Source of copper in the giant Shimensi W-Cu-Mo polymetallic deposit, South China: Constraints from chalcopyrite geochemistry and oxygen fugacity of ore-related granites

Abstract

Porphyry-type copper mineralization is generally considered to be related to ore-forming fluid exsolution from oxidized, magnetite-bearing intermediate- to felsic magmas. However, copper deposits associated with ilmenite-bearing per-aluminous evolved granites also exist and their origin remains obscure. The Shimensi W-Cu-Mo polymetallic deposit, located in the northern segment of the Jiangnan Orogen (South China) composed mainly of the Neoproterozoic Shuangqiaoshan Group meta-sediments/volcanic rocks and biotite granite, is one of the largest tungsten deposits in the world. The W-Cu-Mo polymetallic orebodies occur at the contact zone between the Mesozoic ilmenite-bearing, fluorite-bearing F-rich highly fractionated granite and the Neoproterozoic granite. The reason for the accompanying Cu mineralization within ilmenite-bearing granites is unclear. In this paper, we present a detailed study of chalcopyrite geochemistry including S-Pb isotopes, trace elements and Nd isotopes, and oxygen fugacity (fO2) of ore-related granites in the Shimensi deposit to constrain the origin of copper.Sulfur isotope values of the sulfides give a narrow δ34S interval of −3.3 to −0.5‰ (mean = −1.5‰, n = 32), suggesting the ore-forming fluid is of magmatic origin. The similarity of Pb isotopes of the majority of sulfides and K-feldspars from Mesozoic granites indicates their genetic relationship. However, chalcopyrite samples show a wider range of Pb isotopic compositions than that of K-feldspars, suggesting an extra source for the ore-forming fluids. Chalcopyrite from the ore deposit show features of LREE–enriched patterns, variable Eu negative anomalies (Eu/Eu∗ = 0.26–0.75) and (La/Sm)N values (0.77–5.93), while the two figures for the Mesozoic F-rich evolved granites are Eu/Eu∗ = 0.18–0.45 and (La/Sm)N = 2.42–4.09. This, along with and the larger range of εNd values (−12.4 to −2.7) of the chalcopyrites than for the Mesozoic granites and Neoproterozoic granite therein (−8.6 to −6.2), suggest that the ore-forming fluids cannot be attributed to the Mesozoic granites alone. The local Cu-enriched Neoproterozoic Shuangqiaoshan Group rocks should have contributed to the mineralization. The low bulk rocks Fe3+/Fe2+ ratios (0.02–1.00, average 0.22), Fe2+-enriched biotite compositions (Fe2+/Fe3+ = 5.64–143, average 22), combined with the low Ce4+/Ce3+ ratios (3–509, average 142), negative Eu anomalies (Eu/Eu∗ = 0.05–0.56, average 0.25), and low fO2 [△FMQ-3.4 (±1.9)-△FMQ + 1.5 (±1.7)] for the Mesozoic magmatic zircons, suggest that the granites in the Shimensi deposit are relatively reduced. The low oxygen fugacity (below NNO) of granites is favorable for significant tungsten mineralization accompanying by minor Cu mineralization.