Summary:
Deposit Type
Deposits at the North Island Project are best characterized as Au-rich calc-alkalic porphyry type.
The Hushamu, Red Dog, and Northwest Expo deposits host Cu-Au-Mo-Re mineralization within the upper reaches of porphyry systems, though alteration and mineralization styles diverge from those of typical porphyry deposits, possibly due to strong telescoping and/or epithermal enrichment (Perello, 1995; Sillitoe, 1994).
Hushamu Deposit Mineralization
The Hushamu mineralized zone extends 2,500 metres in a NW-SE direction, dips 25 degrees north, extends 860 metres down dip and has an average thickness of 380 metres. 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 silica-clay-pyrite (SCP) is characterized by very weak enrichment of copper in the form of chalcocite +/- covellite. The zone generally occurs from 60 metre depth to 90 metres below surface. In one hole, EC-187, supergene mineralization was noted at 200 metres depth in fractured rocks proximal to the West Fault.
In the Hypogene Zone, of SCP and chlorite-magnetite (CMG) copper mineralization occurs as blebby and vein chalcopyrite and lesser bornite. The copper grade is highest in CMG-altered volcanics with lesser copper in SCP alteration. Molybdenite and related rhenium concentrations are highest in the SCP altered rocks, however molybdenite is also present in quartz veins in the CMG-altered rocks. Sulphide mineralization decreases where silica flooding is extreme, in the late vertical breccias and surrounding rocks, and in propylitic altered units.
Sulphide mineralization in historical core that has been exposed to the atmosphere has been intensely oxidized and leached by weathering processes. Abundant chalcanthite, brochantite, and other sulfates are observed as precipitates on the core.
Mineralization related to alteration extends over 2.7 km in the NW direction and over 1.3 km in the NE direction. Recently identified quartz vein stockwork feldspar porphyry at the southeast side of Hushamu Mountain opens up the possibility of the mineralized intrusive roots of the system being downfaulted by the Hepler Creek fault zone.
Northwest Expo Deposit Mineralization
At Northwest Expo there is no leached cap as at Hushamu, but similarly the mineralization 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 mineralization 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 mineralization and weak copper grades due it being mobilized into the overlying CMG.
SCP2 has significantly lower to background copper, gold and molybdenum grades.
Red Dog Deposit Mineralization
The Red Dog deposit extends 730 metres 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 quartz-feldspar 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. Mineralization consists of pyrite, chalcopyrite occurring as disseminations and fractures and molybdenite along joints and fractures. Steeply dipping northeast-trending late trachyte dykes cut the mineralization. Grade or tonnage estimates have not been calculated for the Slide Zone due to the difficulty in connecting geology and mineralization between holes.
Pemberton Hills Prospect
Pemberton Hills is an early-stage prospect area where high-level intense hydrothermal alteration (clay, aluminum hydroxides, hydrous sulphates, and hydrous aluminosilicates) suggest potential for underlying Cu-Au mineralization over an ~1 x 2 km area.
Its proximity and analogous alteration styles to that of the Island Copper Mine in the southeast, and the Hushamu and Red Dog deposits to the northwest, the recognition of variably altered high level stocks and dykes, induced polarization (IP) anomalies and the presence of anomalous copper, gold and molybdenum within the alteration footprint are all significant attributes that support further exploration to evaluate the robustness and quality of the Pemberton Hills porphyry copper-gold-molybdenum target (McClintock, 2019).