Abstract
The number I Shitoukengde mafic-ultramafic complex is located in the eastern part of the East Kunlun Orogenic Belt (EKOB). It comprises peridotite (dunite and lherzolite), olivine websterite, pyroxenite (websterite and clinopyroxenite), and gabbro. The Ni-Cu sulfide mineralization is hosted in the peridotite and olivine websterite. Zircon SHRIMP U-Pb isotopic analyses reveal an early Carboniferous age of 333.9 ± 4.2 Ma for the olivine websterite and a Silurian age of 424.7 ± 3.7 Ma for the gabbro. Based on these data and the regional tectonic setting, we suggest that the number I Shitoukengde ultramafic rocks formed during the opening of the Paleotethys Ocean in the East Kunlun area. This geological setting is quite different than that of the Xiarihamu Ni-Co deposit (411 Ma) in the EKOB, which formed in a post-collisional environment. The whole-rock Ni contents of the peridotite (dunite and lherzolite) and olivine websterite are negatively correlated with the FeO contents and positively correlated with the MgO and Fo contents of the olivine, indicating that fractional crystallization of the parent magma of the observed rocks did not play a role in the sulfide saturation process. The ultramafic rocks with high Ni contents, i.e., the peridotite and olivine websterite, have low (87Sr/86Sr)i values, indicating low degrees of crustal contamination. The ultramafic rocks with lower Ni concentrations are in contact with the unmineralized marble country rock. We suggest that the assimilation of marble decreased the sulfide solubility of the magmas that produced the number I Shitoukengde complex. The δ34S values of the Shitoukengde sulfide hosted in the ultramafic rocks range from 1.9 to 4.3‰, with an average value of 2.9‰, indicating that crustal sulfur was added to the magma. In the country rocks, we found minor amounts of sulfide only in the gneiss from the Jinshuikou Group. Therefore, the number I Shitoukengde ultramafic rocks were probably contaminated with some sulfur from the gneiss. We propose that the most prospective ultramafic segments of the number I Shitoukengde complex are adjacent to gneissic country rocks.
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