Geology of the NICO gold-cobalt-bismuth deposit and Sue-Dianne copper-silver deposit, southern Great Bear magmatic zone, Northwest Territories, Canada

CIM Vancouver 2006
Kathryn L. Neale, Robin E. Goad,
Abstract Fortune Minerals Limited owns an 81% interest in the NICO Cobalt-Gold-Bismuth deposit and a 100% interest in the Sue-Dianne Copper Silver deposit located 160km northwest of the City of Yellowknife, Northwest Territories, Canada. The deposits are the only known significant Canadian examples of Proterozoic iron oxide hosted polymetallic deposits, more commonly referred to as hydrothermal iron oxide copper-gold deposits or “Olympic Dam-type”.

NICO and Sue-Dianne are situated in the south part of the Great Bear magmatic zone, the central tectonic subdivision of the Proterozoic, Bear Structural Province. It is a post-Wopmay, collisional plutonic terrane with related continental volcanic rocks dating from 1870 Ma and culminating with the emplacement of A-type rapakivi granite plutons at approximately 1856 Ma. Iron oxide occurrences are widely distributed within the Great Bear magmatic zone, typically occurring along major structural lineaments within the aureoles of anorogenic potassium-rich “A-type” granite intrusions.

The NICO deposit is hosted in iron- and potassium-altered, brecciated subarkosic wacke and siltstone of the Treasure Lake Group at and beneath the volcanic unconformity. The host “black rock” amphibole-magnetite-biotite schists and ironstones are capped by potassium feldspar-magnetite felsite altered rhyolite of the Faber Group. Feldspar-quartz-amphibole porphyritic dykes crosscut the deposit and are feeders to the overlying volcanic pile. The deposit occurs within three, 45-50 degree dipping, stacked stratabound lenses of ironstone, each between 5 and 70 metres in thickness. The lenses can be traced up to 1.5km along strike and are between 100 and 450 metres in width. They contain approximately 10% disseminated and fracture-filling sulphides, including cobaltian arsenopyrite, cobaltite, cobaltian loellingite, native bismuth, bismuthinite, chalcopyrite, and local pyrrhotite and native gold. The 24.6 million tonne deposit is comprised of 9.6 million tonnes of underground resources, grading 2.14g/t gold, 0.15% cobalt and 0.21% bismuth, and 15 million tonnes of open pit resources, grading 0.31g/t gold, 0.11% cobalt and 0.11% bismuth.

The Sue-Dianne deposit conversely, occurs above the volcanic unconformity in a 500 metre long, well-zoned diatreme breccia complex that crosscuts tilted rhyodacite ash flow tuffs of the Faber Group. The Faber Lake rapakivi granite pluton intrudes the base of the complex. Breccias are comprised of potassium feldspar-altered clasts of rhyodacite in a matrix of hematite-magnetite-Fe-silicate within a broader zone of potassium, iron, quartz, and epidote metasomatism and fracturing. The deposit contains approximately 10% disseminated sulphides including chalcopyrite, bornite, locally minor chalcocite, pyrite and molybdenite and with very local pitchblende at the periphery of the deposit. Sue-Dianne contains 24.3 million tonnes, grading 0.56% copper and 2.2 g/t silver, including 10.6 million tonnes, grading 1% copper and 3.3 g/t silver.

At both NICO and Sue-Dianne, paragenetic studies demonstrate that early, reduced, high-temperature mineral assemblages are overprinted by late, oxidative, low-temperature assemblages. These together with stratigraphic relationships, indicate fluid mixing at shallow crustal levels was important in deposit formation. Proximity of the NICO and Sue-Dianne deposits to subvolcanic porphyries, rapakivi granite and various other border phases of the Marian River Batholith, together with geochronology and mineralogy studies, suggest they are all genetically related.
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