t 1 1 s

The Canadian M in e ralo gistVol. 36, pp. ll 15-l 137 (1998)

CRYSTALLOGRAPHY, MINERAL CHEMISTRYAND CHEMICAL NOMENCLATURE OF GOLDFIELDITE,

THE TELLURIAN MEMBER OF THE TETRAHEDRITE SOLID.SOLUTION SERIES

ALFONSO G. TRTJDU'

Department of Eanh Sciences, Monosh University, Claytott, Victoria 3168, Aastrali.a

TJLRICH IC\IffTEL,

Insrttut fi)r Minzralogie und lagerstiinenlehre, Rhzinischc-Westfiilische Technische Hochschule,

Wiillncrstrasse 2, D-52056 Aachzn, Germany

ABsrRAcr

This comprehensive review on goldfieldite, Cu19TeaS13, and tellurian phases of the tetrahedrite solid-solution series(teEahedrit€J, contains a summary of crystallographic properties and compositional data. A new classification scheme for theseminerals is proposed. Furthermore, the association of these minerals to specific ore environments is discussed. The bond angles

and the optical properties of goldfieldite and tetrahedrite are very similar. The cell dimension of goldfieldite is intermediatebetween that of tennantite and tetrahedrite; thus it is strongly controlled by the ionic radius of Te. As expected, Cu vacancies in

goldfieldite reduce the size of the unit cell. The best electron-microprobe analyses are obtained when S is stendardized on

tetrahedrite, bismuthinite or stibnite. The prefened procedure for calculating structural formulae of tellurian elamples of the

tetrahedritess from electron-microprobe results is based on total numbers of atoms, as this distributes possible analytical errors

over all elements. Owing to the occurrence of vacancies in goldfieldite, the calculation should be based on 29 atoms per formula

unt (apfu) for minerals with 2 or less Te apfu Fe/(Te + As + Sb + Bi + Te) < 0.51. For more Te-rich compositions, formulacalculation on the basis of 29 - 4f[e{Ie + As + Sb + Bi) - 0.51 apfu is r*,ommended. A review of available electron-microprobe

data on goldfieldite and tellurian members of the tetrahedrite,, supports previous observations on the substitutions. For up to 2 Te

aplu, substitution of Te4 for (As, Sb, Bi)3+ is coupled with Cu+ for (Fe,Zn)z* subsdrution, whereas higher levels of Te result in

vacancies (reduced Cu contents). A nomenclature grid for goldfieldite and the other Cu-dominant examples of the tetrahedrite""

is proposed on the basis of their semimetal content. The name "goldfieldite" is reserved for those compositions of the tetrahedrite""

that contein more than3 apfu of.Te. For those minerals with 2 to 3 apfuof Te, modifying adjectives such as arsenoan, stiboan or

bismuthoan should be used, according to the second most abundant element. The remaining @11urian minerals of the tetrahedrite""

should be named according to the most abundant semimetal (tennantite for the As-rich member, tetrahedrite for the Sb-rich one)preceded by "tellurian", provided that at least I apfa of Te is present. Most of these tellurian minerals of the tetrahedrite"" are

found in high-sulfidation epithermal Au deposits, regardless whether they are As- or Sb-rich. A few cases have been reported

from porphyry Cu and volcanogenic massive sulfide deposits.

Keywords: goldfieldite, tetrahedrite solid-solution series, electron-microprobe data, substitutiotrs, strucnual formula,

nomenclanue, ore deposits.

SoMrr{arne

Cette dvaluation compr6hensive de la goldfieldite, CuroTeaSrr, et des phases riches en tellure de la solution solide dite

tetraedritess, contient un sommaire des propri6t6s cristallographiques et des intervalles de composition. Nous ptoposons un

nouveau sch6ma de classification pour ces min6raux. De plus, nous discutons de I'asso"136i6a ds sss min6raux avec des contextes

de min6ralisation sp6cifiques. Les angles des liaisons et les propri6t6s optiques de Ia goldfieldite et de la €ra6drite sont trCs

semblables. Le parambre r6ticulaire de la goldfieldite est interm6diaire entre celui de la tennantite et celui de la t6traddrite; iI est

donc fortement d6pendent du rayon ionique du Te. Tel que pr6dit, les lacunes dans la position Cu r6duisent le paramCtre r6ticulaire.

Les meilleures donndes obtenues par nicrosonde 6lectronique sont attendues lorsque la srandardisation pour le soufre s'effectue

sur la t6tra6drite, la bismuthinite ou la stibnite. Le protocole recommand6 pour le calcul d'une formule structurale de cristau( de

la s6rie t6na6drite,, riche en Te i partir des r6sultats de microsonde est fond6 sur le nombre total d'atomes, ce qui distribue les

Present address: CSIRO - Minerals, P.O. Box 83, Kenmore, Queensland 21069, Ausralia E-matl a.ddress: alfonso.trudu@minerals.csifo.auCorresponding author, to whom reprint requests should be addressed. E-mail aidress: knittel@rwth-aachen.de

I 1 1 6

erreurs amlytiques Possibles sur tous les 6l6ments. Vu la pr6sence de lacunes dans la goldfieldite, le calcul devrait supposer29 alomes par t'nit6 formulaire (apzj pour les cas oil le Te 6quivaut A2 apuf ou morns [Te(Ie + As + Sb + Bi + Te) < 0.51. Dansle cas de compositions plus enrichies en Te, le calcul de la formule se fait sur une base de29 -4(lel(Te + As + Sb + Bi) - 0.51apuf. Un examen des donndes disponibles de microsonde dlectronique portant sur la goldfieldite and et les membres enrichis entellure de la sdrie t6tra6drite"" 6taye les observations ant6rieures d propos des sch6mas de substitution. Oil le Te 6quivarlitd2 apttou moins, la substitution de Te# d I'ensemble (As, Sb, Bi)3* est coupl6e au remplacement de Cu+ par I'ensemble (Feln)2+; enrevanche, les niveaux plus 61ev6s de Te mbnent d des lacunes (teneur r6duite en Cu). Nous pr6sentons une grille de nomenclaturepour la goldfieldile et les aufes min6raux d dominance de Cu de la s6rie t6tra6drite"" selon leur teneur en semim6tal. Le nom"goldfieldite" est r6serv6 pour les compositions de la s6rie t6tra6drite"" contenant plus de 3 apuf de Te. Pour les compositionscontenant entre 2 et3 apufdeTe,nous proposons les qualificatifs arsenical, stibi6, ou bismuthifdre, selon lequel des trois 6l6mentsAs, Sb ou Bi est le plus enrichi. Les autres cas porteurs de Te de la s6rie t6tra6drite"" devraient porter le nom du min6ral selonl'€ldment senfm6tellique le plus abondant (tennantite pour le membre arsenical, tdtraedrite pour le membre stibi6) pr6c6d6 par"tellureux", moyennant au moins I apuf deTe.Laplupart des exemples de la s6rie t6tra6drite"" enrichis en tellwium proviennentdes gisernents d'or 6pithermaux h sulfidation 6lev6e, qu'ils soient riches en As ou en Sb. Quelques cas ont 6t6 signal6s dans lesgisements de cuivre de type porphyrique et les gisements volcalogdniques de sulfures massifs.

(Iraduit par la Rddaction)

Mots-cl6s: goldfieldite, solution solide de la t6tra6drite, donn6es de microsonde 6lectronique, subsfitutions, formule structurale,nomenclature, gftes min6raux.

IlrRopucuorl

Goldf ieldite, ideal ly Cu1sTe4S13, as shown byKalbskopf (1974) and Johnson & Jeanloz (1983), is thetellurian end-member of the tetrahedrite-tennantitesolid-solution series (henceforth tetrahedriteJ. The pureend-member has not been found as natural mineral; amaximum of 3.4 Te (out of a possible 4) atoms per for-mula unit (apfu) has been reported (Trudu & Kninel,in press). Goldfieldite, tellurian tetrahedrite andtennantite have been reported to occur in various typesof ore deposit, inclurling epithermal gold deposis, por-phyry-Cu deposits, and volcanogenic massive sulfidedeposits.

Goldfieldite was fint described by Sharwood (1907)and later identified by Ransome et al. (1909) n oresfrom the Mohawk mine at Goldfiel4 Nevada. Thompson(1946) summarized the controversy regarding the inter-pretation of this mineral, which was thought to be pos-sibly a mixture of different known minerals (e.g.,tetrahedrile + calaverite, or famatinite + bismuthinite +calaverite or sylvanite), even if it appeared to be homo-geneous. For a goldfieldite specimen from the Clare-mont mine at Goldfield, he obtained an X-ray pattemanalogous to that of tetrahedrite and hence, proved it tobe a member of the tetrahedrile solid-solution series.

ln his classifications of sulfosalts, Nowacki (1969)regarded goldfieldite as a variefy of tetrahedrite andbelieved that Te substituted for S. Kato & Sakurai(1970a) were the fust to infer that Te substitutes for Asand Sb. Kalbskopf (1974) ndependently came to rhes4me conclusion and suggested the end-member for-mula Cu lqTeaS 1 3. Charlar & lt'vy ( I 974) speculated thatin goldfieldite, Te can be both a cation, substituting forAs and Sb, and an anion, replacing S. Kase (1986) pro-posed that substitution of Tea+ for the trivalenrsemimetals (As, Sb and Bi) is compensated by an in-

crease of monovalent atoms (mainly Cu with minor Ag)from 10 to 12 apfu at the expense of a decrease of thedivalent elements (Fe and Zn) from2 to O apfu leadingto the formula Cu12(As,Sb,Bi)zTezSrs. For higher con-centrations of Te, the charge balance in tetrahedrite"" ismaintained through vacancies in one of the sites occu-pied by monovalent metal atoms (Kalbskopf 1974,Dmitriyeva et al. 1987). These relationships have beenconfirmed for natural goldfieldite (Trudu & Knittell99l,in presso Shimizu & Stanley 1991).

De Medicis & Giasson (l97la) investigated the sys-tem Cu-S-Te at 340"C, but did not report the occur-rence of goldfieldite. In contrast, Karup-Mgller(in prep.) found goldfieldite at 350' and 450'C in hisexperimental investigation of the same system.

We have collected results of over four hundred elec-tron-microprobe analyses of natural tellurian tetra-hedrite," plus a few from synthetic material. These dataform the basis for this review, in which we also summa-rjze and discuss the crystnllography, optical propertiesand mineral chemistry of natural and synthetic telluriantetrahedrite"". We propose a new scheme of classi-fication based on mineral chemistry and discuss theocclurence of these minerals in different types of oredeposits.

Cnysranocnaprilc PARATmTERS

Goldfieldite, as all minerals of the tetrahedrite"", iscubic, a derivative of the sphalerite structure, spacegroup l43m(tt/lensch 1964) with two formulaunits percell. The formula of a half unit cell of tetrahedrite""is generally given as: M*rcfuP*zF*+S13, where M+ =Cu, Ag; lE+ = Fe, Zn, M4 Cd, and Hg, and X = Sb,As, Bi, Te.

Half of the twelve M sites are tetrahedrally coordi-nated (Ml), and the remainder are in a triangular planar

lllT

@

(D

o@@

Frc. l.Hafunit-cellofpuregoldfieldite,modiJiedafterpzuling&Newman(1934)andCharnocketal. (1989a).TheM2sitesare

depicted only 213 full to account for two vacancies.

o.8o.7

r (A) 0.6o.5o.4

1 . 0

r (A)0.8

0.6

o.42 3 4 5 6

coordination number

Frc. 2 . PIot of coorditration number versas ionic radius, r, inAngstrtim (A), for Cu and As, Sb and Bi. Measwed radiifrom Shannon (1976, l98l) are shown by filled symbols,and estimated values by open symbols.

M16 M24 (XYr)oZ

'uM1 =cu

" ' M2=cu'ux=Te

'uY=s

u' z=s

coordination (M2).T\e X atoms occupy four 3-coordr-nated sites, forming a trigonal pyramid. This arrange-ment creates a void in the structure, the site of a lonepair of electrons originating from the X atoms (Moore1981, Johnson & Burnham 1985). In end-membergoldfieldite (Cu1sTeaS13), trvo of the M2 sites are vacanland the others are occupied by monovalent atoms(predominaaly Cu) to balance the charge resulting fromthereplacement of trivalent As, Sb and Bi by tetrava-lent Te (Kalbskopf 1974, Dmitriyeva et al. 1987).Amongst the anions, twelve (usually labeled Y) are in4- fold coordination, and the 13th atom (normallyreferred to as Z; is octahedrally coordinated. A halfunit-cell of goldfieldite is drawn in Figure 1; as they aresupposed to be statistically occupied, the M2 sites areall represented as 2A full.

Radii for five elements relevant to this studv(rvCu*, ryFe2*,rv7n2+'5.2- ̂d S2-) were given by Shan-non (1981). This author also showed that the radius ofSL in sulfides is independent of its coordination number.However, no radii are available for the other elementsthat may be incorporated in the structure. Owing to thislack of data, we must rely on radii measured in oxidesand halides by Shannon (1976). Furthermore, for eachelement, ionic radii are available only for some of thevalence and coordination numbers of interest to thisstudy.

Estimates of ionic radii required for the interpreta-tion of variations in bond leneths and cell dimensions

GOLDFIELDITE, TTIE TELLURIAN TETRAHEDRITESJ

in tellurian members of the tetrahedrite"" ate based onthe following observations (e.9., Shannon 1976): (l) foreach atomic species, the ionic radius decreases linearlywith oxidation state and increases with coordinationnumber, nd (2) for C-u, Fe, and Zn,radti measured insulfides are about 0.1 A smaller than those in oxides. In

-/-) Cu+ (oldde)

./"1cu+ (sulfide)

lf*y1j----tsf*

As3+

Jl

I

I

I

I

I

1 l l 8

TABLE 2. MEAIIURED AI{D CAI.CUI.ATED BONIIITNGIIilIIN SYNTIIETIC GOLDFIBDTT4 ARSENOA}I GOLDFIEIDII4

B@d IR@ uid CN /,.100 IRe EIR'i80d9

neaercd A(elc.-@.)

Ml-Y At:O.63s I M.61 S:1.70 2.335 2.37(2)ffi -O.O35(2)2.313(9) acfl 0.02<9,2.33(E) 16 -{.0048)2344qrsi. -o.00(o

truIVIvIVItrryVIryVIItrryIIIIv

ryVIIu

vtryIV

0.@o.6740.740.n0.740.4950.57o.T2o.4750.600.650.800.E6r0.900.94

o.74

i.tol . l 71.561.W

t Vabs elorlced bllwirg tho procedm dqsibed h ths tem od shm irEgrs 2. Tb nmbq ddigin ie giw o in the oigitrsl !*renco. CN: cordincimnmb6, IR tonicndii rspdsd by Sh@on(19O, SR ndiiin rulfdc rcportedby Sh@@ (l9El). The dat' appfieblototfcaheddt& appsin bold fre

contrast, semimetals have nearly the same ionic radiusin both types ofcrystals Cfable l).

Ionic radii for oxides (Shannon 1976) and sulfides(Shannon l98l) for Cu, Fe,7n, As, Te, Sb, Bi, S and Sein the oxidation states and coordinations relevant to thisinvestigation are listed in Table l. Figure 2 shows thegraphic exffapolation of the radii of Cu, As, Sb and Biin 3-fold coordination. The radius of IIrCu+ in sulfideswas calculated as- the equivalent value measured inoxides minus 0.1 A (Fig. 2).

Because the radii for As, Te, Sb and Bi are very close(t0.03 A or better) in oxides and sulfides (cl tuAss+,ryTe4o vgi:* and vrBi3* in Table l), data obtained fromoxides (e.g., for Sb) and sulfides (e.g., Bi) may be usedinterchangeably. The extrapolation method used to esti-mate the radius of As is similar to that employed for Cu.In0erestingly, although the radius of 52- is different inoxides and sulfides, no such change is observed for Se.

Bon"d lengths and angles

Crystal-structure data has been reported so far onlyby Kalbskopf (197 4), who gathered powder-diffractiondata from synthetic material, and by Dmitriyevaet al. (1987), who studied a single crystal from theYelshishche deposit in Bulgaria. Bond lengths reportedin these studies, as well as data for tetrahedrire(Wuensch 1964) and tennantite (Wuensch et aI. 1966),are summarized in Table 2. The effect of Zn and Fe inthe M2 site on the bond lengths of tennantite andtetrahedrite was neglecled, because the radii of Fe and

TABLE I. IOMC RADtr FOR MA'OR AI{D MINOR ELEMENTSRtrRRE) TOINTEISI WORK

ciddimde

GrCuChF97^ABAgA8A8ArToTssbsbsbsbsbBiBiBisSs

+ 2

+ 3+ 3+ 5+ 5+ 4+ 4+ 3+ 3+ 3+ 5+ J

+ 3+ 3

057rodul0.660.6

o.ort

0.t0

tuD-f 6t:O.57 1 IU 8.61

tufr-Z Qt:0.57 I Itr t.6l

o.a

0^91.1.Clt .15t-70r"st

m 3.92Itrm 6.06m E.61

gtr.7o 2.2'1 2.lqt8Gd 0.11(t2.24q10)scfl 0.6q10)2.2sE9) T@ 0.0149)2.2.rXDTcr 4.@2fi)

S:1.70 227 2.13(3)scdl 0.14(3)2.'l2{Ascdl 0.0t8(7)z2aa\\Ta 0.06(8)2.a4(5lTf! o.6qt

S: 1.70 2.36 2.41(3) 8cfl -0.05(3)S: 1.70 (2.319) 2.364(9)aGfl -0.04{9)S:1.70 2.195 2.245(s)To -O.051(5)S: l.7O 2.56 2.44qrTea 0.114(3)

X-f To:0.66 4( 3 T o + l A s )

As:0.495 3Sb:0.86 3

AIl vatue m in Angffon uitr. The qiddio dst (Gid. stde) of S is iMisbb2-, but its mdincim Dnb@ (cN) m W ad VI fu ths I ad Z rit€clrespeottudy. f fiactimd ioric ah!ruts (s t(E). IR imia ndiu. scfil: d@ orychreic goldfieldm frm Kalbskopf (1974). aGfr: dda o mm goldfislditsftm ffiiyffi

"t aI (19E7). Tu .l"ta m t@ffits to@ Cru@cl (1964). Tet:

dce q trfrahedrile ftm Wu@sch staf (1966).

Zn differ from that of Cu by only 0.02 A. The analyticaluncertainties in the bond lengths measured by Kalbskopf(1974) are relatively large (t0.02 to 10.05 A), becausethey were calculated from powder-diffraction data,whereas the single-crystal data of Dmitiyeva et al.(1987), Wuensch (1964) and Wuensch et al. (1966) areassociated with substantially lower uncertainties(+0.003 to 0.010 A).

There are four distinct bonds in the tetrahedrite""structures. Atoms in the Ml and X sites are bonded to4-coordinated Z anions, whereas the atoms n the" M2sites are bonded to those in both the Y aad Z sites. Asthe bond length in theory is equal to the sum of the radiiof the two atoms linked together, Table 2 also comparestheoretical bond-lengths based on the data of Table 1with the direct measurements of bond lengths intetrahedrite"s. Increasing covalency (Gamble 1974) anddelocalization of electrons (Shannon I976) lead tomeasured bond-lengths tlat are smaller than the sum ofionic radii.

T\e Ml-Ybond-length should be about the same inall Cu-dominated minerals of the tetrahedritess, becausesubstitution in the Xposition does not affect these sites.The data in Table 2 support this interference, consider-ing the uncertainties of the meutsurements. There is alsogood agreement between bond lengths calculated fromionic radii (2.34 A) and measured bond-lengths (2.31-2.37 A).

The MZ-Y bond lengths for tennantite, tetrahedrite,and the arsenoan goldfieldite of Dminiyevaet al. (1987)are very similar, though a decrease from tetrahedrite to

GOLDFIELDME, TIIE TELLURIAN TETRAHEDRITES 1 t 1 9

-vloC'E

I

(J

.Eu

synth.gfd. C!r Gamble (1974)

t tt r M 2 - Yo M 2 - Z

synth.gfd. trI

ars.gfd. O

tenn. o

0 0.10 0.20 0.30

fiFrc. 3. Plot of fractional ionic character (J) versu.s the differ-

ence beflreen calculated and measured bond-lengths (in A).The line represents the linear regression through the data ofGamble (1974), which is represented by filled squares. Seetext for further explanation. synth. gfd.: syntheticgoldfieldite (Kalbskopf 1974), ars. gfd.: arsenoangoldfieldite @mitriyeva et aI. 7987), tenn.: tennantite(Wuensch et aI. 1966), ter.: Etrahedrite (Wlensch 1964).

tennantite to arsenoan goldfieldite is indicated. In con-trasl the synthetic goldfieldite has a significantly shorterM2-Y bond-length (2. 1 6 t 0.0 5 A versus 2.240 t 0.0 I 0A); however, Kalbskopf (1974) proposed that a rela-tively large error might be attached to that value owingto an increased temperafure-factor.

All measured M2-Z bond lengths are shorter thanthe calculated values. Measured bond-lengths shorterthan the sum of the ionic radii of the elements concernedwere observed by Gamble (1974) to be inversely pro-portional to the fractional ionic character (/) in sulfides,selenides and tellurides (Frg. 3), with fi defined as:

fi (A - B) - 1 - exp [-{.25 (xa - xs)2] (l)

@auling 1948), where A and B are the two atoms in-volved in the bond, and 1, their electronegativities, ascalculated by Pauling (1948). However, use of thesmaller radii observed in sulfides (Shannon 1981;Table I of this paper) led to good agreement betweencalculated ard observed bond-lengths for MI-Y ardM2-Y bonds in tetrahedrite, tennantite and arseniangoldfieldite. Only the shorterbond-length in goldfielditewould be consistent with the relationship observed byGamble (L974). Hence, the shorter bond-lengths ingoldfieldite might be due to the vacancies in fhe M2 site,rather than to decreasing ionic character of the bond.

T\e X-Y bond lengths show some contradictoryresults. The measured Te-S and As-S distances insynthetic goldfieldite, arsenoan goldfieldite andtennantite are longer than those calculated, a fact in con-trast with Gamble's (1974) observation. These discrep-ancies may be the result of analytical errors related tothe chemical composition the mineral, errors in thedetermination of the bond lengths, possible changes incoordination of the semimetals as a result of vacanciesin the structure, or inapplicable ionic radii.

The possibility of an analytical error was discussedby Wuensch et al. (1966) in their study of the structureof tennantite; the mineral they used could have con-tained minor amounts of Sb, thus increasing the bondlength. A similar error could not have affected syntheticgoldfieldite. Substantial errors in measurements of thebond lengths are unlikely, though Kalbskopt (1974)

reported a relatively large uncertainry of 10.03 A forthe measured To-S bond length in synthetic goldfieldite.This leaves the possibility that the estimated ionic radiusfor mTe is slightly too low at 0.66 A.

Vacancies are reported only in goldfieldite, but theyshould not affect the coordination of Te (KalbskopfI97 4). If they did, these would result in an increase inthe radius of Te, which would make the differencebefween calculated and measured bond-lengths positive(Table 2), in agreement with Gamble's (1974) generalprediction for all minerals.

Thus, with the data currently available, no viablehypothesis can be proposed to explain why the meas-ured bond-lengths for synthetic goldfieldite, naturalarsenoan goldfieldite and tennantite exceed the calcu-lated values. More accurate measurements can possiblyhelp to solve this problem. The X-Y bond lengths intetrahedrite are in agreement with Gamble' s (197 4) pre-diction, though we used the radii applicable to sulfides.

There are very limited published data regarding thebond angles in goldfieldite. Kalbskopf (1974) teportedthat in pure goldfieldite, the S-Te-S angle is 95.7 + 0.7",which is very close to the corresponding value (95.1t 0.2") for tetrahedrite measured by Wuensch (1964),as are reported values for the two S(I0{u(M1)-S(Dangles in arsenoan goldfieldite @mitriyevaet al. 1987).

Cell dimensions

Currently available data for the cell dimensions ofgoldfieldite are summarized in Table 3, which also listsreference values for synthetic tetrahedrite and naturaltennantite. The prediction of cell dimensions as afunction of the different types of substitutions intetrahedritess was flust attempted by Charlat & I-Evy(1975) and subsequently refined by Mozgova et al.(1979) arrd Johnson et aI. (1986,1987). However, onlyJohnson et al. (1986,1987) provided details on the typeof statistical analysis used. Te-for-(As,Sb) substitutionis only taken into account by the equation given byMozgova et al. (1979):

0 . 1 6

o .12

0.08

0.04

o.00

t120 TIIE CANADIAN MINERALOGIST

TABLB 3. UNIT@I SIU OF GOLDFIELDITETETRAHEDRTTE A},{D TENNANTTTE

40 (A)

Sghecic cytals

Goldfddite frroTeoss 10.263(5)Tstrahe&fte C\luSb4SE 10.3190)Tcrahsddte c\oGe,zntsb{fu 10.383(l)

Ndral dtrtals

Aneoom goL{ffslditgGquAq.rSb@TqrSF 10.304

Arseom goldfddbEGruAq:T%gSsr 10 7(2)

Stibom goldfelditeCu,"nArorSbrrTerS* 10.35

Stibm goHfleldtoOqEAtuSbuTqrSEa 10.32q5)

Sdmlo goldflelditeCqDAsuSboaT%rSurS 10.3t

Tarnorreg 10.186Teffin*te CqFenln,Aqr\** 10.235(tTeanhedrito CurSoorZn"rSbnrSo*t 10.391(1)

raftnkcpf(9+)Hall 0972)HanQYt2)

PDF (29s31)

Dltrihiyswatal (1987)

Sptidmoveral (t9t4)

TqinetaLQgr'

Spirftlmry& Olrtgin (t9S5)PDF (l l-102)Wu@cn €f al- (l 966')Wumsch (1964)

S Chmicat conpodtion is Dot avaih[o * Chmiel 64ssido[ is w€liable md cruimce with sp€afion€'Eic do m ths me mdedal cmpocition protOty cforetD ChrZDuFfuAs$Su (Wi:mch aal 156). *. IGflilts of old estnis SPIitrgE(1959) rcport€d the blwirg coryosfim ftr ndsial A'od thi! l@lit4CurZn*FqrSbnSo.

aa = 10.379 + 0.031 Fe + 0.028 7n + 0.096 Hg +0.007 2MelS - 0.040 As + 0.093 Bi - 0.003 Te +Ag/(21.9-r.or As) (2)

This equation is valid only for tetrahedrite"" with lessthan 3.7 apfu Agand for minerals with a total of 29 apfu(Mozgova et al. 1979). The frst condition is easily meqas the highest Ag content recorded in goldfieldi[e is2.2 apfu (Kovalenker et aI. 1980), bur the secondexcludes crysta.ls with more than 2 Te apfu ow:mg tovacancies inthe M2 site. The validify of Equation 2 hasbeen tested against the measured size of the unit cell ofgoldfieldite reported in Table 3 (PDF #29-531;Spiridonov et al. 1984) with the following results: celldimensions calculated from mineral-chemistry dataare smaller than those measured [10.280 A versus10.304 A for the arsenoan goldfieldite ofPDF #29-531,and 10.32 Aversus 10.35 A for the stiboan goldfielditeof Spiridonov et al. (1984)1. In contrast, Basu et al.(1981) claimed that Equation 2 can predicr cell sizeswithin the analytical uncertainty of the measured value.

As expectedo the cell size of synthetic goldfieldite isintermediate in size between that of tennantite and thatof tetrahedrite in spite of the two vacancies present,However, there is still some debate about the exact cell-dimension of tennantite. Wuensch et al. (1.966) ques-tioned the purity of the natural crystal they^analyze.d,because it yielded a value of 10.235 + 0.005 A, which istoo large ifcompared to the value of 10.19 A previously

reported. L.G. Berry (as cited in PDF#29-531) reporteda value of 10.186 A for natural tennantite containing upto 4.77 wt.Vo Ag; as the Ag-for-Cu substitution in thetetrahedrite," causes an increase in the cell edge (Hall1972), it can be predicted that a pure tennantite end-member (Cu16[Fe,Zn]2As+Sr:) should have a cell sizeslightly smaller than 10.186 A.

On the basis of the available data (Table 3), there isa considerable contraction (about 0. I A) in the cell edgeas the proportion of vacancies increases^ in goldfieldite.The largest cell-edge reportd 10.38 A, measured bySpiridonov & Okrugin (1985), pertains to the selenianvariety. It is evident that the increase in the cell dimen-sion with respect to that of pure goldfieldite is due tothe Se-for-S substitution (c/ Table t).

Orrrcar, Pnopmrns AND SpEcrRoscoprc REsULTS

Quantitative measurements of the reflectance ofgoldfieldite were reported by Spiridonov et al. (1984)lvalues in the range of 29.5 to 3l7o were measured forthe As-bearing variety. High Te contents in goldfielditemarginally increase its reflectance, but large amountsofSb have the opposite effect. These authors concludedthat this mingral is indistinguishable from tetrahedrite,as both are grey with a pinkish to brownish hue. Incontrast, unpublished observations suggest that the Tecontent of goldfieldite may affect its color. One of thepresent authors (UK) notes that Te-rich tetrahedriless isdistinctly pinkish where in contact with Te-freetetrahedrite"", in particular in oil immersion. At the LaMejicana base- and precious-metal epitherrnal deposit(Argentina), two types of goldfieldite were recognized(A. Losada-Calder6n, pers. commun.): a low-Te(a apfu) variety, which occurs at deeper levels and istexturally associa[ed with gold, shows a greenish tintresembling tennantite, and a high-Te (>2 apfu) variety,which is present at shallower levels, is grey, like normalgoldfieldite. The presence of Se, as shown by materialfrom the Ozemoye Au deposit, in Kamchatka, Russia,does not seem to affect the color of the mineral.

Spiridonov et al. (1984) measured also the Vickersmicrohardness (load = 30 g) of goldfieldite and obtainedvalues decreasing from 334 for tetrahedrite with a verylow Te content (0.54 wt.Vo) to 205 for goldfieldite(Te = 14.98 wt.Vo).

At present, there are only two studies concerned withthe spectroscopic examination ofgoldfieldite. In a recentlaser Raman investigation of some geologically impor-lant sulfides, Mernagh & 1ru6u (1993) observed Ramanbands at 354 an d 324 crtrr , which represent an interme-diate position between those of tennantite and oftetrahedrite. They interpreted these results as typicalone-mode behavioro with frequencies decreasingcontinuously from lighter (tennantite) to heavier(tetrahedrite and goldfieldite) minerals of the samegroup.

GOLDFIELDITE, TIIE TELLURIAN TE'TRAHEDRTES 1l2l

M*

3.0

Te

3.5 4.0

TOTAL StSeATOMS

Frc. 4. Plots showing the effect of the choice of the referencematerial for S calibration on the analytical data- All the datain this diagram are ftom the Tirad porphyry Cu-Au pros-pect, northwestem Luzon, Philippines (Trudu and Knittel,in press). Legend: analyses with S calibration carried outon t€trahedrite, stibnite and lis6uthinite (circles in upperdiagram and stippling in lower diagram), and on aneno-pyrite (crosses in upper diagram and harching in lower dia-gram). All concentrations are expressed in aplu.

In the second study, a l2lsb Mtissbauer investiga-tion ofseveral Sb-bearing minerals, Hedges & Stephens(1977) determined that the Sb in goldfieldite is 3-coor-dinated, confirming the result of Kalbskopf (1974).

CarmnnrroN oF ELEcTRoN-MrcRopRoBE DATA

The quality of electron-microprobe data on gold-fi eldite, generally obtained in the wavelength-dispersionmode, surprisingly seems to depend on the choice ofthe reference materials used for calibration, in particu-lar for analysis of the samples for S. Figure 4 shows thatcalibration of S on an arsenopyrite standard gave lowvalues for S in goldfieldite, resulting in too high anumber of atoms per formula unit, when formulae werecalculated on the basis of 13 S. Calibration on stibnite.

bismuthinite and tetrahedrite produced markedly betterresults (Fig. 4). A similar behavior in electron-micro-probe data was reported by Harris (1990) for Ni-Hgsulfides: HgS analyzed with S standardized on NiS at20 kV yielded values up to 7 wt . Vo htgfier than expected,whereas the opposite effect took place when NiS wasanalyzed with S standardization on HgS. Harris (1990)ascribed these poor results to the correction programs,which inadequately dealt with the absorption edge ofthe standards.

Analytical problems with Cu in tetrahedrite that isnot fully substituted have been discussed by Lind &Makovicky (1982). However, the available data suggestthat tellurian tenahedrite"" is in general fi.rlly substituted.

It is not known whether Se, the other anion possiblypresent in goldfieldite, should be calibrated under con-ditions similar to those of S. Se-bearing goldfieldite isvery rare. The only published data pertain to materialsfrom the Ozernoye Au deposit in Kamchatka, Russia.Spiridonov & Okrugin (1985) did not mention whichSe standard they used. For Te and most of the otherelements, standardization on pure metal pfoduced goodresults (Trudu & Knittel, in press). This shows thatmatrix effects have a negligible effect on the measuredconcentrations of the cations, as already pointed out byHanis (1990).

Altogether, 19 elements have been reported to occurin tellurian tehahedrite"", but only the following are rou-tinely detected: S, Cu, Fe,Zn, As, Se, Ag, Sb, Te andBi. Of these, Bi and Se tend to be neglected in routineanalysis of goldfieldite. Bi substitutes for Te, and itstheoretical end-member will be used in the new classi-fication scheme for goldfieldite (see below). Neglect-ing to analyze for Se, even if present in amounts as lowas I wt.%o, can lead to the erroneous assumption thatlow S is caused by a poor calibration of the electronmicroprobe. It must be kept in mind that low concentra-tions of Se may not appear on an elemental spectrum ofgoldfieldite, because critical X-ray absorption lines ofSe partially overlap those of As, which is usually presentin larger amounts than Se in this mineral.

Elements of secondary importance, Au and Pb, andminor or trace amounts of Hg, Mn, Cd, Sn, Co, V andGe have been reported only occasionally.

Cer,curanow oF THE STRUcTuRAL Fonuure

Structural formulae for tetrahedrite"" are usually cal-culated on the basis of 13 anions (S + Se) or 29 totalatoms. Seal et al. (1990) suggested that a calculationprocedure based on 4 semimetal atoms (As, Sb, Bi, Te)be adopted, although they admit that small analyticalerrors for the semimetals can cause larger errors inrecasting the number of sulfur and metal atoms. Thisproblem is also inherent in the calculation of the struc-tural formula on the basis of 13 anions, in particular, ifSe is not sought, but is present in substantial 2mounts.

In addition, analytical data for S may be problematical,

1 3

1 2

1 1

1 0

-d"ail"- - o

* -9td9*"

*..s16?&L**-

o \ o - _

2.52.O

r122

as outlined above. It is even less suitable to calculatethe struchral formula for goldfieldite based on 12metal(Cu, Ag, Zn, Fe,etc.) atoms(e.g., Charlat&Itvy 1975)because vacancies occur for more than 2 atoms of Te(apfu). T\e same reason should make the calculationprocedure based upon a fixed number of total atomssuspect, as this varies from 29 for Te S 2 apfu to27 apfu for end- member goldfieldite.

Keeping in mind all these considerations, we preferto calculate the structural formula of goldfieldite andother tellurian minerals of the tetrahedrite"" using themethod based on the total number of atoms. This proce-dure has the advantage of spreading the analytical errorover all the elements sought. In order to take into accountthe occurrence ofvacancies forTe>2 apfu" we suggestthat l) for Te/(Te + As + Sb + Bi) < 0.5, the calcularionbe based on 29 apfu, as no vacancies occur in the crystalstructure; 2) for Tel(Te + As + Sb + Bi) > 0.5, rhe for-mulae are to be calculated on the basis of

29 - 4lTel(Te + As + Sb + Bi) - O.5l apfu.

Mwenar CrnpusrnY

We have assembled a database containing all resultsof electron-microprobe analyses of tellurian membersofthe tenahedriters group available to us, using an arbi-trary cut-off of 0.3Vo Te (ir all, 354 compositions, asshown in Table 4). No attempt was made to evaluatethe qualiry ofthese data, because various types oferrorscan affect them, e.g., counting statistics, matrix correc-tion, use ofinappropriate standards, etc.,blt we haveexcluded all compositions with totals outside the range97-1O2Vo.In the following.discussiono we will, how-ever, point out inconsistencies in some data sets.

In the following sectionso we summarize what isknown about the distribution of the elements on the dif-ferent structural positions. Where no information isavailable, we will discuss the possibilities based onGoldschmidt's rules of substitution, as summarized inFaure (1991), and with reference to other studies oftetrahedrite-group minerals.

Thc M sites

In tetrahedrite"", there are two sites occupied bymetals, Ml and, M2, whrchare 4- and 3-fold coordinated,respectively. Cu is the main occupant of the M sites; itmay bereplacedby variable amounts of Fe, Zn, Ag, Mn,Cd and Hg. In the tellurian members of the tetrahedrite"",X-ray-diffraction studies (Kalbskopf 1974, Dmiriyevaet al. 1987) have shown that up to two vacancies occurin the M2 site where the Te contents exceed 2 apfu, aspreviously discussed.

11s srystal I ographic positions of the metals (F e, Zn,Ag, Mn, Cd and Hg) substituting for Cu in the tetra-hedrite*, and thus in goldfieldite, must be re-assessedin light of the findings of Charn ock et al. ( t 989a, b), whoused extended X-ray absorption fine structure @XAFS)

TABLE 4. LIST OF SOURGS OF ELECTRON.MIG,OPROBE DATAASSEMBLED FOR TITIS STTJDE AND NTJMBER OF DATASETS

Borisowsra! (1986)Dditiyffistal (1987)Frwzd,et aI (197,Igum(1986)JCPDS (1990)Kmp-MsXq (1992) (sydhstio laKre(1986) l0Kdo & Sakumi (1970a) IKato & Sshmi (1970b) 1Ktritol(tEpubl. dda) 2aKnitel (1989) I IKovaldksetal (1989) 27Kovsloks&Rusitrqv(l9EO 9lKovalmko&Trom(1980) 8Koralskqeta, (1980) 18Kovalskseral (1989)* 20

Totd

' ccluding wa omporitim o.lredy nputed in Kmloku & Ruinov (198Q.Tho omplete dcahs mybo obtaiaed u m E(CEL file fron the snd ailor.

and Mtissbauer spectroscopy, and Makovicky et al.(1990), who studied synthetic letrahedrite by Mdssbauerspectroscopy. These studies in part arrived at differentconclusions relevant to tellurian tetrahedriters.

Copper, the most abundant metal in goldfieldite,occupies both the Ml and. M2 site. Its oxidation statehas been the source of speculation; however, X-rayphotoelectron spectroscopy ofabout twenty sulfides andsulfosalts, including brahedrite, by Nakai et aI. (1976),as quoted by Kase (1986), indicates that Cu tends to bemonovalent in these minerals. Reported concenffationsof Cu in the tellurian members of the tetrahedriters varyfrom 5.88 to 12.74 apfu, but values of less than cc.9.5 Cu apfuwere obtained only for Au-rich goldfieldite(Figs. 5, 6; Kovalenker & Troneva 1980), which prob-ably is not really goldfieldite but a mixture of differentminerals (see discussion below), Problems of electron-microprobe analysis of tetrahedrite not fully substitutedwith Cu were discussed by Lind & Makovicky (1982),but the trend displayed in Figure 6 suggests that Cuinvariably is in excess of lO apfu, where this is requiredby coupled substitution as a result of Te incorporation.

The Cu contents are a function of two types ofsubstitutions, Ag + Au for Cu, and Te for (As, Sb, BD.The plot shown as Figure 5 indicates that the fust typeof substitution is of minor importance, as the concen-tration of the precious metals rarely exceeds 0.5 apfu.The data of Charnock er aI. (1989a, b) indicate thatAg-for-Cu substitution occurs at the M2 site in bothnatural and synthetic tehahedrite. Figure 6 shows therelationships between concentrations of monovalentmetals (mainly Cu) and Te; Cu increases from l0 to12 opfu as Te concentrations reach up to 2 apfu. Forhigher amounts of Te, the Cu content decreases tolO apfu, as vacancies n the M2 site appear (Kase 1986,

Koval@kset4l (1990) 35r.&y(r 7) IIpgircvaal (1983) 4Moz€ow & Ts€'pitr (1983) 8N@gorcdmerql (1978) 19Nwiello (19E9) 3Sakhrcwsral (1984) 6Shimia & Stslsy (1991) I ISpiridmov (1984) ISpiridonov(I987) 3Spiidowstal (1984) 6Spiridow&Okrugh(1985) 3Spdrgs(l%g) 2Tru&&Kdttel(mpm) 32Twpt\etql (l9n) IWilga[iseral (1990) 2

GOLDFIELDITE, TTIE TELLIJRIAN TETRAHEDRITE$ r l23

l 3

1 2

1 1

1 0

c u 9

I

7

6

5

_i.ji"::-:.Eb tr_U

t r A t r - 4

aAI - - - - - - - - . -a

.tr

A

oacrl.isl;,:A

t r t r

-#"6ffi-"-tr

12tr-LFffi. -R: E-S

".O-ad- tr-Ea

A.o--_!Hufrg.tr- --__

{:^:-'-a *o

;-;i>.-- \ <

aa

0.5A g + A u

Ftc. 5. Plot ofthe level ofAg + Au yerszs Cu in tellurian tetrahedrite",. All the values ale inapfu. T\e dashed lines indicate the range of possible substitutions. Compositions ofalleged auriferous goldfieldite (Kovalenker & Troneva 1980) are shown as filled dia-too4.. ftiangles represent samples from the Kochbulashk deposit (Novgorodovaet al.1978, Kovalenker et al. 1980, Kovalenker & Rusinov 1986); squares depict all othersamples (see Table 4).

2.5z.o1 . 51 .00.0

1 3

1 2

1 1

+ 1 0CD

, - 9?

f( J 8

7

60.0 0.5 2.5 3.0

Ftc. 6. Plot of Te content verszs that of monovalent metals M" (Cu, Ag, Au) in telluriantetrahedrite," minerals. All the values are in apfu. The solid line shows the trend of theideal Te versus M+ relattonship, whereas the dashed lines enclose the data that deviateby less than 0.5 apfufromthe ideal relationships. The circles refer to synthetic compo-sitions. Other symbols as in Figure 5.

4.O3.s1 . 51.0 T e

Trudu & Knittel 1991) to balance the increase in chargescaused by the substitution of Te for trivalent semimetals.

Figwe 7 shows the relationship between monova-lent (mainly Cu) and divalent (6ainly Fe and Zn) cationsin tellurian tetrahedrite"s. The plot confirms the substi-

tution of divalent by monovalent atoms to balance theexcess charges resulting from incorporation of tetra-valent Te (up to 2 Te apfu).

Iron occurs in tellurian tetrahedrite"" only if the Tecontent is less than 2 apfu, owing to charge-balance

l t24 T}IE CANADIAN MINERALOGIST

1 ?

=

+ l 1U'

T

= 1 0

(J

9

I

0 .0 0 .5 1 .0 1 .5 2 .O 2 .5 3 .0 3 .5 4 .O

F e + Z n

Hc. 7. Plot of divaleatlt/P* (Fe,Zn)versus M+ (Cu, Ag, Au) metals in tellurian tetrahedrit€""minerals. All the values are n apfu. Symbols as in Figure 6.

oa

- t r nA

A

AA .^

Atrt r t ro o o

A

t r t r* - E a t r

t r t rt r n - a! t u u srt--\A tu- tr

P4^fl

0.0 0.s 1.0 l.s 2.o 2.5 3.0 3.5 4.OT e

Frc. 8. Plot of proportion of divalent ions lVa + (Fe, Zn) versw Te tn tellurian tetrahedritess.All the values ue in apfu. The solid line in the diagram shows the fend of the ideal Teversus Ar12* relationship. Symbols as in Figure 5.

constraints (Frg. 8). Chamocket al. (1989b) investigated ArP * versus M + substitution. This is not the case forthe oxidation state and coordination of Fe in the tellurian tetrahedritesr, as most compositions do plottetrahedrite"s and concluded that divalent and trivalent close to the line ofideal substitution. In contrast to theseFe occur inthe Ml and M2 sites, respectively, but the findings, on the basis of their study of syntheticlower oxidation state predominates in natural minerals. tetrahedrite, Makovicky et al. (199O) found that at lowInsynthetictetrahedrite"",thepresenceoftrivalentFeis iron contents, iron is predominantly ferric, whereasproportional to the excess of monovalent cations (Cu + between Cur rFerSb+Sr: and CuroFezSbaSl3, iron is pre-Ag> lO apfu) resulting in a change in trend for the ideal dominantly ferrous. Whether these findings are relevant

4

F e + Z n

M + + M 2 + = 1 2

f o n a at

trER -Et6Fffift

-rutr

tr A A- A

Atrtr

a a

GOLDFIELDITE, THB TELLURIAN TETRAHEDRITE$ lt25

to tellurian tetrahedritess is uncertain, because Cu inexcess of 10 apfu is compensated by Te4*(Fig. 5). Fur-thermore, Makovicky et al. (1990) concluded thatprob-ably all iron resides atthe M2 site, regardless of its oxi-dation state.

Ttre zinc-for-tron substitution in tetrahedrite"" wasstudied experimentally by Cambi et aI. (1965). As Znonly occurs in the divalent state, it must substitute forFe in the Ml site with 4-fold coordination. A histogramof Fe/@e +Zn) n goldfieldite (Frg. 9) shows that rhereis complete sotd-solution befween pure Fe andZnend-members; however, most compositions analyzed areeither Zn- or Fe-rich. This could be a function of thecomposition of the hydrothermal fluids from whichthese crystals precipitated.

The silver content of tellurian tetrahedriter" rarelyexceeds I apfu; evenin those few cases in which it does(e.9., Kovalenker et al. 1980), the values are erratic.Harris (1990) cautioned that Ag enrichment can t2keplace during electron-microprobe analysis of Ag-richtetrahedrite owing to migration of Ag toward theanalyzed spot.

Using EXAFS spectroscopy, Charnock er a/. (1988)have ascertained that in tetrahedriters, Ag occurs in the3-coordinated M2 site, where it substitutes for Cu.Figure 5 supports this substitution, as previouslydiscussed. However, the lack of data and the lowconcentrations of Ag in the tellurian tetrahedrite""prevent us from evaluating the effect of Ag-for-Cu sub-stitution on the cell dimension. It must be pointed outthat samples of Ag-rich natural tehahedrite are charac-terized by an anomalously small unit-cell Glall 1972,Riley 1974).

Among the other metals reported in goldfieldite,gold is the most common @orjsova et al. 1986, Knittel& Johnson, unpubl. data, Kovalenker & Troneva 1980,Kovalenker et al. 1980, Sakharova et al. 1984, Trudu& Knittel, in press). In general, it occurs in concentra-tions below lVo, the only possible exception beinggoldfieldite from epithermal gold deposits in the C.I.S.(Kovalenker & Troneva 1980, Kovalenker et al. 1980),where amounts of up to 10.68 wt.Vo Att have beenreported. Kovalenker & Troneva (1980) concluded thatAu substitutes for Cu in tetrahedrite"". Spiridonov (1984)disputed the existence of Au-rich goldfieldite on thebasis that the examples of auriferous goldfieldite withAu concenffations in excess of I wt.Vo contain quanti-ties of Fe and Zn (tp to 4.16 apfu in one sample; Fig. 8)that are incompatible with the maximum content of2 apfu of divalent elements in tetrahedrite,". Further-more, X-ray-diffraction patterns of auriferous gold-fieldite by Kovalenker & Troneva (1980) indicate thataralyzed, grains are inhomogeneous (Spiridonov 1984)and may be considered as intergrowths of goldfieldite,Fe-sulfi de and calaverite.

In Figures 5 and 6, the data ofKovalenker & Troneva(1980) for Au-rich goldfieldite display a rrend differenrfrom the l-to-l substitution between Cu and the

precious metals. Furthermore, the structural formulaecalculated for these samples are not charge-balanced.These observations support Spiridonovos (1984) conclu-sion that the examples of auriferous (>l wt.Vo Au)goldfieldite of Kovalenker & Troneva (1980) probablyare mixtures of several minerals. Therefore, it appearsthat only a limited degree of Au incorporation is possi-ble in goldfieldite; we infer that Au, which falls in thesame column of the periodic table as Ag, probablyoccupies M2 sites and is monovalent.

Cadmium concentrations of less than O.lO wt. Vo intellurian tefrahedrite"" phases were reported by Spirido-nov et al. (1984),Boisovaet al. (1986) and Spiridonov(1987). [n these minerals, the Cd-for-(Fe,Zn) substihr-tion may be similal to that proposed for tetrahedrite byJohnson et al. (1986) and confirmed by Charnock et al.(1989a), in spite of the substantial difference in ionicradii between 4-coordinated divalent Fe and Za (0.66and 0.64 A, respectively, from Table l) and Cd (0.84 A:Shannon 1981).

Manganesehas been reporled to occur only in tellu-rian tetrahedrite"" in concentrations up to 0.24 wt. Vo(Spiridonov et al. 1984, Shimizu & Stanley 1991) andis also uncommon in tetrahedrite"" in general. Basu etaI. (1984) reported Mn concentrations as high as5 .7 4 wt.Vo from the Rajpura - Dariba deposit in Raj astan(India), characterized by low contents of Fe and Zn,indicating that Mn in goldfieldite occupies a tetrahedralposition, substituting for divalent Fe and Zn as intetrahedrite.

Mercury, not surprisingly, has been found ingoldfieldite, as there is a pure mercurian end-member,schwazite of the tetrahedrile"" series. However, Hg intellurian tetrahedrite," has been reported only from the

o 10 20 30 4A 50 60 70 80 90 100

100 FeF e + Z n

Flc. 9. Histogram of the ratio 100 fFe/(Fe + Zn)] in telluriantetrahedrite"".

(J coz - -=.Or4l 40&l!

tt26 TT{E CANADTAN MINERALOGIST

Akturpak deposit in Russia (Kovalenker & Rusinov1986). Kalbskopf (1971) showed that divalent Hg is4-coordinated in mercurian tetrahedrite, thus substitut-ing for other divalent metals such as Fe and Zn. ln4-coordination, Hg has a radius of 0.84 A lshannon1981), the same as Cd.

The X site

The X site is of considerable interest, because it iswhere Te-for-(As,Sb,Bi) substitution takes place.Figure 10 shows the occupancy of this site; thereis considerable scatter, if all data are considered(from 3.4 to 4.6 apfu), but the values cluster within the3.9 to 4.2 apfu ntewal. The relatively large spread ofthe data, which does not occur in the data obtained bySpriager (1969), Kase (1986) and Trudu & Knittel (inpress), may, in some instances, be due to the standardi-zation of S on unsuitable reference materials, as previ-ously discussed.

Tellurium. In nature, tellurian tetrahedrite"" crystalscontain up to 3.4 apfu of Te; there are no gaps in thissotd solution (Figs. 6, 8, I 1). Figure 1 1 presents a plotof the concentration of Te yersas the sum of As, Sb andBi concentrations. These data points show a very highcorrelation coefficient (0.97), supporting the assumptionthat Te substitutes for any of the three semimetals.

Arsenic, antimony and, to a lesser extent, bismuth,are the most common elements substituting for Te ingoldfieldite. Johnson et al. (1986) inferred thatSb-for-Te substitution may be prevalent, but our data,plotted as Figure 12, indicates that there is no preferredsubstitution between any of As, Sb and Bi. Whereasvirtually unlimited substitutions is observed among As,Sb and Te (up to a maximum of 3.5 Te ap[u), the c,,r-rently known maximum Bi contents is I apfu (Fig. l2E).This is not surprising, because a pure Bi-bearingtetrahedrite"" end-member has never been doc'mentedto occur in nafure.

TheYan"dZsites

In goldfieldite, the twelve Yand one Z sites are oc-cupied by S and Se, and not by Te, as believed earlier(e.9., Nowacki 1969). Figure 13 presents histograms ofthe total number of anions per formula unit; the upperdiagram ofFigure 13, based on all available data" showsa rather broad spread from 11.3 to 14.0 apfu S + Se,with most data clustering around 13 apfu.ln the lowerhistogram, the data of Kase (1986) and Trudu & Knittel(in press) are displayed; these cover the entire range ofthe Te versus (As + Sb + Bi) substitution and wereobtained under optimal analytical conditions. These datacluster in the very limited range of l3.O X 0.3 apfuS + Se. This suggests that the large spread in fhe com-plete dataset (including data of Knittel 1989) resultsfrom the use of an inappropriate S standard, as previ-ouslv discussed.

S is replaced to a maximum of 2.8 apfu bySe (Fig. l4). Tellurian tetrahedrite with an elevated Secontent was found only in samples from the Ozernoyeepithermal Au deposit (Kamchatk4 Russia: Kovalenkeret al. 1989, Spiridonov & Okrugin 1985), and ftom theWild Dog epithermal Au system on Bougainville Island

@apua New Guinea: Noviello 1989). As the experi-ments at 340"C by de Medicis & Giasson (1971b) onthe system Cu-Te-Se failed to produce the Se-bearinghomologues of goldfielditeo we cannot assess whethercomplete Se-for-S substitution exists in goldfieldite, asit does in sulfides @etlke & Barton 196l).

Other elements

In addition to the aforementioned elements, V, Co,Ge, Sn and Pb have in cases been reported in ffaceamounts in goldfieldite. Vanadium so far has only beenreported by Spir idonov et al. (1984), who found0.02wt.Va V in goldfieldite from Uzbekistan, a value so

TOTAL SEMFMETAL ATOMS PER FORMULA UNIT

Flc. 10. Histograms of the occupancy of the X site by semi-metal (As, Sb, Te and Bi) ions. The upper diagram repre-sents all data on tellurian tetrahedrite""; the lower histogramshows compositions reported by Kase (1986; hatchedpattern) and Trudu and Knittel (in press.; dotted pattem)for comparison.

160

4.44.23.2

30

(Jzlllfcyrl!E,L

4.44.03.8

o

4

E 3+

.cla4, 2

T

tt,

1

o

close to the detection limits that the reality of Vincorporation in goldfieldite may be questioned.Besides, in an exploratory study of minor-element in-corporation in synthetic tetrahedrite, Makovicky &Karup-Mg1ler (1994) found that V does not enter thismineral. Reported contents of cobalt likewise are verylow [Spiridonov et al. (1984) reported 0.03 wt.% Co inthe same sample in which V occursl, but Makovicky &Karup-Mgller (1.994) were able to synthesize theCo end-member of tetrahedrite.

Concentrations of up to O.l7 *.Vo germaniumwerereported by Spiridonov et al. (1984). Considering thatGe is invariably tetravalent (like Te), the most likelylocation for Ge is the X position, where it may substi-tute for As, Sb, Bi or Te. The only problem is thatGe tends to be 4- or 6-coordinated in sulfosalts, whereasthe X site is a trigonal pyramid. This fact probablyaccounts for the very low Ge concentrations ingoldfieldite.

Tin may be a relatively common trace constituent inthe tellurian tetrahedriter" from the Kochbulashkepithermal gold deposit in Uzbekistan: Kovalenkeret al. (1980) reported up to 0.22 wt.7o Sn, andNovgorodova et al. (1978), up to 2.95 wt.% Sn fortetrahedritess from this deposit. However, the data byNovgorodova et al. (1978) referred ro tetrahedritesamples that cont2in only trace emounts (< | wL.Vo) ofTe. Considering that Sn falls in the 54ms sohrmn of theperiodic table as Ge and tends to occur in nature in itstetravalent oxidation state (e.9., in srannite, colusite,etc.), it is very likely that this element can best beaccommodated within the X sites of the tellurian mem-

t l n

bers of the tetrahedrite"", substiruting for the semimetalslike Ge.

Izad, is another element that rarely occurs in gold-fieldite (Kovalenker et al. 1980, Kovalenker & Rusinov1986); its maximum concentration reaches 0.41 wt.Vo.In tetrahedrite"", however, values up to 12.3 wt.%o havebeen recorded by Vavelidis & Melfos ( 1997) and Bishopet al. (1977). These studies show that plumbianteDnantite is low in Fe and Zn; therefore, these authorsinfer that Pb substitutes for these divalent elements. Incontrast, based upon data from letrahedritess mineralswith Pb concentrations of less than 2 w.Vo,Basu et al.(1981) raised the possibility that Pb substitutes for thesemimetals in the X site, because in some sulfides andsulfosalts, it can occur in 3-fold pyramidal coordination,which is the same as that of the X site. They still sug-gested that Pb should be divalent. Makovicky & Karup-Mpller (1994) synthesized tetrahedrite with up toO.4Pb apfu, whereas attempts to synthesize Pb-richtennantite failed (<0.1 Pb apfu). All tetrahedrile"" con-taining significant 4mounts of Pb (both synthetic andnatural material) contain ca. 4 (As,Sb) apfu,hence webelieve that Pb substitr.rtes for Fe and Zn.

The preceding discussion leads us to propose thefollowing chemical formulae:

1) for tellurian tetrahedrite and tennantite (Frg. 17shows that tellurian Bi-bearine tetrahedrite does notoccur in nature):

N ]Cn' a* SF e,Zn Mn,Co,Cd,Hg,Pb)z* z-,1m11Cu,Ag,Au)*6lN[(As,Sb,Bi)3*q-,(Te,Ge,Sn)4"]rv11s,se;i12u11s,se;1

GOLDFIELDITE. TTIE TELLTJRIAN TETRAHEDRITE."

tr -?\l{loo

oG

- I ? trr -tr_ U

A s + S b + B i + T e = 4

2.0T e

Flc. 1 I . Plot of Te contgnt versw the sum of As, Sb, ard Bi. The solid line in the diagramshows the trend of the ideal Te-for-(As + Sb + Bi) substitution. All values arc h apfu,symbols as in Figure 5.

4.O3.53.02.51 . 51 .00.50.0

a

lr28

As

Sb

1.5

Bi 1.0

0.5

0.0

THE CANADIAN MINERALOGIST

4

2.O (c)

+

+#+ + + + - l

. -I .Sf+t+ +f * + + + * . - ' l

i**+ * t f * +

+. r+ -c*i{.-J + ++

12.5 13.0

TOTAL S+SeATOMS

Frc. 13. Histograms of the occupancy of the I and Z sites by(S t Se) in tellurian tetraledrite"". All values ue n apfu.The upper diagram represents all available data on tellu-rian tetrahedrite""; the lower histogram shows analyticalresults of Trudu & Ifuiael (in press; dotted patt€rn) andKase (1986; hatched pattem) for comparison.

Notr,mNcraruns

No official system of nomenclature exists to definetetrahedrite"r minerals according to their Te content.Two definitions of goldfieldite exist: Kato & Sakurai(l97oa) and Spiridonov (1984) suggested 16 name thetetrahedrite"" goldfieldite where Te is the most abundantsemimetalo whereas Dmitriyeva et al. (1987) suggestedthat the mineral be named goldfieldite only where Te isgreater than the sum of the other three semimetals. Theformer definition conforms with the rules on the nomen-clature of the tetrahedrite"" minerals set by the IMA(Spiridonov 1984).

Kovalenker et al. (1990) proposed an equilateraltriangular projection with Te (goldfieldite), Sb + Bi

1 1 . 5

4

TeFtc. 12. Plots of Te content yer.rus (a) As, (b) Sb and (c) Bi. All

the values are n apfu.

where 0 !x<2,and

2) for goldfieldite, which according to Figure 17can be pure, arsenoan or stiboan, the formula is:

rv1cu*61r11Cu,Ag,Au)+6,lIv[(As,Sb,Bi)3+2-,

(Te,Ge,Sn)421rv;1S,Se;t21u11S,Se;1where 0 <x12.

i-f-ifu+ .rrffi. i f*--+

(tetrahedrite) and As (tennantite) at the vertices.They divided i t into six f ields: Sb-goldf ieldite,As-goldf ieldite, Te-tetrahedrite, Te-tennanti te,As-tehahedrite and Sb-tennantite. We believe that thisnomenclature for the tetrahedrite"" is inadequate for thefollowing reasons: 1) it implies that the Sb substitutesfor Bi only, an interpretation that is not always correct(see below). 2) Some misnaming is likely 1s take placenear the vertices, where nearly pure end-members willhave the prefix of the second most abundant elementowhich is present in insignificant concentrations. 3) Theterms used by Kovalenker er aI. (1,990) to name the dif-ferent fields do not conform the IMA recommendations(Nickel & Mandarino 1987): the use of a hyphenatedchemical symbol preceding the name of a mineral isincorrect and is to be avoided.

Thereforeo we suggest a new chemical nomenclaturegdd for tetrahedrite"", in which the M sites are predomi-nantly occupied by Cu, and S is the main anion. Ideally,as there is probably complete solid-solution amongst As,Sb, Te and Bi in the tetrahedrite"", a tetrahedron witheach end-member occupying a vertex constitutes themost appropriate representation (see center of Fig. l5).We neglect ofher elements, such as Ge and Sn, whichmay be accommodated in theXsiteo because they occurin insignificant amounts only in goldfieldite. The valuesused for this plot must be in apfu and normalized to1007o. However, as three-dimensional representationsare very difficult to portray on a sheet of paper, it ismore convenient to use the four faces of the tetrahedronindividually. If only three of the four elements arepresent, then the selection of the appropriate triangle isobvious. If all four elements occur in the mineral, thenan approximation has to be made: it is proposed that the

I t29

triangle containing the three most abundant elements beused. Geometrically, this form of approximation is per-formed by extending the line connecting the vertex ofIeast abundant element to the point inside the tetrahe-dron representing the composition of the mineral untilthis line intersects the face of the tetrahedron. Numeri-cally, this procedlue is followed by recasting the quan-tities of the three most abundant semimetals to lOOVo.This geomeric approximation has the advantage ofretaining the proportions amongst the three most abun-dant semimetals as they occur ia the original structuralformula. Considering that in tetrahedrite""o tle leastabundant semimetal, where present, occurs in very lowconcentrations, the error introduced by this procedureis negligible.

The four triangles of the "exploded" tetrahedronrepresenting the tetrahedrite"" are shown in Figure 15.Regarding the nomenclature of the different fields in thetriangles, we have followed these criteria:

1) the tetrahedrite"" mineral is named after the mostabundant semimetal present, as is usually done.

2) The same principle is applied to distinguishbetween a pure end-member (i.e., it must have a ratiogreater than 3:1 between the two most abundantsemimetals) and one that has substantial amounts ofanother element. In the latter case, the appropriateadjectival modifier has been used, as outlined by Nickel& Mandarino (1987).

3) The lack of an approved name for the Bi-bearingend-member of the tetrahedriters creates some problemsin nomenclature. In the Soviet literature, the term"anniviteo', eponymous after the Annivier Valley inSwitzerland, is commonly used for this mineral. How-ever, this name is not officially recognized by the IMA

S e

GOLDFIELDITE, TTIE TELLTJRIAN TETRAHEDRITESS

r Wild Dog (Noviello, 1989)

a Ozernoye (Spiridonov & Okrugin, 1985)

Kamchatla (Kovalenker et al. 1989)

IT

s \"q

"rj

FIG. 14. Plot of S content yers us Se (apfu) illustrating the substitution between these ele-ments. The line in the diagram corresponds to a total of 13 (S + Se) aplz. Symbols as inFigure 5, except for ttrose for Se-rich compositions, which are identifred in the figure.

t 51 21 i1 0

I 130

@.H. Nickel, pers. commun. 1991), because a naturalmineral close to Culq(Fe, Zn)2BiaS13 has not been found;one of the highest Bi contents in a tetrahedrite"" mineralis 1.60 apfu reported by Lur'ye and coworkers (Jambor& V4nko 1990). Therefore, the name of "Bibearingtetrahedrite" has been choseno bearing in mind that thename 'letrahedrite" does not imply that Bi substitutesexclusively for Sb. The proposed chenical nomencla-ture aims at emphasizing the chemistry of tetrahedrite""minerals. It conforms to the recommendation of the IMAfor temary solid-solutions (Nickel 1992) n the sensethat in each triangle the three mrin areas subdivided bythe 5OVo rule keep the name of tle end-member. Thefurther subdivisions are informal.

The structural formulae of all the samples in thedatabase have been calculated following the criteriapreviously outlined; the results, which exclude syntheticgoldfieldite, are plotted in Figure 16 according to the

THE CANADIAN MINERALOGIST

Ftc. 15. New nomenclature grid for minel4ls of the tetrahedritess group in which theM sites are predominsndy 6ssupied by Cu. The triangles correspond to the faces ofthetetrahedron shown in the center of the figure. Abbreviations: Gfd: goldfieldite, Ten:tennantite, Tet tetrahedrite, BlTet: Bi-bearing tetrahedrite, Ars: arsenoan, Bis:bismuthoan, Stb: stiboan, and Tell: tellurian.

Aesb

nomenclature of Figure 15. Most of the compositionsfall within the Te-Sb-As triangle; only a few plot in theTe-As-Bi triangle; in only four is Te the least abundantsemimetal. Only about llVo of the compositions fallwithin the goldfieldite field, the majority being eitherzrsenoan or stiboan, but not bismuthoan. The remain-ing compositions show an approximately even distribu-tion in all the other fields (tellurian tetrahedrite andtennantite, arsenoan tetrahedrite and stiboan tennantite);only a few cluster in the fields of the pure brahedriteand tennantite.

DrscussroN

From a crystallochemical point of view, two aspectsdeserve further clarification: (a) the substitutions involv-ing atoms with relatively large differences in radii(e.g., Cd for Fe and Zn), and (b) the absence of

As

Te

GOLDFIELDITE, THE TELLURIAN TETRAHEDRITE$ I 131

goldfieldite with a Te content greater than 3.5 apfu innature. Although goldfieldi0e and tellurian tetrahedrite""are ore minerals, they have neverbeen reported to occurin quantities of economic significance within orebodies,unlike tehahedrite, tennantite and freibergite. Sakharovaet al. (1984) regarded goldfieldite as "typomorphic ofgold-telluride deposits localized within volcanicregions". Knittel (1989) suggested thar As-r ichgoldfieldite is more common in "polymetallic Cumineralizationo', whereas the Sb-rich variety seems tobe prevalent in'base-metal-poor precious metal depos-its". We will reassess these hypotheses on the basis ofour database.

Subsfintions of relatively large atoms in telluriante trahe drit e * minerals

One of Goldschmidt's rules (Faure l99l) on substi-tutions in minerals states that the radii of the two atomsinvolved in the substitution should not differ by morethan 157o.In tellurian members of tehahedrite"", thereare a few cases that do not follow this rule: they involveCd and Hg for the Ml site, Ag for M2, and As for X.Considering that tetrahedrite"" minerals have a sfructurethat is sufficiently "flexible" to allow distortions, thereplacement of smaller atoms (e.9., Cu, Fe and Zn) bylarger ones (Cd and Hg) can be accommodated throughrotation of tetrahedra around their 4-fold axes CN.E.Johnson, pers. commun., 1992). This is very likely thecase for goldfieldite, in which the Cd and Hg concen-trations are extremely low.

The substitution of Ag for Cu in tetrahedrite"" haspuzzled scientists for a long time, as with more than4 Ag apfu the cell size decreases, rather than increases,as suggested by the larger size of Ag. Chanock et al.(1988) anributed this to Ag - Sb interaction as deducedftom EXAFS spectroscopic results. In goldfieldite, how-ever, the maximrrm observed Ag content barely exceedsI apfu;thtsitoutside the high-Ag region, and it is likelythat the Ag-for-Cu substitution is accommodatedthrough distortion. The Te-for-As substitution isreflected by a change in the cell dimension, as previ-ously shown.

Absence of end-member goffieldite in nature

There appear to be no crystallographic reasons toexplain why end-member goldfieldite does not form innature. Hydrothermal synthesis of goldfieldite was car-ried out by Kalbskopf (1974): his runs lasted 25 hoursat 230oC under vapor-saturated conditicins and producedgoldfi eldite (88 wt.%o), digenite (8 wt.Vo) and native Te(4wt.Vo). Kalbskopf (1974) infened that 180'C shouldbe considered as the lower l imit of stabi l i ty ofgoldfieldite, and favorable pH conditions are neutral tomi ld ly a lka l ine . Kova lenker & Rus inov (1986)suggested that the formation of goldfieldite requiresoxidizing conditions and high activity of sulfur. Dry

Sb ArsTet StbTen AS

Frc. 16. Classification of 361 samples of natural goldfieldite(electron-microprobe data). In the Te-Sb-As plot, thecrosses indicate compositions for which Bi was not sought,whereas the circles refer to compositions with values forBi. The abbreviations of the fields are as the same as inFigure 15.

Bi-Tet Io "

t t 32 THE CANADIAN MINERALOGIST

synthesis within the system Cu-Te-S was undertakenby S. Karup-Moller (unpubl.) to determine phase rela-tionships at 350o, 450o, 550o, 675', 800' and 900"C.Only runs at 350o and 450oC produced goldfieldite:at the former temperature, it coexists with digenite,covelliteo native Te, phase A (CusoTez:S+z) and a liquidof composition Te13Ss7, whereas at 450"C, goldfielditeis in eqnilibrium with digenite, covellite and a liquid ofcomposition Cu y9.sTes6.5Ss.5. These experiments indi-cate that pure goldfieldite is stable at certain conditionsand could precipitate under natural conditions given theoptimum composition of hydrothermal fluid. However,natural hydrothermal fluids from which telluriantetrahedrites" precipitate probably always contain at leastminor amounts of As, Sb and Bi, hence these elementsare incorporated to some extent in the solid solution.

Ore deposits hosting goldfieldite

Our review shows that tellurian tetrahedrite"" is mostcommonly found in epithennal gold deposits. Althoughwe have only scant information on the mineralogy andalteration of the majority of these deposits, it appearsthat high-sulfidation epithermal gold deposits are themost common host of this sulfosalt. This is not sumris-ing, as high base-metal contents and the pr"."n""of sulfosalts (e.9., enargite - luzonite) are two of thecharacteristics of high-sulfidation systems (White &Hedenquist 1990).

Five occurrences of goldfieldite in porphyry envi-ronments are reported. At Tirad, in the Philippines,goldfieldite is associated with deep-seated advancedargillic alteration (Trudu & Knittel, in press), whichcross-cuts the main porphyry-style mineralization andshows strong similarities to the high-sulfidation l,epantoCu-Au-As deposit, where Gonzalez (1956) reported theoccurrence of tellurian phases. At the Marian golddeposit, also in the Philippines, goldfieldite belongs toan assemblage of minerals typical of low-sulfidationepithermal systems that precipitated in polymetallicveins at the periphery of porphyry-Cu minqall2alisl(Ituittel1989).

At Butte, Montanao Springer (1969) reported theoccurrence ofgoldfieldite in association with colusile,but he did not give precise indications regarding thetiming of precipitation of these minerals. There is onlyvery limited reference to the timing of the deposition ofcolusite at Butte: Thompson (1973) reported it in the"horsetail zone", which is characterized by predomi-nantly sericitic and minor advanced argillic alterations(Meyer et al. 1968), indicating that colusite and theassociated goldfieldite precipitated toward the end of thegeochemical evolution of the porphyry-style minslali-zation at Butte. In the Goldfield area Nevada. orebodiesform irregular sheets and pipes within systems ofsilicified veins hosted by extensively acid-sulfate(alunite - quaftz) altered rhyodacite, and it has beenspeculated that a porphyry-type system may lie at depth

nearby, although probably not directly beneath Gold-field (Wallace 1979).

Finally, at the Murgul copper deposit in Turkey(Willgallis et al. l99O), bismuthoan tennantite with upto 0.87 wlVo Te has been analyzed, but no data on itsparagenetic position were provided. Schneider et al.(1988) showed that "fahlore" is related to the latest stageof alteration, which is siliceous in character; althoughthey infer a volcanogenic origin for the Murgul deposit,most of their observations appear to be indicative of aporphyry-Cu style mineralization.

The occurence of goldfieldite in the Besshi-typedeposits in Japan (Kase 1986) suggests that this mineralalso is associated with volcanogenic massive sulfide(VMS) mineralization. Kase (1986) reported that lower-amphibolite-grade metamorphism affected the Sazareard Ikadazu deposits, and that tennantite and telluriantennantite, together with chalcopyrite, bornite andsphalerite, occur in the interstices of granular pyrite. Thedescription does not provide enough detail to inferwhether the tellurian minerals of the tetrahedrite,,forrned prior or during metamorphism. The Bittibulashkdeposit, in Russia, where tellurian tennantite occurs(Loginov et al. 1983), likewise appears to be a VMSdeposit.

We conclude that goldfieldite and other tellurianmembers of tetrahedrite"" occur in both high- and low-sulfidation epithermal Au mineralization, but also canform during the last stages of porphyry-style minerali-zation and in VMS deposits. Our review also shows thatAs and Sb enrichment in goldfieldite is not related tospecific styles of mineralization, as Knittel (1989)suspected. Triangular Te - Sb - As and tetrahedralTe - Sb - As - Bi plots @g. 17) show that goldfielditefrom the Tirad deposit in the Philippines (Trudu &Knittel, in press) and the Megradzor deposit in Arme-nia are enriched in As. In contrast, at the Goldfielddeposit, Nevada (Knittel & Johnsono in prep.), theAu deposits of the Kamchatka Peninsula, Russia(Kovalenker et al. 1989) and the Au deposits from theKuraminski Range in Uzbekistan (Novgorodova el aL1978), the tellurian members of the tetrahedritess areenriched in Sb. All these deposits display characteris-tics typical of epithermal-style mineralization.

Another interesting aspect of the data displayed inFigure 17 is that in each deposit, the ratio of the twomost abundant trivalent semimetals is approximatelyconstant, whereas the Te content is variable. This mayreflect constant As/Sb and variable Te conlents in thehydrothermal fluids from which the tellurian tefrahedriteprecipitated, but may also reflect a variable abiliry oftetrahedrite"s to accommodate Te, depending on thephysicochemical conditions. Thermodynamic dataregarding the As-Sb-Te exchange are not available.In their study of the thermodynamic properties oftetrahedrite and tennantite, Sack & Loucks (1985)stated that "tetrahedrites-lennantites with intermediateSb/(As + Sb) ratios have greater As/Sb ratio than the

GOLDFIELDITE, TIIE TEIITURIAN TETRAHEDRITEss

Tlrad deposlt, Phlllpplnes(Trudu and Knittd, in prep.)

Megradzor deposit, Armenia, C.l.S.(Kovalenker et al., 1990)

I 133

Sazare and lkadazudeposits, Japan

(Krq 1886)

Karnchatka Peninsuladepodts, Russla, C.l.S.

(Kovalenker et al., 1989)Tet e

Goldfreld deposit,Nevad4 U.s.A.

(Knifiel and Johnsorxin prep)

Kuraminski Range d€Postts'uzbekistan, Cl-S-

(Novgorodova et al., 1978)

As As

Frc. 17. Triangular plots of tellurian tetrahedrite,, minerals from selected ore deposits. The dashed lines represent the best fit to

the data of each olot.

n34 THE CANADIAN MINERALOGIST

hydrothermal solutions from which they precipitated".Therefore, the ratios amongst semimetals in tetra-hedrite"" minerals are not directly indicative of the com-position of the fluid from which they precipitated. Inseveral ore deposits in which tetrahedrite-tennantite isthe only As- and Sb-bearing phase, its Sb content in-creases with the evolution of the hvdrothermal fluid(Sack & Loucks 1985).

Cor.tcl-ustows

In the present study, we show that goldfieldite andother tellurian phases of the tehahedrite"" seem to berelatively rare in comparison to other species of the semegroup, such as tehahedrite, tennantite and freibergite.This may be due to the fact that Te is rarely enriched innature. Furthermore, Te has not been routinely soughtduring electron-microprobe studies of sulfosalts.

Incorporation of Te into tetrahedritess leads to cou-pled substitutions to maintain an overall charge-balance.These substitutions, initially proposed by Kase (1986)and confirmed by data obtained by Trudu & Knittel(1991) and Shimizu & Stanley (1991), can be summa-rized as follows:

For replacement of up to two atoms of trivalent semi-metal by letravalent Te, divalent Fe and Zn are replacedby monovalent Cu. Higher Te contents are compensatedby^ a reduced number of Cu atoms. Interestingly, Cu -Il2 * substitution in goldfieldite takes place in the Mlsite, whereas the vacancies occur in the M2 position.

Where substitutions iavolving elements of differentvalence take place, then the charge balance in the min-eral can be maintained in two ways, either through acoupled substitution or through a vacancy in the struc-ture. In renierite, which according to Bernstein et aL(1989) has the formula Curo(Znr-,Cu")(Gez*,As)FeaS 16where 0 (-r ( 1, on-ly the coupled substitution occurs: astrivalent As replaces tetravalent Ge, monovalent Cutakes the place of divalentZn. An example of the sec-ond mechanism is shown by colusite (S. Merlino, pers.commun., 1992): the charge balance in the solid solu-tion between the Sn-rich (Cu26V2Sn2AsaS32) andSn-poor varieties (Cu2aV2As6S3) is maintained throughvacancies in one of the Cu sites (note that As ispentavalent in this mineral) with a reduction of Cuatoms from 26 to 24. As shown throughout this paper,goldfieldite seems to be unusual in that both mecha-nisms of maintaining charge balance occur, as Te pro-gressively substitutes for As, Sb and Bi.

We hope that this work will make geoscientists moreaware that Te (and also Bi and Se) may be significantcomponents in minerals of the tetrahedrite$ series. Withmore analytical data, we will be able to re-assesswhether a specific signature in mineral composition isfypical of a certaia style of mineralization, Petersen &McMillan (1992) have shown an application of thecompositional data of the letrahedrite"" minerals to min-

eral exploration and mine development; however, theyhave not extended their work to the tellurian membersof this series.

We have experimental work in progress on thetetrahedrite"" with variable Te content in order to meas-ure the coordination of Te through EXAFS spectroscopyand the variation in cell size as a function of crystalchemistry. We hope that one day the work on the ther-modynamic properties of tetrahedrite and tennantite willbe extended to goldfieldite as wello so that we can gaina better understanding of the conditions under which itprecipitates in ore environments.

AcIc.{owLEDGEMENTS

Special fi1anks go to Neil Johnson for thoroughlyreviewing the manuscript. Gus Mumme, Nick Oliverand Mike Solomon commented on some drafu. Discus-sions with Aldo Cundari, Alex Losada-Calder6n andMarc Marshall were very helpful. Ernst Burke, StefanoMerlino and Ernie Nickel made important suggestions.Sven Karup-Mgller kindly provided a preprint of hismanuscript on the Cu-Te-S system. Tom Hall's assist-ance with the geometrical approximation for the classi-fication of goldfieldite is appreciated. Finally, we thankE. Makovicky for a very insightful review, an anony-mous reviewer for his positive review, and the editor,Robert F. Martin, for editorial help.

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Received July 7, 1992, revised manuscript accepted May 20,1998.