Summary:
The Eskay Creek deposit is generally classified as an example of a high-grade, precious metals-rich epithermal volcanogenic massive sulphide (VMS) deposit; however, it has also been suggested to be an example of a subaqueous hot spring gold-silver deposit.
Several distinct styles of stratiform and discordant mineralization are present at the Eskay Creek Project, defined over an area approximately 1,400 m long and up to 500 m wide. The main body of mineralization, which was mined out by previous operators, is the 21B Zone, is a stratiform tabular body of gold-silver-rich mineralization roughly 900 m long, 60 to 200 m wide, and locally exceeding 20 m thick. Individual clastic sulphide beds range from 1–100 cm thick and become progressively thinner up sequence. Mineralized zones are composed of beds of clastic sulphides and sulphosalts containing variable amounts of barite, rhyolite, and mudstone clasts. Imbricated, laminated mudstone rip-up clasts were observed locally at the base of the clastic sulphide-sulphosalt beds, indicating turbiditic emplacement of some beds. In the thickest part of the orebody, pebble and cobble-sized clasts occur in a northward trending channel overlying the Eskay Rhyolite.
Gold and silver occur as electrum and amalgam while silver mainly occurs within sulphosalts. Precious metal grades generally decrease proportionally with decrease in total sulphides and sulphosalts. Clastic sulphoside beds contain fragments of coarse-grained sphalerite, tetrahedrite, lead-sulphosalts with lesser freibergeite, galena, pyrite, electrum, amalgam, and minor arsenopyrite. Stibnite occurs locally in late veins, as a replacement of clastic sulphides, and appears to be confined to the central, thickest part of the deposit., suggesting a locus for late hydrothermal activity. Cinnabar is rare and is found associated with the most abundant accumulations of stibnite. Barite occurs as isolated clasts, in the matrix of bedded sulphides and sulphosalts, and also as rare clastic or massive accumulations of limited extent. Barite is more common towards the north end of the deposit.
Stratiform style mineralization is hosted in black carbonaceous mudstone and sericitic tuffaceous mudstone of the Contact Mudstone located between the Rhyolite and the Hanging Wall Andesite. The stratiform-hosted zones include the 21B Zone, the NEX Zone, the 21A Zone (characterized by arsenic- antimony-mercury sulphides), the barite-rich 21C Mudstone Zone, and the 21Be Zone. Stratigraphically above the 21B Zone and usually above the first basaltic sill, the mudstones also host a localized body of base metal-rich, relatively precious metals-poor, massive sulphides referred to as the Hanging Wall or HW Zone.
The 21A Zone can be subdivided into stratiform- and feeder-style mineralization types. Stratiform mineralization is characterized by a gold-silver-rich sulphide lens that sits on the flank of a small depression at the Rhyolite-Contact Mudstone contact, located 200 m south of the 21B Zone and is bound to the east by the Pumphouse fault. Stratiform-style, mudstone-hosted mineralization is approximately 300 m long by 200m wide and 10 m in thickness. The sulphide lens consists of semi- massive to massive stibnite-realgar ± cinnabar ± arsenopyrite and local angular mudstone fragments. Areas with more concentrated stibnite-realgar ± cinnabar appear to be focused above the interpreted vent locations with relatively limited extent. Visible gold is rare.
The mudstone is underlain by a discontinuous zone of intense Magnesium chlorite alteration and stockwork veining in the Eskay Rhyolite Member. Disseminated stibnite, arsenopyrite, and tetrahedrite also occur in the immediate footwall of the sulphide lens within the intensely sericitized rhyolite. Cinnabar and stibnite are observed in late fractures that cut the sulphide lens, the surrounding mudstone, and locally the rhyolite. Realgar-calcite veinlets locally cut the mudstone in a restricted area adjacent to the sulphide lens.
The main body of mineralization, the 21B Zone, is a stratiform tabular body of gold-silver-rich mineralization roughly 900 m long, 60 to 200 m wide, and locally exceeding 20 m thick. Individual clastic sulphide beds range from 1–100 cm thick and become progressively thinner up sequence). Ore is composed of beds of clastic sulphides and sulphosalts containing variable amounts of barite, rhyolite, and mudstone clasts. Imbricated, laminated mudstone rip-up clasts were observed locally at the base of the clastic sulphide-sulfosalt beds, indicating turbiditic emplacement of some beds. In the thickest part of the mineralized zone, pebble to cobble-sized clasts occur in a northward trending channel overlying the Eskay Rhyolite.
Gold and silver occur as electrum and amalgam while silver mainly occurs within sulphosalts. Precious metal grades generally decrease proportionally with the decrease in total sulphides and sulphosalts. Clastic sulphide beds contain fragments of coarse-grained sphalerite, tetrahedrite, lead-sulphosalts with lesser freibergite, galena, pyrite, electrum, amalgam, and minor arsenopyrite. Stibnite occurs locally in late veins, as a replacement of clastic sulphides, and appears to be confined to the central, thickest part of the deposit, suggesting a locus for late hydrothermal activity. Cinnabar is rare and is found associated with the most abundant accumulations of stibnite. Barite occurs as isolated clasts, in the matrix of bedded sulphides and sulphosalts, and also as rare clastic or massive accumulations of limited extent. Barite is more common towards the north end of the deposit.
The 21C Zone is dominantly characterized by stratabound to stratiform barite-rich mineralization with associated disseminated base and precious metal-rich mineralization in the Rhyolite and is roughly 675 m long by 130 m wide. It occurs at the same stratigraphic horizon as the 21B Zone but is located down-dip and subparallel to it. The two zones are separated by 40 to 50 m of barren Contact Mudstone, roughly 8 to 15 m thick. Mineralization is associated with mottled barite-calcite ± tetrahedrite beds in and near the base of the Contact Mudstone. Precious metal grades are variable. Local areas of brecciation are infilled with sulphides including sphalerite, pyrite, galena, and tetrahedrite. Mineralization in the underlying Rhyolite forms a cryptic, tabular body, sub-concordant to stratigraphy. Aside from containing 1–2% very fine-grained pyrite and trace sphalerite, tetrahedrite, and galena, the rhyolite appears similar to adjacent unmineralized areas.
Precious-metal mineralization near the north end of the 21B Zone extends over top of the anticline into a block bound by segments of the north-south oriented Pumphouse faults. Mineralization of the 21Be Zone is found within a steeply dipping, fault-bounded slab of Contact Mudstone that is complexly folded and faulted. The zone is approximately 530 m long by 115 m wide with an average thickness of 10 m. The silver: gold ratio for the zone is approximately 100 times greater than in the 21B Zone. Stringers of chalcopyrite and chalcopyrite-galena-sphalerite overprint the mineralization. Finegrained pyrargyrite occurs locally in hairline fractures cutting the mudstone and hosts ore-grade mineralization.
The HW Zone forms massive sulphide horizons hosted in the mudstone interbeds within the Hanging Wall Andesite, at a higher stratigraphic level above the Contact Mudstone. The zone is roughly 1100 m long by 140 m wide. The thicknesses of the individual beds range from a few meters up to 20 m. The HW Zone has a higher base metal content compared to other zones, except where tetrahedrite ± sulphosalts are observed, which are associated with significantly higher precious metal grades.