In situ trace elements and Sr isotopes in scheelite and S-Pb isotopes in sulfides from the Shiweidong W-Cu deposit, giant Dahutang ore field: Implications to the fluid evolution and ore genesis

The Shiweidong W-Cu polymetallic deposit, with resources of 300,000 t WO3 and 200,000 t Cu, is one of the major tungsten polymetallic deposits in the giant Dahutang ore field. This study conducted a series of in-situ analyses on scheelite and sulfides, aiming to provide some new insights into the sources, hydrothermal ore-forming processes, fluid evolution history and genesis of this deposit, which is of importance to understand the formation of the super-large Dahutang ore field. Quartz vein- and veinlet-disseminated-type tungsten ore bodies are mainly hosted in Neoproterozoic granodiorite and/or the Yanshanian porphyritic two-mica granites. Four stages of mineralization from early to late are identified in this deposit: 1) pegmatite stage; 2) silicate-oxide stage; 3) oxide-sulfide stage; and 4) sulfide stage. The delta S-34(v)-(CDT) values of stage 3 chalcopyrite and pyrite are from -1.73 parts per thousand to -1.46 parts per thousand and from -1.66 parts per thousand to -0.31 parts per thousand, respectively, indicating a magma-related hydro thermal origin. The (206)pb/(204)pb, (2067)pb/(204)pb and Pb-208/Pb-204 ratios pyrite range from 18.169 to 18.295, 15.650 to 15.679, and 38.496 to 38.525, respectively, revealing a crust origin for the Pb. Trace element compositions of scheelite show that the both stage 2 scheelite (scheelite-II) and stage 3 scheelite (scheelite-III) have high contents of Nb and REEs. The scheelite-II has a narrow range of the Y/Ho ratios (16.2-31.0), obviously negative Eu anomalies (Eu/Eu* = 0.04-0.46) and relatively low Sr-87/Sr-86 ratios of 0.72640-0.73336, whereas scheelite-III shows variable Y/Ho ratios of 20.6-96.9, positive Eu anomalies (Eu/Eu* = 1.31-7.11) and relatively high Sr-87/Sr-86 ratios of 0.73496-0.76098. These data indicated that the Eu anomalies and Sr isotopic compositions of scheelite-II were inherited from magmatic-hydrothermal fluids, whereas those of scheelite-III were sourced from modified fluids due to intense fluid-rock interaction and input of meteoric fluids. Our data also reveal that the REEs of scheelite-II and scheelite-III are mainly controlled by the substitution mechanism of 3Ca(2) = square Ca + 2REE(3+). The W-rich reduced fluids and intense fluid-rock interactions, which could provide enough W and Ca, are likely the key factors for the formation of scheelite in the Shiweidong tungsten deposit.