The Magnum or Churchill Copper mine is situated at the headwaters of Magnum Creek, a tributary of Delano Creek, 8.5 kilometres north-northwest of Mount Roosevelt in the mountainous Muskwa Ranges of the Northern Rocky Mountains, approximately 161 kilometres west of Fort Nelson. It is centred on the 6100-level adit portal in the middle of the Magnum Mine zone (Assessment Report 3535, Map 3; Geology, Exploration and Mining in British Columbia 1971, Figure 11). The Churchill Mine road (present condition unknown) connects the mine workings with the nearby camp, and with the mill (now removed) farther downstream at the confluence of Delano Creek and Racing River. From there the road proceeds northeast to the Alaska Highway, joining it approximately 13 kilometres northwest of Summit Lake, for a total length of 56 kilometres.
The occurrence is in a region known as the Muskwa Anticlinorium, a major north-northwest–trending structure characterized by moderate folding and thrust faulting. The structure consists of Middle Proterozoic (Helikian) rocks of the Muskwa Assemblage, as well as Paleozoic rocks (Geological Survey of Canada Map 1343A; Geological Society of America, Geology of North America, Volume G-2, pages 111, 639). All belong to Ancestral North America (Geological Survey of Canada Map 1713A). Northeast- to northwest-trending Proterozoic diabase dikes are common in the region.
The occurrence lies in the Aida Formation of the Muskwa Assemblage, which comprises shale or slate, dolomitic and calcareous shale, dolostone and minor limestone (Assessment Report 3535; Geology, Exploration and Mining in British Columbia 1971; Geological Survey of Canada Memoir 373). In the area around the Magnum mine, the formation consists of a lower unit of dark-grey, thin-bedded, calcareous shale and interbedded calcareous shale and limestone, and an upper unit of interbedded buff- to orange-weathering dolomitic shale and dolostone, locally containing beds of algal dolostone. A large number of diabase dikes cut the sedimentary rocks, ranging from 1 metre to approximately 100 metres in width and striking from northeast to east. There is minimal contact metamorphism of the sedimentary hostrock, although the adjacent strata are commonly 'bleached' for several metres. The dikes are evenly distributed in the mine area and generally follow the same fracture and alteration zone that contains cupriferous quartz-ankerite veins. In the mine workings and surface showings, dikes are clearly post-mineralization, truncating the veins. Other dikes, locally known as ‘grey dikes’, are known to cut transversely across the zone of mineralization and alteration, and individual veins, striking in a general northwest direction. These dikes are of trachytic composition, contain disseminations and stringers of pyrite, and are generally only a few metres wide.
The sedimentary rocks are deformed into a large number of folds that plunge gently to the south and southeast. These structures range from a metre to several hundred metres in amplitude and are invariably asymmetric, with gently dipping west limbs and steep east limbs, and axial planes dipping to the west and southwest (Geology, Exploration and Mining in British Columbia 1971, Plate 3). The ubiquitous slaty cleavage in the Aida Formation rocks is parallel to the axial planes of these folds. In the Magnum Creek area, diabase dikes, fracture zones and cupriferous veins all have trends that are at a high angle to these fold structures, and are apparently not deformed by them. It appears that the dikes and veins filled a system of fractures, generally striking northeast, that developed after the folding and transverse to the fold axes.
Faults are not common on the property. A number of small faults and shear zones have been mapped, but none appear to be very large except at the Magnum Mine zone, where there has been considerable faulting. Most of these faults lie parallel to the zone and cut both mineralized veins and dikes, but within the mine workings at least two faults have been mapped that strike across the zone, dipping southwest at approximately 40 degrees, and are thought to displace ore shoots in a reverse manner.
Within the Magnum zone itself, the deformation is much more heterogeneous than that described above, shown by highly variable fold axes. The cleavage, partly curved and wavy, strikes predominantly south-southwest, with a dip of approximately 60 degrees to the east. In general, bedding dips gently to moderately southeast and apparently forms the southeast limb of a broad anticline, the hinge zone of which approximately follows Magnum Creek. Also within this zone, the originally calcareous succession is conspicuously non-calcareous, the limestone and calcareous argillite having been extensively altered by decalcification to coarsely crystalline ferrodolomite and ankerite. The same alteration has produced abundant graphite in shale, locally with coarse ankerite crystals. In addition, pyrite was developed in the west part of the zone forming seams and disseminations roughly concordant with bedding.
Mineralization at the Magnum deposit occurs in cupriferous quartz-ankerite veins in the subvertical north- to northeast-striking shear and fracture zones. The local preservation between the principal veins of septa of schistose country rock or brecciated quartz stockworks suggests that the Magnum zone was originally controlled by a narrow shear zone (or zones) that was subsequently exploited by hydrothermal activity and later by dike intrusion. In general, this zone of deformation, alteration, mineralization and dike intrusion trends 035 degrees, dips steeply and is up to 90 metres wide. It has been partly explored for a length of 1375 metres and to a depth of 365 metres. As many as 10 veins have been observed, concentrated in the centre of the zone, although some may prove to be extensions of others. They vary in width from less than 1 metre to as much as 7.6 metres and possess a continuity, both on strike and in depth, which may measure 100 metres or more. As many as three parallel principal veins occur within a width of 45 metres or less across the zone. Numerous subsidiary veins are present, some of which are parallel to the principal veins, and others which have an oblique, north trend and are probably branches of the principal veins.
In more detail, the veins consist of varying proportions of ankerite, quartz, chalcopyrite, and locally pyrite, together with partly replaced remnants of the sedimentary hostrock. Very minor amounts of bornite have also been observed. Malachite and azurite are common on the surface. Pyrite is locally prominent but is generally less than approximately 10 per cent of the total sulphides in the ore. Chalcopyrite is intimately associated with quartz, although in some places the quartz is so sparse that the vein appears to consist of massive chalcopyrite. Chalcopyrite tends to increase noticeably where a vein changes direction. Such jogs occur over only a metre or so and their shape is such as to displace the northern part of the vein west or, alternatively, the upper part westward by a metre (Canadian Institute of Mining, Transactions, 1971). The latter sense of displacement is effected also by at least one of several minor syn- and post-mineralization faults which occur in the northern part of the mine. These mineralized faults dip approximately 40 degrees southwest, and locally displace the upper parts of two principal veins approximately 9 metres west along the strike of the fault.
A post-ore diabase dike of irregular shape and generally steep dip closely follows the southeast side of the vein system and invades it progressively southwards in the zone. The dike is less than 3 metres wide in the northeast of the zone, but widens southwards and splits locally into two or more parallel branches with an aggregate width that may exceed 45 metres. In places, the dike becomes sill like; subsidiary dikes extend west across the vein system. Along part of its length, the main dike is followed by one or more steep faults, with unknown displacement, near which the diabase is propylitically altered. In the northern part of the mine zone, the dike adjoins one or more veins, and locally invades and obliterates them; this occurred more extensively in the southern part of the mine zone.
Work History
The region is known for widespread vein-hosted copper mineralization, generally restricted to fracture systems in Proterozoic sedimentary rocks, but the Magnum mine is the only deposit that has been brought to production. Discovered in 1943, it was explored and developed in the late 1950s and late 1960s. Mining was carried out on four levels intermittently between 1970 and 1975 (Mineral Bulletin MR 223), by which time it had produced 14 673 tonnes of copper from 501 019 tonnes of ore milled. In 1971, the mine was operated until October 1 when, due to low metal prices, it was closed and left under the supervision of a resident caretaker. During the year underground work consisted of 3048 metres of drifting and raising and 5486 metres feet of diamond drilling in 110 holes. Other work included the preparation of a new tailings dam and disposal area. Also in 1971, Churchill Copper Corp. Ltd. completed a geological mapping program on the area as the Mac, Don and ME claims.
Inferred reserves at Magnum at the time of mine closure were 90 710 tonnes grading 3 per cent copper (Northern Miner - May 8, 1975).
In 1998 and 1999, assessment work consisting of Landsat TM (optical) and JERS-1(radar) image studies and structural interpretation, was carried out by Crest Geological Consultants. It was concluded that post-mineralization northwest-trending faults may have truncated several veins. If that structural interpretation is correct, there may be several areas in the vicinity of the Eagle (MINFILE 094K 012), Magnum and Neil (MINFILES 094K 040 and 094K 57) veins that contain more vein structures with accompanying copper mineralization.
In 2005, Twenty-Seven Capital Corp. completed a regionally extensive program of geochemical (rock, silt and soil) sampling and a 9002.0 line-kilometre airborne magnetic survey on the area as the Muskwa property. Also at this time, Aries Resource Corp. and Action Minerals Inc. completed a program of prospecting and sampling on the area as the Angel claims of the Trident Copper project. A chip sample (190077) taken from the area of the Magnum vein assayed 1.97 per cent copper over 0.75 metre (Harrington, E. [2019-07-31]: Technical Report on the Churchkey Property).
Between May and June, 2006, McPhar Geosurveys Ltd. carried out a 4467.0 line-kilometres heliborne magnetic survey for Bradford Mineral Explorations Ltd. over parts of their Trident property. Between August and October 2006, Aeroquest Limited carried out a heliborne magnetic and electromagnetic survey for Bradford Mineral Explorations Ltd. over parts of their Trident property.
In 2007, Action Minerals Inc. and Aries Resource Corp. established a base camp and improved road access to some of the portals before completing seven diamond drill holes, totalling 274.0 metres, on the Missy and Angel claim groups of the Trident Copper property.
During the 2015 field season, prospecting was carried out by C. Warren along the Bonanza Creek drainage.
During late August 2016, R.M. Beck collected 10 samples on the Key property. The samples were taken from quartz-dominant float along Harris and Caribou creeks.
In 2018, Fabled Copper Corp. completed a minor program of geological mapping, rock sampling and petrographic analysis on the Toro property. A rock sample (204312) assayed 0.97 per cent copper (Assessment Report 38031).
In 2021, Fable Copper Corp. completed a program prospecting, rock sampling and UAV photogrammetry surveys on the Church-Key-Neil property. This work focused on the Vein 8a zone, located approximately 1.1 kilometres northwest of the Churchill-Magnum 6100 adit and the Magnum vein. A grab sample (D723397) of vein material hosting 8 to 10 per cent chalcopyrite and 1 per cent bornite from the Vein 8a zone assayed 14.3 per cent copper, whereas sampling of the Magnum vein yielded up to 7.38 per cent copper over 2.0 metres (Assessment Report 39913).
