Trace element substitution and grain-scale compositional heterogeneity in enargite

Abstract

Enargite, Cu₃AsS₄, is a relatively common sulphide mineral and is considered diagnostic for deposits of intermediate- to high-sulphidation type. Analysis of enargite-bearing samples from deposits in the Zijinshan porphyry – high-sulphidation epithermal Cu-Au orefield, southeastern China, provides evidence for the diversity of trace elements that may be hosted within enargite and their range of concentrations. Enargite is shown to host Sb, Te, Sn, Zn and Ge at concentrations up to several thousand ppm. The mineral also incorporates measurable concentrations of Mo, Cd, Bi, Pb, Fe, Se, Ag, Au W, Ga and In. Element mapping (electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry) provides evidence for grain-scale heterogeneity in enargite in the form of oscillatory, grain-scale compositional zonation with respect to Sb, Sn, Te and several other trace elements. Element mapping also clearly shows an inverse correlation between the concentrations of As and Te, and between As and Sn. Incorporation of Sn and Te into the enargite structure is achieved by substitution of Sn⁴⁺ and Te⁴⁺ for As⁵⁺. Charge balance is maintained by incorporation of Fe²⁺, Zn²⁺ and other divalent cations (potentially including Cu²⁺) into the Cu⁺ site. The complex intra-grain zoning results from evolving fluids, multiple phases of growth – in turn leading to an overprinting of primary distribution patterns. Observed patterns are also influenced by equilibrium partitioning between enargite and co-existing minerals. Nevertheless, trace element signatures in enargite from different parts of the Zijinshan ore system show notable differences. Enargite from the high-sulphidation stage typically shows a marked enrichment in Te and Sn whereas enargite from intermediate-sulphidation stage is relatively depleted in Te and Sn, and comparatively enriched in Sb and Se. These differences represent a potential vector for exploration within porphyry – high-sulphidation epithermal systems. Furthermore, the presence of Te-rich enargite may be a prospective guide to high Au-grade mineralization. The notable concentrations of precious metals (Au, Ag) and critical elements (notably Te and Ge) within enargite make this mineral of particular interest from the perspective of potential recovery of these economically important elements. The observed grain-scale zoning and inherent variability within any given sample emphasize that spot analysis of trace elements alone without consideration of such heterogeneity may provide quantitative data of limited use and potentially, lead to misleading interpretations.