The Cromwell workings are on the south side of North Brown Creek. They are at 1850 to 2250 metres elevation on the east side of Silver Cup Ridge, between the headwaters of Brown Creek and North Brown Creek, both of which drain to the northeast into Lardeau Creek. The property consists of eight reverted crown granted mineral claims (Cromwell, 1 & 2 (L.13045/6); Wellington (L.13053), Nelson (L.13050), Tarzan 1 & 2 (L.13048/9), Golden Reef (L.13044) and Tarzan Two (L.13047)). The tenures are on strike with, and approximately 1.5 kilometres to the southeast of, the Triune [082KNW026] vein system. The Cromwell veins were found in the early 1900s and the ground was leased and developed by Myee Exploration Company in 1902. Exploration development and it production a small tonnage of ore in 1900 and 1901. The Cromwell vein was first mentioned in 1900, when good results "$123/ton gold at $20/oz" were reported from oxidized quartz samples collected near-surface. The vein was later trenched and had two adits driven on it. The upper adit was driven at shallow depth to exploit the oxidized ore and the lower adit was collared to access fresher, more pyritic, rock. The latter was driven 170.6 metres to intercept the vein 152 metres below surface; however, it may not have gone far enough. A 2.9 tonne sample from the first 12.2 metres of the upper adit assayed 19.9 grams per tonne gold and 68.6 grams per tonne silver. In the 1920s, the property was acquired by Major Armstead, of New York, who extended the lower tunnel looking for the vein. The Tarzan vein has been trenched, and there is a small shaft. The vein has been traced about 30 metres. In 1953, Cromwell Gold Mines owned and produced 3 tonnes of ore from the vein. In 1981, the property was owned by Mission Minerals Limited.
The Trout Lake area is underlain by a thick succession of sedimentary and volcanic rocks of the Badshot Formation and Lardeau Group near the northern end of the Kootenay arc, an arcuate, north to northwest trending belt of Paleozoic and Mesozoic strata that is now classified as a distinct, pericratonic, terrane. The arc rocks are bordered by Precambrian quartzite in the east and they young to the west, where they are bounded by Jurassic-age intrusive complexes. They were deformed during the Antler orogeny in Devonian-Mississippian time and were refolded and faulted during the Columbian orogeny, in the Middle Jurassic. A large panel, the "Selkirk allochthon", was later offset to the northeast by dip-slip motion along the Columbia River Fault.
The Badshot Formation is composed of a thick Cambrian limestone that is a distinctive marker horizon in the Trout Lake area. It is underlain by Hamill Group quartzite, and it is overlain by a younger assemblage of limestone, calcareous, graphitic and siliceous argillite and siltstone, sandstone, quartzite and conglomerate, and also mafic volcanic flows, tuffs and breccias, all of which belong to the Lardeau Group. The rocks are isoclinally folded and intensely deformed, but only weakly metamorphosed. They occur as intercalated beds of marble, quartzite and grey, green and black phyllite and schist. Fyles and Eastwood (EMPR BULL 45) subdivided the group into six formations (Index, Triune, Ajax, Sharon Creek, Jowett and Broadview) of which the lowermost (Index) and uppermost (Broadview) are the most widespread. The Triune (siliceous argillite), Ajax (quartzite) and Sharon Creek (siliceous argillite) are restricted to the Trout Lake area. The Jowett is a mafic volcanic unit.
There are several veins on the Cromwell property but only two, the Cromwell and Tarzan "quartz fissure veins" are of economic significance. The Cromwell vein is on the Cromwell #2 claim, which straddles North Brown Creek downstream from the Tarzan #1 claim. The creek exposes a partial section through the Lardeau Group and includes rocks of the Triune, Ajax and Sharon Creek Formations. In the early literature, the Ajax quartzite was commonly referred to as the "Cromwell Dyke". In the Cromwell vein area, the rocks are phyllites, carbonaceous shales and phyllites, and schistose quartzites of the Index and Ajax Formations. They are folded, highly deformed and schistose, and bedding and schistosity display the northwest strike and moderate to steep northeast dip commonly found in that part of the Silver Cup anticline. The Cromwell vein is in black slate and quartzite and it contains fragments of locally derived, silicified country rock. The vein has smooth walls and is markedly discordant to stratigraphy and structure. It strikes 015 degrees and dips 74 degrees to the east. Where exposed in the upper adit, it ranges from 0.15 to 1.2 metre in width. It averages 0.76 metre in width in the first 12.2 metres and then tapers to 0.38 metre at the face. The vein consists of white quartz with disseminated pyrite and local, scattered patches of galena and chalcopyrite. Where weathered, the vein contains traces of free gold. The sulphide is unevenly distributed and there is a considerable amount of low-grade mineralization with the high-grade. In 1953, a sample collected across 0.38 metre assayed 7.9 grams per tonne gold and 35 grams per tonne silver. At the same time, one taken over 2.1 metres, approximately 100 metres away, assayed 2.06 grams per tonne gold and 3.4 grams per tonne silver (EMPR AR 1953).
The Tarzan vein is 500 metres to the southwest. It is a quartz filled shear on the Tarzan #1 claim, which covers part of a pot-hole lake at the head of North Brown Creek, near the divide. The tenure straddles a northwest trending fault that separates black argillaceous phyllites of the Sharon Creek Formation, to the northeast, from a mix of Jowett volcanic rocks and green, gritty phyllites of the Broaview Formation, to the southwest. The vein is discordant, has a 040 strike and a 55 to 60 degree dip to the southeast. It is 0.61 metre wide and approximately 30 metres long. It is composed of white quartz with disseminated pyrite and patches of galena and chalcopyrite. The surface rocks are fresh and unoxidized. A sample over 0.5 metre assayed 40.1 grams per tonne gold and 68.6 grams per tonne silver (EMPR Annual Report 1953) and a 2.5 kilogram sample collected from the dump in 1981 assayed 11.21 grams per tonne gold (EMPR Ass RPT 9872).
There are several other quartz veins on the property, particularly on the Tarzan #1 claim. They are up to 0.3 metre wide. Some follow the schistocity of the surrounding rocks and are lens-like and short. Others, follow discordant shears zone and joint planes. There is a notable highly serpentinized greenstone dyke approximately 300 metres north of the Cromwell No.2 vein. It has, intermittently, been examined for its asbestos content.