Bentonite and coal were mined along the old Copper Mountain railway, on the southeast bank of the Similkameen River, across from the town of Princeton.

The deposits lie near the eastern margin of the Princeton Basin, a northerly striking fault-bounded trough filled by Eocene volcanic rocks of mainly intermediate composition comprising the Lower Volcanic Formation, and an overlying Eocene sedimentary sequence of sandstone, shale, waterlain rhyolite tephra and coal, up to 2000 metres thick, comprising the Allenby Formation.

Bentonite occurs in a sequence of sandstone and shale, with minor coal, known informally as the Summers Creek sandstone, in the Allenby Formation (Princeton Group) (Open File 1987-19). Coal occurs in one zone within the Summers Creek sandstone, the Gem-Bromley Vale coal zone. This is one of four significant coal-bearing zones contained in a 530-metre section of the Allenby Formation. This coal zone was also mined at the Granby Company's No. 1 mine (092HSE215), on Bromley Creek to the southwest.

A railway cut exposes a bentonite seam, 4.3 metres thick, overlain and underlain by 0.3-metre thick seams of lignite. The bentonite seam is comprised of the following: a bed of yellow clay 0.9 to 1.2 metres thick, overlain by a thin seam of lignite, followed by 2.1 to 2.4 metres of yellow-green clay. The section is capped by 0.9 metre of brownish clay. The beds strike 089 degrees and dip 20 degrees south. The bentonite outcrops at a slightly higher elevation about 460 metres to the east, along the south wall of the railway cut. Diamond drilling to the southeast, on Lot 2049, encountered bentonite seam thicknesses of between 3 and 3.4 metres. A sample of bentonite analyzed as follows (in per cent) (CANMET Report 626, page 14):

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SiO2 68.60

Al2O3 12.10

Fe2O3 2.00

FeO 0.32

CaO 1.84

MgO 1.84

TiO2 0.14

H3PO4 0.17

Na2O 0.50

K2O 0.23

S nil

H2SO4 0.61

CO2 0.17

C 0.08

H2O (105 C) 7.71

H2O (>105 C) 3.24

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A 1.1-metre thick coal seam (Gem seam) occurs near the west end of the bentonite outcrops. The coal is exposed in the mine, below the railway grade, and along the railway cut above. It appears to underlie the thick bentonite section exposed to the east. The seam dips 15 degrees southwest and contains 0.86 metre of clean coal. Underground workings have followed the seam downdip for 60 metres and along strike for 180 metres. The coal is overlain by sandstone and underlain by carbonaceous shale and bentonite.

The underlying bentonite bed is 1.5 metres thick and comprised of calcium-rich montmorillonite. The bed strikes 126 degrees, dips 6 degrees south, and is underlain by pebble conglomerate. Exchangeable cation analyses and cation exchange capacity (CEC) in milli- equivalents per 100 grams on a sample from a freshly exposed river cut are as follows (Open File 1987-19):

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Sample Magnesium Calcium Potassium Sodium CEC

C86-345C 4.1 36.7 12.4 6.2 40.6

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Princeton-B.C. Colliery Company Ltd. and various other operators mined 771 tonnes of bentonite between 1926 to 1944 (Bulletin 30, page 34). Seven holes totalling 466 metres were drilled for bentonite in 1952 by Princeton Properties Ltd.

The Gem coal seam was first developed in 1921 by the Princeton Coal and Land Company Ltd., operator of the nearby Princeton Colliery (092HSE089) to the northeast. The company produced a small amount of coal in 1922. The mine was reopened and operated briefly during 1929 by Gem Domestic Coal Company Ltd. and the company produced 547 tonnes of coal.