Newly discovered Ordovician Li-Be deposits at Tugeman in the Altyn-Tagh Orogen, NW China

Abstract

Magmatism was widespread in the Proterozoic to Early Paleozoic Altyn-Tagh Orogen of NW China. Rare metal deposits hosted by ca. 472 Ma pegmatite dykes with a monzogranitic geochemistry have recently been discovered in the orogen, including the Tugeman Li-Be deposit. The pegmatites contain spodumene – beryl (– columbite – tantalite – cassiterite), and are coeval with the ca. 482 Ma biotite monzogranite as a late-stage phase of the monzogranite, and both the monzogranite and pegmatite intrude orthogneiss and gneissic syenogranite. Two samples of biotite monzogranite yield weighted mean Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) U–Pb zircon dates of 482 ± 5 and 475 ± 5 Ma, and columbite-tantalite dates of 472 ± 8 Ma for the pegmatite, and minimum dates of 438–412 Ma for hosts of the columbite-tantalite. The gneissic syenogranite and orthogneiss in the deposit’s wallrocks have zircon U–Pb ages of ca. 900 Ma. The geochemistry and Hf-Lu isotopic systematics indicated that the protoliths for the gneissic syenogranite and orthogneiss have a mantle source with a contribution from the Proterozoic crust, and the biotite monzogranite has a Proterozoic crustal source. The Tugeman Li-Be deposit, therefore, has a Precambrian crustal source and was deposited during the Ordovician, which we propose was a post-collision extensional event and a previously unknown mineralising event in China.