Browsing by Author "Howarth, Geoffrey"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    Classification and petrogenesis of the Tongo dike-01 from the Tongo-Tongoma cluster, Sierra Leone: constraints from bulk rock geochemistry
    (2021) Mathafeng, Katleho; Howarth, Geoffrey
    The Man Craton in West Africa, like the Kaapvaal Craton in southern Africa, hosts diamondiferous kimberlites. However, West African kimberlites are commonly micaceous and unusual relative to archetypal South African kimberlites. Petrographically, they appear more similar to orangeites (aka Group II kimberlites), which represent a type of olivine-lamproite. A suite of 14 representative samples from the Cretaceous Tongo dike-01, Sierra Leone have been analysed for their bulk-rock major and trace elements as well as Sr-Nd-Pb isotope geochemistry. The primary objectives of this study are: 1) provide detailed petrographic observations of the dike, 2) classify the dike relative to kimberlites worldwide, 3) constrain the geochemical effects of secondary processes on bulk-rock analyses, 4) provide an estimate of the close-to-primary parent magma composition, and 5) constrain the petrogenesis of these diamondiferous rocks. The major element chemistry of the Tongo dike-01 reflects concentrations that are similar to both archetypal kimberlites and orangeites. Major elements such as SiO2 (~28.20 ± 3.90 wt. %) and CaO (~ 12.50 ± 1.80 wt. %) display archetypal kimberlite concentrations whereas Cr2O3 (~0.20 ± 0.01 wt. %) and P2O5 (~1.65 ± 0.60 wt. %) resemble those that define orangeites. The high abundance of phlogopite in this dike is illustrated by the high bulk-rock concentrations of K2O (~3.03 ± 0.50 wt. %) and Al2O3 (~4.08 ± 1.00 wt. %), similar to those of orangeites. Like the major element geochemistry, the trace element geochemistry of the Tongo dike-01 also displays mixed archetypal kimberlite and orangeite traits. Trace elements such as Nb (~365.0 ± 50.4 ppm) and Y (~18.77 ± 6.60 ppm) possess concentrations that are similar to kimberlites whereas Rb (~160.0 ± 14.8 ppm) and Th (~36.22 ± 5.30 ppm) resemble orangeites. Trace element ratios are no different, ratios such as Ce/Pb (16-82), Ba/Nb (1-8), La/Nb (0.6-1.2) and La/Sm (11-13) resemble those of kimberlites while La/Yb (280-520) are more similar to orangeites. However, unlike major and trace element geochemistry, the Sr-Nd-Pb isotope geochemistry of the Tongo dike-01 solely resembles those of archetypal kimberlites (87Sr/86Sr)i ~0,7039, (206Pb/204Pb)i ~18.88, (208Pb/204Pb)i ~40.02 and (143Nd/144Nd)i ~0.51253 ± 0.00001. Prior to interpretation of primary processes and parent magma composition estimates, the effects of secondary processes were first evaluated. These secondary processes include crustal contamination, ilmenite contamination and olivine entrainment/fractionation. Samples that had experienced these secondary processes were excluded and a suite of unaltered/least contaminated samples was compiled in order to constrain the close-to-primary magma composition of the Tongo dike-01 and interpret primary petrogenetic processes effecting the kimberlites. To determine a representative parent magma composition, a total of six out of the fourteen samples were excluded from consideration. The estimated close-to-primary magma composition for the Tongo dike-01 is suggested to be SiO2 ~28.20 ± 3.90 wt. %, Fe2O3 ~10.20 ± 1.70 wt. %, TiO2 ~1.70 ± 0.30 wt. %, Al2O3 ~4.08 ± 1.00 wt. %, K2O ~3.03 ± 0.50 wt. %, La ~363.12 ± 25.43 ppm, Gd ~15.87 ± 4.20 ppm, Yb ~1.020 ± 0.06 ppm, ( 87Sr/86Sr)i ~0.7039, ( 206Pb/204Pb)i ~18.88, ( 208Pb/204Pb)i ~40.02 and ( 143Nd/144Nd)i ~ 0.51253 ± 0.00001. Although the petrography and major element concentrations are similar to orangeites found in South Africa, the trace element and Sr-Nd-Pb isotope geochemistry of the Tongo dike-01 reflects a kimberlite composition. Thus, the Tongo dike-01 is more consistent with being classified as a relatively rare type of ‘mica-rich' kimberlite rather than orangeite. Kimberlites from around the world derive from the same asthenospheric mantle reservoir and their major element chemistry is controlled by the compositions/mineralogy of the lithospheric mantle assimilated during kimberlite evolution. The similarity of the trace element ratios and Sr-Nd-Pb isotopes of the Tongo dike-01 in this study relative to archetypal kimberlites worldwide strongly implies that the Tongo dike-01 derives from the same asthenospheric reservoir as these kimberlites, although mineralogically the Tongo dike-01 is different and has a different parent melt major element composition. This is interpreted to reflect the contribution of lithospheric mantle material that is mineralogically different to that assimilated by archetypal kimberlites during the ascent of the Tongo dike-01 parent magma through the sub-continental lithospheric mantle (SCLM). In the case of the Tongo dike-01, its primary melt is K2O-rich and must have assimilated more K2O-rich material in the SCLM. Such material is typically present as metasomatic products, e.g., Phlogopite-Ilmenite-Clinopyroxene (PIC) xenoliths observed in South Africa kimberlites. These xenoliths tend to possess abundant phlogopite. Thus, the main difference between the Tongo kimberlite and archetypal SA kimberlites is the fact that Tongo kimberlite assimilated more K2O-rich metasomatised material in the SCLM during its evolution.
  • No Thumbnail Available
    Item
    Open Access
    Petrology and geochemistry of the diamondiferous K-richteriteand leucite-bearing Kareevlei Kaapvaal lamproite
    (2023) Qashani, Zuko; Howarth, Geoffrey
    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.
  • Thumbnail Image
    Item
    Open Access
    The petrogenesis of Liberian diamondiferous rocks, Man Craton, West Africa
    (2022) Ndimande, Njabulo; Howarth, Geoffrey
    Kimberlites and lamproites are ultramafic igneous rocks believed to originate from the convective asthenosphere and sub-continental lithospheric mantle, respectively, however, the genetic link between these rock types is still a debate among researchers. Recently, it has been shown (e.g., in the Dharwar Craton, India) that both kimberlites and lamproites may be derived from the same asthenospheric mantle source. During their ascent to the surface, they assimilate differently metasomatized SCLM material. However, it is not clear if this process applies to all cratonic regions. The Man Craton, West Africa, is a host to diamondiferous rocks that share petrographic and geochemical characteristics with both kimberlites and lamproites, thus not easy to classify. The Camp Alpha (possibly Precambrian in age) and Neoproterozoic-aged Weasua clusters are the selected locations for this study. To further assist with the classification of peculiar West African diamondiferous rocks and evaluate their genetic link and origin, I provide detailed petrography, as well as phlogopite and spinel chemistry, bulk-rock geochemistry (for the Camp Alpha samples) and perovskite chemistry (for the Weasua samples). The Camp Alpha rocks contain abundant macrocrysts of olivine (that are completely altered) and ilmenite occurring in a groundmass that comprises spinel, phlogopite (the samples are phlogopite-poor), perovskite, calcite, serpentine, and rare apatite. Most importantly, there is no evidence of diopside in the Camp Alpha samples. The Camp Alpha phlogopite is enriched in Al2O3 but slightly depleted in FeO and TiO2 and the spinel exhibits a kimberlitic compositional evolution (i.e., trend 1). The bulk-rock trace element ratios Ba/Nb (0.91 - 9.55), La/Nb (0.12 - 1.22; mostly < 1.10) and Ce/Pb (6.77 - 99.2; mostly > 22), fall within the ranges defined by kimberlites. Additionally, primitive mantle normalised trace element patterns are similar to kimberlite patterns (e.g., Rb, K, Sr and P 2 show negative anomalies). As a result of these characteristics, the studied Camp Alpha rocks are classified as kimberlites. A close to parent melt composition of the Camp Alpha kimberlite is provided in this study, based on bulk rock geochemistry. The composition of the estimated melt is similar to that of kimberlite magmas and characterised by low K2O (of 0.80 wt.% relative to 3.63 ± 1.40 wt.% of orangeites for example; reflective of mica-poor nature). The Weasua rocks, in this study, are micaceous (phlogopite-rich) and contain primary groundmass diopside and calcite, which suggests that the studied Weasua rocks represent carbonate-rich olivine lamproites. From the inversion of trace element data for the perovskites using known partition coefficients, the composition of the melt in equilibrium with perovskite at the time of crystallisation is determined. The composition of this Weasua parent melt is similar to that of kimberlite magmas (i.e., similar primitive mantle-normalised trace element patterns, Ba/Nb, Ce/Pb and La/Nb ratios). Whereas the Camp Alpha and Weasua rocks are classified as kimberlite and lamproite, respectively, the trace element composition of the parent melt is similar to that of kimberlite magmas for both cases. This suggests a common asthenospheric magma source for the Camp Alpha kimberlite and Weasua lamproite. En route to the surface, these rock types assimilated differently metasomatized SCLM material (i.e., the Weasua lamproite magma assimilated mica-diopside-rich wall rock). The Camp Alpha kimberlite was shown to be approximately 800 Ma old, similar to the Weasua lamproite. Therefore, the Camp Alpha and Weasua magmas were interpreted to be broadly coeval based on the similarity in age and location, In addition to the Indian Dharwar Craton, the hypothesis that kimberlites and lamproites share a common convective asthenospheric magma source but assimilate differently metasomatized SCLM material en route to the surface is confirmed in the Man Craton, West Africa.