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.

The results of oxygen isotope analysis of hydrothermally altered volcanic rocks are used to define the thermal and isotopic characteristics of the ore-forming fluids which produced several volcanogenic massive sulfide deposits in the Noranda district. These deposits (Corbet, Ansil, Norbec, Horne, and Mobrun) formed during temporally distinct events that span much of the volcanic stratigraphy in the district.The data show that the delta 18 O values of the altered rocks and the size (economic tonnage) of each deposit increase upward through the volcanic stratigraphy from low delta 18 O values at the Corbet (-2.2 to 4.8ppm) and Ansil (-0.8 to 5.0ppm) deposits, to intermediate delta 18 O values at the Amulet (3.6-6.7ppm) and Norbec (3.6-10.5ppm) deposits, to high delta 18 O values at the Horne (4.2-11.6ppm) and Mobrun (6.0-13.8ppm) deposits. A corresponding increase is indicated for the delta 18 O values of the discharging hydrothermal fluids, from -2 + or - 2 per mil at the Corbet deposit to +3.0 + or - 1.5 per mil at the Horne deposit. Alteration in most of the deposits is shown to have resulted from the flow of fluids having temperatures of 300 degrees + or - 50 degrees C, with the exception of the pyrite-rich Mobrun deposit where slightly lower temperatures (200 degrees + or - 50 degrees C) are indicated.These observations are interpreted to indicate that the duration of hydrothermal discharge (as indicated by deposit size) at a given site increased upward through the volcanic stratigraphy. Since the fluid, and thus the altered rock, delta 18 O values normally evolve from low to high with time in a volcanogenic massive sulfide-forming system (e.g., Cathles, 1983), the duration of fluid discharge is a primary factor in controlling the isotopic signature retained at a given deposit. In the Noranda district, the duration of fluid discharge depended strongly on the rate of volcanic accumulation, with short-lived systems (e.g., Corbet, Ansil) forming during periods of rapid accumulation of extrusive volcanic rocks, and longer-lived systems forming during more quiescent periods (e.g., Norbec), or at sites where the accumulation of extrusive material was limited by topography (e.g., Horne).

Low K rhyolites of the Archean Blake River Group that host the Horne volcanogenic massive sulfide deposit are altered to quartz-sericite-chlorite-albite (quartz-sericite zone) and chlorite +/- quartz (chlorite zone) assemblages. The quartz-sericite zone accounts for approximately 90 percent of the 5 km3 of the altered hanging-wall and footwall rocks. Chlorite zone alteration is limited to portions of the footwall and flanks of the Cu-rich H orebodies. Clusters of gold-bearing chlorite-quartz veinlets crosscut large volumes of the quartz-sericite zone rocks.Alteration has added an average of 13 wt percent SiO2, 2.3 wt percent K2O, 430 ppm Ba, and 46 ppm Rb to the system and leached 3.6 wt percent Na2O, 2.2 wt percent CaO, and 41 ppm Sr. Middle and light REE were increasingly mobile from Yb to La; they were leached from the chlorite zone rocks and added to quartz-sericite zone rocks. Chlorite was more Fe rich (Fe/Fe + Mg = 0.65-0.85) in the chlorite zone rocks than in other altered lithologies (Fe/Fe + Mg = 0.2-0.5).Sulfur isotope values for massive ores are in the range delta-S34 = -2.1 to 1.9 per mil. Mineral separates from altered rocks yielded delta-O-18 values of 10.2 to 12.1 per mil for quartz, 7.2 to 9.5 per mil for calcite, 2.2 to 3.0 per mil for chlorite, and 0.1 to 1.0 per mil for magnetite. Oxygen and sulfur isotope fractionation between coexisting minerals suggests formation from fluids having delta-O-18 = 3 to 4 per mil at temperatures between 275-degrees to 380-degrees-C. Oxygen isotope compositions of host rocks are delta-O-18 = 8 +/- 1 per mil for freshest rhyolite, delta-O-18 = 6.6 to 11.6 per mil for quartz-sericite zone rocks, and delta-O-18 = 3.9 to 4.4 per mil for chlorite zone rocks. These measured values compare closely to estimated whole-rock delta-O-18 values based on mass balance considerations using normative mineralogies.Alteration of wall rocks encompassing the massive sulfide orebodies is characterized by intense sericitization, a high degree of silicification, and a marked positive delta-O-18 anomaly. Calculations based on SiO2 solubilities and probable K contents of the hydrothermal fluids suggest a water-rock mass ratio of 50 to 300 during the mineralization event.