Abstract

Dr. M. J. Le Bas said he had two questions.

1. Is this occurrence of a layered kimberlite sill with intercumulus calciteunique ?

2. The calcite which forms 10 to 20% of this sill is stated by the authors to be genetically linked with carbonatite. Whilst many of the petrological and geochemical features support this, no account has been made in the surrounding rocks of any marginal fenitization, which is the 'hall mark' of carbonatites and is normally shown by the metasomatic growth of K-feldspar, aegirine or Na-amphibole. Is there any such fenitization ?

The Authors replied that to the best of their knowledge the Benfontein kimberlite sill is unique. Magmatic sedimentation features are known in the kimberlite sills (e.g. those in S. W. Greenland) but the fine-scale layering, concentration of intercumlus calcite in the upper parts of layers, and the small-scale diapirism, have not been recorded in kimberlite previously.

2. Whilst not being 'fenitisation' in the generally accepted sense, potassium metasomatism is known in many kimberlite intrusions. This is mainly manifested in the form of phlogopitisation of the basic and ultrabasic xenoliths, though classical potassium fenitisation is known at the contacts of a kimberlite dyke in the Seguela River area of the Côte d'Ivoire. In the specific case of the Benfontein sills, metasomatic mica is developed in the peripheral parts of some dolerite xenoliths.

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