Mineralogy and ore-forming fluid characteristics of Shaliuquan rare metal pegmatites at northern margin of Qaidam Basin
In this contribution,systematic mineralogical and fluid inclusion study were performed on the Shaliuquan No.1 beryl-type pegmatite dike at northern margin of Qaidam Basin based on its internal zonation to explore the magmatic differentiation and evolution of ore-forming fluid within the pegmatite.The evolutionary trends of mi-crocline and mica compositions suggest that the evolution of magmatic differentiation of the No.1 pegmatite from margin to core is mainly controlled by fractional crystallization,and the degree of differentiation increases gradu-ally.The differentiation of the pegmatite-forming melt is accompanied by the decrease in the fluid salinity.The ore-forming fluids evolved to CO2-bearing,weakly acidic fluids during the beryl crystallization.The ore-forming fluid hosed by quartz belongs to the NaCl-H2O system with medium-high temperature(200～427℃)and low salin-ity(w(NaCleq)3.2％～9.9％),whereas that hosted by beryl belongs to the NaCl-H2O-(CO2)system with medium-high temperature(213～363℃)and low salinity(w(NaCleq)1.1％～10.2％).The early-stage NaCl-H2O fluid evolved to two distinctly different fluids,namely a NaCl-H2O fluid and a NaCl-H2O-CO2 fluid at the late stage.The presence of abundant CO2-rich fluid inclusions and daughter minerals including siderite(FeCO3)indicates that the crystallization of beryl was facilitated by fluids enriched in CO2(CO32-).The Be mineralization hosted by the Shaliuquan No.1 pegmatite is likely the consequence of both melt differentiation and fluid evolution.
万方数据
维普
