Tuffisitic kimberlites from the Wesselton Mine, South Africa: Mineralogical characteristics relevant to their formation

Tuffisitic kimberlites from the Wesselton Mine consist, in order of formation, of the following primary components: chloritized olivine macrocrysts and phenocrysts; magmaclasts; cryptocrystalline diopside-phlogopite-rich mantles, and a smectite–chlorite interclast matrix. Magmaclasts consist of one to several crystals of chloritized olivine set in a microcrystalline groundmass of diopside, apatite, perovskite, spinel and chloritized and fresh phlogopite, the latter commonly rimming chloritized olivines. Magmaclasts have some similarities to holocrystalline hypabyssal kimberlite but lack monticellite, carbonate, carbonate-serpentine segregations and atoll spinels. Spinels in the magmaclasts show only a limited compositional evolution relative to spinels in spatially-associated hypabyssal kimberlite. Pre-existing solids, including discrete olivine grains, magmaclasts and most xenoliths, are mantled by acicular diopside and phlogopite. The interclast matrix is now represented by mixed layer phyllosilicates (chlorite–smectites) that are poorer in alumina and iron than chlorite pseudomorphing olivine and microlitic phlogopite and diopside. The interclast chlorite–smectite is considered to represent former phlogopite which has undergone late-stage deuteric hydrothermal-like modification. The interclast matrix crystallized from the volatile-rich remnants of the magma. None of the constituents of tuffisitic kimberlites, including the chlorite or chlorite–smectites, represent material formed from externally-derived fluids. These primary textures are unique to kimberlites and only form in certain circumstances. Tuffisitic kimberlites formed by progressive crystallization and volatile exsolution-induced segregation and/or disruption within a kimberlite magma (magmaclasts, mantles, interclast matrix) during a continuum of rapidly changing conditions in a subsurface (subvolcanic) magmatic system. This continuum represents a transition from a degassing magmatic system to a magmatic-derived hydrothermal-like fluid. Rapid drops in temperature with extensive exsolution of CO2 and incorporation of large amounts of locally-derived country rock xenoliths resulted in explosive diatreme formation and within-diatreme modification of the crystallization and deuteric replacement sequence of typical kimberlite magma resulting in a distinctive mineralogy and texture.