The Big Onion property lies on the north edge of the Late Triassic-Early Jurassic Iron Mask batholith. The Big Onion deposit occurs along the contacts between Pothook unit diorite and Upper Triassic Nicola Group volcanic rocks which have been intruded by Sugarloaf unit diorite. The deposit is hosted in Sugarloaf unit diorite where alteration is characterized by an early mineralized, pervasive albitic alteration, crosscut by fracture-controlled K-feldspar-biotite-magnetite-chalcopyrite veins. The albitic alteration and K-feldspar veins control mineralization. Picrite also occurs in close proximity to the deposit and does not play an active role in mineralization, but does denote the presence of major faults.
There are at least four main areas of workings on the property: the Larsen shaft, Night Hawk shaft 46 metres south of Larsen, Bonnie Etta shaft 335 metres east of Larsen, and Norma shaft 244 metres south of Larsen. In the Larsen workings the main or No. 1 zone of mineralization strikes westerly in diorite on the north side of a faulted contact with picrite basalt which is altered to pyroxene, hornblende and biotite. The diorite is brecciated and flooded with K-feldspar-magnetite-chalcopyrite veins. Chalcopyrite veins range up to 15 centimetres or more. Gangue mineralogy also includes calcite, epidote and gypsum with occasional well-crystallized fluorite, prehnite and zeolite. Talc and gypsum occur on slips in the diorite. Locally on the Night Hawk claim (Lot 1747) diorite contains disseminated chalcopyrite. The Larsen shaft is located on the Night Hawk claim and was deepened from 10.6 to 27.4 metres slope depth in 1951. In 1952, the shaft was again deepened, a first level station cut at 27.4 metres slope depth, and a drift run west for 19.8 metres. Minor chalcocite is reported to occur in material on the dump.
The Night Hawk shaft is located on Lot 5622 (Champion No. 1) about 46 metres south-southeast of the Larsen shaft and exposes diorite mineralized with disseminated chalcopyrite.
The Bonnie Etta workings (adit and shaft) are on an easterly striking fault zone about 1.8 metres wide located on Lot 5623 (Champion No. 2 Fr.) about 335 metres east of the Larsen shaft. The workings are in diorite intersected by K-feldspar and chalcopyrite veins. In 1929, 30 tonnes of ore was shipped from the Bonnie Etta claim yielding 249 grams gold, 809 grams silver and 2897 kilograms copper.
The Norma shaft is on Lot 1066 (Erin Fr.) about 244 metres south of the Larsen. This shaft is sunk vertically to a single level 61 metres below the collar. A raise connects this level with the Iron Mask 228 metre level (092INE010), which is estimated to be 49 metres lower in elevation. The Iron Mask, Erin and Norma workings are interconnected.
On the Big Onion property, reserves are stated to be 3,266,000 tonnes grading 0.71 per cent copper and 0.44 gram per tonne gold (Lang, pers. comm., 1994 in CIM Special Volume 46).
The property is located west and south of Bowers Lake, about 7 kilometres west of Kamloops. The claims adjoin the Iron Mask mine (092INE010) on the north. The Bonnie Etta claim was staked in about 1897 and Crown granted (Lot 881) to J. Armstrong and B. Wallace in 1902. The workings include an adit 16.7 metres long and a shaft of unknown depth. Ore was shipped from these workings in 1929. The Victor (Lot 1340) and Norma (Lot 1302) claims were Crown granted in 1905 to The British Columbia Trust Co. Ltd. The Night Hawk claim (Lot 1747) was owned in 1899 by O.S. Batchelor and J.C. McLaren and reported to be under bond to G.F. Monckton, representing English capital; the workings at that time included a 15 metre adit. By 1905 the claim was owned by J. Fenton and under option to Messrs. Gwin, Shaw, Jowell and McGee. A shaft (Larsen shaft) had been sunk and a drift run on the vein. The claim was Crown granted in 1915 to J.F. Shaw. Kamloops Copper Company Ltd. was incorporated in October 1951 to acquire about 40 claims, including the above Crown grants and the adjacent Iron Mask property. Early in 1952 Berens River Mines Limited, a Newmont Mining Corporation subsidiary, optioned the property. The Larsen shaft, inclined 60 degrees south, was deepened to 30 metres. At 27 metres a drift was driven westerly for 19.8 metres and 121.9 metres of diamond drilling done from the face. An electromagnetic survey was carried out and four surface diamond-drill holes put down in the vicinity of the Larsen and Bonnie Etta workings. This work failed to find ore and the option was dropped. In 1956, the company dewatered the Larsen shaft and from the 27 metre level drove a crosscut southwesterly for 88 metres. Drifts were run 30 metres east and two others, 19.8 metres and 41 metres west, respectively. Four holes totalling 426.7 metres were diamond drilled from the end of the crosscut. An electromagnetic survey was carried out over an area measuring 1829 by 1829 metres. The company name was changed in 1964 to Kamloops Copper Consolidated Ltd. Vanco Explorations Limited optioned this and a number of adjacent properties in September 1965. A ground magnetometer survey (62.7 kilometres) and diamond drilling were carried out until May 1966 when the option was terminated. The company, Kamloops Copper Consolidated, amalgamated with Midland Petroleums, Ltd. and Consolidated Prudential Mines Ltd., in February 1972 to form Davenport Oil & Mining Ltd. In March 1972, an exploration and development option was given to Comet Industries Ltd. and a subsidiary company, Initial Developers Corporation Limited. A large anomaly was outlined by an induced polarization survey and confirmed by magnetometer and electromagnetic surveys. The anomaly, referred to as the Big Onion zone, is located between Ironmask and Bowers lakes on Lots 1302 (Norma) and 1342 (Mint Fr.) and extends from the Davenport property westerly into the adjoining property of Comet Industries. Drilling to October 1972 indicated reserves of 3,628,400 tonnes averaging 0.6 per cent copper (Western Miner, October 1972, page 85). The 1972 work included 1981.2 metres of surface diamond drilling in 13 holes, 365.7 metres of rotary drilling in 2 holes, and 1280.1 metres of percussion drilling in 12 holes. In December 1972, Getty Mining Pacific, Limited optioned 120 contiguous claims in the combined properties (Victor, Iron Mask, 092INE010, Iron Cap, 092INE018 and DM, 092INE030) held by Comet, Initial and Davenport. Work by Getty in 1973 included an induced polarization survey over 93.3 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 3.2 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 and the adjacent Rainbow property for a total of some 2185 hectares. 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 5579 metres of diamond drilling in 34 holes. This work indicated insufficient tonnage and the option was terminated in September 1981. Kamloops Copper in 1972 amalgamated with others to form Davenport Industries Ltd. Ownership of the property was subsequently held by Davenport (30 per cent) and associate companies Comet Industries Ltd. (40 per cent) and Initial Developers Limited (30 per cent). Work by the owners in 1983 included geophysical surveys over 13 kilometres and diamond drilling. Work to date by owners and optionees has been interpreted to give the Big Onion zone an estimated reserve of 2,400,000 tonnes at 0.84 per cent copper, 0.4 gram per tonne gold and 8 grams per tonne silver (Initial Developers Limited, Filing Statement 164/86). Teck Corporation and Metall Mining Corporation, through Afton Operating Corporation, in August 1987 obtained from Davenport, Comet and Initial a 10 year exploration option on the Victor property and adjacent ground.