Sulfur and copper isotopic signatures of chalcopyrite at Kalatongke and Baishiquan: Insights into the origin of magmatic Ni-Cu sulfide deposits

Abstract

Sulfide Cu isotope systematic has been used as a metallogenic tracer in various deposits; however, only rarely in magmatic Ni-Cu sulfide deposits. Here we present chalcopyrite δ34S and δ65Cu data for the Kalatongke and Baishiquan Ni-Cu sulfide deposits in the Central Asian Orogenic Belt. The Kalatongke deposit yields δ34S of −3.99‰ to 3.30‰ and δ65Cu of −1.32‰ to 0.07‰, whereas chalcopyrite from the Baishiquan deposit has δ34S values ranging from 1.70‰ to 4.98‰ and δ65Cu from −0.40‰ to 0.59‰. These isotopic ratios show no correlation with intrusion/orebody shape, location of sample in the orebody, lithofacies or type of mineralization. Based on Kalatongke and Baishiquan δ34S data, our multicomponent modeling suggests that, with an increase in the relative crustal component contribution, the measured δ34S values in sulfide ores approach the value of the contaminant. Mass ratios (R′) of silicate magma/(xenomelt + sulfide xenomelt + resitite + crystalline olivine) of 250–800 in the Kalatongke and 10–1000 in the Baishiquan deposit, yield δ34S values of −1.78‰ to 1.74‰ and 1.70‰ to 4.98‰ in these deposits. Contamination by host tuff and shale material with δ34S values of −7.2‰ to 3.3‰ reasonably explains the negative δ34S in the Kalatongke ores. A lack of correlation between the Kalatongke and Baishiquan chalcopyrite δ65Cu values and Cu or S contents (or Cu/Ni ratios) indicates that Cu isotopic fractionation was not controlled by sulfide fractional crystallization, but instead by contamination and the mantle source. Multicomponent modeling shows that an R′ factor between 10 and 1000 can reproduce most δ65Cu signatures in sulfide ores from the Kalatongke and Baishiquan deposits, with the exception of a few negative δ65Cu values. Undetected contamination and heterogeneous mantle resulting from different degrees of partial melting and metasomatism in the mantle source may explain these negative values and the wide range of δ65Cu noted in magmatic sulfide deposits worldwide. The Cu and S isotopes in sulfide from the magmatic Ni-Cu sulfide deposits can be utilized as indicators of crustal contamination and the nature of the mantle source.