Abstract

The Bathurst-Newcastle mining district of northern New Brunswick contains over 20 Ordovician volcanogenic massive base metal sulfide deposits, which occur in a broadly circular, domal volcanic-sedimentary complex about 50 km across. Volcanic rocks of this complex form a bimodal rhyolite-basalt suite and stratigraphically overlie sediments of continental origin. Most of the felsic volcanic rocks are submarine ash-flow tuffs and are older than the basaltic rocks. The sulfide deposits occur at several stratigraphic levels in the sequence, but most occur in sedimentary horizons near the outer or upper contact of the felsic volcanic pile. They can be divided into two types: (1) those occurring as bulges within laterally persistent, semicontinuous mineralized horizons that are spatially associated with rhyolitic ash-flow tuffs and (2) those occurring as discontinuous lenses associated with brecciated felsic lavas.The presence of a resurgent caldera in the district is indicated by the broadly circular distribution of rock types, their domal structure, and the occurrence of the sulfides in a large rhyolitic ash-flow field. The caldera hypothesis is also supported by other evidence in specific areas. Development of the volcanic-sedimentary complex prior to Taconian deformation is related to the stages of a hypothetical resurgent caldera cycle, and it is concluded that type 1 deposits formed after caldera collapse and during resurgence, whereas type 2 deposits formed during later moat rhyolite dome extrusion.

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