THE PERALKALINE TIN-MINERALIZED MADEIRA CRYOLITE ALBITE-RICH GRANITE OF PITINGA, AMAZONIAN CRATON, BRAZIL: PETROGRAPHY, MINERALOGY AND CRYSTALLIZATION PROCESSES

The 1.818 Ga magmatic, subsolvus, Madeira albite-rich granite of the Pitinga province, exploited at the Pitinga mine, in northern Brazil, crystallized from a F-rich melt, also enriched in Sn, Rb, and HFSE. It is composed of a peralkaline, cryolite-bearing core facies and a peraluminous to metaluminous, oxidized, fluorite-bearing border facies, generated by autometasomatic processes after the crystallization of the core facies. A metaluminous to peralkaline hypersolvus granite is comagmatic with the albite-rich granite. Petrographic studies indicate that quartz was the first phase to crystallize, at similar to 700 degrees C; the quartz-K-feldspar cotectic was attained at around 650 degrees C, and the ternary feldspar solvus and onset of albite crystallization, down to the solidus, estimated at around 500 degrees C. Massive cryolite and pegmatitic rocks found in the center of the stock were derived from the residual melt. The albite-rich granite probably originated by crystallization of a dense, F-rich, peralkaline phase derived from a peralkaline to metaluminous parental melt by immiscibility.