Interaction between granite magmas and the origin of tin-mineralized potassic episyenites from the Madeira granite, Pitinga mine (AM), Brazil

Abstract

The petrographic study in samples from the contact zone between the border albite-rich granite and the porphyritic hypersolvus alkali feldspar granite, in the western border of the Madeira pluton, allowed for the recognition of rocks with inherited textural and mineralogical features of both facies. This is interpreted as resulting from the interaction between two coeval granite magmas, which were simultaneously emplaced in the crust. Later, in subsolidus stage, the action of hydrothermal fluids caused a pervasive potassic metassomatism, identified as episyenitization, that initially caused the quartz dissolution of the granitic rocks, generating vugs and increasing the altered rocks’ permeability. At the same time, the albite of the border albite-rich granite had been dissolved and replaced for microcline, and, in low-temperature hydrothermal stages, cavities were filled by quartz, microcline, fluorite, hematite, cassiterite, phengite, chlorite, pyrite, sphalerite, galena and chalcopyrite. The potassic episyenites are dark red to brown colored, medium to fine grained, microcline-rich, locally porous rocks and commonly with vugs filled with quartz and sulfide. The cassiterite crystals of the episyenitized rocks show almost pure composition and are Nb-poorer than those magmatic ones. The potassic episyenitization may have been triggered by an oxidized, K-rich, alkaline, silica-undersaturated fluid. Two hypotheses have been proposed to its origin: 1) exsolution of a fluid phase from the magmatic liquid associated with hypersolvus granite; 2) the same fluid which caused the autometassomatic alteration of the albite-rich granite core facies.