Geological Setting and Exploration of Diverse Styles of Gold Mineralization in the Red Lake Camp, Northwestern Ontario

Mining Rocks! CIM Toronto 2005
Abstract The Red Lake Camp has produced >645 t of gold from 17 mines that were active at various times since production began in 1930. The bulk of the gold production (>480 t) has been from the prolific Campbell – Red Lake deposit, the only currently producing deposit, being mined separately by Goldcorp Inc and Placer Dome Inc. Based on host rock associations, alteration assemblages, and vein mineralogy, the gold deposits of the Red Lake camp can be classed into at least two main groups; volcanic-hosted (vein and wall-rock) replacement deposits and quartz-(tourmaline) vein deposits, accounting for approximately 93% and 7% of total camp Au production respectively.

Volcanic-hosted replacement (VHR) deposits are preferentially developed in the Meso-Archean Balmer Assemblage, but and appear to have a spatial relationship to unconformities with younger volcanic assemblages. Large (>1 M oz) VHR deposits display zones of potassic (biotite+/-microcline), carbonate (and or calc-silicates in amphibolite facies), chloritic, and aluminosilicate wall-rock alteration. Carbonate-quartz veins with colloform and cockade textures are preserved in greenschist and lower amphibolite facies domains but become overgrown by hornblende and diopside with increasing metamorphism. Silicification (quartz-sericite) accompanies mineralization and replaces carbonate-quartz veins and previously carbonatized wall-rocks. Large high-grade vein-hosted orebodies occur in structurally complex segments of fault zones, which have dismembered folded stratigraphy, and are favourably controlled by competency contrasts between adjacent units, especially between tholeiitic basalt and basaltic komatiite. VHR deposits are typically have high Au:Ag (3 to >10), accompanied by a As-Sb-(Zn-Hg-Se) signature.

Quartz-(tourmaline) deposits are typically hosted in, and centred around, intermediate to felsic plutonic rocks however veins may occur some distance away within adjacent volcanic rocks. The veins may occur as distinct sub-parallel to conjugate vein sets, stockworks, or as quartz-tourmaline breccia bodies. Wallrock alteration surrounding the veins is typically narrow and consist of silicification-(sericitization) with or without pink hematization. This inner zone may be flanked by an outer chloritic-carbonate+/-biotitic zone. Aluminous alteration appears to be absent in this style of mineralization. Quartz tourmaline deposits are more silver-rich (Au:Ag ranges 3:1 to 1:11) than volcanic-hosted deposits and are also anomalous in Cu-Zn-(As-Te-Pb-Bi).

Challenges for exploration include recognition of dilational vein geometries versus modified geometries produced by superimposed deformation, understanding the timing of pre- syn- and post-ore faults / shear zones, recognition of rock types overprinted by locally intense alteration and deformation.
Keywords: Red Lake, Campbell-Red Lake, Archean, Gold
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