Advances in metallurgy, coupled with the anticipated expansion of power and transportation infrastructure in the Yukon, have induced
The junior is convinced Howards Pass can be rendered economic at current zinc prices.
“Our vision is a bulk-mining situation whereby the mining costs are reduced significantly through a large-scale operation running at a minimum of 20,000 tonnes per day,” said Copper Ridge President Gerry Carlson, who guided The Northern Miner on a recent tour of the property.
Situated in mountainous, glaciated terrain near the border of the Yukon and Northwest Territories, the project is 1,375-1,500 metres above sea level. It is accessible by fixed-wing aircraft or helicopter from Whitehorse (370 km away), Ross River (180 km) or Watson Lake (300 km), as well as by winter road, from Flat Lake, just north of Tungsten, N.W.T.
In early July, Copper Ridge acquired an option to purchase a 100% interest in the property from 51%-owner
Subsequently, Copper Ridge entered into a deal with Billiton Metals Canada, a subsidiary of London-based
Following the completion of the concept study, Billiton can elect to acquire a half-interest by spending $5.5 million toward the completion of a prefeasibility study. The major has the right ultimately to earn up to a 70% interest in the project by carrying all costs to feasibility.
The initial 2,000-metre diamond drill program, recently completed, saw one of two rigs test known mineralization at the Anniv deposit, as well as extensions to this mineralization. Here, Copper Ridge hopes to intersect near-surface mineralization amenable to open-pit mining. The other rig tested a relatively flat-lying portion of mineralized strata near the surface in the Don Valley area, 3 km southeast of Anniv. Assays are pending.
The drilling also provided material for further metallurgical tests, results from which will be incorporated into the concept study.
Discovered in 1968 by Placer Development (now Placer Dome), Howards Pass is, by tonnage, the largest shale-hosted zinc resource known to exist. Estimates run as high as 476 million tonnes grading 5% zinc and 2% lead, plus 9 grams silver per tonne, in three distinct deposits: OP, Anniv and XY.
Between 1973 and 1982, Placer drilled and evaluated Howards Pass; the work partly entailed driving a 367-metre decline on the XY deposit. U.S. Steel joined the project in 1975, providing $10 million, out of a total exploration budget of $15 million, to earn a 49% stake. In the end, the partners had drilled off a resource of 110.5 million tonnes grading 5.4% zinc and 2.3% lead, based on a cutoff grade of 4% combined lead and zinc.
Prefeasibility evaluations indicated that the Howards Pass could not be mined economically, owing to lack of infrastructure and to the high cost of traditional milling and smelting. The cost of transporting the zinc to market would have been prohibitive.
Fine-grained
Metallurgically, the project was also problematic. Like many shale-hosted exhalative deposits, Howards Pass has fine-grained mineralization that requires long grinding times and large mills. It is also rich in graphite, making it difficult to separate the lead from the zinc for the purpose of producing concentrates.
Placer performed bench-scale tests using three different grind sizes from three bulk samples taken from the XY deposit. The results indicated a 62-68% zinc recovery to a zinc concentrate, and a 40.2-48.2% lead recovery to a lead concentrate. The overall recovery for zinc and lead — reporting to a lead, a zinc and a bulk zinc-lead concentrate — was 85% and 86%, respectively.
In 1977, the Sheridan Research Centre completed a bench-scale test on a zinc-rich concentrate, as well as a bulk lead-zinc concentrate using a pressure oxidation process. Zinc recoveries of 98.6-to-99.8% were attained with a lead sulphate and pyrite residue.
Copper Ridge intends to take advantage of recent technological advances in hydrometallurgy to overcome the previous metallurgical barriers.
“Our concept is to maximize the zinc recovery,” says Carlson. “We’re not interested in the lead, so a rough flotation, or maybe even a gravity separation, will do.”
Copper Ridge hopes to produce zinc metal on-site through autoclave leaching and/or bioleaching and electrowinning, and thereby cut concentrate transportation costs.
Natural gas
The proposed hydrometallurgical-electrowinning plant would consume a considerable amount of electricity. However, with the recent discovery and development of natural gas reserves in northeastern British Columbia and the western Northwest Territories, relatively inexpensive gas turbine power could be developed close to the project (Placer originally proposed diesel-generated power).
Copper Ridge envisages producing bulk zinc, lead, cadmium and silver concentrate in a mill circuit comprising gyratory crushing, semi-autogenous grinding, and flotation. Concentrates would be transported south via a slurry pipeline to a pressure leaching, purification and electrowinning plant at the Robert Campbell Highway, near Frances Lake, or possibly at Watson Lake. Annual production is pegged at 350,000-500,000 tonnes of zinc metal. Capital costs are estimated to be $1.2 billion.
The concept study is to be completed by December 1, 2001, at a cost of $3 million, followed by a prefeasibility study, which will cost between $6 million and $8 million. Following a final feasibility study, construction would begin in 2004, and startup, a year later.
The Howards Pass deposits consist of three complexly folded and faulted saucer-shaped bodies that host laminated to massive sulphide mineralization. These bodies are referred to as XY, Anniv and OP and are believed to have formed separately in anoxic (oxygen deficient) sub-basins along the base of a paleoslope in the eastern Selwyn basin. The Anniv deposit is 22 km northwest of the XY deposit, whereas the OP zone is an additional 8 km northwest of Anniv. To date, the areas between the sub-basins have not been drilled to determine if the mineralized bodies are connected along strike.
The Anniv deposit has an indicated resource of 55.5 million tonnes grading 7.1% combined zinc-lead, plus an additional 261 million tonnes of inferred resources at an unspecified grade. The inferred resource estimates were based on projected extensions of potential mineralization and have not been drill-tested. As a result, no grade estimates were assigned.
The deposit is hosted in a complexly folded and faulted sub-basin that is cut off to the northwest by a fault. Anniv is divided into the West, Central and East zones.
The Central zone contains the largest tonnage of high-grade resource, pegged at 48 million tonnes grading 8.1% combined zinc and lead using a 4% combined zinc-lead cutoff. Placer calculated a preliminary minable diluted resource of 8.9 million tonnes grading 10.6% zinc and 5.5% lead. The ore horizon in this zone varies in thickness from 3 to 9.5 metres and locally up to 12 metres.
The XY deposit hosts an indicated resource of 55 million tonnes grading 8.3% combined zinc-lead, plus an additional 113 million tonnes of inferred resource at an unspecified grade. The deposit represents a complexly folded and faulted lens of intercalated chert, limestone, mudstone and sulphide, which occurs within a syncline. The deposit has been subdivided into six zones, referred to as the Northeast, Nose, East Central, West and Brodel.
The main sulphide minerals at Howards Pass are sphalerite and galena, with minor pyrite. Quartz and calcite can be seen as veins and nodules.
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