Tin-bearing sodic episyenites associated with the Proterozoic, A-type Agua Boa granite, Pitinga mine, Amazonian craton, Brazil

This paper reports the first occurrence of tin-mineralized episyenites in the Amazonian craton. The Agua Boa and Madeira plutons in the Pitinga region are stanniferous Proterozoic A-type granites of the rapakivi series featuring metasomatic episyenitization. The biotite granite facies of the Agua Boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite. The sodic episyenites formed by: (a) albitization of K-feldspar, (b) vug formation by dissolution of magmatic quartz, (c) vug filling by albite, chlorite, lithian muscovite, cassiterite +/- fluorite +/- K-feldspar and (d) deposition of late quartz cassiterite in remaining cavities. In the potassic episyenites, vugs produced by quartz dissolution are filled by secondary K-feldspar, which also replaces magmatic phases. Micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites. In contrast to the sodic episyenites, the potassic and micaceous episyenites contain only traces of cassiterite. Relative to unaltered biotite granite, the sodic episyenites are strongly depleted in SiO2, K2O, LREE and Sr, moderately depleted in HREE, and enriched in Na2O, Al2O3, Rb and Sn. The potassic episyenites resemble the sodic episyenite in SiO2 and Al2O3 but exhibit higher values of K2O, Ba, Y and Rb; their Na2O, REE, and Sn concentrations match those of the biotite granite host. Metasomatic episyenitization is attributed to: (1) a high temperature gradient and fluid-rock ratio during subsolidus cooling of the Agua Boa pluton, (2) quartz dissolution in the biotite granite facies by reaction with an high-temperature, silica-undersaturated fluid of probable magmatic origin, (3) hydrothermal cavity filling by a low-temperature, silica-saturated fluid of meteoric, or mixed magmatic/meteoric character. Significant tin values in metasomatic sodic episyenites near the Pitinga mine suggest that exploration of similar rocks in other A-type granites may be worthwhile.