Genesis of Garnets within pegmatites in the northwestern margin of the Mufushan granite (central Jiangnan Orogen)，and its implication for rare metal mineralization: comparison from mineralized and unmineralized pegmatites

The Mufushan area，in the central Jiangnan Orogen，South China，has become one of the most significant rare metals resource bases in China. The Duanfengshan pegmatite-type Nb-Ta deposit is the only large-sized deposit in the northwestern margin of the Mufushan granitic batholith. However，its genesis and physicochemical conditions are not well studied，in particular of the newly discovered microcline pegmatite-type Nb-Ta mineralization. This study focuses on garnets in the mineralized and unmineralized microcline pegmatites in the Duanfengshan area. Garnets were observed using cathodoluminescence (CL) and backscattered electron (BSE) imagery. The major and trace elements are determined by EPMA and LA-ICP-MS，which are used to discuss the genesis of garnet and indication for Nb-Ta mineralization of microcline pegmatites. Garnets in mineralized and unmineralized pegmatites in the Duanfengshan area both belong to the solid solution series of almandite-spssartite. Almandite (Sps42.56Alm54.63) dominantly occurs in the mineralized pegmatites，whereas spssartite (Sps58.93Alm37.18) mainly distributes in the unmineralized pegmatites. Garnets both are characterized by enrichment in heavy rare earth elements (HREE)，and depletion in light rare earth elements (LREE) with Eu negative anomaly (Eu/Eu*=0-0.09). These mineralogical and geochemical features suggest a magmatic origin for garnets within microcline pegmatites in the Duanfengshan area. The two types of pegmatite would be derived from different magmatic sources in the same period. Both formed in medium-high temperatures and medium-low pressures. The low Mn contents (12.07-19.00 wt.%) of garnet in the mineralized pegmatites is caused by the crystallization of niobium-tantalum minerals. Rare metal mineralization is related to the low degree of liquidus undercooling of pegmatite veins in the outer contact zone. The Y/Gd and Y /Dy ratios of garnets，and the appearance of tourmalines in pegmatites confirm that the pegmatitic magmas are enriched in volatiles，which is beneficial to the enrichment of rare metal elements. However，the undeveloped zoning of unmineralized pegmatites is due to the high degree of liquidus undercooling in the inner contact zone.