Summary:
The North Island Project is underlain predominantly by andesitic volcanics, volcaniclastics and epiclastic sediments of the early to mid-Jurassic Bonanza Group which are intruded by diorite, quartz monzonite, and quartz-feldspar porphyries of the coeval Island Plutonic Suite. Several facies of breccia are present, including hydrothermal and tectonic as well as possible magmatic breccias in the deposit areas.
Mineralisation - Hushamu
The Hushamu mineralized zone extends 2,500 m in a NW-SE direction, dips 25 degrees north, extends 860 m down dip and has an average thickness of 380 m. Three mineralized zones have been recognized in the Hushamu Deposit related to and defined by the alteration zones.
LEA, the Leached Zone is typical of ground water acid leaching occurring at the top of porphyry systems. In BC, leached caps of the majority of porphyry systems have been removed by erosion and/or glacial processes. This leached rock is generally bleached, the majority of sulphide minerals have been removed, while abundant clay minerals formed by the leaching process and silica-rich minerals remain. This zone generally occurs at the top of the deposit, however there are minor discontinuous, leached zones throughout Hushamu Mountain. Copper has been partially to completely removed but minor molybdenite and gold remain.
The rarely formed supergene zone at the base of LEA within the SCP is characterized by very weak enrichment of copper in the form of chalcocite +/- covellite. The zone generally occurs from 60 m depth to 90 m below surface. In one hole, EC-187, supergene mineralisation was noted at 200 metres depth in fractured rocks proximally to the west fault.
Mineralisation - Northwest Expo
At Northwest Expo there is no leached cap as at Hushamu, but similarly the mineralisation is within the SCP and CMG alteration zones.
SCP1 at its upper levels has background to low copper-gold-molybdenum-rhenium assay grades. Towards the basal 50 metres of the contact between SCP1 and CMG generally the gold grade becomes significant (>0.1g/t Au) along with molybdenum (>250ppm Mo) and associated rhenium (>0.5 ppm Re) grades. This increases down to the contact and diminishes progressing down through the CMG alteration. Conversely, copper grades are mostly low to background in SCP and generally increase abruptly at the SCP1 / CMG contact. Molybdenite occurs on fracture surfaces, as lesser veins and as blebs.
In CMG copper mineralisation occurs as disseminated chalcopyrite and minor bornite and chalcocite associated with hydrothermal magnetite. Rarely, covellite has been observed along with bornite replacing it, indicating a second mineralizing Phase. The gold grades at Northwest Expo are significantly higher than Red Dog and Hushamu. In the deeper portions of the deposit, porphyry related vein stockworks host weak gold mineralisation and weak copper grades due it being mobilized into the overlying CMG.
Mineralisation - Red Dog
The Red Dog deposit extends 730 m NW-SE by 400 metres width and 120 metres thickness occurring predominantly in a 350 x 150 m WNW-trending quartz-magnetite breccia localized in altered Bonanza Group rocks adjacent to quartzfeldspar porphyritic dykes. This breccia consists of angular clasts of fine grained finely veined CMG cemented by white to light grey silica. Chalcopyrite and pyrite occur as disseminations, blebs and fracture fillings with lesser amounts of bornite and molybdenite.
A second mineralized zone termed the Slide Zone lies 400 m east of the Red Dog deposit in altered Bonanza Group volcanics south of the Red Dog Intrusion stock. Mineralisation consists of pyrite, chalcopyrite occurring as disseminations and fractures and molybdenite along joints and fractures. Steeply dipping northeast-trending late trachyte dykes cut the mineralisation. Grade or tonnage estimates have not been calculated for the Slide Zone due to the difficulty in connecting geology and mineralisation between holes.
Deposit Types
The Hushamu, Red Dog, and Northwest Expo deposits host Cu-Au-Mo-Re mineralisation within the upper reaches of porphyry systems, though alteration and mineralisation styles diverge from those of typical porphyry deposits (e.g. Sillitoe, 2010), possibly due to strong telescoping and/or epithermal enrichment (Perello, 1995; Sillitoe, 1994). Hushamu is similar in grade and size to the past-producing Island Copper Mine 29 km to the east, which produced 345 Mt of mineralized material with average grades of 0.41% Cu, 0.017% Mo, 0.19 g/t Au and 1.4 g/t Ag (Perelló et al., 1995).
Deposits at the North Island Project are best characterized as Au-rich calc-alkalic porphyry type; these deposits commonly form in sub-circular zones of brecciated and hydrothermally altered rock in and around the apex of a quartz diorite to quartz monzonite stock. Deposits developed in relatively high-level, subvolcanic environments are commonly associated with multiple dyke and breccia Phases. However, deposits formed at greater depth are more often associated with broad zones of faulting in plutonic rocks (Panteleyev, 1995).
The juxtaposition of porphyry-style mineralisation with intense lithocaps-style alteration and the relatively high Au contents suggest a shallow level of formation for the North Island deposits and/or a significant reduction in paleosurface elevation during the life of the hydrothermal activity (Sillitoe, 1994). Disseminated chalcocite at Northwest Expo (Mulja, 2024) suggests possible epithermal Au enrichment. The morphology of the interpreted paleohorizontal semi-tabular lens of mineralisation within lithocaps-style advanced argillic alteration is atypical of porphyry deposits in B.C., though several deposits in Southeast Asia have similarities (e.g. Batu Hijau, Lepanto-Far Southeast, Onto), possibly due to the tropical latitudes of Wrangellia at the time of formation.
Mineralisation is hosted within the intrusive rocks and/or the host rocks and consists of quartz stockworks, veinlets, disseminations and replacements within large hydrothermally altered systems. Metallic mineralisation is comprised of chalcopyrite, pyrite, bornite, molybdenite, magnetite, hematite and chalcocite. The large (up to 10 km2 ) hydrothermal systems are marked by distinctive alteration assemblages. The core of porphyry systems exhibits potassic alteration assemblages (potassium feldspar-biotite-magnetite ± anhydrite-diopside-garnet and commonly host the strongest CuAu mineralisation as chalcopyrite and bornite. Peripheral to the potassic core extensive propylitic alteration consists of albite, chlorite, epidote, calcite, diopside, actinolite and pyrite. Potassic and propylitic alteration are often overprinted by phyllic (quartz-sericite-pyrite), argillic and, in the uppermost parts of deposits, advanced argillic alteration (Giroux and Pawliuk, 2005).
Advanced argillic alteration is prevalent on the North Island Project, often with full textural replacement of the protolith resulting in gusano texture (SIM) above the hydrothermal fluid upflow zone. The presence of magnetite-rich bodies enclosed by advanced argillic alteration is unexpected because normally the acidic fluids responsible for lithocaps formation are magnetite destructive. Laterally extensive lithocaps environment with horizontal planar mineralisation at Northwest Expo may be analogous to Lepanto, an epithermal Cu-Au deposit that formed in the lithocaps environment above the associated Far Southeast porphyry deposit in Luzon, Philippines (Hedenquist et al., 1997). In the Onto lithocaps-hosted high-sulfidation porphyry deposit on eastern Sumbawa Island, Indonesia, copper occurs predominantly as covellite deposited during advanced argillic alteration, though mineralized intrusions imply a magmatic source (Burrows et al., 2020).