Mineralogy, isotope geochemistry and ore genesis of the miocene Cuonadong leucogranite-related Be-W-Sn skarn deposit in Southern Tibet

Abstract

The Himalayan orogenic belt is characterized by widespread leucogranites, which are mostly peraluminous with less than 5% mafic minerals. Previous studies mainly focused on the petrological characteristic, genetic mechanism, magmatic evolution and tectonic significance of these leucogranites. However, recently, a skarn-type Be–W–Sn polymetallic deposit, which is spatially related to the Cuonadong leucogranites of the Himalayan leucogranites belt, was discovered. The orebodies occur in the inner and outer contact zones between the Cuonadong leucogranites and their surrounding marble. This study focuses on the the characteristics of the skarn mineral assemblages. The deposit shows a typical skarn mineral zonation, from inside to outside, including dark pyroxene-garnet skarn zone, red garnet zone, dark green amphibole-specularite zone, light-green vesuvianite-epidote ribbon, and vesuvianite-chlorite zone. Zircon U-Pb dating shows that the ore-related leucogranite was emplaced at ca. 18 Ma. Geochemically, the leucogranite has high SiO2, Al2O3, Na2O, K2O and low CaO contents belonging to peraluminous granite. The Cuonadong leucogranite is characterized by right-dipping chondrite-normalized REE patterns with distinct negative Eu anomalies. Importantly, all samples show Lanthanide tetrad effect on chondrite-normalized REE patterns, indicating significant melt-fluid interaction has occurred due to strong fractional crystallization. Additionally, the Cuonadong leucogranite is enriched in large ion lithophile elements (such as Rb) and relatively depleted in high field strength elements (such as Nb, Zr, Hf), suggesting a crustal source region. High F and B contents in the magma could have significantly increased the solubility of water in the magma, while the magmatic-hydrothermal solution enriched in F and B should have remarkably enhanced the metal-carrying capacity and increased the permeability of surrounding rocks, which in turn effectively facilitated water–rock reaction.