Petrology of the hypabyssal kimberlite of the Kroonstad group II kimberlite (orangeite) cluster, South Africa: Evolution of the magma within the cluster

Geology Department, Rhodes University, P.O. Box 94, Grahamstown, South Africa
Lithos (Impact Factor: 4.48). 07/2011; 125(1-2):795-808. DOI: 10.1016/j.lithos.2011.05.001


We report here a detailed petrographic study accompanied by the first whole-rock major and trace element geochemical analyses as well as the first Sr and Nd isotope data for the Kroonstad Group II Kimberlite Cluster. Hypabyssal kimberlite is described from the Lace, Voorspoed and Besterskraal North kimberlite pipes. The Lace kimberlite is a monticellite phlogopite kimberlite with mineralogy and geochemistry more similar to typical Group I kimberlites. The Voorspoed kimberlite is an aegirine and sanidine bearing kimberlite and along with the geochemistry shows a more evolved composition relative to the Lace kimberlite. The Besterskraal North kimberlite is a highly evolved type Group II kimberlite containing abundant K-richterite, sanidine, aegirine and leucite. The magma evolutionary trend is linked to a variation in olivine macrocryst abundance. The Lace unevolved magma contains abundant olivine macrocrysts and has not been affected by significant fractionation whereas the Voorspoed and Besterskraal North kimberlite contain far fewer olivine macrocrysts, which indicates that significant fractionation has affected these magmas en route to the surface. We evaluate the potential for fractionation to result in differentiation of a common parent magma or magma generated from a common source en route to the surface. This process is complicated by the fact that olivine macrocrysts are essentially xenocrysts within the magma and the loss of these xenocrysts will not affect the primary magma composition. Furthermore we present two alternate hypotheses for the evolution of the magmas; a) contamination en route to the surface and b) varying degrees of partial melting at the source. The evolutionary trend is also linked with diamond characteristics. In particular diamond parcels at the Voorspoed pipe between 1906 and 1911 were deficient in large stones, which indicate that fractionation has likely also removed the large stones from the Voorspoed magma along with the olivine macrocrysts.Research highlights► Kimberlite magma evolution within a single cluster. ► Evolution linked to olivine macrocryst/phenocryst abundance a ratio. ► Evolution linked to possible early contamination by crustal material. ► Identification of pseudomorphed monticellite and melilite in a Group II kimberlite.

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    ABSTRACT: The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ~ 1600 m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically < 2 mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy of the infill: outer east/west zone with a sequence bsBreccia–BVK–mMVK/softMVK; a central zone with a sequence mMVK–BVK–jrMVK–mMVK and an outer north zone with a uniform mMVK sequence.The evidence suggests that the volcaniclastic infill has formed through complex processes involving re-working of extra-crater tephra back into the vent and coeval pyroclastic deposition. It is also likely that there is significant mixing of the resedimented and pyroclastic deposits. Distinction between deposits formed through re-working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re-working extra-crater tephra from the surface (with possible coeval pyroclastic eruption) is probably impossible. For this reason we interpret the mMVK as being deposited by dominantly resedimentation of tephra whereas the jrMVK has been deposited dominantly by pyroclastic processes. The presence of BVK and fgVK deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater ~ 2000 m in depth was periodically open.The identification of the complex, layered, variable internal structure of the volcaniclastic infill at the Voorspoed has significant implications for diamond ore evaluation in South Africa. Typical South Africa kimberlite pipes are infilled with tuffisitic kimberlite, which are characterised by their homogenous nature over large vertical intervals (> 1000 m). This is not the case at the Voorspoed pipe where MVK layers are variable in thickness (< 10 m up to 230 m).
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    ABSTRACT: The Lace and Voorspoed kimberlites occur on the Kaapvaal Craton (South Africa), and form part of the Kroonstad Group II kimberlite (orangeite) cluster. The Lace kimberlite is composed of a main pipe and a satellite blind pipe, the latter of which does not reach the current land surface (~ 30 m below the current land surface), and is not observed connecting with the main pipe at depth. The main pipe increases in size from ~ 100 m to ~ 250 m in diameter at depth. The Voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers (up to 200 m thick) of resedimented volcaniclastic kimberlite (RVK). Coherent kimberlite (CK), identified at all three pipes, is described here in order to constrain their formation.
    No preview · Article · Dec 2013 · Journal of Volcanology and Geothermal Research
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    Full-text · Article · Apr 2014 · Journal of Asian Earth Sciences