Zoned orthopyroxenes in the Ni-Co sulfide ore-bearing Xiarihamu mafic-ultramafic intrusion in northern Tibetan Plateau, China: Implications for multiple magma replenishments

Abstract The Xiarihamu mafic-ultramafic intrusion is situated in the East Kunlun Orogenic Belt (E-KOB), northern Tibetan Plateau, China, with Ni-Co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite. Whole-rock and mineral compositions show cyclic variations in drill-hole intersections through the Xiarihamu mafic-ultramafic intrusion. Microbeam X-ray fluorescence (XRF) mapping shows reverse zoning (Cr-poor core, Cr-rich rim) and oscillatory zoning (Cr-poor core, Cr-rich mantle and Cr-poor rim) in orthopyroxene of olivine orthopyroxenite and orthopyroxenite. Zoned orthopyroxenes are euhedral or subhedral, and there are relatively sharp interfaces between zones with distinct Cr contents. Crystal-size distribution (CSD) curves of orthopyroxenes in olivine orthopyroxenite show concave-upward curvature at size larger than 3.0 mm, and larger orthopyroxenes commonly display zoning, implying a secondary recrystallization and grain growth of larger orthopyroxenes. Quantitative analyses show that Cr and enstatite contents of cores are relatively lower, but incompatible element (e.g., Ti, Zr, Ga and Sr) contents of cores are higher than rims in reverse zoned orthopyroxenes, and chemical profiles of unzoned orthopyroxenes resemble those of rims (but different from those of cores) in reverse zoned orthopyroxenes. Chemical profiles of orthopyroxenes indicate that Cr zoning is not due to the change in Cr partitioning behavior between orthopyroxene and silicate melt, which shows significant temperature and redox dependence. Textural feature and crystal-size distribution of orthopyroxenes suggest the formation of Cr-rich zones attests to the multi-stage growth in the process of magma replenishment, rather than a reactive liquid flow model or in-situ trapped liquid overgrowth. Orthopyroxene zoning, together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the Xiarihamu mafic-ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition.