The geologic setting of the Aldermac copper deposit, Noranda, Quebec

CIM Bulletin, Vol. 76, No. 851, 1983

A.D. HUNTER and J.M. MOORE, JR. Department of Geology, Carleton University, Ottawa, Ontario

The Aldermac deposit and associated prospects occur in the Noranda Archean volcanic centre of the Abitibi greenstone belt. Over two million tons of ore grading 1.65% copper, with minor gold and silver, were mined between 1933 and 1943. The deposit has features in common with the Vauze and Millen-bach mines in the same district. Recent detailed mapping, geochemistry and petrography combined with data from former diamond drilling and mining operations permit a re-interpretation of the Aldermac area. The orebodies and spatially related sulphidic zones are coneluded to be volcanogenic. The host rocks are a chemically bimodal suite of mafic and rhyolitic flows, volcaniclastics and related subvolcanic intrusions. Massive sulphide lenses are spatially related to a quartz-feldspar-phyric rhyolite dome and siliceous, volcanic-exhala-tive sedimentary rocks. The succession is cut by gabbro and syenite, related to later stages of volcanism, and by post-Kenoran diabase dykes. Regional metamorphism attained the low greenschist fades, but the rocks lack penetrative deformation. Hydrothermal alteration is primarily an intense silicification of mafic volcanic rocks enveloping, or lying below, sulphidic zones. Chlorite and sericite alteration zones, including dalma-tianite, are more locally developed and have a closer spatial relation to mineralization, as do epidote, calcite and calcic garnet. The structure is complex and cannot be explained by isoclinal folding; rather it reflects concurrent faulting, and extrusive and intrusive activity. Fault blocks were rotated and exhibit opposed facing; the original setting was possibly a submarine graben. Sulphidic zones occur within mafic andfelsic volcanic rocks and at the interface between the two; there is thus potential for discovery throughout the known stratigraphic succession. Exploration conducted according to a volcanogenic model is advisable, particularly in the light of recent discoveries in similar rocks of the Amulet area.
Keywords: Geology, Ore deposit, Aldermac Deposit, Massive sulphides, Abitibi greenstone belt, MacKay Lake zone, Chance showing, North Chance, North Chance tuff, Archibald Prospect.