Zircon Th/U ratios suggest a post-collision extensional setting for the Permian Ni-Cu sulfide deposits in the Eastern Tianshan, NW China

Abstract

Magmatic Ni-Cu sulfide deposits have been discovered in orogenic belts worldwide. However, the optimal period for extensive basaltic magmatism generating Ni-Cu sulfide mineralization during the development of orogenic belts has not been well addressed. Zircon Th/U may be correlated with end-member tectonic stress regimes (continental extension vs. compression) that correlate with distinct magmatic conditions. Here, zircon Th/U ratios from the Permian Huangshan-Jingerquan Ni-Cu metallogenic belt in the Eastern Tianshan are revisited, including magmatic zircons from mafic and felsic rocks and detrital zircons from sedimentary rocks. These zircons are filtered to minimize the effect of amagmatic conditions (e.g., metamorphism, alteration, inclusion and metamictization). After filtration, zircon Th/U ratios in the Huangshan-Jingerquan belt are likely linked to the silica content of the host melt and crystallization temperature under zircon growth with fractionating melts. They display high Th/U ratios (up to 2.5) and δEu values (up to 1.35) during the early Permian. In contrast, the others with low Th/U (most < 1.0) ratios and δEu (<0.8) appear in the pre-Permian except the late Devonian. This indicates that the crust in the region was thickened induced by collision at the end of late Carboniferous and then extended during the early Permian. Conclusively, we suggest that the post-collision extensional setting is the preferred stage for the Permian short-lived (<5 Ma) Ni-Cu sulfide deposits during the development of the Tianshan Orogenic Belt.