Trace and minor elements in sphalerite: A LA-ICPMS study

Abstract

Sphalerite is an important host mineral for a wide range of minor and trace elements. We have used laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) techniques to investigate the distribution of Ag, As, Bi, Cd, Co, Cu, Fe, Ga, Ge, In, Mn, Mo, Ni, Pb, Sb, Se, Sn and Tl in samples from 26 ore deposits, including specimens with wt.% levels of Mn, Cd, In, Sn and Hg. This technique provides accurate trace element data, confirming that Cd, Co, Ga, Ge, In, Mn, Sn, As and Tl are present in solid solution. The concentrations of most elements vary over several orders of magnitude between deposits and in some cases between single samples from a given deposit. Sphalerite is characterized by a specific range of Cd (typically 0.2–1.0 wt.%) in each deposit. Higher Cd concentrations are rare; spot analyses on samples from skarn at Baisoara (Romania) show up to 13.2 wt.% (Cd 2+ ↔ Zn 2+ substitution). The LA-ICPMS technique also allows for identification of other elements, notably Pb, Sb and Bi, mostly as micro-inclusions of minerals carrying those elements, and not as solid solution. Silver may occur both as solid solution and as micro-inclusions. Sphalerite can also incorporate minor amounts of As and Se, and possibly Au (e.g., Magura epithermal Au, Romania). Manganese enrichment (up to ∼4 wt.%) does not appear to enhance incorporation of other elements. Sphalerite from Toyoha (Japan) features superimposed zoning. Indium-sphalerite (up to 6.7 wt.% In) coexists with Sn-sphalerite (up to 2.3 wt.%). Indium concentration correlates with Cu, corroborating coupled (Cu +In 3+) ↔ 2Zn 2+ substitution. Tin, however, correlates with Ag, suggesting (2Ag +Sn 4+) ↔ 3Zn 2+ coupled substitution. Germanium-bearing sphalerite from Tres Marias (Mexico) contains several hundred ppm Ge, correlating with Fe. We see no evidence of coupled substitution for incorporation of Ge. Accordingly, we postulate that Ge may be present as Ge 2+ rather than Ge 4+. Trace element concentrations in different deposit types vary because fractionation of a given element into sphalerite is influenced by crystallization temperature, metal source and the amount of sphalerite in the ore. Epithermal and some skarn deposits have higher concentrations of most elements in solid solution. The presence of discrete minerals containing In, Ga, Ge, etc. also contribute to the observed variance in measured concentrations within sphalerite.