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Nigel Cook, University of Oslo (Norway)
William Skinner, Ian Wark Research Institute (Australia)
Allan Pring, South Australian Museum (Australia)
Cristiana Ciobanu, University of Adelaide (Australia)
Masaaki Shimizu, University of Toyama (Japan)
Leonid Danushevskiy, University of Tasmania (Australia)
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Sphalerite is an important host mineral for a wide range of minor and trace elements. The distribution of trace elements carries implications for economic ore exploitation, mineral processing and environmental remediation. We have used LA-ICP-MS techniques to understand the distribution of Ag, As, Bi, Cd, Co, Cu, Fe, Ga, Ge, Hg, In, Mn, Mo, Ni, Pb, Sb, Se, Sn and Tl in samples from more than 20 ore deposits, including material with wt.% levels of Mn, Cd and In. Our aim was to constrain ranges of solid solution in natural samples, and to compare this with published phase equilibria, distinguish between solid solution and microscale inclusions, and to identify potential geological or genetic controls on sphalerite geochemistry.
Analysis by LA-ICP-MS provides accurate trace element data, confirming that Cd, Co, Ga, Ge, Hg, In, Mn, Sn and Tl are present in solid solution. The concentrations of most elements vary significantly between deposits, over several orders of magnitude, and in some cases between single samples from a given deposit. Sphalerite is, however, characterized by a specific range of Cd (typically 0.2-1.0 wt.%) in each deposit. Higher concentrations are rare; spot analyses show up to 13.2 wt.% (Cd2+ Zn2+ substitution). On the other hand, Pb, Sb and Bi are most commonly encountered as micro-inclusions, rarely as solid solution. Silver may be both as solid solution and as microinclusions. Sphalerite does appear to incorporate minor amounts of both As and Se. We observe inverse correlation between Mn and Fe in several samples. Mn-enrichment (up to ∼4 wt.%) does not appear to enhance incorporation of other elements.
Sphalerite from Toyoha, Japan contains complex zoning. In-sphalerite (up to 6.7 wt.% In) coexists with Sn-sphalerite (up to 2.3 wt.%). Indium concentration correlates with Cu, corroborating a coupled Cu+In3+ for 2Zn2+ substitution. Tin, however, correlates with Ag, suggesting a coupled substitution involving Ag and Sn. We see little evidence for Cu incorporation by itself. The dataset does not reveal any evidence of coupled substitution being involved for incorporation of Ge. We accordingly postulate that Ge may be present as Ge2+ rather than Ge4+.
Trace element concentrations and fractionation of a given element into sphalerite are influenced by type of deposit, crystallization temperature, metal source, cooling history and proportion of sphalerite in the ore. In a broad sense, epithermal and some skarn deposits have higher concentrations of most elements in solid solution with local metal source, crystallization temperature and cooling history contributing to the measured concentrations and coexistence/absence of associated trace minerals containing In, Ga, Ge etc.
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