Geology of the Red Lake District

The Red Lake District is underlain by Archean rocks of the Superior Province of the Canadian Shield. Rocks of four subprovinces are found in the Red Lake District:

1. Uchi Subprovince rocks in the Red Lake District comprise the Red Lake and Birch-Confederation Lake greenstone belts in which the bulk of exploration and mining activity has taken place. The supracrustal rocks of the Red Lake greenstone belt can be subdivided into several assemblages with ages ranging from ca. 2990 Ma to ca. 2700 Ma. Major granitoid intrusions show a range from ca. 2734 Ma to 2699 Ma.

2. English River Subprovince rocks, south of the Uchi Subprovince, are predominantly metasedimentary and host minor intrusive rocks similar to those in the Quetico Subprovince.

3. To the north, the Berens River Subprovince formed the core of a microcontinent. This area is underlain by ca. 2750-2690 Ma felsic plutonic rocks interpreted as a magmatic arc formed at an Andean-style margin that culminated in the Kenoran Orogeny. These plutonic rocks intruded an older substratum (North Caribou terrane) on which Mesoarchean volcanic rocks of the Red Lake belt are also interpreted to have formed.

4. The Sachigo Subprovince comprises crustal blocks ranging from Paleoarchean (>3.4 Ga) to Neoarchean (ca. 2.7 Ga) in age.

Geology of the Red Lake Belt (adapted from Sanborn-Barrie et al. 2001)

The Red Lake greenstone belt is dominated by the (ca. 2990 Ma) mafic-ultramafic Balmer assemblage, an oceanic plain sequence; minor calc-alkalic volcanic rocks of arc-like affinity terminate the assemblage. The majority of lode gold deposits in the camp are hosted by the basal mafic-ultramafic sequence. A later diverse lithologic association, the Ball assemblage, appears to represent a shallow marine, volcanic edifice built upon the Balmer substrate.
Widespread ca. 2894 Ma calc-alkaline volcanism is represented in Red Lake by the Bruce Channel assemblage. Overlying this is the ca. 2850 Ma Trout Bay assemblage which includes substantial basaltic and gabbroic rocks in western Red Lake which are prospective for PGE mineralization, and which includes minor intermediate pyroclastic rocks throughout central Red Lake. The Trout Bay assemblage may correlate with Woman assemblage rocks of the Confederation Lake belt.

A regional angular unconformity is interpreted to separate the Mesoarchean assemblages from the Neoarchean Confederation assemblages. Volcanogenic massive sulphide mineralization is associated with the younger sequence. A significant number of felsic units are classed as FII and FIII type rhyolites, considered highly prospective for large (Kidd Creek/Noranda type) massive sulphide deposits (Parker 1999).

A newly recognized component of the Neoarchean supracrustal package is the Huston sedimentary assemblage that includes surface exposures of polymictic cobble- to pebble-conglomerate and argillite; clasts include jasperoidal chert iron formation, massive sulfide pebbles, mafic flow rocks as well as well-bedded, graded turbiditic wacke and argillite. The Huston assemblage conformably to unconformably overlies the McNeely sequence (Confederation assemblage) and underlies the Graves assemblage (Sanborn-Barrie et al. 2004). The U-Pb age of detrital zircons give single age peaks of 2743 and 2746 Ma at the cemetery and Madsen sites, respectively (Skulski et al. 2001), indicating single source derivation from erosion of pre-existing Confederation age rocks, and deposition after ca. 2743 Ma.

In the Campbell-Red Lake deposit area the conglomerate defines the interface between the Balmer or Bruce Channel assemblages and the Confederation assemblage. Exposures on 16 Level of the Red Lake Mine reveal that
..the [Huston] conglomerate is a polymictic proximal conglomerate or breccia (debris flow) dominated by subangular to subrounded laminated cherty clasts with local jasper-rich and green mica fragments; it is similar to the Temiskaming conglomerate… it also contains clasts of andalusite-rich altered basalt and a few local clasts of layered (possibly sheeted) carbonate veins with small crustiform banding and cockade texture.. (Dubé et al. 2004)

Detrital zircon analyses from the conglomerate on 16 Level of the Red Lake Mine define a single population with a mean age of 2747±4 Ma (Dubé et al. 2004). This implies that the former two assemblages were exposed at surface by 2747 Ma.

The presence of local andalusite-rich clasts and clasts of carbonate vein material in an unaltered matrix demonstrates that there was a period of at least some aluminous and carbonate alteration prior to deposition of the Huston conglomerate (Dubé et al. 2004). The position of colloform-crustiform iron-carbonate±quartz veins in the Campbell-Red Lake deposit underneath the interpreted subaerial unconformity also leads Dubé et al. (2004) to interpret the veins as near-surface, epithermal-epizonal products, part of a protracted hydrothermal alteration event spanning pre- to post-Huston assemblage time, a period of more than 35 m.y.

Recent age dating (Skulski et al. 2001) has also yielded multiple ages of detrital zircons from a fragmental unit thought to correlate with the Austin "tuff" ore zone at the former Madsen mine. Most of the Meso- and Neoarchean assemblages exposed in Red Lake are represented in this unit. Maximum age of deposition is consequently ≤2700±6 Ma.

Dubé, B., Williamson, K., McNicoll, V., Malo, M., Skulski, T., Twomey, T. and Sandborn-Barrie, M. 2004. Timing of gold mineralization at Red Lake, Northwestern Ontario, Canada: New constraints from U-Pb Geochronology at the Goldcorp High-Grade Zone, Red Lake mine, and the Madsen mine; Economic Geology, v. 99, No.8, pp. 1611 to 1641.

Parker, J.R. 1999. Exploration potential for volcanogenic massive sulphide (VMS) mineralization in the Red Lake greenstone belt; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, Open File Report 6000, p.19-1 to 22-26.

Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario, in Current Research 2001-C19, Geological Survey of Canada, 32p.
Skulski, T., Sanborn-Barrie, M. and Sanborn, N., 2001. New U-Pb geochronology in the Red Lake greenstone belt, Western Superior NATMAP, unpublished poster.

The map below depicts the general geology of the Red Lake District