SULFUR ISOTOPE COMPOSITION OF THE BRUNSWICK-NO-12 MASSIVE SULFIDE DEPOSIT, BATHURST-MINING-CAMP, NEW-BRUNSWICK - IMPLICATIONS FOR AMBIENT ENVIRONMENT, SULFUR SOURCE, AND ORE GENESIS

The Brunswick No. 12 massive sulphide deposit occurs within a Middle O rdovician bimodal volcanic and sedimentary sequence that is thought to have formed in a continental back-are rift covered with a thick succe ssion of carbonaceous hemipelagic and turbiditic sedimentary rocks. Th e deposit consists of three en echelon lenses that are zoned from Vent Complex to Bedded Ore and Bedded Pyrite facies. The Bedded Ore facies has the lowest average delta(34)S values (14.2 parts per thousand), b ut are only slightly less positive than laminated pyrite in footwall s edimentary rocks (delta(34)S(mean) = 15.1 parts per thousand). delta(3 4)S values for the bedded sulphides show an upward increase from 14.2 parts per thousand, in Bedded Ore to 16.5 parts per thousand in Bedded Pyrite. Average delta(34)S values for Vent Complex (15.8 parts per th ousand) and underlying stringer sulphides (16.1 parts per thousand) ar e consistently more positive than those for Bedded Ore. In carbonaceou s shales and siltstone of the Patrick Brook Formation that underlie th e deposit, delta(34)S values that range between 13.8 and 25.6 parts pe r thousand and the similarity of these values to those of the Brunswic k No. 12 deposit indicate major bacterial reduction of sulphate to sul phide under closed or partly closed conditions, and that most of the S in the deposit originated from ambient sulphidic bottom waters. Furth ermore, the average delta(34)S value for Brunswick No. 12 bedded ores lies on the Selwyn Basin pyrite evolutionary curve and indicates that anoxic conditions within the Tetagouche back-are basin reflect a globa l anoxic episode. The Brunswick No. 12 deposit probably formed, theref ore, by the mixing of hydrothermal metals with dissolved sulphide of s eawater origin during periods of ocean anoxia. The increase of delta(3 4)S values towards the Vent Complex may reflect the addition of isotop ically heavy S formed by the inorganic reduction of seawater sulphate.