The Iron Mask batholith lies in the southern part of the Quesnel trough, also known as the Nicola belt. The most important pre- Tertiary rocks in this belt are Upper Triassic volcanic and sedimentary rocks of the Nicola Group. The batholith is a subvolcanic, multiple intrusion which is comagmatic and coeval with the Nicola rocks. It is situated along the southwest side of a regional northwest trending fracture zone and is itself cut by numerous northwesterly faults. The batholith comprises two major northwest trending plutons separated by 6 kilometres of Eocene Kamloops Group volcanic and sedimentary rocks. The Tertiary rocks occupy what appears to be a graben structure resulting from renewed fault movement around the margins of the plutons during Paleocene or Early Eocene time (Bulletin 77). The larger pluton, the 18 kilometre long southern part of the batholith, is called the Iron Mask pluton. The smaller Cherry Creek pluton farther northwest, outcrops on either side of Kamloops Lake. The combined exposure of the batholith, including the intervening younger rocks, is about 33 kilometres long and 5 kilometres wide.
Sedimentary and volcanic rocks of the Kamloops Group unconformably overlie the Nicola rocks and the Iron Mask batholith. These include tuffaceous sandstone, siltstone and shale with minor conglomerate, as well as basaltic to andesitic flows and agglomerates with minor dacite, latite and trachyte.
In the vicinity of the batholith, the Nicola Group is dominated by volcanic and volcaniclastic sedimentary rocks. They are generally recognized by albitization of feldspars, occurrence of patchy epidote, and/or rare hematite alteration. On the southwestern flank of the Iron Mask pluton, well-indurated, massive and bedded tuff, breccia and interbedded flows and flow breccia are prominent and are weakly metamorphosed. On the northeast flank, less well-indurated and less altered tuff and tuff breccia predominate. However, adjacent to the intrusive contact, these rocks are also well indurated and epidotized and are locally mineralized with sulphides. At the southeastern tip of the Iron Mask pluton and locally along the southwestern flank, the Nicola rocks comprise distinctive porphyritic augite-hornblende basalt.
The Iron Mask pluton comprises four major, successively emplaced units designated as the Iron Mask Hybrid, Pothook, Sugarloaf and Cherry Creek units. Locally, an additional Picrite unit also occurs which is probably not genetically related to the batholith. The smaller Cherry Creek pluton consists entirely of the Cherry Creek unit. Isotopic dates (194 to 204 Ma +/- 6 Ma) indicate that all of these units are of Late Triassic or earliest Jurassic age (Bulletin 77). The component units (except the Picrite unit) of the multiphase batholith are largely controlled by major systems of northwesterly, northerly and northeasterly trending fractures or faults. Most units show some degree of alteration and/or contamination which may be intense locally. Weak to moderate saussuritization is ubiquitous in all batholithic rocks while potassium feldspathization is more prominent in rocks of the Cherry Creek unit. The units are briefly described in order of oldest to youngest (determined mainly on crosscutting relationships).
The Iron Mask Hybrid unit forms the spine of the Iron Mask pluton. It is mostly agmatitic, consisting of rounded to angular fragments of various sizes, texture and composition in a dioritic matrix. The fragments include mainly coarse and fine-grained diorite and coarse-grained gabbro with lesser amounts of medium to coarse-grained hornblendite and scattered xenoliths of Nicola Group volcanic rocks. All rock varieties in the unit contain magnetite which is often more than 10 per cent by volume. Mineralization, particularly of iron and copper, is almost ubiquitous in this unit. The Iron Mask mine (092INE010), a former copper producer, is located in this unit, but is also associated with picrite.
The Pothook unit occurs mainly in the northwestern half of the Iron Mask pluton, appearing frequently as narrow, gradational zones between the Iron Mask Hybrid and Cherry Creek units. Rocks of this unit are uniformly of dioritic composition and are medium to coarse grained. The Pothook unit is locally mineralized with copper and iron.
The Picrite unit consists of rocks of basaltic composition with abundant clinopyroxene and serpentinized olivine phenocrysts. These rocks generally occur as steeply dipping, poorly exposed and relatively small lenticular bodies in many parts of the batholith. They appear to be associated with recurring, northwesterly trending fracture systems and copper mineralization frequently occurs in their vicinity. Because picrite basalt has been observed far from the two component plutons of the batholith, it is probable that this unit is not part of the batholith.
The Sugarloaf unit occurs mainly along the southwest side of the Iron Mask pluton and as small enclosed bodies in the southern half of the pluton. Rocks of this unit are mainly porphyritic with hornblende, minor clinopyroxene and plagioclase in a greyish green matrix. They are of fairly uniform diorite-andesite composition. Several copper occurrences are hosted by the Sugarloaf rocks. The Ajax deposit (092INE012) east of Jacko Lake is located within brecciated and albitized Sugarloaf rocks.
The Cherry Creek unit is the most widely distributed phase of the batholith. It constitutes the entire Cherry Creek pluton. The unit consists of rocks with composition ranges from diorite, monzonite, syenite to their porphyritic and fine-grained equivalents as well as local intrusive breccias. Copper and minor iron mineralization is prominent in the Cherry Creek unit, particularly in zones of intense brecciation associated with alkali metasomatism. Afton mine (092INE023) lies at the western termination of a narrow, 4 kilometre long, easterly trending zone of intense intrusive brecciation that is located at the northern edge of the Iron Mask pluton.
The Crescent deposit is 3500 metres east of the main Afton pit. It is centred in and around a tabular body of Cherry Creek unit diorite breccia which straddles the northeast striking contact between Cherry Creek monzonite and diorite on the southeast and latite porphyries on the northwest. It is in fault contact with older Pothook unit diorites to the south. The deposit is elongated in a northeast direction and has average horizontal dimensions of 152 by 46 metres. Better grade mineralization is confined to the intrusive breccia and dips 60 degrees to the southeast. Steeply dipping normal faults with westerly to northwesterly strikes control emplacement of the intrusive breccia unit. Potassic feldspathization and copper mineralization appear to be genetically related to this breccia intrusion. Chloritization is intense and pervasive in the mineralized area. Propylitic alteration and a pyrite halo extend beyond the better grade copper zone.
Chalcopyrite is the only copper mineral of economic significance. It occurs as blebs and disseminations, in fractures, veinlets and microveinlets, and occasionally in breccia and vugs with accompanying calcite. Pyrite occurs with chalcopyrite as well as forming a halo peripheral to copper mineralization. Bornite and molybdenite are present only in trace amounts. Magnetite is associated with the breccia intrusion as disseminations and veinlets. Gold mineralization is closely associated with the chalcopyrite mineralization. However gold-copper ratios are variable suggesting more than one pulse or period of mineralization. On average, the gold-copper ratios are low compared to the Afton and Pothook orebodies.
The property is located in the vicinity of Ironmask Lake approximately 10 kilometres west of Kamloops. An area roughly identified as lying between Ironmask Lake and the DM shaft (092INE030) 1.5 kilometres to the west has been held for many years by a number of Crown grants and various located claim groups. The Truth group, comprising the Truth, Dakota, Hope, Jennie, and Pearl claims was owned by A.G. McDonald and Joseph McGee. Development working during the period 1899 to 1906 inclusive included a shaft to 24.3 metres with a 30.4-metre drift from it. A second shaft was sunk to 9.1 metres and an adit was driven more than 30.4 metres to drain the workings. Ore mined in 1906 was sold to the nearby Iron Mask operation (092INE010) for fluxing purposes. The Con Verdant claim (Lot 1341) was Crown granted to the British Columbia Trust Co. Ltd. in 1905. The May Fraction (Lot 1311) was Crown granted to A.R. Thomas in 1908. The Sodium Fraction (Lot 4666) and Winty (Lot 4667) claims were Crown granted in 1941 to W.E. Wintemute and Marcus Smaby, respectively. Berens River Mines Limited, a subsidiary of Newmont Mining Corporation, held a number of claims in 1952. An electromagnetic survey was carried out and 5 diamond-drill holes put down on the Iron Cap (092INE018) and adjacent claims. Graham Bousquet Gold Mines, Limited in 1956 acquired the DM group, in part a restaking of the Truth group, and other claims in the vicinity. Work included geophysical and geochemical surveys. The company amalgamated with several other companies in 1958 to form Cadamet Mines Limited. Noranda Exploration Company, Limited optioned the property in 1958 and carried out further geophysical surveys. The option was given up later in the year. Comet Mining Corporation Ltd., incorporated December 1964, optioned the Lorna 1 4 claims from Lorne H. Stephens. The company also acquired the Iron Cap and some 62 located claims in the RO, DM, and Monzo groups. In 1965, Vanco Explorations Limited optioned this and a number of adjacent properties. Work by the company included geological, geophysical and geochemical surveys. Diamond drilling totalling 1612.3 metres in 15 holes was carried out on the Lorna claims. The option was terminated in May 1966. Comet Mining and Krain Copper Resources Ltd. amalgamated in May 1966 to form Comet Krain Mining Corp. Ltd. In October 1966, the company optioned the Con Verdant, May Fraction, Sodium Fraction and Winty Crown grants from Isabelle Wintemute. The company name, Comet Krain was changed in April 1971 to Comet Industries Ltd. An exploration and development option agreement was made in January 1972 with Initial Developers Corporation Limited, an associate company. Work by Initial on the Lorna, DM and HO groups included geological mapping, magnetometer, electromagnetic and induced polarization surveys over 48 line kilometres, a geochemical survey, 2286 metres of diamond drilling in 25 holes, and 2011.6 metres of percussion drilling in 22 holes. Work on adjacent groups (EB, ID and RO 47-52) owned by Initial included magnetometer, induced polarization and electromagnetic surveys over 16 line kilometres. This work "indicated some 2,267,750 tonnes mineable reserves assaying 0.5 per cent copper in the Crescent zone" (Western Miner, October 1972, page 85). In December 1972, Getty Mining Pacific, Limited optioned 120 contiguous claims in the combined properties (Victor, Iron Mask, Iron Cap and DM) held by Comet, Initial and Davenport. Work by Getty in 1973 included an induced polarization survey over 93 line kilometres, a magnetometer survey over 86.9 line kilometres, 2084.5 metres of rotary drilling in 8 holes, 564.4 metres of diamond drilling in 2 holes, and 15,513.4 metres of percussion drilling in 159 holes. Among these holes, 25 were spaced over a 3 by 4.8 kilometre area. The option was given up in 1974. Davenport Oil & Mining changed its name in 1973 to Davenport Industries Ltd. Initial Developers Corporation in May 1974 amalgamated with North Pacific Mines Ltd. under the name Initial Developers Limited. Canadian Superior Exploration Limited optioned the above combined properties (some 2185 hectares) in 1975. Work in 1976-78 included a magnetometer survey over 89 line kilometres, 8064 metres of diamond drilling in 47 holes and 4211 metres of percussion drilling in 48 holes. The option was terminated in 1978. Craigmont Mines Limited in February 1981 obtained an exploration agreement on the above four properties and on the adjacent Rainbow property (092INE028) of Pacific Seadrift Resources. Craigmont completed some 5791.2 metres of diamond drilling in a 7 month program. This work indicated insufficient tonnage and the option was terminated in September 1981. Comet Industries Ltd in 1983 carried out exploration on the combined Lorna, Iron Cap and DM properties, including magnetometer and electromagnetic surveys over 13 kilometres and 294 metres of diamond drilling in 3 holes. Crescent zone reserves were reported as possible 1,300,000 tonnes at 0.51 per cent copper (Initial Developers Limited, Filing Statement 164/86). This property, and the adjacent Iron Mask, Victor and Iron Cap properties were owned in 1986 by Comet Industries Ltd. (40 per cent), Davenport Industries Ltd. (30 per cent) and Initial Developers Limited (30 per cent). Work reported on the Lorna and DM in 1986 included geochemical sampling (50) and 209 metres of diamond drilling in 2 BQ size holes. Teck Corporation and Metall Mining Corporation, through Afton Operating Corporation, in August 1987 obtained a two year option agreement to earn a 60 per cent interest in the DM property (5 Crown grants, 317 hectares) by placing the property in production. Work in 1987-88 included 1968 metres of diamond drilling in 23 holes and open-pit development. Reserves were reported as 1,070,000 tonnes at 0.46 per cent copper 0.206 gram per tonne gold (Mineral Exploration Review, 1988, page 29). Production began in October 1988, the ore being hauled to the nearby Afton mill. The Crescent deposit was mined by open-pit methods from October 1988 to March 1989 and was part of the Afton mine operations. See Afton mine (092INE023) for production figures.
In 2004, Abacus Mining conducted a very aggressive exploration program on their large Afton area property. Work included a large 3-D induced polarization survey and more than 27,000 metres of drilling, split between the Rainbow deposit (092INE026), the DM-Audra-Crescent area, and the Coquihalla zone. Abacus’ work in this area in 2004 had several encouraging drillholes including 308.9 metres grading 0.42 per cent copper and 0.20 gram per tonne gold in a hole through the DM deposit, and 98 metres grading 0.45 per cent copper and 0.60 gram per tonne gold in a hole at the Audra (Exploration and Mining in BC 2004, page 58). The latter is located between the DM deposit and the Crescent pit (092INE026), where Teck mined a small amount of ore in 1988-89. On the DM zone, drilling by the company and previous operators has defined a zone of copper-gold mineralization up to 200 metres in depth within an area 150 metres by 350 metres. The zone is open on three sides and at depth. The historical drilling indicates that two known mineralized DM intrusive breccia bodies appear to be coalescing at depth. On a larger scale, geophysical induced polarization anomalies also indicate that the DM, Audra and Crescent zones may also join at depth.
The following resource figures were reported in the Northern Miner on July 7, 2005. The DM-Audra-Crescent area hosts a drill-indicated resource of 59.9 million tonnes at 0.217 per cent copper and 0.116 gram gold at an 0.1 per cent copper cutoff grade. This drops to 6.3 million tonnes at 0.437 per cent copper and 0.246 gram gold at a cutoff grade of 0.35 per cent copper. The DM-Audra-Crescent area also hosts an inferred resource of 48.9 million tonnes at 0.186 per cent copper and 0.081 gram gold at a cutoff grade of 0.1 per cent copper, or 2.7 million tonnes at 0.393 per cent copper and 0.18 gram gold if the cutoff is raised to 0.35 per cent.
See DM (092INE030) and Rainbow (092INE028) for further details.