Geochemical characteristics of cryolite-tin-bearing granites from the Pitinga Mine, northwestern Brazil — A review

Abstract

Mineralization in granites bearing cryolite, tin and rare elements (F, Zr, Nb, Ta, Y, REE, Rb) in the Pitinga region is related to the Madeira and Agua Boa Granites. These alkaline granitic bodies have Proterozoic Rb/Sr ages of 1689 Ma, and are interpreted to be cogenetic due to strong similarities in their petrological, geochemical and isotopic characteristics. The Madeira Granite is composed of an early rapakivi facies (RPK) intruded by a biotite granite (BTG) with an apogranitic (APG) facies at the core. The RPK is mildly metaluminous, the BTG is metaluminous to peralkaline and the APG ranged from peraluminous to peralkaline, due to different stages of albitization, microclinization, or greisenization. The Agua Boa Granite also has an early rapakivi facies that is intruded by a fine-grained porphyritic granite (FRG) and by a late biotite granite. All facies are peraluminous. The apogranite, which is closely associated with primary mineralization of cassiterite, cryolite, malacon, xenotime, REE, Nb, Ta, Y and Rb, can be subdivided into a marginal subfacies and a core subfacies. The marginal subfacies is strongly oxidized, with red staining and with fluorite and cassiterite as accessory phases. The core subfacies consists grey, peralkaline rocks with seams enriched in riebeckite and cryolite that are associated with strongly albitized bands with peraluminous affinity. Evidence linking the APG to metasomatic/late magmatic processes includes the occurrence of high albite with 0% An, high contents of F, Zr, Nb, Ta, Y, Li, Ga, Na 2O, Rb and HREE, low contents of MgO, TiO 2, P 2O 5, LREE, Ba and Sr, and very low normalized La/Yb.