Browsing by Author "Erlank, A J"
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- ItemOpen AccessGeochemistry and mineralogy of selected Atlantic Ocean basalts(1980) Le Roex, Anton Powter; Erlank, A J; Reid, A MBulk rock compositional variations in lavas from four localities in the Atlantic Ocean are evaluated quantitatively in terms of fractional crystallisation and partial melting models. Samples studied are from the Mid-Atlantic Ridge at 36°49 'N (FAMOUS), from the Islas Orcadas fracture zone on the Southwest Indian Ocean Ridge, from the Conrad fracture zone on the America-Antarctica Ridge and from Bouvet Island, situated at the southernmost tip of the Mid- Atlantic Ridge and in the vicinity of both the fracture zones mentioned above. Bulk rock major and trace element variations in selected basalts from the FAMOUS area, in conjunction with a detailed study of the chemistry of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationships between the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. The NiO content of these phenocrysts in the former three basalt types is low, or a given Mg Mg+Fe²⁺ atomic ratio of the mineral, relative to the phenocrysts in the olivine basalts. Consideration of the Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase- pyroxene basalts but, in addition, distinguishes the picritic basalts from the former three basalt types.
- ItemOpen AccessThe geochemistry of the Olivine Melilitetes and related rocks of Namaqualand Bushmanland, South Africa(1979) Moore, Andrew Edward; Erlank, A JEarly Tertiary olivine melilitites and associated non-diamondiferous kimberlites occur in two pipe clusters in the Namaqualand and Bushmanland regions of South Africa. The more northerly cluster is centred about the town of Gamoep and the second is located south of Garies. The olivine melilitites are characterized by olivine phenocrysts which exhibit a broad range in composition on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipes studied: (a) Rare anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the remaining olivines in the same rock; (b) euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than HILN olivines in the same rock. Some of the euhedral olivines show abnormal zonation patterns ("unusual" hoppers). Hopper and HILN olivines contain fluid and carbonate inclusions which apparently record the separation of a vapour phase and an immiscible carbonate liquid during magma ascent: (c) large rounded olivines (megacrysts) up to 40 mm across. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg with Ni (F092, 0.36% Ni to F075, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock. [d] In addition to these main populations, there also occur rare magnesium-rich (F091) anhedral olivines which show strained extinction and are believed to be xenocrysts.
- ItemOpen AccessThe geochemistry, kinematics and geodynamics of the Gannakouriep dyke swarm(1992) Ransome, Ian G D; Erlank, A J; Reid, DaveThe Gannakouriep dyke swarm comprises a linear swarm of north to northeast striking mafic dykes emplaced during late Proterozoic rifting in the Gariep belt. The swarm has a strike length of over 300 km crossing terrane boundaries of the Gordonia sub-province, Richtersveld igneous sub-province and 8 ushmanland sub-province of the Namaqua Mobile Province. The main axis of dyke intrusion is centred within the 2.0 Ga Richtersveld igneous sub-province where the density of dyking increases westwards across strike towards the Gariep belt, prior to disappearing within the sediment/basement contact of the para-autochthonous Port Nolloth Assemblage. An internal Rb-Sr mineral isochron, together with pyroxene K-Ar plateau and whole rock model TCHUR ages, indicate that the dyke swarm was intruded around 720 Ma. The majority of the dykes comprise subophitic relict gabbros and dolerites of alkali affinities replaced by metamorphic minerals of both greenschist and amphibolite facies. The greenstones are encountered within the NE striking eastern sector of the dyke swarm which records the original tensile stress field, whilst amphibolites are encountered in the northerly striking western sector of the swarm which has been rotated 26° anticlockwise during Pan-African (500-550 Ma) deformation in the Gariep belt. The latter age of regional metamorphism and deformation is supported by available K-Ar age data on whole rock samples and amphibole mineral separates. Integrated field relationships, continuum mechanics modelling and geochemical studies suggest that the swarm was initiated as a series of low level crustal magma chambers which subsequently gave rise to a series of dyke complexes that comprise the Gannakouriep dyke swarm. The geochemical variability between individual dyke complexes across the entire swarm is negligible; only slight differences being recorded by their high-field-strength element concentrations. The latter suggests that all dykes are genetically linked to a single mantle diapir, with only slight source characteristic heterogeneities. All dykes are ferro-tholeiites with no primitive (picritic) member being present. Geochemical trends recorded by the swarm are easily reconcilable in terms of fractionation of the phenocryst assemblage olivine, plagioclase, clinopyroxene, (ilmenite and Ti-magnetite). However an earlier phase of fractionation of essentially olivine and plagioclase at the base of the crust is suggested since constraints on the degree of partial melting (3-6%) imposed by REE patterns would derive an alkali basalt parental melt. The geodynamic relationship between the Gannakouriep dyke swarm and rifting in the Gariep belt is accounted for by a lithospheric plate model containing non-coincidental crustal and mantle weaknesses represented by late Namaquan D4n extensional faults and a mantle weakness possibly resulting from crustal thickening within the Richtersveld igneous sub-province during Namaquan tectonics.
- ItemOpen AccessThe geochronology and geochemistry of Karoo volcanics in the Lebombo and adjacent areas(1980) Bristow, John William; Erlank, A J; Duncan, A RKaroo volcanics of Jurassic to Cretaceous age crop out in the eastern areas of southern Africa. These volcanics are well preserved in the Lebombo monocline which straddles the borders of South Africa, Swaziland and Mozambique, and in the Nuanetsi and Tuli synclines and Sabi monocline of south-east Zimbabwe. Geochronological and geochemical data have been used to evaluate the geochemistry of the rock types found in the above areas, particularly with respect to the mafic volcanics of the Lebombo. In the eastwardly dipping Lebombo monocline, nephelinites overlain by picrite basalts occur in the northern part of the volcanic succession whereas over the remaining area olivine-poor basalts are dominant. Rhyolitic volcanics in the form of massive flows overlie the basaltic rocks and attain their maximum thickness in Swaziland and the southern Lebombo. The rhyolites are notably thinner to the north of Swaziland and are overlain by an upper sequence of olivine-poor lavas with interdigitated rhyolite flows. Basic dykes are ubiquitous in the Lebombo whereas acid dykes, though present, tend to be localised. In Nuanetsi, picrite basalts, olivine-poor basalts and rhyolite flows with interbedded basic lavas crop out ina major synclinal structure. Dyke rocks are common and late Karoo intrusive complexes cut the lavas. Volcanic rocks in the Tuli syncline consist predominantly of picrite basalts and olivine poor lavas, whereas nephelinites, picrite basalts and olivine-poor lavas crop out in the Sabi Monocline.
- ItemOpen AccessHydrogeochemical exploration at Tsumeb(1980) Marchant, James William; Orren, Michael J; Erlank, A JThe Precambrian Otavi dolomites in the district of Tsumeb in Namibia are host to base metal mineralization but large tracts of these rocks occupy a mature pediplain and are deeply buried by transported overburden of Kalahari age. This sandy material makes exploration for minerals very difficult. Hydrogeochemistry was considered as a possible answer to the problem of prospecting in this region but a major obstacle was the lack of an undisturbed site for orientation studies. The Tsumeb mine was therefore used as a "surrogate" orientation target and water samples collected there were analyzed for about forty elements or ions. The data were plotted on long-sections through the orebody and examined with the aid of simple statistical calculations. The Na/Cl⁻ ratio proved to be a powerful index for identifying groups of waters with a common origin. It appeared that the most important of these groups of waters were not natural groundwaters but were being recycled. Millions of litres of water are pumped from the mine daily and a significant proportion finds its way back into the workings through seepage. This conclusion was supported by chemical data and by calculations in which the rates of pumping and recharge were balanced. Over the years the process of recycling has caused a rise in the concentrations of sodium, chloride and sulphate in the water arriving at the sumps in the mine. The other major components of these waters - Ca, Mg and HCO₃⁻ - are not affected to the same extent by recycling because of controls imposed by the carbonic system. It was not generally possible to establish rational links between the distributions of the trace elements, the major groups of related waters and the proximity of ore. Although some trace elements tended to be more abundant in samples from the upper parts of the mine, where exposure to broken, weathered ore is greatest, the value of this association was diminished by the fact that these waters are recycled effluents that cannot be equated with the natural groundwaters that would be in contact with any undiscovered ore-bodies similar to the Tsumeb lode. This conclusion was reinforced by the fact that some of the trace elements were relatively abundant in waters that were comparatively saline but which had not been exposed to oxidized ore. It was concluded that it would not be possible to interpret the trace element data from the mine waters unambiguously. Nevertheless it was clear that sulphate, Cu, Pb, Zn, Hg, Cd, Se, Te and perhaps U were being mobilized from the ore and that these were therefore all good "candidate'' pathfinders. Less probable candidates were Mo, Ge, As, Na and chloride. A second orientation study was then undertaken, using normal groundwaters from boreholes and springs on surface. Sampling was concentrated around the Tsumeb and Kombat mines as targets and waters were also obtained from background areas where dolomites were exposed. The chemical data were plotted on maps by computer and were examined with the aid of simple statistical calculations such as the cumulative frequency distribution. Although anthropogenic effects were apparent at both mines they were not severe and it was possible to be more confident about the prospective roles of the various chemical species in hydrogeochemical exploration for ores in the Otavi dolomites. These prospective roles may be summarized as follows: undetectable or of virtually no interest: pH, temperature, phosphate, K, Ti, Fe, li, Rb, Cs, V, Cr, Mo, Ag, Au, Cd, Hg, Tl, Ge, As, Sb, Bi, Se and Te; useful as iridicators of regional hydrogeochemical features not directly related to mineralization: Si, F⁻, Sr and Al; weak regional pathfinders: Ni and Co; probably good regional pathfinders: sulphate, Cu, Pb and Zn. Bicarbonate, Cl⁻, Na, Ca and Mg are not pathfinders but it is useful to have these data when considering the nature and significance of the samples and the abundances in them of the trace elements. Despite the identification of a suite of pathfinders it was not feasible to establish orientation criteria such as contrast ratios. An attempt was made to apply the findings of this orientation survey in an area to the northeast of Tsumeb, where dolomites are buried under thick sand and calcrete. An area of five thousand square kilometres was selected and all usable sources of groundwater within it were sampled. The following data were collected: total dissolved solids, pH, Ca, Mg, Na, bicarbonated chloride, sulphate, Cu, Pb and Zn. Hydrological data suggested that the regional movement of groundwater was from the exposed dolomites into the pediplain. The hydrogeochemical data showed that during this migration the major element composition of the groundwater changed ("metamorphosed") radically. Because of this metamorphism, very severe difficulties stood in the way of applying in the sandy pediplain what had been learned in the areas of well-exposed dolomite. An attempt was therefore made to provide tighter control between the lithology of the geological profile and the hydrogeochemistry of the associated formational waters in the pediplain. This work consisted of (a) a sampling program to try to ''fingerprint" specific formational waters. This proved to be impossible. (b) drilling four diamond drill holes near selected anomalies and studying the cores in detail. These revealed that the Kalahari beds were very thick (46 to >103m). In all cases the base of the overburden was far below the present water table and in some cases the overburden was underlain by unmineralized rocks other than dolomite. These results indicated that the hydrogeochemical anomalies were probably spurious and it was concluded that hydrogeochemical exploration in this kind of Kalahari terrane was not practicable.
- ItemOpen AccessAn investigation into the determination of some volatile elements in silicate rocks employing d.c. arc emission spectroscopy in artificial atmospheres(1981) Baumgartner, Friedrich Carl; Erlank, A JThe determination of 14 trace elements, namely As, Ag, Bi, Cd, Cu, Ga, Ge, Hg, In, Pb, Sb, Sn, Tl and Zn, in silicate rocks using d.c. arc optical emission spectrography (O.E.S.) and X-ray fluorescence spectroscopy (X.R,F.) was investigated. X.R.F, was shown to be capable of determining Cu, Ga, Pb and Zn in normal silicate rocks and Sn, As and Ge in samples enriched in these latter three elements. Improvement of the sensitivity of the trace elements selected employing d.c. arc excitation in artificial atmospheres was examined in detail. A Margoshes and Scribner (M & S) design gas jet was constructed, and proved to have numerous advantages over those gas jets which incorporate a glass canopy. Excitation in argon, argon-oxygen and nitrogen resulted in many advantages. The most prominent were, remarkable enhancement of the element Zn, and selective distillation of the volatile elements when the d.c. arc was surrounded by argon. The drastic reduction 1.n electrode temperature associated with excitation in argon resulted in an unacceptable decrease in the volatilization rates of the more abundant volatile elements. Several successful methods of overcoming these reduced volatilization rates were found. Increased amperage coupled with a special electrode design was one while the use of a small carrier electrode was another. It was also found that the carrier design electrode was most suitable for use with argon and nitrogen atmospheres as its use promoted selective distillation of the volatile elements. The lack of improvement in volatilization characteristics through the use of additives was attributed to the dominating influence of argon on electrode temperature. The principal advantage of excitation in argonoxygen was the suppression of CN emission essential for ultimate sensitivity of the element Tl. While excitation in nitrogen improved the volatilization characteristics of the more abundant elements, severe CN emission and increased spectral background limited the use of nitrogen to those elements with spectral lines not affected by CN emission.
- ItemOpen AccessProterozoic crustal evolution of the Awasib Mountain terrain, southern Namibia, with speical reference to the volcanic Haiber flats formation(1989) Hoal, Brian Garner; Reid, Dave; Erlank, A JThe middle to late Proterozoic Awasib Mountain terrain (AMT) straddles the boundary between the Rehoboth and Gordonia subprovinces in southern Namibia. The AMT is made up of two major crustal components, the older of which is correlated with the Namaqualand Metamorphic Complex (NMC), and the younger with the Sinclair Sequence.
- ItemOpen AccessThe reaction between ferrocytochrome c and hydrogen peroxide(1968) Hulett, Leslie Graham; Ahrens, L H; Erlank, A JThe reaction between ferrocytochrome c and hydrogen peroxide has been studied to determine possible reaction mechanisms. This was done by investigating the effect of various physical factors, such as hydrogen ion concentration, ionic strength and temperature, on the reaction and by studying the kinetics of the reaction. The study of pH and ionic strength showed that an ionization occurred at pH 7.8 which influenced the reactivity of the cytochrome c. The reaction was studied kinetically at pH 8.1 and pH 6.6. At pH 8.1 simple kinetics showed an uncomplicated reaction between ferrocytochrome c and hydrogen peroxide. The kinetic studies at pH 6.6 showed that ferrocytochrome c, hydrogen peroxide and ferricytochrome c (acting as a product catalyst) were involved in the reaction. The product catalysis complicated the kinetics and difficulty was experienced in fitting a suitable reaction mechanism. Copper ions have been found to catalyse reactions similar to that studied here. The effect of a copper histidine complex was therefore investigated on the reaction at pH 8.7. The copper complex was found to catalyse the oxidation of ferrocytochrome c by hydrogen peroxide and the kinetic studies of this oxidation showed that the reaction could be fitted to a simple mechanism which did not involve the formation of intermediate complexes. An unusual feature of the reaction was the ability of the cytochrome c, after complete oxidation, to undergo reduction, which is dependent on the presence of both hydrogen peroxide and copper ions.