Geology of the Raposos gold mine, Minas Gerais, Brazil

The gold ore bodies are mineralized segments of folded Precambrian iron formations separated by shear zones, diabase dikes, and barren iron formation. The type of iron formation that acts as principal host is banded siderite-quartz rock. Belts of this rock are enclosed in chlorite schist, some of which is chromium-rich. Gold-bearing hydrothermal solutions, rising along the axes of plunging folds, preferentially replaced bands of siderite in the iron formation. Early solutions altered the schist wall rock to green chromian sericite schist and deposited pyrite. Arsenopyrite, pyrrhotite, chalcopyrite, quartz, ankerite, and most of the gold were introduced during the later, principal stage of mineralization, from an unknown source or sources.