Petrology and geochemistry of the diamondiferous K-richteriteand leucite-bearing Kareevlei Kaapvaal lamproite

Qashani, Zuko

Petrology and geochemistry of the diamondiferous K-richteriteand leucite-bearing Kareevlei Kaapvaal lamproite

Thesis / Dissertation

2023

Abstract

The Kareevlei diamond mine is the first mine on the evolved leucite-bearing Kaapvaal lamproite. These rock types have generally been believed to be non-diamondiferous or at least sub-economic and as a result, their petrogenesis has remained poorly constrained as the exploration programs focused mainly on the unevolved subvarieties of Kaapvaal lamproites, such as at the Finsch mine. To further assist with the petrogenesis of these uncommon diamondiferous rock types, 13 Kaapvaal lamproite samples of hypabyssal texture from the Kareevlei diamond mine have been analyzed for their petrography, mineral chemistry (phlogopite, Krichterite, and diopside), and bulk-rock major and trace elements as well as Sr–Nd isotope compositions. The Kareevlei Kaapvaal lamproites comprise completely altered olivine macrocrysts (3–12 vol.%) set in a groundmass of olivine microcrysts (3–19 vol.%), abundant phlogopite (32–58 vol.%), diopside (12–36 vol.%), pseudo-leucite (0–27 vol.%), K-richterite (0–25 vol.%), and interstitial material containing carbonate minerals. Two distinct mineralogical varieties are identified based on the presence/absence of leucite and K-richterite: (1) phlogopite-diopside lamproites and (1) leucite-richterite lamproites. Phlogopite laths in these lamproite varieties show two distinct core populations characterized by high-Cr2O3 (Cr2O3 = 0.89–1.97 wt.%) and lowCr2O3 concentrations (Cr2O3 = 0.04–0.68 wt.%), both mantled by TiO2 and FeO-rich rims. High Cr2O3 cores are interpreted as xenocrysts from phlogopite peridotite xenoliths, whereas the low Cr2O3 cores resemble MARID xenolith phlogopite. The phlogopite rims have compositions similar to typical Kaapvaal lamproites and represent direct crystallization by the parent magma. The Kareevlei leucite-richterite lamproites are characterized by raised bulk-concentrations of SiO2 (44.8 – 47.9 wt.%), Al2O3 (6.34–7.34 wt.%), and Na2O (0.78–1.99 wt.%), and lowered MgO (16.2–17.1 wt.%) concentrations compared to phlogopite-diopside lamproites (SiO2 = 40.8–42.9 wt.%; Al2O3 = 5.44–6.22 wt.%; Na2O = 0.42–0.74 wt.%, MgO = 20.7–23.0 wt.%). The two mineralogical varieties have distinct incompatible trace element concentrations, with the leucite-richterite samples being depleted in light REEs (LREE/Chondrite: La = 656–828; Ce = 518–597; Nd = 234–261) compared to the phlogopite-diopside samples (LREE/Chondrite: La = 1125–1391; Ce = 817–1029; Nd = 333–415). Additionally, these lamproite varieties exhibit clear separation in incompatible trace element ratios, with leucite-richterite lamproites having lower La/Yb = 101–143, Gd/Yb = 6.28–7.73, and La/Sm = 10.0–11.0, relative to phlogopite-diopside lamproites (La/Yb = 167–205; Gd/Yb = 8.13–9.55; La/Sm = 12.4–13.4). However, these mineralogical varieties have similar 87Sr/86Sri and 143Nd/144Ndi ratios with ranges of 0.7071–0.7073 and 0.5118–0.5119, respectively. The two distinct lamproite varieties identified as phlogopite-diopside and leucite-richterite lamproites in this study suggest relative evolution at Kareevlei, marked by the complete absence of K-richterite and leucite in the groundmass of phlogopite-diopside lamproites. The major element compositional trends of lamproites commonly reflect the primary mineralogy. In Kareevlei lamproites, these trends (e.g., MgO depletion with SiO2, Al2O3, and Na2O enrichment) appear to be controlled by relative mantle xenocryst accumulation rather than evolution through fractional crystallization as the incompatible trace elements and their ratios are not consistent with fractional crystallization control on Kareevlei magma evolution. The Sr-Nd isotopes suggest that both Kareevlei phlogopite-diopside and leucite-richterite lamproites are derived from an isotopically homogeneous mantle source within the sub-continental lithospheric mantle (SCLM). The higher incompatible trace element ratios (e.g., La/Yb, Gd/Yb, and La/Sm) in phlogopite-diopside lamproites are regarded as a consequence of derivation by lower degrees of partial melting. In contrast, the leucite-richterite lamproites with their low La/Yb, Gd/Yb, and La/Sm ratios are indicative of derivation by greater degrees of partial melting. It is concluded that the Kareevlei lamproite varieties are generated by variable degrees of partial melting of MARID-veined peridotite lithologies in the SCLM. While these lamproites varieties appear to be derived from an isotopically homogenous source, the variation in their groundmass mineralogical assemblages is a consequence of variable degrees of partial melting rather than evolution through fractional crystallization en route to the surface. This hypothesis can be tested in the future for other Kaapvaal lamproite clusters across the Kaapvaal craton to see if variable degrees of partial melting are the primary process responsible for the relative evolution observed.

Keywords

Geological Sciences

Reference:

Collections

Masters

Full item page