The Brunswick No.6 volcanogenic massive sulphide (VMS) Cu-ZONE, Bathurst Camp, New Brunswick, Canada: Petrology, geochemical composition, and petrogenesis
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Date
1998
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University of New Brunswick
Abstract
Cu-rich massive sulphides envelope the north-end and base of the Brunswick No. 6 Pb-Zn massive-sulphide lens. Preliminary ore reserve calculations indicate 1.7 Mt grading 0.9% Cu. Mineralogically, the principle minerals identified are: pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, magnetite, trace arsenopyrite, cobaltite, and cassiterite. Generally, chalcopyrite and pyrite are fine grained, although cataclastically deformed pyrite porphyroblasts (porphyroclasts) and pyritic massive sulphide boudins are hosted in a remobilized and recrystallized pyrrhotite-rich matrix. In this study, 11 sample intervals 5 feet long (1.62 m) from 1 O diamond drill holes (DOH) into the Cu zone were re-assayed yielding an average of 0.90% Cu, 1.28% Zn, 0.42% Pb, 28.6 g/t Ag, 0.046% Bi and 0.225 g/t Au, as well as 0.131 % As, 0.030% Sb, 0.069% Co and Sn values below the detection limit of 0.005%. Intervals in diamond Drill hole B-259 into the Pb-Zn (exhalative) zone were re-assayed (n = 6) and yielded an average of 0.78% Cu, 1.08% Pb, 3.46% Zn, 0.051% Bi, 0.311 % As, 0.063% Sb, 0.07% Co, 58.62 g/t Ag and 0.495 g/t Au. A similar trend occurs at the Brunswick No. 12 deposit. There is a notable decrease in Zn, Cu, Pb, Ag, As, and Sb concentrations with increased depth into the sheath-shaped basal Cu zone. Bi and Au concentrations exhibit a "U" shaped trend with the lowest concentrations occurring at approximately the centre of the Cu zone. Bulk sulphur analyses (n = 11) conducted on 1 O drill holes within the Cu zone yielded an average o 34S value of 14.6 per mil. Additionally, Hole B-259 sampled (n = 6) from the Pb-Zn (exhalative) zone averaged o 34S=14 per mil creating an increasing trend of of 34S values entering the Cu zone.
The high Cu and low base-metals within the basal massive-sulphide zone compared to the Zn-Pb-Ag exhalative massive sulphides in both the No. 6 and No. 12 deposits is common in proximal VMS deposits. It is usually interpreted as a hydrothermal zone- refining feature located above the stockwork feeder zone, which is consistent with; 1) the relatively high pyrrhotite to pyrite abundance and higher abundance of chalcopyrite, arsenopyrite, bismuthinite, and cassiterite that have higher temperature-sensitive solubilities than the base metals that form the exhalative part of the deposit, 2) lower sphalerite, galena, tetrahedrite/tennantite, and argentite concentrations, and 3) it's occurrence above the stockwork feeder zone represents the main hydrothermal fluid discharge conduit that formed the deposit.