REGIONAL GEOLOGY AND LITHOGEOCHEMISTRY, IN THE VICINITY OF THE CHESTER VMS DEPOSIT, BIG-BALD-MOUNTAIN AREA, NEW-BRUNSWICK, CANADA

Volcanic rocks of the Ordovician Tetagouche Group are host to a number of VMS deposits in the Bathurst mining camp of northern New Brunswick . Recent geological mapping and lithogeochemical studies have led to a better understanding of the stratigraphic position and depositional s etting of the VMS deposits in the southwestern part of the camp. The b imodal volcanic assemblage of the Big Bald Mountain area possesses che mical characteristics consistent with generation in an ensialic back-a re basin. Pervasive potassic metasomatism of the felsic volcanic rocks may be related to widespread, low-temperature interaction with sea wa ter in the vicinity of a major eruptive center. The Chester VMS deposi t is hosted by plagioclase-rich crystal tuff and overlain by aphyric t o sparsely porphyritic felsic flows. The extensive Cu-rich stockwork z one at Chester is interpreted as the conduit for metalliferous fluids that fed massive Pb-Zn-rich lenses deposited as sulfide mounds on the sea floor. The quartz-chlorite alteration halo associated with the foo twall stockwork zone is enriched in Fe, Mg, and Si and depleted in Ca, Na, K, and absolute REE reflecting chloritization of feldspar and dil ution by silica. Light and middle REEs are depleted in the silicic-pyr itic alteration halo in the hanging wall of the massive sulfide lenses indicating that hydrothermal activity continued after base-metal depo sition. The lithogeochemical characteristics of the extensive alterati on halos surrounding the Chester deposit provide a useful exploration guide in the search for blind VMS deposits hosted by the Tetagouche Gr oup. However, the depositional environment of individual deposits in t he Miramichi Highlands vary considerably so that the specific criteria developed from the study of the Chester deposit need to be integrated into a more general exploration model for the Bathurst mining camp as a whole.