Behavior of Mg and C-O isotopes during mafic magma-carbonate interaction at the Jinchuan Ni-Cu deposit, North China Craton

Abstract

The Jinchuan Ni-Cu deposit is one of the largest magmatic sulfide deposits in the world, but the cause of sulfide saturation remains unclear. It has been suggested that sulfide saturation in the Jinchuan magma was triggered by oxidation through assimilation of fluids related to de‑carbonation of marbles. This study uses Mg and C-O isotopes to evaluate the process of carbonate assimilation. The measured values of δ18O and δ13C for carbonate minerals from country rock marbles range from 13‰ to 22‰ and from −1.7‰ to 1.3‰, respectively. Carbonate minerals from xenoliths and hybrid rocks in the contact zone have lower C-O isotope ratios, with δ18O from 11‰ to 16‰ and δ13C from −4.7‰ to −2.3‰, respectively. The carbonate δ26Mg values of xenoliths and hybrid rocks are from −1.71‰ to −1.36‰, similar to the values of the marbles (−1.74‰ to −1.34‰). The whole-rock δ26Mg values of hybrid rocks samples associated with xenoliths vary from from −1.69‰ to −0.92‰. The δ18O values of pyroxene at the margin and the center of the Jinchuan intrusion vary from 5.1‰ to 7.8‰ and from 5.3‰ to 6.5‰, respectively. Despite varying distances from contacts with country rocks, pyroxene in these samples are characterized by δ26Mg values from −0.32‰ to −0.16‰, which are similar to the typical value of the Earth's mantle (−0.25‰ ± 0.07‰).The C-O isotopic variations are consistent with de‑carbonation. Rayleigh fractionation models suggest from 40% to 80% CO2 loss from xenoliths and hybrid rocks. The Mg isotopic compositions in the contact zone reflect reactive assimilation between xenoliths and mafic magma. Product olivine and diopside inherited the δ26Mg value of reactant dolomite. The transport of released CO2 from xenoliths into magma is consistent with increasing oxygen fugacity and potentially triggering sulfide saturation. However, Mg isotope exchange was confined to the contact zone, and appears to have been unrelated to the ore-forming process.