Assessing Scheelite As an Ore-deposit Discriminator Using Its Trace-element and REE Chemistry

Scheelite (CaWO₄) collected from 37 different ore-deposit settings (orogenic, sediment- and greenstone-hosted, skarn, porphyry, greisen, volcanogenic massive sulfide, breccia, and polymetallic deposits) globally was examined and evaluated using a combination of cathodoluminescence (CL), SEM-EDS, LA-ICP-MS, and δ¹⁸O analytical methods. These data provided the basis to assess whether the crystal-chemistry of scheelite could be used to distinguish between differing environments of formation. The results show: (1) scheelite can record very complex growth histories based on CL images and elemental mapping; (2) predominant elemental substitutions involve As⁵⁺ or Mo⁶⁺ ↔ W⁶⁺ and Sr²⁺ or REE³⁺ ↔ Ca²⁺; (3) highly variable trends for the REEs that vary in terms of ΣREEs (below detection to 10⁴ range in CN values), the degree and nature of fractionation (i.e., flat, convex, concave), and the nature of Eu_N values, which are both positive and negative, with Eu anomalies (Eu_A) ranging from <0.1 to >20–30; and (4) δ¹⁸O values varying from –4.6 to +12.7‰, which apparently do not relate to a deposit-specific fluid signature (δ¹⁸O_H2O). The variability in REE patterns suggests that many factors affect both ΣREE and incorporation, hence the observed fractionation, into scheelite (e.g., fluid chemistry, coprecipitation of other REE-bearing minerals). Substitutions involving Mo and As are of particular relevance, as both elements are redox sensitive and their enrichment is reflected in ore-deposit settings such that reduced orogenic-type Au-W deposits are As and Mo depleted in contrast to the more oxidized intrusion-related deposits where Mo is enriched (>100 ppm). Similarly, both the Eu_A values and Sr are relatively enriched in orogenic settings versus intrusion-related settings. These observed chemical differences for Sr, Mo, and Eu_A in scheelite from different deposit settings were used to construct the first ever discriminant diagram for scheelite, which is summarized in a plot of Eu_Aversus Sr/Mo, which successfully discriminates the samples used in this study. Thus, the crystal-chemistry of scheelite is shown to be both a strong indicator of ore-forming conditions and an ore-deposit discriminator.