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TitleAssessing scheelite as an ore-deposit discriminator using its trace-element and REE chemistry
AuthorPoulin, R S; Kontak, D J; McDonald, A; McClenaghan, M B
SourceCanadian Mineralogist vol. 56, 3, 2018 p. 265-302, https://doi.org/10.3749/canmin.1800005
Image
Year2018
Alt SeriesNatural Resources Canada, Contribution Series 20200174
PublisherMineralogical Assoc Canada
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
Subjectsgeochemistry; mineralogy; Nature and Environment; Science and Technology; scheelite; indicator elements; ablation; mass spectrometer analysis; minerals; mineral processing; chemical analyses; oxygen isotopes; mineral deposits; rare earths geochemistry; rock analyses, rare earth elements
Illustrationstables; figures; plots; geochemical plots
Released2018 05 30
AbstractScheelite (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 delta O-18 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 Sigma REEs (below detection to 10(4) 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) delta O-18 values varying from -4.6 to +12.7 parts per thousand, which apparently do not relate to a deposit-specific fluid signature (delta O-18(H2O)). The variability in REE patterns suggests that many factors affect both Sigma REE 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 EuA 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-A versus 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.
GEOSCAN ID326586

 
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