Browsing by Author "Frimmel, Hartwig E"
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- ItemOpen AccessAspects of the geology, geochemistry and metamorphism of the lower orebody, Broken Hill deposit, Aggeneys(1993) Hoffmann, Dennis; Frimmel, Hartwig EThe Broken Hill deposit, Aggeneys, is a metamorphosed stratiform Cu-Pb-Zn-Ag sulphide deposit situated in the mid-Proterozoic supracrustal sequence of the Bushmanland Subprovince in the Namaqualand Metamorphic Complex. The deposit comprises two superposed orebodies, each consisting mainly of massive sulphide lenses and iron formation which are hosted within a metapelitic schist close to major quartzite horizons. This study is concerned with the tectonically lower orebody (LOB). The iron formation is magnetite-rich and contains varying proportions of (Fe,Mn)-rich silicates (garnet, amphibole, olivine, orthopyroxene, pyroxenoid), quartz and Cu-Pb-Zn-sulphides. These minerals occur in mm- to 5 cm-thick bands and are often traceable over tens of metres. The well preserved banding is considered to represent bedding. Five different varieties of silicate-rich mesobands in the iron formation can be distinguished based on the predominant mineral assemblage: (a) amphibole-olivine-quartz +/- garnet, (b) amphibole-quartz, (c) garnet-apatite-quartz +/- amphibole, (d) garnet-apatite-quartzorthopyroxene, (e) pyroxferroite-quartz +/- amphibole and (f) quartz. These rocks all contain magnetite, and Ba-rich biotite is common but is not always present.
- ItemOpen AccessChemostratigraphic correlation and Pb-Pb dating of carbonate sequences in the external Gariep belt and Kango inlier of the Saldania belt in Namibia and South Africa(2000) Fölling, Peter Gerhard; Frimmel, Hartwig EWorldwide most Neoproterozoic sequences are characterised by an intimate association of glaciogenic diamictite with marine carbonate deposits, indicating possibly rapid climatic changes from icehouse to greenhouse conditions. Such sequences also occur in the Pan-African Gariep Belt and in the Kango inlier of the Saldania Belt in Namibia and South Africa. Correlation of these sequences with others has been problematic because of poor age control, structural complications and poor exposure in the Saldania Belt.
- ItemOpen AccessFluid evolution and mineralising potential in the outer margin of the southern Gariep Belt(1998) Board, Warwick Stuart; Frimmel, Hartwig EFluid inclusion analyses of quartz veins of various generations from the outer margin of the southern Gariep Belt reveal the presence of two chemically distinct fluid populations: the pre-orogenic, rift-related Rosh Pinah ore-forming fluid and the orogenic fluids. The oreforming fluid is more saline and chemically more complex than the orogenic fluids, probably representing hot, chemically evolved hypersaline brines induced by rift-related magmatism. Four generations (D₁/D₂, D₃, D₄ and D₅) of orogenic veins are recognised on the basis of their structural relationships. The orogenic fluids are chemically similar, dominantly aqueous and generally of moderate to low salinities (<10 wt% NaCleq), with the few higher salinity estimates (10-16 wt% NaCleq) being attributed to the interaction of some of the orogenic fluids with evaporitic horizons. A trend of decreasing temperature with a decrease in age (D₄> D₁/D₂>D₃>D₅) is developed within the orogenic fluids, with the exception of those fluids related to the emplacement of the Kuboos Pluton, which yielded the highest temperatures. The D₁/D₂ fluids from the northern and Vanrhynsdorp areas were hotter than the fluids from the central area, whereas the D₃ and D₅ fluids exhibit no apparent temperature variation as a function of area. D₄ fluids show a decrease in temperature further away from the pluton. Estimates of the maximum metamorphic pressure recorded in the outer margin of the belt have been further constrained, using microthermometry results in conjunction with mineral assemblages, to 2.9-3.6 kbar, 2.9-3.1 kbar and 1.8-3.1 kbar for the northern, central and Vanrhynsdorp areas, respectively. Field observations and fluid inclusion analyses suggest channeled fluid flow on a regional rather than on an intra-formational scale, even though a rock-buffered system is suggested by oxygen isotope results. A different, largely rock-dominated fluid system was developed within the relatively impermeable carbonate units compared with the fluid system developed within the rest of the outer margin of the belt. The D₁/D₂ fluids probably represent circulating formation and metamorphic waters that were advectively forced out of the orogen and into the foreland, whereas channeled meteoric fluid flow most likely dominated during the transtensive phase of the orogeny. The D₄ fluids probably contain a magmatic component, in addition to the meteoric component. Comparisons between the ore-forming fluid and the orogenic fluids indicate that the latter have neither the required salinities, nor the required chemistry necessary in order to transport sufficient metals to produce a massive sulphide deposit the size of Rosh Pinah. The chemistry of the orogenic fluids are, however, similar to those associated with gold remobilisation and as such the potential for gold mineralisation related to these orogenic fluids should not be ruled out. Base metal sulphide exploration within the outer margin of the belt should be confined to rocks of the Hilda Subgroup. The recognition of highly saline, chemically complex nonorogenic quartz vein-hosted fluid inclusions is considered critical from the point of view of base metal sulphide exploration.
- ItemOpen AccessGeochemical fingerprinting of carbonate wall rock alteration at major base metal sulphide deposits in the Otavi Mountain Land, Namibia(1998) Chetty, Deshenthree; Frimmel, Hartwig E; Le Roex, AntonThe Otavi Mountain Land is a base metal ore province in which base metal sulphide deposits are hosted by platform carbonates in a foreland fold-and-thrust belt on the northern edge of the PanAfrican Damara Belt. Deposits have been classified as the Berg Aukas- and Tsumeb- types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at each of these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Optical cathodoluminescence imaging, electron probe micro analysis, X-ray fluorescence spectrometry, X-ray diffraction, high performance ion chromatography, proton probe micro analysis, stable isotope techniques, and fluid inclusion microthermometry were employed (i) to differentiate between carbonate generations associated with the alteration and mineralisation, particularly for the more economic Tsumeb-type deposits, represented by Tsumeb and Kombat, as well as in comparing between Berg Aukas- and Tsumeb-type deposits; (ii) to set constraints on the fluids effecting such alteration and associated mineralisation; (iii) to determine the relationship of the Khasib Springs deposit, for which little geochemical data exists, to deposits of the Tsumeb-type, and (iv) to identify those parameters which are diagnostic of Tsumeb-type mineralisation.
- ItemOpen AccessLate Proterozoic bedrock geology and its influence on neogene littoral marine diamondiferous trapsites, MA1-Sperrgebiet, Namibia(2001) Jacob, Jana; Frimmel, Hartwig E; Bluck, BrianNamibia's south western coast, the Sperrgebiet, hosts one of the world's largest diamond placer deposits. Diamond distribution in this placer deposit is directly related to the presence of a diamond-carrying gravel, the degree of reworking of the gravel and the quantity and quality of the diamond trapsites. The diamond-carrying gravel is present in the form of palaeo-beaches. Six Plio-Pleistocene beaches have been identified lying onshore between the Orange River mouth and Chameis Bay. The Plio-Pleistocene beaches are underlain by Late Proterozoic footwall. The Late-Proterozoic footwall has undergone extensive marine erosion and development of marine platforms during different sea-level stillstands. Diamond trapsites in the form of potholes and gullies are incised into palaeo platforms formed during the sea-level stillstands. The marine erosion palaeo-platforms have been cut into the predominantly siliciclastic rocks of the Gariep Belt. The Late Proterozoic Gariep Belt is divided into an eastern para- aulochthonous passive continental margin zone, the Port Nolloth Zone, and a western allochthonous Marmora Terrane. Previous work suggested that the Marmora Terrane had been thrust on top of the Port Nolloth Zone in a south-easterly direction and that the Marmora Terrane could be subdivided into three tectonostratigraphic units: the Schakalsberge Complex, the Oranjemund Complex and the Chameis Complex. In addition, previous work indicated that the Oranjemund Complex had been stacked tectonically between the Chameis Complex in the northwest and the Schakalsberge Complex in the southeast. However, this study questions the subdivision of the Marmora Terrane into three separate tectonic complexes. The rocks in the study area forms part of the newly defined Oranjemund Group, Oranjemund Sub-terrane.
- ItemOpen AccessA metamorphic and geochemical study of mafic rocks across the Pencksökket-Jutulstraumen Discontinuity, western Dronning Maud Land, East Antarctica(2005) Grosch, Eugene Gerald; Frimmel, Hartwig EA petrological and metamorphic comparison of Mesoproterozoic metabasic rocks on the easern margin of the Archaean Grunehogna Craton and the adjacent Maud Belt in western Dronning Maud Land, East Antarctica, revealed a difference in peak metamorphic conditions from T = ~275° to 730°C and P=2 to 10.7 kbar over a distance of only 30 km across a major glacial valley. The lower grade constraints were derived from average P-T calculations using THER-MOCALC and thermodynamic modeling of phase equilibria together with chlorite geothermometry. The high-grade P-T constraint for the westernmost part of the Maud Belt closest to the glacier, derived from hornblende-plagioclase thermometry and geobarometric calculations with a garnet amphibolite assemblage, is very similar to that reported for the eastern Maud Belt and, therefore, does not support the concept of a westward decreasing metamorphic field gradient as previously proposed. In conjunction with a recent geochronological study on the eastern Maud Belt, this study suggests that the inferred sub-glacial boundary between the Grunehogna Craton and the Maud Belt, known as the Pencksökket-Jutulstraumen Discontinuity, may represent a major thrust that developed during Pan-African orogenesis (possibly as the continuation of the East African Mozambique Belt into East Antarctica) prior to extension and its development as a normal listric fault or succession of fault slices during the Mesozoic break-up of Gondwana.
- ItemOpen AccessA study of the berg Aukas-type Pb-Zn-V deposits in the Otavi mountain land, Namibia(1993) Chadwick, Peter J; Frimmel, Hartwig EThis study is focussed on the petrographic and isotopic aspects of the crystallization processes taking place in carbonate-hosted Berg Aukas-type Zn-Pb-V deposits of the Otavi Mountain Land in Northern Namibia. Numerous studies have shown that ore formation in Mississippi Valley-type deposits bears a close relationship with carbonate diagenesis and basin evolution. The application of isotope (Sr , O, C) techniques as regards to timing and mechanisms of ore-forming processes, as a possible tool in exploration, is especially emphasized in this study. Detailed petrographic and isotope-geochemical investigations were carried out in the Late Precambrian deposits of Berg Aukas, situated just north of the town of Grootfontein, in addition to the Odin Prospect situated to the east of Kombat Mine. Both ore deposits display many similarities, but also show differences concerning the facies-bound character of the host rock, which is related to the paleogeographic position and the tectonic framework. In spite of these differences, similar complex diagenetic textures and structures suggest a comparable diagenetic evolution. Isotope-geochemical analyses have been performed on samples of consecutive crystallization generations, or dolomite types. This was checked petrographically using cathodoluminescence, and sample homogeneity was monitored by means of X-ray diffraction. The results reveal significant isotopic trends, which are interpreted to reflect the diagenetic evolution of the respective basin. A consistent strontium isotope trend is noted, and in both cases studied, the later crystallization generations are enriched in radiogenic strontium, as compared to the earlier ones, which tend to approximate the strontium isotope composition of contemporaneous seawater. The increase in ⁸⁷Sr/⁸⁶Sr ratios in subsequent diagenetic generations is explained with the pore fluids evolving towards more radiogenic compositions with advancing diagenetic stages. The influence of basinal brines enriched in radiogenic strontium as a consequence of the reaction with Rb-bearing phases, especially detrital (clay) minerals hosted by underlying basement rocks, is considered to be most likely. Stable isotope investigations carried out on identical samples also show certain trends, though these are less pronounced compared to the strontium studies. In general, the later crystallization generations are depleted in the heavy isotope of carbon and oxygen, which is explained in terms of changing physico-chemical conditions of the system, in combination with changing fluid compositions during the course of diagenetic evolution. By combining petrographic and isotopic observations, the genetic hypothesis for the ore deposits at Berg Aukas and Odin can be significantly constrained. In both cases, a model is proposed based on basinal brine migration during advanced diagenetic stages. These brines were enriched in radiogenic strontium due to the interaction with basement rocks. Thus, the combination of petrographic and isotope-geochemical methods has proved to constitute a powerful tool in tracing the diagenetic evolution, and in an exploration sense may be used to identify imprints of fluid circulation, which mobilized and concentrated metals during various stages of basin evolution.
- ItemOpen AccessTectono-thermal evolution of Gjelsvikfjella, Western Dronning Maud Land, Antarctica(2005) Bisnath, Avinash; Frimmel, Hartwig EEast Antarctica is dominated by the East Antarctic Craton, which consists of fragments of Archaean continental crust surrounded by polydeformed orogenic belts (e.g. Maud Belt). The Maud Belt in Dronning Maud Land preserves a high-grade polyphase tectono-thermal history. This belt has traditionally been interpreted as a Mesoproterozoic-age (1.3-0.9 Ga) mobile belt that was later affected by thermal overprint associated with little or no deformation during the Neoproterozoic (0.6-0.5 Ga). Therefore the Maud Belt has been used as a piercing point in Mesoproterozoic supercontinent reconstruction. However, with the growing number of geochronological data pointing to major structural reworking and not just a thermal overprint of the Mesoproterozoic crust during the Pan-African orogeny thereby challenging early models of Mesoproterozoic supercontinent reconstruction. Areas east and west of Gjelsvikfjella record major Pan-African reworking. Thus the outcrops of Gjelsvikfjella provide the linkage between the two areas. Yet, so far little data has been available for this crucial sector. Therefore, detailed fieldwork in the Gjelsvikfjella coupled with petrographic, geochemical and geochronogical data obtained from carefully selected samples reveal a complex tectono-thermal history for this part of the Maud Belt. The SHRIMP U-Pb zircon data obtained make it possible to differentiate between a series of magmatic and metamorphic events. The oldest event recorded is the formation of an extensive, 1140-1130 Ma, volcanic arc. This was followed by 1104 ± 8 Ma granitoids that might represent, together with so far undated mafic dykes part of a decompression melting-related bimodal suite. Only very few relics of late Mesoproterozoic granulite-facies metamorphism is constrained in 1070 Ma strain-protected domains.