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 Mark or Valenciennes River 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 (Assessment Report 20580).
Four diatremes and a series of subparallel crosscutting dikes comprise the Mark occurrence. Breccia dikes crop out at the northern end of the area. The diatremes and dikes intrude subhorizontal carbonate country rocks which are strongly foliated, as are the diatremes. The diatremes are identified as tuff breccia, crystal-lithic ash-lapilli tuff and lithic ash-lapilli tuff (Assessment Report 20580).
The two southern diatremes are foliated at the margins and massive in the core. The rock is rusty weathered with a pale green fresh surface. Angular fragments of carbonates, shales and a few quartzites comprise 30 per cent of the rock volume. Altered spinel peridotite xenoliths occupy 1 to 3 per cent of the rock volume. Their modal size is 2 centimetres, though 15-centimetre clasts are present. Altered brown olivines and dark green spinels each make up about 2 per cent of the rock.
The two northern diatremes are narrow and smaller, and do not exhibit the variety of clast types that characterize the larger southern ones. They are well foliated with angular clasts comprising 20 per cent of the rock volume. Dark green spinels are sparsely distributed.
Thin sections show the diatreme phase is tuffaceous with rounded and fractured quartz grains, autolithic fragments and sedimentary fragments. Locally it contains 40 per cent polymorphous inclusions ranging in size from 0.1 to 60 millimetres consisting of serpentine, serpentine and calcite, or calcite and quartz. Fractured red-brown spinels, round or angular, are present in trace amounts in the groundmass and within the polymorphous inclusions. The groundmass is composed of a dusty carbonate, spinels and pyrite (Exploration in British Columbia 1988, page B43).
A dozen dikes, 1 to 2 metres wide, outcrop in the area. They are generally subparallel, though locally crosscutting and they cut the diatremes. The dikes are also subparallel to foliation in the hostrocks. The dikes differ from the diatremes as they contain very few foreign fragments and are cut by quartz and calcite veins. The dikes are porphyritic in hand sample with characteristic sieve- textured brown olivine pseudomorphs, altered euhedral clinopyroxenes, fine-grained micas and rare spinels.
In 1983, a 30-kilogram portion of a 160-kilogram bulk sample of a diatreme produced one ilmenite and thirteen chromites and one 0.00015820-carat microdiamond fragment (Assessment Report 13596, page 12). Further examination, sampling, processing and analysis in 1986, 1988 and 1990 has identified ilmenite, chromite and garnet but failed to reveal or substantiate the presence of macro or microdiamonds (Assessment Reports 15151, 17753 and 20580). However, scanning electron microscope (SEM) studies identified corundum - several blue sapphires were present in the fused concentrates of three samples of diatreme material (Assessment Report 20580, page 11).
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).
