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Physical and chemical constraints on mineralization in the Eskay Creek deposit, northwestern British Columbia : evidence from petrography, mineral chemistry, and sulfur isotopes Roth, Tina

Abstract

The Eskay Creek deposit is an unusual, polymetallic, precious metal-rich, volcanogenic massive sulfide and sulfosalt deposit located in northwestern British Columbia. Combined production and current reserve and resource estimates total 2.34 million tonnes, grading 51.3 g/t Au and 2 326 g/t Ag, and are contained in a number of stratiform and stockwork vein zones that display a variety of textural and mineralogical characteristics. The bulk of the ore is hosted in the 21B zone, a tabular stratiform lens that consists of well-bedded, clastically reworked sulfides and sulfosalts interbedded with unmineralized, carbonaceous argillite. In addition to extremely high precious metal grades, Eskay Creek is distinguished from conventional VMS deposits by its association with elements of the 'epithermal suite' (Sb-Hg±As), sulfosalt-rich mineralogy, and the dominance of clastic sulfides and sulfosalts. Physical and chemical constraints on the deposition of the Eskay Creek deposit have been investigated by detailed field studies, supplemented by petrographic, geochemical and stable isotopic analysis of the ore minerals. The deposit formed during two periods of hydrothermal activity, reflecting stratiform lenses occurring at two stratigraphic levels within argillite at the contact between rhyolite and basalt. During the first stage of activity, early sphalerite, Ag-rich tetrahedrite, galena, pyrite and electrum deposited on the sea floor near the center of the present 21B zone. The resulting seafloor mound and chimneys were periodically fragmented, transported, and redeposited in a basin adjacent to the vent site. Hydrothermal fluids migrated laterally along the permeable clastic bed to deposit progressively more Sb and Hg-rich sulfides and sulfosalts including dominantly boulangerite and bournonite, associated with significant electrum, followed by stibnite. Later minerals are progressively more restricted towards the center of the deposit. The latest stage of hydrothermal overprint in the 21B zone is characterized by minor cinnabar. The second stage of hydrothermal activity resulted mainly in deposition of the stratigraphically higher HW massive sulfide lens. The HW zone is typical of mineralized lenses where sulfides were not transported from the vent site. Ore of this type is characterized by replacement textures, presence of chalcopyrite and absence of lead sulfosalts. Silver is hosted principally in near end-member tetrahedrite and in electrum. Sphalerite, tetrahedrite and electrum locally contain significant Hg due to reaction with late stage, Hg-rich fluids. Lowtemperatures of formation are reflected in the low FeS contents in sphalerite, as well as the sulfosalt-rich mineral assemblage, and are optimal for transport of gold as a sulfide complex. Sulfur isotope signatures in the sulfides are consistent with a magmatic source, either derived by leaching the underlying volcanic pile, direct magmatic input, or both. Sulfur isotope values in pyrite from the host argillite grade rapidly to biogenic signatures within tens of metres from the edge of the orebodies, indicating that hydrothermal fluids are focussed and had a limited extent of lateral migration. Processes responsible for formation of the Eskay Creek deposit are not unique in the VMS environment, but require the coincidence of several favourable conditions to optimize the precious metal grade in the deposit. Continued removal of material deposited at the vent site is essential to prevent sealing and subsequent increase of temperatures of the hydrothermal system, therefore providing sustained low temperature conditions favourable for transport of gold. Redeposition of the clastic sulfides adjacent to the vent site allowed access for hydrothermal fluids to deposit additional gold. A reduced basinal environment with an appropriate depositional geometry is therefore necessary to preserve transported sulfides.

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