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Abstract Volume ABSTRACT VOLUME THE 9th IGNEOUS AND METAMORPHIC STUDIES GROUP MEETING 15-18 January 2017 Glenburn Lodge, Muldersdrift Organised by: Marlina Elburg, Jérémie Lehmann, Trishya Owen-Smith & Sebastian Tappe Department of Geology, University of Johannesburg Sponsored by: UNUSUAL FEATURES WITHIN THE STRATIGRAPHY OF THE CRITICAL ZONE SOUTH OF MOKOPANE Acheampong, K.O1., Kinnaird, J.A1., Nex, P.A.M.1 1 School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa; [email protected] The mafic rocks of the Bushveld Complex differ to the north and south of the Ysterberg-Planknek Fault, part of the array that is the Thabazimbi-Murchison Lineament. To the north, there is sporadic Lower Zone, succeeded by a PGE-Cu-Ni-bearing Platreef, Main Zone and Upper Zone successions. To the south is Lower Zone, the Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member and Main Zone. This investigation on farm of Grasvally aims to provide a better understanding of the mafic rocks south of Mokopane, in order to provide an improved understanding of the relationship between the geology and mineralisation of this region. The farm Grasvally is situated approximately 20 km south of Mokopane. The geology of the Grasvally area comprises mainly mafic to ultramafic Bushveld rocks of Lower Zone to Main Zone affinity which were emplaced above sediments of the Pretoria Group. The GNPA member was originally divided into two major sub-units by Hulbert (1983) but more recently it has been divided into Lower Mafic Unit (LMF), the Lower Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO) (de Klerk, 2005). The GNPA member may be the equivalent of the Platreef to the north of the Ysterberg-Planknek Fault. The Thabazimbi- Murchison Lineament may have acted as a barrier to the flow of magma causing the GNPA member to be compartmentalised and follow a different evolution from Critical Zone magma in the eastern or western limbs or the Platreef to the north. Previous work carried out at Grasvally was minimal, apart from detailed work by Hulbert (1983). Smith et al. (2011) provided an in depth account of the petrography and mineralogy of silicates, oxides and base metal sulphides in the GNPA member to the north of this study area. In this project, 20 borehole cores have been studied and logged and the stratigraphy compared and tentatively correlated with the work of de Klerk (2005). These cores were drilled in 2004 by Samancor and were made available for this study by Sylvania Platinum Ltd. Core logging indicated that the GNPA member overlies the Magaliesberg quartzites of the Pretoria Group and the Lower Zone to the east and the west of the farm respectively. Previous work (Smith et al., 2014) suggests PGE mineralisation is associated with sulphides and is not restricted to one lithological unit within the GNPA member. Within the LGN blebs (<1 cm) of fine-grained cumulus magnetite within norite have not been documented previously. Additionally, below this unit, within the LMF are disseminated to well-developed layers of chromite, as well as isolated pods of chromitite within norite and gabbronorite which are not easily correlated with the known stratigraphy. The boreholes also intersect a pyroxenite unit within the LMF that has visual similarities, mineralisation, the same hanging and footwall, as the UG 2 chromitite layer. This pyroxenite comprising orthopyroxene, interstitial plagioclase and clinopyroxene hosts disseminated to well-developed chromitite layer with associated disseminated to blebby pyrrhotite and chalcopyrite. Two well-developed chromitites ~60-90 cm are intersected within the Lower Zone (LZ). The chromitites, disseminated chromite, pyrrhotite, chalcopyrite and minor pentlandite mineralisation are hosted within the harzburgites and pyroxenites of the LZ. de Klerk, L. 2005. Bushveld Stratigraphy on Rooipoort, Potgietersrus Limb [abs.], 2nd Platreef Workshop, Mokopane, South Africa, October, Abstracts Hulbert, L. J., 1983, A petrological investigation of the Rustenburg layered suite and associated mineralization south of Potgietersrus. Smith, J., Holwell, D., and McDonald, I., 2011, The mineralogy and petrology of platinum-group element- bearing sulphide mineralisation within the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, south of Mokopane, northern Bushveld Complex, South Africa: Applied Earth Science, v. 120, no. 4, p. 158-174. Smith, J., Holwell, D. A., and McDonald, I., 2014, Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement: Mineralium Deposita, v. 49, no. 6, p. 667-692. 2 EUDIALYTE OR NO EUDIALYTE - CONTRASTING TRENDS OF AGPAITIC CRYSTALLIZATION IN NEPHELINE SYENITE OF THE PILANESBERG COMPLEX, SOUTH AFRICA Tom Andersen1,2, Marlina A. Elburg1 & Muriel Erambert2 1 Department of Geology, University of Johannesburg, South Africa; [email protected] 2 Department of Geosciences, Oslo University, Norway, [email protected], [email protected] Peralkaline nepheline syenites can roughly be divided in two families: the miaskitic, in which Zr and Ti are hosted in zircon, titanite and ilmenite, and the agpaitic, in which these common HFSE minerals are not stable. Common to most agpaitic rocks is the presence of members of the eudialyte group (complex Na-Ca- Mn-Fe-Zr-Cl silicates), but other minerals such as Na-Ca-Ti-Zr-F disilicates and aenigmatite (Na2Fe5TiSi6O20) are also diagnostic. There is general consensus that the main driving force behind the miaskitic-to-agpaitic transition is an increase in the alkali content of the magma but other factors such as halogen and water content are also important (e.g. Andersen et al., 2010). Two types of peralkaline nepheline syenite from the Pilanesberg complex that show different magmatic crystallization histories have been investigated in this study: white foyaite from the outer ring segment (the Matooster type of Retief, 1963) and members of the green foyaite suite from the southern rim of the complex. Although the major and trace element composition of the rocks are different (Elburg et al., 2017), both groups have crystallized magmatic mineral assmblages with alkali feldspars, nepheline, sodalite, fluorite and sodic pyroxene and amphibole.. The white foyaite magma started out as a miaskitic system, crystallizing titanite and ilmenite together with sodic-calcic amphibole and minor biotite. In interstitial aggregates and pegmatitic patches representing trapped residual melts, the mafic and HFSE silicate assemblages evolved through mildly agpaitic assemblages with sodic amphibole and pyroxene, astrophyllite and aenigmatite, to highly agpaitic final assemblages with lorenzenite (Na2Ti2Si2O9). Throughout this 2+ sequence, Zr remained contained in the NaFe 0.5Zr0.5Si2O6 component in aegirine until hilairite (Na3Zr(SiO3)3·3H2O) crystallized as the final magmatic mineral. In contrast, in the green foyaite, eudialyte crystallized relatively early, together with aegirine and aenigmatite. There are indications of high-Zr pyroxene cores, but zirconium content in aegirine dropped once eudialyte started crystallizing. At a later stage, normandite (NaCa(Mn,Fe)(Ti,Zr)(Si2O7)2O2F2) crystallized. There is no evidence that astrophyllite, lorenzenite or hilairite ever have formed in the samples investigated. A chemographic analysis of melt-mineral equilibria suggests that the main driving force behind the evolution towards more highly alkaline mineral assemblages in both rock types is indeed an increasing degree of peralkalinity of the melt, probably driven by alkali feldspar fractionation (Elburg et al., 2017). However, the contrast between the crystallization regimes of the white and green foyaite is controlled by differences in water activity, the interstitial assemblages in the white foyaite having crystallized at higher aH2O than the green. The trapped melts in the white foyaite evolved as closed systems until the end of magmatic crystallization, whereas the green foyaite magma may have lost an aqueous fluid phase at an earlier stage of evolution. References: Andersen, T., Erambert, M., Larsen, A.O., Selbekk, R. 2010. Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zirconium silicate mineral assemblages as indicators of alkalinity and volatile fugacity in mildly agpaitic magma. Journal of Petrology, 51, 2303-2325. Elburg, M.A., Cawthorn, R.G., Andersen, T., 2017. Whole rock geochemistry of the Pilanesberg Complex: Reflecting or controlling mineralogy? Abstracts, IMSG 2017 Retief, E.A., 1963. Petrological and mineralogical studies in the southern part of the Pilanesberg alkaline complex, Transvaal, South Africa. Unpubl. D. Phil. thesis, Oxford Univ. 263 pp. 3 GRANULITE FACIES U, Th, REE OCCURRENCES IN THE NAMAQUALAND METAMORPHIC COMPLEX: A REAPPRISAL M.A.G. Andreoli1 1 School of Geoscience, University of the Witwatersrand, Johannesburg, South Africa; [email protected] Introduction. Over a period of more than 30 years the author was a geoscientist of the South African Nuclear Energy Corporation and mapped, inspected and logged cores from numerous airborne/ground radiometric anomalies within the Namaqualand Metamorphic Complex (NMC). Previously published results of these studies are summarized as follows. Geology of U radiometric anomalies in the O’okiep Copper district. In this area late stage, small intrusions, marginal to the batholitic ~1200 Ma U, Th-enriched Concordia Granite, may constitute potential
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