Fe and Mg isotopes as tracers of the mantle sources and ore-forming processes of the Xiarihamu magmatic Ni-Cu sulfide deposit

Abstract

Mg and Fe isotopes increasingly are utilized to investigate the origins of magmatic sulfide deposits and interactions between the crust and mantle. However, further evaluation of the validity of using Fe isotopes as a constraint to assess crustal contamination is necessary, particularly using more mafic-ultramafic samples. To address this issue, we analyzed the Mg and Fe isotopic compositions of silicate and sulfide minerals in the Xiarihamu magmatic Ni-Cu sulfide deposit, which has high Ni reserves and is located in the East Kunlun Orogenic Belt. The clinopyroxene and orthopyroxene separates from the Xiarihamu deposit have δ⁵⁶Fe values of −0.048 ‰ to +0.172 ‰ and a Δ⁵⁶Fe_Cpx-Opx value of 0.068 ‰. The pentlandite separates have δ⁵⁶Fe values of −0.475 ‰ to +0.062 ‰ and Δ⁵⁶Fe_Opx-Pn values of −0.001 ‰ to 0.487 ‰. The clinopyroxene and orthopyroxene separates have δ²⁶Mg values of −0.376 ‰ to −0.141 ‰ and a Δ²⁶Mg_Opx-Cpx value of −0.014 ‰. Most of the δ²⁶Mg values of the silicate minerals align with those of unmodified mantle. The observed Fe and Mg isotope variations of the minerals in this deposit exceed those caused by mantle melting. The δ⁵⁶Fe and δ²⁶Mg values of the mineral separates do not exhibit correlations with the whole-rock loss on ignition (LOI), orthopyroxene Mg/(Mg + Fe) molar ratio, olivine forsterite content, and sulfide δ³⁴S, indicating that post-magmatic hydrothermal alteration, crystallization differentiation, and crustal sulfur addition cannot sufficiently explain the Mg and Fe isotope variations in the Xiarihamu deposit. The δ²⁶Mg and δ⁵⁶Fe values are lower than mantle values, indicating that the mantle source was modified by a carbonate melt. The δ⁵⁶Fe values exhibit a weak positive correlation with the f_O2 value, and the relationship between the Pd tenor and δ⁵⁶Fe value is consistent with the variations in the silicate melt to sulfide liquid mass ratio (R factor), illustrating that the Fe isotope variations in the Xiarihamu deposit were controlled by both the redox conditions and R factor. The Mg-Sr-Nd mixing calculations revealed that crustal assimilation was crucial for triggering the sulfide saturation in the Xiarihamu deposit. Therefore, Fe isotopes may be unsuitable for identifying crustal contamination; however, when combined with Mg isotopes, they can be useful in determining the mantle sources and metallogenic processes in igneous Ni-Cu mineralization systems. These findings have important implications for future research on the formation of Ni-Cu sulfide deposits, and the mechanisms of Fe isotope fractionation.