Geology of the Moreton's Harbour area, Newfoundland with emphasis on the environment and mode of formation of the arsenopyrite veins

Abstract

Basic volcanic rocks of Ordovician age near Moreton's Harbour, Newfoundland (pillow lavas, massive flows, and pyroclastic rocks) are intruded by numerous basic and acidic dykes. The rocks are cut by a network of thin veins containing abundant arsenopyrite and some stibnite, associated with pyrite, pyrrhotite, and minor sphalerite, galena, and chalcopyrite. In a few veins, stibnite predominates. Both arsenopyrite and stibnite veins have been mined on a small scale for their associated gold content. The widespread arsenopyrite has been used to determine P-T conditions of formations of the veins, applying the methods of L.A. Clark and A.H. Clark. -- X-ray diffraction analysis of 30 specimens shows that the arsenic content of the arsenopyrite varies between 30 and 34 atomic per cent (d131 spacing from 1.6301 to 1.6338Å.), indicating that the probable temperature range of crystallization extended from about 610°C to 390°C under confining pressures of 1500 bars. -- The hydrothermal veins are related structurally to the acidic dykes (the most competent rock in the area) and genetically to granodioritic intrusive rocks of Devonian age outcropping about five miles east of Moreton's Harbour.