Coupled Substitutions of Minor and Trace Elements in Co-Existing Sphalerite and Wurtzite

Abstract

The nature of couple substitutions of minor and trace element chemistry of expitaxial intergrowths of wurtzite and sphalerite are reported. EPMA and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses display significant differences in the bulk chemistries of the two epitaxial intergrowth samples studied. The sample from the Animas-Chocaya Mine complex of Bolivia is Fe-rich with mean Fe levels of 4.8 wt% for wurztite-2H and 2.3 wt% for the sphalerite component, while the sample from Merelani Hills, Tanzania, is Mn-rich with mean Mn levels in wurztite-4H of 9.1 wt% and for the sphalerite component 7.9 wt% In both samples studied the wurtzite polytype is dominant over sphalerite. LA-ICP-MS line scans across the boundaries between the wurtzite and sphalerite domains within the two samples show significant variation in the trace element chemistries both between and within the two coexisting polytypes. In the Merelani Hills sample the Cu + + Ga 3+ = 2Zn 2+ substitution holds across both the wurztite and sphalerite zones, but its levels range from around 1200 ppm of each of Cu and Ga to above 2000 ppm in the sphalerite region. The 2Ag + + Sn 4+ = 3Zn 2+ coupled substitution does not occur in the material. In the Animas sample, the Cu + + Ga 3+ = 2Zn 2+ substitution does not occur, but the 2(Ag,Cu) + + Sn 4+ = 3Zn 2+ substitution holds across the sample despite the obvious growth zoning, although there is considerable variation in the Ag/Cu ratio, with Ag dominant over Cu at the base of the sample and Cu dominant at the top. The levels of 2(Ag,Cu) + + Sn 4+ = 3Zn 2+ vary greatly across the sample from around 200 ppm to 8000 ppm Sn, but the higher values occur in the sphalerite bands.Crenshaw [7] reported the transformation occurs at 1020 • C at 1 atm H 2 S, but subsequent researchers reported different inversion temperatures [8][9][10][11]. The transition temperature is reported to be lowered significantly by substitution of Fe, Mn, and Cd [12,13]. Sphalerite and wurtzite are essentially hydrothermal minerals generally forming at much lower temperatures, e.g., in Mississippi Valley-type deposits [2]. As far back as 1934, it was proposed that stabilization of the sphalerite was linked to an excess of sulfur, and wurtzite to sulfur deficiency [14]. Pankratz and King [15] published very accurate analyses of synthetic sphalerites and wurtzites showing that the M:S ratio for sphalerite of down to 0.997 and for wurtzite up to 1.002. Scott and Barnes [16] demonstrated by hydrothermal experiments that the crystallization of ZnS in either the sphalerite form or the wurtzite form was dependent on the sulfur fugacity, ƒS 2 , with the formation of the wurtzite polytypes restricted to low ƒS 2 (~10 −20 ).The major divalent impurities in ZnS, Fe, Mn, and Cd, are said to be all more soluble in wurtzite than sphalerite and, therefore, stabilize wurtzite relative to sphalerite under a given set of conditions [12,13,16]. Lepetit et al. [17] showed that the upper solubility limit of Fe in sphalerite has a str...

Trace element composition and U-Pb ages of cassiterite from the Bolivian tin belt

“…Previous studies on the concentration of high-tech metals in deposits from the Bolivian tin belt have focused mostly on the sulfide mineralization (Schwarz-Schampera and Herzig 2002;Ishihara et al 2011;Artiaga et al 2013;Murakami and Ishihara 2013;Jiménez-Franco et al 2018;Cacho et al 2019;Torres et al 2019;Torró et al 2019a,b;Pring et al 2020; summary of reported maximum concentrations of In, Ge and Ga shown in Table 3). In the following lines we assess the potential of cassiterite from the Bolivian tin belt as a source of high-tech, critical elements, chiefly Nb, Ta, In, Ge and Ga, by comparing our results with bibliographic values.…”

Richardsite, Zn2CuGaS4, A New Gallium-Essential Member of the Stannite Group from the Gem Mines near Merelani, Tanzania

“…The Merelani sphalerite and wurtzite are Mn-rich, and were found to contain several trace elements (e.g., Fe, Cu, Se, and Cd) with concentrations greater than 500 ppm and a discernable differentiation between the sphalerite and wurtzite. Noteworthy, 1450 ppm Ga in the wurtzite and 1750 ppm in the sphalerite phases were also reported [8], with estimated standard deviations of 30 and 80 ppm, respectively. In both the sphalerite and wurtzite phases, trace Ga and Cu concentrations were consistent with the coupled substitution Cu + + Ga 3+ ↔ 2Zn 2+ .…”

“…The mines are also host to well-formed and uncommonly large crystals of pyrite, alabandite, and wurtzite as well as several rare sulfides, including clausthalite (PbSe), germanocolusite (Cu 13 VGe 3 S 16 ), and merelaniite (Mo 4 Pb 4 VSbS 15 ) [5][6][7]. A detailed study of the chemistry of intergrown sphalerite and wurtzite, which included samples the Merelani mines and from the Animas-Chocaya Mine complex, Quechisla district, Bolivia, was recently published [8]. The Merelani sphalerite and wurtzite are Mn-rich, and were found to contain several trace elements (e.g., Fe, Cu, Se, and Cd) with concentrations greater than 500 ppm and a discernable differentiation between the sphalerite and wurtzite.…”

Noble Metal Speciations in Hydrothermal Sulphides

“…Both forms of ZnS are important in mineralogy as many minerals crystallise in the same structures. For example, there are würtzite and spha-lerite forms of AgI (iodargyrite and miersite) and CdS (greenockite and hawleyite) [109]. Most of our gas vapour transport and salt flux experiments led to the formation of pure cubic sphalerite.…”