Summary:
The Turnagain deposit is a magmatic sulphide resource hosted by an Alaskan-type intrusion.
While Turnagain exhibits these Alaskan-type characteristics, it is unusual in that it hosts magmatic sulphide mineralization in appreciable abundance. Nixon (1998) concluded that the iron-nickel-copper (Fe-Ni-Cu) sulphides in the Turnagain complex are primary and magmatic (as opposed to having resulted from a post-emplacement process) and that wall rock inclusions which have been observed in drill core may have provided a mechanism for sulphur saturation and precipitation of Fe-Ni-Cu sulphides. This has been confirmed by sulphur and lead isotope results reported by Scheel (2007).
Disseminated and rare net-textured mineralization at Turnagain is hosted in dunite, wehrlite, olivine clinopyroxenite and clinopyroxenite and serpentinized equivalents. Sulphides include pyrrhotite, pentlandite, chalcopyrite and trace bornite. Valleriite is sometimes present where serpentinization is intense.
Property Geology
The Early Jurassic (190 ±1 million years ago [Scheel, 2007]) Turnagain complex comprises a central core of dunite with bounding units of wehrlite, olivine clinopyroxenite, clinopyroxenite, representing crystal cumulate sequences, hornblende clinopyroxenite and hornblendite. The complex is elongated and broadly conformable to the northwesterly trending regional structural grain.
The ultramafic rocks are generally fresh to mildly serpentinized; however, more intense serpentinization and talccarbonate alteration occur along faults and restricted zones within the complex. The central part of the ultramafic body is intruded by granodiorite to diorite and hornblende-plagioclase porphyry dykes and sills.
Mineralization
Showings of semi-massive and massive sulphides have been identified by work to date. These semi-massive and massive zones, plus broad zones of disseminated sulphides, are generally hosted by dunite and wehrlite near the southern and eastern margins of the ultramafic body. The central and northern dunite is largely devoid of sulphide minerals although their highly magnesian olivine is more enriched in nickel (up to 0.20 to 0.30 weight percentage) than the olivine in the peridotites and pyroxenites of the Horsetrail and Northwest zones, which may be nickeldepleted in areas of sulphide mineralization. Nixon (1998) suggests that these features are further evidence of fractional crystallization of ultramafic magma.
Primary sulphide minerals consist of pyrrhotite with lesser pentlandite (iron-nickel sulphide) and minor chalcopyrite. Some bornite has been reported. Interstitial and blebby sulphides, with grain sizes ranging from 1 to 4 mm, are evident in widespread disseminated zones seen in drill cores. With increasing concentrations, these intercumulus sulphide grains coalesce to form net-textured sulphides. Semi-massive and massive sulphides and rare sulphide matrix breccias were also noted in drill cores over intervals not exceeding a few tens of centimetres.
Narrow fracture-filling sulphide lenses, commonly featuring chalcopyrite and minor pentlandite along with the more prevalent pyrrhotite, appear to be products of remobilization of primary sulphides adjacent to dykes, altered xenoliths and serpentinized areas.
Secondary nickel and copper sulphides, including violarite and valleriite, have been noted in serpentinized zones and both primary and secondary sulphides are associated with graphite (Nixon, 1998). Microscope and microprobe studies of drill core samples from the Horsetrail Zone (Kucha, 2005) have identified additional nickel sulphide minerals including mackinawite, heazlewoodite, godlevskite and millerite. PGE minerals identified to date include vysotskite, a palladium-iron-nickel sulphide and sperrylite, a platinum arsenide mineral.
The principal mineral zones identified to date on the Turnagain property include the following:
• The Horsetrail Zone and surrounding area have been the focus of most of the historic and recent diamond drilling. Results to date suggest a northwest to west-northwest trend for these zones, which consist of broadly dispersed, disseminated to intercumulus sulphide mineralisation in both dunite and wehrlite and serpentinised equivalents. Sulphide grains range in size from 0.5 to 5 mm and commonly occupy interstices between olivine grains. Drill core samples from the Horsetrail Zone have a median of 0.23% total nickel with grades ranging from 0.01% to 4.89% total nickel. Total cobalt grades range from 0.001% to 0.480% with a median of 0.013% Co. There appears to be a spatial relationship between graphitic xenoliths, increasing clinopyroxene content in the ultramafic host rocks and the incidence of sulphide mineralisation. Where present, chalcopyrite occurs along the margins of pyrrhotite and in narrow veinlets. Relatively unaltered dunite adjacent to the Horsetrail Zone may contain total nickel values of 0.20% to 0.30%, virtually all of which is in the crystal lattices of the silicate mineral olivine and consequently is not of economic importance.
• The Northwest Zone is contiguous with, and lies northwest of, the Horsetrail Zone. This zone has mineralisation styles and grades similar to the Horsetrail Zone, but is intruded by several mafic and felsic dikes which dilute the overall grade. Drill core samples from the Northwest Zone have a median grade of 0.20% total nickel with grades ranging from 0.01% to 2.86%. Total cobalt grades range from 0.001% to 0.166%. The Horsetrail and the Northwest Zones form a zone approximately 2,000 m long in the east-west direction, and 550 m wide from north to south and have been tested by 251 drill holes.
• The Hatzl Zone mineralisation consists of disseminated and net textured pyrrhotite and pentlandite hosted by dunite and wehrlite. This mineralisation is similar to, and may be continuous with, the Horsetrail Zone. The Turnagain River flows between the two zones and the region below the river has not been sufficiently drill tested to exclude the potential of additional mineralisation. The Hatzl Zone is 1,150 m long in a northeast direction and 300 m wide in a northwest direction and has been tested by 17 drill holes.
• The Duffy Zone mineralisation lies 500 m northeast of the Horsetrail Zone and consists of disseminated sulphides similar to those within the Horsetrail Zone. Grades range from 0.014% to 0.525% total nickel. The Duffy Zone is 300 m in diameter and does not crop out. It was discovered by exploration drilling in 2006. The zone has been tested by six drill holes.