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

Abstract

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 3Ca2+ = 2(REEs, Y)3+ + □Ca, whereas those incorporate into SchA-2, SchA-3, SchB-2, and SchB-3 via mechanisms of Ca2+ + W6+ = (REEs, Y)3+ + Nb5+ and 3Ca2+ = 2(REEs, Y)3+ + □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 (fO2) 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 ∼ 159 Ma. And the age of ∼ 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.