VOLCANIC FACIES ARCHITECTURE AND HYDROTHERMAL ALTERATION OF THE JURASSIC ESKAY CREEK DEPOSIT, NORTHWESTERN BRITISH COLUMBIA
The footwall rhyolite at Eskay Creek formed by multiple rhyolite generations involving extrusive and intrusive emplacements. Extrusive volcanism involved phreatic-hydrothermal explosive eruptions and the deposition of associated volcanic rocks. At least locally, rhyolite sills intruded along the contact of the still wet and unconsolidated deposits of the phreatic-hydrothermal explosions and the mudstone that was deposited on top of the extrusive parts of the footwall rhyolite. The rhyolite sills have been subject to hydrothermal alteration. As the bulk of the ore is hosted by the carbonaceous mudstone, the ore-forming event must, therefore, have overlapped with the youngest intrusive emplacement of rhyolite. At the same time, basalt sills occurring within the carbonaceous mudstone were affected by hydrothermal alteration. This indicates that the onset of basaltic volcanism occurred prior to the waning of the hydrothermal activity.
Volcanic facies analysis allowed the identification of several key geological factors. The Eskay Creek deposit formed within an extensional environment characterized by the coincidence of felsic and mafic volcanic rocks. The deposit is located in a near-vent setting and formed late during the evolution of the footwall rhyolite volcano. There is no evidence for a shallow-water setting although the occurrence of phreatic-hydrothermal eruptions is a conspicuous feature of Eskay Creek. Widespread carbonate alteration may suggest that the hydrothermal fluids were unusually gas-rich.