Ore-forming process of the W-Sn and Cu skarn mineralization in the Huangshaping deposit (Nanling Range): Constraints from scheelite geochemistry and cassiterite U-Pb geochronology

The Nanling Range in South China is endowed with abundant Jurassic W-Sn and Cu skarn deposits; however, the ore-forming age and process of these deposits lack adequate comparative studies. The Huangshaping deposit in the Nanling Range coincidently develops both W-Sn and Cu skarn mineralization systems, providing a good window to address these issues. Multiple generations of hydrothermal scheelite were identified at Huangshaping, including SchA-1, SchA-2, and SchA-3 in the W-Sn system, and SchB-1, SchB-2, and SchB-3 in the Cu system. Scheelite in the two systems has similar textural evolution, presenting as regular oscillatory zoning (SchA-1, SchB-1), chaotic oscillatory zoning (SchA-2, SchB-2) truncating the regular oscillatory zoning of SchA-1 and SchB-1, respectively, and homogeneous texture (SchA-3, SchB-3) replacing the early scheelite in the respective system. Rare earth elements (REEs) substitute into SchA-1 and SchB-1 mainly via mechanism of 3Ca(2+) = 2(REEs, Y)(3+) + (sic)(Ca), whereas those incorporate into SchA-2, SchA-3, SchB-2, and SchB-3 via mechanisms of Ca2+ + W6+ = (REEs, Y)(3+) + Nb5+ and 3Ca(2+) = 2(REEs, Y)(3+) + (sic)(Ca). Continuously decreasing Mo concentrations and increasing Eu anomalies from SchA-1 to SchA-3 and from SchB-1 to SchB-3 reflect progressively decreasing oxygen fugacity (fO(2)) of hydrothermal fluids in the two systems. The higher Sr concentrations in scheelite from the W-Sn system than that from the Cu system indicate stronger fluid-rock interaction in the W-Sn system, consistent with its more development of skarn. Cassiterite U-Pb geochronology from the W-Sn and Cu systems at Huangshaping yielded lower intercept ages of 159.1 +/- 0.6 Ma (MSWD = 1.8, N = 38) and 161.5 +/- 0.8 Ma (MSWD = 1.6, N = 25), respectively. Combined with previously published age data from the regional W-Sn and Cu skarn deposits in the Nanling Range, it is inferred that Cu skarn mineralization may have formed earlier than W-Sn with age boundary of similar to 159 Ma. And the age of similar to 159 Ma might represent an important shifting time for the Cu mineralization to W-Sn mineralization in the Nanling Range. This study highlights that the textural and geochemical evolution of scheelite can provide important constraints on the ore-forming process in different skarn mineralization systems.