GEOLOGY, LITHOGEOCHEMISTRY AND VOLCANIC SETTING OF THE ESKAY CREEK AU-AG-CU-ZN DEPOSIT, NORTHWESTERN BRITISH-COLUMBIA

Mineralization at Eskay Creek occurs within mid-Jurassic marine Volcan ic and sedimentary strata of the uppermost Hazelton Group of the Stiki ne terrane. Within the 21 Zone, gold-silver-rich sulfide-sulfosalt dep osits occur within carbonaceous argillites which overlie rhyolite, and which are overlain by basaltic pillow lavas and sills. The economic m ineralization of the 21B zone (1.08 Mt grading 65.5 g/t Au, 2930 g/t A g, 5.6% Zn, and 0.77% Cu) forms a lens-shaped body up to 900 by 300 by 20 m in size, mainly in the form of stratiform beds of elastic ore in the lower argillites while the underlying rhyolite hosts vein systems that are interpreted as feeders for the stratiform mineralization (Ed munds and Kuran, 1992; Rye, 1992; Edmonds et al., 1994). The rhyolite comprises flow banded, perlitic and spherulitic facies of massive lava , and also flow breccias, sediment-interaction breccias, and beds of r hyolite-dominated volcaniclastic material. Immobile element relations (Ti, Al, Zr, Th, Nb, and Yb) indicate that virtually all of the rhyoli tes, regardless of degree of alteration, have been derived from a chem ically near-homogenous precursor The least altered rhyolite was low in TiO2 (0.08%), with moderate Zr (similar to 170 ppm), high Y (similar to 55 ppm), high Nb (similar to 30 ppm), and fairly high REE abundance s with (La/Yb)(n) ratios of 2 to 4. These features suggest that the Es kay rhyolite has a tholeiitic affinity, although its trace element com position is much different from that of oceanic-are tholeiitic rhyolit es. The basalts display a narrow compositional range, with 1.3 to 2.0% TiO2, 60 to 90 ppm Zr, 25 to 40 ppm Y, 2 to 6 ppm Nb, 250 to 350 ppm Cr2O3, and low REE abundances with near-flat patterns. These features indicate that the Eskay basalt has a tholeiitic N-MORB affinity, proba bly with a small component of E-MORB and a minor subduction signature. The Eskay rhyolite and basalt have some chemical similarities with bi modal volcanic rocks in extensional, continental margin rift and buck- are settings. Rhyolite in the vicinity of mineralized zones is altered to variable proportions of sericite, Mg-chlorite, K-feldspar, and qua rtz. Intense chlorite-sericite alteration is confined to the upper 20 m of rhyolite below the outline of the 21B orebody, but silica and K-f eldspar-alteration extend further laterally and deeper in the rhyolite . Calculated mass changes for intensely altered rhyolites indicate ext reme ranges in silica, from absolute (wt %) losses of 40 to 60% due to conversion of rhyolite to pure chlorite-sericite, to gains of 200 to 300% which result from multiple silica infillings of breccias, primary voids and secondary veins. Chloritized rhyolites show MgO gains of 14 to 24%, whereas K-feldspar-silica-altered rhyolites have gained up to 7% K2O and 50% SiO2. Zones of extreme chlorite-sericite alteration pr obably represent vent-proximal areas in the upper rhyolite where seawa ter was mixed with discharging, more acidic fluids. The K-feldspar-sil ica alteration probably occurred under cooler, more neutral conditions generally peripheral to the main feeder zones. Hangingwall basalts ar e little to moderately altered, but unmineralized. The tholeiitic Eska y basalt is spatially restricted to the area of the Eskay rhyolite, al though the basalt was erupted later. The rhyolite could be linked gene tically to the basalt through a high degree of fractional crystallizat ion, or by partial melting of crustal rocks due to heating caused by i ntrusion of basaltic magma, although incompatible trace element ratios suggest that the rhyolite magma assimilated a small component of more evolved crustal material. The tholeiitic rhyolite and basalt at Eskay Creek contrast with the volcanic rocks in the underlying Hazelton Gro up, which contains a greater proportion of intermediate, calc-alkaline rocks that previously have been interpreted as a subaerial to shallow marine marginal volcanic-are assemblage. The location of the Eskay Cr eek deposit initially may have been influenced by deep faulting which allowed unfractionated mafic magma derived from the upper mantle to pe netrate to an upper crustal location. Localized near-surface faulting may have then alllowed eruption of rhyolite and eventually basalt, and also promoted seawater circulation through footwall strata of the 21 Zone. Although subsidence accompanying rifting of the partly submerged , marginal calc-alkaline are had already led to open marine sedimentat ion by the time of tholeiitic rhyolite-basalt volcanism and 21B zone m ineralization, water depths were sufficiently shallow (< 1500 m) to al low boiling of hydrothermal fluids during mineralization. Extrusion of basalt was the last magmatic event in the area, and was succeeded by deposition of the thick turbidite-pelagic mudstone sequence of the Bow ser Basin. (C) 1997 Canadian Institute of Mining, Metallurgy and Petro leum.