GOLD EMPLACEMENT AND HYDROTHERMAL ALTERATION IN METABASIC ROCKS AT THE MOUSKA MINE, BOUSQUET DISTRICT, ABITIBI, QUEBEC, CANADA

Abstract

The Mouska mine, in the Bousquet region of the Abitibi greenstone belt, Quebec, exploits a sulfide-rich quartz-vein-type gold deposit hosted by a metavolcanic sequence of basalt and andesite. The ore zones constitute three main structural and lithologic systems, named 07, 08 and 22, comprising both lenses of massive and disseminated sulfides and quartz veins. Gold, varying from microscopic to visible, is hosted by both sulfide and quartz veins. The ore minerals consist of pyrrhotite and chalcopyrite, together with minor amounts of pyrite. Pyrite in particular, consists of two generations. Pyrite I is fine-grained (100 to 200 μm), and encloses micro-inclusions of gold (10 to 12 μm), chalcopyrite and pyrrhotite. Pyrite II is late, coarse-grained and cataclastic, and lacks micro-inclusions of gold. Pyrite I is rare, and partially to completely replaced by chalcopyrite and pyrrhotite. It may well represent a remnant of the first paragenetic assemblage (with chalcopyrite and pyrrhotite inclusions) in the deposit. Gold in the Mouska deposit exhibits a wide range of occurrences and habits. The gold micro-inclusions in pyrite I contain 4 to 6% Ag; gold in any other habit contains up to 25% Ag. Compared to the common Archean auriferous quartz-vein deposits, the Mouska deposit has a higher sulfide content of the veins, and the variably altered and deformed metabasic rocks show both distal and proximal halos of alteration. Such halos result from a complex and progressive interaction between hydrothermal-predeformational (sulfide event) and tectonometamorphic (quartz event) imprints. Mineralogical, geochemical and isotopic studies show that alteration assemblages surrounding the ore zones not only vary with lithology (basalt to andesite), but indicate a complex hydrothermal history, where the mafic protoliths have undergone several transformations during the Au–sulfide and Au–quartz depositions. In the proximal alteration, the mass-balance calculations display a clear addition of K, which may account for the observed enrichment in biotite and white mica toward the ore zones. However, these calculations show substantial addition of SiO2 only in the altered basalts, which can be explained by the massive destruction of ferromagnesian minerals.