The petrogenesis and metallogenesis of the Chakabeishan Li-Be pegmatitic deposit in Qinghai, NW China: Evidence from Geochronology, Geochemistry, and mineral geochemistry

Abstract

In recent years, a newly rare metal metallogenic belt has been discovered in the North Qaidam Terrane (NQT), NW China, of which the Chakabeishan (CKBS) Li-Be pegmatitic deposit is an important part. In this study, CMS (chemical composition-mineral assemblage-structural geology) classification is applied on CKBS pegmatitic rocks, based on their CMS features, the rocks are identified as pegmatite type. The CKBS pegmatites exhibit unique internal vertical zonation characteristics, and six zones are identified: biotite-muscovite-albite zone, graphic zone, layered tourmaline-albite zone, beryl-albite zone, spodumene-beryl-albite zone, and spodumene-albite zone. In this article, we present U-Pb zircon ages, Hf isotope dating, whole-rock geochemistry, and mineral geochemistry to elucidate pegmatite evolution. The zircon U-Pb ages of the spodumene-bearing pegmatite are 258.1 ± 1.4 Ma and 219.6 ± 6.3 Ma, indicate that the magma experienced two stage crystallization. The negative εHf(t) values range from −14.55 to −9.4 with T2DM model ages of 1845–2173 Ma, indicate that the magma originated from ancient rocks anatexis. The first anatectic event occurred during the Paleo-Tethys Ocean subduction process, as the magma originated from the anatexis of ancient metasedimentary rocks, provided enough rare elements (Li, Be, Nb, Ta). However, the second anatectic event occurred during post-orogenic stage and intruded in the host rocks forming pegmatite veins in the Late Triassic. Subsequently, intruded magma experienced mild fractionated crystallization, melt-fluid immiscibility and subsequent metasomatism, forming the vertical zonings.