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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 ofREE fractionation patterns (flat, convex, concave) are variable and both positive and negative Euanomalies (<0.1 to >20-30) may be present; and (4) the δ18O values are highly variable, rangingfrom -4.6 to +12.7‰. The broad ranges do not independently fix a single parent fluid duringscheelite 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 ofthe 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 withmetamorphic-related systems and where the zonation is pronounced, the scheelite is associatedwith magmatic-related systems. The variability in REE patterns suggest that other factors arelikely controlling the incorporation of REEs into scheelite (e.g., changes in fluid chemistry andmineral precipitation). The presence of substitutions involving Mo and As are particularlyrelevant, as both are redox sensitive. In particular, enrichments in Mo (>100 ppm) suggest<br/>formation in highly oxidizing environments wherein, Mo may be mobile. The crystal-chemistryof scheelite has been demonstrated to be both a strong indicator of ore-forming conditions andthe general geological environment.