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Abstract

The Eskay Creek deposit is an unusual high-grade precious and base metal volcanogenic massive sulphide and sulphosalt deposit hosted in volcanic and sedimentary rocks of the Lower to Middle Jurassic Hazelton Group. This deposit consists of several zones distinguished by differing ore mineralogies and grades. Published geological reserves for the deposit are 4.3 million tonnes grading 28.8 grams per tonne gold and 1 027 grams per tonne silver. The 2 1A zone, the focus of this study, contains an estimated 0.97 million tonnes grading 9.6 grams per tonne gold and 127 grams per tonne silver. The lowermost stratigraphic units in the Eskay Creek 21A zone are marine sedimentary and volcanic rocks. Stockwork and disseminated mineralization occur in an overlying flow banded and brecciated rhyolite that forms the footwall to probable stratiform sulphide mineralization hosted in contact argillite. The argillite is overlain by a thick sequence of massive to pillowed and brecciated basalt, intercalated with argillite and turbidite. The units related directly to mineralization apparently were generated in an extensional rifled arc environment. From lithogeochemical analyses, the overlying basalts reflect a back-arc environment of formation. Alteration in the 2 IA zone, most intense in the footwall rhyolite, is characterized by varying mineral abundances in an assemblage of quartz - sericite - pyrite ± potassium feldspar ± chlorite. Alteration (determined by petrography, X-ray diffraction, transmitted electron microscope and lithogeochemistry) progresses from early devitrification through silicification, potassic alteration and sericitization (dominantly illite) to pervasive replacement of rhyolite by clinochiore. Increasing hydrothermal alteration is traced geochemically, mainly in the mobility of silica, potassium and magnesium. The most intense alteration in the footwall rhyolite, is a chlontic pipe defined by a discontinuous stockwork that underlies apparently stratiform mineralization. The overlying basalt sequence is not markedly altered. Mineralization within the 21A zone consists of a small, semi-massive, probably stratiform, stibnite-realgar rich lens in a small defineable basin which is underlain by stockwork and disseminated suiphides in the rhyolite that is described above. An overall vertical zonation was observed in sulphides and distribution of metals. Veinlets in the rhyolite contain mainly sphalerite - galena - pyrite - tetrahedrite ± chalcopyrite. Chalcopyrite occurs mainly in the lower part of the rhyolite. Near the upper contact, intensely altered rhyolite contains disseminated arsenopyrite and stibnite. Stratiform semi-massive mineralization in the contact argillite consists dominantly of realgar, stibnite, arsenopyrite and cinnabar; few intervals contain base metal rich sulphides. Assay distribution within the rhyolite also varies vertically. Gold and silver increases with proximity to the upper contact of the rhyolite. Highest gold grades are in the semi-massive sulphides. Highest silver grades are in the 50 metres immediately below the top of the rhyolite.