Petrology, Geochemistry, and Genesis of the Copper zone at the Brunswick No. 6 Volcanogenic Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick, Canada

Abstract

A Cu-rich pyrrhotite-pyrite zone that occurs at the base of the Brunswick No. 6 Pb-Zn massive-sulfide lens is part of a south-plunging synclinal sheath fold. To the north of the unmined open pit, this Cu zone’s preliminary ore-reserve calculations indicate >1.7 Mt grading 0.9% Cu. Pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite are the major opaque minerals, and are accompanied by trace amounts of arsenopyrite-cobaltite, bismuthinite, and cassiterite. Most of the chalcopyrite and pyrite is fine grained, but cataclastically deformed pyritic porphyroblasts, porphyroclasts, and boudins of pyritic massive sulfide are hosted by a matrix of remobilized and recrystallized chalcopyrite-bearing pyrrhotite. Eleven 1.6 m-long intervals were sampled near the mid point of massive sulfide intersections from 10 diamond-drill holes (DDH) intersecting the Cu zone. Re-assaying of these samples yielded averages of 0.96% Cu, 0.10% Zn, 0.06% Pb, 12.2 g/t Ag, 0.04% Bi, 0.08 g/t Au, 0.03% As, 0.01% Sb, and Sn values below the detection limit of 50 ppm. Six 1.6 m core intervals in the exhalative Pb-Zn zone (DDH B-259) were also re-assayed, yielding averages of 0.79% Cu, 1.08% Pb, 3.46% Zn, 0.051% Bi, 58.6 g/t Ag, 0.50 g/t Au, 0.311% As, 0.063% Sb, and Sn values of 80 to 670 ppm. The concentrations of Zn, Cd, Pb, Ag, As, Sb, Mo, Ca, and Sr decrease with increased depth into the sheath-shaped basal Cu zone, which has notably higher Ba, Se, and Te contents. In contrast to the chemical differences, the bulk δ 34 S values for both zones range from 13‰ to 15‰, which are similar to the values for other deposits within the Tetagouche Group. The contrasting distribution of major and trace elements suggests that the zoning is a syngenetic feature, modified by D 1 deformation and related metamorphism. Geobarometry of sphalerite shielded within pyrite indicates peak D 1 pressures of >7 kb, similar to those at the nearby Brunswick No. 12 deposit. Late re-equilibration in the presence of pyrrhotite resulted in very high mole % FeS contents in sphalerite. The high Cu and low Pb-Zn contents within the Cu zone compared with those in the overlying, contiguous Zn-Pb-Ag exhalative massive sulfide zone, is a pattern commonly observed in proximal VMS deposits. The metals’ distribution is interpreted to reflect a higher temperature zone-refining within the massive sulfides, which are located above a stockwork feeder zone that has been transposed to the north. The interpreted zone refining is consistent with: (1) the relatively high pyrrhotite-to-pyrite abundance and the higher abundance of chalcopyrite; (2) lower sphalerite, galena, tetrahedrite-tennantite, arsenopyrite, and cassiterite abundances in the Cu zone; and (3) the low S/Se ratio typical of other Cu-rich zones. This interpretation is consistent with the similarity of δ 34 S values for the Cu and Pb-Zn zones.