Dissolution of Pyrochlore and Microlite in Alkaline, Subaluminous, and Peraluminous Granitoid Melts

Abstract

The data of experiments are presented on the Ta and Nb concentrations in acidic magma melts of various compositions upon dissolution of pyrochlore and microlite at 650‒850°C and 100‒400 MPa, which allow one to obtain quantitative characteristics to create physicochemical models of the genesis of Ta–Nb deposits. Upon the dissolution of pyrochlore in granitoid melts at 100 MPa, the maximum Nb content of 0.7‒1.9 wt % was found in alkaline solutions, with a decrease to 0.06–0.38 wt % and a moderate increase to 0.11–0.41 wt % in sub- and peraluminous melts, respectively. The growth in temperature increased the solubility of pyrochlore; the effect of an increase in pressure was ambiguous. Pyrochlore was unstable in peraluminous granite melts. Upon the dissolution of microlite, the Nb/Ta ratio in the melt was nearly doubled with an increase in the alumina content in the melt.