The Cu-Ni mineralization potential of the Kaimuqi mafic-ultramafic complex and the indicators for the magmatic Cu-Ni sulfide deposit exploration in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China

Abstract

Abstract The East Kunlun Orogenic Belt experienced two periods of mantle-derived magmatic activity during its post-collisional stages. One occurred during the Late Silurian-Early Devonian, and the other occurred during the Middle-Late Triassic. The Late Silurian-Early Devonian Xiarihamu mafic-ultramafic intrusions were found to be associated with a giant magmatic sulfide Ni-Co deposit discovered in 2012. The U-Pb age of websterite in the Kaimuqi mafic-ultramafic intrusion is 221.1\u202f±\u202f2.3\u202fMa. None of the Middle-Late Triassic mafic-ultramafic intrusions in the East Kunlun Orogenic Belt contain Cu-Ni mineralization. Based on simulations by the software MELTS, we found that the crystallization process leads to a drop in the sulfur content at sulfide saturation. The mineralized Late Silurian-Early Devonian mafic-ultramafic intrusions exhibit both a broader range of m/f values and higher m/f values than the non-mineralized intrusions in the East Kunlun Orogenic Belt, which means that the mineralized intrusions experienced a long process of crystallization differentiation. This process is probably responsible for Cu-Ni mineralization in the Early Devonian Xiarihamu mafic-ultramafic intrusion. Compared to the olivine and clinopyroxene in the Xiarihamu giant Cu-Ni deposit, the Late Triassic Kaimuqi olivine contains significantly lower MgO, Fo and SiO2 contents and higher CaO and FeO contents, and the Kaimuqi clinopyroxene contains higher FeO and CaO contents. Therefore, we infer that the Kaimuqi intrusion formed from a melt with higher FeO and CaO contents and a lower SiO2 content. This melt required more sulfur to achieve sulfide saturation. This characteristic most likely hindered mineralization in the Kaimuqi mafic-ultramafic intrusion. All of the mafic-ultramafic intrusions that host Cu-Ni deposits in the East Kunlun Orogenic Belt contain lherzolite, harzburgite, and orthopyroxenite, indicating that the pyroxene in these intrusions is mainly the orthopyroxene. If an intrusion contains more clinopyroxene, it indicates that its magma contains more calcium. The magma with higher calcium content will need a higher sulfur content to achieve sulfide saturation. Although websterite occurred in the Kaimuqi intrusion, the clinopyroxene and orthopyroxene content in the websterite is 75% and 20%, respectively, and the peridotite lithofacies is wehrlite. Therefore, orthopyroxene is rare in the Kaimuqi mafic-ultramafic intrusions. Thus, the Kaimuqi mafic-ultramafic intrusion may not have had the potential to form Cu-Ni ore deposits. Finally, this paper summarizes some geological and geochemical indicators for Cu-Ni mineralization in the mafic-ultramafic intrusions in the East Kunlun Orogenic Belt. These indicators provide some enlightenment to the exploration of Cu-Ni deposits in mafic-ultramafic intrusions, especially for the geological exploration bureaus or companies, which do not have the budget for large numbers of geological dating, isotopes, major elements, and platinum group elements during the exploration evaluation process in the current global mining recession situation.