Yields at Yukon - valuable deposits

Materials World magazine
,
1 Dec 2010
Three-dimensional render of the topographic map of Yukon, Canada

Canada’s smallest federal territory is rich in mining heritage and mineral discoveries, yet there are further valuable deposits in the Yukon landscape. Michael Forrest reports

The western Cordillera of Canada is a formidable barrier, and, in 1887, the most efficient way to penetrate the interior was by boats that plied the inlets in the Pacific coast. At the head of one of these inlets is Skagway, and beyond that is the famous White Pass that connects the coast to Whitehorse, Dawson and the Klondike, 600 miles away.

The gold finds of over a century ago were the beginning of a vibrant mining industry in the Yukon Territory that continues today. ‘Mining is an important part of our economy’, says Dr Harvey Brooks.

In addition, the region has a progressive range of incentives designed to promote and enhance mineral prospecting and exploration activities. The maximum levels are US$15,000 for grassroots prospecting, US$25,000 for focused regional exploration and US$50,000 for target evaluation. Taxation is also favourable based on a cumulative progressive scale, starting at three per cent for profits exceeding US$10,000 and rising to 12% for profits over US$35m. The regime allows deductions for capital and operating costs involved in constructing or repairing community infrastructure, including education programmes and environmental remediation both within and adjacent to mining properties.

Other tax deductibles include exploration and pre-production costs that may be pooled and carried forward to be amortised over the life of the mine.

Shelf life

The major geological components trend to the northwest and can be referenced to the subduction of the Pacific oceanic plate beneath the North American continent.

More than 190 million years ago, the western edge of the Ancient North American continental craton extended far out into the ancient Pacific Ocean. This submerged continental shelf is composed of crystalline basement rocks – akin to the Canadian Shield – that are at least 1.7bln years old. These rocks provided a stable continental platform upon which sediments, predominantly limestone and sandstone, accumulated. The most easterly part is the foreland, a stable block of sedimentary rocks accumulated over one billion years resting on the crystalline basement. The south-eastern edge is marked by the Tintina fault zone, separating the platform from numerous dissimilar crustal fragments to the southeast, which were amalgamated prior to their accretion to the ancient North American margin.

The accretionary process gave rise to metamorphism and intrusions along with fluids that provided the mechanisms for the emplacing of many mineralising systems.

One of the largest deposits is the porphyry copper-gold Casino deposit being developed by Western Copper Corp. Some 914Mt grading 0.2% copper 0.24g/t gold and 0.02% molybdenum have been identified in the proven and probable categories. ‘Another success is the Wolverine mine in the final stages of development and funded by Chinese investment in the Yukon,’ notes Brooks.

Gold is the focus of a number of companies, particularly those exploring the Tintina gold belt. Golden Predator’s CEO, William Sheriff, announces that his company has six advanced gold projects. The mineralisation is related to the Tombstone granite plutons emplaced during the mid-Cretaceous at Gold Dome, while further south at Grew Creek, gold mineralisation occurs as quartz-adularia veins and vein stock-work within permeable felsic pyroclastic tuffs contained within the Tintina fault graben structure.

‘The belt offers some exceptional opportunities and gold grades from up to 16g/t at our Cynthia advanced project, to 28.6 g/t at Grew Creek’ says Sheriff.

‘At Golden Dome, we mapped a 30km2 bismuth soil anomaly at the headwaters of placer deposit that has been worked on since 1906, yet no source had been identified. We have recently completed a core and reverse circulation drill programme that has produced results including 25m grading 11g/t.’

Opportunity knocks

Susan Craig, President and CEO of Northern Freegold Resources (NFR) in Vancouver, Canada, is an experienced prospector and member of the Yukon Mineral Advisory Board. Her company has staked a 198km2 belt within the Tintina trough. ‘Since 2006, NFR has identified 20 mineralised zones, of which two have 43-101 compliant resource statements, with the Nucleus zone containing over one million ounces of gold,’ she says.

‘These projects are within a new emerging mine camp including the Fort Knox operating mine of Kinross Gold, the Casino Deposit of Western Copper and the Pogo mine of Sumitomo Metal Mining. There is no doubt that the current intensive exploration programme will bring results.’

In 2009, the company completed initial metallurgical testing on three separate composite samples that were representative of bulk tonnage low grade oxidised (oxide) and non-oxidised (sulphide) samples, as well as higher grade, sulphide-rich material – previously referred to as skarn – that comprises the Nucleus deposit.

Initial results have oxide and sulphide samples returning recoveries of 98% gold on a 48-hour cyanidation bottle roll test. Higher grade sulphide-rich samples recovered 86% gold on a similar test, which increased to 92% when combined with gravity concentration. Gold contained in these samples responded well to extraction by either a combination of cyanidation and gravity concentration, or cyanidation alone.

Silver lining

The older sedimentary basins to the northwest of the Tintina fault are not without mineralisation. Within the Selwyn basin there is one of the richest silver deposits in North America. The local geology is characterised by three sedimentary rock units that metamorphosed to greenschist facies assemblages during the Middle Cretaceous. Some 217Moz of silver has been recovered from mines at Keno Hill between 1913 and its closure in 1989.

Today, Alexco Resources, also based in Vancouver, is poised for production, according to Chief Financial Officer David Whittle. ‘Since 2005, when we gained control from the administrator, we have developed highgrade silver resources in the Bellekeno property and others at Keno Hill. In September 2010, we completed mine and mill construction with a 400t/day conventional flotation concentrator within three per cent of budget. Keno is still as prospective as ever.’

Drill results in October provided values of 1,518 g/t silver over 10m and 834 g/t over 7m from near the old Onek mine. An historical resource estimate, including proven, probable and inferred mineralisation, totals 85,734t grading 393g/t silver, 4.71% lead, 16.41% zinc and 0.3g/t gold.

‘Although believed by Alexco to be relevant and reliable, we are working to convert to 43-101 compliant resources,’ confirms Whittle.

Copper Ridge Exploration in Vancouver has also identified mineralisation in sedimentary basins northwest of the Tintina fault. Clear Lake is a barite associated, shale-hosted sedimentary exhalative massive sulphide deposit. President Greg Dawson notes, ‘The orebody is 1,000m long and up to 120m wide and has an inferred resource of 7.65Mt grading 7.6% zinc and 1.08% lead. SEDEX mineral deposits such as Clear Lake often occur in clusters and we have made significant discoveries during the year.’

Mike Burke, Head Mineral Services in the territory, ‘We believe we have in place a proactive environment for mine development, however, prospective ground is the most important factor, and historical and present production testifies to metallogenesis of the Yukon. The Territory’s Geological Survey has a large inventory of data that can assist exploration.’

Further information

Mike Burke, Head of Mineral Services Yukon Geological Survey. Email: mike.burke@gov.yk.ca