Pb isotope compositions of pyrite from the C quartz-tourmaline vein of the Siscoe gold deposit, Val D'Or, Quebec: constraints on the origin and age of the gold mineralisation.

Pb isotope compositions were studied of hydrothermal pyrite separates taken from the C quartz-tourmaline vein and its wall rock. Pyrite is abundant in the wall rock where it occurs as grains with inclusions of chalcopyrite, gold, tetradymite, pyrrhotite, rutile, silicates and carbonates. In the vein, it forms euhedra generally devoid of inclusions. Locally, the euhedra are fractured and the fractures are filled with native Au, calcite and tetradymite. Textural relationships suggest that pyrite precipitated synchronously with and/or after Au in the wall rock, but predated Au in the vein. The Pb ratios yield a model age which is interpreted to date the cooling of the hydrothermal system that introduced and/or remobilised Au into the C quartz-tourmaline vein. The results indicate that widespread hydrothermal circulation occurred between 2.64 and 2.55 Ga, after the collision of the Pontiac and Abitibi sub-provinces, and was caused by dewatering and S-type magmatism in the thickened unstable continental crust. The long-lived hydrothermal system promoted remobilisation of Au from earlier veins and possibly introduction of Au from deeper zones, enhancing the Au resources in the deposit.
Pb isotope compositions were studied of hydrothermal pyrite separates taken from the C quartz-tourmaline vein and its wall rock. Pyrite is abundant in the wall rock where it occurs as grains with inclusions of chalcopyrite, gold, tetradymite, pyrrhotite, rutile, silicates and carbonates. In the vein, it forms euhedra generally devoid of inclusions. Locally, the euhedra are fractured and the fractures are filled with native Au, calcite and tetradymite. Textural relationships suggest that pyrite precipitated synchronously with and/or after Au in the wall rock, but predated Au in the vein. The Pb ratios yield a model age which is interpreted to date the cooling of the hydrothermal system that introduced and/or remobilised Au into the C quartz-tourmaline vein. The results indicate that widespread hydrothermal circulation occurred between 2.64 and 2.55 Ga, after the collision of the Pontiac and Abitibi sub-provinces, and was caused by dewatering and S-type magmatism in the thickened unstable continental crust. The long-lived hydrothermal system promoted remobilisation of Au from earlier veins and possibly introduction of Au from deeper zones, enhancing the Au resources in the deposit.