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Carmen Conde, Instituto Geológico y Minero de España (Spain)
David Lentz, University of New Brunswick (Canada)
Jim Walker, Geologic Surveys Branch (Canada)
Fernando Tornos, Instituto Geológico y Minero de España (Spain)
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The Brunswick No.6 VMS deposit produced approximately 12 Mt of 7% Zn + Pb and 0.45 wt. % Cu to the end of mine life in 1983. New sampling from surface and underground drillholes from the past-producing Brunswick No.6 ore deposit shows the continuity of the Cu zone at depth. It was previously described as enveloping the S-plunging, sheath-folded deposit. In this study, we present a detailed petrographic and geochemical study of the remaining resource of 5 Mt of Cu-rich massive sulphide that forms the basal part of the orebody.
The mineral assemblage includes pyrite, pyrrhotite and chalcopyrite with minor amounts of sphalerite, galena, magnetite, chlorite and quartz. Arsenopyrite, bismuthinite, native bismuth and cassiterite occur as trace minerals. Texturally, it is characterized by the presence of subhedral to euhedral porphyroclasts of pyrite that range from 0,5 mm to 4 cm. Pyrrhotite and chalcopyrite occur as anhedral minerals filling fractures and as irregular grains, locally having galena and sphalerite as inclusions. The main trace mineral is euhedral arsenopyrite that occurs intergrown with pyrite. Native Bi, bismuthinite and cassiterite form irregular distributed masses. The whole Cu-rich zone locally shows foliation and ductile to cataclasitic deformation, with widespread recrystallization.
The geochemical signature of the Cu-rich zone is characterized by the presence of high concentrations of Co and Cu relative to the Pb-Zn ore and the massive pyrite. The chemical data shows a strong positive correlation with the Cu grades (r'=0.8). These data could be indicate the presence of cobaltite or stoichiometrical substitution of Co for As and Fe in the arsenopyrite and pyrite, respectively. Pb, Zn, As and Sb appear in lower proportions. The Sn and Se contents are rarely above the detection limit (30 and 50 ppm, respectively). Furthermore, the assay data display good correlations between Ag and Au (r'=0.60), whereas Au shows a very significant correlation with Bi (r'=0.77) and As (r'=0.78).
Thus, on the basis of the previous works and this new petrographic and analytical study the genesis of the Cu-rich zone could be interpreted as a consequence of a massive sulphide zone refining process, resulting from its position immediately above of the high temperature fluid that is venting through the feeder zone.
This research was supported by Xstrata Canada Corporation, Xstrata Zinc Canada Division and it's a contribution to the IGCP-502 project "Global comparisons of volcanic-hosted massive sulphide districts".
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