Strata-bound massive sulfide deposits in Silurian-Devonian volcanic rocks at Harborside, Maine

Abstract

Strata-bound polymetallic massive sulfide ores of Silurian-Devonian age occur within the Coastal volcanic belt of Maine and New Brunswick. This terrain, overprinted by greenschist-facies regional metamorphism, is thought by some to represent volcanism above a consuming plate margin, and by others a tensional tectonic environment. Along the east side of Penobscot Bay, Maine, volcanic rocks of the Castinc Formation are a well-preserved bimodal assemblage of mafic and felsic lavas, pyroclastites, and volcanogenic sedimentary rocks that record cyclic volcanic episodes.At Harborside, massive bodies of pyrite, sphalerite, and chalcopyrite were mined by Callahan Mining Corporation from open pit and underground workings. From 1968 to 1972, production totaled 800,000 tons of ore averaging 5.5 percent Zn, 1.28 percent Cu, 0.5 percent Pb, and 17.1 g/short ton Au. The massive sulfide bodies are on the flank of a black rhyolite dome and are assorted with Mg-rich rocks including talc, talc-carbonate, and talc-chlorite assemblages. The orebodies are strata-bound within dome-derived oligomictic rhyolite breccias, which are in turn enveloped by coarse polymictic fragmental rocks characteristic of vent facies volcanism. The nature of the volcanism and of the volcanic products, as well as the association of clusters of sulfide bodies about brecciated domes, compares closely with many of the massive sulfide deposits in the Kuroko district, Japan.