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Notes: The metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan-African Gariep orogenic belt in south-western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro-) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio-riebeckite with little ferro-glaucophane component in a highly metasomatized albite-rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low-P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio-riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.

Notes: Summary Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na+ and Ca2+, with Cl− or HCO 3 − being the dominant anion, whereas K+, Mg2+, and SO 4 2− are subordinate. Most fluid inclusions have elevated NH 4 + concentrations which are directly correlated with those of N03. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected. A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and δ18Ofluid values around O%o. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7–7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.

Notes: Summary The auriferous conglomerate horizons (reefs) in the Witwatersrand Basin of South Africa are in many places cut by hydrothermal quartz veins that frequently contain sulphide, bitumen, and, less commonly, free gold. New Pb isotopic results for the Ventersdorp Contact Reef which has experienced particularly intense hydrothermal alteration, reaffirm the radiogenically enriched nature of the Pb in this reef and provide additional insight into its origin. This study focuses on analyses of galena, chalcopyrite, pyrrhotite, and bitumen from quartz veins, which presumably formed during the 2.020 Ga Vredefort meteorite impact event. The radiogenic, mainly uranogenic, component of the Pb appears to have been derived almost entirely from uraninite in the surrounding reef rock. Assigning a 2.02 Ga age of mineralization and constructing secondary isochrons for paragenetically early galena and chalcopyrite, ages of the source uraninite are calculated as 2.6–2.4 Ga. No special significance is ascribed to these source ages, which likely reflect extensive radiogenic Pb loss from originally somewhat older detrital uraninite during transport, sedimentation, and post-burial alteration. Analyses of detrital(?), syngenetic, and epigenetic pyrite from a reef conglomerate define a subsidiary linear array with a considerably shallower slope. Interpreted as a secondary isochron, the array gives an implausibly young mineralization and/or source age indicative of a superimposed isotopic disturbance. Five analyses of paragenetically late chalcopyrite and pyrrhotite plot on a207Pb/204pb versus206Pb/204Pb diagram with nearly constant207Pb/204Pb of 24.1–24.6 despite a huge range in206Pb/204Pb from 60–230. This trend is further revealed by Pb with similar207Pb/204Pb but still higher206Pb/204Pb ratios (up to 949) in bitumen globules deposited on quartz crystals lining cavities in the veins. This nearly horizontal array cannot be interpreted as a secondary isochron, and requires the addition of virtually pure206pb to a more normal, radiogenically-enriched Pb. The most plausible explanation for this decoupling of the238U and235U decay schemes is that an intermediate daughter isotope, most likely222Rn, diffused from uraninite and was selectively captured by the bitumen where it subsequently decayed to206Pb. Whether the 206 Pb was acquired mainly at the time of hydrothermal activity by fluids at elevated temperatures, or more or less continuously until the present remains unresolved.

Notes: Summary In the West Rand Group of the 3.07–2.71 Ga old Witwatersrand Supergroup, South Africa, a series of banded iron-formations occur. They are of chemical origin and were deposited in an offshore shelf environment. The coarser-grained, in places pyrite-bearing, and partly auriferous metasedimentary rocks forming the bulk of the Witwatersrand Supergroup are regressive. The iron-formations, however, were deposited during transgression. The presence of allogenic pyrite in the fluviatile metaconglomerates and that of magnetite and, in places, haematite in the marine iron-formations suggests a lower pH and higher sulfur activity for the Archaean meteoric environment than for recent hydrothermal fluids on the ocean floor. Post-depositional alteration of the Witwatersrand rocks includes burial metamorphism at temperatures between 300 and 350 °C and pressures around 2.5 kbar, and multiple hydrothermal inflitration events at slightly lower temperatures, coeval with the brittle deformation of the basin fill during the deposition of the Transvaal Supergroup and the Bushveld Vredefort events. Additional thermal metamorphic overprint of the iron-formations around the Vredefort Dome caused the growth of orthoamphiboles. They show a wide range of compositions between ferro-anthophyllite and ferrous alumino-gedrite, suggesting that the crest of the solvus curve for Fe-rich orthoamphiboles is below 500 °C. Chlorite and amphibole compositions, and the presence of Fe-oxide-bearing horizons between pyrite-bearing ones indicate that the fluid composition during post-depositional alteration was largely controlled by the bulk rock composition of the infiltrated stratigraphic horizons and not by some external source.

Notes: Abstract The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct.

Notes: Abstract Advective-dispersive fluid flow through permeable and porous rock causes systematic alteration of the infiltrated rock. O and Sr isotopes can be used as tracers to monitor exchange processes between mineralizing hydrothermal solutions and carbonate host rocks. Fluid infiltration into rock of initially uniform isotopic composition leads to characteristic changes in δ18O and ɛ87Sr in the rock, that depend on infiltration distance, fluid velocity, diffusivity and reaction kinetics. For fast fluid-solid equilibrium during advective flow, the shape of such alteration profiles can be described by the dimensionless Peclet number (P e ), which expresses the fluid flux, infiltration distance and diffusivity. In the case of mineralization from low to moderate temperature solutions, isotope exchange is not instantaneous, but kinetically controlled, and a similar parameter, the Damköhler number (N d ) expressing fluid flux, isotope exchange rate and infiltration distance, becomes more useful. N d - and P e -values have been estimated for various examples of carbonatehosted hydrothermal mineralization in the Eastern Alps (Austria) and the Benue Trough (Nigeria) by modelling O and Sr isotope data from ore (or gangue minerals) and host rock. The values obtained enable a rough differentiation between pervasive and channelled fluid flow as the prevailing transport mechanism to be made. Results indicate that both highly channelled fluid flow through fractures without significant interaction with the wall-rock, and pervasive fluid flow with infiltration distances into the percolated host rock of the order of 100 m, occur near mineral deposits. In the latter case, the extent of O and Sr isotopic alteration of the host rock can be used as an effective exploration tool.

Notes: Summary Phase relations and mineral compositions indicate a polymetamorphic history of carbonate and siliciclastic sediments constituting the Kaaimans Group of the Pan-African Saldania Belt along the south coast of South Africa. Regional metamorphism during Pan-African orogeny did not exceed greenschist facies conditions but was superimposed locally onto low- to medium-grade contact metamorphism related to the intrusion of syn-orogenic granitoids. Petrogenetic grids, constructed for the System CaO-MgO Al2O3-SiO2-H2O-CO2 and based on available thermodynamic data, reveal that vesuvianite, grossular-rich garnet and wollastonite bearing minerai assemblages, found in cale-silicate rocks adjacent to the syn-orogenic Rooiklip granite gneiss sheet, were formed at temperatures possibly as low as 450°C and pressures between 2 and 3 kbar. These mineral assemblages were in equilibrium with an H2O-CO2 fluid that was characterized by a CO2/(CO2 + H2O) mole fraction of less than 0.02. The fluid composition in the cale-silicate rocks was buffered entirely externally through magmatic/hydrothermal fluids related to the granite intrusion. The fluid flux was enhanced by syn-intrusive bedding-parallel shearing in the sedimentary rocks. Vesuvianite, which is of highly variable but generally Fe-rich composition, contains up to 2.1 wt.% F and hence acted as sink for fluorine. A regional increase in Pan-African metamorphic grade from the tectonically lowest to highest formation suggests an overturned position of the whole Kaaimans Group. Later metamorphism during the Cape Orogeny caused only very low- to low-grade retrogression.

Notes: Abstract Analyses of gold particles from the Ventersdorp Contact Reef in the Klerksdorp and Carletonville goldfields of the Late Archaean Witwatersrand Basin reveal a wide range in Au, Ag and Hg concentrations of 80.9 to 92.9, 6.0 to 17.6, and 0.6 to 5.8 wt.%, respectively. However, individual gold particles are generally homogeneous. This observation is predicted by modelling of Ag and Hg diffusion through gold at the peak metamorphic temperature of about 300 °C. The only exception is gold in hydrothermal quartz veins, the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids, as a consequence of the 2023 Ma Vredefort meteorite impact event. Inter-particle homogenisation on a hand-sample scale was generally not achieved. The mean gold composition for the same reef in different mines (kilometre scale) can vary drastically. These results confirm the model of a metamorphosed and hydrothermally altered placer deposit. Furthermore they indicate that, in spite of a significant post-depositional modification of the composition of individual gold particles, the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles.

Notes: Abstract The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The δ 18O-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6‰ (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in δ 18O-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average δ 13C-value of the carbonate host is +0.5 ± 1.2‰ (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower δ 13C-values, on average -1.4 ± 0.8‰ (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower δ 13C-values of -4.2 ± 0.6‰ and-4.7 ± 0.7‰, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average δ 13C-value of -2.9 ± 0.7‰, and matrix siderite/ankerite an average value of-4.1 ± 0.4‰. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystallization and partial mobilization of the various carbonate phases.