Paper No. 4
Presentation Time: 9:45 AM


MONECKE, Thomas, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 and HANNINGTON, Mark, Department of Earth Sciences, University of Ottawa, Marion Hall, 140 Louis Pasteur Street, Ottawa, ON K1S 0X7, Canada,

Eskay Creek represents an unusual precious metal-rich, polymetallic volcanic-hosted sulfide and sulfosalt deposit (2.180 Mt of ore grading 49.5 g/t Au and 2407 g/t Ag) located in the Iskut River area of British Columbia. The deposit comprises several ore zones that consist of stratiform clastic sulfide and sulfosalt beds and laminations that are hosted by a thick interval of carbonaceous mudstone at the contact between felsic volcanic rocks and overlying basalt. In addition, economic zones of discordant sulfide veins and disseminations occur in the footwall of the stratiform ores.

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.