MINERAL CHEMISTRY OF CASSITERITES FROM PITINGA PROVINCE, AMAZONIAN CRATON, BRAZIL

Abstract

Tin mineralization in the Pitinga Province is associated with Paleoprotero zoic (l .82 Ga) Madeira and Água Boa plutons. The primary mineralization occurs as (l) disseminated casgiterite in the rnagmatic albite granite facies ofthe Madeira pluton; (2) hydrothermal ðassiteiite, related with gleisen and episyenites in the Água Boa pluton. Another type ofmineralization is secondary and associated with the lateritic profile developed over the albitè granite. The aim of this paper is to discuss the rnineralogical and chemical characteristics of these cassiteritès and the relaiionships between them and the style of mineralization. The cassiterites of gJoup ( I ) show þigtt"J EçO, NbrO., and TarO, and lower TiO. contents wheri compared to those of the ôther groups. A coupled substitution 3 Sno*++2 (Nb, Ta))* + Fer* is dorñinänt and bì,esi explain the chémical variation obsèr'ved in this group. The cassiterites from the group (2) and the laterites have comparatively higher TiO, coñtents indicating that the mechanism Sna*