Saltpeterkop (Saltpetre Kop)

Saltpeterkop is a conical hill (De Wet, 1975, Plate 2) rising 250 m above the surrounding country and composed of breccias that occupy a vent with a diameter of 1x1.5 km. Beds of the Beaufort Series are nearly horizontal in the region but at about 4 km from the vent begin to tilt away and this inclination increases until it is close to vertical near the contact. This produces a spectacular ring structure which has been described in some detail by Verwoerd (1990, Figs 1 and 4), who interpreted the structure as a dome overlying an alkaline-carbonatite ring complex which is obscured by a volcanic superstructure that has only been partly eroded. Around the vent are numerous smaller pipes, du Toit (1954) says there are 19, and many dykes mostly filled with breccia and tuff of which, according to du Toit, there are 46. Rock types represented include trachyte, carbonatite, olivine melilitite and pyroclastic kimberlitic rocks. The Saltpeterkop vent is filled with lithic pyroclastic rocks amongst which Karoo rocks predominate with some quartzite and dolerite; xenocrysts of hornblende, ilmenite and biotite are rare. Blocks are up to a metre across but most are tuff-sized and cemented by silica or iron oxides, but in the vicinity of carbonatite outcrops the matrix is calcite. At Silver Dam, 3 km west of Saltpeterkop, three small breccia pipes, up to 30 m diameter, contain a distinctive breccia with a high content of coarse ferromagnesian minerals and ilmenite (De Wet, 1975). Olivine melilitite forms dykes and, less commonly, sills in which phenocrysts of olivine up to 5 mm across are prominent as is phlogopite up to 2.5 cm. Other major phases are melilite, as phenocrysts and in the groundmass, and spinels with chromite cores surrounded by titanomagnetite; minor phases include diopside, Ti-rich garnet, nepheline, sodalite, perovskite, apatite and calcite. Olivine phenocrysts may have rims of either monticellite, spinel or Ba- and Ti-bearing micas (Boctor and Yoder, 1986). Rock and mineral analyses will be found in McIver and Ferguson (1979) and Boctor and Yoder (1986). Trachyte forms dykes, some of which are choked with country rock inclusions, and consists of sanidine phenocrysts in a matrix of feldspar microlites, iron oxides, a little nepheline, apatite, patches of fluorite and scarce biotite. The trachytes are strongly potassic, as indicated by analyses in De Wet (1975) and McIver and Ferguson (1979). Carbonatites are represented by thin dykes and by a larger body on the western side of the Saltpeterkop vent. Four types are distinguished by De Wet (1975), namely sovite, ankeritic carbonatite, which is a local facies of the sovite, alvikite, the most widespread variety, and biotite metacarbonatite, which is a biotite-rich rock extensively replaced by calcite. The sovite consists essentially of calcite with dolomite forming tiny inclusions in the calcite and larger patches along grain boundaries; pyrochlore is also present. The same carbonates occur in the alvikites together with abundant biotite, feldspar, which is probably xenocrystal, melilite, which lines vugs in some dykes, and occasional wollastonite and barite. Verwoerd et al. (1995) describe a mineralized zone enriched in rare elements and an extensive suite of minerals in which these elements are concentrated. Rock and mineral analyses are in the references already quoted and in Duncan et al. (1978).