Browsing by Author "Gurney, John"

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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 TiOâ‚‚ 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