Abstract
The pattern of hydrothermal alteration associated with a gold prospect located within a differentiated iron-rich gabbro sill in the Chibougamau region has been investigated in order to establish the alteration mechanism. The gold mineralization, occurring in east-west trending shear zone, shows a close spatial relationship to zones of intense alteration. Ankerite-sericite-pyrite/ankerite-fuchsite-chlorite/ankerite-sericite-chlorite, and chlorite-calcite-magnetite form four distinct alteration facies which extend outward from the mineralized zones. Detailed mineralogical, petrological and chemical studies show that the iron content of chlorite increases whereas the iron content of the carbonate decreases from the mineralized zone outwards. The hydrothermal alteration pattern is shown to be derived from a single fluid with a high initial CO2/H2O ratio. At first, reducing reactions prevailed and, with progressive fluid-rock interaction, the CO2/H2O ratio was lowered and oxidation reactions dominated.
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