Sulfur isotopes and the origin of the sulfide deposits of the Bathurst-Newcastle area of northern New Brunswick

Abstract

The regional geology of the Bathurst-Newcastle area is briefly outlined. There are 2 types of sulfide deposits in the area: a) epigenetic fissure deposits, and b) massive sulfide deposits. Geological evidence can be variously interpreted to suggest a syngenetic, a source bed, or a magmatic hydrothermal origin for the massive sulfide deposits. S 32 /S 34 ratios are presented for more than 300 sulfide samples from the host rocks and sulfide deposits. Four granite bodies are dated by the K-Ar method. Model Pb ages are presented for 5 galena samples. Sulfides disseminated through the black shale and argillites of the host rocks are enriched in S 32 (mean value 22.31) relative to the meteoritic ratio, and broad range of values. S 32 /S 34 ratios for the sulfide deposits fall into 2 distinct groups. In general one group (21.88-22.05) represents the massive sulfide deposits, while the other group (22.18-22.21) represents the fissure deposits. This suggests 2 separate, but probably closely related sources for the S. Granites intrusive into the host rocks of the sulfide deposits have a mean age of 386 + or - 10 m.y. Pb minerals in the sulfide deposits have a mean model Pb age of 370 + or - 20 m.y. Thus the Pb minerals and the granite bodies were emplaced during the same major orogenic event. The sulfide deposits of the Bathurst area have a narrow range of S 32 /S 34 values. Sulfide deposits thought to have a magmatic hydrothermal origin all have a narrow range of S 32 /S 34 values. It therefore seems reasonable that well homogenized magmatic hydrothermal solutions are the most logical source for the sulfide deposits. The homogeneity of the Pb isotope abundance data gives strong independent evidence that the ores are of a magmatic hydrothermal origin. The problem of why the massive sulfide deposit are enriched in S 34 is open for further investigation.