Organic acids, including humic acid, play a significant role in the weathering of minerals containing metals such as Pt and Pd. They are also among the reactants which are under consideration for new hydrometallurgical methods of liberating unconventional PGE ores (such as the oxidised ores of the Great Dyke, Zimbabwe, and at locations in the Bushveld Complex of South Africa where PGE-bearing layers have been exposed to weathering). In order to better understand the processes operating during weathering of PGE-bearing chromitite seams of the Bushveld Complex, chromite concentrate originating from a South African chromium mine was subjected to reaction with different concentrations of synthetic humic acid. The results confirm the greater mobility of palladium in the environment compared to platinum. Crushed chromite concentrate showed greater mobility of Cr, but not of Pd or Pt, compared to uncrushed concentrate. Increasing the concentration of humic acid increased the amount of Pd and Pt in solution. These experiments give insight into the processes that govern the weathering of chromitite in the Bushveld Complex. The main Pd– and Pt-bearing minerals are not enclosed within chromite but occur at grain boundaries. Thus, they can be liberated by disaggregation of chromite and infiltration of water along chromite grain boundaries. Once in solution, Pd is more mobile than Pt and is dispersed further. Organic acids play an important role during the weathering process as they are capable of enhancing the mobility of the PGE, particularly Pd.
{"title":"The influence of humic substances on the weathering of PGE in chromitite of the Bushveld Complex: An experimental simulation of the weathering environment","authors":"E. Kotzé, S. Schuth, S. Goldmann, F. Holtz","doi":"10.25131/sajg.125.0020","DOIUrl":"https://doi.org/10.25131/sajg.125.0020","url":null,"abstract":"\u0000 Organic acids, including humic acid, play a significant role in the weathering of minerals containing metals such as Pt and Pd. They are also among the reactants which are under consideration for new hydrometallurgical methods of liberating unconventional PGE ores (such as the oxidised ores of the Great Dyke, Zimbabwe, and at locations in the Bushveld Complex of South Africa where PGE-bearing layers have been exposed to weathering). In order to better understand the processes operating during weathering of PGE-bearing chromitite seams of the Bushveld Complex, chromite concentrate originating from a South African chromium mine was subjected to reaction with different concentrations of synthetic humic acid. The results confirm the greater mobility of palladium in the environment compared to platinum. Crushed chromite concentrate showed greater mobility of Cr, but not of Pd or Pt, compared to uncrushed concentrate. Increasing the concentration of humic acid increased the amount of Pd and Pt in solution. These experiments give insight into the processes that govern the weathering of chromitite in the Bushveld Complex. The main Pd– and Pt-bearing minerals are not enclosed within chromite but occur at grain boundaries. Thus, they can be liberated by disaggregation of chromite and infiltration of water along chromite grain boundaries. Once in solution, Pd is more mobile than Pt and is dispersed further. Organic acids play an important role during the weathering process as they are capable of enhancing the mobility of the PGE, particularly Pd.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-10-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42704621","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 floor rocks of the northern lobe of the Bushveld Complex host several sill-like mafic-ultramafic bodies. In the present paper we evaluate whole rock data generated by exploration companies for sills on the farms Townlands, Amatava, Uitloop, Turfspruit and Rietfontein, located to the north of Mokopane, in order to constrain the origin of the sills and their mineralisation. Key observations include: (i) The sills have geochemical affinities to the Lower Zone (LZ) or Lower Critical Zone (LCZ). (ii) Most sills are enriched in sulphides and platinum-group elements (PGE) relative to most other LZ and LCZ cumulates. (iii) Most PGE mineralised intrusives have been emplaced into the carbonaceous-pelitic Duitschland Formation. (iv) The sills are spatially associated with the Mokopane gravity anomaly, possibly representing a major feeder zone to the Bushveld Complex. (v) The sills show evidence for assimilation of the sedimentary host rocks in the form of locally elevated δ34S, incompatible trace element contents and the presence of carbonaceous and pelitic country rock xenoliths. (vi) There is no correlation between PGE abundance and indicators of crustal contamination. Based on these data we propose that in the vicinity of the putative Mokopane feeder zone relatively fertile, unevolved magmas ascended through the crust initially as dykes. When intersecting the relatively fissile Duitschland Formation the mode of magma emplacement changed to one of sills. This facilitated contamination with sulphide- and graphite-rich carbonate and shale, triggering sulphide melt saturation. The sulphides were locally entrained and upgraded within the sills before precipitating, likely within flow dynamic traps.
{"title":"Petrogenesis of PGE mineralised intrusions in the floor of the northern Bushveld Complex","authors":"W. Maier, A. Brits, D. Grobler","doi":"10.25131/sajg.125.0019","DOIUrl":"https://doi.org/10.25131/sajg.125.0019","url":null,"abstract":"\u0000 The floor rocks of the northern lobe of the Bushveld Complex host several sill-like mafic-ultramafic bodies. In the present paper we evaluate whole rock data generated by exploration companies for sills on the farms Townlands, Amatava, Uitloop, Turfspruit and Rietfontein, located to the north of Mokopane, in order to constrain the origin of the sills and their mineralisation. Key observations include: (i) The sills have geochemical affinities to the Lower Zone (LZ) or Lower Critical Zone (LCZ). (ii) Most sills are enriched in sulphides and platinum-group elements (PGE) relative to most other LZ and LCZ cumulates. (iii) Most PGE mineralised intrusives have been emplaced into the carbonaceous-pelitic Duitschland Formation. (iv) The sills are spatially associated with the Mokopane gravity anomaly, possibly representing a major feeder zone to the Bushveld Complex. (v) The sills show evidence for assimilation of the sedimentary host rocks in the form of locally elevated δ34S, incompatible trace element contents and the presence of carbonaceous and pelitic country rock xenoliths. (vi) There is no correlation between PGE abundance and indicators of crustal contamination. Based on these data we propose that in the vicinity of the putative Mokopane feeder zone relatively fertile, unevolved magmas ascended through the crust initially as dykes. When intersecting the relatively fissile Duitschland Formation the mode of magma emplacement changed to one of sills. This facilitated contamination with sulphide- and graphite-rich carbonate and shale, triggering sulphide melt saturation. The sulphides were locally entrained and upgraded within the sills before precipitating, likely within flow dynamic traps.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44564178","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. Höhn, H. Frimmel, T. Will, N. Brodtmann, W. Price
The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments. We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.
{"title":"The depositional environment of the Koeris Formation in the Aggeneys-Gamsberg ore district, South Africa","authors":"S. Höhn, H. Frimmel, T. Will, N. Brodtmann, W. Price","doi":"10.25131/sajg.125.0018","DOIUrl":"https://doi.org/10.25131/sajg.125.0018","url":null,"abstract":"\u0000 The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments.\u0000 We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-08-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47385662","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 existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.
从最近的澳大利亚和南非古地磁数据来看，在新太古代早期，Vaalbara，即新太古代至古元古代Kaapvaal-Pilbara超克拉通的组合，其存在是值得怀疑的。显著相似的~2.7~2.5Ga地质序列（具有近层与近层的相关性）支持了新太古代末期的相干Vaalbara。在这里，我们报道了南非Klipriviersberg群的古地磁和岩石磁学结果，这是最初用于定义Vaalbara的最古老的岩石序列。正同褶皱测试表明，Witwatersrand向斜形成期间获得了高温剩磁分量。这种褶皱结构早于Vredefort撞击结构，其形成与中太古宙中央兰德群的沉积和克里普里维尔斯伯格群的挤压同步。所研究的Klipriviersberg群岩石没有直接定年，但根据最下层Ventersdorp超群的碎屑锆石年龄和侵入Witwatersrand超群的镁铁质岩床的U-Pb baddeleyite年龄，大多数岩石的年龄小于2 780至2 789 Ma，但比上覆的2 720至2 750 Ma Platberg群还要古老。Klipriviersberg群极点位于27.7°S，32.7°E，A95为11°。对Kaapvaal和Pilbara克拉通的中太古宙至新太古宙古极点的比较表明，它们的共同漂移路径穿过极圈，因此支持了Vaalbara在2.78至2.70 Ga层段的存在。
{"title":"A positive syn-fold test from the Neoarchaean Klipriviersberg Group of South Africa: Quo vadis Vaalbara?","authors":"M. de Kock, I. Malatji, H. Wabo, L. Maré","doi":"10.25131/sajg.125.0017","DOIUrl":"https://doi.org/10.25131/sajg.125.0017","url":null,"abstract":"\u0000 The existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-07-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43500522","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}
Mineralogically distinctive layers in the Bushveld Complex, South Africa, can usually be traced for hundreds of km in both the eastern and western limbs. They are remarkably uniform laterally in mineral chemical composition. There is one notable exception, namely the Pyroxenite Marker in the middle of the Main Zone in the eastern limb. It defines the boundary between the Lower Main Zone and Upper Main Zone. Toward the south in the eastern limb mineral compositions become more evolved, and ultimately the Pyroxenite Marker layer itself disappears and is replaced by magnetite-bearing rocks. In all previously published profiles through the eastern Bushveld, through a 400 m interval with the Pyroxenite Marker in the middle, there is a regular prolonged reversal in the anorthite content of plagioclase and mg# of pyroxene of about ten units, attributable to magma addition. In contrast, in the western limb there is no actual outcrop of this layer, but it has been located in the BK borehole from the centre of the limb. A similar magnitude of reversals in mineral compositions as in the east was reported in a previous study. In this study, a second profile was taken close to the eastern limit of the western limb to test if there was lateral variation comparable to that observed in the eastern limb. More mineralogical data are also reported from the borehole intersection. These two sections from the western limb show extremely similar changes in mineral compositions. In the eastern limb the location of the Pyroxenite Marker also shows an upward, abrupt change from primary pigeonite (below) to primary orthopyroxene (above). The same change occurs in the west, and at the same mineral compositions as in the east, and so this boundary can be defined by the pigeonite to orthopyroxene transition even in the absence of an actual pyroxenite layer. Hence, the term Pyroxenite Marker Transition is more applicable. Both in the east and west more primitive plagioclase compositions occur well above this boundary, and so magma addition and/or mixing continued well into the Upper Main Zone.
{"title":"Lateral uniformity of the Pyroxenite Marker Transition in the western Bushveld Complex, South Africa","authors":"R. G. Cawthorn","doi":"10.25131/sajg.125.0010","DOIUrl":"https://doi.org/10.25131/sajg.125.0010","url":null,"abstract":"\u0000 Mineralogically distinctive layers in the Bushveld Complex, South Africa, can usually be traced for hundreds of km in both the eastern and western limbs. They are remarkably uniform laterally in mineral chemical composition. There is one notable exception, namely the Pyroxenite Marker in the middle of the Main Zone in the eastern limb. It defines the boundary between the Lower Main Zone and Upper Main Zone. Toward the south in the eastern limb mineral compositions become more evolved, and ultimately the Pyroxenite Marker layer itself disappears and is replaced by magnetite-bearing rocks. In all previously published profiles through the eastern Bushveld, through a 400 m interval with the Pyroxenite Marker in the middle, there is a regular prolonged reversal in the anorthite content of plagioclase and mg# of pyroxene of about ten units, attributable to magma addition. In contrast, in the western limb there is no actual outcrop of this layer, but it has been located in the BK borehole from the centre of the limb. A similar magnitude of reversals in mineral compositions as in the east was reported in a previous study. In this study, a second profile was taken close to the eastern limit of the western limb to test if there was lateral variation comparable to that observed in the eastern limb. More mineralogical data are also reported from the borehole intersection. These two sections from the western limb show extremely similar changes in mineral compositions. In the eastern limb the location of the Pyroxenite Marker also shows an upward, abrupt change from primary pigeonite (below) to primary orthopyroxene (above). The same change occurs in the west, and at the same mineral compositions as in the east, and so this boundary can be defined by the pigeonite to orthopyroxene transition even in the absence of an actual pyroxenite layer. Hence, the term Pyroxenite Marker Transition is more applicable. Both in the east and west more primitive plagioclase compositions occur well above this boundary, and so magma addition and/or mixing continued well into the Upper Main Zone.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48995079","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}
M. Ncume, N. Hicks, L. Hoyer, J. Bristow, G. Botha
The Bumbeni Complex represents an Early Cretaceous volcanic complex that developed at the southern termination of the Lebombo mountain range in northern KwaZulu-Natal, South Africa. The volcano-sedimentary complex intrudes and disconformably overlies the Lebombo Group of the Karoo Large Igneous Province. The volcano-stratigraphy is defined by an assemblage of volcanic and volcaniclastic rocks as well as late stage quartz-syenite intrusions. Based on current geological and geophysical studies the complex is considered to represent the remnants of a collapsed cauldron structure. The stratigraphic succession is intricate and laterally discontinuous, comprising mafic and felsic lava flows, welded and non-welded tuffs, ignimbrites, lahar deposits, rhyolite domes and dykes and coarse-grained syenitic rocks. Rhyolite dome formation and collapse, and associated plinian eruptions and column collapse, were largely responsible for the variety of volcaniclastic rocks, including classic air-fall tuff deposits. The complex forms part of a northeast trending volcanic basement lineament known as the Bumbeni Ridge defined by aeromagnetic data.
{"title":"Lithostratigraphy of the Bumbeni Complex and its associated subdivisions, South Africa","authors":"M. Ncume, N. Hicks, L. Hoyer, J. Bristow, G. Botha","doi":"10.25131/sajg.125.0012","DOIUrl":"https://doi.org/10.25131/sajg.125.0012","url":null,"abstract":"\u0000 The Bumbeni Complex represents an Early Cretaceous volcanic complex that developed at the southern termination of the Lebombo mountain range in northern KwaZulu-Natal, South Africa. The volcano-sedimentary complex intrudes and disconformably overlies the Lebombo Group of the Karoo Large Igneous Province. The volcano-stratigraphy is defined by an assemblage of volcanic and volcaniclastic rocks as well as late stage quartz-syenite intrusions. Based on current geological and geophysical studies the complex is considered to represent the remnants of a collapsed cauldron structure. The stratigraphic succession is intricate and laterally discontinuous, comprising mafic and felsic lava flows, welded and non-welded tuffs, ignimbrites, lahar deposits, rhyolite domes and dykes and coarse-grained syenitic rocks. Rhyolite dome formation and collapse, and associated plinian eruptions and column collapse, were largely responsible for the variety of volcaniclastic rocks, including classic air-fall tuff deposits. The complex forms part of a northeast trending volcanic basement lineament known as the Bumbeni Ridge defined by aeromagnetic data.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46985994","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}
Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession. Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ‘stacks’ of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts.
{"title":"The petrogenesis and emplacement mechanism of layered ultramafic-mafic complexes of the Barberton Greenstone Belt: Insights from the Stolzburg Complex, South Africa","authors":"M. M. Tau, R. Bolhar, A. Wilson, C. Anhaeusser","doi":"10.25131/sajg.125.0013","DOIUrl":"https://doi.org/10.25131/sajg.125.0013","url":null,"abstract":"\u0000 Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession.\u0000 Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ‘stacks’ of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46509500","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}
K.J. Beaton, R. Gibson, G. Bybee, S. Walker, J. Diener, E. Baxter
Metamorphism in the lower Witwatersrand Supergroup exposed in the Vredefort Dome has previously been proposed to be related to elevated heat flow linked to the 2.06 Ga Bushveld magmatic event; however, there are no unambiguous chronological data to confirm this timing. Microtextural and mineral compositional analysis of a garnet-bearing metapelite in the northwestern collar of the Vredefort Dome suggests at least two metamorphic events with distinctly different P-T conditions preceding the Dome-forming meteorite impact at 2.02 Ga. THERMOCALC mineral equilibrium calculations yield P-T conditions of 500°C, 3.1 kbar for the M1 mineral assemblage garnet1-plagioclase-muscovite-biotite-chlorite-ilmenite. Thin, discontinuous, garnet2 overgrowths on the garnet1 porphyroblasts define a subsequent, M2, event with P-T conditions of 530°C, 5 kbar. Garnet Sm-Nd chronology yields an isochron age of 2 796.0 ± 1.5 Ma, indicating an early Ventersdorp (Klipriviersberg) timing of M1 metamorphism. Although the M2 garnet overgrowths are volumetrically too small to date, the calculated M2 pressure is consistent with the predicted overburden thickness above the lower Witwatersrand Supergroup during emplacement of the Bushveld Complex. While elevated, the M2 apparent geotherm (30°C/km) is significantly lower than the M1 apparent geotherm (46°C/km); however, thermal modelling suggests both events benefitted from local perturbations caused by contemporaneous sill emplacement. Our results thus show that the initial garnet-forming, mid-amphibolite facies metamorphism in the collar of the Vredefort Dome is not related to the emplacement of the Bushveld Complex, but rather to the early stages of magmatism associated with the Ventersdorp Large Igneous Province (LIP). Nonetheless, elevated heat flow associated with the Bushveld LIP also reached comparable amphibolite facies conditions.
{"title":"Amphibolite facies metamorphism in lower Witwatersrand Supergroup rocks exposed in the Vredefort Dome – a Ventersdorp LIP connection","authors":"K.J. Beaton, R. Gibson, G. Bybee, S. Walker, J. Diener, E. Baxter","doi":"10.25131/sajg.125.0016","DOIUrl":"https://doi.org/10.25131/sajg.125.0016","url":null,"abstract":"\u0000 Metamorphism in the lower Witwatersrand Supergroup exposed in the Vredefort Dome has previously been proposed to be related to elevated heat flow linked to the 2.06 Ga Bushveld magmatic event; however, there are no unambiguous chronological data to confirm this timing. Microtextural and mineral compositional analysis of a garnet-bearing metapelite in the northwestern collar of the Vredefort Dome suggests at least two metamorphic events with distinctly different P-T conditions preceding the Dome-forming meteorite impact at 2.02 Ga. THERMOCALC mineral equilibrium calculations yield P-T conditions of 500°C, 3.1 kbar for the M1 mineral assemblage garnet1-plagioclase-muscovite-biotite-chlorite-ilmenite. Thin, discontinuous, garnet2 overgrowths on the garnet1 porphyroblasts define a subsequent, M2, event with P-T conditions of 530°C, 5 kbar. Garnet Sm-Nd chronology yields an isochron age of 2 796.0 ± 1.5 Ma, indicating an early Ventersdorp (Klipriviersberg) timing of M1 metamorphism. Although the M2 garnet overgrowths are volumetrically too small to date, the calculated M2 pressure is consistent with the predicted overburden thickness above the lower Witwatersrand Supergroup during emplacement of the Bushveld Complex. While elevated, the M2 apparent geotherm (30°C/km) is significantly lower than the M1 apparent geotherm (46°C/km); however, thermal modelling suggests both events benefitted from local perturbations caused by contemporaneous sill emplacement. Our results thus show that the initial garnet-forming, mid-amphibolite facies metamorphism in the collar of the Vredefort Dome is not related to the emplacement of the Bushveld Complex, but rather to the early stages of magmatism associated with the Ventersdorp Large Igneous Province (LIP). Nonetheless, elevated heat flow associated with the Bushveld LIP also reached comparable amphibolite facies conditions.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48576928","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}
L. L. Le Bras, L. Milani, R. Bolhar, G. O’Sullivan
Uranium-lead dating of apatite was undertaken by Laser Ablation-Sector Field-Inductively Coupled Plasma Mass Spectrometry (LA-SF-ICPMS) in situ on apatite from principal rock types of the Loolekop phoscorite-carbonatite intrusion within the Phalaborwa Igneous Complex, South Africa. In situ U-Pb analysis on selected apatite produces U-Pb ages of 2 083.9 ± 41.9 Ma (n = 33; MSWD = 0.87), 2 020.4 ± 116.7 Ma (n = 18; MSWD = 0.91) and 2 034.3 ± 39.0 Ma (n = 17; MSWD = 0.6) for phoscorite, banded carbonatite and transgressive carbonatite, respectively, with a combined age of 2 054.3 ± 21.4 Ma (n = 68; MSWD = 0.86), which we interpret to indicate the timing of emplacement. Apatite U-Pb dates are similar to dates reported in previous studies using zircon and baddeleyite U-Pb systems from the same rock types, showing that apatite can be used as geochronometer in the absence of other commonly used U-Pb-bearing accessory minerals, not only in carbonatite-phoscorite complexes, but in all mafic igneous intrusions. Similar ages for zircon, baddeleyite and apatite indicate little to no re-equilibration of the latter, and suggest that the Loolekop Pipe intrusion cooled below 350°C within ~21 Ma of emplacement. This conclusion is supported by apatite BSE images and trace element systematics, with unimodal igneous trace element characteristics for apatite in each sample. The combination of in situ U-Pb geochronology, trace element geochemistry and BSE imaging makes apatite a useful tool to investigate the emplacement mechanisms of carbonatite-phoscorite complexes, which is particularly advantageous as apatite is one of the main mineral phases in these rock suites.
{"title":"Applying U-Pb chronometry and trace element geochemistry of apatite to carbonatite-phoscorite complexes – as exemplified by the 2.06 Ga Phalaborwa Complex, South Africa","authors":"L. L. Le Bras, L. Milani, R. Bolhar, G. O’Sullivan","doi":"10.25131/sajg.125.0015","DOIUrl":"https://doi.org/10.25131/sajg.125.0015","url":null,"abstract":"\u0000 Uranium-lead dating of apatite was undertaken by Laser Ablation-Sector Field-Inductively Coupled Plasma Mass Spectrometry (LA-SF-ICPMS) in situ on apatite from principal rock types of the Loolekop phoscorite-carbonatite intrusion within the Phalaborwa Igneous Complex, South Africa. In situ U-Pb analysis on selected apatite produces U-Pb ages of 2 083.9 ± 41.9 Ma (n = 33; MSWD = 0.87), 2 020.4 ± 116.7 Ma (n = 18; MSWD = 0.91) and 2 034.3 ± 39.0 Ma (n = 17; MSWD = 0.6) for phoscorite, banded carbonatite and transgressive carbonatite, respectively, with a combined age of 2 054.3 ± 21.4 Ma (n = 68; MSWD = 0.86), which we interpret to indicate the timing of emplacement. Apatite U-Pb dates are similar to dates reported in previous studies using zircon and baddeleyite U-Pb systems from the same rock types, showing that apatite can be used as geochronometer in the absence of other commonly used U-Pb-bearing accessory minerals, not only in carbonatite-phoscorite complexes, but in all mafic igneous intrusions. Similar ages for zircon, baddeleyite and apatite indicate little to no re-equilibration of the latter, and suggest that the Loolekop Pipe intrusion cooled below 350°C within ~21 Ma of emplacement. This conclusion is supported by apatite BSE images and trace element systematics, with unimodal igneous trace element characteristics for apatite in each sample. The combination of in situ U-Pb geochronology, trace element geochemistry and BSE imaging makes apatite a useful tool to investigate the emplacement mechanisms of carbonatite-phoscorite complexes, which is particularly advantageous as apatite is one of the main mineral phases in these rock suites.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42812372","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}
H. El Atfy, M. Kora, R. Spiekermann, A. Jasper, D. Uhl
Fossil evidence for wildfires, in form of fossil charcoal, is known from a large number of Cretaceous localities worldwide and it has repeatedly been argued that wildfires were connected to the evolution and radiation of angiosperms during this period. The present study provides new evidence (in form of macro-charcoal) for the occurrence of wildfires during deposition of the Lower Cretaceous (pre-Aptian) Malha Formation at Wadi Budra of the Sinai Peninsula, Egypt. Despite growing evidence for the worldwide occurrence of wildfires during the Cretaceous, the available database for pre-Aptian wildfires is still rather scarce for large regions when seen on a global scale, hampering causal interpretations concerning the interactions between fire ecology and the evolution of ecosystems during these stages.
{"title":"Further evidence for Cretaceous wildfires: macro-charcoal from the Malha Formation at Wadi Budra, west-central Sinai, Egypt","authors":"H. El Atfy, M. Kora, R. Spiekermann, A. Jasper, D. Uhl","doi":"10.25131/sajg.125.0011","DOIUrl":"https://doi.org/10.25131/sajg.125.0011","url":null,"abstract":"\u0000 Fossil evidence for wildfires, in form of fossil charcoal, is known from a large number of Cretaceous localities worldwide and it has repeatedly been argued that wildfires were connected to the evolution and radiation of angiosperms during this period. The present study provides new evidence (in form of macro-charcoal) for the occurrence of wildfires during deposition of the Lower Cretaceous (pre-Aptian) Malha Formation at Wadi Budra of the Sinai Peninsula, Egypt. Despite growing evidence for the worldwide occurrence of wildfires during the Cretaceous, the available database for pre-Aptian wildfires is still rather scarce for large regions when seen on a global scale, hampering causal interpretations concerning the interactions between fire ecology and the evolution of ecosystems during these stages.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"1 1","pages":""},"PeriodicalIF":1.8,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41605909","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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