Abstract

Massive Cu-Zn sulfide (+ or - Au + or - Ag) orebodies are found at a number of localities in the Betts Cove ophiolite, the main ones being mined at Betts Cove and Tilt Cove. The mineralization is mainly pyrite and chalcopyrite, with lesser amounts of sphalerite, as well as traces of galena. Magnetite is locally abundant along with coarse specular hematite in the East mine. About 42,000 oz of gold was produced from both the massive ore and associated with sphalerite in banded and stringer sulfides within the ophiolitic pillow lavas. Mineralization is accompanied by intense black chloritization as well as more restricted stockwork pyrite and silica. The sulfide minerals exhibit the same tectonic fabrics as the host rocks, indicating their predeformation origin.Chemical data for 150 samples from the Tilt Cove area show primary chemical trends related to both olivine fractionation and advanced partial melting of the upper mantle. Silica, Fe, Mg, Ca, Na, and K were highly mobile during alteration with major variations related to gain or loss of silica--the lowest silica rocks approaching compositions of pure chlorite. Several trace elements were relatively immobile during alteration, including Ti, Zr, Y, P, Cr, and Ni. The lavas can be clearly separated into a high Zr group which overlaps samples from the ophiolite in the Betts Cove area and the compositional fields of mid-oceanic ridge basalts and island-arc tholeiites, and a low Zr group extremely depleted in both Ti and Zr. The low Zr samples, which are all from the Tilt Cove area where the largest massive sulfide orebodies are found, have a number of geochemical characteristics analogous to boninites. Rare earth element concentrations are lower in the lower Zr lavas, which have concave-upward patterns, than in the higher Zr lavas, which have convex-upward patterns.We endorse the suggestions of earlier workers that these lavas were formed by wet melting of depleted upper mantle caused by fluids produced through dehydration during subduction of previously metasomatized oceanic crust. We would go further, however, and suggest that these fluids were also enriched in base metals and sulfur, which were also incorporated into the lavas, either in solution or as immiscible sulfide droplets. The Tilt Cove orebodies were subsequently derived from these lavas by the generally accepted mechanism, i.e., leaching of metals by circulating seawater. It is estimated that the separate 3- to 5-million-ton sulfide orebodies at Tilt Cove were formed by leaching of approximately 2 km 3 of pillow lava containing approximately 300 ppm S, with no more than a approximately 200-m depth of circulation over a very broad area of approximately 12 km 2 . A smaller leaching volume would be adequate if higher concentrations of sulfur were available in the source rocks.

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