Genesis of Kalqiaer Super–large Fluorite Zone in Altyn Tagh Area: Chronology, Rare Earth Elements and Sr–Nd Isotopes Constraints

In recent years, great breakthroughs have been made in fluorite prospecting of Altyn–Tagh which new discovery of the super–large fluorite ore belt in the Kalqiaer area. The typical fluorite ore deposits such as Kalqiaer, Xiaobaihegou, Kumutashi, Layidan and Gaijike are closely related to and mainly distributed in the outer contact zones of the flesh red alkali feldspar granite. The host rocks are mainly biotite plagioclinal gneiss and carbonate rocks in Altyn rock group. Their orebodies obviously controlled by NE direction fault structure and the main ore types are veined, brecciated, massive and banded ore which major minerals are fluorite and calcite. The genesis of the deposit belongs to hydrothermal filling deposit. Zircon LA–ICP–MS dating yields concordant ages of 455.8±2 Ma for the alkali feldspar granite in the Kalqiaer super–large fluorite deposit which indicating it was formed in the Middle to late Ordovician and was the product of magmatic activity in the transitional tectonic setting from the compressional to extensional segimes. Gneissic potassium feldspar granite obtains the concordant age of 914.5±4.1 Ma respectively indicating it was formed in the early Neoproterozoic and related to the convergence event of the Rodinia supercontinent. The rare earth element characteristics show that the distribution pattern of rare earth in fluorite and calcite was a rightward light rare earth enrichment type, with negative Eu anomalies. The REE patterns of fluorite and calcite are similar to the ore–forming rock and ore–hosting strata, indicating a genetic relationship. Fluorite deposits in Kalqiaer area are hydrothermal origin, showing the characteristics of homologous and homochronous ore–forming fluids, and the ore–forming environment is medium–low temperature under reducing conditions. The Sr–Nd isotopic composition of fluorite in Kalqiaer area shows that the ore–forming materials are all derived from the crust. It is suggested that Ca may be mainly derived from the leaching extraction of the strata by magmatic hydrothermal, while F element may be mainly derived from alkali feldspar granite.