Gross Brukkaros

Gross Brukkaros is a ring-shaped structure with a 2 km broad central volcanic vent of tuffs. The circular rim is built of domed sandstones of the Fish River Series, the dome having a diameter of some 8 km. The flat-lying sandstones start to dip away from the mountain about 2 km from the ring and they reverse to inward dips near the ring, so defining a circular anticlinal structure. The inward dipping sandstones are overlain by 150 m of shales and these in turn by rocks referred to by Janse (1969) as microbreccias, which consist mainly of bedded, finely fragmented materials. The central area is occupied by unstratified microbreccias (tuffs). The layered microbreccias, which are illustrated by Janse (1969) and Ferguson et al. (1975b), display graded- and cross-bedding and cut and fill structures. They comprise a wide variety of fine-grained mineral and rock fragments cemented by carbonate, K-feldspar, hematite and limonite. Mineral grains, in order of abundance (Ferguson et al., 1975b), are quartz, plagioclase, K-feldspar, colourless pyroxene, aegirine, calcite and dolomite, while rock fragments include feldspathic quartzite, shale, chert, limestone, microbreccia and rare syenite, granophyre and granite, the last comprising alkali feldspar, quartz and aegirine. One variety of breccia is rich in carbonate as poikilitic plates up to 3 cm in diameter and as patches and veins 1-4 cm wide containing abundant aegirine needles. Concentrates from microbreccias yielded rare kimberlitic ilmenite and heavy mineral concentrates from soils within the crater kimberlitic pyrope and chrome diopside. All the microbreccias have thin veins and vugs of calcite, dolomite, quartz, opaline silica, chert, barite and zeolite. Using 20 chemical analyses of the sediments and microbreccias Ferguson et al. (1975b) concluded that the microbreccias have been fenitized, with the addition notably of K, P and C and a range of trace elements. Stachel et al. (1995) have described juvenile carbonatite lapilli from the epiclastic caldera fill. The lapilli are of calcio- and magnesiocarbonatite for which they provide microprobe data as well as bulk rock analyses of the sediments and their pyroclastic and metasomatised components. Numerous dykes radiate from Gross Brukkaros. They are only about 2-25 cm in width but up to 5-6 km in length. They consist of calcite, dolomite and ankerite with fragments of shale and in places granite and granite gneiss. Stachel et al. (1994), as a result of detailed investigations of the stratigraphy of the crater fill, attribute the genesis of the dome structure to depletion of a shallow intrusion (laccolith-shaped) by volcanic activity, subsidence and then infilling of the depression by a complex series of sedimentary processes. About 45 satellite vents occur on the flanks and at the foot of the mountain and they are filled with coarse breccias cemented by calcite with varying proportions of olivine, phlogopite and carbonate which, according to Ferguson et al. (1975b), can be divided into carbonatitic and kimberlitic vents. Lorenz and Kurszlaukis (1997) describe in detail an explosive carbonatite pipe. One vent at Blue Hills, described by Janse (1971), is a monticellite-bearing peridotite consisting of olivine phenocrysts and aggregates of magnetite up to 1.5 cm in diameter in a groundmass of monticellite, nepheline, olivine, magnetite, perovskite and calcite. The geochemistry and mineralogy of Blue Hills has been investigated in more detail by Kurszlaukis and Franz (1997) who distinguished monticellite picrite, phlogopite-carbonate picrite, mica-olivine-carbonate picrite, late sills of phlogopite-carbonate rock and dykes and sills of carbonatite. Further analyses of this rock are in Janse (1971 and 1975) and Ferguson et al. (1975b). Verwoerd (1967) and Janse (1975) include analyses of beforsite dykes.