Like the Hemlo gold deposits in Ontario, the Eskay Creek polymetallic deposits will be discussed by geologists for decades. Eskay Creek is a unique discovery, from both an academic and an economic point-of-view, in that it represents a new and important type of mineralization to be found in northwestern British Columbia. Until its discovery, advanced projects in the region typically revealed gold mineralization associated with shear-hosted quartz and carbonate lodes.
The project is also unique in that the deposit itself reveals a spectrum of mineralized styles, ranging from those often thought to signify epithermal gold-silver, to other, more economically important mineralization similar to that found in volcanogenic massive sulphide deposits. In any case, the success of exploration drilling at Eskay Creek is attributed to systematic drilling of a projected trend in an otherwise strata-bound zone.
The Eskay Creek deposits are underlain by extrusive felsic volcanic rocks of the Jurassic Mt. Dillworth formation. Mineralization is hosted within a thin mudstone unit that occurs at the contact with overlying Salmon Formation pillowed andesite flow rocks.
Exploration drilling to the end of July, 1990, totalling some 135,330 metres in 660 holes, has delineated a mineralized body known as the 21 zone. It is 1,450 metres long by up to 300 metres wide, and is still open along strike to the northeast. Within the drilled area of the 21 zone, stratigraphy is subdivided into a hangingwall andesite unit, the contact mudstone unit, the rhyolite unit and the footwall dacite unit.
The 21 zone itself is sub-divided into the 21A deposit (or South zone) and the 21B deposit (Central and North zones), on the basis of differences in mineralogy and grade continuity. In addition, drilling during the winter months of early 1990 discovered two parallel mineralized areas known as the Pumphouse Lake and 21C zones.
The 21B deposit, with its uniquely high precious metal grades and associated base metal values, contains the bulk of reserves at Eskay Creek, which now total 1.55 million tons (1.4 million tonnes) grading 1.34 oz. (46 grams) gold, 36 oz. (1.2 kg) silver, 1.9% lead and 4.7% zinc. This calculation is based on drill data to the end of 1989 and on a cutoff grade of 0.25 oz. (8.6 grams) gold.
The 1990 winter drill program completed numerous in-fill holes that confirmed grade and thickness within established reserve blocks and that also linked together numerous isolated blocks of probable reserves. At the same time, step-out holes extended the mineralization north to the property boundary. A revised reserve estimate should result in a sharp increase in the number of tons and contained ounces.
The 21B zone is about 1,150 metres long, from 130 metres to 300 metres wide, and is two to 11 metres thick in strata-bound ore, with adjacent footwall mineralized areas extending over 60 metres deeper. The southernmost portion, about 650 metres, contains the best grades and geological continuity.
Within the 21B deposit are several different mineralization types, the most significant being a sheet of precious-metals-rich massive sulphide mineralization hosted within the contact unit. The deposit also contains more widespread, but less continuous, disseminated mineralization within the rhyolite unit, as well as evidence of hangingwall unit-hosted stacked sulphide lenses at greater depths in the northern portion of the zone.
Gold mineralization occurs as electrum, a natural alloy of gold and silver. The bulk of silver values report to tetrahedrite. Other sulphide minerals present include sphalerite, galena, chalcopyrite, pyrite and minor boulangerite and bournonite.
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