Statistical Analysis Of Geochemical Data Research Articles

The mid-Cretaceous bauxites of SE France: Geochemistry, U-Pb zircon dating and their implications for the paleogeography at the junction between Alpine Tethys and Pyrenean Rift

Karst bauxites mark episodic exhumation of carbonate platforms, thus providing key information for basin analysis and paleoclimate processes at the regional to continental scale. Most karst bauxite deposits of SE France lie between Jurassic platform carbonates in the footwall and Upper Cretaceous marine to continental sediments in the hanging wall. These deposits delineate a stratigraphic gap coeval with the Durance extensional tectonics, which led to the separation of the Vocontian and South Provence Basins, and shaped the junction between the Alpine Tethys and the Pyrenean Rift.Our new mineralogical and geochemical data show that SE France karst bauxites were affected by in-situ weathering and alteration in conjunction to sedimentary reworking. Statistical analysis of geochemical data indicates that bauxite deposits have a similar chemical footprint, likely pointing to a common and long-lasting bauxitization process. New U-Pb zircon data from the Provence deposits confirm that the source material for the SE France bauxites is to be primarily found in the Hercynian basement. The data suggest the presence of an additional exotic zircon source deriving either from Avalonia and/or Baltica. The comparison of the new U-Pb zircon data with those on coeval bauxites evidences a marked difference between the SE France and the Sardinian deposits. This difference indicates diversification in the detritus provenance of the two areas since the Lower Cretaceous, setting the bauxites of SE France and Sardinia in lateral continuity with the bauxites occurring on the two opposite margins of the Pyrenean Rift, and supporting the development of these deposits on the shoulders of the rift-related basin formed at the eastern termination of the Pyrenean Rift.

Features of Carbonaceous Rocks Surrounding the Kara Astrobleme: Statistical Analysis of Geochemical Data

Features of Carbonaceous Rocks Surrounding the Kara Astrobleme: Statistical Analysis of Geochemical Data

3D Modeling and Determination of Factors Responsible for Zinc‐Lead Mineralization in the Mehdiabad Deposit, Central Iran, based on Statistical Analysis of Geochemical Data

AbstractAlthough the Mehdiabad zinc‐lead deposit is one of the most well‐known deposits in the central Iran structural zone, the genesis of the deposit remains controversial. The host rock of the ore is a dolomitic limestone of the Lower Cretaceous Taft Formation. In the two main orebodies of the deposit, which includes the Black Hill and East Ridge ore zones, the oxide and sulfide ores are observed at the surface and at depth, respectively. The elements Zn, Fe, Mn and Mg are more abundant in the East Ridge ore zone (in both sulfide and oxide ores), with Ba, Pb, Ag and Cu being more abundant in the Black Hill oxide ore. Based on the distribution of elements and their correlation with each other in these ore zones, the elements are divided into three general groups, that of terrigenous elements, chemically‐deposited elements and ore‐forming (hydrothermally deposited) elements, a division that is supported by the results of factor analyses. The spatial distribution of elements is jointly affected by contact with host rocks, the boundary of oxide‐sulfide ores and fault zones. The main factors governing the distribution of elements are the mechanical transfer of detrital sediments, chemical sedimentation, transfer by hydrothermal fluids, oxidation and surface dissolution, all of which affected the spatial distribution of elements. The ore‐forming elements are mostly affected by hydrothermal fluids and oxidation. This study not only provides additional information about the genesis of the Mehdiabad deposit, but also could assist in the exploitation of ore and further exploration purposes. The results of this study can aid in the exploration and exploitation of the Mehdiabad deposit and similar deposits in the region.

Exploration and Sources of Bauxite Deposit in the Boloven Plateau, Southern Laos

The Boloven bauxite deposit occurs either in the weathered basalt (alkali basalt and tholeiite), or in the Cretaceous sandstone. It is generally agreed that the bauxite deposits/laterites overlying the alkali basalt and tholeiite were derived from weathering of underlying basalt, however, the origin of bauxite deposit overlying the sandstone remains controversial. Chondrite-normalized REE patterns show that the bauxite ores/laterites overlying the sandstone exhibit quite similar chondrite-normalized REE patterns to those overlying the alkali basalt. Diagram of Al-Ti-Zr shows that the bauxite ores/laterites overlying the sandstone, tholeiite and alkali basalt are close to each other and to the calc-alkaline suite, however, significantly different from the sandstone and shales. Binary diagram of log Nb/Y vs log Zr/Ti further indicates that the parent rocks of bauxite ores/laterites overlying the sandstone belong to the suit of ultra-alkali to alkali basalt. Multivariate statistical analysis of geochemical data exhibit that the geochemical characteristics of HREE, Y, LREE and Al2O3 for bauxite ores/laterites overlying the sandstone are similar to those overlying the alkali basalt (15.7 Ma), obviously different from those overlying the tholeiite (1.2 to 0.5 Ma). Consequently, it can be inferred that the bauxite deposits/laterites overlying the sandstone were derived from the alkali basalt.

Characterization of Origin and Evolution of Formation Water in Buried Hill of Jizhong Depression, China, Using Multivariate Statistical Analysis of Geochemical Data

Groundwater samples from buried hill of Jizhong Depression were evaluated using two statistical analyses: hierarchical cluster analysis (HCA) and principal component analysis (PCA). The samples were classified into four clusters, C1–C4, in HCA and the hydrochemical types of C1–C4 are HCO3-Na, Cl·HCO3-Na, Cl-Na, and Cl-Na·Ca. From C1 to C2, C3, and C4, the water-rock interaction becomes increasingly intensive, and rNa/rCl gets lower while total dissolved solids and r(Cl-Na)/rMg get higher. Three components of PCA explain 86.87% of the variance. Component1 (PC1), characterized by highly positive loadings in Na+ and Cl−, is related to evaporation concentration. Component2 (PC2) is defined by highly positive loading in HCO3- and is related to influence of atmospheric water. With high positive loadings in Ca2+ and high negative loadings in Na+ and SO42-, component3 (PC3) suggests plagioclase albitization. The combination of HCA and PCA within the hydrogeological contexts allowed the division of study area into five dynamic areas. From recharge area to discharge area, the influence of atmospheric water gets weaker and water-rock interactions such as evaporation concentration and plagioclase albitization become intensive. Therefore groundwater in buried hill showed paths of hydrochemical evolution, from C1, to C2, C3, and C4. Buried hill reservoir in Jizhong Depression is mainly distributed in hydrodynamic blocking and discharge area; therefore the two regions can be the favorable areas for petroleum migration.

A city scale study on the effects of intensive groundwater heat pump systems on heavy metal contents in groundwater

As a result of the increasing use of shallow geothermal resources, hydraulic, thermal and chemical impacts affecting groundwater quality can be observed with ever increasing frequency (Possemiers et al., 2014). To overcome the uncertainty associated with chemical impacts, a city scale study on the effects of intensive geothermal resource use by groundwater heat pump systems on groundwater quality, with special emphasis on heavy metal contents was performed. Statistical analysis of geochemical data obtained from several field campaigns has allowed studying the spatiotemporal relationship between temperature anomalies in the aquifer and trace element composition of groundwater. The relationship between temperature and the concentrations of trace elements resulted in weak correlations, indicating that temperature changes are not the driving factor in enhancing heavy metal contaminations. Regression models established for these correlations showed a very low reactivity or response of heavy metal contents to temperature changes. The change rates of heavy metal contents with respect to temperature changes obtained indicate a low risk of exceeding quality threshold values by means of the exploitation regimes used, neither producing nor enhancing contamination significantly. However, modification of pH, redox potential, electrical conductivity, dissolved oxygen and alkalinity correlated with the concentrations of heavy metals. In this case, the change rates of heavy metal contents are higher, with a greater risk of exceeding threshold values.

A novel method to estimate mineral compositions of mudrocks: A case study for the Canadian unconventional petroleum systems

The mineral composition of mudrocks is an essential attribute in controlling the reservoir quality of unconventional petroleum systems. The present study introduces a semi-quantitative method to estimate mineral phases of mudrocks in various Canadian unconventional hydrocarbon systems using total elemental analysis (inductively coupled plasma-mass spectrometry (ICP-MS)) and Rock-Eval data (total organic carbon (TOC) and mineral carbon (MinC)).This method involves statistical analysis based on a sound knowledge of hydrocarbon source rock inorganic geochemistry. The workflow can be divided into four steps: (i) converting major elements (Si, Al, Fe, K, Na, Ca, Mg, Ti, and P) to their oxides, (ii) inferring modes of occurrence of elements using statistical analysis of geochemical data (major elements, TOC, and MinC), (iii) identifying the mineral types (oxide, aluminosilicates, carbonates, sulfide, and phosphate) according to elemental occurrences and calculating mineral phase concentrations, and (iv) verifying the results by comparing to XRD data on selected samples. The results, especially for brittle minerals such as quartz, carbonates (e.g. calcite, dolomite, and ankerite), and pyrite, show that the estimated mineral compositions correspond closely and consistently with measured mineralogy obtained from XRD. This method takes advantage of bulk geochemical data already available for hydrocarbon potential and chemostratigraphic studies, without devoting additional samples and cost for XRD analysis.

Application of factor analysis and electrical resistivity to understand groundwater contributions to coastal embayments in semi-arid and hypersaline coastal settings

Groundwater contributions and sources of salinity to Oso Bay in south Texas were investigated using multivariate statistical analysis of geochemical data and multitemporal electrical resistivity tomography surveys. Both analysis of geochemical data and subsurface imaging techniques identified two commonalities for the investigated system: 1) hypersaline water occurs near the groundwater/surface water interface during wet conditions creating reverse hydraulic gradients due to density effects. The development and downward movement of these fluids as continuous plumes deflect fresher groundwater discharge downward and laterally away from the surface; and 2) more pronounced upwelling of fresher groundwater occurs during drought periods when density inversions are more defined and are expected to overcome dispersion and diffusion processes and create sufficiently large-enough unstable gradients that induce density-difference convection. Salinity mass-balance models derived from time-difference resistivity tomograph and in-situ salinity data reaffirm these findings indicating that groundwater upwelling is more prominent during dry to wet conditions in 2013 (~545.5m3/d) and is less pronounced during wet to dry conditions in 2012 (~262.7m3/d) for the 224m2 area surveyed. Findings show that the highly saline nature of water in this area and changes in salinity regimes can be attributed to a combination of factors, namely: surface outflows, evapoconcentration, recirculation of hypersaline groundwaters, and potential trapped oil field brines. Increased drought conditions will likely exacerbate the rate at which salinity levels are increasing in bays and estuaries in semi-arid regions where both hypersaline groundwater discharge and high evaporation rates occur simultaneously.

Sediment geochemistry as a provenance indicator: Unravelling the cryptic signatures of polycyclic sources, climate change, tectonism and volcanism

AbstractInterpretation of bulk‐sediment geochemistry is one of several approaches for determining sediment provenance. This study investigates the value added by bulk‐sediment geochemical analysis in interpreting provenance in a passive margin clastic basin, the Upper Jurassic–Lower Cretaceous deltaic sediments of the Scotian Basin. Provenance studies in this basin are challenging because source tectonic terranes are parallel to the basin margin and polycyclic sediment sources are abundant. More than 400 samples of mudstone and sandstone representing the geographical and stratigraphic range of interest were analysed for 57 elements. Diagenetic processes added calcium to many samples and removed potassium in rocks buried below 3 km, thus impacting principal component analysis and published weathering indices. However, multiple geochemical approaches to assessing the degree of weathering showed climatically controlled changes in weathering in the Tithonian and Barremian, and changes in supply from major tectonic events, such as the top‐Aptian uplift in the Labrador rift. Covariance of elements in heavy minerals demonstrates the varying magnitude of polycyclic supply and stratigraphic changes in sources. Geochemical analyses revealed a previously unsuspected Tithonian alkali volcanic sediment source, characterized by high niobium and tantalum. The lack of highly contrasting sources means that geochemistry alone is inadequate to determine sediment provenance. Published discrimination diagrams are of limited value. Statistical analysis of geochemical data is strongly influenced by diagenetic processes, episodic volcanic inputs and polycyclic concentration of resistant heavy minerals in sandstones. Single indicator elements for particular sources are generally lacking. Nevertheless, careful consideration of geochemical variability on a case by case basis, integrated with detrital mineral studies, provides new insights into palaeoclimate, sediment provenance and, hence, regional tectonics. Although there is no simple template for such analysis, this study demonstrates an approach that can be used for other basins.

Geochemical fractionation of manganese in the Riogrande II reservoir, Antioquia, Colombia

The Riogrande II reservoir in Colombia has a total storage capacity of 240 million m3 and lies 2,270 m above sea level. The reservoir is used for power generation, water supply and environmental improvement. Dissolved manganese (Mn) is removed from reservoir water dedicated to domestic use by purification processes. Removal of Mn, however, poses a major challenge to purification processes and warrants the study of ways to naturally reduce dissolved Mn levels in the reservoir. The source of Mn within the reservoir is not well understood, however, presumably arises from sediment mobilization initiated by variation in pH, redox potential (ORP or Eh), dissolved oxygen (O2) and ionic strength conditions. This study investigated conditions within the reservoir to further understand Mn transfer from the sediment into the water column. O2, pH, oxidation–reduction potential (ORP or Eh), organic matter content and electric conductivity were measured in water samples and sediment from the reservoir. Sequential extraction (SE) procedures were used to test the specific effects exerted by each of these conditions on Mn mobilization from the sediments. The European Community Bureau of Reference (BCR) sequential extraction procedure was used to quantify metals in sediment (referred to as the BCR extraction below). Statistical analysis of geochemical data from water samples (both water column and sediment pore water) and sediments demonstrated the conditions under which Mn can be released from sediments into the water column. The results indicated a primarily oxic water column and anoxic reducing conditions in the sediment (ORP or Eh ≤ −80 mV). The pH of water in contact with bottom sediments varied from 7.6 to 6.8. The pH of sedimentary pore water varied from 6.8 to 4.7. The sediments contained significant amounts of organic matter (20 %). Chemical extractions showed that the exchangeable fraction contained over 50 % of the total Mn within sediments. Microscopic analysis using scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) indicated that Mn does not occur within well-crystallized mineral phases in the Riogrande II sediments. A large proportion of Mn exists instead as material adsorbed onto the surfaces of recently deposited sediment particles. Bacterial oxidation of organic matter may cause the observed anoxic conditions at the bottom of the reservoir. Mineralization of organic matter therefore contributes to reducing conditions within the sediments. Mobilization of Mn from the sediment into the water column may result from reductive dissolution of this fraction. Manganese release by this mechanism diminishes the water quality of the Riogrande II reservoir and warrants further study.

Background levels of trace elements and soil geochemistry at regional level in NE Italy

Trace elements of both lithogenic and anthropogenic origin occur in soils distributed in various solid state forms, in the organic fraction and in the soil solution, and their fate in the environment depends upon several factors. The distribution patterns of element concentration are primarily influenced by the lithology of parent material, and secondly by the soil forming processes, that modify the basic geochemical composition and redistribute the content of metals within the soil profile. Alteration processes may lead to the release of potentially toxic elements, particularly heavy metals, into the environment. In this study, our objectives were to evaluate the background level and the concentration range of heavy metals in soils of a famous environmental scenery in northern Italy, to ascertain metal distribution and to identify possible contamination of some sites, and the related environmental hazard, with special reference to the pollution of the Dolomites area, which is a unique and delicate ecosystem. A regional soil survey was carried out in recent years in forested areas of the Trentino Region, in northern Italy. The territory investigated extends over approximately 400,000 ha (70% of the whole region), mostly in mountain areas. Approximately 460 soil samples from more than 120 representative soil profiles were collected and analysed for routine analyses and total Cu, Ni, Pb, Cd, Zn, Cr, Fe, Mn contents. Soils were grouped into 5 groups according to their pedogenetic characteristics. Most soil horizons (260) have acid reaction (mean pH = 6.1), though many are developed from calcareous rocks. Soil texture varies from sandy loam to silty loam. Cation exchange capacity varies consistently with the nature of parent material, as well as base saturation. The organic matter content is generally high at surface, and decreases with depth, the richest horizons (up to 11%) being those from calcareous materials. Background levels of trace elements in the soils investigated are consistent with currently recorded trace element concentration of soils from Western Europe. A geological matrix effect may be accounted for metal release by parent material weathering. Therefore, no contamination is recorded in the whole Trentino region. Nevertheless, metal accumulation in surface horizons at some sites may be ascribed to atmospheric input from local agricultural or industrial activities. Statistical analysis of geochemical data enhanced identification of pedogenetic processes responsible for the formation of soil groups with different weathering stages, and proved a useful tool to outline pedogenetic trends.

The use of multivariate statistical analysis of geochemical data for assessing the spatial distribution of soil contamination by potentially toxic elements in the Aljustrel mining area (Iberian Pyrite Belt, Portugal)

Aljustrel mine is located in SW Portugal, in the western sector of the Iberian Pyrite Belt. The Aljustrel village was developed around the exploitations of massive polymetallic sulphides that occur in the area (4 orebodies mined, 2 in exploration phase). The pyrite ore was extensively exploited from 1850 to 1993, when production was discontinued. A mining restart occurred in 2008, only during a few months. The objectives of the study were to assess the levels of soil contamination, to determine associations between the different chemical elements and their spatial distribution, as well as to identify possible sources of contamination that can explain the spatial patterns of soil pollution in the area. Principal component analysis combined with spatial interpretation successfully grouped the elements according to their sources and provided evidence about their geogenic or anthropogenic origin. From this study, it is possible to conclude that soils around Algares/Feitais tailing deposits, Estéreis and Águas Claras mine dams and S. João mine show severe contamination. The highest concentrations of As (up to 3,936 mg kg−1) and certain heavy metals (up to 321.7 mg kg−1 for Bi, 5,414 mg kg−1 for Cu, 20,000 mg kg−1 for Pb, 980.6 mg kg−1 for Sb, and 22 mg kg−1 Cd) were obtained near Algares area while the highest concentration of Cd (up to 61.6 mg kg−1) and Zn (up to 20,000 mg kg−1) were registered in samples collected in the S. João area. The highest pollution load index (>4.0) was recorded at the Algares area where the metal concentrations exceed typical soil background levels by as much as two orders of magnitude.

Statistical analysis of geochemical data: a tool for discriminating between kaolin deposits of hypogene and supergene origin, Patagonia, Argentina

AbstractThe numerous kaolin deposits located in Patagonia, Argentina, have been formed by hypogene or supergene processes. The primary origin has been established from O18 and D isotopic composition of the main minerals, kaolinite and/or dickite, and from the behaviour of certain elements during the alteration. The aim of this paper was to find if there is a tool, other than oxygen-deuterium data, to establish the origin of the Patagonian kaolin deposits. To handle the large number of variables per sample, a statistical multivariate study was used. The Principal Component method defines, on one hand the variables that better characterize each deposit and, on the other hand, the correlation between them. Fifty seven elements were considered and those that were not explained using these two components (which represent 75% of the total variance of the model) were discarded. As a result, the contents of Fe2O3, P2O5, LOI, Sr, Y, Zr, V, Pb, Hf, Rb, S and REE were used and the results show that the two components separate the deposits into two fields that are consistent with the process of formation. The first component indicates that Fe2O3, Y, Rb, U and HREE are more abundant in the supergene deposits, whereas, Sr, Pb, S and V are more abundant in the hypogene deposits. The second component shows that S, P2O5 and the LREE are enriched in the hydrothermal deposits, whereas Zr is more abundant in those formed under weathering conditions.

Multivariate statistical analysis of geochemical data of groundwater in Veeranam catchment area, Tamil Nadu

The study of hydrogeochemistry of the Mio-Pliocene sedimentary rock aquifer system in Veeranam catchment area produced a large geochemical dataset. Groundwater samples were collected at 52 sites over 963.86 km2 area and analyzed for major ions. The large number of data can lead to difficulties in the integration, interpretation and representation of the results. Two multivariate statistical methods, Hierarchical cluster analysis (HCA) and Factor analysis (FA), were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples, and to identify geochemical processes controlling groundwater geochemistry. Hydrochemical data for 52 groundwater samples were subjected to Q- and R- mode factor and cluster analysis. R-mode analysis reveals the inter-relations among the variables studied and the Q-mode analysis reveals the inter-relations among the samples studied. The R-mode factor analysis shows that Ca, Mg and Cl with HCO3 account for most of the electrical conductivity, total dissolved solids and total hardness of groundwater. The ‘single dominance’ nature of the majority of the factors in the R-mode analysis indicates non-mixing or partial mixing of different types of groundwater. Both Q-mode factor and Q-mode cluster analyses indicate an exchange between the river water and the groundwater in the vicinity. The rock water interaction like flood basin back swamp deposits of silty clayey formation is the major cause for the cluster II classification. Cluster classification map reveals that 58% of the study area comes under cluster II classification.

Environmental assessment of the arsenic-rich, Rodalquilar gold–(copper–lead–zinc) mining district, SE Spain: data from soils and vegetation

The Rodalquilar mineral deposits (SE Spain) were formed in Miocene time in relation to caldera volcanic episodes and dome emplacement phenomena. Two types of ore deposits are recognized: (1) the El Cinto epithermal, Au–As high sulphidation vein and breccia type; and (2) peripheral low sulphidation epithermal Pb–Zn–Cu–(Au) veins. The first metallurgical plants for gold extraction were set up in the 1920s and used amalgamation. Cyanide leaching began in the 1930s and the operations lasted until the mid 1960s. The latter left a huge pile of ~900,000–1,250,000 m3 of abandoned As-rich tailings adjacent to the town of Rodalquilar. A frustrated initiative to reactivate the El Cinto mines took place in the late 1980s and left a heap leaching pile of ~120,000 m3. Adverse mineralogical and structural conditions favoured metal and metalloid dispersion from the ore bodies into soils and sediments, whereas mining and metallurgical operations considerably aggravated contamination. We present geochemical data for soils, tailings and wild plant species. Compared to world and local baselines, both the tailings and soils of Rodalquilar are highly enriched in As (mean concentrations of 950 and 180 μg g−1, respectively). Regarding plants, only the concentrations of As, Bi and Sb in Asparagus horridus, Launaea arborescens, Salsola genistoides, and Stipa tenacissima are above the local baselines. Bioaccumulation factors in these species are generally lower in the tailings, which may be related to an exclusion strategy for metal tolerance. The statistical analysis of geochemical data from soils and plants allows recognition of two well-differentiated clusters of elements (As–Bi–Sb–Se–Sn–Te and Cd–Cu–Hg–Pb–Zn), which ultimately reflect the strong chemical influence of both El Cinto and peripheral deposits mineral assemblages.

Emplacement mechanisms of sill complexes: Information from the geochemical architecture of the Golden Valley Sill Complex, South Africa

The well-exposed Golden Valley Sill Complex, Karoo basin, South Africa, consists of four large sills (ca. 100 m thick; long axes: 13–24 km), one small sill (55–80 m thick; long axis: 4 km; forming an appendix to one of the large sills), and two large dykes (15–20 m thick; 25 and 70 km long), plus some minor intrusions. Field mapping shows physical connections between the small sill and one of the large sills, but no other connection between the large sills, or connections between the sills and the large dykes. Geochemical variation diagrams for 327 dolerite samples from the different sills and dykes in the Golden Valley Sill Complex showed similar compositional ranges for the dolerite bodies, but some differences in ratios between strongly incompatible elements. These differences were probed by Forward Stepwise-Discriminant Function Analysis using all major and trace elements as variables, and the height major sills and dykes as populations. The Forward Stepwise-Discriminant Function Analysis identified six different geochemical signatures among the sill-dyke populations. The six geochemical signatures were interpreted as representing six magma batches of distinct chemical characteristics that were involved in the formation of the Golden Valley Sill Complex. Four different magma batches gave rise to two of the large saucer-shaped sills and the two dykes. The exposed large dykes thus do not represent feeders to the main sills. A fifth magma batch gave rise to the two other large saucer-shaped sills as well as the smaller one. The statistical analysis also gave information about the connection between different dolerite exposures that could not be obtained by field, and made it possible to reconstruct the different sills through complex areas where several sills meet, and through widely spaced exposures. On the basis the field mapping and statistical analysis of geochemical data the geochemical architecture of the complex was established. Our study gives support to both the main models proposed earlier to explain the emplacement of sill complexes: (1) each sill in a sill complex is fed by different magma batches, and (2) the different sills in a sill complex feed one another, forming a group of nested sills generated by one single batch of magma. Furthermore, our study indicates that the interconnected sills in the Golden Valley Sill Complex are connected along their outermost edges through lateral overflow. The statistical analysis establishes a link between minor dolerite bodies and the main intrusion, thereby improving the geological mapping of the sill complex. The different units in the Golden Valley Sill Complex show strong geochemical affinity to lavas in the Lesotho Formation, the younger of the two formations in the Lesotho lava remnant. This suggests that these sills were emplaced at the same time as the Lesotho Formation, and that magmas with the same chemical characteristics as the Lesotho Formation were emplaced over a significantly larger area than covered by the remnants of the Lesotho Formation.

Multivariate statistical analysis of geochemical data as indicative of the hydrogeochemical evolution of groundwater in a sedimentary rock aquifer system

Summary The study of groundwater hydrogeochemistry of the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Quebec produced a large geochemical dataset. Groundwater samples were collected at 153 sites over a 1500 km2 study area and analyzed for major and minor ions. The large number of data can lead to difficulties in the integration, interpretation and representation of the results. Two multivariate statistical methods, hierarchical cluster analysis (HCA) and principal components analysis (PCA), were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples, and to identify geochemical processes controlling groundwater geochemistry. This subgroup consisted of 144 samples and 14 parameters (Ca2+, Mg2+, Na+, K+, HCO 3 - , Cl−, SO 4 2 - , Fe2+, Mn2+, Br−, Sr2+, F−, Ba2+, HS−). Seven geochemically distinct clusters, C1–C7, resulted from the HCA. Samples from clusters C3, C4, C6 and C7 are mostly located in preferential recharge areas. The majority of these samples have Ca–Mg–HCO3 recharge groundwater (C3, C6, C7) and Na–HCO3 evolved groundwater (C4). Samples from the other three clusters (C1, C2, C5) are characteristic of an aquifer system under confined conditions. The majority of these samples have Na–HCO3 evolved groundwater (C1, C5) and Na–Cl ancient groundwater that exhibits elevated concentrations in Br− (C2). In addition to recognizing the importance of hydrogeological conditions on groundwater geochemistry, the distribution of clusters also showed the importance of the geological formations on minor and trace elements, such as Fe2+, Mn2+, Sr2+, F− and Ba2+. The first five components of the PCA account for 78.3% of the total variance in the dataset. Component 1 is defined by highly positive loadings in Na+, Cl− and Br− and is related to groundwater mixing with Champlain Sea water and solute diffusion from the marine clay aquitard. The high positive loadings in Ca2+ and Mg2+ of component 2 suggest the importance of dissolution of carbonate rocks in this aquifer system. From their characteristic loadings, the first two components are defined as the “salinity” and “hardness” components, respectively. Components 3–5 are related to more local and geological effects. The integration of the HCA and the PCA, with conventional classification of groundwater types, as well as with the hydrogeological and geological contexts, allowed the division of the region into four main geochemical areas, providing an improved regional picture of the aquifer system dynamics and hydrogeochemical evolution of groundwater. The following factors were recognized as influencing the evolution of groundwater identified in every geochemical area: (1) geological characteristics including sedimentary rock type and till mineralogy; (2) hydrogeological characteristics represented by the level of confinement and the hydraulic gradient; and (3) the geological history including the latest glaciation and the Champlain Sea invasion. With its integrated approach, this hydrogeochemical study contributes to the characterization and understanding of complex groundwater flow systems, and provides an example of the long-term geochemical evolution of hydrogeological systems after a major perturbation, in this case seawater invasion.

Medical geochemistry research in Spišsko-Gemerské rudohorie Mts., Slovakia

This study presents an assessment of the potential impact of geological contamination of the environment on the health of the population in Spissko-Gemerské rudohorie Mts. (SGR Mts.). The concentration levels of potentially toxic elements (mainly As, Cd, Cu, Hg, Pb, Sb, and Zn) were determined in soils, groundwater, surface water, and stream sediments as well as in the food chain (locally grown vegetables). A medical study included some 30 health indicators for all 98 municipalities of the study area. The As and Sb contents in human fluids and tissues were analyzed in one municipality identified to be at the highest risk. Based on element content, environmental and health risks were calculated for respective municipalities. Out of 98 municipalities 14 were characterized with extremely high environmental risk and 10 were characterized with very high carcinogenic risk from arsenic (groundwater). Extensive statistical analysis of geochemical data (element contents in soils, groundwater, surface water, and stream sediments) and health indicators was performed. Significant correlations between element contents in the geological environment and health indicators, mainly cancer and cardiovascular diseases, were identified. Biological monitoring has confirmed the transfer of elements from the geological environment to human fluids and tissues as well as to the local food chain.

Log-ratios and geochemical discrimination of Scottish Dalradian limestones: a case study

Abstract Geochemical data are used widely to help correlate lithostratigraphical sequences, particularly where they are unfossiliferous and/or affected by metamorphism and deformation. In this study, geochemical data for variably impure metamorphosed limestones from the Neoproterozoic — Cambrian Dalradian Supergroup of Scotland have been used to aid lithostratigraphical discrimination and correlation in a region where apparently similar sequences crop out in widely separated regions affected by major deformation. The key problem in the statistical analysis of geochemical data is that the data are constrained to a constant sum and cannot, therefore, be analysed in raw form by conventional statistical techniques. To overcome this problem, the data have been transformed into log-ratios. Subsequent statistical analysis of the transformed data focuses on determining the simplest sub-composition that effectively discriminates between the limestones. The results show that the Fe 2 O 3 -MgO-CaO subcomposition is sufficient in this regard, with the additional benefit that it reflects the composition of the carbonate component in the limestones. Mann-Whitney tests of the log-ratio Fe 2 O 3 /MgO show that limestones from most different lithostratigraphical levels are statistically different from each other at high levels of significance. The results have clarified interpretations of the lithostratigraphy and subsequent interpretations of the tectonic history of the region.

Trace-metal pollution of soils in northern England

Data from a regional geochemical survey of topsoils (n=818) in rural and peri-urban areas over a single parent material (Coal Measures) are used to identify two types of trace-metal pollution – severe local contamination at 20 sites and widespread, diffuse pollution in more densely populated areas. Median concentrations of several trace metals in topsoils were significantly higher in areas of high, compared to low, population density (percentage increases in parenthesis): As (31), Cu (39), Fe (7), Mo (26–36), Ni (29), Pb (20), Sn (40), and Zn (11). Four potential pathways of diffuse trace-metal pollution are postulated: coal-ash dispersal, atmospheric aerosols derived from coal combustion, the historical spreading of sewage waste, and those related to road vehicles. The statistical analysis of geochemical data classified by local, human population density can be an effective means of identifying the magnitude and extent of diffuse pollution, and could help to establish natural background levels.