Petrology, geochemistry, and distribution of the copper zones at the Brunswick No. 12 volcanogenlc massive sulfide deposit, Bathurst Mining Camp, New Brunswick

Abstract

The Brunswick No. 12 volcanogenic massive sulfide deposit of the Bathurst Mining Camp is a vent-proximal, exhalative deposit formed above a well-developed feeder pipe (stockwork zone). The deposit is divided into three mineralogically and texturally distinct lenses: 1) massive pyrite-pyrrhotite-chalcopyrite (basal Cu zone); 2) lead-zinc ore; and 3) massive pyrite. There are five overprinting folding events recognized at the Brunswick No. 12 mine. The basal Cu zone is found in the nose of an F1-F2 asymmetrical recumbent fold; the region experienced upper-greenschist grade metamorphism with temperatures up to 425°C and pressures in excess of 700 MPa. Samples totaling 78 were collected from 9 drill cores on the 1125m level of the mine; all the drill holes intersect pyrite-pyrrhotite-chalcopyrite intervals along the entire strike of the deposit. Re-assaying of these drill hole samples yielded an average of 1.28% Cu, 1.05% Zn, 0.28% Pb, 48.2 g/t Ag, 0.52 ppm Au, 0.03% Bi, 0.013% Sb, 0.077% Co, 0.4% As, low Sn values ( < 30 ppm), and low Se values ( <50 ppm). A significant Spearman Rank correlation exists (> 95% Cl) between Cu and Ag, Bi, and Co, whereas a weak to moderate positive correlation exists between Pb and Zn with Cu. This deposit is petrochemically similar to the Brunswick No. 6 deposit located approximately 10 km southeast of Brunswick No. 12. Although the Brunswick No. 12 deposit is being mined primarily for Zn and Pb, respectively, significant amounts of Cu can be found in three sections of the deposit: the hanging wall zone, footwall zone, and the "425 level" zone. The combined Cu zones are estimated to contain 64.5 Mt grading 0.98% Cu, of which 5.4 Mt grades 1.53% Cu. The principle opaque minerals identified in the Cu zone are pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, and galena. Minor minerals include quartz, magnetite, native bismuth, bismuthinite, chalcocite, bomite, cassiterite, ilmenite, and rutile. Pyrite occurs as euhedral grains (0.02mrn to 0.84mrn), porphyroblasts and porphyroclasts, which have overgrown chalcopyrite, pyrrhotite, and sphalerite, and are hosted within a matrix of recrystallized chalcopyrite and pyrrhotite. Pyrrhotite within the Cu zone is abundant (up to 60%) and dominantly monoclinic, although hexagonal and triclinic pyrrhotite are also present. Chalcopyrite-diseased sphalerite grains are present in the Cu zone, and they account for less than 10% of the total sphalerite grains within the zone. The average composition of the diseased sphalerites is X (FeS)sp = 0.14, whereas the X(FeS)sp = 0.17 for non-diseased sphalerite grains within the Cu zone. The relatively high Cu and low base metal contents within the basal Cu zone of the deposit is consistent with its occurrence above the stockwork feeder zone; the proximity of the basal Cu zone to the vent is distribution of metals within the deposit is interpreted to reflect a high-temperature zone refining system. This is consistent with the high abundance of chalcopyrite, arsenopyrite, and native bismuth all of which have higher temperature sensitive solubilities than sphalerite and galena, resulting in high Cu/Zn and Cu/Pb within the Cu zone.