Titanite U-Pb dating and geochemical constraints on the Paleozoic magmatic-metamorphic events and Nb-Ta mineralization in the Yushishan deposit, South Qilian, NW China

Abstract

Titanite, as one significant petrogenetic tracer, occurs in the ore-bearing alkaline unit (syenite and aegirine-augite syenite dikes) and in the barren calc-alkaline unit (monzodiorite) and metamorphosed strata (called as leptynite) in the recently found large Yushishan Nb-Ta deposit from South Qilian, NW China. Most titanite grains in the alkaline unit display obvious shape-preferred orientation and zonation, coexisting with allanite, zircon and alkali feldspar. Some titanite grains from the alkaline unit have been partially or completely altered into fine-grained euxenite and/or Nb-rich rutile, but retained their original crystal shapes. Although some titanite grains in the igneous rocks show obvious core-rim textures, both of them yield almost the same U-Pb ages. Combined with U-Pb dating of zircon from these different rock units, the complex magmatic-metamorphic events in the Yushishan district can be subdivided into two different periods: the first one occurring in the Neoproterozoic (ca. 834–790 Ma, zircon U-Pb ages) that are related to the regional volcanism and metamorphic activities and the formation of leptynite during the metamorphism, while the second one occurring in the early Paleozoic (ca. 490–470 Ma, titanite and zircon U-Pb ages) that are related to the formation of granitic intrusions and subsequent metamorphic process and Nb-Ta mineralization. Geochemically, titanite from these igneous rocks is consistent with the typical features of igneous titanite, such as high Fe and REE contents, and negative Eu anomalies. The zoned titanite in both alkaline (mainly syenite) and calc-alkaline units uniformly displays a core-to-rim decreasing trend for the Fe, REE, Nb and Ta contents and Th/U ratios, corresponding to a secular variation from the magmatic to metamorphic evolution. Titanite from the leptynite is interpreted to be of metamorphic origin, with low Fe and REE contents, weakly positively sloping REE patterns, along with positive Eu anomalies. It is suggested that the titanite can be used as an indicator of metal fertility and ore potential for the host rocks/fluids. The high Nb and Ta features in titanite from the alkaline unit suggest a genetic relationship between the alkaline granitic magmatism and Nb-Ta mineralization in the Yushishan deposit. In addition, the high and variable Nb and Ta contents in titanite from the leptynites reveal an inherited signature of the metamorphic fluids. Considering the absence of titanite in the ore-bearing leptynites, we speculate that titanites with high and variable Nb and Ta contents of titanite in the barren leptynites have the potential to provide abundant metals for the precipitation of Nb-Ta-bearing minerals to form economic orebodies.