Petrology and geochemistry of the lower group chromitites and host rocks on the farm Zandspruit 168 JP, Western Bushveld Complex
- Authors: Botha, Michael James
- Date: 1988
- Subjects: Petrology -- Africa, Southern , Geochemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4905 , http://hdl.handle.net/10962/d1001565
- Description: The eight Lower Group chromitite layers of the Ruighoek Pyroxenite in the area west of the Pilanesberg (LGl - LG7), on the farm Zandspruit 168 JP, were sampled in borehole cores drilled through the succession. The stratigraphic succession faIls within Cameron's (980) units B to E of the Critical Zone. The lowermost layer of the Lower Group, the LG 1 chromitite, is located some 440 metres below the MG 2 (Middle Group) chromitite layer, above which the first appearance of cumulus plagioclase in the Complex is seen. By convention, this horizon is designated the top of the Ruighoek Pyroxenite in the Western Bushveld Complex. Homogeneous units of chromite-bearing orthopyroxenite (bronzitite), exhibiting inconspicuous layering defined in terms of variations in orthopyroxene grain-size host all but one of the Lower Group layers; the LG 4 chromitite layer is exposed within an olivine-rich subunit 23 metres in thickness (C₃ subunit). The cumulative thickness of chromitite is 2,92 metres or 0,8 per cent of the studied section, which is 381 metres in thickness. Minimum and maximum thicknesses of the LGl - LG7 layers exposed in drill core are 17 and 81 centimetres, respectively, with minor chromitite layers ranging between 2 and 5 centimetres in thickness. Weighted mean Cr₂0₃ contents of units B to E vary between 1,17 and 3,22 per cent, with the latter estimate representative of the D₂ subunit which hosts the LG 6 chromitite layer. The LG 6 is correlated with the Steelpoort layer of the Eastern Bushveld Complex, and varies between 76 and 81 centimetres in thickness under a large portion of the farm Zandspruit. An undisturbed succession striking N15°E and dipping 12 - 15°E is depicted within the studied area, which is bounded on the eastern side by the north-striking Frank fault. Major folding of the layered succession is evident to the north of the area, where the layering adjacent to the trace of the fault dips 35° to the southwest. Particular attention is paid in the present study to (a) the nature of chromitite layers and their host rocks, (b) the contrast between the mineral chemistry of weakly disseminated chromite and grains within massive ore layers, (c) concentrations of Cr, V, Ni, Co, Sc and Ti in orthopyroxene in relation to stratigraphic height, and levels of Sr, Ba and Zr associated with hypothetically pure, intercumulus plagioclase feldspar, and (d) possible mechanisms which induce crystallization of chromitite layers containing 50 per cent Cr₂0₃ from magma with a Cr content of less than 1 000 ppm. Electron microprobe studies of chromite in relation to mineralogical and textural environment clearly reveal that (a) the proportions of Cr and Al cations are linked to paragenesis: higher Al/Cr ratios characterize olivine-bearing domains, whereas grains intergrown with plagioclase feldspar exhibit low AI/Cr ratios, and (b) Al contents rise with a decline in Mg/(Mg + Fe²⁺) from high values to a value of 0,450, then decrease with a further decline in Mg/(Mg + Fe²⁺). The paragenetically later trend is emphasized in a large population of chromite grains which escaped early encapsulation in orthopyroxene crystals and continued to grow in the environment of intercumulus plagioclase. Within-and between-sample compositional variation of grains in silicate-rich domains is modelled in terms of in situ growth increments, diffusive homogenization of zonal structures, and residence time within interstitial melt. Fractionation trends, as measured by Mg/(Mg + Fe²⁺) ratios in whole-rock and/or microprobe studies of orthopyroxene, are reversed in relation to stratigraphic height towards the top of the B unit and in the overlying C unit. These data are supported, for example, by lower vanadium contents and higher Ni/Sc ratios in hypothetically pure orthopyroxene. Small olivine crystals in chromite-rich domains are enriched in Ni relative to coarse-grained olivine in adjacent dunite: a feature attributed to early isolation of primocrysts from magma in the former case, and in situ equilibration between olivine crystals and Ni-depleted residual melt in the latter case . Similarly, rising Ni contents and Mg/(Mg + Fe²⁺) ratios of orthopyroxene with increasing stratigraphic height in the footwall of the LG 6 chromitite layer, linked to a progressive decline in orthopyroxene grain-size, are effects which may arise out of early separation of interstitial melt from orthopyroxene cumulates. A model is thus proposed which (a) links the thickness of chromitite layers to the vertical separation between successive layers or the thickness of fine-grained orthopyroxenite in the footwall, (b) ascribes copious nucleation of chromite to liquid mixing of this footwall derived, Cr - depleted contaminant with influxes of hot, primitive magma, and (c) tenders the notion that the present modal proportion of mesostasis in the footwall of a chromitite layer serves as a reciprocal measure of the volume of fractionated exudate
- Full Text:
- Date Issued: 1988
- Authors: Botha, Michael James
- Date: 1988
- Subjects: Petrology -- Africa, Southern , Geochemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4905 , http://hdl.handle.net/10962/d1001565
- Description: The eight Lower Group chromitite layers of the Ruighoek Pyroxenite in the area west of the Pilanesberg (LGl - LG7), on the farm Zandspruit 168 JP, were sampled in borehole cores drilled through the succession. The stratigraphic succession faIls within Cameron's (980) units B to E of the Critical Zone. The lowermost layer of the Lower Group, the LG 1 chromitite, is located some 440 metres below the MG 2 (Middle Group) chromitite layer, above which the first appearance of cumulus plagioclase in the Complex is seen. By convention, this horizon is designated the top of the Ruighoek Pyroxenite in the Western Bushveld Complex. Homogeneous units of chromite-bearing orthopyroxenite (bronzitite), exhibiting inconspicuous layering defined in terms of variations in orthopyroxene grain-size host all but one of the Lower Group layers; the LG 4 chromitite layer is exposed within an olivine-rich subunit 23 metres in thickness (C₃ subunit). The cumulative thickness of chromitite is 2,92 metres or 0,8 per cent of the studied section, which is 381 metres in thickness. Minimum and maximum thicknesses of the LGl - LG7 layers exposed in drill core are 17 and 81 centimetres, respectively, with minor chromitite layers ranging between 2 and 5 centimetres in thickness. Weighted mean Cr₂0₃ contents of units B to E vary between 1,17 and 3,22 per cent, with the latter estimate representative of the D₂ subunit which hosts the LG 6 chromitite layer. The LG 6 is correlated with the Steelpoort layer of the Eastern Bushveld Complex, and varies between 76 and 81 centimetres in thickness under a large portion of the farm Zandspruit. An undisturbed succession striking N15°E and dipping 12 - 15°E is depicted within the studied area, which is bounded on the eastern side by the north-striking Frank fault. Major folding of the layered succession is evident to the north of the area, where the layering adjacent to the trace of the fault dips 35° to the southwest. Particular attention is paid in the present study to (a) the nature of chromitite layers and their host rocks, (b) the contrast between the mineral chemistry of weakly disseminated chromite and grains within massive ore layers, (c) concentrations of Cr, V, Ni, Co, Sc and Ti in orthopyroxene in relation to stratigraphic height, and levels of Sr, Ba and Zr associated with hypothetically pure, intercumulus plagioclase feldspar, and (d) possible mechanisms which induce crystallization of chromitite layers containing 50 per cent Cr₂0₃ from magma with a Cr content of less than 1 000 ppm. Electron microprobe studies of chromite in relation to mineralogical and textural environment clearly reveal that (a) the proportions of Cr and Al cations are linked to paragenesis: higher Al/Cr ratios characterize olivine-bearing domains, whereas grains intergrown with plagioclase feldspar exhibit low AI/Cr ratios, and (b) Al contents rise with a decline in Mg/(Mg + Fe²⁺) from high values to a value of 0,450, then decrease with a further decline in Mg/(Mg + Fe²⁺). The paragenetically later trend is emphasized in a large population of chromite grains which escaped early encapsulation in orthopyroxene crystals and continued to grow in the environment of intercumulus plagioclase. Within-and between-sample compositional variation of grains in silicate-rich domains is modelled in terms of in situ growth increments, diffusive homogenization of zonal structures, and residence time within interstitial melt. Fractionation trends, as measured by Mg/(Mg + Fe²⁺) ratios in whole-rock and/or microprobe studies of orthopyroxene, are reversed in relation to stratigraphic height towards the top of the B unit and in the overlying C unit. These data are supported, for example, by lower vanadium contents and higher Ni/Sc ratios in hypothetically pure orthopyroxene. Small olivine crystals in chromite-rich domains are enriched in Ni relative to coarse-grained olivine in adjacent dunite: a feature attributed to early isolation of primocrysts from magma in the former case, and in situ equilibration between olivine crystals and Ni-depleted residual melt in the latter case . Similarly, rising Ni contents and Mg/(Mg + Fe²⁺) ratios of orthopyroxene with increasing stratigraphic height in the footwall of the LG 6 chromitite layer, linked to a progressive decline in orthopyroxene grain-size, are effects which may arise out of early separation of interstitial melt from orthopyroxene cumulates. A model is thus proposed which (a) links the thickness of chromitite layers to the vertical separation between successive layers or the thickness of fine-grained orthopyroxenite in the footwall, (b) ascribes copious nucleation of chromite to liquid mixing of this footwall derived, Cr - depleted contaminant with influxes of hot, primitive magma, and (c) tenders the notion that the present modal proportion of mesostasis in the footwall of a chromitite layer serves as a reciprocal measure of the volume of fractionated exudate
- Full Text:
- Date Issued: 1988
Geology of the central and southern domains of the Koras Group, northern Cape Province
- Sanderson-Damstra, Christopher Gerald
- Authors: Sanderson-Damstra, Christopher Gerald
- Date: 1983 , 2013-04-17
- Subjects: Geology -- South Africa -- Cape of Good Hope
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5017 , http://hdl.handle.net/10962/d1006203 , Geology -- South Africa -- Cape of Good Hope
- Description: The Central and Southern Domains of the Koras Group, situated on the Doornberg Lineament, are the structurally preserved remnants of a once more widespread late-syntectonic cover sequence. Detailed examination of the field relationships, lithology and petrography together with new geochemical data (30 analyses) has resulted in the proposal of a new geological succession consisting mainly of two cycles of bimodal basaltic-rhyolitic volcanics with interbedded, immature conglomerates and lithic greywackes. These two cycles, named the Boomrivier and Leeudraai Formations, are overlain by the immature, polymictic orthoconglomerates and red arkosic sandstones of the Kalkpunt Formation. The first volcanic cycle commenced with the Lambrechtsdrif basaltic andesites and was followed, after a short hiatus, by the Swartkopsleegte rhyodacites. The second cycle comprises the Rouxville basalts and basaltic andesites and the Swartkop and Kenilworth rhyolites. Field evidence suggests that eruption of the rhyolitic and basaltic volcanics in the second cycle was contemporaneous. Geochemically, the volcanics can be classified as an "average-K" to high-K, tholeiitic, subalkaline association which exhibits general similarities to other Southern African bimodal associations e.g., the tholeiitic lavas of the Wilgenhoutsdrif Group. The Koras Group is petrologically similar to the Sinclair Sequence which is presently considered to be its coeval equivalent, but the dominantly calc-alkaline character of the Sinclair rocks distinguishes them from the dominantly tholeiitic Koras lavas. In a short literature review, the four main hypotheses for the petrogenesis of bimodal associations: liquid immiscibility, crystal fractionation, two-stage partial melting and separate magma sources, are described and the most feasible explanation for the origin of the Koras lavas is thought to be a "separate magma source" hypothesis in which two cycles of mantle-derived basalts and crustal-derived rhyolites were produced in a zone of high heat flow and erupted in an area of crustal weakness. The middle- or late-Proterozoic Koras Group was formed during unstable tectonic conditions, in a depositional setting that was probably controlled by late folding of the underlying pre-Koras sequences as well as the major strike-slip movement and subordinate dip-slip faulting in the Doornberg Lineament.
- Full Text:
- Date Issued: 1983
- Authors: Sanderson-Damstra, Christopher Gerald
- Date: 1983 , 2013-04-17
- Subjects: Geology -- South Africa -- Cape of Good Hope
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5017 , http://hdl.handle.net/10962/d1006203 , Geology -- South Africa -- Cape of Good Hope
- Description: The Central and Southern Domains of the Koras Group, situated on the Doornberg Lineament, are the structurally preserved remnants of a once more widespread late-syntectonic cover sequence. Detailed examination of the field relationships, lithology and petrography together with new geochemical data (30 analyses) has resulted in the proposal of a new geological succession consisting mainly of two cycles of bimodal basaltic-rhyolitic volcanics with interbedded, immature conglomerates and lithic greywackes. These two cycles, named the Boomrivier and Leeudraai Formations, are overlain by the immature, polymictic orthoconglomerates and red arkosic sandstones of the Kalkpunt Formation. The first volcanic cycle commenced with the Lambrechtsdrif basaltic andesites and was followed, after a short hiatus, by the Swartkopsleegte rhyodacites. The second cycle comprises the Rouxville basalts and basaltic andesites and the Swartkop and Kenilworth rhyolites. Field evidence suggests that eruption of the rhyolitic and basaltic volcanics in the second cycle was contemporaneous. Geochemically, the volcanics can be classified as an "average-K" to high-K, tholeiitic, subalkaline association which exhibits general similarities to other Southern African bimodal associations e.g., the tholeiitic lavas of the Wilgenhoutsdrif Group. The Koras Group is petrologically similar to the Sinclair Sequence which is presently considered to be its coeval equivalent, but the dominantly calc-alkaline character of the Sinclair rocks distinguishes them from the dominantly tholeiitic Koras lavas. In a short literature review, the four main hypotheses for the petrogenesis of bimodal associations: liquid immiscibility, crystal fractionation, two-stage partial melting and separate magma sources, are described and the most feasible explanation for the origin of the Koras lavas is thought to be a "separate magma source" hypothesis in which two cycles of mantle-derived basalts and crustal-derived rhyolites were produced in a zone of high heat flow and erupted in an area of crustal weakness. The middle- or late-Proterozoic Koras Group was formed during unstable tectonic conditions, in a depositional setting that was probably controlled by late folding of the underlying pre-Koras sequences as well as the major strike-slip movement and subordinate dip-slip faulting in the Doornberg Lineament.
- Full Text:
- Date Issued: 1983
The extrusive and intrusive basaltic rocks of the Molteno-Jamestown area
- Authors: Mitchell, Andrew Alexander
- Date: 1980
- Subjects: Basalt -- South Africa -- Cape of Good Hope
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4900 , http://hdl.handle.net/10962/d1001559
- Description: The Karoo basalt outliers between Molteno and Jamestown in the north-eastern Cape Province are associated with two central volcanic vent complexes, referred to in the text as the Brosterlea and the Modderfontein complexes. The basalts, particularly those associated with the Brosterlea complex, show geochemical variations throughout the sequence, a factor which has facilitated the subdivision of the Brosterlea basalts into a series of discrete units, each having its own chemical characteristics. Most of the basalt units at Brosterlea can be correlated with units identified in the Barkly East basalt suite. As is the case around Barkly East, the Brosterlea basalt units cannot be related to one another by any simple crystal fractionation or partial melting process, and the most feasible alternative explanation lies in the existence of inhomogeneities in the upper mantle at the time of generation of the magmas. New electron microprobe data are presented for the silicate phases in the Karoo basalts. Analyses of augites from a limited number of slides indicate that pyroxenes from different basalt units define different trends on the Ca - Mg - Fe triangular diagram. The plagioclases in the Brosterlea basalts are fairly Ca-rich (average 70% An), and the K-content of the plagioclases is shown to vary with the K-content of the parent basalt. A comparison of the Karoo Central Province with the younger Columbia River and Deccan Trap Provinces shows many similarities in the evolutionary history of the three provinces. The Karoo Province, however, is distinct from the other two provinces in some aspects of the geochemistry. Broadly speaking, the Karoo basalts are depleted in the incompatible elements relative to the Columbia River and Deccan basalts, and often enriched in the transition metals, most specifically Cr
- Full Text:
- Date Issued: 1980
- Authors: Mitchell, Andrew Alexander
- Date: 1980
- Subjects: Basalt -- South Africa -- Cape of Good Hope
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4900 , http://hdl.handle.net/10962/d1001559
- Description: The Karoo basalt outliers between Molteno and Jamestown in the north-eastern Cape Province are associated with two central volcanic vent complexes, referred to in the text as the Brosterlea and the Modderfontein complexes. The basalts, particularly those associated with the Brosterlea complex, show geochemical variations throughout the sequence, a factor which has facilitated the subdivision of the Brosterlea basalts into a series of discrete units, each having its own chemical characteristics. Most of the basalt units at Brosterlea can be correlated with units identified in the Barkly East basalt suite. As is the case around Barkly East, the Brosterlea basalt units cannot be related to one another by any simple crystal fractionation or partial melting process, and the most feasible alternative explanation lies in the existence of inhomogeneities in the upper mantle at the time of generation of the magmas. New electron microprobe data are presented for the silicate phases in the Karoo basalts. Analyses of augites from a limited number of slides indicate that pyroxenes from different basalt units define different trends on the Ca - Mg - Fe triangular diagram. The plagioclases in the Brosterlea basalts are fairly Ca-rich (average 70% An), and the K-content of the plagioclases is shown to vary with the K-content of the parent basalt. A comparison of the Karoo Central Province with the younger Columbia River and Deccan Trap Provinces shows many similarities in the evolutionary history of the three provinces. The Karoo Province, however, is distinct from the other two provinces in some aspects of the geochemistry. Broadly speaking, the Karoo basalts are depleted in the incompatible elements relative to the Columbia River and Deccan basalts, and often enriched in the transition metals, most specifically Cr
- Full Text:
- Date Issued: 1980
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