Zonalidade vertical no corpo mineralizado da mina de ouro Schramm (SC) – detalhamento do modelo genético

Abstract

The petrography of ore samples collected from level 140 to 180 of the Schramm gold mine displayed a hydrothermal phase not previously discerned, occurred before the ore genesis, which crystallized magnetite-hydrotermalites. A vertical zonation, was also displayed, characterized by three different ore paragenesis. Bellow level 155, the carbonated ore is hosted by the magnetite-hydrotermalites and it is composed by lamellar hematite, magnetite, sphalerite and gold, with subordinated pyrite, calcopyrite, arsenopyrite, gersdorffite and siegenite. This paragenesis typifies an environment where the primary ore fluid was oxidized and desulphurized, what caused gold deposition. Around the 155 level, the carbonated ore has few sulfides and a paragenesis in which siegenite, gersdorffite and sphalerite are very common minerals, with gold and subordinated pyrite and calcopyrite. At this level ore is hosted by ultrabasic granulites, and gold was deposited as a consequence of the low pH-intermediate fO 2 environment. Above the 155 level, the main ore minerals are Bi-rich galena, sphalerite and gold, with subordinated gersdorfite, pirrotite, pyrite and cosalite. Gold is mainly in carbonate but also in quartz veins hosted by granulites. These minerals characterize a reduced fluid and a high pH environment. The ore body was formed through at least four phases: (1) Genesis of magnetite-hydrothermal veins, at temperatures around 230oC. (2) Genesis of the oxidized ore inside magnetite-hydrothermal veins and the siegenite-millerite-gersdorffite ore hosted by ultrabasic granulites, at temperatures between 260o and 310oC. (3) Period that the main vein stay opened, filled by fluids that precipitated euhedric, prismatic, siderite, quartz and chlorite with little gold, at temperatures between 310o and 320oC. (4) With the vein walls coated by minerals crystallized previously, the primary, reduced, ore fluid could not react with host rocks. Gold deposited due the temperature reduction caused by mixing of ore fluid and meteoric water. This new fluid precipitated milky-quartz until the vein closure and the end of the hydrothermal system.