Summary:
The Cu-Au-Ag deposits of the Carmacks Project, and the related Minto deposit, are considered rare examples of metamorphosed porphyry Cu systems (Kovacs et al., 2020).
The following section regrading the Carmacks Project mineralization has been modified from Kovacs (2018). Modifications have been made based on recent work by Granite Creek.
Mineralization of the Carmacks Copper Cu-Au-Ag deposit occurs within a 3 km–long, north-northwest trending belt that is separated into a northern Zone 1, 4 & 7, central Zones 2000S and southern zones 13 and 12.
Three different varieties of hypogene copper mineralization are recognized, with distinct mineralogical and textural characteristics: disseminated, foliaform, and net textured.
Disseminated copper sulphide mineralization is interpreted to be the least modified by metamorphism because it typically occurs in the undeformed metamorphic lithologies and augite gabbro. Disseminated chalcopyrite and pyrite comprise a minor portion of the hypogene mineralization and is typical in the undeformed, hornblende porphyroblastic amphibolite, granoblastic quartz-plagioclase-biotite schist, and augite gabbro. Disseminated copper sulphide minerals also occur in leucosome within the quartz- plagioclase-biotite schist unit. Chalcopyrite is typically fine-grained (0.3-0.7 mm) and occurs as anhedral to wavy, skeletal grains intergrown with also fine-grained (0.1 mm), minor subhedral pyrite.
Foliaform copper sulphides are restricted to the amphibolite and the quartz-plagioclase-biotite schist and occur as chalcopyrite-dominant stringers that parallel the dominant foliation (). Chalcopyrite is fine-grained (0.3-0.5 mm) and forms elongated anhedral grains. Bornite is less common in the foliaform mineralization, but where present forms fine-grained (0.2-0.5 mm), anhedral, blebby intergrowth with foliaform chalcopyrite. Pyrite intergrown with chalcopyrite is rare, but it is euhedral where present. The foliaform nature of copper sulphides is interpreted as the result of Late Triassic deformation and temporally associated upper amphibolite facies metamorphism.
In contrast, mineralization hosted by the migmatite occurs as net-textured intergrowths of bornite and chalcopyrite, which comprises as much as 20%-40% of the rock by volume, with typical bornite-chalcopyrite ratios of 3:1. Pyrite is absent in the migmatite. Bornite and chalcopyrite occur together in irregular nettextured domains up to 2–4 mm across forming low interfacial angles that are clearly interstitial with respect to silicate grains. Both chalcopyrite and bornite are commonly replaced by digenite along 50 mm-wide fractures and grain margins and is interpreted as the latest copper phase due to secondary oxidation. Net-textured bornite is especially abundant in melanosome, where it typically forms higher-grade (1-2% Cu) domains. The net-textured domains contain numerous inclusions of fine-grained native bismuth, Au-Ag tellurides, and bismuth tellurides. Molybdenite is commonly intergrown with net-textured copper sulphides and occurs as kinked anhedral grains separated along cleavage surfaces or as euhedral undeformed grains.
The pyrite content of the deposit is notably low (~1%). Gold is principally associated with bornite and occurs as 10-20 µm inclusions of electrum or native gold, or more commonly as gold telluride (calaverite), or solidsolution gold-silver tellurides, as determined by SEM-EDS spectrometry. Silver is present as hessite inclusions in bornite. Because gold and silver are typically associated with bornite, the bornite-chalcopyrite ± digenite zone is precious - metal enriched and the migmatite contains higher copper, gold, and silver grades than the amphibolite and quartz-plagioclase-biotite schist sequence.
Copper grades increase progressively northwards from the lower grade material found in the southern deposit Zone 12 and 13 deposits through to the highest-grade material at the Zone 1 deposit. This change in grade is interpreted to be caused by the increasing northward metamorphic gradient of the inliers which is also reflected in the general depth of emplacement of the batholiths.
Deep oxidation of the deposits has oxidized primary sulphides to copper oxides and copper carbonates with approximately 15% of the copper in the oxide domain occurring as remnant copper sulphide, in the form of chalcopyrite. This oxidation profile has led to the formation of an oxide cap that can be over 200 metres thick at zone 1 to ~40m thin in Zone 12. The majority of the copper found in oxide are in the form of the secondary minerals malachite, cuprite, azurite, tenorite (copper limonite) and crednerite with 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.
Within the oxidized portion of the deposit, pyrite is virtually absent and pyrrhotite is absent. Oxidation 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.
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.