The Anvil lead/zinc/silver district is 200 km northeast of Whitehorse, Yukon. Five known stratiform sedimentary exhalative deposits occur in a well-defined trend within a narrow Cambrian stratigraphic interval. The first to be mined was the Faro orebody, which in the words of Gregg Jilson, Vice-president, Exploration, was a “phenomenal deposit” containing 60 million tonnes. The Faro pit and underground workings are being wound down this year after 22 years. At the start of 1991, there were just over a million tonnes of ore left in the Faro pit and 251,000 tonnes in the underground mine. The other deposits include Vangorda (currently being mined), Grum (awaiting pre-production stripping), Dy and Swim. Together, they form a curvilinear array that has been squeezed by deformational events and are located between the Tintina Fault Zone, an ancient, dormant rift, and the Anvil Batholith. Along this trend is a remaining inventory of 85 million tonnes in the five deposits. Reserves for Faro, Vangorda, Grum and Dy at the beginning of 1992 were 44.7 million tonnes averaging 5.3% Zn, 3.6% Pb, 55 g/t Ag and 0.8 g/t Au.
In comparison to Faro, Vangorda “is high-grade, has good metallurgy and is shallow,” Jilson said. (For details, please see the story on our tour of the Faro mine.) Also setting apart the deposits on the Vangorda Plateau from Faro itself are the higher gold values. At 0.8 g per tonne, gold values are about eight times the values of the Faro deposit. (Gold recoveries should help offset the costs of ore haulage from the plateau to the Faro concentrator, roughly 14 km. away.)
The open-pit Grum deposit displays three sulphide horizons in a complex fold structure, Jilson said. Ultimately, this will be a deeper pit than Vangorda. The Dy deposit, mineable only by underground methods, is probably a downfaulted extension of the Vangorda orebody. Jilson also suspects that all the deposits are remnants of a larger structure which once was several times the current size of the deposits. Geological events, however, might have robbed much of the ore. “We’re about 80 million years too late,” he joked.
Jilson, who has nearly 20 years of experience on the massive sulphides of the Anvil Range and elsewhere in the Yukon, said the company now has “a very good model” to work with in finding more such deposits. The targets, from an areal perspective, tend to be large because, despite their structural complexity, they are flat-lying overall. However, if the drills miss any portion of the deposit, there is no tell-tale sign to suggest, at the least, proximity to a mineralized structure.
“You have to drill through the deposit to see it. There is no chemical signature or trace. For that reason, you have to drill densely,” Jilson said. The exploration chief is eager to carry on with magnetic and electromagnetic surveys in the area, in addition to drilling. These tools have not been used in the area for 15 years and new insights are expected because of improved technology.
For each of these deposits, sulphide deposition is thought to have occurred when geothermally heated fluids moved up fracture zones and were exhaled on the sea floor and deposited with reduced basinal sediments, mainly shales. The deposits formed as layers on the sea floor, and some of the orebodies consist of several layers stacked one above the other with sediment deposited between them. Each layer contains massive sulphides and disseminated sulphides in quartzite. The layers are strongly zoned; they can be visualized as a very large fried egg, the yolk being massive sulphides and the white, the disseminated sulphides. The upper part of the massive sulphides is commonly baritic. The wall rocks beneath many of the layers are bleached and altered. All deposits are within 150 m of the contact between the lower, non-calcareous Mt. Mye phyllites and the upper calcareous Vangorda phyllites and calc-silicates.
According to company literature on the geology of this area, the rocks of the Anvil district have been complexly deformed by two major deformational events. Structures related to these events significantly affect the geometry of the orebodies. Structural features vary according to depth within the metamorphic pile. Metamorphism increases with proximity to the Cretaceous granitic rocks of the Anvil plutonic suite. The first deformational event was associated with northeast-verging (shaped like a Z’ looking northwest) tight folds and thrusts.
Phase I folding affected the sulphide deposits and developed a steeply, southwesterly-dipping pervasive axial planar cleavage. Phase I axial planar cleavage is best preserved in the deposits of greenschist metamorphic level (Grum, Vangorda, Dy). Overprinting the first phase of folding is a second generation of folds which are of smaller wavelength and amplitude. Second-phase folds are ubiquitous in the region and are generally tight to isoclinal; they have shallowly dipping axial planes and are southwest verging (shaped like an S’ looking northwest). The Phase I and II folds are approximately co-axial, trending northwest-southeast and plunging shallowly in either direction. The second-phase axial planar cleavage is pervasive throughout all of the developed deposits and tends to overprint most of the earlier fabrics. The emplacement of the Anvil Batholith during the Cretaceous Period domed the metasediments, forming a northwest-trending arch. Large-scale, late extensional faulting associated with the uprise and emplacement of the Anvil Batholith also affects the geometry of the ore deposits.
Grum
The Grum deposit was discovered in 1973 when a diamond drill hole was collared to test a geophysical anomaly north of the Vangorda deposit. Surface drilling continued in 1974 and 1975. To further define the deposit, an underground operation was conducted in 1975 and 1976. The underground workings consist of an access decline which splits into two parallel decline ramps following the trend of the orebody for 700 m and extending to a depth of 200 m. A total of 525 surface and underground drill holes (80 km. in total length) have delineated the Grum deposit. Since acquiring the deposit in 1985, Curragh has drilled 112 holes totalling 12,500 m to better define early production ore and obtain fresh metallurgical samples.
Stratigraphically, the Grum horizons span an interval from approximately 100 m above and 50 m below the Mt. Mye/Vangorda formation contact. The deposit has a strike length of 2,000 m and is 500 m wide with up to a 35-m thickness for individual ore layers. The spatial distribution and geometry of the orebody is strongly controlled by Phase I and II folding, as well as late extensional faulting. Grum ore geometry is defined by a Phase I anticline syncline pair, which has been refolded on coaxial Phase II folding. The axial traces of both phases of folding plunge gently (11deg) to the northwest. Late, large-displacement, normal faults associated with the emplacement of the Anvil Batholith truncate the Grum orebody at both its northwest and southeast ends.
Grum in-situ geologic reserves as of Jan. 1, 1992, are 33.9 million tonnes grading 8.92% combined Pb and Zn, with additional potential downplunge to the northwest. The current 25.2-million-tonne mining reserve consists of approximately 58% carbonaceous quartzite and 32% massive sulphides. The average grade is 5.0% Zn, 3.0% Pb, 50 g/t Ag and 0.8 g/t Au.
Vangorda
The Vangorda pit is at the location of the first discovery zone of all the sulphide deposits in the Anvil Range. The discovery area was an oxidized sub-crop by Vangorda Creek and has since been mined out.
The Vangorda deposit is in the Mt. Mye formation, 50 m to 75 m below the Vangorda/Mt. Mye formation contact. There are two major ore horizons as well as several smaller lenses. The uppermost is overlain and underlain by phyllites; the lower, and more important horizon, is overlain by phyllites, but underlain by a thick unit of low-grade massive to semi-massive sulphides, pyritic quartzites and altered wall rock. This footwall unit is enriched in copper and gold relative to zinc and may be a siliceous footwall alteration zone. The deposit is believed to be partly on the overturned limb of a large second-phase fold. The lower horizon is thought to be the same stratigraphic horizon as the lowest horizon of the Grum deposit. “We can recognize these horizons from deposit to deposit. But they aren’t always of equal importance in their contribution to reserves,” Jilson said. For example, while the lower horizon at Vangorda contributes the bulk of mineable tonnage, at Grum it is a relatively minor component.
The entire Vangorda deposit is cut by late faults related to the intrusion of the Anvil Batholith. These faults commonly dip to the north and northeast. The faults commonly have limited displacement, but are of great importance in grade control. The deposit is cut off at the northwest end of the pit by a major north/south-trending fault, which dips steeply to the east. The orebody is roughly 1,000 m long by 200 m wide with an average thickness of 25 m, although this varies locally. The orebody plunges gently to the northwest at 5deg overall. There are three main ore types: baritic ore, which is the most abundant; graphitic quartzites; and mineralized footwall quartzites. The latter type is considered “bonus” ore because it is unpredictable in its mineralization and, therefore, not included in scheduled reserves. The metamorphic grade at Vangorda, like Grum, is greenschist compared with amphibolite facies at the Faro deposit. The difference is due to the greater distance from the Anvil Batholith. Remaining reserves at Vangorda as of January, 1992, are 4.2 million tonnes at a combined 4.2% Zn, 3.3% Pb, 42 g/t Ag, and 0.7 g/t Au.
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