyanide,such gold in natural waters also is possible,but tlte resulrs as steel and galvanic industries, fertilizers, and of workers who have considered this problem domesticsewage, can elevatethe cyanidecontent of (Golevaeta|.1970, Gosling et al. 1971,Lomonosov streams to levels as high as 200 pg/L, The same et al. 1985) are not in agreementover their impor- respectiveestimates were usedfor the concentrations tance. Clearly, there is a greatneed for further work of SCN- as for CN- in Table 3, as it was assumed in this field. that cyanide would be present in similar concantra- Of the ligands that can form strong Au complexes, tions whetheror not the solutionswere sufficiently only a few occur at sufficient concentrations in sulfur-rich to convert the CN- to SCN-. GOLD.RICH RIMS ON ELECTRUM IN PLACERS 221

TABLE 3. PREDICTED GOLD.COMPLEX CONCENTRATIONS

Au-corytx(r) hgttlgliwe) hgtcmlr.(3) lt ttUtLs(O bgtorrl"r(9 Ref.(O TRIVALENT GOLD COMPLEXES

FOIIR,LrcANI}S

A(cNL- :1.42 -8.02 -5.E1 -8.15 DFA Au(NH3ia3+ -851 -9.11 {51 -55.49 BGA AuI+- :1.42 -t4.14 :1.42 -6L91 CA A(oIIbB12 -E.00,-5.35 -fl.96 {.m,6.35 -66:13 CA Au(SCN)a- :1.42 -$a -5.81 -61.64 DA AuBr4- {.35 -x.15 {.35 :t3.93 CA

FTgD,LIGN{I}S Au(SCPrz- 4.4 -x:14 -5.81 -67.49

SIXLIGANI}S

Au(SCN)63- -7t25

MONOVALENT COLD COMPLEXES ..--...-.-

ONELIGA}ID

AuS- --(r) :1,99 -tt.32 EFA AuIIS(q) :l.gg -t3rl EFA AUSOr: -t325 -3t.4 EFA AuS2O3- -13.$ -3tcl EFA

TIVOLIGANIIS

A'{cNb- :1.42 :732 -5.Et -6.ll CEA Au(OII)2- {.00 -823 -E.(x) -24.49 A AuOIICI- {.00, -3.65 -9.89 {.00, -3.55 -26.15 c Au12- :7.42 -9.n :7.42 -mB CA AuOHBf -E.00-6.35 -1050 -8.00,5.35 -m75 CA Au(SCN)2- :1.42 -11.88 -5.81 -2l,.14 DA Au(NHfir -851 -11.93 -851 -25.19 BGA AuCt2- -3.65 -tLzl -3.65 -8.47 CA AuB12- 4.35 -30.64 CA Au(HS)2-^ :1.9 -1616 EFA Au(SO!2r- = :, -r3,25 -30.04 BFA Au(S2Oj25- -t3.47 -31.19 EFA

DI4OLI' COMILEXES

Au2@S)2S2- :l.gg -lE.62 BFA Au2S2z- :L99 -2tl.3 BFA

(l) Cmentalim daa is not pedicrcd fc AI{OII)2FA- or Au[CS(NH)212+, even though their "Sg" prmctfrr frm Table 2regreaerthan -1.65, becausecdc€ntratim,{rtafothe ligmds inneal systrrns aerptavailable. a) logtliglsw = log activity (* cmemado) of tbe ligd in averagestrem wrt€r& cl) log[onx]r* = log activity (- e6s€n6alirl of the Au-coqrlex in averagecrean waters. (4) logtltglrs = bg activity (- cmcentatim) of lbe ligard in avemgeredrcing solutios in agmicomer*ich sediment& (5) logtcrtxlrc = log activity (= cmc.) of the Au-coplex in averageredrcing solutims in ugmic-moer-rich sedims. (o References: A - Bam l9q B - F€rh 1966: c - Fuge l978a,b,c; D - Knnz l98l; E - Psr l98l; F-Wagmmetal. l9ee C-mcrzka 198. CI Redrcedsulinspecieseenor$ableina€ra[odstremwrt€rs. Althoght@rnaybereleasedfrmcgmic-rnd€r-rich sedin€nts ed persist rretasubly in tte strean waters, lhere ue ro repqts of measrable cfic€otratims of S2-, HS-, SO3z-, or StOrz' in urpolftned srem waers. 222 THE CANADIAN MINERALOGIST

The NH3 concentration used in Table 3 was appropriate Eh and pH conditions of streamsedi- determinedusing a pK value of -9.25 for the reac- ments. The HrS concentrationvalue usedfor these tion NHf, - NH: * H*, and by assumingthat the calculationswas taken from Pan (1981),who meas- "ammonia" concentrations of natural waters ured the concentrationof this speciesin a neutral (repofied inconsistently as either NH, or NHf) paddy soil that wasadjusted to different pH values. actually representsthe total of both species.The The reportedconcentrations are likely to be closeto number of measurednatural concentrationsis not those found in water-loggedstream sediments con- large,but a rangeof -5 ppb to -5,000 ppb appears taining some organic matter. This approach is likely to be common for ocean water, river water, rain to underestimatethe concentrationsof the sulfoxy water, and groundwater. River water and ground- ligands, as thesespecies are formed by partial oxi- water tend to have concentrationsin the middle to dation of HrS and may persist metastably at higher upper portion of this range, but as a conservative concentrations (Giggenbach 1974, Hoffman 1977). estimate,100 ppb was chosenas the total ammonia The discovery of numerous small crystals of copper concentration,yielding an activity of NH3 for use sulfide growing on the surfacesof recentcoins found in Table 3 equalto 3.12x l}e m. while panning in Brush Creek subslantiates the The concentrationsof the HS-, S2Of, and SOr2- presenceof significant amounts of sulfide in these ligands in sediment solutions were calculated by stream sediments (Groen et al. 1986). The crystals assuming chemical equilibrium with H2S for the have two basic types of morphology, roughly

3 L2'

I e' + 0.2502+ 05H2O>OH'

Difrrion Conoding Dewbping Boundary Bulk Eecrum Gol4Rich Rim |.apr Solulion Ftc. 10. Self-electrorefining model for the formation of a gold-rich rim. (a) Schematic illustration of the self- electrorefining process occurring on the surface of a placer electrum grain entailing the following steps (see cor- responding small circled numerals on diagram): (l) transport of Au- and Ag-complexing ligands (LzJ to the cor- roding surface of the electrum, (2) dissolution of the electrum by the complexing agents, with coincident liberation of three electrons(e-), (3) loss of the more solubleAg-complex to the bulk solution, (4) dissociationof the Au- complex by reaction with the free electrons at the pure gold surface bringing about the precipitation of pure gold, (5) consumption of extra free electrons through reduction of O2 and H2O to OH- (diagram patterned after Fig. I of Strickland& Lawson 1973),(b) Back-scatteredelectron image of a potshed placergold grain from Brush Creek showing a gold-rich rim (brightest, high-electron-yield areas), with textural features nearly identical to those seen in the schematic of part (a). GOLD.RICH RIMS ON ELECTRUM IN PLACERS 223 orthogonal prisms and trigonal to hexagonalplates, Au(CN)! + Fd+ + 2H2O : the latter being more common. The hexagonalplates Auo+FeOOH+2HCN+H+. are believedto be covellite, and the orthogonal crystals, chalcocite.Based on the log a(S) versuslog Strongevidence supporting this type of processis a(O) diagram for copper (Garrels & Christ 1965,p. provided by the common occurrenceof iron hydrox- l@), covellite is only stable at log a(S) greater than ide coatings on many of the recoveredplacer grains, about -21.0 [og a(S2) = -33.2]. Theseestimates and the presenceof iron hydroxideinclusions in the are consistentwith those given by Pan (1981). gold-rich rims, as noted above.There are also numer- The concentrationsof the gold complexesare ous reports of the common and intimate association generallymuch higher for streamwaters than for the of the two phases@esborough 1970, Lesure 1971' sedimentsolutions, because the more oxidizing con- Mann 1984,Wilson 1984,Dilabio et ol. L985,Freys' ditions in the stream waters promote oxidation of sinetet a|.1989). A precipitativeorigin for gold-rich elementalgold to gold ions. The most likely com- rims is furthermore supported by the fact that the plexesfor significant transport ofgold in oxidizing textures observed on natural grains are commonly stream waters indicated by Table 3 are Au(CN)j, indistinguishable from those texture$resulting from Au(OlI)j, Au(NH3)l+, AuOHCf, AuOHBT- and the hydrometallurgical processof cementation (Fig. Au(I)j, whereasin the stream-sedimentsolutions 10). Strickland& Lawson (1973)described "cemen- Au(CN)! and Au(HS)z are potentially important, tation" as: "...the electrochemicalprecipitation of although Au(CN)! is by far predominant. These a metal, usually from an aqueoussolution of its salts, complexesare the main ones likely to yield concen- by a more electropositivemetal." In the presentcase, trations of dissolvedgold in typical fresh waters,near however, the cementationprocess is driven by the or abovethe upper limit of natural measuredcon- oxidation of ferrous iron, rather than by the oxida- centrations(Fig. 9). tion of a solid metal substrate. Table 3 indicates that cyanide is by far the most effective complexing agent of gold in both oxidiz- Se(-electrorefining ing and reducing conditions; unfortunately, the small number of measuredCN- concentrations in natural Electrorefining is a hydrometa[urgical processin watersmakes it difficult to obtain a reliable estimate which a multicomponent alloy is electrochemically of its averageconcentration. dissolved, with the subsequentprecipitation of a Thiosulfate has been credited as a likely trans- generally pure phase of the most noble dissolved porter of gold in acidic, oxidizing sulfide ore deposits metal. The driving force for this processis an elec- (Golevaet al.1974, Stoffregen1986). Table 3 indi- trochemical potential. This potential can be brought cates that this ligand is not especially important in about in two ways: (1) with a battery, (2) as a result normal stream waters, however, where total sulfur of the electromotiveforce @MF) betweentwo dis- activity is considerablylower and Eh-pH conditions similar metalsin a solution whoseEh is higherthan are markedly different. that in which the starting alloy is stable. Our con- Precipitation of gold complexesfrom solution cerns are with natural systems,where batteries are takes place by the reduction of the Aul* or Au3* not involved, and so the term "self-electrorefining" ions asthey encountersolutions with a lower Eh. A will hereafter be used to refer to electrorefining steepredox boundary existsat the interface betwpen brought about by the secondprocess listed above. stream waters and organic-matter-rich stream sedi- Fontana's (1986)description of the dezincification ments. In Brush Creek, for example,crystals of cop- corrosion proce$sshows that the dissolutionof the per sulfide grew on coin surfacesonly one or two brassrequires oxidizing solution conditions and that centimeters below oxidizing stream waters. This the EMF formed by the copper-brasscouple causes redox boundary is a very likely site of gold precipi- pure copper to precipitate. According to Fontana tation. Sharp redox boundaries also occur at the (1980: "The commonly acceptedmechanism con- water table, and the largegold nuggetsfound down sistsof threesiep$, as follows: (l) the brassdissolves, to, but not below, the water table in the southeastern Q) the zinc ions stay in solution, and (3) the copper United States(Cook 1987)may havegrown at such platesback on." For our concernsthen, dezincifi- a redox boundary. cation is a form of self-electrorefining,even though The precipitationof aqueousgold complexeswas it involvesman-made materials, because it proceeds examinedby Mann (1984)and Stoffregen(1980. The without an external source of power. precipitationmechanism suggested by Mann (1984) Mann (1984)performed electrochemicalexperi- usesFd+as a reducing agent. The Fd* ion, liber- ments, which showedthat in 0.03 la HCI - I ru NaCl ated from weathering rocks, migrates in solution solutions, a distinct increasein Au-Ag alloy stabil- until it encountersand releasesits available electron ity occurs with increasing fineness as measured to a gold complex. This reaction simultaneously againsta pure gold electrode,demonstrating lower precipitatesnative gold and ferric hydroxide: reduction potentials for lower fineness alloys. 224 THE CANADIAN MINERALOGIST

Mann's (1984)results with syntheticalloys indicate between fine, organic-matter-rich sediment and that placer electrum grains can oxidize, releasingdis- oxidizing streamwater, or at the water table). Where solvedAg and Au into solution @g. l0a). Oncein gold precipitation resulting from an Eh gradient is solution, the lesselectropositive (and now isolated the dominantprocess, it is possiblethat the gold-rich or "refined") dissolvedAu speciesaccepts the avail- rim is an incipient stageof a large "grown" nuggeq able electronsat the gold-rich areason the electrum thus gold grains that happen to reside in a soil surface,and immediatelyprecipitates to yield the tex- horizon at the water table, for example,might grow tures describedbelow. The more easilyoxidized sil- to the remarkable and enigmatic sizesreported from ver is washedaway. The resulting electrolyticreac- various areas in the southeasternUnited States tion is self-perpetuatingas a result of the EMF (Crayon 1857).The validity of this idea could be betweenthe gold-rich areasand the unrefined elec- tested by analysis of large gold nuggets. If they trum. This processdoes not require a sharpEh gra- formed by an overgrowth process,the finenessof dient to precipitatea gold-rich rim, just an ambient the outer portion of suchgrains should be very close solution that is oxidizing enoughto initiate dissolu- ro 1000. tion of the primary electrum.This processwould be acceleratedby the presenceof ligands such aSCN- CoNcrusroNs , OH-, NH3, Cl-, I- Br, or HS-; as they increasethe solubility of the electrum components.Rudimentary Gold grainsundergo both physicaland chemical forms of this idea were suggestedmore than 50 years changesas they are transportedby streams.Newly ago by Fisher(1935). liberatedgold grainsare usually irregularly shaped, The electrorefiningprocess frequently produces and as a result of progressivestream transport they surfacesbearing lobate protrusions of the deposited become semispherical,wafer-shaped, and finally metal (crystallinegrowths also havebeen reported). flake-shaped.Their surfacesevolve from smooth and Theseprotrusions form becausethe surface of the clean to pitted, hackly, and eventually,to lobate- more electropositive metal must maintain sitesthat textured, although variations in the stream'scom- remain exposedto the aqueousmedium so that dis- position, energyand sedimenttype somewhatmodify solution and subsequentcharge-transfer c:rn occur; this generaltrend. Grain sizedecreases with increas- hence"channelways" developbetween the growing ing distanceof transport. metal deposits on the surface @ig. 10). The During transport, many grains of placer gold branching-coraltexture of Figure 2 and the magni- developan outer rim of nearly pure gold on the more fied view of the lobate gold-rich rimming of Figure silver-rich electrumcore. The rim has a very sharp 4a (shown in Fig. 10b)are strikingly similar to the contact with the core, and appearsto be the result schematicrepresentations of cementationtgxtures in of electrochemicalprocesses active in the streamor Strickland& Lawson (1973)(after which Fig. lOa is stream sediments(or both). The rim generally is patterned),and suggestthat theserims wereformed thickest on flake-shaped(most transported)grains by precipitation from an electrorefining process.The and thinnest or absenton irregular (east transported) sizeof the protrusionsand the ratio of gold-coated grains. surfacearea to surfacearea of the dissolvingelec- Three proposed mechanismsof gold-rich rim trum core are highly variable on natural placer developmentwere evaluatedon the basisof obser- grains, as would be expectedfor grains of differing vational evidenceand theoretical calculations. Selec- finenessof the core electrumin solutions of differ- tive leachingof silver from the margin of gold grains ing ambient composition(yielding variable reaction appear$to be impossible,because there is no means rates and availabilities of gold for precipitation). We by which the solutionscan physicallyextract the sil- do believe,however, that the samebasic processes ver from more than the outer few 0ngstrdmsof the are responsible for the range of microtextures grain. Diffusion of silver through the electrum to observedin the gold-rich rims. make the silver availableto solution cannot aid this Gold-rich rims and their associatedsurface tex- process,owing to the extremelyslow ratesof diffu- tures appear to be the only likely products of sion at low temperatures.Gold dissolution, trans- extendedexposure of electrumgrains to streamand port, and cementationare stepsin a viable process sedimentwaters. Gold-rich rims are very common of growth of the gold-rich rims. There are sufficient on placergold grains from the streamsthroughout amounts of CN-, OH-, NH3, Cl-, I- Br-, and HS- the presentstudy area, as well as from other locali- in averagestream and sedimentsolutions to account ties that we, and other investigators@esborough for the highest gold concentrations reported for 1970,Giusti & Smith 1984,Freyssinet et al. 1989), streams.Several sulfur-bearing ligands are poten- have sampled.These rims appearto form by elec- tially capable of significant transport of gold, trorefining where sharp Eh gradients do not exist (in provided they can maintain their concentrationsin flowing stream water), and by cementationwhere oxidizing waters through metastablepersistence. sharp Eh gradients do exist (e.9., the interface Someorganic complexesalso show potential for gold COLD-RICH RIMS ON ELECTRUM IN PLACERS 225 transpofi; however,there simply are not enoughdata BoworsH, F.W. (1983): Fire assayingfor gold and sil- regarding thesespecies for a quantitative evaluation. ver. .Iz Proceedings: International Gold/Silver Con- Sharp redox boundaries,such as the stream water ference III (Y.S. Kim, ed.). Nevada Inst. Technol- - sedimentinterface and the water table, would cause ogy, Reno, Nevada. gold precipitation. Self-electrorefiningof placer elec- A.K. & RvaN, D.E. grains process produce Bnoors, R.B., CnerrEnrrr, trum is also a that can gold- (1981): Determination of gold in natural waters at rich rims. The required conditions are presentin most the parts-per-trifion (pg cm-i) level. Chem. Geol, streamenvironments, and the texturesof the gold- 33, 163-169. rich rims match those reported from controlled hydrometallurgicalexperiments with other alloys. Cr:rnrvavnv, A.M., CnrruwevEva, L.E., YEREluEYEve., Thus, a combination of self-electrorefiningand M.N. & ANDReyEv,M.I. (1969): Hydrogeochemis- cementationcan explainthe formation of gold-rich try of gold. Geochem.Int. 6,348-358. rims, as well as observedsurface textures on the (1969): gra- grains of placer gold. Coor, H.E. & Httueno, J.E. Effect of dient energy on diffusion in gold-silver alloys. ./. Appl. Phys. N, 219l-2197.

ACKNoWLEDGEMENTS Coor, R.B., Jn. (1987):Notable gold occurrencesof Georgia and Alabama. Mineral. Rec. lE, 65-74. The authors expresssincere gratitude to Robert F. Martin, ScottA. Wood, and an unnamedreferee Cnarc, J.R. & CanaHar, J.E. (1990): Paleoplacer for their insiebtful reviewsof this manuscript. Sam- gold in the Lilesville sand and gravel deposits, North ples of placergold were graciouslydonated for use Carolina. Georgia Geol. Sum., Bal/. (in press). in this study by the U.S. Museum of Natural His- (1985): tory and Mr. JamesR. McCloud. This researchwas & Rrrr,rsrnr,J.D. Gold: compositional variationsations of naturallynaturallv occurring alloys.allovs. Gaol. Soc. supported by the Department of the Interior's Am., Abstr. Programs17, 555. Mineral Institutes program administered by the Bureau of Mines under allotment grant number SoLrEnc,T.N, & Snans,C.E. (1980): Gll8415l. Appalachiangold. Placerdeposits of Montgomery County, Virginia. Geol. Soc.Am., Abstr. Programs RSF'BnsNcss t2.175.

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