Alluvial Samples Research Articles

Platinum-group-element minerals (‘osmiridium’) from Waratah Bay, Victoria, Australia, and a review of other reported Victorian occurrences

Minerals consisting of the platinum-group elements iridium, osmium and ruthenium (IPGE) are widely dispersed in alluvial deposits derived from ultramafic igneous complexes originating in ophiolite belts. Rock types, such as cumulate dunite–pyroxenite–harzburgite, and their serpentinite derivatives are especially fertile sources of IPGE minerals, which are colloquially termed ‘osmiridium’. IPGE minerals have been recorded from five localities in Victoria—beach sands at Waratah Bay, and stream concentrates from Turtons Creek, Stockyard Creek and Horseshoe Creek in South Gippsland. A grain of IPGE was recently found in a gully near the Dolodrook River further north. These occurrences are all poorly documented by comparison with the northwest Tasmanian deposits, and only Waratah Bay has provided enough material to enable full characterisation. Microprobe data show that all four minerals currently defined in the Os–Ru–Ir system occur at Waratah Bay, including rutheniridosmine. Gold and chromite are also present in the alluvial samples from Waratah Bay and Turtons Creek. Attempts to correlate compositions of alluvial chromite with those from likely proximal serpentinite source rocks proved to be inconclusive due mainly to inadequate representative sampling. However, the chromite compositions are broadly characteristic of those found in the early Paleozoic ultramafic ophiolite complexes in northwestern Tasmania, in which IPGE minerals and chromite are abundant. Regional aeromagnetic trends across Bass Strait suggest that the Victorian IPGE occurrences represent disaggregated and metamorphosed extensions of the Tasmanian sequences.

Sediment Transport Capacity in a Gravel-Bed River with a Sandy Tributary

Bedload transport in a river is a deeply analyzed problem, with many methodologies available in the literature. However, most of the existing methods were developed for reaches of rivers rather than for confluences and are suitable for a particular type of material, which makes them very inaccurate in cases where the sediments are comprised of a mix of different types of soil. This study considers the effect of two different bed sediment sizes, gravel and sand, in relation to bed load transport in a confluence. Five well-known and validated equations (namely Meyer-Peter and Müller, Parker + Engelund and Hansen, Ackers and White, and Yang) are applied to the case study of the Tagus–Alberche rivers confluence (in Talavera de la Reina, Spain), where main and tributary rivers transport different materials (sand and gravel). Field works in the area of the confluence were conducted, and a set of alluvial samples were collected and analyzed. The previously mentioned methods were employed to analyze the geomorphology in the confluence area and downstream of it under different flooding scenarios, concluding different trends in terms of deposition/erosion in the area under historic flooding scenarios. When the trends show erosion, all methods are very consistent in terms of numerical predictions. However, the results present high disparity in the estimated values when the predictions suggest deposition, with Parker + Engelund and Hansen yielding the highest volumes and Meyer-Peter and Müller the lowest (the latter being around 1% of the former). Yang and Ackers and White predict deposits in the same range in all cases (around 15% of Parker and Engelund Hansen). Yang’s formula was found to be suitable for the confluences of rivers with different materials, allowing for the estimation of sediment transport for different grain sizes. The effect of different flow regimes has been analyzed with the application of Yang’s formula to the Tagus-Alberche confluence.

Adamas as diamond and its occurrence near Ancient Philippi in Macedonia, northern Greece

The adamas of Ancient peoples is proved to be the precious diamond. An ancient and uconventional diamond occurrence, the mines of Philippi in Macedonia (northern Greece), is geologically explained here. The most probable source is believed to be flat subduction-related (shoshonitic) lamprophyres that originated at a depth of >110 km within the mantle in the diamond stability field. The thick lithospheric mantle beneath Greece supports the occurrence of diamondiferous lamprophyric rocks in this area. However, no lamprophyre outcrops have been found, except for a few lamprophyre dikes 14 km SE of Philippi. Another explanation for the occurrence would implicate detrital diamonds that have been preserved in the local greenschist facies rocks. The XRD analysis of Philippi paragneiss showed the existence of the mantle mineral magnesiowüstite, which is a possible diamond indicator. The Philippi occurrence deserves detailed investigation involving alluvial sampling and ground magnetic surveys to locate the diamond source.

Denudation of the Golan Heights basaltic terrain using in-situ 36Cl and OSL dating

While erosion rates of carbonates along the Dead Sea fault (DSF) and its margins have been determined in several studies, information about denudation of basalts along the DSF are absent. Here we report concentrations of in situ cosmogenic 36Cl in bedrock and sediment samples combined with OSL ages of terraces from the Meshushim basin, a typical basaltic basin in the Golan Heights, northern Dead Sea Rift (DSR).Denudation rates of exposed and soil-covered bedrock basalts, range from 12.3 ± 1.4 mm/ka to 176.1 ± 47.4 mm/ka, well within the range of DSR values, regardless of lithology and climate. In all locations where exposed and soil-covered bedrock were samples next to each other, the soil-covered basalt erodes faster. This is regardless of notable variations in average annual temperature and precipitation. This means that bedrock outcrops were not covered and exposed very frequently, and that chemical weathering at the bedrock-soil interface is a significant erosional factor.Sediments in the Meshushim basin are stored in soil pockets and blankets on the upper plateau, colluvial material on slopes, and alluvial terraces. The difference in 36Cl concentrations between soil-covered and exposed bedrock indicates coverage duration of ca. 3–6 ka. The residence time estimate of only several thousands of years for sediments stored in alluvial terraces is 170 ± 20 y to 4000 ± 290 y. Nowhere in the Meshushim drainage system did we find evidence for storage of sediment longer than a few thousands of years.36Cl concentrations in colluvial and alluvial sediments are similar to those measured in soil-covered basalts. This similarity suggests that sub-surface weathering of basalts is the major source for sediments in upper and central segments of the basin, concurrent with the higher denudation rates of buried bedrock. However, very low 36Cl concentrations measured in samples collected in the lower parts of the basin indicate involvement of landslides which instantaneously expose previously shielded sediments that were incorporated into the drainage system. The effect of landslides is also manifested by the high basin scale denudation rates calculated from alluvial samples in various locations, as well as the geomorphological profile of the Meshushim basin. Sediment yield calculated from 36Cl concentrations measured in this study suggest temporal changes in denudation rates over the Holocene and up to the last ∼60 ka, with higher past denudation rates and sediment yield even in most of the Holocene related to present rates and yields.Overall, the results of this study show that the fact that the Golan lithology is basaltic does not result in different denudation process and rate from other locations investigated along the Dead Sea Rift, which are mostly of carbonate lithology and that relief and climate are stronger parameters than lithology in controlling denudation.

Geochemical mapping by unmixing alluvial sediments: An example from northern Australia

Alluvial sediments have long been used in geochemical surveys as their compositions are assumed to be representative of areas upstream. Overbank and floodplain sediments, in particular, are increasingly used for regional to continental-scale geochemical mapping. However, during downstream transport, sediments from heterogeneous source regions are carried away from their source regions and mixed. Consequently, using alluvial sedimentary geochemical data to generate continuous geochemical maps remains challenging. In this study we demonstrate a technique that numerically unmixes alluvial sediments to make a geochemical map of their upstream catchments. The unmixing approach uses a model that predicts the concentration of elements in downstream sediments, given a map of the drainage network and element concentrations in the source region. To unmix sedimentary chemistry, we seek the upstream geochemical map that, when mixed downstream, best fits geochemical observations downstream. To prevent overfitting we penalise the roughness of the geochemical model. To demonstrate our approach we apply it to alluvial samples gathered as part of the Northern Australia Geochemical Survey. This survey gathered samples collected over a ∼500,000 km2 area in northern Australia. We first validate our approach for this sample distribution with synthetic tests, which indicate that we can resolve geochemical variability at scales greater than 0.5–1° in size. We proceed to invert real geochemical data from the total digestion of fine-grained fraction of alluvial sediments. The resulting geochemical maps for two elements of potential economic interest, Cu and Nd, are evaluated in detail. We find that in both cases, our predicted downstream concentrations match well against a held-out, previously ‘unseen’ subset of the data, as well as against data from an independent geochemical survey. By performing principal component analysis on maps generated for all 46 available elements we produce a synthesis map to identify the significant geochemical domains in this part of northern Australia. This map shows strong spatial similarities to the underlying lithological map of the area. Finally, we compare the results from our approach to geochemical maps produced by interpolating alluvial geochemical data using two standard methods: kriging and inverse distance weighting. We find that, unlike the method presented here, both these methods can generate maps that are dampened relative to expected magnitude, as well as being spatially distorted. We argue that the unmixing approach is the most appropriate method for generating geochemical maps from regional-scale alluvial surveys.

Trace Element Geochemistry of Alluvial TiO2 Polymorphs as a Proxy for Sn and W Deposits

The Segura mining field, the easternmost segment of the Góis–Panasqueira–Segura tin–tungsten metallogenic belt (north–central Portugal), includes Sn-W quartz veins and Li-Sn aplite-pegmatites, which are believed to be genetically related to Variscan Granites. Sediment geochemistry indicates granite-related Ti-enrichments, locally disturbed by mineralization, suggesting magmatic and metamorphic/metasomatic titaniferous phases. Therefore, Segura alluvial samples and the geochemistry of their TiO2 polymorphs (rutile, anatase, and brookite) were investigated, and their potential as exploration tools for Sn and W deposits was evaluated. The heavy-mineral assemblages proved to be good proxies for bedrock geology, and TiO2 polymorph abundances were found to be suitable indicators of magmatic and/or metasomatic hydrothermal processes. The trace element geochemistry of Segura’s alluvial rutile, anatase, and brookite is highly variable, implying multiple sources and a diversity of mineral-forming processes. The main compositional differences between TiO2 polymorphs are related to intrinsic (structural) factors, and to the P-T-X extrinsic parameters of their forming environments. Anomalous enrichments, up to 9% Nb, 6% Sn and W, 3% Fe, 2% Ta, and 1% V in rutile, and up to 1.8% Fe, 1.7% Ta, 1.2% Nb, 1.1% W 0.5% Sn and V in anatase, were registered. Brookite usually has low trace element content (<0.5%), except for Fe (~1%). HFSE-rich and granitophile-rich rutile is most likely magmatic, forming in extremely differentiated melts, with Sn and W contents enabling the discrimination between Sn-dominant and W-dominant systems. Trace element geochemical distribution maps show pronounced negative Sn (rutile+anatase) and W (rutile) anomalies linked to hydrothermal cassiterite precipitation, as opposed to their hydrothermal alteration halos and to W-dominant cassiterite-free mineralized areas, where primary hydrothermal rutile shows enrichments similar to magmatic rutile. This contribution recognizes that trace element geochemistry of alluvial TiO2 polymorphs can be a robust, cost- and time-effective, exploration tool for Sn(W) and W(Sn) ore deposit systems.

Dose dependency of aliquot sizes and age models from modern alluvial fan deposits of Helan Mountain, China

Alluvial fan deposits are identified as evidence of regional climatic variations and tectonic events; therefore, it is crucial to establish absolute time series by dating alluvial fan deposits. Limited exposure to light poses a challenge to accurately estimating the buried ages for alluvial deposits with optically stimulated luminescence (OSL). This challenge has been positively developed by controlling the number of grains on each disk to measure and choosing suitable statistical models for the equivalence dose (De) distribution to analyze. In this research, three modern alluvial samples and one modern loess-like sample were collected from the Dashuigou alluvial fan of Helan Mountain, China. The De distributions of these four samples were studied by the application of small aliquots (1-, 3-, and 5-mm aliquots) of quartz OSL with the average dose model, central age model, unlogged minimum age model (MAM), lowest 5%, internal/external consistency criterion minimum age model (IEU), and finite mixture model. It is concluded that an overestimation of De lower than 1 Gy can be obtained using quartz OSL dating of 1-mm aliquots (∼50 grains) with MAM and IEU for the alluvial sediments. The lowest 5% method may underestimate the De values of the 1-mm aliquots for young samples. This research makes the dose dependency of aliquot sizes and age models more definite and opens up the possibility of dating paleoalluvial deposits to establish a chronological framework to decipher the implications of paleoclimates and tectonics.

A simplified multiple aliquot regenerative dose protocol to extend the dating limit of K-feldspar pIRIR signal

The post-infrared infrared stimulated luminescence signal (pIRIR) from K-feldspar shows significantly less athermal fading than the conventional IRSL signal, thus it has been widely used for dating Quaternary deposits. With the single aliquot regenerative dose (SAR) protocol, the test dose signal is influenced by the preceding regenerative dose, and the characteristic saturation dose (D0) of the pIRIR signal is mostly in the range of 200–600 Gy, depending on the size of the test dose. According to the D0 value, the dating range of pIRIR is restricted to <300 ka with the SAR protocol. Here we present a multiple aliquot regenerative dose (MAR) protocol using low-temperature thermoluminescence (TL) signal for inter-aliquot normalisation. The multiple-elevated-temperature (MET) pIRIR signals up to 290 °C are investigated. The D0 increases to 850–1300 Gy with this protocol, which should represent the intrinsic D0 of IR or pIRIR signals of K-feldspar. Based on the higher D0, this protocol has the potential to date samples with equivalent doses (Des) up to 2000 Gy, i.e. up to ∼600 ka considering a dose rate of ∼3.5 Gy·ka−1. Applications of this MAR protocol on two loess samples from the Chinese Loess Plateau provide ages consistent with expected ages up to 350 ka, with the MET-pIRIR signal at 290 °C. However, tests on three fluvial and alluvial samples show age overestimation with the new MAR protocol as well as SAR protocol, which may result from the hard-to-bleach property of the high-temperature pIRIR signals. Simulations of fading correction indicate that the age will be over-corrected when the laboratory dose response curve has a D0 smaller than the true D0 of the signal. Caution should be taken in fading corrected ages measured with the conventional SAR protocol, as the D0 values are always underestimated in this case.

Pollen distribution and transportation patterns in surface sediments of Liaodong Bay, China

Understanding pollen transport pathways and dispersal mechanism from the land to sea is a prerequisite for marine palynological study. Palynological analysis of 164 surface sediment samples in Liaodong Bay, and 39 analogous surface alluvium samples from its five inflowing rivers, identifies the distribution patterns, pathways and possible sources of pollen and spores. The results show that pollen and spore assemblages in surface sediments are well correlated to regional vegetation distribution, and the variations of pollen assemblage in different parts of Liaodong Bay reflected local vegetation changes along the coast. High pollen concentrations are mainly distributed in the estuaries of inflowing rivers, coastal waters and sea muddy areas. The pollen assemblage characteristics of alluvial samples are similar to those from coastal waters with water depths <8.5 m. Samples from the alluvium and surface sediments of coastal waters were dominated by herbaceous pollen taxa including Artemisia, Amaranthaceae, Poaceae, Cyperaceae and Typha. Herbaceous pollen percentages and concentrations decreased as the water depth increased, indicating that pollen and spores in the coastal waters of Liaodong Bay are mainly carried by the inflowing rivers. However, pollen assemblages for samples with water depth >8.5 m are significantly different from those of the alluvium. In samples taken below a depth of 8.5 m, the arboreal pollen is dominated by airborne Pinus, and there is a high number of the waterborne Selaginella fern spores, both of which are sourced from a wider region. In the Liaodong Bay, both wind and ocean current transportation determines the pollen distribution patterns in deeper waters, while fluvial and longshore current transportation determines the pollen assemblages found in shallow waters. The dispersal characteristics of pollen assemblages between the land and the sea in Liaodong Bay provide a theoretical basis for the interpretation of fossil pollen assemblages and past sea level changes.

Investigation on the possible control of grain size on development of ‘mud cracks’: a preliminary experimental study of recent Gangetic alluvial samples, West Bengal, India

The so-called mud cracks, possibly the most common deformation feature in sediment, are evident in almost all dried-up exposed sediment surfaces. Several experimental attempts have so far been made to explain the evolution of mud cracks. However, most of the experiments were carried out with bentonite clay or other artificial substances in controlled heating condition. Therefore, those experiments could not or were not performed to elucidate the control of grain size on crack development although it has been observed that cracks develop in almost all types of soil. The present work is an attempt to understand the control of sediment grain size in development and evolution of these cracks. This experimental study was carried out in near-natural condition. Sediment samples of three different grain-size fractions ranging from sub-sand size to very coarse sand size were used for the experiment. The 10-day-long experiment with watering and drying of the samples under sunlight shows that the time gap between watering and appearance of cracks, smoothness of crack walls, number of cracks and also number of polygons with cracks as their arms are dependent on grain size of the sediment.

Regional-scale distributions of pollen and spore assemblages in alluvium around the Bohai Sea: An essential step toward understanding marine palynological sources in China

To better understand the provenance and transport paths of modern pollen and spores in the Bohai Sea, we analyzed 104 surface alluvial samples from its sixteen inflowing rivers and 56 modern surface pollen samples from around Bohai Sea in the East Asia Surface Pollen Database. Our results revealed that the pollen assemblages of the inflowing rivers were dominated by non-arboreal pollen types, which accounted for an average of 60% of the pollen and were represented primarily by Chenopodiaceae; the percentages of arboreal pollen reached an average of 23%; fern spores accounted for the lowest percentage of the spores and pollen. The characteristics of the pollen assemblages in the alluvium of the inflowing rivers, especially large rivers, reflected the vegetation of upstream sampling points rather than the characteristics of the vegetation around the sampling points. The vegetation types represented by the pollen and spores in the short-source rivers or seasonal rivers were beyond the range of the drainage basin, especially the trees and ferns, and differences of the watershed vegetation of the inflowing rivers led to different pollen assemblages in the alluvium in the study area. The Yellow River is the main source of modern surface pollen in the Bohai Sea. Unlike the pollen of the inflowing rivers, the pollen assemblages in the Bohai Sea were dominated by arboreal pollen, especially Pinus. The fern spores were dominated by Selaginella, but non-arboreal pollen accounted for relatively low percentages of the pollen.

Regional specificity of radionuclide fixation in river basins due to soil petrology and mineralogy

The main goal of this research was to compare and reveal the difference in the character of Cs-137 distribution and fixation in fine fractions of alluvial soils formed by the rivers draining areas of different geochemical origin with the corresponding mineral composition of clay fraction and to discuss the results in light of possible ecological perspectives. Alluvial soil samples were collected in the Yenisey (Central Siberia) and Iput (the East European Plain) river basin areas contaminated by 137Cs. Granulometric fine fractions (>0.01, 0.01–0.005, 0.005–0.001, <0,001 mm) were obtained by pipette technique; clay minerals were determined by a universal Carl-Zeiss Jena X-ray diffractometer (Germany); and 137Cs activity was measured by a CANBERRA gamma spectrometer with HPGe detector (USA). Clay mineralogy demonstrated regional specificity of its composition due to different origins of the soil-forming rocks. Clay fraction of alluvial soil samples collected in the Yenisey river floodplain consists mainly of chlorite-vermiculite and chlorite-smectite interstratifications while in the Iput river basin it is mainly composed of hydromicas and mica-smectite interstratifications. We infer that this may result in a higher potential ability of clay fraction for the radionuclide exchange and release in the former region. Specificity of local clay minerals as radionuclide sorbents should be of particular ecological concern in relation to long-term exchange processes and involvement of the radioisotope in local biogeochemical cycles including its transfer to food chains. The phenomenon is considered in light of the developed scientific field inaugurated as petro-mineralogical ecology and needs further investigation.

Groundwater Contamination by Uranium and Mercury at the Ridaura Aquifer (Girona, NE Spain).

Elevated concentrations of uranium and mercury have been detected in drinking water from public supply and agricultural wells in alluvial and granitic aquifers of the Ridaura basin located at Catalan Coastal Ranges (CCR). The samples showed high concentrations of U above the U.S. standards and the World Health Organization regulations which set a maximum value of 30 µg/L. Further, high mercury concentrations above the European Drinking Water Standards (1 μg/L) were found. Spatial distribution of U in groundwater and geochemical evolution of groundwater suggest that U levels appear to be highest in granitic areas where groundwater has long residence times and a significant salinity. The presence of high U concentrations in alluvial groundwater samples could be associated with hydraulic connection through fractures between the alluvial system and deep granite system. According to this model, oxidizing groundwater moving through fractures in the leucocratic/biotitic granite containing anomalous U contents are the most likely to acquire high levels of U. The distribution of Hg showed concentrations above 1 μg/L in 10 alluvial samples, mainly located near the limit of alluvial aquifer with igneous rocks, which suggests a possible migration of Hg from granitic materials. Also, some samples showed Hg concentrations comprised between 0.9 and 1.5 μg/L, from wells located in agricultural areas.

Assessing pollen distribution patterns and provenance based on palynological investigation on surface sediments from Laizhou Bay, China: an aid to palaeoecological interpretation

Assessing pollen distribution patterns in the sea is essential to understanding pollen provenance, upon which subsequent interpretation of Quaternary pollen data from marine sediments in the same study area should be soundly based. Here we present palynological data from seafloor sediments in Laizhou Bay and analogous samples from its inflowing rivers to elucidate the distribution patterns and provenance of modern palynomorphs. Our results demonstrate that the pollen assemblages of marine areas are dominated by arboreal pollen types, especially Pinus, and herbaceous pollen types are dominated by Chenopodiaceae and Artemisia. The higher pollen concentrations occur in the southwest area of Laizhou Bay, and the lower pollen concentrations are mainly distributed in the nearshore area of the Yellow River estuary. Such pollen distribution patterns are tightly correlated with the distribution features of grain size of the surface sediments. Specifically, the sediments with higher pollen concentrations are of very fine and fine silts, whereas those with lower pollen concentrations are of fine sands. The results of PCA analysis suggest that pollen grains in the nearshore areas are mainly transported by water flows, of which herbaceous pollen show relatively higher concentrations; while pollen grains in the offshore areas are largely carried by winds, especially those pollen types with sacs, such as Pinus. Dissimilarity analysis between marine and alluvial samples indicates that the pollen spectrum show close linkage with the regional vegetation in the surrounding watersheds. Pollen quantitative characteristics and pollen assemblage changes in different parts of the marine areas indicate various pollen provenance and changing landscape of terrestrial vegetation, particularly along the inflowing rivers. Our Assessments on pollen distribution patterns and provenance based on palynological investigation would strongly aid Quaternary palaeoecological and palaeoclimatological interpretation for our study area.

Using geochemical indicators to distinguish high biogeochemical activity in floodplain soils and sediments

A better understanding of how microbial communities interact with their surroundings in physically and chemically heterogeneous subsurface environments will lead to improved quantification of biogeochemical reactions and associated nutrient cycling. This study develops a methodology to predict potential elevated rates of biogeochemical activity (microbial “hotspots”) in subsurface environments by correlating microbial DNA and aspects of the community structure with the spatial distribution of geochemical indicators in subsurface sediments. Multiple linear regression models of simulated precipitation leachate, HCl and hydroxylamine extractable iron and manganese, total organic carbon (TOC), and microbial community structure were used to identify sample characteristics indicative of biogeochemical hotspots within fluvially-derived aquifer sediments and overlying soils. The method has been applied to (a) alluvial materials collected at a former uranium mill site near Rifle, Colorado and (b) relatively undisturbed floodplain deposits (soils and sediments) collected along the East River near Crested Butte, Colorado. At Rifle, 16 alluvial samples were taken from 8 sediment cores, and at the East River, 46 soil/sediment samples were collected across and perpendicular to 3 active meanders and an oxbow meander. Regression models using TOC and TOC combined with extractable iron and manganese results were determined to be the best fitting statistical models of microbial DNA (via 16S rRNA gene analysis). Fitting these models to observations in both contaminated and natural floodplain deposits, and their associated alluvial aquifers, demonstrates the broad applicability of the geochemical indicator based approach.

A comparison between single- and multi-objective optimization to fit spectral induced polarization data from laboratory measurements on alluvial sediments

Spectral induced polarization measurements on unconsolidated and saturated alluvial samples, sand-clay mixtures and well sorted sandy samples, are modelled with the generalized Cole–Cole phenomenological model and two simplified models: the standard Cole–Cole and the Cole–Davidson model. The goodness of fit is evaluated, as a first step, through the root mean square error, weighted on the data errors of the real and the imaginary component. At a later stage a multi-objective optimization is proposed, based on two different indicators for the resistivity amplitude and phase misfit. The analysis of the misfits variations among all the tested parameters associations is conducted to identify the Pareto set of optimal solutions. Both procedures lead to model parameter estimates comparable with literature values. However, the multi-objective approach provides information about the uncertainty of the parameter estimates and highlights the presence of more than one characteristic value for the relaxation time and the frequency exponent in many samples, thus suggesting the possible occurrence of different polarization processes in the investigated frequency range.

Microchemical signature of alluvial gold from two contrasting terrains in Cameroon

The microchemical signature of alluvial gold particles has wide application in constraining their primary sources. In this study, we apply this concept to investigate the composition of gold-bearing alloys from alluvial samples draining two geologically distinct terrains in southern and eastern Cameroon where the search for primary gold has remained elusive. The first set of gold grains (Lom grains) are from the Lom river drainage system with predominantly metasedimentary Pan-African rocks in the catchment region while the second set of grains (Nyong grains) are from the Mbal and Ebap tributaries of the Nyong river draining over a Paleoproterozoic complex comprising metamorphosed ultramafic rocks, amphibolites and granulitic gneisses. The gold grains recovered from these fluvial networks after panning were first studied under an electron microscope in order to evaluate their morphological features and subsequently embedded in epoxy resin, polished, and their compositions determined by both electron microprobe (EMPA) and laser ablation (LA-ICP-MS) techniques. The Lom grains are irregular to sub rounded with extensively pitted surfaces while the Nyong grains are predominantly rounded, oblong and with smooth surfaces. Nyong grains are devoid of inclusions while galena and pyrite are entombed in the Lom grains. Both set of grains are essentially Au–Ag alloys although the Ag content of the cores of the Nyong grains from both EMPA and LA-ICP-MS analytical techniques are significantly lower (0.05–6.07 wt% Ag; 93.54–99.29 wt% Au) than for Lom gold (0.06–22.75 wt% Ag; 78.76–99.86 wt% Au). X-ray element distribution maps do not show any zonal variation in core composition suggesting the gold grains derived from lode sources with single episode of hydrothermal gold deposition. Also the Nyong grains have significant amounts of Pt, Pd and Cr suggesting a link with ultramafic rocks while the Lom grains have substantial Sb and Zn levels pointing to hydrothermal quartz veining as the possible primary mineralization system. The grains from both localities have Ag-depleted rims. The results demonstrate that the microchemical signature of gold is useful to study the hypogene environment of a given area and a field for hydrothermal-orogenic gold is proposed on the ternary Au–Ag–Hg and Au–Ag–Cu plots.

Optically stimulated luminescence dating of Holocene alluvial fans, East Anatolian Fault System, Turkey

In this study, the optically stimulated luminescence dating technique was used to determine the time of deposition of alluvial sediment samples from the Türkoğlu-Antakya segment of the East Anatolian Fault System (EAFS) in Turkey. The double-single aliquot regenerative dose protocol on fine grain samples was used to estimate equivalent doses (De). Annual dose rate was computed using elemental concentration of uranium (U) and thorium (Th) determined by using thick-source alpha counting and potassium (K) concentrations using X-ray fluorescence and/or atomic absorption spectroscopy. The environmental dose was measured in situ using α-Al2O3:C chips inside plastic tubes for a year. The two different bulk sediment samples collected from the Islahiye trench yielded ages of 4.54 ± 0.28 and 2.91 ± 0.23 ka. We also obtained a 2.60 ± 0.18 ka age for the alluvial deposit in the Kıranyurdu trench and 2.31 ± 0.14 ka age for an excavation area called Malzeme Ocağı. These ages were consistent with the corresponding calibrated Carbon-14 (14C) ages of the region. The differences between the determined ages were insufficient to clearly distinguish the disturbance event from the effects of bioturbation, biological mixing, or other sources of De variation in the region. They provide a record of alluvial aggradation in the region and may determine undocumented historical earthquake events.

ALLUVIAL AND ELUVIAL PLATINUM-GROUP MINERALS FROM THE BUSHVELD COMPLEX, SOUTH AFRICA

The present work provides an initial description of detrital platinum-group minerals (PGM) collected from alluvial sediments of rivers draining the Bushveld Complex, and from eluvial concentrations at the Onverwacht platiniferous pipe. During a field campaign, sediments were sampled at nine localities around the Bushveld Complex, and heavy mineral assemblages were investigated using optical and scanning electron microscopy (SEM) as well as microprobe analysis (EPMA) of PGM. All concentrates from the alluvial samples contain discrete PGM grains with grain sizes in the range from ~50 to 150 μm (maximum 600 μm). The overall PGM proportions are: native Pt, Pt-Pd and Pt-Fe alloys (together 54%), sperrylite (33%), cooperite/braggite (11%), and stibiopalladinite (2%). This PGM assemblage distinctly contrasts to the suite of PGM in the pristine, sulfide-bearing mineralization in the Merensky, UG-2 and Platreef, the assumed sources of the detrital PGM. Specifically, PGE-bismuthotellurides and -sulfarsenides, common in the primary ores, are missing in the assemblage of detrital PGM in the fluvial environment. Nearly all detrital PGM (98%) are Pt minerals, corroborating earlier findings that Pd-dominated PGM are unstable and are dissolved in the supergene environment, and that PGE-bismuthotellurides and -sulfarsenides, common in the PGM assemblages of the pristine ores are unstable during weathering and mechanical transport. The eluvial material collected at Onverwacht contained ca. 150 PGM grains with sizes mainly in the range 100 to 300 μm range (maximum 1.87 mm). The PGM assemblage comprises grains of Pt-Fe alloys (66%), sperrylite (14%), and many rarer PGM including stibiopalladinite, hollingworthite, laurite, PGE arsenides and PGE sulfides. The suite of eluvial PGM observed is similar to the PGM assemblage described previously from the Onverwacht pipe proper, including the type locality minerals genkinite, irarsite and platarsite, as well as some additional and possibly new PGM. Most of the relatively rare PGM detected in the suite of eluvial grains from Onverwacht were also reported in the detrital PGM assemblage from the Moopetsi river, farm Maandagshoek (Oberthur et al., 2004), indicating that many of the latter grains originate from platiniferous pipes and not from the Merensky or UG-2 reefs. Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, this work also highlights the fact that the nowadays somewhat neglected field methods and basic techniques have their merits and value in mineral exploration, especially if they are combined with modern micro-analytical methods. The systematic recovery of PGM from stream sediments, soils and till should regain wider application in mineral exploration as these tools can provide useful indicators of platinum mineralization.

Alluvial xenotime and heavy minerals assemblage from the northern edge of Nisa-Albuquerque Batholith, eastern Portugal: Provenance and geochemical implications

The xenotime-bearing heavy mineral assemblages present in the alluvial samples from Vila Velha de Ródão, Nisa, Póvoa e Meadas and Sto. António das Areias, localized in the northern edge of Nisa-Albuquerque Batholith, Eastern Portugal, were studied in detail. The insights for mineral provenance came from the analysis of the drainage network and from the composition of the regional alluvial samples and outcropping lithologies. Since xenotime is a mineral with economic interest, the motivation of the current work is its contribution for exploration studies, considering the potential information contained in the alluvial samples. In the study region, the alluvial heavy minerals and their morphology reflect clearly the mineralogy and relative distance to their source, respectively. In alluvial samples with direct provenance from the Nisa granite (late Carboniferous), the increase in xenotime concentration and decrease in monazite, apatite, zircon, ilmenite and iron oxide concentration, from W (Nisa) to E (Sto. António das Areias), matches the increase in P2O5 and decrease in REE, CaO, Zr, TiO2, and Fe2O3 contents in granite rocks.The geochemical signature of xenotime studied reveals an igneous source, characterized by the characteristic strong Eu negative anomaly in REE patterns. Their YPO4 values (72–78mol%) are similar to xenotime from Erzgebirge (Germany) granites. The high values of Y/Ho (50–71) and, in some cases, downward kinks at Ho in REE plots, suggest generation in late stages of crystallization and association with a siliceous evolved magmatic system. The slight decrease in HREE contents in xenotime, from W to E, and Ho anomalies in the normalized REE pattern are similar to those identified in the Nisa granite, which supports the sourcing of xenotime from these granitic rocks. The genetic development of the batholith chemical zonation (more evolved to E) seems to have favoured the generation of xenotime instead of apatite, monazite and zircon, in the eastern end area (Sto. António das Areias).Supporting the geochemical signature, the alluvial zircons also display morphological typologies that are compatible with provenance from peraluminous granites formed at temperatures of 600–700°C, possibly from the Nisa granite. Since all xenotime grains show the same magmatic affinity, this leads to the hypothesis that the xenotime from the Vila Velha de Ródão sample, with provenance from Cenozoic sedimentary rocks, has been transported to this northern area, after been disaggregated from Nisa batholith granites, most probably by braided anastomosing and erratically sandy channels, that explains its actual position in the opposite bank of the Tagus river. This phosphate has been deposited with other sediments in tectonic depressions and subsequently included in the formation of sedimentary rocks. The greater development of Ca, Th and U phosphate inclusions/substitutions in these xenotime grains can be explained by the chemical mobility provided during all the geological processes.