Genesis of Maanshan Gabbro Associated with Crossroads Cu-Ni Mineralization in Karakoram: Constraints from Elemental Geochemistry and Sr-Nd-S Isotopes

The Triassic mafic-ultramafic complex at Maanshan in the Chalukou area of the West Kunlun, Xinjiang, represents a recently discovered copper-nickel mineralization occurrence. However, its petrogenesis and ore-forming potential remain poorly constrained. This study conducts systematic petrographic, whole-rock geochemical, Sr-Nd isotopic, and in situ sulfur isotopic analyses of sulfide minerals from the Maanshan gabbro, aiming to reveal its magmatic evolution, source characteristics, tectonic setting, and sulfur saturation mechanisms, thereby enhancing the understanding of regional magmatism and mineralization processes and providing insights for prospecting breakthroughs. The gabbro exhibits right-inclined chondrite-normalized rare earth element patterns with light rare earth element enrichment and heavy rare earth element depletion. Its trace element signatures show enrichment in high field strength elements (e.g., Nb, Ta) and depletion in large ion lithophile elements (e.g., Ba, K), consistent with ocean island basalt (OIB) affinities. Elevated (Th/Yb)_PM ratios (1.35–7.10) and low ratios of Ce/Pb (10.74–22.51), and Nb/Ta (12.27–16.05) indicate significant crustal contamination. Positive correlations between MgO and Cr/Ni suggest fractional crystallization of olivine and clinopyroxene during magmatic evolution. Whole-rock initial (⁸⁷Sr/⁸⁶Sr)_i ratios range from 0.70350 to 0.70358, with ε_Nd(t) values of +5.0 to +5.5, reflecting a strongly depleted mantle source. Low ratios of La/Nb (0.80–1.17), La/Ta (10.3–14.6), and Dy/Yb (2.16–2.43) further suggest that the primary magma was derived from a depleted asthenospheric mantle dominated by peridotite. Positive δNb values (0.13~0.14) and high mantle potential temperatures (Tp = 1574–1616 °C) of the Maanshan gabbro, combined with regional basaltic magmatism and deep-marine volcanic-sedimentary sequences, support a tectonic setting related to mantle plume activity, analogous to ocean island-seamount basalts. In situ δ³⁴S values of disseminated pyrite and chalcopyrite range from +3.27‰ to +4.23‰ and –3.59‰ to –2.06‰, respectively, deviating from typical mantle sulfur values and indicating that crustal contamination was the primary mechanism triggering sulfur saturation and sulfide segregation. The occurrence of surface annabergite together with the relatively low M/F ratio (0.94–1.15) of the gabbro in the Maanshan intrusion suggests favorable prospecting potential at depth, warranting further exploration.