Abstract

Paleoproterozoic gold is found in two different volcano-sedimentary sequences and intrusive bodies separated by 600 km within the Guiana Shield. The 2148 Ma St-Elie granite intruded volcanic-arc sequences of the Paramaca Group, whereas the 2096 Ma Omai stock penetrated back-arc/island-arc sequences of the Barama-Mazaruni Group. Positive epsilon Nd values suggest that both suites lack Archean crustal contaminants, which is consistent with an origin in an oceanic arc setting. The St-Elie deposit is characterized by quartz-dominant vein systems with minor metallic minerals, represented chiefly by pyrrhotite and pyrite. Biotite, calcite, and chlorite are the main gangue minerals. The alteration haloes at St-Elie feature a strong potassic zone, which is absent at Omai. The ore-controlling structures are ductile and the vein emplacement is related mainly to contemporaneous, high-angle ductile shear zones. The St-Elie deposit formed at temperatures >350 degrees C. The stable isotopes values are compatible with a magmatic or metamorphic source. The Omai deposit is characterized by quartz-dominant vein systems with minor metallic minerals, represented mainly by pyrite. Ankerite, scheelite, albite, and chlorite are common gangue minerals. The vein emplacement is syn-to post-deformation and it is related mainly to brittle shear zones with low-angle, stockwork networks and breccia zones. The Omai deposit formed in a temperature range between 250 degrees C and 170 degrees C, much lower than the St-Elie deposit. The stable isotope values are compatible with mixing between deep-seated fluids of metamorphic or magmatic origin with surface-derived fluids. These differences between St-Elie and Omai are interpreted in terms of their depth of formation. Alteration patterns, structural elements, and paragenetic assemblages indicate that the gold at St-Elie was most probably emplaced at a mesozonal depth, whereas the Omai setting yields evidence for epizonal conditions.

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