Summary:
The Carmacks copper-gold deposit is similar to the Minto deposit, located 50 km to the northwest (Sinclair, 1976; Pearson, 1977), except that the Minto deposit is flat lying and primarily a sulphide deposit. A number of theories for the genesis of the Carmacks deposit have been postulated over the years and by different operators. The Cu-Au- Ag metal tenor and association with Late TriassicEarly Jurassic granodiorites would appear to suggest a link between mineralization at Carmacks and the porphyry copper deposits of the same age that occur across British Columbia. However, the linear deposit shape, lack of mineralized material-stage veining and lack of porphyry alteration show clearly that the Carmacks copper deposit is not a classic porphyry system. Evidence from the drilling campaigns suggests the deposit was formed by assimilation of older, copper bearing volcano-sedimentary rocks into the Jurassic Granite Batholith. These “rafts” of mineralized rock would have been variably metamorphosed, and in places completely assimilated into the granodiorite. The volcano-sedimentary rafts would tend to pull apart along bedding planes forming large tabular sheets as observed in Zones 1, 4, 7, 7A, 8, 12, 13 and 2000S. Evidence suggests the sulphide mineralization has been remobilized out of the rafts into the surrounding diorite. At a later time, when the upper parts of the batholith where eroded and the rocks were exposed to the atmosphere and meteoric waters, the sulphide mineralization began to oxidize and precipitate as the oxide minerals.
Deep oxidation of the deposit has led to the formation of an oxide cap that can be over 200 m thick. The majority of the copper found in oxide are in the forms of the secondary minerals malachite, cuprite, azurite and tenorite (copper limonite) with very minor other secondary copper minerals (covellite, digenite, chalcocite). Native copper occurs as dendritic secondary precipitates on fractures, disseminated grains or thin veinlets. Other secondary minerals include limonite, goethite, specular hematite and gypsum. Primary copper mineralization, occurring below the oxidized level, is present as bornite and or chalcopyrite. Other primary minerals include magnetite, gold, molybdenite, native bismuth, bismuthinite, arsenopyrite, pyrite, pyrrhotite, and carbonate. Molybdenite, visible gold, native bismuth, bismuthinite, and arsenopyrite occur rarely.
The copper in the upper 200 m in Zones 1, 4, 7, and 7A is oxidized; whereas in Zones 13 and 12 the oxide cap can be as thin as ~40 m. Within the oxidized area, pyrite is virtually absent and pyrrhotite is absent. Weathering has resulted in 1% to 3% pore space and the rock is quite permeable. Secondary copper and iron minerals line and in-fill cavities, form both irregular and coliform masses, and fill fractures and rim sulphides. Primary sulphide minerals and magnetite are disseminated and form narrow massive bands or heavy disseminations in bands. Gypsum occurs as microveinlets. Carbonate occurs as pervasive matter, irregular patches, or microveinlets, not commonly but on the order of 1% where present. Gold occurs as native grains, most commonly in cavities with limonite or in limonite adjacent to sulphides, but also in malachite, plagioclase, chlorite, and rarely in quartz grains. Gold is rarely greater than five microns in size.
Primary (hypogene) copper mineralization appears to be associated primarily with the amphibolitegneiss units and the early-formed diorites, whereas secondary copper mineralization does not appear to be preferential to a particular rock type. This is owing to the remobilization of copper during supergene processes. In the north half of Zone 1, copper mineralization forms high and low grade zones that are reasonably consistent, both along strike and down dip, and these zones are broadly constrained to the deformed rocks and diorites, but transcend local lithological boundaries. Higher grades tend to form a footwall zone, while lower grades form a hanging wall zone. Primary mineralization, below the zone of oxidation comprises mainly of chalcopyrite, bornite, molybdenite, magnetite, pyrite and pyrrhotite. Primary copper mineralization appears to be zoned from bornite on the north to chalcopyrite, and finally to minor pyrite-pyrrhotite in the south. Narrow veinlets of anhydride were found in the deepest drill hole.
Alteration minerals associated with the mineralizing include K-feldspar and biotite. Epidotization and some K-feldspar are related to pegmatite dyke intrusion, which is a post- mineralization event. Clay (montmorillonite type) and sericite development are clearly weathering products. Silica introduction, usually as narrow veinlets, is not common and may be related to aplite dyking or metasomatism. Chloritization of mafics, biotitization of hornblende, rare garnets, carbonate, and possibly anhydrite all appear related to metasomatism and assimilation of precursor rocks to the gneissic units.
In Zone 1, oxide copper grades increase with depth in both the footwall and hanging wall. There is no association of copper values with mafic mineral content, or grain size. Gold values are higher in the north half of the deposit. They average 0.75 g/t compared with 0.27 g/t in the south half. There is no apparent increase in values with depth and the highest grade gold values are not associated with the highest copper values; however, gold values in the northern half are higher in the footwall section. This lack of increase in gold values with depth suggests that the gold distribution reflects a primary distribution rather than a secondary distribution such as oxide copper values. As with oxide copper, the gold content does not correlate with rock type, mafic constituents or grain size.