Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.     

Should the use of standardized baselines in the CDM be mandatory?

There has been considerable debate on the merits of standardized baselines (SBLs) in the clean development mechanism (CDM), and how such baselines could reduce transaction costs for CDM projects. It has not been considered whether the voluntary versus mandatory use of SBLs by CDM project developers can affect the environmental integrity of the CDM. An example is given in which SBLs are applied to a homogeneous output industry in order to illustrate how the voluntary use of SBLs could lead – even with relatively stringent benchmarks – to over-crediting of emission reduction credits.     

Cambrian sedimentary dykes in the Proterozoic basement of the Västervik area (southeast Sweden): episodic formation inferred from macro- and microfabrics

Fabrics of Cambrian sedimentary dykes formed in Proterozoic granites of the Västervik area (Southeast Sweden) evidence repeated opening/filling and mineralisation/cementation events under varying conditions. Diagnostic features include (1) wall-parallel boundaries between epiclastic fillings and (2) early formed dyke sediments that appear as lithoclasts in subsequently formed sedimentary fillings. The psammitic components mostly consist of well-rounded quartz grains related to a coastal environment and fragments from the granitic host rock. Platy calcitic fragments embedded in the epiclastic matrix originally formed as microveins within already-lithified dyke sediments and the adjacent host rock. Convex downward-pointing, internal sagging structures, together with the preferred orientation of compositional boundaries and long axes of grains/rock fragments parallel to the dyke walls, are interpreted as the result of suction-controlled flow of unconsolidated fillings during episodes of downward dyke growth. Pressure solution of quartz grains are evidence of extensional phases with dyke propagation that were interrupted by phases of horizontal compression normal to the dyke walls. The N–S and NE–SW striking sedimentary dykes formed by opening of a pre-existing joint set during NW–SE oriented rifting during the Cambrian.     

Investigation of runoff generation in a pristine,poorly gauged catchment in the Chilean Andes I: A multi‐method experimental study

Catchment scale hydrological process studies in southern Chile are of special interest as little research at this scale has been carried out in this region. In particular, the young volcanic ash soils, which are typical for this area, are not well understood in their hydrological behaviour. In addition, extensive land use changes require detailed knowledge of hydrological processes in disturbed as well as undisturbed catchments in order to estimate resulting risks of erosion, eutrophication, floods and droughts. This study focuses on data collection and experimental determination of relevant processes in an undisturbed forested catchment in the Andes of southern Chile. The here gained understanding of runoff generation can serve as a reference for comparison with sites subject to human intervention, improving estimation of the effects of land use change. Owing to the lack of long‐term data for this catchment it was necessary to replace long time series by a multitude of experimental methods covering as many aspects of the runoff generation process as possible. The methods used in this investigation include: measurements of streamflow, rainfall, throughfall, water chemistry, soil water dynamics, groundwater dynamics, soil physics, soil mineralogy, geo‐electrical sounding, and tracer techniques. Methods and equipment used during field campaigns are described and evaluated for usefulness versus expenditure (labour and financial costs). Selected results and the hypotheses developed from these findings are presented. The results suggest the importance of fast processes for rainfall runoff response on the one hand as well as considerable dampening effects of a large subsurface storage on the other hand. Copyright © 2008 John Wiley & Sons, Ltd.     

Contrasting punctuated zircon growth in two syn-erupted rhyolite magmas from Tarawera volcano: Insights to crystal diversity in magmatic systems

Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9 ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~ 3–5 μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200 ka. The dominant age peak in the T1 magma is ~ 30 ka with subordinate peaks at ~ 45, ~ 75, and ~ 100 ka, whereas the T3 magma has a dominant zircon interior age peak at ~ 90 ka with smaller modes at ~ 35 and ~ 150 ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~ 90 ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~ 100 ka) that were punctuated by asynchronous hiatuses of up to 30 ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (< 10²–10³ years) to result in extensive zircon growth. The contrasting, punctuated zircon histories argue against closed-system fractional crystallization models for silicic magmatism that require protracted cooling times following a mostly liquid starting condition.     

Dating of Pleistocene uranyl phosphates in the supergene alteration zone of Late Variscan granites by Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry with a review of U minerals of geochronological relevance to Quaternary geology

Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) has proved to be a reliable tool to date metatorbernite–bassetite solid solution series in Pleistocene weathering pits developing on the surface of Late Variscan granites. The maximum age of weathering in these granites in NW Spain is 1.61 ± 0.01 Ma. The likelihood to find uranyl phosphates or silicates amenable to LA-ICP-MS dating increases with increasing silica content in the magmatic bedrocks (granites) and with decreasing grade during regional metamorphism (low-grade-stage/greenschist metamorphics) of the parent material. Uranyl vanadates may be used for geochronological studies during the Quaternary as well but they are confined to arid morphoclimatic zones.     

Nature of magmatism and sedimentation at a Columbia active margin: Insights from combined U–Pb and Lu–Hf isotope data of detrital zircons from NW India

Detrital zircons from a Palaeoproterozoic quartzite, deposited between 1.85 and 1.82 Ga in the northern Aravalli orogen of NW India, show a distinctive age peak of ca. 1.85 Ga and variable, but largely subchondritic εHf_{1.85 Ga} between ? 1.3 and ? 21.0 corresponding to hafnium model ages of 2.5 to 3.6 Ga. These data indicate an important period of reworking of ancient (Eo- to Neoarchaean), strongly heterogeneous continental crust at this time. Prevalence of ca. 1.85 Ga subduction-related granitoids, almost identical U–Pb age spectra and εHf_t of detrital zircons in ca. 1.85 Ga metasedimentary rocks in the Aravalli orogen and the inner Lesser Himalaya indicate similar sediment provenances and thus a geological connection between these two terranes during late Palaeoproterozoic. All together, the data constrain a rapid succession of sedimentation, metamorphism and subduction-related magmatic activity and support the interpretation of an active geodynamic realm along the entire north Indian margin at ca. 1.85 Ga. Comparison of detrital zircon data in conjunction with published paleomagnetic data from north India and other crustal blocks of the Columbia supercontinent, additionally, suggest a close affinity of north India with Madagascar, the Cathaysia block of South China and South Korea during Columbia times.