|Abstract||The Kootenay Arc contains numerous carbonate-hosted Zn-Pb deposits (Fig. 1), including several past-producers (e.g., Reeves MacDonald, Jersey-Emerald, HB, Mastodon) and a number of advanced exploration
projects (e.g., Oxide, Jackpot, Duncan, Abbott, Wigwam). The largest deposits range in size from 6 to 10 Mt and contained on average 3-4% Zn, 1-2% Pb, 0.4% Cd, and traces of Ag. The deposits are hosted by deformed and metamorphosed Lower Cambrian
platformal carbonates of the Badshot Formation, or its equivalent, the Reeves member of the Laib Formation. |
The present distribution and form of the deposits are controlled by the regional structure. They are essentially stratabound, lenticular
concentrations of sulphides (sphalerite, galena, pyrite, local pyrrhotite and rare arsenopyrite) confined to dolomitic and/or siliceous zones, which are folded isoclinally (Fig. 2). These structures mimic the large-scale regional structures, and are
commonly elongated parallel to the regional structural grain. As well, the sulphides and their dolomitic envelopes have been affected by regional and local contact metamorphism. Field and microscopic observations indicate that the sulphides appear to
have formed primarily by replacement of the host carbonates, with minor open space filling of breccia zones, fractures and vugs.
The near-surface portions of several carbonate-hosted sulphide deposits are weathered, strongly oxidized, and consist
of extensive Zn- and Pb-bearing iron oxide gossans and base metal-bearing nonsulphide mineralization (Fig. 3). The Reeves MacDonald, Jersey-Emerald, HB, Lomond, and Oxide group of deposits exhibit the best examples of carbonate-hosted nonsulphide
base metal mineralization in the southern Kootenay Arc. Sulphide oxidation post-dates Middle Jurassic deformation (i.e., age of regional metamorphism and first phase of deformation) and may have started before or slightly after the last glacial
maximum (17 000-14 000 BP). The relatively warm and dry climate, which prevailed some 10 000-7000 BP, may have been particularly favourable for supergene oxidation of sulphides and formation of carbonate-hosted nonsulphide base metal mineralization
in the southern Kootenay Arc (Simandl and Paradis, 2009). However, the recent documentation of nonsulphide Pb-Zn mineralization north of the study area (Paradis et al., 2010) suggests that, under favorable geological conditions, this type of
mineralization should be expected in the northern portion of the Kootenay Arc as well.
The origin of the sulphide deposits of the Kootenay Arc is enigmatic because of the intense deformation that has modified most of their original features.
However, Re/Os isotope analyses on the Reeves MacDonald sulphides indicate that the mineralization occurred during the Devonian period and that the deposits are epigenetic [i.e., Mississippi Valley-type (MVT)]. They share many characteristics with
other Cordilleran MVT deposits including: (1) they are hosted by altered dolomitized and/or silicified carbonates; (2) the deposits contain simple mineral assemblage of pyrite, sphalerite, and galena; (3) the mineralization is zinc-rich (high
sphalerite/galena ratios: Zn:Pb from 10:1 to 2:1); (4) they display
complex ore textures and alteration assemblages; (5) pyrite, sphalerite, and galena grains have isotopically heavy sulfur (delta34S values = 9.0 to 26.4 per mil); and (6) galena
and sphalerite have radiogenic Pb.
The new Devonian age confirms the epigenetic nature (as opposed to syngenetic) of the sulphide deposits, and improves our understanding of the genesis of these deposits in the context of the Cordilleran
tectonics. The Devonian was an important metallogenic epoch that saw the proliferation of base metal deposits along the continental margin of North America with coeval development of volcanic-hosted massive sulphide (VHMS) deposits in arcs and
back-arcs of the pericratonic terranes, sedimentary exhalative (SEDEX) deposits in continent-margin basins, and finally MVT deposits in continental platforms. From the prospecting point of view, determination of the physical properties of the
sulphide and nonsulphide mineralization and their host rocks in the Kootenay Arc is one of the essential steps that will lead to improved interpretation of existing and future geophysical surveys. The use of portable XRF in exploration for white
Zn-rich nonsulphide deposits appears as one of very promising and potentially cost-effective exploration tools.