AN OCCURRENCE OF MAGMATIC SPHALERITE, GRANITEVILLE QUARRY, STATEN-ISLAND, NEW-YORK

Despite the thermally refractory character of zinc sulfide, the deposi tion of sphalerite is almost invariably the result of relatively low-t emperature hydrothermal processes. However, a few rare occurrences are known in which sphalerite has been reported to be a magmatic mineral. We describe an occurrence of microscopic crystals of sphalerite enclo sed selectively in the albite component of the quartz-albite granophyr e, which constitutes the groundmass of a pyroxene trondhjemite that wa s produced by the marginal fusion of a xenolith of Lockatong argillite enclosed in the Palisades diabase sill in the Graniteville quarry, St aten Island, New York. The petrographic characteristics arid the euhed ral character of the sphalerite strongly support the interpretation th at these crystals are part of the magmatic suite. Based on phase-equil ibrium diagrams for the system albite - quartz and the system diopside - nepheline - silica, we conclude that the zinc sulfide probably crys tallized from a silicate melt between 1073 degrees and 1062 degrees C. Possible sources of the zinc and sulfur are (I) the sedimentary xenol ith, which is now a hornfels containing 20-54 ppm of zinc, and (2) the basaltic magma of the sill, which crystallized to a diabase containin g 50 ppm of zinc. The latter hypothesis involves diffusion of zinc and sulfide ions across the liquid-liquid boundary between the co-existin g basaltic magma of the sill and the trondhjemitic magma derived from the xenolith.