Geology, lithogeochemistry and volcanic setting of the Eskay Creek Au-Ag-Cu-Zn deposit, northwestern British Columbia.

Barrett, T.J. and Sherlock, R.L., 1996a. Exploration and Mining Geology, v. 5, p. 339-368.
Mineralization at Eskay Creek occurs within mid-Jurassic marine strata of the uppermost Hazelton Group of the Stikine terrane in British Columbia. Au-Ag-rich sulfide-sulfosalt deposits of the 21 Zone occur within graphitic argillites overlying rhyolite, and are overlain by basaltic pillow lavas and sills.

The economic mineralization of the 21B zone (1.08 Mt grading 65.5 g/t Au, 2930 g/t Ag, 5.6% Zn, and 0.77% Cu) forms a lens-shaped body up to 900 x 300 x 20 metres in size, mainly in the form of stratiform beds of clastic ore in the lower argillites. The underlying rhyolite hosts vein systems that are interpreted as feeders for the stratiform mineralization. The rhyolite comprises flow banded, perlitic and spherulitic facies of massive rhyolite, and also flow breccias, sediment-interaction breccias, and zones of rhyolite-dominated fragmental material.

Immobile element relations (Ti, Al, Zr, Th, Nb and Yb) indicate that virtually all of the rhyolites, regardless of degree of alteration, have been derived from a chemically near-homogenous precursor. The least altered rhyolite was low in TiO2 (0.08%), with moderate Zr (150-180 ppm), high Y (50-60 ppm), high Nb (30-35 ppm), and fairly high REE abundances with (La/Yb)n ratios of 2-4. These features suggest that the Eskay rhyolite has a tholeiitic affinity. The basalts display a narrow fractionation range, with 1.5-2.0% TiO2, 70-90 ppm Zr, 25-40 ppm Y, 3-8 ppm Nb, 250-350 ppm Cr, and low REE abundances with near-flat patterns. These features indicate that the Eskay basalt has a tholeiitic N-MORB affinity, probably with a small component of E-MORB. The Eskay rhyolite and basalt have some chemical similarities with bimodal volcanic rocks in extensional, continental margin back-arc settings.

Rhyolite in the vicinity of mineralized zones is altered to variable proportions of sericite, Mg-chlorite, K-feldspar and quartz. Intense chlorite-sericite alteration is confined to the upper 20 metres of rhyolite below the outline of the 21B orebody, but silica and K-feldspar-alteration extend further laterally and deeper in the rhyolite. Calculated mass changes for the altered rhyolites indicate huge ranges in silica, from absolute (weight %) losses of 40-60% due to conversion of rhyolite to pure chlorite-sericite, to gains of 200-300% which result from multiple silica infillings of primary voids and secondary fissures. Chloritized rhyolites show MgO gains of 14-24%, whereas K-feldspar-silica-altered rhyolites have gained up to 7% K2O and 50% SiO2. Zones of extreme chlorite-sericite alteration probably represent vent-proximal areas in the upper rhyolite where seawater was mixed with discharging, more acidic fluids. The K-feldspar-silica alteration probably occurred under cooler, more neutral conditions peripheral to the main feeder zones.

The rhyolites could be linked genetically to the basalts through a high degree of fractional crystallization. If so, certain immobile incompatible element ratios suggest that the rhyolite magma assimilated a small component of more evolved crustal material. Alternately, the rhyolite could have been formed by partial melting of tholeiitic crust due to heating caused by intrusion of basaltic magma. The rhyolite was erupted before the basalt, with a significant hiatus between these events during which argillites accumulated and the 21B zone mineralization was deposited. The tholeiitic rhyolite and the basalt at Eskay Creek contrast with the volcanic rocks in the underlying Hazelton Group, which contains a greater proportion of intermediate, calc-alkaline rocks which have been previously interpreted as an subaerial to shallow marine volcanic-arc assemblage. The location of the Eskay Creek deposit may have been influenced by deep faulting which allowed unfractionated mafic magma derived from the upper mantle to penetrate to a shallow crustal location. Shallower secondary faults may have allowed eruption of rhyolite and basalt, as well as promoting fluid and heat transfer to the surface. Subsidence accompanying rifting had led to open marine conditions by the time of mineralization and tholeiitic rhyolite-basalt volcanism, although the water depths were sufficiently shallow to allow boiling of hydrothermal fluids. 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 Bowser Basin.

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