GEOCHEMICAL AND ISOTOPIC (ND, O) DATA FROM ORDOVICIAN FELSIC PLUTONICAND VOLCANIC-ROCKS OF THE MIRAMICHI HIGHLANDS - PETROGENETIC AND METALLOGENIC IMPLICATIONS FOR THE BATHURST MINING CAMP

Middle Ordovician felsic magmatism contemporaneous with Bathurst Camp Pb-Zn volcanogenic massive sulphide (VMS) deposits consists of strongl y altered volcanic to subvolcanic rocks, belonging to the Tetagouche G roup, and relatively unaltered granitoid plutons, which are divided in to northern, central: and southern groups within the Miramichi Highlan ds. Calc-alkalic felsic volcanic rocks and northern plus central pluto ns have E-Nd(T) values ranging from -8.2 to -1.9 and -4.0 to +0.3, res pectively. They exhibit within-plate-type volcanic and transitional I- to A-type granite geochemical characteristics. Granitoid rock delta(1 8)O values range from +8.0 to +10.1 parts per thousand. Published gran itoid rock Pb isotopic compositions overlap unpublished galena data fr om Bathurst VMS deposits. Field, geochemical, and isotopic evidence in dicate that these volcanic and granitoids rocks are consanguineous and mainly derived from Proterozoic or older infracrustal sources. Alkali c felsic volcanic rocks, and associated alkaline basaltic rocks, are m ore juvenile (epsilon(Nd)(T) = +3.2 to +4.2) and were possibly derived from slightly enriched mantle sources. Southern plutons exhibit conti nental are-type features. The felsic magmatism and VMS deposits likely formed in an Okinawa-type back-are basin developed from rifting the E arly Ordovician Popelogan continental are, of which the southern pluto ns are remnants. Correlations between pluton groups and volcanic forma tions indicate that felsic magmatism was erupted through and onto the Miramichi Group. As most felsic volcanic formations lack plutonic equi valents, the Tetagouche Group probably does not represent disrupted sl ices of an originally conformable stratigraphic section. This supports a model in which thrust slices juxtapose remnants of volcanic centres erupted at different locations within a back-are basin.