The Key Tuffite, Matagami mining district: Origin of the tuff components and mass changes
The Key Tuffite is a thick (0.3 m to 6 m) and laterally extensive (—10 km) cherty, sulfide-bearing tuff unit in Archean greenstones in the Matagami mining district in northwestern Quebec. It lies at the contact of the Watson Lake Group and overlying Wabassee Group volcanic rocks, and is host to major Zn-Cu volcanogenic massive sulfide deposits. The tuff component is highly altered and its volcanic precursors are not identifiable by petrographic means. The tuff is mixed with a volcanic exhalative component comprising chert, carbonate and sulfides (pyrite, pyr-rhotite, and minor sphalerite and chalcopyrite). The normative mineral composition of the Key Tuffite is 33% quartz, 28% sulfides, 16% sericite, 8% carbonate, 6% chlorite and 9% other minerals. In the vicinity of the Bell AUard and Orchan mines, the precursor tuff components are identified using immobile major and trace elements, as Watson Lake tholeiitic rhyolite (Zr/Y = 1.5 to 3.5, LaN/YbM = 1.7 to 2.8, SiO2 = 72.5 ± 1.2 wt %, and Wabassee calc-alkaline andesite (Zr/Y = 9, LaN/YbN = 4 to 8, SiO2 = 56% to 65%). Immobile element computation techniques were used to calculate mass additions and losses in the conversion of the tuff to the Key Tuffite. Average net amounts of FejOj (27% total iron), SiO2 (14%), CaO (8%), S and K2O ( + 2%), were added, Na2O was removed (-2.8%), and MgO was over-all unchanged. Total net mass change was + 63%, which comprises mainly exhalative additions, but also includes changes owing to hydrothermal alteration of the tuff component.