Browsing by Author "Gurney, John J"
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- ItemOpen AccessA detailed geochemical investigation of diamond-bearing eclogite xenoliths from the Kaalvallei kimberlite, South Africa(2000) Kiviets, Gail Beverly; Gurney, John JThe eclogites are essentially bimeneralic assemblages of garnet and clinopyroxene. Thirteen xenoliths contain accessory diamond with graphite on diamond surfaces. One has accessory ilmenite. The rocks are well equilibrated and are classified as Group 1 eclogites, based on their mineral textures and compositions. The calculated equilibrium temperatures for the eclogites range from 1157 °C to 1245 °C, assuming a pressure of 50 kbar. Two populations of eclogites are defined in terms of the calcium content of the garnet, equilibration temperatures and trace element abundances.
- ItemOpen AccessThe development of the national accelerator centre proton microprobe as an analytical tool in geochemistry(1995) Van Achterbergh, Esmé; Le Roex, Anton; Gurney, John JThis thesis describes work performed to establish and demonstrate a quantitative trace element microanalysis technique for geological material using protons accelerated by the Van de Graaff Accelerator at the National Accelerator Centre (NAC) in Faure near Cape Town. The method relies on the analysis of Proton Induced X-ray Emission (PIXE) spectra, interpreted with the help of the GeoPIXE software package. The use of the Si(Li) energy dispersive detector provides simultaneous multi-element detection at the parts-per-million (ppm) level, and a scanning beam facility permits trace element distributions to be studied at these levels. The calibration of the detector efficiency and the thicknesses of selectable X-ray attenuating filters was performed using pure elemental samples. This involved the accurate determination of the target to detector distance, the thickness of the active volume of the Si(Li) detector crystal, the thicknesses of all the absorbing layers between the sample and the detector crystal, and the assessment of the effects of incomplete charge collection in the detector.
- ItemOpen AccessDiamonds and their mineral inclusions from the Sloan diatremes of the Colorado-Wyoming State Line kimberlite district, North America(1989) Otter, Marshall Lorrence; Gurney, John JThe Sloan diamonds were investigated for their physical characteristics, inclusion mineralogy and composition, and carbon isotope composition. The relationships between these features are described and interpreted with respect to diamond genesis. The physical characteristics investigated include crystal state, crystal regularity, primary morphology, resorption morphology, primary and secondary sizejmass, colour, surface features and inclusion content. Significant relationships between these characteristics were found. The proportion of an individual crystal, lost during resorption, decreases with increasing diamond size. Larger crystals and diamonds displaying brown colours appear to have been more susceptible to breakage relative to smaller crystals and diamonds of other colours, respectively. Brown colours were more common on smaller diamonds relative to larger stones and, further, were more common on single crystal forms relative to twinned/aggregate crystals. Variation in diamond physical characteristics between the various kimberlite phases in the Sloan 1 & 2 complex has been documented. The Sloan 2 phase is characterized by larger and less resorbed diamonds relative to those in other kimberlite phases in the diatreme. In addition, corrosion sculpture is much more common on Sloan 2 diamonds.
- ItemRestrictedEclogite xenoliths from the Premier kimberlite, South Africa: Geochemical evidence for subduction origin(2006) Dludla, Siyanda; Le Roex, Anton P; Gurney, John JA suite of mantle eclogite hosted within the Premier kimberlite on the Kaapvaal craton can be classified on the basis of Na2O content in garnets as group I type, although textures are ambiguous. No accessory phases of note occur, but rutile and phlogopite are found in a few samples. Clinopyroxenes show variable light rare element (LREE) enrichment (La/Ybn = 2–48), and the garnets are strongly LREE depleted relative to chondrites (La/Ybn = <0.04). Four pyroxenite samples include both garnet clinopyroxenite and garnet orthopyroxenite; clinopyroxenes in these samples are strongly LREE enriched (La/Ybn = 57–65). Calculated equilibration temperatures of the eclogites range from 999 ± 32 to 1168 ± 14° C with an average temperature of 1102 ± 37° C, assuming a pressure of 50 kbar. Relative to a shield geotherm of 40mW/m2, these temperatures suggest a sampling depth of 135 to 165 km. A single, calcium-rich sample gives an equilibration temperature of 1296 ± 32° C at the same assumed pressure. Calculated equilibrium temperatures and pressures for the garnet pyroxenites are 887 to 987° C and 26 to 39 kbar (clinopyroxenite) and 1135 to 1156° C and 48 ± 2 kbar (orthopyroxenite). Reconstituted bulk rock compositions of the eclogites indicate the presence of low- and high-MgO groups. The MgO-poor eclogites (8 to10.5 weight % MgO) have jadeite-rich clinopyroxenes and except for lower silica contents are similar to mid-ocean ridge basalts in major element composition, with slight negative Euanomalies (Eu/Eu*=0.83 to 0.96), indicative of (low-P) plagioclase fractionation. The MgO-rich eclogites (13.6 to 18 weight % MgO) are similar in composition to oceanic gabbro. In combination the geochemical data suggest that the Premier eclogite suite represents a fragment of a once composite oceanic crustal section; the protolith to the low-MgO eclogites was recycled oceanic crustal layer two metabasalt, which experienced silicic melt loss during subduction; the protolith to the high-MgO suite was oceanic crustal layer three cumulate gabbro/pyroxenite.
- ItemOpen AccessA geochemical study of diamonds from Cullinan diamond mine, South Africa(2005) Whitehead, Kerryn; Richardson, Steve; Gurney, John JThe Cullinan kimberlite is a Group I kimberlite and is located in the northeastern region of the Kaapvaal Craton, South Africa. The kimberlite pipe has been dated at 1180 ± 30 Ma and intrudes the Bushveld Igneous Complex (2.05 Ga). This study explores the geochemistry of a suite of one hundred selected diamonds and their associated mineral inclusions. The majority of the diamonds described here are peridotitic (94%) and the remainder are eclogitic. The peridotic inclusions may be further subdivided into harzburgitic and lherzolitic parageneses.
- ItemOpen AccessA geochemical study of diamonds, diamond inclusion minerals and other mantle minerals from the Klipspringer kimberlites, South Africa(2000) Westerlund, Kalle; Gurney, John J; Richardson, SteveThe Klipspringer kimberlites occur in the north-eastern part of the Kaapvaal craton which is poorly represented in kimberlite and mantle studies. The kimberlites have been dated at 148+-4 Ma and the current study presents the geochemical characteristics of their diamonds, diamond inclusion and mantle macrocrysts/bimineralic nodules. The Klipspringer lithosphere is dominated by eclogite and Iherzolite with harzburgite and dunite present to a lesser extent.
- ItemOpen AccessA geological study of the River Ranch kimberlite pipe and associated diamonds and mantle minerals : Limpopo Mobile Belt, Zimbabwe(1997) Muusha, Miracle; Gurney, John JThe River Ranch kimberlite is a 5.2 hectare diatreme from which the original surficial crater facies material has been removed by erosion. Proof of the prior existence of such a feature is provided by down rafted blocks of epiclastic and pyroclastic rocks exposed in the diatreme during open pit mining operations. Six intrusive kimberlite phases have been recognised in the diatreme by careful mapping and confirmed by petrographic observations, particularly variations in groundmass mineralogy. Subsequent to emplacement, the diatreme has been cut by intrusive tholeitic dolerites of apparently Karoo age. The River Ranch occurrence is classified as a Group I kimberlite although the definitive isotopic evidence is lacking due to pervasive alteration of the exposed rocks. The absence of megacrystic and groundmass ilmenite and the presence of groundmass diopside are unusual for the group of rocks. However the presence of monticellite and the low abundance of phlogopite argue against a Group II classification. The overwhelmingly peridotitic nature of the mantle mineral macrocrysts in the kimberlite is consistent with the observations of Kopylova et al (1995) that the diamonds at River Ranch are predominantly peridotitic and are likely to have formed in a single process. The approximate equilibration temperature for a small suite of coarse grained lherzolite from the kimberlites is 1200°C, suggesting a geothermal gradient rather higher than seen in the Kaap-Vaal craton. The diamonds at River Ranch are predominantly brown, strongly resorbed and have less than average value. It is predicted that a combination of kimberlite petrography, micro-diamond measurements and mantle macrocryst studies should be a valuable aid to grade control and mine planning.
- ItemOpen AccessPetrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms, South Africa(2004) Coe, Nancy; Le Roex, Anton; Gurney, John JThe Swartruggens (156 Ma) and Star (128 Ma) kimberlites are two Group II, diamondiferous, hypabyssal kimerlite dyke swarms, situated in the Northern Province and the Free State respectively, South Africa. Representative samples from all dykes exposed in the mining operations, the Main and Changhouse Dykes, South Fissure and the barren Muil Dyke at Swartruggens, and the Wynandsfontein, East Star, Clewer, Byrnes and Barren dykes at Star, have been analysed for their major and trace element contents and Sr, Nd and Hf isotope compositions. Primary kimberlite magma chemistry is subjected to considerable modification due to the incorporation of both mantle and crustal material during ascent to the surface, crystal fractionation, and post-emplacement alteration by deuteric fluids. This study aims to constrain the effects of these processes, and thus to identify least-modified, close-to-primary, parental magma compositions, with the view to understanding the source region characteristics of, and the petrogenetic processes giving rise to, these kimberlites.
- ItemOpen AccessPetrology of the Sutherland Commonage melilitite intrusives(1988) Viljoen, K S; Gurney, John JThe petrology of the Sutherland Commonage olivine melilitite intrusives have been investigated using petrographic and chemical methods. The occurrence consists of a ring dyke which surrounds a centrally located sill complex. The rock of the ring dyke is a typical melilitite which consists of olivine in a groundmass of melilite, clinopyroxene, opaque spinel, nepheline and perovskite. The sill complex is a multiple intrusion and is comprised of a lower green melilitite and an overlying (and younger) grey melilitite. The green melilitite is deuterically altered and the original mineralogy is destroyed to a large extent. The grey melilitite contains autoliths of the green and is a fairly typical monticellitic melilitite in which phenocrysts of olivine are set in a groundmass of melilite, monticellite, opaque spine!, nepheline and perovskite. Microprobe analyses of clinopyroxenes indicate that they are aluminous titanian diopsides and salites which exhibit complex zonation patterns. They record magmatic conditions ranging from the intrusive stage to a final phase of magmatic evolution during which a vapour phase evolved after the majority of the groundmass minerals had crystallised. The chemistry of olivine phenocrysts suggests that the parent magma to the Commonage intrusives accumulated in a temperature-zoned reservoir at the base of the lithosphere. Large, unzoned olivine phenocrysts crystallised in this chamber. Subsequent rupture of the chamber and ascent of magma led to supercooling and the crystallisation of abundant, strongly zoned phenocrysts of smaller size. Olivine crystallisation continued until the magma reached crustal levels. It is inferred from the chemistry of chromites and magnetites that the magma in the ring dyke was more evolved than those in the sill complex and that very oxidising conditions prevailed in the grey melilitite during the crystallisation of magnetite in this intrusive type. The high fO₂ may have resulted from the degassing of CO₂ after intrusion. Major and trace elements have been analysed for in eleven whole rock samples and the ⁸⁷Sr/⁸⁶Sr ratio was determined for seven of the same samples. The results of the geochemical study suggest that the Commonage melilitites were derived by the melting of a recently metasomatised region of the asthenosphere, probably under the influence of an ocean-island-type hotspot situated in the lower mantle.
- ItemOpen AccessSome aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa(1989) Hops, Jennifer Jane; Gurney, John JThe Jagersfontein kimberlite contains an abundance of both deformed high-temperature peridotites and Cr-poor megacrysts. The Cr-poor megacryst suite is represented by olivine, orthopyroxene, clinopyroxene and garnet. The megacrysts show features which are unique to Jagersfontein, a particularly notable feature being the absence of ilmenite and ilmenite-silicate intergrowths. Major element and REE compositions of the Cr-poor megacryst suite are consistent with a magmatic fractionation sequence. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the Cr-poor clinopyroxene megacrysts indicate a source similar to that of non-DUPAL ocean island basalts. Deformed peridotites at Jagersfontein have high calculated temperatures of equilibration (1132-1361°C), which are slightly lower but which overlap with those of the Cr-poor megacryst suite. Both the high-temperature peridotites and the Cr-poor megacrysts yield similar pressures of equilibration (51±2 kbar), indicating their association with a thermal perturbation and supporting a close spatial association between them. Olivine and pyroxenes in the high-temperature peridotites appear homogeneous, but compositional gradients were observed in several garnet porphyroclasts. These garnets show rim enrichments in TiO₂ and Na₂O. This zonation in the garnets is evidence for enrichment of the deformed peridotites shortly before kimberlite eruption. This enrichment is likely to be due to interaction with the megacryst magma. In addition, the high-temperature peridotites show a general enrichment in Fe, Ti, Na and Al with decreasing Ca/(Ca+Mg). Such features support a magmatic aureole model, in that temperature and degree of enrichment might be expected to increase with proximity to the megacryst magma body. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of clinopyroxene separates from the high-temperature peridotites are similar to those from oceanic peridotites. Modal abundances and olivine forsterite contents of the high-temperature peridotites are consistent with an origin as residues of partial melting events involving basalt formation. It is suggested that partial melting events, in the upper mantle beneath Jagersfontein, resulted in the formation of a depleted protolith which underplated the base of the Archaean lithosphere. This depleted protolith was subsequently enriched by interaction with the Cr-poor megacryst magma just prior to kimberlite eruption. The high-temperature peridotites therefore represent samples from the base of the lithosphere rather than from the convecting asthenosphere.
- ItemOpen AccessSome aspects of the mineral chemistry of the peridotite xenolith suite from the Bultfontein Diamond Mine, Kimberley, South Africa(1978) Lawless, Patrick Joseph; Gurney, John JThe xenolith suite from the Bultfontein Mine, Kimberley, South Africa, comprises lherzolites, harzburgites, and wehrlites with and without garnet; garnet olivine websterites and polymict peridotites. Many of the peridotites contain phlogopite and/or potassic-richterite which are interpreted as metasomatic minerals together with ilmenite, rutile, sulphides, and rare zircon. The variations in texture, mineral chemistry, equilibration conditions and bulk chemistry are described. No diamond-bearing assemblages have been found. The textures vary from coarse to porphyroclastic and to fluidal mosaic and laminated and disrupted. The deformed xenoliths have textures which may vary over distances of centimetres in individual specimens. The range of textures is greater than that reported for the xenoliths from Thaba Putsoa and Matsoku kimberlite pipes of northern Lesotho. There is no clear correlation between texture, mineral chemistry, or bulk chemistry in the Bultfontein xenoliths. Temperatures and pressures of equilibration have been calculated using three different geothermometers and three geobarometers, and the uncertainties involved in their application have been discussed. Some of the deformed xenoliths appear to have equilibrated at marginally higher temperatures and pressures than the undeformed xenoliths. The four-phase garnet lherzolites do not define a "perturbed geotherm" similar to that found in several other kimberlite localities in southern Africa and elsewhere.
- ItemOpen AccessA study of the diamonds, diamond inclusion minerals and other mantle minerals from the Swartruggens Kimberlite, South Africa(2001) McKenna, Neil; Gurney, John JWhile a clear and unequivocal model for the formation of diamonds is still lacking, the past few decades have seen a 'revolution' in the scientist's perception of mantle processes, particularly that regarding diamond formation. Kramers (1977) fIrst clearly recognised the ancient origin of diamonds from his studies of composited sulphide inclusions. However, it was not until mid-Archean ages were obtained from syngenetic garnet inclusions in diamond by Richardson et al. (1984), that it became generally accepted that most diamonds were in fact very much older than their kimberlitic hosts, and could therefore not have been precipitated from the kimberlite magma.
- ItemOpen AccessA study of unusual diamonds from the George Creek K1 Kimberlite dyke, Colorado(1995) Chinn, Ingrid Lee; Gurney, John JCathodoluminescence photomicrographs of diamonds from the George Creek Kl (section 28) kimberlite dyke in Colorado reveal complex intergrowth relationships between CO?-free and CO?-bearing diamond growth generations. The distribution of the CO?-bearing diamond in some specimens suggests that this generation is younger than the CO?-free diamond growth generation, although the age relationships are mostly ambiguous. CO?-bearing diamond appears to have crystallized from fluids which invaded fractures and etched embayments in the CO?-free diamond growth generation, which shows evidence of plastic deformation. The CO?-free diamond growth generation commonly exhibits features caused by extreme plastic deformation during mantle residence time. Abundant yellow-green plastic slip planes transect zones of customary blue cathodoluminescence in many diamonds, and raised lamination lines have been recognized on resorption surfaces. The complexity and intensity of surface etch features in most George Creek diamonds, including the CO?-bearing growth generation, suggests that the diamonds were subjected to multiple episodes of etching and resorption. Extensive development of hexagonal and trigonal etch pits resulted from the action of oxidizing CO?-H?O fluids, and some late-stage etching is believed to have occurred in the hypabyssal dyke system prior to kimberlite eruption.
- ItemOpen AccessSulphide Re-Os characterisation and nitrogen aggregation state of the Ellendale diamonds, Kimberley Province, Australia(2008) Smit, Karen Vena; Gurney, John J; Richardson, Steve; Le Roex, Anton; Shirey, Steve; Smith, CraigA suite of sulphide-bearing diamonds recovered from the Ellendale 4 and 9 pipes in the Ellendale diamond province of lamproite intrusions in north-western Australia have been investigated for their nitrogen aggregation state and the Re-Os isotope geochemistry of the sulphide inclusions. The Ellendale lamproites, dated at ca. 20 Ma (Allsopp et at., 1985) intrude the King Leopold Mobile Belt just south of the Kimberley craton and are thus an atypical, off-craton diamondiferous locality. The diamonds contain roughly equal proportions of peridotitic and eclogitic inclusions (Hall & Smith, 1984; Griffin et at., 1988; Jaques et at., 1989). The diamonds in this study range in size from 0.26 to 0.92 carats and are dominated, with the exception of one octahedron, by highly resorbed tetrahexahedroida.
- ItemOpen AccessThe use of upper mantle derived ilmenite to predict preservation of diamond parcels in Kimerlite(2001) Horwood, Steven J; Gurney, John JBibliography: leaves 80-95.