The Kruidfontein Carbonatite Complex, South Africa: Geology, Petrology, Geochemistry and Economic potential

Abstract

The Kruidfontein Carbonatite Complex (1243 ± 171 Ma) is an example of an intracontinental caldera system which is related to carbonatite activity and mineralisation. It had a two-stage history, beginning with a cone-building period of predominantly nephelinitic pyroclastics and ignimbrites followed by cauldron subsidence. The second stage was carbonatitic in nature, and was also followed by cauldron subsidence. Today, the eroded volcano comprises an approximately circular carbonatitic inner zone, surrounded by an outer zone of silicate pyroclastics. The cauldron subsidence contributed to the preservation of the inner zone carbonatite sequence. Structures related to the caldera formation are radial and ring faults, along with several mineral occurrences and potential ore deposits. The soil geochemistry, together with the high-resolution radiometric survey, depict several positive anomalies throughout the inner and outer zones. Of these the following are important: (a) the ferruginous lapilli tuft unit in the eastern and north-eastern part of the inner zone; (b) the northern part of the inner zone associated with vents (characterised by vent breccia and proximal debris flow deposits, carbonatite dykes, plugs and faults), and (c) an area towards the southern boundary of the inner zone, characterised by dykes, vents and associated breccia and debris flow deposits. The anomalous Au, Pb and Zn levels are mainly associated with vent breccia and secondary ferruginisation. The strong Ba anomaly occurs in zones of fenitisation and displays a degree of structural control. Also notable is the correlation between high Ba values and the presence of calcite-carbonatite and fluorite-calcite-carbonatite dykes and barite fluorite veins. No Bouguer high is present on the Complex, suggesting that no buried high-density body is present in depth. The structure of the Complex, however, is clearly delineated by the high-resolution magnetic survey. A pronounced magnetic anomaly is situated along the northern boundary of the inner zone, the extent of which is limited to the assumed position of the caldera-collapse fault. In addition, a north-south section derived from magnetic data across the Complex indicates gently-dipping rocks in the south, whereas in the north the magnetic zone is almost vertical. Anomalously high fluorite contents are present along the southern boundary of the inner zone. Here, the inward dipping altered carbonatite rocks are replaced by fluorite mineralisation. Core from the KD01 borehole, drilled in the NE part of the inner zone has revealed great detail about the carbonatitic pyroclastic sequence. The sequence is dominated by ash fall and ash flow tufts. With the exception of a unit 32 m below the top of the borehole, the sequence has been replaced by a fine-grained intergrowth of potassium feldspar, ankerite and chlorite. This was followed by a period of veining characterised by intergrown, very fine-grained chlorite, apatite, anatase and pyrite. The earlier feldsparankerite- chlorite assemblage has been overprinted by apatite, followed by fluorite crystallisation. Both minerals occur either as euhedral crystals or as small, scattered grains. The latest alteration to affect the rock was the introduction of calcite, which has caused some fine-scale brecciation and some replacement of the pre-existing mineral assemblage. One unit of the borehole core contains ash grains with exceptionally well-preserved primary textures, including quench microphenocrysts, and less abundant vesicles and larger phenocrysts. There are also groups, or "clots" of tiny microphenocrysts, which have not previously been observed in carbonatite lavas. These ash grains have erupted as droplets of lava of calcite-carbonatite and silicate, and formed an ash deposit containing some altered silicate lava fragments. The primary mineralogy of the droplets has been completely masked by late Sr-bearing calcite, chlorite and apatite. The latest stage of mineralisation is represented by anatase, apatite, chlorite and pyrite.