Wolframite geochronology and scheelite geochemistry of the Yangwuchang W-Au deposit and Dashegou W deposit in the Yangxie ore district, the North Qinling, China: Implications for W-Au mineralization

The characteristics of W-Au mineralization have received great attention, however still under debate. The Yangwuchang W-Au deposit and Dashegou W deposit are two newly discovered quartz-vein type deposits in the Yangxie Au-W ore district, the North Qinling Orogenic Belt, China. The characteristics of W-Au mineralization in these deposits remain poorly understood. Hence, in order to determine the mineralization timing and its genetic link with magmatism, as well as the source, nature, and evolution of the ore-forming fluids, we undertook a combined study of wolframite geochronology with scheelite texture and geochemistry. In-situ U-Pb dating of wolframite shows that the timing of W mineralization is 141.8-139.1 Ma. This age range is perfectly matched with the published Au metallogenic age in the ore district. Hence, the W-Au mineralization should be associated with Early-Cretaceous magmatism, ruling out the genetic relationship with the Late-Triassic magmatism. Based on cathodoluminescence (CL) imaging and in-situ trace elements analyses, two generations of scheelite (Sch-I and Sch-II) can be discriminated. The CL images of Sch-I show a weak intensity, distinct boundary, and zonation pattern. In contrast, Sch-II is characterized by a bright and homogeneous CL response that encloses or crosscuts Sch-I. Sch-I shows depletion in Mo, Mn, Nb, and Y, and slight enrichment in Sr and U. It is also characterized by MREE-depleted chondrite-normalized patterns with strongly positive Eu anomalies. Compared with Sch-I, Sch-II has elevated concentrations of trace elements and MREE-enriched patterns with slightly positive Eu anomalies. The changes of MREEs, Y, and Mn contents are the main factor causing the CL intensity difference. Our data reveals that 3Ca2+ = (2REE)3+ + Ca (site vacancy) is the main substitution mechanism that controls REEs into the scheelite. The strong positive Eu anomalies of Sch-I in the Yangwuchang deposit are related to the Eu2+ released from plagioclase in the country rock during greisenization. Other Eu anomalies of Sch-I and Sch-II were inherited from ore-forming fluids. The positive Eu anomalies and low contents of Mo and As point out a strongly reduced environment. The geochemical features of scheelite decipher that the ore-forming fluids of the Yang-wuchang W-Au deposit and Dashegou W deposit were single-derived reduced magmatic fluids. The decompo-sition of plagioclase in the host rocks provided significant Ca for scheelite precipitation. The cooling and depressurization of W-Au-rich magmatic fluids may be the main mechanism accounting for W-Au mineralization.