Geology and lithogeochemistry of the Buchans polymetallic sulfide deposits, Newfoundland

Abstract

The Buchans orebodies are polymetallic stratiform massive volcanogenic sulfide deposits which exhibit numerous similarities to the Kuroko deposits of Japan. Since 1928 approximately 16 million tons of ore have been mined with an average grade of 14.88% Zn, 7.70% Pb, 1.36% Cu, 3.73 oz Ag, and 0.045 oz Au. The ore is intimately associated with volcaniclastic siltstones and laharic breccias within dacitic pyroclastic flows which form a part of lowermost cycles of calc-alkaline volcanism. It was deposited in paleotopographic depressions by precipitation of base metals from (sea?) water which was enriched in these components by submarine fumarolic activity during quiescent phases in volcanism. Most of the massive sulfide deposits are underlain by less important stringer and stockwork mineralization which is distinct in both time and space from the major orebodies.Over 300 chemical analyses of diamond drill core samples for 11 major and 14 trace elements were performed on a single cross section 3 km long by 1.5 km deep which contains a representative portion of most lithologic units and three major orebodies. Studies of elemental distributions in the vicinity of the orebodies indicate that dispersion of ore metals is generally restricted to within 30 meters of the orebodies. Study of lateral elemental variations within lithologic units indicates a broad increase of base metals toward the area of stockwork mineralization over a distance of about 3 kilometers.The abundances of Ba, Pb, and Zn are distinctly higher in volcanic rocks and sediments related to ore compared with those unrelated to ore. These differences are consistent throughout the entire area of study and are statistically significant at a high level of confidence. Cu, Ag, and Hg in some instances are likewise higher in units associated with mineralization. The source of these elements is attributed to a metalrich volatile phase which accompanied volcanism that produced the lithologic units related to mineralization.