Browsing by Author "Gurney, John"

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    A detailed investigation into diamond-bearing xenoliths from Newlands Kimberlite, South Africa
    (2001) Menzies, Andrew; Gurney, John
    This study involves a detailed investigation of diamond-bearing peridotite and eclogite from Newlands kimberlite, located on the Kaapvaal craton, South Africa. Eighteen peridotitic garnet macrocrysts (1 to 3 cm in size) containing diamond, and one with graphite, consist predominantly of lilac garnet with diamond (or graphite) ± chromite and altered silicates. The garnets are predominantly high-Cr(> 8 wt¾), Ti-depleted, and extremely sub-calcic {< 3 wt¾). However, three specimens have high-Cr calcic compositions. The chromites are Cr-rich, Ti-depleted, whilst the solitary olivine is Fo 93 .4. The sub-calcic garnets display a narrow range of trace element signatures, depleted in Zr, Y, Ga, and Ti, whilst Sr and LREE's are enriched. The [REE] 0 patterns are all very similar and display a sinusoidal shape. The most calcic diamondiferous garnet analysed at Newlands yields a [REE] 0 pattern similar to "fertile" mantle garnets. The major and trace element geochemistry of the sub-calcic garnets is similar to inclusions in diamonds and diamond-bearing peridotites world-wide. Geothennometry yields temperatures ranging from 900 to 1050 °C, equivalent to pressures of 40 to 55 kbar (based on a 37-38 mW/m2 geothenn determined from Newlands coarse peridotites). This P-T range is within the diamond stability field. Four diamond-bearing peridotitic garnet macrocrysts are Re depleted and yield a range of ages spanning from the Proterozoic (minimum TRD 1.77 Ga) to the mid-Archrean (maximum TMA 3.52 Ga). Seventeen eclogites (2 to 6 cm in size) containing diamond are dominated by coarse-grained garnet and clinopyroxene, with primary trace phases including sulphides and rutile. Alteration is pervasive and ranges from minor grain boundary effects to massive mineral (primarily clinopyroxene) replacement. The garnets have Na2O concentrations greater than 0.07 wt%, whilst the clinopyroxenes have K2O concentrations greater than 0.08 wt%. Both phases are Fe-rich and Ca- and Mg- poor relative to other eclogites from Newlands. The garnets display a very restricted range of trace element compositions and are relatively enriched in Ga, Zn, Mn and Ti, and depleted in Cr and Ni relative to most diamond-free eclogites from Newlands. The garnets have similar [REE] 0 patterns that are [HREE]n enriched. The clinopyroxenes also display a restricted range of trace element compositions (with one exception), and are relatively enriched in Ga, Zn, Zr, Mn and Ti, and depleted in Ni relative to diamond-free eclogites from Newlands. The clinopyroxenes have similar [REE]n patterns that are [LREE]n enriched. Bulle rock major element compositions (calculated assuming a 50:50 gamet-clinopyroxene ratio) are compositionally similar to ancient (Proterozoic or Archrean?) magnesian basalts. A statistical analysis of eclogites at Newlands indicates that those associated with diamonds have a discernible geochemistry. The diamond-bearing eclogites, with one exception, have textures and mineral geochemistry equivalent to Group I (and Group B) eclogites (depending on the classification scheme used). Geothermometry yields temperatures between 920 to 1080 °C, equivalent to pressures between 42 and 58 kbar (based on a 37-38 mW/m 2 geotherm), which lies within the diamond stability field. Re-Os systematics for the diamond-bearing eclogites are indicative of formation ages in the Archrean. These eclogites had 187 Os/1 88 Os substantially higher than chondritic mantle at circa 3 Ga. The Re-Os systematics, major and trace element mineral chemistry, and stable isotopes (albeit preliminary) of the diamond-bearing eclogites from Newlands are consistent with a protolith that has interacted within surficial environments. This implies the operation of plate tectonics since the early part of the Earths history. The peridotitic diamonds occur in two primary forms, namely (< I mm) single octahedra or octahedral aggregates. The eclogitic diamonds range in size from approximately 100 μm within diamond aggregates to large single crystals up to 2 mm. The dominant morphology is octahedral but there are also significant numbers of cubes and dodecahedrons, and some cubo-octahedrons. Cathodoluminescence indicates that the eclogitic diamonds grew in at least three distinct periods. FTIR spectra indicate that the peridotitic and eclogitic diamonds display different nitrogen concentrations and platelet peak positions. Specifically, the vast majority of harzburgitic diamonds are Type II (or near Type II) whilst all eclogitic diamonds are Type I, with significantly higher N concentrations. Based on these criteria, the majority of diamonds at Newlands are of eclogitic origin. Furthermore, this difference implies that the peridotitic and eclogitic diamonds at Newlands are derived from separate sources. Both sets of diamonds have type IaAB aggregation states ranging from 0 to 20 %, consistent with diamond formation in the Archrean for both parageneses. Time-averaged temperatures imply that the diamonds formed at higher temperatures than their ambient levels at the time ofkimberlite eruption. The early Proterozoic to mid-Archrean Re-Os ages obtained for both the diamond-bearing garnet macrocryst and diamond-bearing eclogites at Newlands overlap the major crustal building periods of the Kaapvaal craton ( de Wit et al., 1992). This implies that cratonic root stabilisation beneath the Kaapvaal craton is potentially coeval with crustal formation (Kramers, 1979; Richardson et al., 1984; Richardson et al., 1990; Richardson and Harris, 1997). The presence of diamond implies that the SCLM may have extended to depths of approximately 200 km by the end of the Archrean. In addition, the apparent lack of any systematic age variation with calculated equilibrium conditions implies that the SCLM and overlying continental crust nuclei have been coupled since formation. Once formed, these assemblages remain coupled and, under favourable circumstances, can survive the destructive forces of plate tectonics.
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    A study of mantle inclusions in the Koffiefontein Kimberlite pipe, South Africa
    (1980) CARDOSO_P; Gurney, John; Le Roex, Anton
    Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusuaLly scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general, they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration, and one appears to link tog~ther the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in Tio2 to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenolith s, or megacrysts. A small number of concentrate grains have compositions ma tched by the diamond inclusions. Attempts to a pply geothe r mobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which areascribe d to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromi te.
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    A pertographic and geochemical study of selected peridotitic and pyroxenitic xenoliths from the three kimberlite localities in the Lac de Gras region, Northwest Territories, Canada
    (2002) Doyle, Patricia Mary; Le Roex, Anton; Gurney, John
    Eighty-five peridotitic and pyroxenitic xenoliths from the Arnie, Pigeon and Misery kimberlites in the Lac de Gras region, Northwest Territories, Canada were selected for inclusion in this study. The three kimberlites are situated within a 40 km radius of one another on the BHP property, and all are diamond-bearing. The Misery kimberlite is presently being mined, and the Pigeon kimberlite is part of · the future BHP-DiaMdt Ekati mining plan. A petrographic study of the xenoliths using both transmitted light microscopy and binocular microscopy was followed by major anti trace element analysis. Major element compositions of individual minerals were determined using a wavelength dispersive electron microprobe, and trace element abundances were determined using laser ablation ICP-MS. Pressures and temperatures of equilibration were then determined using 'garnet-olivine, garnet-orthopyroxene and trace element geothermobarometers (TNi, Per).
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    A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa
    (1980) Cardoso, P; Gurney, John
    Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusually scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho, and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration and one appears to link together the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in TiO₂ to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenoliths, or megacrysts. A small number of concentrate grains have compositions matched by the diamond inclusions. Attempts to apply geothermobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which are ascribed to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromite.
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    A study of the kimberlites, diamonds and associated rocks and minerals from the monastery mine, South Africa. Volume 1: Text
    (1986) Moore, Rory O; Gurney, John
    The hypabyssal quarry kimberlite is the most abundant phase at Monastery. Four petrographically distinct varieties are recognized. Mineralogically, the Ql and Q4 kimberlites are opaque oxide-rich serpentine-phlogopite kimberlites, the Q2, a phlogopite-monticellite kimberlite and the Q3, a monticellite-phlogopite kimberlite. The East-end kimberlite is an opaque oxide-rich serpentine-monticellite kimberlite, but is poorly exposed and highly weathered. The breccia kimberlite hosts abundant country rock fragments in a soft serpentinous matrix. It is an opaque oxide-rich phlogopite serpentine kimberlite breccia. The precursor kimberlite dyke associated with the diatreme is an opaque oxide-rich calcite kimberlite. The ultramafic xenoliths at Monastery are predominantly coarse grained and exhibit a high incidence of modal metasomatism. Some textures intermediate between porphyroclastic and granuloblastic were noted. Significant annealing has occurred. Garnet, orthopyroxene and clinopyroxene may have been derived by exsolution from high temperature aluminous orthopyroxenes. Minerals in the peridotites and pyroxenites have similar compositions to those from other localities. Two groups of phlogopite composition have been noted. Wehrlitic rocks have phlogopite similar to that in richterite-bearing peridotites from Kimberley.
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    The geochemistry and origin of xenoliths from the Roberts Victor Mine
    (1978) Hatton, Christopher John; Gurney, John
    The results of an investigation into the geochemistry of xenoliths from the Roberts Victor mine are reported. 750 nodules were examined in hand specimen at the Roberts Victor mine, and in accordance with previous observations greater than 95% are eclogite xenoliths. The paucity of peridotite nodules is attributed to selective sampling by the kimberlite, and to the preferential abrasion of the more readily altered peridotites. 250 samples were selected for further study in slabbed section. Mineral analyses by electron microprobe of 100 samples, and whole rock analyses by X-ray fluorescence of 40 samples were carried out. Orthopyroxene bearing eclogites (garnet websterites) constitute a significant portion of the eclogite suite, and orthopyroxene plays an important role in the evolution of the Roberts Victor eclogites. Three categories of inhomogeneous eclogites, clinopyroxene-megacryst-bearing eclogites, inhomogeneous kyanite eclogites and chrome-rich eclogites were examined in detail. (i) Megacryst-bearing eclogites originated by crystallisation of clinopyroxene megacrysts within volatile-rich portions of an eclogite magma and the subsequent entrapment of the megacryst within biminerallic eclogite. (ii) Inhomogeneous kyanite eclogites consist of biminerallic eclogite enclosing a kyanite eclogite zone of markedly different mineral and whole rock chemistry. The kyanite eclogite zone most likely originated by liquid immiscibility. (iii) Most chrome-rich eclogites examined exhibit marked variation in chemistry over very short distances. These variations have been attributed to fluctuations in volatile content during crystallisation, disequilibrium partial melting of garnet lherzolite and to the preservation of a diffusion gradient between two bodies of different composition. The diverse peridotite suite at Roberts Victor includes dunite, wehrlite, harzburgite, garnet harzburgite, lherzolite and garnet lherzolite. A connecting link between peridotite and eclogite is represented by an altered garnet lherzolite in which a thin layer of biminerallic eclogite is preserved on one edge. Detailed studies of the mineral chemistry of an eclogite containing both diamond and graphite indicate that this rock equilibrated at 40 to 45 kbar in the temperature range 1000 to 11 so 0 c. Diamond and graphite a'."e thought to have formed by the reduction of CO2 , introduced together with Ti, Kand S, during the melting of garnet lherzolite to produce the eclogitic magma from which the rock crystallised. The Roberts Victor eclogite suite is thought to have originated by volatile induced partial melting of garnet lherzolite. The diversity of the eclogite suite is related to the relative importance of the volatile input and the thermal input during partial melting, with high volatile, low temperature melting leading to the production of heterogeneous, rapidly crystallised eclogite bodies (type II eclogites) and low volatile, higher temperature melting leading to the production of a larger, relatively homogeneous eclogite body (type I eclogite).
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    The megacryst suite from the Schuller kimberlite, South Africa
    (1991) de Bruin, Deon; Gurney, John