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. 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 northeastwards to the Alaska Highway, joining it about 13 kilometres northwest of Summit Lake, for a total length of 56 kilometres.
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. Inferred reserves at Magnum at the time of mine closure were 90,710 tonnes grading 3 per cent copper (Northern Miner - May 8, 1975).
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 diabase dykes of Proterozoic age are common in the region.
The showing occurs 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 dykes cut the sedimentary rocks, ranging from a metre to about 100 metres in width and striking from northeast to east. There is minimal contact metamorphism of the sedimentary host rock, although the adjacent strata are commonly 'bleached' for several metres. The dykes 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, dykes are clearly post-mineralization, truncating the veins. Other dykes, locally known as "grey dykes", are known to cut transversely across the zone of mineralization and alteration, and individual veins, striking in a general northwest direction. These dykes 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 which 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 dykes, 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 dykes 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 dykes, but within the mine workings at least two faults have been mapped which 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) which was subsequently exploited by hydrothermal activity and later by dyke intrusion. In general, this zone of deformation, alteration, mineralization and dyke 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 ten 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 a hundred 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, northerly 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 host rock. 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 about 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 about 9 metres west along the strike of the fault.
A post-ore diabase dyke 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 dyke 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 which may exceed 45 metres. In places, the dyke becomes sill-like; subsidiary dykes extend west across the vein system. Along part of its length, the main dyke 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 dyke adjoins one or more veins, and locally invades and obliterates them; this occurred more extensively in the southern part of the mine zone.