Regional Geological Setting of Gold Deposits, Eastern Red Lake Area, Northwestern Ontario

Special Volume, Vol. SV 24, No. 1982, 1982

The major Archean gold deposits in the Red Lake greenstone belt, which have produced over 374 million grams of gold since 1930, occur near the top of the lower komatiitic to tholeiitic mafic to ultramafic metavolcanic sequence. The upper calalkalic to felsic rocks form three discrete and separate ediftces overlymg this lower sequence. The gold deposits are classified according to their stratigraphic or lithologic associations into: 1) mafic-volcanic-hosted deposits; 2) felsic-intrusive-hosted deposits; and 3) stratabound deposits. Group 1 and to a lesser extent Group 2 deposits occur within zones of alteration several square kilometres in extent. The nature of this pervasive alteration can be characterized by comparison of petrochemical data from the altered rocks and their fresher equivalents some distance away. The altered rocks have substantial addition of Si02> C02, K20, As and Sb and subtraction of Na20, CaO, MgO and total Fe. Cr, Ni, Co and Zr appear to have been relatively immobile. Hydrothermal alteration and gold deposition probably accompanied early fumarolic activity and continued during subsequent deformation of the metavolcanic sequence, with gold eventually being mobilized and concentrated into major vein structures to form the present orebodies. The smaller Group 2 deposits formed by emplacement of felsic intrusions into already altered, anomalously gold-bearing mafic metavolcanics, with the heat from the intrusions setting up parasitic hydrothermal systems which mobilized the gold and deposited it along with minor sulphides in fractures in the cooling intrusions.

Keywords: Gold, gold mining, geology, gold deposits, mafic rocks, felsic rocks, alteration