Underlying Basement Rocks Research Articles

Variability in mineralization of the petrified forests in the UNESCO Global Geopark of Lesvos, Greece

Miocene petrified tree fossils in tuffs from Lesvos Island show a variety of types of mineralization. Trees from the older part of the volcanic succession, including the well-known Petrified Forest near Sigri, are brilliant yellows, reds and browns as a result of Fe–Mn hydrothermal alteration post-dating silicification. Trees from the younger part of the volcanic succession on the southeastern flank of the volcanic chain include some in which the original carbon has not been replaced, others that are completely silicified or completely replaced by calcite, and yet others with cell walls silicified and the lumina filled with a variety of carbonate and sulfide minerals. Widespread Fe–Mn hydrothermal veins are found only in the older part of the volcanic succession and in underlying basement rocks and date from the inversion of the Sigri basin in the mid-Burdigalian and the intrusion of the Mesotopos dykes and laccoliths. The variable petrifaction of fossil wood is thus essentially a consequence of the variability of diagenetic and hydrothermal processes. This study demonstrates the importance of tectonic history in localising volcanic hydrothermal circulation and hence diversity in style of petrifaction.

What is the source of magnesium in hydrothermal dolomites? New insights from coupling δ26Mg - ∆47 isotopes

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18Ofluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18Ofluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆47-derived temperatures, it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids.

North Kara and Vize-Ushakov Composite Tectono-Sedimentary Elements, Kara Sea

Abstract North Kara Shelf and adjoining Severnaya Zemlya Archipelago represent a region of a complex tectonic interplay between two large Arctic fold-and-thrust belts: the Early Mesozoic Pay-Khoy-Novaya Zemlya–South Taimyr in the SW and the Late Paleozoic Taimyr–Severnaya Zemlya–in the S-SE and E, and an inferred Devonian/Carboniferous deformation zone in the N. Contractional deformations of different ages formed a series of basement highs and inverted sediment-cored swells that separate deeply subsided sedimentary basins. In the framework of the volume's terminology, this region consists of two composite tectono-sedimentary elements (CTSE), the Vize-Ushakov (VU CTSE) and North Kara (NK CTSE). The principal difference between these elements is in the age of underlying basement rocks, which, in turn, determines the stratigraphic span of the overlying sedimentary successions. In this chapter we characterize the geology of this remote Arctic region based on the available seismic data supported by the published results and our own studies.

Association and textural-compositional evolution of pyrite-organic matter in coals of the Tarakan, Barito, and Pasir Basins, Kalimantan, Indonesia

The Barito, Pasir, and Tarakan Basins in Kalimantan Island is the most extensive coal-bearing sedimentary basin in Indonesia, mined since 1989. The coal is notable in having high sulfur coal content, exceeding >10% in some cases, contributed by the presence of pyrite. The latter issue contributed to interest in the coal, forming economic mineral deposits hosted by the sedimentary pyrite and organic matter. Novel findings of this research providing valuable information of trace element behavior during the diagenetic conditions. This paper documents the textural and compositional evolution of pyrite-organic matter of coals using coal petrographic, proximate, ultimate, total sulfur content, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), electron probe micro analyser (EPMA), and Raman spectroscopy. Coals from Kalimantan are mainly characterized by medium-high sulfur content (>3%) and the maximum temperatures reached by organic matter during diagenesis are 166 ± 10 °C for the Pasir Basin, 136 ± 11 °C for the Barito Basin, and 132 ± 12 °C for the Tarakan Basin. Pyrite is mainly found as large size framboids (mostly >5 μm), indicating formation in the sedimentary layer with dysoxic bottom water conditions during early–late diagenetic stages. The framboidal pyrite was likely formed in detrovitrinite and gelovitrinite under topogenous mire conditions, due to the degradation of macerals and biochemical gelification, which provided the organic templates. High concentrations of trace elements (TE) of framboidal pyrite, such as 1) Cu, Ni, and Co in the Pasir Basin and 2) As in the Barito Basin, likely reflect the local sedimentary provenance and underlying basement rocks. Pyrite texture evolved with increasing diagenetic temperatures from 1) framboidal pyrite, 2) recrystallized framboidal pyrite rims, and 3) anhedral, anhedral-euhedral pyrite transition, and euhedral massive pyrite. This study shows a similar average TE concentrations of various pyrite texture, suggesting that the TEs are not remobilized during the evolution of pyrite at the diagenetic stages.

Geochemical and Isotopic (Nd, Sr) Tracing of the Origin of REE Enrichment in the Cambrian Georgina Basin Phosphorites

AbstractPhosphorites of the Georgina Basin (northern Australia) are an established economic source of phosphate and have recently been recognized to be a potential source of rare earth elements (REE). Previous bulk‐rock geochemistry work focused on the eastern margin of the basin revealed that phosphorites from the southern region have significantly higher (up to 0.5%) REE contents than equivalent prospects further to the north. In this study, we examine the origin of REE enrichment in the Georgina Basin phosphorites using an integrated geochemical, petrographic, mineralogical, and isotopic (Sm‐Nd and Sr) approach. The trace element geochemistry of primary phosphate minerals is consistent with a seawater origin for the REEs in phosphorites, with minimal input from the underlying basement rocks. Variations in total REE concentrations are controlled largely by the seawater composition, depositional environment, and the texture of the phosphorite (grainstone vs. mudstone phosphorite). Grainstone phosphorite yields on average higher concentrations of REEs compared to the mudstone phosphorite, likely due to the higher surface‐to‐mass ratio of the grainstone phosphorite resulting in the uptake of higher concentrations of REEs during coastal reworking. The formation of the low‐REE northern and central deposits took place during early transgression in oxidized, shallow supratidal to subtidal platformal environments. During the subsequent main transgressive event, fault‐controlled topography in the southern domain allowed phosphogenesis along basement highs to be facilitated along a redox boundary, where more saline and REE‐rich deep anoxic bottom waters, along with Fe‐cycling and bacterial mediation played a major role in the enrichment of REEs in the southern phosphorites.

INSIGHT INTO ROCK THERMAL CONDUCTIVITIES IN THE NORTH CROATIAN BASIN THROUGH IN SITU MEASUREMENTS

The thermal conductivity of rocks represents one of the significant variables when investigating geothermal potential of an area on a local scale as well as regionally when performing basin analysis with the aim of estimating hydrocarbon potential. While steady-state methods of measuring thermal conductivity are presumed to yield more reliable results, transient methods allow for in situ measurements, thereby considerably simplifying and reducing measurement costs. This study was performed with the goal to expand the understanding of thermal conductivity of rocks typical for the North Croatian Basin (NCB) infill, as well as the underlying basement rocks. The measured values reveal distinct ranges across various lithologies. The thermal conductivity values measured in crystalline rocks are quite consistent, showing narrow ranges of values for each lithotype: for granite the measured values are between 2.317 and 2.486 W m-1 K-1, the value range for gneiss is between 3.332 and 3.565 W m-1 K-1 and the thermal conductivity of amphibolite is in the range between 1.549 and 1.623 W m-1 K-1. In contrast, the thermal conductivity values of sedimentary rocks vary within broader ranges – the values in sandstones range between 1.778 and 2.433 W m-1 K-1, for marlstones the registered range is between 0.917 and 2.323 W m-1 K-1, the values measured in shales range between 0.894 and 2.304 W m-1 K-1 and biocalcarenites show values of thermal conductivity between 0.990 and 2.023 W m-1 K-1. The greater variability in values measured for sedimentary rocks is attributed to the variability in porosity and fluid saturation, as well as the greater variability of mineral composition. Further research is needed to determine which factor has the greatest influence on the variability of thermal conductivity values, i.e. to establish to which extent each of the factors contributes to the measured values.

Geochronology and geochemistry of ore-hosted zircon reveal the genesis of typical Sb-(Au-W) deposits in South China

The Xikuangshan and Woxi Sb-(Au-W) deposits are the most important Sb-related mineralization in the Xiangzhong metallogenic province, South China. In this study, trace element, U-Pb dating, and Hf isotopic results of zircon in typical ore samples from the Xikuangshan and Woxi deposits provide new insights into metal/fluid sources and mineralization processes. The zircons in different samples from the Xikuangshan deposit have a similar distribution of zircon U-Pb ages with two major groups: 400–500 Ma and 700–900 Ma, which are consistent with those from their host rocks (Devonian strata) and underlying basement rocks (Lengjiaxi and Banxi Groups). By comparison, the zircons from the Woxi deposit have younger U-Pb ages which are the same as contemporaneous magmatism in South China. The 176Hf/177Hf ratios, εHf(t), and TDM1 further proved an original relationship between the younger zircons (with ages < 500 Ma) and magmatism. The systematic trend in the size and roundness of zircons captured from basement rocks for Xikuangshan might be resulting from gravity separation in the sedimentary process which required a stable and long-lasting mineralization process. On the other hand, the homogenous size and roundness of zircons from the Woxi deposit, regardless of zircon ages and sampling locations, might have been caused by a relatively short and intense mineralization process. The differences in the mineralization process for the two deposits lead to different alteration degrees of zircons and are reflected by the variable concentrations of Y, Hf, Pb, and Th, as well as rare earth elements. In the deposit scale, some samples are considered to be altered at high grade and represent the main mineralization stages for the Xikuangshan and Woxi deposits, as their zircons have variable concentrations of trace and rare earth elements. Combined with previous studies, the sources of zircons and mineralizing material are distinct for the Xikuangshan and Woxi deposits: originated from basement rocks (Xikuangshan deposit) and magmatism (Woxi deposit). In addition, stable and long-lasting deep-circulated meteoric water is considered the mineralizing fluid for the Xikuangshan deposit, whereas the mineralizing fluid for the Woxi deposit might be related to magmatism. This research highlights the use of zircon geochronology and geochemistry in revealing the genesis of Sb deposits, making this method applicable worldwide to similar vein-type mineralization.

In-situ trace elements and sulfur isotopic analyses of stibnite: Constraints on the genesis of Sb/Sb-polymetallic deposits in southern China

The Xiangzhong metallogenic province (XZMP), the largest polymetallic mineralization province in southern China, is known for its large number of Sb and Sb-polymetallic deposits (>100). In this study, detailed microscope observation, trace element, and sulfur isotopic analyses were performed on stibnite from the Xikuangshan, Woxi, and Banxi deposits in the XZMP to constrain the ore genesis.Stibnite from three deposits appears similar under backscattered electron imaging and has similar major elements contents. Detectable trace elements in stibnite are Fe, Cu, Zn, As, and Pb. Base metals (Cu, Pb, and Zn) are enriched in stibnite from the Woxi deposit but are depleted in stibnite from the Xikuangshan and Banxi deposits. Gold is detectable only in stibnite from the Woxi deposit. Cu, As, and Pb mainly occur as solid solutions within the crystal lattice in stibnite, while micro-scale inclusions of Fe- and/or Zn-rich phases are also observed. The δ34S values of stibnite from three deposits are distinct but with narrow ranges: +6.8 to +10.2 ‰; +5.5 to +6.5 ‰; −4.2 to +0.1 ‰ for the Xikuangshan, Banxi, and Woxi deposits, respectively. The δ34S values of stibnite suggest that the underlying basement rocks are the potential source of sulfur for the Sb mineralization in the Xikuangshan and Banxi districts while sulfur for the Sb mineralization of the Woxi district may have originated from a deep magmatic source. In addition, a positive trend between the δ34S values of stibnite and sampling depth suggests an input of the 34S-rich basin brine during the mineralization in the Woxi district.We propose a genetic model for the Sb mineralization in the XZMP: Fluids rich in Sb, Au, W, Cu, Pb, and Zn related to igneous activities ascended via regional deep faults and mixed with 34S-rich basin brine. Ore minerals of Sb, Au, and W were deposited within structural traps resulting in the Woxi-type deposits. The basement-derived deep fluids driven by orogenic leached Sb and related metals from the basement metamorphic rocks, which migrated via the regional faults and mixed with cold near surface meteoric water to form the Xikuangshan-type deposits.

LA-ICP-MS Mapping of Barren Sandstone from the Proterozoic Athabasca Basin (Canada)—Footprint of U- and REE-Rich Basinal Fluids

The Proterozoic Athabasca Basin hosts a large number of high-grade, large-tonnage unconformity-related uranium (U) deposits, many of which are also enriched in rare earth elements (REE). The basin also contains hydrothermal REE mineralization unassociated with U. Previous studies postulated that U and REE were derived from either the basin or the basement; however, the exact source of the metals remains ambiguous. This study provides evidence of U- and REE-rich fluids throughout the Athabasca Basin through laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping of barren sandstone distal to mineralized areas. The results indicate that elevated U and REE concentrations mainly occur in the matrix; there are strong positive correlations between U and REE, Th, P and Sr, and moderate positive correlations between U and Zr, Ba, Fe, Al, K and Ca, but the few spots with the highest U are unrelated to these elements. Quantitative evaluation of the element correlations, together with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, suggests that most of the elevated U and REE are hosted in aluminum phosphate sulfate (APS) minerals rather than apatite and monazite. As the APS minerals are of diagenetic-hydrothermal origin, the results testify to the presence of U- and REE-rich fluids within the Athabasca Basin. The elevated Th/U ratio (~10) and REE pattern (strong heavy rare earth element (HREE) depletion) are consistent with a model in which large amounts of U and REE (especially HREE) were leached from the sandstone within the Athabasca Basin and contributed to U and REE mineralization near the unconformity between the sedimentary rocks in the basin and underlying basement rocks. This study demonstrates that LA-ICP-MS mapping can be effectively used to evaluate microscale distribution of elements and their mobility in sedimentary rocks to address mineralization related problems.

The Norwegian mountains: the result of multiple episodes of uplift and subsidence

The elevation of the mountains in Norway is geologically young. Much of the present‐day land surface was buried below a thick cover of relatively young sediments in the early Miocene, 23 Ma, when Scandinavia started to be uplifted. Big river systems eroded deeply into the rising landscape and transported sand and gravel from Norway and Sweden to Denmark where the detritus was deposited in a large delta. In Norway, the erosion formed an extensive plain near sea level that included the present‐day mountain plateau of Hardangervidda and extended across a thick pile of sediments that covered the present‐day coastal areas of Norway. Hardangervidda was uplifted to its present elevation of about 1200 m after a second phase of uplift that began about 5 Ma, in the early Pliocene. The hard bedrock of Hardangervidda has preserved this part of the plain as an elevated plateau, but the part of the plain that extended across the sediments has been eroded away, exposing the underlying basement rocks. That re‐exposed basement surface was shaped in the Jurassic when the climate was warm and humid. The basement rocks were weathered where rainwater seeped into fracture zones. Erosion of the weathered rocks has left a terrain of fracture valleys and hilly relief that contrasts with the sub‐horizontal plain of Hardangervidda. This weathered landscape is today exposed on the slope between the west coast and Hardangervidda. While the elevation of the mountains is young, today's landscape has a long history.

Sediment Routing and Anthropogenic Impact in the Huanghe River Catchment, China: An Investigation Using Nd Isotopes of River Sediments

AbstractThe Huanghe once had a sediment flux of &gt;1,000 Mt/yr, but this has decreased by ∼90% as its river sediment routing systems have undergone dramatic changes influenced by human activities such as dam construction. However, the way in which the sediment geochemistry of the river has responded to the altered sediment routing processes is not well known. This study investigates the sediment source‐to‐sink routing regime of the Huanghe River using Nd isotope fingerprinting. Four major source terranes, namely the Songpan‐Ganzi (SG) Block, Ordos Desert (OD), Chinese Loess Plateau (CLP) and North China Craton (NCC) are recognized according to their distinct Nd isotopes. The gradual downstream decrease in εNd values in sediments of the upper Huanghe indicates a decreasing contribution of material from the SG Block and a corresponding increase contribution of local underlying basement rocks, which is inferred to be related to sediment capture by a cascade of hydroelectric dams. A gradual increase in εNd from Yinchuan to Tongguan suggests an increasing contribution from the CLP under intense erosion. Relatively low εNd in the downstream sediments suggest a contribution from proximal NCC basement, consistent with the shift from deposition to erosion in the lower channel in recent years. The marked heterogeneity in Nd isotopes in the Huanghe sediments corresponds well to sediment source‐to‐sink processes in response to increasing human impacts. In a setting of global rivers facing strong anthropogenic impacts, the ways in which altered sediment routing systems affect river sediment geochemistry deserve more research attention.

Bouguer anomaly inversion and hydrocode modeling of the central uplift of the Araguainha impact structure.

Araguainha is a mid-sized complex impact structure formed in sedimentary and underlying basement rocks of the Paraná Basin, Brazil. The structure has strongly deformed sedimentary strata surrounding a granitic core. The central uplift is a region of high geological complexity, comprising different types of sedimentary, igneous (granite) and metamorphic lithologies, plus breccias and impact melt sheets. New ground gravity data was collected to produce a Bouguer anomaly map and to perform a 3-D inversion in order to obtain a 3-D density model of the central uplift. This 3-D density model is consistent with iSALE numerical modeling results, which shows that the rocks in the innermost portion became brecciated and/or melted after undergoing pressure/temperature peaks. The positive anomaly of Furnas and Ponta Grossa formations associated with the numerical model shows that the central uplift is ~16 km wide. Thus, the granite's uplift caused the uplift of the entire stratigraphic package, from its Devonian-aged units to the Permian ones, forming a bull's eye pattern around the granitic core. The results also indicate that Araguainha was formed by a 3 km diameter impactor, and the rocks of the granitic basement rocks were uplifted by ~2 km.

Lithologic modeling using VES method for subsurface structure delineation with geological constraints, eastern margin of Chiang Mai Basin and southwestern margin of Mae On Depression, San Kamphaeng District, Chiang Mai Province, northern Thailand

We conducted a vertical electrical sounding (VES) survey over the Quaternary sediments covering a transitional region between the eastern part of the Chiang Mai rift basin and the Mae On Depression, northern Thailand and constructed 1D lithologic subsurface models for delineating geological structures of the underlying basement rocks. Thirteen VES stations were collected and divided into northern and southern sub-parallel alignments trending WSW-ENE across the high magnetic contrast zone. We employed a Schlumberger array with the maximum half current electrode separations around 465–500 m, using the ABEM Terrameter SAS4000 resistivity meter. The VES data were inverted using EarthImager™ 1D program. We classified four geoelectric units composed of (1) very-low-resistivity saturated sediments (17–39 Ωm), (2) low-resistivity unsaturated sediments (33–581 Ωm), (3) medium-resistivity sedimentary rocks (57–110 Ωm), and (4) high-resistivity mafic volcanic rocks (176–804 Ωm). Shallow zones of the geoelectric models were interpreted as sediment fill with layered strata with variable resistivity because of different water saturation and clay content. The sediment fill thickens westward to the Chiang Mai rift basin. Deeper zones of higher resistivity are interpreted as the underlying basement rocks. These basement rocks show contrasting resistivities with the western part modeled as clastic sedimentary rocks of sandstone and the eastern part as mafic volcanic rocks. Geological observation evidence of westward-dipping faults and chronostratigraphic results integrated with the 1D lithologic models and drilled well information support a structural model of a reverse fault trending SW-NE between the Permian volcanic and Carboniferous sedimentary basements. This structure is part of the Chiang Mai-Chiang Rai Suture Zone associated with the eastward subduction model during the Permo-Triassic Indosinian Orogeny in northern Thailand.

Application of geological and electrical resistivity methods for groundwater studies at Tatiko, North Central Nigeria

The success probability of water wells constructed in crystalline rock terrains is largely dependent on the size, location and interconnectivity of the fractures present in them as well as the amount and nature of the material that may clog the fractures. Geological and geoelectrical studies were proposed and conducted at Tatiko to determine the structures in the underlying basement rocks and evaluate their groundwater potential. Geological study was used to delineate the structural control on groundwater availability while the geoelectrical study (Vertical Electrical Sounding) was used to investigate the properties of the subsurface materials and determine their potential to host sufficient quantity of groundwater.Structural data were plotted on a rosset diagram and showed major structural control trending in the NW – SE direction. Quantitative interpretation of the geoelectric data indicates that the major fractures are within the weathered to competent basement indicating good hydraulic connection between the weathered and fresh basement. The fractures are deepest within the region that showed thicker weathering and are as shallow as 7 meters in other regions. Apparent resistivity of the fractures ranges between 100Ωm and 1200Ωm in most parts of the area with the highest values in the west. An integration of the result clearly showed that the NW and SW parts of the study area are the most favourable for drilling and further groundwater studies and drilling is recommended to a depth of 40m to 50m.&#x0D; Keywords: Groundwater, Geoelectrical study, Fractures, Tatiko, Resistivity, Basement Complex.

Paragenetic and petrographic study of uranium mineralization along the Midwest Trend, Northeastern Athabasca Basin

The Athabasca Basin in northern Saskatchewan is host to many world-class uranium deposits associated with the unconformity between the Paleoproterozoic sandstone of the basin and the underlying crystalline basement (Jefferson et al., 2007). While the style and tonnage of these deposits vary, the current genetic model for unconformity-related uranium deposits has been a practical tool for exploration in the Athabasca Basin. However, the factors which control the location and formation of these deposits is still not fully understood. A paragenetic and petrographic study of mineralization along the Midwest Trend, located on the northeastern margin of the Athabasca Basin, aims to refine the current model and to address the general problem: What are the factors which control mineralization and non-mineralization? The Midwest Trend will be used as a "modèle réduit" for uranium mineralization, as it displays many features characteristic of unconformity type deposits. The Midwest Trend comprises three mineral leases that encompass two uranium deposits, the Midwest Main and Midwest A (Allen et al., 2017a, b). Mineralization occurs along a NE-trending graphitic structure, and is hosted by the sandstone, at the unconformity, and in much lesser amounts in the underlying basement rocks. Petrographic observations aided by the use of RAMAN spectroscopy and SEM-EDS, have been used to create a paragenetic sequence of mineralization (Fig.1). Future work will focus on fluid inclusion studies using microthermometry, LA-ICP-MS, and mass spectrometry of contained gases.&#x0D;

A long term baseline and variability of natural radionuclides in groundwater at the Vaalputs low-level radioactive waste disposal facility, Namaqualand, South Africa: regional implications

Abstract Vaalputs is the only South African nuclear waste disposal facility; it is licensed to dispose of low and intermediate level radioactive waste. Possible disposal of long-lived waste, potentially including spent fuel rods, prompted a study to establish a baseline of naturally-occurring radionuclides from the U and Th decay chains in the groundwater of Vaalputs. This baseline was created by investigating a dataset containing 25 years of analyses of the groundwater at the Vaalputs site. The behaviour over time, of 23 radionuclides in groundwater samples from 13 annually and three quarterly sampled boreholes, was evaluated and integrated in order to establish changing trends in the activities of radionuclides, the relative contributions of individual radionuclides to total radiation levels and how these were influenced by groundwater conditions. 238U levels showed a natural anomaly in the near-field of the disposal site that was attributed to the underlying basement rocks (granites) which are anomalously enriched in U. A set of duplicate samples from 2009 determined that the activity ratio of 234U/238U in the groundwater of Vaalputs is 4.1; this is ascribed to more rapid leaching of 234U relative to 238U, because of increased crystal structural damage around sites surrounding 234U atoms. High levels of 226Ra, unsupported by 238U, were found in groundwater from boreholes on the western side of the property: this groundwater yielded low values for 234U/238U, lower pH and stronger oxidizing conditions than groundwater from the rest of the area. The absence of overlying sedimentary rocks, which tend to ameliorate the effects of high radiation from granites by absorption, is suggested as the reason for the different geochemical conditions of the groundwater from boreholes in the west. This suggestion may apply to those boreholes elsewhere in Namaqualand with low published concentrations of 238U, but exceptionally high levels of α -and β-emitters. 232Th and its daughter radionuclides yielded radiation levels far below the guideline of 1 Bq.L-1 specified by the World Health Organization; this result is in line with the known low mobility of Th in groundwater systems.

Assessment of Hydrochemistry, Facies Type and Processes Contributing the Major Ions to the Groundwater of Ajakanga Area, Ibadan, South Western Nigeria

Resulting from insufficient supply of potable water and risk of possible contamination from anthropogenic sources, 14 samples were collected from wells at Ajakanga and environs in Ibadan, Southwestern Nigeria, in order to determine the shallow groundwater quality and desirability for drinking purposes. Physical parameters such as pH, EC, and TDS were measured at site using a combined EC, TDS and pH meter. Major ions analyzed included Na+, Ca2+, Mg2+, K+, NO3-, Cl-, HCO3- and SO42-. The anions were measured using titrimetric method, and analyzed by Genesys 2.0 Spectrophotometer, while the cations were analyzed using the flame photometry method. The hydrochemical data were subjected to a series of statistical analysis and conventional graphical plots. Results showed that all the chemical parameters analyzed, excepting Cl- fall within the World Health Organization permissible limit for drinking water. The shallow groundwater is therefore fit for drinking purposes. Cations concentration in the water is in the order of Ca>Mg>Na>K, while for the major anion HCO3>Cl>SO4>NO3. Scatter plots and Gibbs diagram showed that the cations were released into the water as a result of weathering of the underlying basement rocks. Trilinear Piper diagram indicated two water facies in the area, with CaHCO3 water being dominant, while CaMgHCO3 is subordinate. Application of Chloro Alkaline Indices and the Ca/HCO3 ratio indicated that reverse ion exchange reaction is responsible for the dominance of Ca2+ and Mg2+ in the waters.

Miocene fragmentation of the Central Andean foreland basins between 26 and 28°S

We present new U-Pb LA-ICP-MS data from the Central Andean foreland basins combined with new and published stratigraphic information in order to reconstruct the Miocene fragmentation of the Andean foreland between 26 and 28°S. The disruption of this foreland basin and the subsequent development of elevated intermountain basins have been the focus of several studies. However, the absence of temporal constraints in the Miocene to Pliocene sedimentary record of the low elevation Choromoro and Tucuman foreland basins has presented an obstacle for precise paleogeographic reconstructions. We describe 11 discontinuous stratigraphic sections and use the U-Pb LA-ICP-MS method to date 10 pyroclastic-bearing sediments in order to reconstruct the stratigraphic evolution of the Choromoro and Tucuman basins. We combine our results with published stratigraphic and thermochronologic data from adjacent basins to present a refined Miocene paleogeographic model. In a first stage, a continuous Early Miocene foreland lacustrine basin developed, filling up the preexisting Paleogene topography. The second stage is characterized by basin unroofing around ∼12 Ma; the easily eroded sedimentary cover was removed, leading to the uplift of the underlying basement rocks and the segmentation of the lacustrine system. In the third stage, relief increase took place after ∼6 Ma due to the low erodibility of the basement blocks; as a result, stable fluvial systems developed. Progressive relief development caused pronounced unconformities in the basins and the development of proximal fluvial-gravitational depositional systems after 3 Ma. This model emphasizes on the relations between tectonics, climate, and erodibility, and their control on the evolution of the depositional systems and relief.

Geotechnical and geochemical appraisal of IFON clay, Southwestern Nigeria: Implications for industrial utilization

Clays are geological materials of economic potential in the manufacturing and environmental industries. Geotechnical properties and geochemical composition of Ifon clay deposits in Ondo State, Nigeria were investigated with the aim of determining their economic potentials. The geotechnical investigation includes consistency limits, strength, permeability and firing tests, while, X-ray fluorescence (XRF) method was used for determination of the bulk elemental composition of the clays using standard techniques. Geotechnical appraisal of the clay deposits revealed that high proportion of clay size fractions (52.3 – 66.1%) makes the clays suitable in their natural state for industrial raw materials. Activity of the clays was found to be low at approximately 0.3 – 0.7 and classified as inactive clays. Unconfined compressive strength varies between 3.4 N/mm2 and 4.7 N/mm2 and moderate shrinkage values (≤ 12%) indicate that the clays could be used as building construction materials. Coefficient of permeability of 7.75 × 10-8 to 1.72 × 10-5 cm/sec described the clays as low to impermeable soils suitable as barrier soils. SiO2 (64.41 – 68.74%) and Al2O3 (22.98 – 28.45%) contents dominate the clay geochemistry followed by iron oxide, Fe2O3 (3.57 – 8.31%) content among other oxides in traces, imply that they originated from underlying basement rocks subjected to varying degree of weathering. Low MgO, CaO, K2O and Na2O indicate no expandable clay minerals in the clays. The clays show low heavy metal concentrations which guarantee their suitability as raw materials in chemical industries. Thus, Ifon clay deposits are suitable for the production of ceramics, refractory bricks, paper, paints, rubber and fertilizer. Â

Landscape evolution and geochemical dispersion of the DeGrussa Cu-Au deposit, Western Australia

Sedimentary systems associated with ore deposits may act as mechanical dispersion agents of pathfinder elements at local and regional scales. The DeGrussa Cu-Au Volcanogenic Massive Sulfide (VMS) deposit (12.4 Mt @ 4.7% Cu and 1.8 g/t Au) was discovered in a deeply weathered landscape in Western Australia. The DeGrussa deposit is geochemically characterized as being enriched in Cu, Zn, Au and Ag, as well as significant enrichments in In, Mo, Se, Te, Bi, As, Cd and Co, and depletions in Nb, Hf, Th, REE, Cr, Ni, V, Sc and Ba. The Cenozoic palaeochannel sedimentary package that overlies the DeGrussa deposit displays elevated Cu, Zn, Au, and Ag concentrations, as well as significant enrichments in Cr, Ni, V, Sc, In, Mo, Se, Te, Bi, As, Cd and Co, and a remarkable depletion in REE.The palaeochannel sedimentary package shows similarities in its chemical signature to the DeGrussa deposit and associated basement rocks. However, due to the complexity of the landscape evolution and weathering impacts, there is uncertainty regarding the source(s) of this anomalous signature. It may represent (1) the geochemical signature of the immediately underlying basement rocks and/or the DeGrussa deposit; (2) concentration related to the weathering intensity experienced by the sediments, (3) concentration related to the cannibalistic nature of the sedimentary system, resulting in the reworking of previous deposits, or (4) a combination of the above. The Cenozoic channel clay deposits display strikingly depleted values of total REE (TREE) content, which has been interpreted to indicate that the palaeochannel clay units were mostly derived from weathering of proximal dolerite, with no mixing with other weathering products. This, in combination with high Mo, In and Se, and elevated Cu, Zn, Au, and Ag concentrations, may represent the relics of the DeGrussa footprint. Chromium Ni, V, and Sc are interpreted as being sourced mainly from the mafic basement units.Despite the geochemical signatures reported, this study has discovered that the metal geochemical footprint observed in the palaeochannel sedimentary package represents the footprint of the DeGrussa deposit. Hence the geochemistry of the transported cover can be used as an efficient tool for mineral exploration in specific landscape contexts, since the cover can preserve key elemental vectors of the mineral deposit and the geochemical signature of the basement rocks.