The Distribution of Ag and Sb in Galena: Inclusions Versus Solid Solution
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American Mineralogist, Volume 78, pages 85-95, 1993 The distribution of Ag and Sb in galena: Inclusions versus solid solution Tnorras G. Sn.lnpx Departmentof Geology,Arizona StateUniversity, Tempe,Arizona 85287-1404,U.S.A. PBrBn R. Busncr Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287-1404,U.S.A. Ansrnacr The distributions of Ag and Sb in galena samplesfrom La Paz and Zacatecas,Mexico, were investigatedusing electron microprobe analysis,backscattered-electron imaging, and high-resolution transmission electron microscopy. Both samples contain numerous rod- shapedinclusions of diaphorite (PbrAg3Sb3S8),and the LaPaz samplesalso contain franck- eite [(Pb,Sn).SnrSbrFeS,o].Although diaphorite is a common Ag-bearing inclusion in ga- lena, franckeite has not been previously reported. Both sampleshave Ag concentrations nearly equal to those of Sb, indicating coupled substitution of Ag* and Sb3*for 2 Pb,*. The average Ago,Sbo,S contents of the bulk La Paz and Zacatecassamples (including inclusions)are approximately1.37 and0.44 molo/0,respectively. Of thesetotal AgorSborS amounts, 0.48 and 0.32 molo/ooccur in solid solution in galenain the La Paz andZacatecas samples,respectively. Inclusions ofdiaphorite are from 20 nm to 50 pm long, but no evidencefor Ag-Sb atom clusterswas found. Rod-shapeddiaphorite inclusions are oriented with their c axes (rod axes)parallel to the ( 100) directions ofgalena and their a and b axesparallel to the ( I l0) galenadirections. The diaphorite-galenainterfaces are semicoherent,with misfit disloca- tions every 6 to 10 nm. Disk-shaped inclusions of franckeite in the LaPaz samplesare oriented with their a*, b, and c axes parallel to the (100) directions of galena.The top and bottom interfacesofthe disks are parallel to the (100) layers offranckeite and appear to be coherentwith the galena.The franckeite inclusions have a modulated-layer structure with a wavelength of 3.55 nm, which is significantly smaller than that of previously de- scribed franckeite. Both diaphorite and franckeite inclusions appearto have resulted from coherent exsolution at low temperatures.Diaphorite appearsto occur metastably relative to freieslebenite(AgPbSbSr), suggesting that there is a coherent solvus betweendiaphorite and galena. INrnooucrroN from material that is structurally bound in solid solution The distribution of precious metals in sulfide minerals is crucial for understandingthe distributions ofprecious is of economic, crystallographic, and geologic impor- metals and the processesof their incorporation and re- tance. Accurate knowledge of precious metal distribu- covery. This is particularly difficult for invisible Au, which tions is of economic importance for maximizing metal occurs as either submicroscopicparticles or in solid so- recovery and improving metallurgical techniques. The lution with sulfides, sulfarsenides,and arsenides.Ag oc- crystallographic significanceof precious metal distribu- curs in more minerals than Au and is common in many tions is in understanding how "foreign" atoms are ac- base-metalminerals, either in the form of Ag-bearing in- commodated in sulfide structures and in the crystallo- clusions or in solid solution. By overlooking Ag in such graphiccontrols on exsolution.Understanding the primary common minerals, significant amounts of Ag are lost in vs. secondarynature of precious metals, as well as the processing(Cabri, 1987, 1992). processesand conditions of ore deposition, is of signifi- Microbeam techniquesare very useful for determining cant geologicimportance. the concentrations and distributions of precious metals The occurrenceof precious metals in sulfide minerals in minerals (Cabri, 1992), but such techniques cannot and their economic importance has been addressedby, always distinguish precious metals in solid solution from among others, Cabri (1987, 1992) and Chryssoulis and microscopic inclusions of precious-metal-bearingminer- Cabri (1990). Distinguishing submicroscopic inclusions als. If such inclusions are not accountedfor in analyses, the result could be unrealistically high solid-solution es- - p..*"t BayerischesGeoinstitut, Universitiit Bay- timates or incorrectly interpreted substitution mecha- "Odress: reuth,Postfach 10 12 51,8580 Bayreuth, Germany. nisms. SEM imaging of samplescan be usedto determine 0003-o04x/93/0102-o085$02.00 85 86 SHARP AND BUSECK: Ag AND Sb IN GALENA the presenceof microinclusions that are larger than about galena solid solution below 420 to 390 "C (Hoda and 100 nm, but the high spatial resolution of transmission Chang, 1975; Amcofl 1976) thar may be useful as an electron microscopy (TEM) is neededto observe smaller indicator of deposition temperatures.However, our re- particles.In addition, the structural relations betweenin- sults, as well as previous studies (Czamanskeand Hall, clusions and the host mineral can be obtained with TEM 1976;Laflammeand Cabri, 1986a,1986b), suggest that to interpret better the origins of inclusions. High-resolu- diaphorite (PbrAg.SbrSr),rather than freieslebenite,is the tion transmission electron microscopy (HRTEM) has re- major inclusion type in Ag- and Sb-bearinggalena. cently been applied to the problem of invisible Au in The purpose ofthe present study is to investigate the arsenopyrite(Cabri et al., 1989) and disseminatedAu in distribution of Ag and Sb between solid solution and in- Carlin-type deposits(Bakken et al., I 989). In the first case clusions in galena samples from two Ag deposits. The (Cabri et al., 1989),Au particles were not observed,sug- compositions of the galenaand inclusions are determined gestingthat the Au is structurally bound, whereasBakken with EMPA. The type and size distribution of the inclu- et al. (1989) found Au particles less than 20 nm in di- sions are determined with TEM, as are the structural re- ameter that had not been previously observed. lations to the host galena.The origins of the inclusions High-sensitivity analytical techniques,such as proton- are discussedin terms of their morphologies and crystal- induced X-ray emission (Cabri, 1987; Cabri et al., 1984, lographic relations to galena. 1985)and secondary-ionmass spectrometry (Mclntyre et al., 1984;Chryssoulis et al., 1986; Chryssoulisand Cabri, AND CHARACTERIZATToNTECHNTQUES 1990; Cabri et al., 1989, l99l), combined with careful S,l'rvrpr-rs inspection for inclusions, have been used to show that The Ag- and Sb-bearing galena samples used in this trace concentrations of precious metals occur in many study were from the La Paz and Zacalecas Ag districts, sulfide minerals. Samplesof galena containing relatively Mexico. Most publishedstudies of Ag-bearinggalena have high concentrationsofAg, Bi, and Sb have been studied focused on coupled substitution of Ag with Bi; for this using electron-microprobe analysis (EMPA) (Laflamme study sampleswere chosenthat contain up to I wto/oAg and Cabri, I 986a,I 986b;Foord et al., I 988),but HRTEM and Sb, with no detectableBi and only trace amounts of (Sharp and Buseck, 1989) and scanning tunneling mi- As, Sn, and Te. Both samplescontain Ag-Sb inclusions, croscopy(STM) (Sharpet al., 1990)have only recently predominantly diaphorite, and minor amounts of other been applied to Ag-bearing galenas. sulfosalts.These galena samples were also chosenfor their ConcentrationsofAg in galenaare variable and depend large crystal size, which was required for previously re- on the presenceof other elementssuch as Sb and Bi. The ported STM investigations(Sharp et al., 1990). simple substitution of Ag for Pb (2Ag- : Pb'*) is limited In addition to the Ag- and Sb-bearingsamples, Ag-free to a maximum of 0.4 molo/oAg.S at 700 oC and less at galena (from an unknown location) and synthetic Ag- lower temperaturesbecause half of the Ag must go into bearing and Sb-Bi-free galena(provided by Louis Cabri) interstitial sites(Van Hook, 1960;Karup-Moller, 1977). were examined. These sampleswere used as controls to Where Ag* substitution for Pb2* is coupled with substi- determine whether observed defects correlate with Ag and tution of Sb3*or Bi3* [Ag* + (Sb,BD3*: 2Pb"f, the charge Sb concentrations. and cation-anion ratio are balanced, allowing higher Ag Chemical analyseswere obtained with a Jeol JXA 8600 concentrations. Galena with combined Ag, Sb, and Bi Superprobeelectron microprobe using an acceleratingpo- concentrationsgreater than 0.5 wto/ois called galenasolid tential of 15 kV. Cleavage fragments of galena were solutionby Foord et al. (1988),who report samplescon- mounted in epoxy resin, polished,and C coated,resulting taining as much as 9.4 and 18.5 wto/oAg and Bi, respec- in polished surfacesnearly parallel to (100) planes.Back- tively. Whereas excessAg relative to Sb or Bi is limited scattered-electronimaging (BEI) was used to locate sul- by interstitial substitution of Ag, excessSb or Bi is more fosalt inclusions and to observetheir textural relations to easilyaccommodated by the substitutionmechanism 2(Sb, the galena host. Inclusions and the galenahost were an- Bi;'* * tr : 3Pb'?*),resulting in vacancieson Pb sites. alyzed with wavelength-dispersive X-ray spectroscopyfor The lattice constantsfor the AgSbSr-galenasolid solution Pb, S, Ag, Sb, Bi, As, and Sn, using common sulfidesand show a negative deviation from ideality, which is inter- Bi metal as standards.The galena host (between inclu- preted as evidence for clustering of the solute atoms in sions), was analyzedusing a 50-nA beam current to ob- the solid solution (Wernick, 1960; Godovikov and Nena- tain high count rates