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Cathodoluminescence and trace elements of scheelite: Constraints on ore-forming processes of the Dabaoshan porphyry Mo-W deposit, South China
Scheelite, a good proxy to record the complex history of hydrothermal evolution, has been investigated in the Dabaoshan porphyry Mo-W deposit, South China. Based on cathodoluminescence (CL) textures, mineral paragenesis, and chemical characterization, three generations of scheelite were identified (Sch1–Sch3). Sch1 is disseminated in porphyry rocks, or occasionally occurs in early quartz-molybdenite veins. It is CL-dark and contains high concentrations of V, Sr, Mo, Nb, Ta, and rare earth elements (REE), low U/Th ratios (0.37 mean) and light REE (LREE)-enriched patterns with negative Eu anomalies. Sch2 and Sch3 mainly occur in late quartz-pyrite-scheelite ± chalcopyrite veins. Sch2 is CL-bright and commonly rims Sch1, and/or is overgrown by late oscillatory Sch3. Compared with Sch1, Sch2 shows depletion in V, Sr, Mo, Nb, and Ta, but slight enrichment in U and Th, and is characterized by middle REE (MREE)-enriched patterns with negative Eu anomalies. Sch3 grains show reddish-brown color under transmitted light and exhibit oscillatory zonation under CL. Sch3 has elevated concentrations of Mn, U (up to 4484 ppm), and Th (up to 18.8 ppm) contents, and high U/Th ratios (1175 mean). Chemical mapping reveals a negative correlation between U concentrations and CL intensity, which is attributed to the radiation damage caused by the radioactive decay of U. Based on the REE patterns, Sch3 is subdivided into Sch3a and Sch3b. Sch3a shows MREE-enriched patterns with slight negative to positive Eu anomalies, while Sch3b is LREE-enriched and shows gradual depletion in MREE and HREE with increasing positive Eu anomalies in consecutive growth zones.
The Dabaoshan porphyry and all generations of scheelites show similar Y/Ho ratios, indicating that ore-forming fluids were mainly derived from the Dabaoshan porphyry. Our results reveal that REE chiefly substitute into scheelite by the mechanism Ca2+ = REE3+ + Na+ while U substitutes for W into scheelite. Sch1 containing the highest concentrations of Mo, V, Nb, and Ta precipitates from the magmatic fluids at high temperature. It inherits the LREE-enriched patterns and negative Eu anomalies from the causative porphyry. From early to late generations, the gradual depletion of Mo, Nb, Ta, and V in scheelite is ascribed to the precipitation of molybdenite, rutile, and scheelite, whereas the increasing concentrations of U and Th in scheelite indicates the gradual enrichment of U and Th in ore-forming fluids. The increasing U/Th ratios in scheelites may result from not only the differential solubility between U and Th in fluids, but also the preferential incorporation of U relative to Th in scheelite. Compared with Sch1, Sch2 and Sch3a are depleted in LREE, which is interpreted to reflect the precipitation of apatite. Sch3b with various REE patterns within single grains is interpreted as precipitation from a closed-system accompanied by intermittent fluid replenishment. This study demonstrates that REE patterns of scheelite are controlled by multiple factors, including substitution mechanism, fluid composition, hydrothermal alteration, precipitation of REE-rich minerals, and hydrothermal dynamics during scheelite growth. The systematic variation of trace elements in scheelite suggests that scheelite chemistry can be used to monitor the evolution of ore-forming fluids in the Dabaoshan deposit.
Publication titleOre Geology Reviews: Journal for Comprehensive Studies of Ore Genesis and Ore Exploration
Department/SchoolSchool of Natural Sciences
PublisherElsevier Science Bv
Place of publicationPo Box 211, Amsterdam, Netherlands, 1000 Ae
Rights statement© 2019 Elsevier B.V. All rights reserved.