Massive sulfide deposits of the Noranda area, Quebec. V. The Corbet mine

Abstract

The Corbet deposit is located in the upper part of the Flavrian andesite, the lowest volcanic formation within cycle III of the Noranda Central Mine Sequence. The deposit consisted of one main lens of massive sulfides, and several smaller lenses, and contained 2.7 × 106 t of mineable ore at 2.92% Cu, 1.98% Zn, 20.6 g/t Ag, and 1.0 g/t Au. The orebody had a massive pyrrhotite–chalcopyrite core, which passed laterally into massive pyrite–sphalerite. The northwestern part of the ore-body is overlain by 50–100 m of Flavrian andesite, whereas the southeastern portion is overlain by Northwest rhyolite.Lavas within the Flavrian andesite and the lower Northwest rhyolite are low in K2O (<0.5%), and are partly of tholeiitic affinity, and partly of transitional affinity. The tholeiitic volcanic series has Zr/Y ratios of 2.8 to 4.5, whereas the transitional series has Zr/Y ratios of 4.5 to 7.1. The two series are also distinct in plots of Nb–Zr, Yb–Zr, Nb–Y, and the rare earth elements. These data indicate that two slightly different magma types existed in the chamber that fed this portion of the extrusive Central Mine Sequence.Alteration is most intense in the breccias of the upper Flavrian andesite, within ~50 m of the orebody, and is almost entirely chloritic. There is no zone of silicification, although moderate sericitization occurs lateral to and above the orebody. Mass-change calculations indicate that large amounts of SiO2 and CaO + Na2O were leached from the rocks by hydrothermal solutions, whereas large amounts of hydrothermal FeO and seawater MgO were added.Oxygen isotope depletions are among the largest in the Noranda area and extend laterally and vertically up to 300 m from the orebody. Within this volume, δ18O values of altered volcanic rocks have been decreased to values as low as 2 to −2‰. These depletions result from reactions with seawater at ~ 250–300 °C and from strong silica leaching, as indicated by mass-change calculations. The hydrothermal system at Corbet can be assessed using two lithogeochemical tools: calculated mass changes and oxygen isotope ratios, both of which are sensitive to water–rock ratio and temperature.