Genesis of the Proterozoic Mangabeira tin–indium mineralization, Central Brazil: Evidence from geology, petrology, fluid inclusion and stable isotope data

Abstract

The Mangabeira deposit is the only known Brazilian tin mineralization with indium. It is hosted in the Paleo- to Mesoproterozoic Mangabeira within-plate granitic massif, which has geochemical characteristics of NYF fertile granites. The granitic massif is hosted in Archean to Paleoproterozoic metasedimentary rocks (Ticunzal formation), Paleoproterozoic peraluminous granites (Aurumina suite) and a granite–gneiss complex. The mineralized area comprises evolved Li-siderophyllite granite, topaz–albite granite, Li–F-rich mica greisens and a quartz–topaz rock, similar to topazite. Two types of greisens are recognized in the mineralized area: zinnwaldite greisen and Li-rich muscovite greisen, formed by metasomatism of topaz–albite granite and Li-siderophyllite granite, respectively. Cassiterite occurs in the quartz–topaz rock and in the greisens. Indium minerals, such as roquesite (CuInS2), yanomamite (InAsO4·2H2O) and dzhalindite (In(OH3)), and In-rich cassiterite, sphalerite, stannite group minerals and scorodite are more abundant in the quartz–topaz rock, and are also recognized in albitized biotite granite and in Li-rich muscovite greisen. The host rocks and mineralized zones were subsequently overprinted by the Brasiliano orogenic event.Primary widespread two-phase aqueous and rare coeval aqueous-carbonic fluid inclusions are preserved in quartz from the topaz–albite granite, in quartz and topaz from the quartz–topaz rock and in cassiterite from the Li-rich muscovite greisen. Eutectic temperatures are −25°C to −23°C, allowing modeling of the aqueous fluids in the system H2O–NaCl(–KCl). Rare three-phase H2O–NaCl fluid inclusions (45–50wt.% NaCl equiv.) are restricted to the topaz–albite granite. Salinities and homogenization temperatures of the aqueous and aqueous-carbonic fluid inclusions decrease from the topaz–albite granite (15–20wt.% NaCl equiv.; 400°C–450°C), to the quartz–topaz rock (10–15wt.% NaCl equiv.; 250°C–350°C) and to the greisen (0–5wt.% NaCl equiv.; 200°C–250°C). Secondary fluid inclusions have the same range of salinities as the primary fluid inclusions, and homogenize between 150 and 210°C.The estimated equilibrium temperatures based on δ18O of quartz–mica pairs are 610–680°C for the topaz–albite granite and 285–370°C for the Li-rich muscovite greisens. These data are coherent with measured fluid inclusion homogenization temperatures. Temperatures estimated using arsenopyrite geothermometry yield crystallization temperatures of 490–530°C for the quartz–topaz rock and 415–505°C for the zinnwaldite greisens. The fluids in equilibrium with the topaz–albite granite have calculated δ18O and δD values of 5.6–7.5‰ and −67 to −58‰, respectively. Estimated δ18O and δD values are mainly 4.8–7.9‰ and −60 to −30‰, respectively, for the fluids in equilibrium with the quartz–topaz rock and zinnwaldite greisen; and 3.4–3.9‰ and −25 to −17‰, respectively, for the Li-rich muscovite greisen fluid. δ34S data on arsenopyrite from the quartz–topaz rock vary from −1.74 to −0.74‰, consistent with a magmatic origin for the sulfur. The integration of fluid inclusion with oxygen isotopic data allows for estimation of the minimum crystallization pressure at ca. 770bar for the host topaz–albite granite, which is consistent with its evolved signature.Based on petrological, fluid inclusion and isotope data it is proposed that the greisens and related Mangabeira Sn–In mineralization had a similar hydrothermal genesis, which involved exsolution of F-rich, Sn–In-bearing magmatic fluids from the topaz–albite granite, early formation of the quartz–topaz rock and zinnwaldite greisen, progressive cooling and Li-rich muscovite greisen formation due to interaction with meteoric water. The quartz–topaz rock is considered to have formed in the magmatic-hydrothermal transition. The mineralizing saline and CO2-bearing fluids are interpreted to be of magmatic origin, based on the isotopic data and paragenesis, which has been documented as characteristic of the tin mineralization genetically related to Proterozoic within-plate granitic magmatism in the Goias Tin Province, Central Brazil.