A study of the crystal chemistry, cathodoluminescence, geochemistry and oxygen isotopes in Scheelite: application towards discriminating among differing ore-deposit systems

Abstract

Scheelite (CaWO4) from a total of 37 of world-wide, differing ore-deposit settings (orogenic, sediment- and greenstone-hosted, skarn, porphyry, greisen, volcanogenic massive sulfide, breccia and polymetallic deposits) were examined using cathodoluminescence, SEMEDS, LA-ICP-MS and whole-grain stable isotopic analyses. The goal of this study was to assess whether the crystal-chemistry of scheelite (i.e., major, minor and trace elements, stable isotopes, CL response) could be used to distinguish between differing environments of formation. Results show: (1) weak to strong CL responses, these varying from grains with complex, oscillatory zonation patterns, to discordant patterns to those where no zonation is evident; (2) the predominant elemental substitutions involve As5+ or Mo6+ ↔ W6+, and Sr2+ or REE3+ ↔ Ca2+; (3) the trends in REE vary in terms of ΣREEs (104 range in CN values). The degree and type of REE fractionation patterns (flat, convex, concave) are variable and both positive and negative Eu anomalies (<0.1 to >20-30) may be present; and (4) the δ18O values are highly variable, ranging from -4.6 to +12.7‰. The broad ranges do not independently fix a single parent fluid during scheelite formation. The intensity of CL zonation was found to correlate with Mo content: increases in Mo ↔ W substitution correlates with a reduction in CL signal. Further, the nature of the type zonation revealed by CL was found to directly correlate with geological environment: where zonation is absent (i.e., a homogeneous CL response), the scheelite is associated with metamorphic-related systems and where the zonation is pronounced, the scheelite is associated with magmatic-related systems. The variability in REE patterns suggest that other factors are likely controlling the incorporation of REEs into scheelite (e.g., changes in fluid chemistry and mineral precipitation). The presence of substitutions involving Mo and As are particularly relevant, as both are redox sensitive. In particular, enrichments in Mo (>100 ppm) suggest formation in highly oxidizing environments wherein, Mo may be mobile. The crystal-chemistry of scheelite has been demonstrated to be both a strong indicator of ore-forming conditions and the general geological environment.