Petrogenesis and indicator mineral chemistry of the K-richterite- and leucite-bearing diamondiferous Kareevlei Kaapvaal lamproite.

Kareevlei is the first diamond mine to be hosted in a K-richterite- and leucite-bearing Kaapvaal lamproite (formerly termed Group II kimberlite). To constrain the petrogenesis and lithospheric setting of the Kareevlei Kaapvaal lamproites, we present petrography, groundmass mineral compositions, bulk-rock major and trace element concentrations and Sr Nd isotopes for 13 coherent-textured samples, along with garnet indicator mineral compositions and Ni-in-garnet thermobarometry. Two distinct mineralogical varieties of Kaapvaal lamproites are identified: (1) phlogopite-diopside olivine lamproites and (2) leucite-richterite olivine lamproites. Phlogopite macrocryst and microcryst laths have two distinct core populations, interpreted as xenocrysts from phlogopite peridotites and MARIDs, whereas rims represent crystallisation from the magma. The leucite-richterite lamproites are characterised by higher bulk-rock SiO 2 , Al 2 O 3, and Na 2 O, and lower MgO contents compared to the phlogopite-diopside lamproites. The two mineralogical varieties have distinct incompatible trace element concentrations, with the leucite-richterite samples having lower concentrations of light REEs (LREE: La = 656–828; Ce = 518–597; Nd = 234–261 times chondrite) compared to the phlogopite-diopside samples (La = 1125–1391; Ce = 817–1029; Nd = 333–415 times chondrite). Additionally, these two groups exhibit clear separation in trace element ratios, with higher La/Sm and Gd/Yb in the phlogopite-diopside lamproites, which is interpreted to represent formation by lower degrees of partial melting relative to the leucite-richterite lamproites. However, both lamproite varieties have similar 87Sr/86Sr i and 143Nd/144Nd i ratios, suggesting derivation from an isotopically homogeneous source in the SCLM. The indicator mineral suite at Kareevlei comprises, in decreasing abundances, oxides (predominantly ilmenite and chromite), garnet, corundum, and rare clinopyroxene. Garnets are largely eclogitic (59%) with only smaller proportions of peridotitic G9 (19%) and G10 (8%) garnets observed. Peridotitic garnet thermobarometry indicates a paleogeotherm with an estimated surface heat flux of ∼44.3–44.9 mW/m−2 and an estimated lithospheric thickness of ∼187–195 km, which is within error of the nearby highly diamondiferous Finsch Kaapvaal lamproite (cf. 192 ± 6 km). Single mineral garnet thermobarometry suggests that most peridotitic garnets at Kareevlei were sampled by the lamproite at relatively shallow depths above the diamond stability field. The dominance of eclogitic garnet and the absence of peridotitic garnet that equilibrated within diamond stability suggests that the high-value diamonds extracted from Kareevlei are predominantly eclogitic in origin. • Petrology of richterite- and leucite-bearing Kareevlei diamondiferous lamproites • Mineralogical / bulk-rock geochemistry related to variable degrees of partial melting • Indicator minerals suggest LAB at ±188 km • High-value diamonds likely eclogitic in origin [ABSTRACT FROM AUTHOR]