The Peralkaline Tin-Mineralizes Madeira Cryolite Albite-Rich Granite of Pitinga, Amazonian Craton, Brazil: Petrography, Mineralogy and Crystallization Processes.

Abstract

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, cryolitebearing 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 ~700°C; the quartz – K-feldspar cotectic was attained at around 650°C, and the ternary feldspar solvus and onset of albite crystallization down to the solidus, estimated at around 500°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.