Summary:
The KSM deposits display many diagnostic features of porphyry Cu ± Au ± Mo systems. The deposits are centered on intrusive complexes composed of Early Jurassic, Texas Creek suite porphyry stocks and dykes. The Kerr, Mitchell, East Mitchell, and Iron Cap zones display the typical lateral and vertical zoning sequence of alteration assemblages observed in many porphyry systems: deep central potassic alteration, peripheral propylitic alteration, and shallow sericitic alteration. Mineralization is associated with stockwork quartz veinlets and arrays of sheeted quartz veinlets, with vein density decreasing in later intrusive phases. Host rocks may be mineralized for up to several hundred meters from the intrusions. The Kerr, Mitchell, and East Mitchell zones also feature small remnants of advanced argillic alteration. The structurally complex Sulphurets deposit does not feature the same clear alteration zoning patterns observed at the other three deposits, due to its dismembered and fragmental nature. However, the Lower Panel fault block at Sulphurets, which hosts the bulk of the mineralization, features potassic alteration and mineralization typical of porphyry Cu ± Au ± Mo systems.
Kerr Zone
The Kerr Zone is centered on an Early Jurassic, north-south trending, steep westerly dipping, tabular intrusive complex. Drilling demonstrates that the Kerr Zone has an extent of 2,400 m along strike, a width of roughly 800 m, and a vertical extent of at least 2,200 m. The flattened morphology and relatively elevated copper:gold ratio of the Kerr Zone distinguishes Kerr from the other deposits in the district. The surface expression of the Kerr deposit is a large and elongated, northerly trending, pyritic gossan, primarily exposed in a cirque on the northern flank of Kerr peak.
The Kerr intrusive complex is composed of a suite of northerly-striking and steeply west-dipping dykes and intrusions emplaced into a sequence of rhythmically bedded siltstones, sandstones, conglomerates, and debris flows belonging to the Lower Jurassic Hazelton Group. Wall rocks adjacent to the intrusions have been hornfelsed and hydrothermally altered, but generally contain marginal metal grades. The complex is composed of an east and west limb separated by a thin wedge of intensely altered sedimentary wall rock. The west limb is up to 500 m thick, and the east limb is up to 300 m thick.
The dominant copper mineral is chalcopyrite, which typically occurs as isolated grains about 0.2 mm to 2 mm across, disseminated and clustered in quartz veins, fractures, and surrounding haloes. Bornite is present almost exclusively in the north half of the east leg, within a QABX zone containing >50% crackled quartz veins, and is accompanied by coarse grained chalcopyrite and minor tennantite. Tennantite-tetrahedrite is rare, but widely distributed in late quartz-carbonate veins, mostly in wall rocks, along with minor sphalerite, rare galena, and arsenopyrite. Visible gold has not been observed except under microscopic examinations, where it is observed as less than 100 µm inclusions within sulphides, mainly chalcopyrite, and sulphide grain boundaries.
Iron Cap Zone
The Iron Cap deposit is the northernmost porphyry gold-copper-molybdenum deposit in the KSM district, and structurally above the Mitchell deposit, in the panel of rocks between the MTF and STF. It is now known to be hosted by an Early Jurassic intrusive complex that is roughly contemporaneous with those in the Mitchell and Kerr zones. The Iron Cap mineralized zone forms a tabular body striking roughly north-south and dipping ~60° to the west, with approximate true dimensions of 1,500 m in strike, 1,500 m down dip, and up to 800 m in thickness. Mineralization remains open down dip.
Only the southern and eastern margins of the Iron Cap deposit outcrop at surface, where weak to moderate gold + copper ± molybdenum mineralization is primarily hosted in sedimentary strata of the Hazelton Group.
The Early Jurassic Iron Cap intrusive complex is composed of multiple intrusion and breccia phases. One of the earliest phases is a pre-mineral, medium-grained plagioclasehornblende-phyric diorite with porphyritic texture found on the southeast side of the complex (“P2 East”). A second plagioclase-hornblende-phyric diorite phase, finer-grained than the pre-mineral diorite, is northwest of the complex (“P2 West”). The P2 West phase is thought to be syn-mineral, as it hosts significant volumes of early quartz-sulphide veins (A-type and B-type veins, based on the vein classification scheme of Gustafson and Hunt, 1975), and because this diorite phase is spatially associated with some of the highest gold and copper grades observed at Iron Cap. Syn-mineral, plagioclase-K-feldsparhornblende(-quartz)-phyric monzonite intrusions (“P3 East”) are in the central to eastern parts of the Iron Cap intrusive complex. The P3 East monzonites are typically mediumgrained, with seriate texture, and include several thin dykes as well as a ~100 m thick tabular intrusion striking roughly north through the centre of the Iron Cap deposit, and dipping ~60° west. As with the syn-mineral diorite, portions of the P3 East monzonite host significant volumes of early quartz-sulphide A-type and B-type veins and strong goldcopper mineralization. Finally, a suite of weakly-mineralized monzonite dykes and intrusions, coarser-grained than the central syn-mineral monzonite, occurs on the western side of the intrusive complex (“P3 West”).
Mineralized hydrothermal breccias are significantly more abundant and voluminous at Iron Cap than at the Mitchell, Sulphurets, or Kerr deposits. Tabular, steeply dipping, and northerly striking mineralized hydrothermal breccia bodies, ranging from ~10 m to ~250 m wide, occur within central Iron Cap. The breccias are matrix-supported and contain poorly-sorted subangular clasts. The clasts within the breccias are commonly too altered to permit protolith identification but clasts of P3 monzonite are sometimes observed. Jigsaw hydrothermal breccias are also common along the margins of the P2 East intrusion, with subangular to angular monomict clasts of the P2 East intrusion in a hydrothermally-altered rock flour matrix.
Total quartz vein abundances at Iron Cap typically range from 10 vol.%) abundances of sheeted A-veins, which correspond to zones of particularly elevated gold and copper grades. Such zones are notably observed within the P3 East and P2 West intrusive phase. The vein types present at Iron Cap include: early quartz – chalcopyrite ± magnetite “A-veins”, magnetite ± chalcopyrite veins with selvedges of pink hydrothermal K-feldspar ± magnetite; quartz – chalcopyrite ± molybdenite ± pyrite “B-veins,” with a distinctive sulphide-bearing central suture surrounded by quartz; gray quartz veinlets with a variable sulphide-sulphosalt assemblage of pyrite ± tennantite ± tetrahedrite ± chalcopyrite ± sphalerite, which crosscut the earlier A-veins and B-veins; pyritic “D-veins” with sericite-pyrite selvedges; and post-mineral quartz – carbonate ± chlorite ± sphalerite ± galena ± chalcopyrite ± tennantite ± tetrahedrite veins, which are thought to be associated with mid-Cretaceous deformation. High silver values are generally associated with presence of galena and sphalerite.
Microscopic examinations of polished thin sections confirm that Iron Cap was also subjected to post-mineral deformation, as evidenced by widespread mylonitic textures.
The Iron Cap Zone terminates at the south along the north-dipping Iron Cap Fault (ICF). South of the fault, hornfelsed sedimentary rocks are mineralized with marginal gold and copper grades similar to intervals above the MTF at Mitchell. A few holes through this area contain higher than average molybdenum grades, including in interval of 133 m with 0.10% molybdenum.