Geological Relationships Between Massive Sulfide Bodies and Ophiolitic Volcanic Rocks near York Harbour, Newfoundland

Small lenses of massive copper- and zinc-rich sulfide occur in ophiolitic volcanic rocks capping the Bay of Islands Igneous Complex near York Harbour, Newfoundland. Ore was discovered in 1897 and about 50 000 tons were produced between 1898 and 1912. Recent exploratory work has outlined several ore bodies totaling 282 000 tons that contain 1.92% Cu and 4.67% Zn, assuming 20% dilution.The deposits are stratabound, occurring along an intensely altered zone that separates two members of the layered pillow lava succession. Two types of mineralization are present. Lenticular massive bodies ranging in size up to 37 000 tons are composed of brecciated pyrite, sphalerite and chalcopyrite. These are stratigraphically underlain by chalcopyrite-rich stringer mineralization in altered lava.Microscopic and petrochemical investigations indicate that the host lavas have undergone pervasive spilitic alteration and intensive propylitic alteration near ore. Geochemical studies of Co:Ni ratios in pyrite show that Ni content is highest in disseminated pyrite from the propylitized lavas and Co is highest in pyrite from stringer ore. The Co:Ni ratio for all analysed pyrite is greater than unity, suggesting a magmatic-volcanic derivation for the deposits.The geological evidence suggests that the sulfides are volcanogenic and were formed from hydrothermal fluids rich in Fe, Zn, Cu and S. These moved upward along zones of primary brecciation in the lavas, leached Fe, Ni, and Ti from the ferromagnesian minerals and deposited the massive lenses by chemical sedimentation or precipitation from the metal-rich exhalations on or near the sea floor.Comparison of the York Harbour deposits to the recent heavy metal bearing sediments of the Red Sea 'deeps', and to the Cretaceous lavas and orebodies of Cyprus, supports the above genetic interpretation, and indicates that these ores are clearly an integral part of ophiolite complexes. The comparison provides new information about sulfide mineral paragenesis during initial sedimentation, diagenesis, burial, and later metamorphism.