THE INDIUM-RICH SULFIDES AND RARE ARSENATES OF THE Sn-In-MINERALIZED MANGABEIRA A-TYPE GRANITE, CENTRAL BRAZIL

Abstract

Indium minerals and In-rich phases in Brazil are restricted to Paleo- to Mesoproterozoic tin-bearing within-plate magmatic zones in central Brazil. The most important concentration of indium is in the Mangabeira granitic massif, which is also the type locality of yanomamite (InAsO4·2H2O). The Sn–In-mineralized area of this massif is comprised of Li-rich siderophyllite granite, topaz–albite granite, quartz – Li-mica greisens and a vein of In-rich quartz–topaz rock, with up to 1 wt% In, mainly composed of quartz, topaz, zinnwaldite, arsenopyrite and cassiterite. Accessory minerals are sphalerite, ferberite, lollingite, chalcopyrite, bismuthinite, galena, stannite-group minerals, tennantite, argentite and roquesite. Secondary minerals comprise bornite, digenite, covellite, scorodite, phenakite, native copper, yanomamite, dzhalindite, metazeunerite and rare arsenates, like pharmacosiderite, segnitite, chenevixite, and goudeyite, and unknown Bi, Ba and Sn arsenates. Indium-rich stannite, as well as red In-rich and brown In-poor varieties of sphalerite, also are present. Zinc is replaced by indium, copper and iron in the sphalerite structure, probably according to the scheme Cu + In + Fe = 3Zn. Indium-rich sphalerite intergrown with roquesite, forming a texture interpreted as the product of roquesite exsolution, is documented here for the first time. The data obtained are consistent with the existence of the pseudoternary system stannite – sphalerite – roquesite and of a discontinuous solid-solution between yanomamite and scorodite.