Alkaline diatremes and dikes of probable Paleozoic age occur in three areas of southeastern British Columbia. The Ospika River diatreme in the north, the central Golden cluster and the Cranbrook-Bull River group in the south. The cluster of diatremes and associated crosscutting dikes northeast of Golden is situated within a Cambro-Ordovician stratigraphic and structural unit and may be coeval. There are five diatremes in the Golden cluster and are named: Bush River (Larry), Lens Mountain (Jack, 082N 088), Mons Creek (Mike), Valenciennes River (Mark, 082N 089) and HP (Exploration in British Columbia 1988, page B39).
Sedimentary rocks in the Jack or Lens Mountain occurrence area consist of an upper dolomite sequence, a middle limestone and shale sequence, and a lower massive limestone unit. These may correlate with the lower part of the Middle Ordovician Skoki Formation, the Lower Ordovician Outram Formation and the uppermost part of the Lower Ordovician Survey Peak Formation. All of these units are characterized by a well defined, moderately to steeply southwest dipping (60 to 80 degrees), northwest striking axial plane (?) cleavage that is essentially parallel to the axis of a nearby anticline.
The Jack multiphase diatreme underlies a narrow ridge trending northwest between two permanent snowfields. From edge to edge, there is a variation in texture and clast size and clast/matrix ratio. To the southeast, the diatreme is foliated with an orange weathered surface and light green fresh surface; it contains 25 per cent sedimentary rock inclusions ranging in size from 0.2 to 2.0 centimetres consisting of limestone clasts and sand grains. In the saddle of the ridge, the rock is light green and aphanitic with disseminated pyrite and an absence of foreign clasts. To the northwest are alternating outcrops of limestone 30 to 40 metres across, and coarse diatreme material containing 20 per cent subangular limestone clasts averaging 5 to 10 centimetres across. The northern diatreme phases weather dark red with a dark grey fresh surface. The diatremes are breccias, tuff breccias and lapilli tuff breccias.
In thin section, the "sand-grain rich" phase consists of 25 per cent rounded quartz grains, 20 per cent fine-grained carbonate clasts, 5 per cent elongated relict lapilli and 3 per cent subhedral to anhedral, altered grains replaced by calcite and rimmed by very fine grained sphene and opaque minerals. The matrix is fine-grained carbonate.
Thin sections from the saddle contain up to 10 per cent disseminated pyrite, and lapilli rimmed with pyrite. Apatite phenocrysts are altered in the core. The matrix consists of fine-grained carbonate and opaque minerals.
The coarse breccia phase consists of subangular clasts of limestone and relict phenocrysts in a carbonate matrix. This porphyritic rock contains 15 per cent phenocrysts now entirely pseudomorphed by fine-grained quartz and/or calcite. Altered crystals possibly of titanamphibole or annealed recrystallized sphenes have been replaced by calcite but retain a rim and inclusions of very fine grained sphene. The groundmass is extremely fine grained grey material with calcite patches.
In 1983, treatment of seven bulk samples from an upper breccia portion of the diatreme produced pyrope garnets, ilmenites and chromites. More significantly, one 29.5-kilogram bulk sample of "sandy marl" (possibly the sand-grain rich phase mentioned previously) from the diatreme breccia produced an excellent quality octahedral microdiamond weighing 0.00037320 carats (Assessment Report 13597). Further sampling and analysis and diamond drilling in 1985 and 1986 failed to confirm the presence of macro or microdiamonds (Assessment Reports 15289 and 16195).
Petrographic examination does not support the designation of these rocks as either kimberlites or lamproites, two rock types which are mined for diamonds (Exploration in British Columbia 1988, page B39).