Geochemistry of mineralized and barren, late proterozoic felsic volcanic rocks in southeastern Cape Breton Island, Nova Scotia (Canada)

Abstract

Abstract Late Proterozoic volcanic rocks of southeastern Cape Breton Island, Nova Scotia (Canada), comprise part of the Avalon Composite Terrane of the Canadian Appalachians. They occur in several fault blocks (from southeast to northwest: Coastal, Stirling, East Bay Hills and Coxheath). The blocks are composed mainly of Late Proterozoic volcanic suites which are predominantly bimodal and were emplaced in a volcanic arc setting. The basaltic rocks range from tholeiitic (Coastal and Stirling) to calc-alkalic (East Bay Hills and Coxheath). The felsic volcanic rocks of the Stirling block host a massive sulphide deposit but no economic mineralization has been discovered in the volcanic rocks of the other blocks. The felsic volcanic rocks display significant differences in composition amongst the fault blocks. The Coastal and Stirling blocks contain low-K rhyolites with low-k abundances of strongly incompatible trace elements and relatively flat chondrite-normalized REE patterns. These rocks were probably derived by the melting of mafic igneous rocks compositionally similar to tholeiitic basalts of Archean greenstone belts or island arc tholeiites. Compared to the Coastal block rocks, the Stirling rhyolites are depleted in Eu, Sr and Al 2 O 3 indicating extensive feldspar fractionation. Trace element characteristics of the Stirling block rhyolites are closely comparable to those of the Archean felsic metavolcanic rocks from the Superior Province of the Canadian Shield which are preferentially associated with massive sulphide mineralizations. The similarities suggest that the low-K felsic volcanic rocks with flattish REE patterns and negative Eu and Sr anomalies are also indicative of a promising environment for massive base-metal sulphide mineralization in volcanic belts at least as young as Late Proterozoic. The rhyolites of the Coxheath block have calc-alkalic affinities, higher contents of strongly incompatible elements than the Coastal and Stirling rhyolites and their REE are patterns are enriched in light REE. The East Bay Hills felsic tuffs resemble the Coxheath rhyolites. They both could be derived by melting of a source with a composition of the average lower crust.