The Larry showing is located 90 kilometres north-northwest of Golden, immediately west of the British Columbia-Alberta border. Access to the area is gained via the northern end of the Bush River logging road.
Numerous diatremes are located along the British Columbia- Alberta border between 50 to 90 kilometres north of Golden. Most of the diatremes are hosted in Upper Cambrian carbonate rocks and in most cases, comagmatic dykes are also present. Micro diamonds are reportedly recovered from heavy mineral separates taken from two breccia pipes in this swarm. Preliminary investigation of these rocks suggests that they are neither kimberlites nor lamproites (Open File 1987-17).
Three petrologically, geographically and temporally distinct suites of ultrabasic diatremes can be recognized in British Columbia. The first suite, examples of which are located in the area north of Golden, including the Larry showing, and in the Ospika area, is characterized by macrocryst-rich breccias and dykes. The macrocrysts consist of titaniferous augite, phlogopite, green diopside, spinel and olivine with either augite or phlogopite in most abundance. In some cases, microphenocrystic melilite is present in small amounts. These rocks are tentatively classified as ultramafic lamprophyres; those with melilite are alnoites; those without are aillikites (Open
Breccia pipes commonly contain multiple phases of intrusion characterized by variable proportions of xenoliths, macrocrysts, and accretionary or pelletal lapilli (Open File 1987-17).
Rubidium/strontium age dates of 334+/-7 and 348+/-7 Ma have been obtained from phlogopite separates from two of these pipes. They indicate emplacement in Devono-Mississippian time (Open File 1987-17).
At the Larry showing a diatreme dyke pipe swarm containing diamond indicator minerals intrudes gently folded Upper Cambrian to Lower Ordovician McKay Group and Upper Ordovician to Lower Silurian Beaverfoot Formation marine sediments. Lithologies include siltstone, shale, limestone and dolomite. These sediments have been intruded by a complex, north trending system of diatreme pipes and dykes continuous over at least 1200 metres length and up to 120 metres width on the Larry 1 claim. An additional diatreme breccia pipe, 300 metres long by 509 metres wide, is located on the Larry 2 claim (Assessment Report 17752).
Three diatremes were examined by J. Pell in 1986 revealing two breccia types. The first is a rusty weathering, clast-dominated megabreccia. The clast to matrix ratio is about 3 to 2. Over 99 per cent of the clasts are subrounded to subangular fragments of the hosting carbonate lithologies. The average size is 10 to 40 centimetres. Altered granitoids and less commonly, gabbroic rocks comprise the remainder of the xenolith population. The matrix is predominantly carbonate and quartz sand grains (Open File 1987-17).
The second breccia type is a clast dominated breccia; also massive, but is rusty to dark green weathering. The clast to matrix ratio is greater than the first breccia type. Clast population is more varied with approximately 50 per cent of the clasts being subangular sedimentary rock fragments of carbonate, shale and orthoquartzites. Two to 5 per cent contain angular to rounded, granitic material 5 to 15 centimetres across. Rounded, 8 to 25 centimetre-sized clasts comprise 10 to 20 per cent of the xenoliths. These clasts consist of coarse, randomly oriented carbonate grains, chrome mica and opaque oxides. An additional 5 to 10 per cent of the clast population is made up of accretionary or pelletal lapilli fragments ranging from a few millimetres to 3 centimetres across. The matrix of the breccia consists of chlorite > calcite > quartz >> trace apatite. Silvery mica macrocrysts, up to 3 centimetres across, are abundant (Open File 1987-17).
Both homogeneous and zoned dykes are common thoughout the area.
A total of four bulk samples of diatreme material have been collected from the showing and analyzed for diamonds and diamond indicator minerals. Microdiamonds were not found in the samples analyzed. The mineralogy of the diatremes was determined to be chromium diopside, chromite, G5 (eclogite and/or primitve mantle source) magnesium almandine garnet, magnetite, grossular garnet, forsterite and clinopyroxene. The presence of G5 garnets, in particular, is consistent with a diamond paragenesis. But the compositions of the other indicator minerals are known to occur in either diamondiferous or nondiamondiferous lamproite or nondiamondiferous kimberlite. G5 garnets are in some instances found as inclusions in diamond, therefore can form at the same pressure and temperature conditions as diamonds (Assessment Report 17752).
Microscopic examination of sample 36 from the Larry 1 claim concluded that the sample consisted of juvenile lapilli in which some areas appear to be welded. Xenoliths of probable country rock are also present. Numerous phenocrysts of phlogopite comprise the only discernible primary material (Assessment Report 17752).
