The Oro Viejo dolomite occurrence outcrops on both sides of the Goldstream River near its confluence with the Columbia River, approximately 78 kilometres north-northwest of Revelstoke.

The region is underlain by a northwest trending succession of metamorphic rocks of the Lower Proterozoic Horsethief Creek Group, the Lower Cambrian Badshot Formation and the Lower Paleozoic Hamill and Lardeau groups. This sequence is intruded by post-Cambrian granitic masses.

A body of dolomite comprising the Badshot Formation trends north-northwest for 7 kilometres, crossing the Goldstream River a kilometre east of the Columbia River. The unit is underlain by banded light grey to dark grey limestone of the Horsethief Creek Group and overlain by silver grey phyllites and carbonaceous slate of the Lardeau Group. The entire succession dips gently to the west. An average foliation strikes 090 degrees an dips 36 degrees north. Local thickening of the Badshot Formation may be caused by a decollement.

The deposit consists of snow white, microcrystalline, earthy to chalky, massive dolomite displaying zones of tectonic brecciation along strike. A zone of high purity dolomite (95 to 99 per cent dolomite) occurs over a 4-kilometre strike length with a width of 500 metres in the north and over 1500 metres in the south. Outcrops at various elevations, suggest that the high purity zone extends down dip for a vertical depth of at least 548 metres. This zone is estimated to contain 300 million tonnes of dolomite over an average width of 1000 metres to a depth of 30 metres, with over 25 million tonnes grading at least 98 per cent dolomite (Assessment Report 18028, p. 11). Twenty-seven grab samples, collected in 1987 over a 7.3-kilometre strike length, displayed the following average and range in compositions (in per cent) (Assessment Report 16604, p. 10a):

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Average Range

From To

CaO 33.98 29.94 49.66

MgO 18.15 4.59 21.72

SiO2 0.41 0.04 4.06

Al2O3 0.15 0.02 1.28

Fe2O3 0.21 0.09 0.60

MnO 0.024 0.006 0.090

TiO2 0.023 0.02 0.07

Na2O 0.013 0.003 0.100

K2O 0.018 0.001 0.308

Ig. Loss 46.21 42.25 47.15

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Nineteen of the samples contained greater than 21 per cent MgO and seventeen contained less than 0.10 per cent SiO2. Of an additional thirteen samples collected in 1988, nine contained greater than 20 per cent MgO (Assessment Report 18028).

This property was mapped and sampled by R.G. Komarechka in 1987 and by G.C. Hurlburt in 1988.