Formation of the Heishan magmatic Ni-Cu sulfide deposit in the Central Asian Orogenic Belt: Implications from magma oxygen fugacity and sulfur isotopes

Abstract

The genesis of magmatic Ni-Cu sulfide deposits in convergent tectonic settings is primarily controlled by magmatic oxygen fugacity (fO₂) reduction and/or external sulfur assimilation. To test two competing genetic hypotheses for the Heishan magmatic Ni-Cu deposit in the Central Asian Orogenic Belt, we employ sulfur isotope analysis of sulfide minerals and fO₂ estimates derived from olivine-orthopyroxene-spinel (Ol-Opx-Spl) equilibria to constrain the sources of sulfur and the redox conditions during mineralization. The δ³⁴S values of sulfide minerals vary from –1.1 to + 7.5 ‰, showing systematic variations among lithologies. The δ³⁴S values of sulfides from sparsely disseminated mineralized harzburgite (–1,1 to + 1.6 ‰) fall within the range of mantle-derived sulfur, whereas those from disseminated mineralized lherzolite (1.1 to + 7.5 ‰) indicate crustal sulfur contributions. Magmatic fO₂ conditions span a range of ΔFMQ + 0.69 to ΔFMQ + 1.03. The measured values are > 1 order of magnitude lower compared to those observed in the Xiadong sulfide-barren ultramafic intrusion within the Central Asian Orogenic Belt. Combining geochemical constraints from sulfur concentrations in primitive arc basalts (2,000–3,000 ppm) with thermodynamic modeling of sulfur solubility under variable redox conditions in hydrous basaltic systems, we confirm that exogenous sulfur input alone is insufficient to achieve sulfide segregation in the primary magma of the Heishan deposit. A substantial decrease in primary magma fO₂ is necessary to trigger sulfide saturation for the Heishan deposit, while the introduction of external sulfur can enhance the degree of mineralization. This study establishes a dual-control paradigm: fO₂ reduction governs the initiation of sulfide melt segregation, whereas external sulfur influx modulates metal endowment.