Genesis of Kumutashi Fluorite Deposit in the West Altyn-Tagh Orogen, NW China: Constraints from Apatite in situ U-Pb Dating, Sr-Nd Isotope and Chemistry

In recent years, significant breakthroughs in fluorite prospecting have been made in the western Altyn-Tagh Terrane, and Kaerqiaer, Kumutashi and other deposits have been discovered successively, however, the research on metallogenic epoch and ore-forming processes are still unclear. In this paper, the closely symbiotic apatite with fluorite were selected as the research object to carry out the main microanalysis of apatite, U-Pb dating and in situ Sr-Nd isotopic test analysis, so as to explore the metallogenic epoch and the genesis of deposit. The apatite often has a self-semi-automorphic structure with uniform surface and nearly transparent under monopolarized light, mainly symbiotic with fluorite, calcite, tainiolite, bastnaesite and other minerals. The study shows that the U-Pb isotope age of apatite microregion is 448±27 Ma, and the fluorite mineralization is closely related to the invasive activity of alkali feldspar granite, all of which are the products of the late Ordovician tectonic-magmatic activity. The F content of apatite is 4.20% to 5.12%; the Cl content is less than 0.02%, and the very low Cl content indicates a low dissolved fluid Cl content. The content of rare earth elements is high (908×10⁻⁶～2164×10⁻⁶), and the partition curve of rare earth shows strong Eu negative anomaly and positive Ce negative anomaly. This anomaly is obviously consistent with its associated fluorite, calcite and alkali feldspar granite, which may be closely related to the dissolution of massive fluid in the magma-hydrothermal stage. The ratio of ⁸⁷Sr/⁸⁶Sr of apatite is from 0.70913 to 0.71047, the ratio of ¹⁴³Nd/¹⁴⁴Nd is from 0.51138 to 0.51153, and εNd(t) is from −13.27 to −10.26, reflecting that the ore-forming materials have the characteristics of crust-mantle mixing. Comprehensive studies show that the ore-forming age of fluorite in the western Altyn-Tagh Terrane is Ordovician, closely related to the same period alkali feldspar granite, formed in the post-collision extension stage, the ore-forming fluid may be derived from the melt-fluid evolution of alkali feldspar granite, and it is a magmatic hydrothermal filling type deposit.