Higher prices for palladium and platinum have generated renewed interest in exploring for these metals around the world and in the Canadian Cordillera.
Recent prices reflect the growing use of these metals in advanced technologies, as well as the fact that gold has fallen on hard times, with market values stuck below US$300 per oz. British Columbia has seen limited production of platinum from placers, and byproduct platinum and palladium from lode deposits. However, solid prices for palladium, coupled with a growing demand for all platinum group elements (PGEs), signal an opportunity to explore for new types of deposits.
PGEs occur as mineralization in a variety of environments, but most production has been from large mafic to ultramafic intrusions and placers. The large intrusions are currently the dominant source of the world’s PGEs. They can be divided into layered mafic to ultramafic complexes or flood basalt-associated mafic intrusions. Placer production is minor, though regions in Alaska and Russia produced significant quantities of platinum in the last century.
The layered mafic-ultramafic complexes, such as the Bushveld of South Africa and Stillwater in the U.S., exhibit mineral bands that can be traced for tens of kilometres. Within these complexes, the PGEs are confined to stratabound ore horizons that contain either platinum-rich chromite layers or copper-nickel sulphide lenses with byproduct platinum and palladium. Typically only the largest intrusions have economic concentrations of PGEs. Large, layered complexes are typically found in areas of stable continental crust, not in Cordilleran environments.
In the Canadian Cordillera, the Axelgold layered gabbro intrusion of north-central British Columbia is the only large intrusion that exhibits similarities to layered mafic-ultramafic complexes hosting PGEs. Although chromite is reported from peridotite layers and the intrusion contains weakly disseminated sulphides, there are no reports of PGEs. A similar layered gabbro-norite intrusion with basal portions of dunite and peroditite in Alaska, called La Perousse, hosts the Brady Glacier nickel-copper deposit with minor associated PGEs.
Mineralization associated with flood basalt-associated mafic intrusions are best represented by the Noril’sk-type deposits of Russia, which are the world’s second-largest producer of PGEs. In 1999, the mines in the Noril’sk and nearby Talnakh regions produced 28% of the world’s PGEs and were also selling palladium from their stockpiles. The copper-nickel-palladium-platinum mineralization occurs in gabbro and diabase intrusions (usually sills) related to extensive overlying Triassic flood basalts.
Voluminous Triassic flood basalts with associated mafic-ultramafic intrusive complexes are found in Wrangellia — a geological terrane which extends along the west coast of British Columbia up the Alaska Panhandle and into the Yukon. The volcanics are known as the Karmutsen Formation in the south and the Nikolai basalts in the north. Copper-nickel-PGE mineralization is associated with the 600-km-long Kluane mafic-ultramafic belt, which extends from the southwestern Yukon into northern British Columbia. The Wellgreen deposit, 250 km west-northwest of Whitehorse, Y.T., has reserves of 49.9 million tonnes grading 0.36% nickel and 0.35% copper, plus 0.51 gram platinum and 0.34 gram palladium per tonne. It was mined for nickel and copper in the early 1970s, when it produced 171,652 tonnes of ore. The Chilkat and Mansfield are two similar PGE-bearing occurrences that occur in the British Columbian portion of the Kluane belt. Karmutsen flows and associated sills are extensive on Vancouver Island and the Queen Charlotte Islands. Tofino Nickel, Kitkat and Swede are some of the small number of copper-nickel-PGE occurrences that have been found on these islands. However, it is not clear if any of these occurrences are related to Karmutsen magmatism. Tofino Nickel has high-grade grab samples with up to 18.7 grams per tonne palladium and 6.9 grams per tonne platinum.
Alaskan-type ultramafic intrusive complexes are commonly zoned and form sills, stocks or irregular intrusive bodies. They are known to contain platinum with minor associated iridium, osmium and rhodium hosted by thin chromitite layers, concentrations of cumulus magnetite or clinopyroxenite. There has been relatively limited lode platinum production from these deposits, though they are associated with significant placers in Russia, Alaska and around the Tulameen complex near Princeton, B.C. Lode platinum Alaskan-type occurrences in British Columbia are found in the Tulameen complex (Grasshopper Mountain, Olivine Mountain, Lodestone Mountain), the Turnagain complex near Dease Lake, and the Wrede Creek complex west of Williston Lake. Typically, the best grades of platinum are associated with chromitite schlieren. The Tulameen and Turnagain Alaskan-type intrusions have attracted the most exploration attention in the past decade.
Another intrusive setting for nickel and copper deposits, which can have associated byproduct PGEs, is gabbroid intrusions in greenstone and orogenic belts. The sulphides occur as lenses, net-textured stockworks and disseminations of pentlandite, chalcopyrite and pyrrhotite hosted by small-to-medium-size stocks. In British Columbia, the Giant Mascot mine and Nickel Mountain prospect are examples of this type of deposit. Platinum and palladium grades of up to 2 and 3 grams per tonne, respectively, have been reported for the mine.
Several poorly understood copper-PGE mineral occurrences are associated with alkalic intrusive rocks in the Okanagan and Greenwood areas. These intrusive rocks have been correlated with Eocene Coryell intrusions based on composition and texture. These include the Dobbin, Sappho and Maple Leaf and other occurrences in the Franklin camp. On the Dobbin property, drilling has intersected 15 metres grading 0.54% copper, plus 1.36 grams platinum and 0.947 gram palladium per tonne. The Maple Leaf, also known as the “Platinum Blonde,” has been known to contain platinum since its original development in 1915.
Podiform chromites hosted by ophiolites can have anomalous concentrations of ruthenium, osmium and iridium. Since the individual pods are small and the chromite is lower-quality, these are not highly prospective for PGEs.
Shales can host thin, laterally extensive layers of sulphides, such as pyrite, vaesite, jordisite and sphalerite. These layers are typically thin and metallurgically complex but do contain multiple economically attractive commodities, including PGEs. Several occurrences with minor PGEs have been identified in the Yukon, including the Old Nick and Taiga prospects. Similar-age shales occur in sedimentary strata along the eastside of the Rocky Mountain Trench in British Columbia.
Hydrothermal PGE occurrences may occur in Cordilleran environments; some of the occurrences in British Columbia classified as veins could have formed in this manner.
In British Columbia, about 100 mineral occurrences are reported to have anomalous concentrations of platinum or palladium. About a third of these occurrences are placers containing gold and platinum. There is only one PGE placer, called “Van Winkle Bar.” The Tulameen and Similkameen river placers produced 620,000 grams platinum between 1889 and 1936. Many of these placers are related to Alaskan-type intrusions. British Columbia lode occurrences are related to Alaskan-type, gabbroid-associated, flood basalt-associated and vein deposits.
— The author is a geologist with the British Columbia Geological Survey. The above was first published on www.bc-mining-house.com, the web site of the B.C. and Yukon Chamber of Mines.
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