Regional Geochemical Patterns Research Articles

Application of self-organizing map (SOM) and K-means clustering algorithms for portraying geochemical anomaly patterns in Moalleman district, NE Iran

In this paper, in order to reveal the regional geochemical patterns of regularly sampled stream sediment data, we have employed the K-means and self-organizing map (SOM) as clustering methods in the Moalleman district, northeast Iran. Initially, a set of analyzed elements of geochemical data was subjected to isometric log-ratio (ilr) transformation to address the closure problem related to geochemical data, then, ordinary principal component analysis (PCA) was utilized for recognizing the internal relations between selected elements (As, Au, Cu, Pb, Sb and Zn). Subsequently, the K-means and SOM as unsupervised clustering methods were applied based on PC1 (Cu-Pb-Zn aggregation) and PC2 (Au-As-Sb aggregation) to distinguish different populations of multi-element geochemical indicators. In this regard, Silhouette Width (SW) was implemented for computing the optimal cluster number in K-means clustering method. In the next step, due to the presence of numerous copper mineral deposits/occurrences in the study area, we opted to implement the supervised SOM on ilr-transformed values of Cu-Pb-Zn elements for delineating high anomalous zones. For this purpose, a confusion matrix based on training and out-of-bag (OOB) data was developed for the supervised SOM model and the results indicated the accuracy of 96.27% and 94.26%, respectively. Moreover, success-rate curves were used for assessing the overall performance of K-means and SOM (unsupervised and supervised) models. Experimental outcomes represented the superiority of SOM models (especially the supervised SOM) over K-means in delineating the geochemical anomaly targets which can be used as an effective and powerful tool for discovering the complex patterns among variables in exploratory geochemical data.

Multi-element geochemical data mining: Implications for block boundaries and deposit distributions in South China

To explore the macroscopic characteristics of geological block boundaries, Mesozoic magmatic rocks, and various ore deposit distributions in South China, we processed large-scale stream sediment geochemical data covering the entire South China region. Two unsupervised data mining algorithms, cluster analysis and correlation analysis, were used to identify the geochemical distribution patterns. The combined geochemical patterns of nine major elements show two gradient lines that divide the South China region into three blocks. These gradient lines reveal the eastern and western boundaries of the intracontinental orogenic belts between the Yangtze and Cathaysia blocks. We suggest that the Nanning-Pingxiang segment is located on the southern end of the Yangtze-Cathaysia block boundary, and the Jishou-Kaili segment is a part of the boundary between the Yangtze block and the eastern orogenic belt. In the Mesozoic, the distribution of magmatic rocks migrated from the inland to the southeast coast. Meanwhile, the mantle-derived compatible elements were gradually depleted, whereas the crust-derived incompatible elements were gradually enriched. The migration and enrichment of geochemical elements provide material sources for the formation of various ore deposits. High-temperature metallogenic deposits related to Mesozoic magmatic activity are widely developed in eastern South China, while low-temperature metallogenic, sedimentary, and metamorphic deposits are more developed in inland South China. The regional geochemical patterns indicate the distributions of deposits to varying degrees.

Multivariate Analysis of Stream Sediment Geochemical Data for Gold Exploration in Delijan, Iran

Muteh is a major Iranian gold mine that is located in Delijan, within in the Sanandaj-Sirjan metallogenic zone. Previously, the gold mineralization in Muteh was interpreted to be formed in Precambrian. However, recent geological models propose younger age, i.e., late Eocene, for the gold mineralization, therefore, the geologic units of age and older than late Eocene can be considered as targets for gold exploration. The objective of this paper is to investigate Muteh type gold mineralization potential in the area with respect to the new geological models. Statistical analysis of 644 stream sediment samples was used to determine anomalous concentrations Au and elemental correlations of Au geochemical pathfinders. Using the factor analysis, an eight-factor model was established for the stream sediment geochemical data explaining 74.76% of the total variance. Factor 4 was found to be representative of Al-Ca-Na-K-P, which defines the lithological variation in the data. Factor 6 represents the variation of Pb-Ag-Sb-Ba-Tl, which is related to the hydrothermal mineralization. The factor 7 includes Au-S-Mo and is representative of regional geochemical patterns of gold. The correlation of Au with S and Mo signifies the inclusions of Au in pyrite minerals of Muteh mine. Factor 7 specifies three exploration targets. Moreover, the fourth prospective area representing significant geochemical signatures for gold mineralization was not detected in factor analysis. The results show that data reduction techniques, such as factor analysis, have great potential for geochemical applications; however, it is essential to perform careful analysis in addition to using these techniques. Factor analysis highlights the regional geochemical patterns and may slight the smaller subsystems.

Regional geochemical patterns in SE Scotland: source lithology, inheritance and glacial overprinting

Synopsis The Lower Palaeozoic Southern Uplands terrane shows a NE–SW, strike-parallel coincidence of regional geochemical lineaments defined from stream-sediment data, and major lithostratigraphically significant structures. West of Moffat, Silurian strata of the Gala and Hawick groups are divided by geochemical lineaments coincident with the Laurieston Fault. East of Moffat, the geochemical lineaments diverge towards the NE with Gala-type regional geochemistry (high Sr and Ba, low CaO) to the north of the Moffat Valley Fault, whilst Hawick-type regional geochemistry (low Sr and Ba, high CaO) continues to the south of the Laurieston Fault; between the faults lies the recently-defined Ettrick Group. Beyond the eastern end of the Lower Palaeozoic outcrop, its geochemical lineaments extend across the unconformable cover of Devonian ‘Old Red Sandstone’. This is partly a drainage catchment effect, but two other possible explanations are considered: a localized derivation of the Devonian clastic rocks, such that they inherit the compositional patterns of the underlying Lower Palaeozoic sandstones; or a geochemical signature imposed by glacial deposits derived from the Lower Palaeozoic outcrop. The local effect of glacial dispersion is confirmed by the distribution of TiO 2 ; anomalously high levels associated with Carboniferous mafic lavas have been transported NE across the low-TiO 2 Carboniferous sedimentary rocks of the Tweed Basin.

Major advances in exploration geochemistry, 1998–2007

ABSTRACT Exploration geochemistry continues to be a major contributor to mineral exploration programs at all scales. The decade has spanned a complete economic cycle in the minerals industry, during which there has been a change in exploration emphasis from the dominance of Au and diamonds to the addition of a range of base metals and the recent revival of U exploration. Increasing metal prices and the development of new geochemical techniques have allowed exploration activity to expand in previously under-explored terrains, including those characterized by thick, transported regolith cover. There has been significant evolution and refinement of geochemical dispersion models for transported regolith, especially processes associated with oxidizing sulphide mineralization. These models have prompted further developments in sampling methods, analytical techniques and approaches to anomaly detection. The routine measurement of fundamental soil and groundwater properties, such as pH, has been demonstrated to assist in detecting geochemical and mineralogical effects of buried mineralization on regolith. New lithogeochemical techniques have been developed for terrane selection and early discrimination of potentially mineralized units. The production of regolith-landform maps, including 3-D models, and more detailed techniques for differentiating the origin and nature of regolith materials now underpins the design of exploration programs in a number of terrains. Various exploration techniques, such as biogeochemistry and soil gas, have been revitalized by the application of new technologies. Ongoing improvements to detection limits and the reliability of multi-element analytical methods have permitted development and routine application of a wide range of selective geochemical extraction methods aimed at detecting subtle geochemical haloes developed in transported regolith cover. The results from a plethora of selective extraction orientation studies have, however, indicated the reliability of such methods to be uncertain, especially in arid terrains. Low-density geochemical mapping of large regions has been undertaken by government agencies for a variety of resource, environmental and health purposes, including attracting mineral exploration. Such maps typically demonstrate the dominance of underlying geology on regional geochemical patterns in regolith, even in industrialized regions. Philosophical and numerical approaches to defining and detecting geochemical anomalies continue to be reviewed. There is a need to supplement classical parametric outlier-detection approaches, routinely applied to geochemical data, with statistical techniques that reflect a recognition that subtle, multivariate patterns related to the geochemical effects of mineralization may emanate from within the main data cloud and not outlying or distinct clusters.

A regional soil and sediment geochemical study in northern California

Abstract Regional-scale variations in soil geochemistry were investigated in a 20,000-km 2 study area in northern California that includes the western slope of the Sierra Nevada, the southern Sacramento Valley and the northern Coast Ranges. Over 1300 archival soil samples collected from the late 1970s to 1980 in El Dorado, Placer, Sutter, Sacramento, Yolo and Solano counties were analyzed for 42 elements by inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry following a near-total dissolution. These data were supplemented by analysis of more than 500 stream-sediment samples from higher elevations in the Sierra Nevada from the same study site. The relatively high-density data (1 sample per 15 km 2 for much of the study area) allows the delineation of regional geochemical patterns and the identification of processes that produced these patterns. The geochemical results segregate broadly into distinct element groupings whose distribution reflects the interplay of geologic, hydrologic, geomorphic and anthropogenic factors. One such group includes elements associated with mafic and ultramafic rocks including Cr, Ni, V, Co, Cu and Mg. Using Cr as an example, elevated concentrations occur in soils overlying ultramafic rocks in the foothills of the Sierra Nevada (median Cr = 160 mg/kg) as well as in the northern Coast Ranges. Low concentrations of these elements occur in soils located further upslope in the Sierra Nevada overlying Tertiary volcanic, metasedimentary and plutonic rocks (granodiorite and diorite). Eastern Sacramento Valley soil samples, defined as those located east of the Sacramento River, are lower in Cr (median Cr = 84 mg/kg), and are systematically lower in this suite compared to soils from the west side of the Sacramento Valley (median Cr = 130 mg/kg). A second group of elements showing a coherent pattern, including Ca, K, Sr and REE, is derived from relatively silicic rocks types. This group occurs at elevated concentrations in soils overlying volcanic and plutonic rocks at higher elevations in the Sierras (e.g. median La = 28 mg/kg) and the east side of the Sacramento Valley (median 20 mg/kg) compared to soils overlying ultramafic rocks in the Sierra Nevada foothills (median 15 mg/kg) and the western Sacramento Valley (median 14 mg/kg). The segregation of soil geochemistry into distinctive groupings across the Sacramento River arises from the former presence of a natural levee (now replaced by an artificial one) along the banks of the river. This levee has been a barrier to sediment transport. Sediment transport to the Valley by glacial outwash from higher elevations in the Sierra Nevada and, more recently, debris from placer Au mining has dominated sediment transport to the eastern Valley. High content of mafic elements (and low content of silicic elements) in surface soil in the west side of the valley is due to a combination of lack of silicic source rocks, transport of ultramafic rock material from the Coast Ranges, and input of sediment from the late Mesozoic Great Valley Group, which is itself enriched in mafic elements. A third group of elements (Zn, Cd, As and Cu) reflect the impact of mining activity. Soil with elevated content of these elements occurs along the Sacramento River in both levee and adjacent flood basin settings. It is interpreted that transport of sediment down the Sacramento River from massive sulfide mines in the Klamath Mountains to the north has caused this pattern. The Pb, and to some extent Zn, distribution patterns are strongly impacted by anthropogenic inputs. Elevated Pb content is localized in major cites and along major highways due to inputs from leaded gasoline. Zinc has a similar distribution pattern but the source is tire wear.

Semi-hierarchical correspondence cluster analysis and regional geochemical pattern recognition

Semi-hierarchical correspondence cluster analysis (SHCCA), firstly developed in this paper, extracts the main advantages of correspondence analysis, hierarchical and non-hierarchical cluster analysis, and unifies the R- and Q-mode cluster analysis of large data set. A systemic program to recognize the regional geochemical patterns is built up based on this method. With this program, the complex tasks for data interpretation can be achieved by simple processes, and important geochemical information can be displayed by a single diagram, i.e. the multivariate regional geochemical image. As one of the applied examples of this program, the regional geochemical pattern recognition for a shallow covered area around Tahe in Heilongjiang Province is introduced. The results show that many hidden geochemical patterns related to the lithologies, structures, ore-forming conditions and prospecting targets etc are revealed by the geochemical image, and that the main geochemical patterns are related with certain geological and gravitational patterns. By finding contrasts between geochemical patterns and geological or gravitational patterns, the SHCCA results assist the geological mapping in this area. Geochemical data obtained in Chinese regional geochemical exploration provides useful information regarding geology and minerals, and the method described in this paper provides a new way to examine this type of resource.

Fluorosis and arsenicosis: An update

Geoscience Australia and the Cooperative Research Centre for Landscape Environments and Mineral Exploration, in collaboration with State agencies are conducting a series of pilot baseline geochemical surveys (BGS). These aim to characterize regional geochemical patterns and contribute to what is presently a limited research direction in Australia. BGS can help (1) establish baselines to monitor future change; (2) target mineral exploration; (3) develop informed environmental policies; and (4) geomedical studies on plant and animal well being. In 2004-05, sampling at an average density of 1 sample per 1100 km 2, was conducted in the Gawler Craton, South Australia. In contrast to our pilot region in the Riverina of New South Wales and Victoria, the Gawler lacks well-developed drainage systems and is dominated by aeolian dunes in its western sector. One of the key aims of the Gawler study is to determine whether elemental excesses or deficiencies exist in the regolith and the implications of these for plant, animal and human health. Top (0--10 cm depth) and bottom (~55--90 cm depth) sediment samples were collected in the lower parts of 42 catchments. The compositions of the 2.95) in 4 samples (3 sites) are dominated by iron oxides, spinels, ruffle, zircon and barite. Some Cr and V may be related to heavy minerals such as spinels, limiting their bioavailability. Cu, Se and Zn are potentially deficient in parts of the region, but once identified in agricultural areas can easily be remedied through the application of suitable fertilizers. Dispersion of elements in the region appears to be on a catchment scale. The pilot studies conducted highlight the need for more comprehensive national guidelines for the assessment of elements in the natural environment. Results from this study are based on total elemental concentrations, and do not account for bioavailability, which plays an important role in assessment of geochemical hazards. However, assessment of baseline regional-scale geochemical trends greatly assists in focusing research.

Pleistocene recharge to midcontinent basins: effects on salinity structure and microbial gas generation

The hydrogeochemistry of saline-meteoric water interface zones in sedimentary basins is important in constraining the fluid migration history, chemical evolution of basinal brines, and physical stability of saline formation waters during episodes of freshwater recharge. This is especially germane for interior cratonic basins, such as the Michigan and Illinois basins. Although there are large differences in formation water salinity and hydrostratigraphy in these basins, both are relatively quiescent tectonically and have experienced repeated cycles of glaciation during the Pleistocene. Exploration for unconventional microbial gas deposits, which began in the upper Devonian-age Antrim Shale at the northern margin of the Michigan Basin, has recently extended into the age-equivalent New Albany Shale of the neighboring Illinois Basin, providing access to heretofore unavailable fluid samples. These reveal an extensive regional recharge system that has profoundly changed the salinity structure and induced significant biogeochemical modification of formation water elemental and isotope geochemistry.New-formation water and gas samples were obtained from Devonian-Mississippian strata in the Illinois Basin. These included exploration wells in the New Albany Shale, an organic-rich black shale of upper Devonian age, and formation waters from over- and underlying regional aquifer systems (Siluro-Devonian and Mississippian age). The hydrostratigraphic relations of major aquifers and aquitards along the eastern margin of the Illinois Basin critically influenced fluid migration into the New Albany Shale. The New Albany Shale formation water chemistry indicates significant invasion of meteoric water, with δD values as low as −46.05‰, into the shale. The carbon stable isotope system (δ13C values as high as 29.4‰), coupled with δ18O, δD, and alkalinity of formation waters (alkalinity ≤24.08 meq/kg), identifies the presence of microbial gas associated with meteoric recharge. Regional geochemical patterns identify the underlying Siluro-Devonian carbonate aquifer system as the major conduit for freshwater recharge into the fractured New Albany Shale reservoirs. Recharge from overlying Mississippian carbonates is only significant in the southernmost portion of the basin margin where carbonates directly overlie the New Albany Shale.Recharge of dilute waters (Cl− <1000 mM) into the Siluro-Devonian section has suppressed formation water salinity to depths as great as 1 km across the entire eastern Illinois Basin margin. Taken together with salinity and stable isotope patterns in age-equivalent Michigan Basin formation waters, they suggest a regional impact of recharge of δ18O- and δD-depleted fluids related to Pleistocene glaciation. Devonian black shales at both basin margins have been affected by recharge and produced significant volumes of microbial methane. This recharge is also manifested in different salinity gradients in the two basins because of their large differences in original formation water salinity. Given the relatively quiet tectonic history and subdued current topography in the midcontinent region, it is likely that repeated cycles of glacial meltwater invasion across this region have induced a strong disequilibrium pattern in fluid salinity and produced a unique class of unconventional shale-hosted gas deposits.

Palimpsest glacial dispersal trains and their significance for drift prospecting

Palimpsest glacial dispersal trains are residual trains that are produced when the lithic components of an earlier dispersal trains are incompletely re-entrained by a subsequent glacial movement in a different direction. As evidence for multiple ice-flow directions comes from an ever growing number of regions, palimpsest trains increasingly appear as common features of glacial dispersal patterns produced by Quaternary ice sheets. The major characteristics of these trains, particularly their trend and outline, are predominantly related to an earlier glacial movement and only partly related to the glacial movement that produced the till sheet in which they occur. Palimpsest dispersal trains owe their characteristics to the cumulative effects of two or more succeeding ice-flow directions. The concept of palimpsest trains leads not only to a better understanding of the factors which control glacial dispersal patterns but also to an integrated classification of dispersal trains. The three main types of glacial dispersal trains, ribbon-shaped, fan-shaped and amoeboid, owe their characteristics features to the direction, number and succession of former glacial movements. The concept of palimpsest dispersal trains is supported by an empirical glacial dispersal model which do not only constrains the conditions under which these trains may form but also identifies two key factors of subglacial clastic dispersal: (1) the travel distance of basal ice and (2) the rate of subglacial erosion. The basis of this glacial dispersal model is a corollary of Krumbein's exponential decay model: the concentration of lithic components of a rock type in subglacial debris increases exponentially until its distal contact is crossed, at which point the exponential increase of lithic components of rock types lying downglacier will cause the concentration of lithic components of the first rock type to decrease exponentially. The proposed model carries out multiple iterations during each of which compositional data obtained by the previous iteration are redistributed through an exponential decay function. Our work shows that not only is it important to know the sequences of ice-flow directions which occurred in glaciated terrains but also to recognize that older glacial movements are most likely to have residual effects on dispersal patterns produced during subsequent glacial movements. The recognition of palimpsest dispersal trains provides a new tool to better assess these residual effects on regional geochemical patterns in glacial sediments and thus to adjust drift prospecting strategies.

Sr, C and O isotope systematics in the Pucará basin, central Peru

A combined Sr, O and C isotope study has been carried out in the Pucara basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucara basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucara basin. The San Ramon granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucara Group is accompanied by only a slight decrease in δ 18O values and essentially no change in δ 13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ 13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ 18O values in carbonate rocks from the lower part of the Pucara Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucara sequence and cooler, dilute brines possibly representing local fluids within the Pucara sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucara Group.

Geochemistry of aquatic and terrestrial sediments, precambrian shield of Southeastern Ontario

Lake water and sediment samples from approximately 2200 lakes and glacial sediment (sub-solum) samples from about 1800 sites were collected throughout a 38000 km2 rectangular area extending from Georgian Bay east to the Ottawa and St. Lawrence Rivers, Ontario, Canada. Lake water alkalinity and pH patterns are similar to the distribution of carbonate components in glacial drift. Carbonate-rich drift derived from the Paleozoic limestone terrain on the northeast flank of the Precambrian Frontenac Arch has been dispersed in a south-westward direction across a variety of non-calcareous metasedimentary and igneous rocks of the Canadian Shield, providing a buffering capacity to lakes situated in granitic terrain. The distribution patterns of mobile trace and minor elements are influenced by geochemical processes associated with subaerial weathering, ground and surface water transport, and the geochemical environment within the lakes themselves. Although composition of the drift is generally reflected by lake geochemistry, these post depositional processes can cause significant variations between patterns derived from the two sample types. Anions and cations such as SO 4 = , Cl−, Na+, and F− exhibit concentration patterns thought to reflect both anthropogenic inputs and natural variations due to differences in the geology. All regional geochemical patterns may show evidence of local enhancement caused by high concentrations of chemically distinctive minerals in drift or nearby bedrock.

Dredged basalts from the mid-oceanic ridge north of Iceland

An examination of the concentrations of the incompatible elements in tholeiites from the Kolbeinsey Ridge indicates that plume magmas have probably not overflowed into that area from Iceland or Jan Mayen Island. The results also show that the chemistry of plume magmas may be intimately related to regional geochemical patterns, and it is possible that Iceland represents a ‘weak spot’ or decompression focus, which causes complex flow patterns within and from the asthenosphere.