G. Chagondah, A. Hofmann, M. Elburg, L. Iaccheri, J. Kramers, A. Wilson
An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton. In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.
{"title":"Petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton","authors":"G. Chagondah, A. Hofmann, M. Elburg, L. Iaccheri, J. Kramers, A. Wilson","doi":"10.25131/sajg.126.0004","DOIUrl":"https://doi.org/10.25131/sajg.126.0004","url":null,"abstract":"\u0000 An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton.\u0000 In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45945423","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Geological, lithological, petrographical, geochemical, and geochronological data in the Palaeoproterozoic Richtersveld Subprovince/Magmatic Arc (RMA) of Precambrian basement rocks of the Vioolsdrif Domain in southern Namibia, strongly support linkages in the history of formation between the volcanic rock types of the Orange River Group (ORG) and the plutonic rocks of the Vioolsdrif Suite (VS). Previous age dating indicates volcanics of the ORG are more-or-less synchronous with granitic phases of the VS. Geochemical, mineralogical, and comprehensive field and petrological characteristics of the volcanic and granitic rocks suggest genetically linked, parallel igneous-effusive rock suites through processes of fractional crystallisation. Intermittent tapping of evolving residual magmas produced an extrusive carapace of volcanic rocks covering the granitic rocks in a classic magmatic differentiation context of parallel geochemical and lithological evolution. As the magma chamber fractionated plutonic phases of the VS at depth, it extruded residual liquids as volcanic flows and ejecta at surface to form the ORG. A first principal model, based on Ba, Rb and Sr trace element and SiO2, MgO, Al2O3, TiO2 major element behaviour, is supported by multivariate modelling of 28 major and trace elements in 129 rock analyses using Principal Component Analysis (PCA). PCA indicates the first four components account for more than 80% of the total compositional variance in all rock types lending comprehensive support for a linked geochemical differentiation model for both the igneous and the volcanic suites. Strong evidence for the magmatic co-evolution of the plutonic and effusive series includes the Cu-mineralisation event, which manifests itself as a resolvable, orthogonal fifth principal component, geochemically overprinting the intermediate ‘porphyry monzogranite’ differentiation stages, but not related to the terminal leuco-granites. The full spectrum of geochemical relationships is consistent with current models of a direct genetic relationship between evolving high Sr/Y magmas and hydrothermal porphyry Cu deposits, in overall agreement with the full context of field, rock, mineralogical, geochemical, and economic geology interpretations presented here.
{"title":"Geochemical evolution of magmatic rocks in the Vioolsdrif Domain, Namibia","authors":"R. Minnitt, K. Esbensen","doi":"10.25131/sajg.126.0001","DOIUrl":"https://doi.org/10.25131/sajg.126.0001","url":null,"abstract":"\u0000 Geological, lithological, petrographical, geochemical, and geochronological data in the Palaeoproterozoic Richtersveld Subprovince/Magmatic Arc (RMA) of Precambrian basement rocks of the Vioolsdrif Domain in southern Namibia, strongly support linkages in the history of formation between the volcanic rock types of the Orange River Group (ORG) and the plutonic rocks of the Vioolsdrif Suite (VS). Previous age dating indicates volcanics of the ORG are more-or-less synchronous with granitic phases of the VS. Geochemical, mineralogical, and comprehensive field and petrological characteristics of the volcanic and granitic rocks suggest genetically linked, parallel igneous-effusive rock suites through processes of fractional crystallisation. Intermittent tapping of evolving residual magmas produced an extrusive carapace of volcanic rocks covering the granitic rocks in a classic magmatic differentiation context of parallel geochemical and lithological evolution. As the magma chamber fractionated plutonic phases of the VS at depth, it extruded residual liquids as volcanic flows and ejecta at surface to form the ORG. A first principal model, based on Ba, Rb and Sr trace element and SiO2, MgO, Al2O3, TiO2 major element behaviour, is supported by multivariate modelling of 28 major and trace elements in 129 rock analyses using Principal Component Analysis (PCA). PCA indicates the first four components account for more than 80% of the total compositional variance in all rock types lending comprehensive support for a linked geochemical differentiation model for both the igneous and the volcanic suites. Strong evidence for the magmatic co-evolution of the plutonic and effusive series includes the Cu-mineralisation event, which manifests itself as a resolvable, orthogonal fifth principal component, geochemically overprinting the intermediate ‘porphyry monzogranite’ differentiation stages, but not related to the terminal leuco-granites. The full spectrum of geochemical relationships is consistent with current models of a direct genetic relationship between evolving high Sr/Y magmas and hydrothermal porphyry Cu deposits, in overall agreement with the full context of field, rock, mineralogical, geochemical, and economic geology interpretations presented here.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45179970","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Ncube, H. Wabo, T. Owen-Smith, A. Gumsley, N. Beukes
The Puduhush gabbro is located on the western margin of the proto-Kalahari Craton in Southern Africa. This gabbro intrudes the Volop Formation, which conformably overlies the Hartley Formation lava of the late Palaeoproterozoic Olifantshoek Group. Here we report a new U-Pb ID-TIMS baddeleyite age as well as petrographic, whole-rock geochemical and palaeomagnetic results for the Puduhush gabbro. The gabbro shows a well-preserved sub-ophitic texture between clinopyroxene and plagioclase, with minor amounts of amphibole, olivine, biotite and Fe-Ti oxides. The new U-Pb ID-TIMS baddeleyite age of 1 881 ± 1 Ma reported here for the Puduhush gabbro, together with existing ages for the Hartley Formation, define a ca.1 916 to 1 881 Ma age bracket for the Volop Formation. Our 1 881 ± 1 Ma age is also within error of ages reported for the oldest episode (so-called Episode 1) of the ca.1.89 to 1.83 Ga magmatism in the eastern and northern parts of the proto-Kalahari Craton. Our geochemical results also suggest compositional similarities between the Puduhush gabbro and Episode 1 magmatism, particularly the post-Waterberg sills. The virtual geomagnetic pole calculated here for the Puduhush gabbro (VGP: 1.6°N; 352.0°E; A95 = 14.2°) is consistent with the Episode 1 pole. All data are therefore combined to produce a new palaeomagnetic pole (11.7°N; 8.8°E, A95 = 9.3°) for Episode 1 magmatism. The present study provides the first evidence that the ca.1.89 to 1.83 Ga magmatism had a wider footprint that previously thought, extending to the western margin of the proto-Kalahari Craton. This wide-scale magmatism, previously proposed to be related to a back-arc extension setting, is here reinterpreted in the context of a mantle plume. Our results are consistent with the lithostratigraphic-based notion that at least parts of the red-bed successions (i.e., Olifantshoek and Waterberg Groups) that are hosts to the ca.1.89 to 1.83 Ga magmatism could be correlative units, representing an extensive sedimentary sequence that once covered large expanses of the proto-Kalahari Craton.
{"title":"The Puduhush gabbro in Griqualand West, South Africa: extending ca. 1.89 to 1.83 Ga intraplate magmatism across the proto-Kalahari Craton","authors":"S. Ncube, H. Wabo, T. Owen-Smith, A. Gumsley, N. Beukes","doi":"10.25131/sajg.126.0006","DOIUrl":"https://doi.org/10.25131/sajg.126.0006","url":null,"abstract":"\u0000 The Puduhush gabbro is located on the western margin of the proto-Kalahari Craton in Southern Africa. This gabbro intrudes the Volop Formation, which conformably overlies the Hartley Formation lava of the late Palaeoproterozoic Olifantshoek Group. Here we report a new U-Pb ID-TIMS baddeleyite age as well as petrographic, whole-rock geochemical and palaeomagnetic results for the Puduhush gabbro. The gabbro shows a well-preserved sub-ophitic texture between clinopyroxene and plagioclase, with minor amounts of amphibole, olivine, biotite and Fe-Ti oxides. The new U-Pb ID-TIMS baddeleyite age of 1 881 ± 1 Ma reported here for the Puduhush gabbro, together with existing ages for the Hartley Formation, define a ca.1 916 to 1 881 Ma age bracket for the Volop Formation. Our 1 881 ± 1 Ma age is also within error of ages reported for the oldest episode (so-called Episode 1) of the ca.1.89 to 1.83 Ga magmatism in the eastern and northern parts of the proto-Kalahari Craton. Our geochemical results also suggest compositional similarities between the Puduhush gabbro and Episode 1 magmatism, particularly the post-Waterberg sills. The virtual geomagnetic pole calculated here for the Puduhush gabbro (VGP: 1.6°N; 352.0°E; A95 = 14.2°) is consistent with the Episode 1 pole. All data are therefore combined to produce a new palaeomagnetic pole (11.7°N; 8.8°E, A95 = 9.3°) for Episode 1 magmatism. The present study provides the first evidence that the ca.1.89 to 1.83 Ga magmatism had a wider footprint that previously thought, extending to the western margin of the proto-Kalahari Craton. This wide-scale magmatism, previously proposed to be related to a back-arc extension setting, is here reinterpreted in the context of a mantle plume. Our results are consistent with the lithostratigraphic-based notion that at least parts of the red-bed successions (i.e., Olifantshoek and Waterberg Groups) that are hosts to the ca.1.89 to 1.83 Ga magmatism could be correlative units, representing an extensive sedimentary sequence that once covered large expanses of the proto-Kalahari Craton.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49245521","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores. The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively. It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth. The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 μM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies
南部非洲的中太古宙（2.96至2.91 Ga）Witwatersrand Mozaan序列包含多个单元，这些单元表明在大氧化事件发生约5亿年前，海洋水柱中存在游离分子氧。Mozaan群和West Rand群的Thalu和Brixton组的岩石地层相关性现在为含氧水柱提供了进一步的证据。这两个地层包含多个含碳酸锰泥岩层。本研究记录了这些地层及其地层学、矿物学、岩石学、全岩地球化学以及来自深层岩芯的稳定碳和氧同位素。含碳酸锰矿层出现在向上粗化单元的底部，表明在海平面上升期间沉积，随后是回归。泥岩在含锰碳酸盐结核周围表现出尖锐的接触和压实，表明后者早成岩生长。结核由菱锰矿岩芯和钾铁矿边缘组成，或由钾铁矿岩芯和铁白云石边缘组成，表明锰浓度从岩芯到边缘降低。相对于周围的泥岩，碳酸盐结核的锰含量明显富集。归一化为页岩的结核中的稀土元素和钇含量显示出重稀土元素过轻稀土元素富集以及铕和钇的正异常，表明沉淀来自混合海洋热液。结核碳酸盐的碳同位素和氧同位素相对于Pee Dee Belemnite分别贫13C（-12.2至-21.30/00）和18O（-14.9至-23.90/00）。结果表明，铁和锰的主要沉积方式是由铁和锰氧化细菌促进的沉淀。铁和锰来源于远端高温热液羽流中溶解的Fe2+和Mn2+。所研究单元的地层位置表明，沉积发生在中大陆架到外大陆架上，在那里，铁的氧化和沉积进行得足够远，环境可以转变为锰的氧化和沉淀。然后，这些沉淀物与有机碳反应形成菱锰矿，该菱锰矿非常局部地成核以生长结核。由于可用的锰全部被还原并结合到碳酸盐中，Fe3+氢氧化物也被任何过量的有机碳还原，并在生长的后期结合到结核边缘中。所提出的结核形成模型的主要含义是，在架子的水柱中可以获得游离分子氧，使氧化锰的微需氧化学石自养生物发挥作用。氧气浓度超过约5μM。这种浓度，以及所研究单元的沉积环境，都在先前研究的范围和参数内，这些研究表明了太古宙海洋“氧绿洲”的条件。
{"title":"Manganese carbonate-bearing mudstone of the Witwatersrand-Mozaan succession in southern Africa as evidence for bacterial manganese respiration and availability of free molecular oxygen in Mesoarchaean oceans","authors":"A. Smith, N. Beukes, J. M. Cochrane, J. Gutzmer","doi":"10.25131/sajg.126.0005","DOIUrl":"https://doi.org/10.25131/sajg.126.0005","url":null,"abstract":"\u0000 The Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores.\u0000 The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively.\u0000 It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth.\u0000 The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 μM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies ","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-02-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42771952","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
New evidence was sought for sub-pan silcrete formation along the southern margin of Ntwetwe Pan in the Botswana Makgadikgadi basin. This was achieved by integrating drillhole and surface sample descriptions with element and isocon analysis. The silcrete deposit which lies ~1.0 m below the pan surface, comprises Ntane sandstone fragments and detrital infill, cemented by amorphous silica. The deposit probably evolved during sequential periods of palaeo-lake infilling and drying since at least the Mid-Pleistocene. Changes in basin water pH brought about during fresher water infill interspersed with brine evapo-concentration and later evaporative pumping, induced the mobilisation and precipitation of the silica cement immediately below the pan floor.
{"title":"Factors leading to sub-surface pan silcrete formation in north-central Botswana","authors":"S. Ringrose, L. Cassidy, S. Diskin, S. Coetzee","doi":"10.25131/sajg.126.0003","DOIUrl":"https://doi.org/10.25131/sajg.126.0003","url":null,"abstract":"\u0000 New evidence was sought for sub-pan silcrete formation along the southern margin of Ntwetwe Pan in the Botswana Makgadikgadi basin. This was achieved by integrating drillhole and surface sample descriptions with element and isocon analysis. The silcrete deposit which lies ~1.0 m below the pan surface, comprises Ntane sandstone fragments and detrital infill, cemented by amorphous silica. The deposit probably evolved during sequential periods of palaeo-lake infilling and drying since at least the Mid-Pleistocene. Changes in basin water pH brought about during fresher water infill interspersed with brine evapo-concentration and later evaporative pumping, induced the mobilisation and precipitation of the silica cement immediately below the pan floor.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-02-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48460846","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
An earthquake (magnitude, ML = 3.8) occurred on 31 October 2019 at 11:19 hours Greenwich Mean Time (GMT) in the Sunduza area of the KwaZulu-Natal (KZN) Province in South Africa. The earthquake was located near the broad fracture zone of the Ntlakwe-Bongwan fault and was felt along the eastern coast of South Africa. The effects of the event were assessed by conducting macroseismic investigations through interviewing members of the public and completing questionnaires. Analysis of all the collected macroseismic data showed that maximum shaking with Modified Mercalli Intensity of IV-V and V were experienced near the epicentral area as well as along the coast. Similar intensity values were also experienced in some parts of Durban about 100 km northeast of the epicentre. Shaking at such long distances was attributed to ground motion amplification due to site effects. A fault plane solution of the event obtained using both the first motion polarities and amplitude ratios showed normal faulting along a fault oriented in a strike of 185.0°, dipping at 68.0° with a rake of -52.0°. The strike of the focal mechanism solution aligns with the strike of the Ntlakwe-Bongwan fault zone and nearby faults suggesting that the event might have ruptured along one of the fracture segments of this fault mapped to the north. The obtained dip is also in line with a high angle fault comparable to that observed along the Ntlakwe-Bongwan fault. The epicentral region is highly faulted although the activity is not known.
{"title":"Macroseismic analysis and the determination of a focal mechanism of the 31 October 2019, KwaZulu-Natal earthquake in South Africa","authors":"B. Manzunzu, V. Midzi, T. Zulu, K. Mphahlele","doi":"10.25131/sajg.126.0002","DOIUrl":"https://doi.org/10.25131/sajg.126.0002","url":null,"abstract":"\u0000 An earthquake (magnitude, ML = 3.8) occurred on 31 October 2019 at 11:19 hours Greenwich Mean Time (GMT) in the Sunduza area of the KwaZulu-Natal (KZN) Province in South Africa. The earthquake was located near the broad fracture zone of the Ntlakwe-Bongwan fault and was felt along the eastern coast of South Africa. The effects of the event were assessed by conducting macroseismic investigations through interviewing members of the public and completing questionnaires. Analysis of all the collected macroseismic data showed that maximum shaking with Modified Mercalli Intensity of IV-V and V were experienced near the epicentral area as well as along the coast. Similar intensity values were also experienced in some parts of Durban about 100 km northeast of the epicentre. Shaking at such long distances was attributed to ground motion amplification due to site effects. A fault plane solution of the event obtained using both the first motion polarities and amplitude ratios showed normal faulting along a fault oriented in a strike of 185.0°, dipping at 68.0° with a rake of -52.0°. The strike of the focal mechanism solution aligns with the strike of the Ntlakwe-Bongwan fault zone and nearby faults suggesting that the event might have ruptured along one of the fracture segments of this fault mapped to the north. The obtained dip is also in line with a high angle fault comparable to that observed along the Ntlakwe-Bongwan fault. The epicentral region is highly faulted although the activity is not known.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"1 1","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42043522","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Epembe Complex is one of the Mesoproterozoic (~1200 Ma) carbonatite alkaline complexes situated along the southern margin of the Congo Craton in northwestern Namibia. Nepheline syenites and minor syenites constitute the main lithologies, cross-cut by a calcite-carbonatite dyke. In order to constrain zircon forming-processes and magma sources, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Syenite zircons are generally unaltered and are characterised by positively sloping REE patterns in a chondrite-normalised diagram, with positive Ce anomalies. Syenite zircon further displays significant negative Eu anomalies attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognised. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon underwent low temperature fluid alteration and displays a cloudy appearance. Type 2 zircon is characterised by enrichment in LREE, Nb and Ti when compared to magmatic type 1 zircon. Carbonatite zircon displays a variety of textures and variable chemical compositions suggestive of the presence of both xenocrystal, altered and magmatic zircon. The Hf concentration and Hf isotope composition of type 1 and type 2 zircon are similar suggesting that zircon alteration did not affect the Hf isotope systematics. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+1.5 ± 0.2 to +1.9 ± 0.1) is consistent with the melts having been derived from a moderately Depleted Mantle.
{"title":"A zircon trace element and Hf isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline carbonatite complex, Namibia","authors":"M. Tshiningayamwe, R. Bolhar, P. Nex","doi":"10.25131/sajg.125.0021","DOIUrl":"https://doi.org/10.25131/sajg.125.0021","url":null,"abstract":"\u0000 The Epembe Complex is one of the Mesoproterozoic (~1200 Ma) carbonatite alkaline complexes situated along the southern margin of the Congo Craton in northwestern Namibia. Nepheline syenites and minor syenites constitute the main lithologies, cross-cut by a calcite-carbonatite dyke. In order to constrain zircon forming-processes and magma sources, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Syenite zircons are generally unaltered and are characterised by positively sloping REE patterns in a chondrite-normalised diagram, with positive Ce anomalies. Syenite zircon further displays significant negative Eu anomalies attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognised. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon underwent low temperature fluid alteration and displays a cloudy appearance. Type 2 zircon is characterised by enrichment in LREE, Nb and Ti when compared to magmatic type 1 zircon. Carbonatite zircon displays a variety of textures and variable chemical compositions suggestive of the presence of both xenocrystal, altered and magmatic zircon. The Hf concentration and Hf isotope composition of type 1 and type 2 zircon are similar suggesting that zircon alteration did not affect the Hf isotope systematics. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+1.5 ± 0.2 to +1.9 ± 0.1) is consistent with the melts having been derived from a moderately Depleted Mantle.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49163310","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Mansour, S. Tahoun, F. Oboh-Ikuenobe, M. Ahmed, T. Gentzis
This study investigates the Bahariya Formation using 39 rock-cutting samples from the Abu Gharadig Basin in the north Western Desert, Egypt. An approach combining palynomorph composition and palynofacies analysis with lithological composition and geophysical gamma ray log values is used to assess biostratigraphic age constraints, prevalent depositional palaeoenvironments, and relative sea level changes. A moderately diverse assemblage comprised mainly of terrestrial palynomorphs versus minor content of marine palynomorphs is identified. Marker spore and pollen taxa are used to name three interval zones of early Cenomanian, and early-middle Cenomanian ages. Quantitative distribution of particulate organic matter using cluster analysis reveals two palynofacies assemblages: an older assemblage dominated by phytoclasts (mainly wood particles); and a younger assemblage characterised by moderate abundances of amorphous organic matter and phytoclasts. Palynofacies analysis suggests deposition of the Bahariya Formation in fluvio-deltaic to inner neritic shelf environments. Based on the pronounced stratigraphic variations in the particulate organic matter composition, including terrestrial:marine palynomorph ratio, lithological and gamma-ray log data, three complete transgressive-regressive sequences and one incomplete sequence and their systems tracts are defined. The transgressive-regressive trends reflect 3rd order sequences. Additionally, successive changes in relative sea level indicate a continuous rise at the topmost part of the succession.
{"title":"Palynomorph composition and palynofacies analysis of the Cenomanian Bahariya Formation in the north Western Desert of Egypt: Depositional palaeoenvironment and sequence stratigraphic implications","authors":"A. Mansour, S. Tahoun, F. Oboh-Ikuenobe, M. Ahmed, T. Gentzis","doi":"10.25131/sajg.125.0023","DOIUrl":"https://doi.org/10.25131/sajg.125.0023","url":null,"abstract":"\u0000 This study investigates the Bahariya Formation using 39 rock-cutting samples from the Abu Gharadig Basin in the north Western Desert, Egypt. An approach combining palynomorph composition and palynofacies analysis with lithological composition and geophysical gamma ray log values is used to assess biostratigraphic age constraints, prevalent depositional palaeoenvironments, and relative sea level changes. A moderately diverse assemblage comprised mainly of terrestrial palynomorphs versus minor content of marine palynomorphs is identified. Marker spore and pollen taxa are used to name three interval zones of early Cenomanian, and early-middle Cenomanian ages. Quantitative distribution of particulate organic matter using cluster analysis reveals two palynofacies assemblages: an older assemblage dominated by phytoclasts (mainly wood particles); and a younger assemblage characterised by moderate abundances of amorphous organic matter and phytoclasts. Palynofacies analysis suggests deposition of the Bahariya Formation in fluvio-deltaic to inner neritic shelf environments. Based on the pronounced stratigraphic variations in the particulate organic matter composition, including terrestrial:marine palynomorph ratio, lithological and gamma-ray log data, three complete transgressive-regressive sequences and one incomplete sequence and their systems tracts are defined. The transgressive-regressive trends reflect 3rd order sequences. Additionally, successive changes in relative sea level indicate a continuous rise at the topmost part of the succession.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45519654","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Main Karoo Basin, and its Late Carboniferous to Early Jurassic stratified fill, the Karoo Supergroup, is the largest geological entity in South Africa and one of the economically most strategic because of its coal resources and potential shale gas resources. It also has a potential future in renewable energy in the form of geothermal energy. Although the basin has not received sufficient dedicated attention from a thermal perspective, numerous heat flow studies have resulted in the gradual accumulation of a substantial data base of the thermal conductivity of the constituent rocks, geothermal gradients and heat flow data. The main purpose of this paper is to collate this information, as well as new observations and thus generate a reference for future geothermal investigations. Approximately 900 conductivity measurements have resulted in well-established average conductivities for most stratified rock types, which vary by an order of magnitude from less than 0.3 W m-1 K-1 for coal to more than 5.0 W m-1 K-1 for sandstone. The maximum recorded temperature in the deepest parts of the basin (greater than 5 km) is 160°C and the overall average thermal gradient is approximately 29 K/km, which is greater than most other geological environments in southern Africa. The heat flow at 74 localities varies in the range 41 to 83 mW m-2 and the average for the central and southern parts of the basin is 62 ± 11 mW m-2 (34 values); this is approximately equal to the average heat flow for the mid-Proterozoic Namaqua tectonic province that underlies the southern half of the basin. The heat flow decreases to less than 50 mW m-2 in the northern and north-western parts of the basin, which values are typical of the underlying Archaean Kaapvaal Craton. Simple models illustrate the application of the heat flow and thermal conductivity data for calculating crustal temperature in the Karoo Basin.
卡鲁主盆地及其晚石炭世至早侏罗世分层充填卡鲁超群是南非最大的地质实体，也是经济上最具战略意义的地质实体之一，因为其煤炭资源和潜在的页岩气资源。它在地热能形式的可再生能源方面也有潜在的未来。尽管从热的角度来看，该盆地没有得到足够的专门关注，但大量的热流研究已经逐渐积累了组成岩石的热导率、地热梯度和热流数据的大量数据库。本文的主要目的是整理这些信息以及新的观测结果，从而为未来的地热调查提供参考。对于大多数层状岩石类型，大约900次电导率测量得出了公认的平均电导率，其变化幅度为一个数量级，从煤炭的小于0.3 W m-1 K-1到砂岩的大于5.0 W m-1。盆地最深处（大于5公里）的最高记录温度为160°C，总体平均热梯度约为29 K/公里，高于南部非洲的大多数其他地质环境。74个地区的热流在41至83 mW m-2之间变化，盆地中部和南部的平均值为62±11 mW m-2（34个值）；这大约等于位于盆地南半部下方的中元古代Namaqua构造省的平均热流。盆地北部和西北部的热流降至50 mW m-2以下，这是下伏太古宙卡瓦尔火山口的典型值。简单的模型说明了热流和热导率数据在卡鲁盆地地壳温度计算中的应用。
{"title":"Heat flow in the Main Karoo Basin, South Africa","authors":"M.Q.W. Jones, S. Scheiber-Enslin","doi":"10.25131/sajg.125.0022","DOIUrl":"https://doi.org/10.25131/sajg.125.0022","url":null,"abstract":"\u0000 The Main Karoo Basin, and its Late Carboniferous to Early Jurassic stratified fill, the Karoo Supergroup, is the largest geological entity in South Africa and one of the economically most strategic because of its coal resources and potential shale gas resources. It also has a potential future in renewable energy in the form of geothermal energy. Although the basin has not received sufficient dedicated attention from a thermal perspective, numerous heat flow studies have resulted in the gradual accumulation of a substantial data base of the thermal conductivity of the constituent rocks, geothermal gradients and heat flow data. The main purpose of this paper is to collate this information, as well as new observations and thus generate a reference for future geothermal investigations. Approximately 900 conductivity measurements have resulted in well-established average conductivities for most stratified rock types, which vary by an order of magnitude from less than 0.3 W m-1 K-1 for coal to more than 5.0 W m-1 K-1 for sandstone. The maximum recorded temperature in the deepest parts of the basin (greater than 5 km) is 160°C and the overall average thermal gradient is approximately 29 K/km, which is greater than most other geological environments in southern Africa. The heat flow at 74 localities varies in the range 41 to 83 mW m-2 and the average for the central and southern parts of the basin is 62 ± 11 mW m-2 (34 values); this is approximately equal to the average heat flow for the mid-Proterozoic Namaqua tectonic province that underlies the southern half of the basin. The heat flow decreases to less than 50 mW m-2 in the northern and north-western parts of the basin, which values are typical of the underlying Archaean Kaapvaal Craton. Simple models illustrate the application of the heat flow and thermal conductivity data for calculating crustal temperature in the Karoo Basin.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44548027","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it. Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established. The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets. The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.
{"title":"New constraints on the age of ore at Black Mountain mine, Bushmanland Ore District, South Africa","authors":"D. Cornell, A. Moses, T. Cawood, M. Richter","doi":"10.25131/sajg.125.0024","DOIUrl":"https://doi.org/10.25131/sajg.125.0024","url":null,"abstract":"\u0000 The chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it.\u0000 Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established.\u0000 The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets.\u0000 The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45034588","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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