Regional Stream Sediment Survey Research Articles

Statistical Studies of Soil Geochemical Data from Gbongan-Odeyinka Area, Southwestern Nigeria

Statistical Studies of Soil Geochemical Data from Gbongan-Odeyinka Area, Southwestern Nigeria In order to follow up some geochemical anomalies discovered in a regional stream sediment survey in parts of southwestern Nigeria, a medium density geochemical soil survey was carried out in Gbongan-Odeyinka area of Osun State, southwestern Nigeria. As a result, soil geochemical data on 13 elements (Ag, As, Au, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sn, and Zn) obtained from 186 samples from the study area were subjected to statistical analysis involving frequency distribution, Pearson correlation and R-mode varimax rotated factor analysis techniques. The frequency distribution plots revealed that all the elements investigated except Ag and Fe, which display approximately normal distribution, are positively skewed with Au and Zn being the strongest. The Pearson correlation study and factor analysis enabled separation of the 13 elements into the following three factors or metal associations: 1) Pb-Co-Ni-Fe-Cr- As-Mn-Cu; 2) Sn-Ag-Au-Cd; and 3) Zn-Cd, which are interpreted to reflect the influence of both environmental and mineralization factors on the soil of the study area. Of the three associations only the Sn-Ag-Au-Cd is considered related to metallic mineralization in the study area based on the existing knowledge of Au and Sn mineralization in the adjacent Ife-Ilesa area. The gold is believed to be hosted by schists and the tin by pegmatites. The other metal associations are interpreted in terms of environmental factors. On the basis of the presence of Fe and Mn in the Pb-Co-Ni-Fe-Cr-As- Mn-Cu association, it is inferred that the scavenging action of iron and manganese oxides was at least partly responsible for forming such trace elements association in the soil. In the case of the Zn-Cd association, both metals are considered to be anthropogenic, being possibly introduced to the soil through the application of pesticides on cocoa and kolanut plantations of the study area, although the possibility of Zn being derived from other sources cannot be ruled out.

Geochemical exploration for gold in the Nile Valley Block (A) area, Wadi Allaqi, South Egypt

The present work aims to estimate Au-anomalous distribution patterns, the optimum grain size fraction and pathfinder elements for gold placer. The obtained data of analyzed elements (Au, Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Ga, Hg, Li, Mn. Mo, Ni, Pb, Rb, Sb, Se, Sr, Te, Ti, V, Y and Zn) in both the fine (−0.25 mm) and coarse (−1 mm+0.25 mm) grain size fractions of 32 dry stream sediment samples, which have been derived from metamorphosed Island Arc volcanic rocks of Late Proterozoic age, revealed that most of the analyzed elements in both grain size fractions are asymmetrically distributed and did not pass the tests of normality. The coarse grain size fraction appears to be the better size fraction for Au detection and can be considered as the optimum grain size fraction for future application in regional stream sediment surveys. The presence of Au anomalies in the upper part of the stream and beside the mine is either sourced by the main Au-mineralization or new potential extension of the mineralization in the study area. Silver, Cd, Se, Zn and Te can be considered as useful pathfinder elements for Au in the coarse grain size fraction, and they may be used for future geochemical exploration for Au in the area. Recommendation of the authors is pointing to perform lithogeochemical survey in the eastern and western parts of the mine.

Overbank sediments as appropriate geochemical sample media in regional stream sediment surveys for gold exploration in the savannah regions of northern Ghana

Conventional stream sediment sampling in which sediments are taken from the active channels during reconnaissance regional geochemical surveys in gold exploration has over the years failed to delineate prospective target zones in northern Ghana, where the relict is flat. Whereas the technique has been successful in the south western Ghana, which is characterised by moderate to high relief, generally the savannah north is associated with low relief, commonly with flat topographies and generally decoupled stream channels. Geochemical comparison of active stream and overbank sediments in this study demonstrate that active stream channels may contain contaminated materials of recent origin, but overbank sediments, except for the uppermost horizons, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, composite samples taken from the overbank sediment layers were relatively less erratic and are considered to be an appropriate geochemical medium in delineating potential regional gold targets for follow up. The results show that overbank sediment sampling can be used as a cost-effective method to define broad anomalous zones; and the technique must be considered useful during reconnaissance geochemical surveys in the savannah regions.

Overbank sediment sampling in Greece: a contribution to the evaluation of methods for the ‘Global Geochemical Baselines’ mapping project

The different steps in the development of the floodplain or overbank sediment sampling procedure for continental-scale geochemical mapping are described with examples from the Greek pilot project of the Western European Geological Surveys’ Working Group on ‘Regional Geochemical Mapping’. The Greek overbank and stream sediment results were compared statistically with those of the national regional stream sediment survey. The purpose was (1) to compare the geochemistry of overbank sediment, collected from a single site, with that of several tens or hundreds of regional stream sediment samples taken from the upstream drainage basin, and (2) to develop a cost-effective procedure to assess the geochemistry of a drainage basin. ‘ Composite’ samples taken over the whole thickness of the overbank sediment sequence showed that element contents were close to the median, or within the interquartile range, of the same elements in the regional stream sediment samples. Overbank sediment samples collected from pre-defined intervals were not considered satisfactory, for they were taken across overbank sediment layers. Therefore, the method was subsequently refined by collecting samples from different overbank sediment layers, with the recommendation to collect samples at 25-cm intervals when thick layers of over 1 m were encountered. In selected river basins, surficial overbank and active stream sediments reflect contamination, whereas overbank sediment layers at depth show natural element contents, or pristine conditions. Such data are required for a realistic assessment of the degree of contamination of individual drainage basins. After considering results from Greece and other countries, it is recommended to sample both the surface overbank sediment (0–10 cm), and the deepest possible layer in order to measure pristine or pre-industrial conditions. The Greek results demonstrate that overbank sediments are useful for mapping natural geochemical conditions and baseline concentrations, as well as assessing anthropogenic contamination. The cost-effectiveness of overbank sediment sampling for the ‘Global Geochemical Baselines’ project of the International Union of Geological Sciences (IUGS) and International Association of GeoChemistry (IAGC) is verified.

Models of Spatial Structures of Regional Multi‐element Geochemical Anomalies over Copper‐Polymetallic Orefields

Abstract Regional stream sediment surveys at a 1:200,000 scale reveal positive and negative regional multi‐element geochemical anomalies over medium to large copper‐polymetallic orefields of different genetic types in China. Regional geochemical anomalies of orefield refer to those geochemical anomalies that are related to metallogenesis of an orefield in a certain area. The anomaly area is typically 10 to 100 km2. The regional multi‐element anomalies related to mineralization can be divided into three groups, that is, the ore‐element anomaly association, indicator element anomaly association, and metallogenic environmental element anomaly association. Their common spatial distributions over ore deposits or orefields possess unique structures.The model of spatial structure of regional multi‐element geochemical anomalies (RAGSS) of an orefield delineates structural feature possessed by orderly spatial distributions of different groups of multi‐element anomaly associations related to orefield metallogenesis. It is used to outline the common metallogenetic anomaly visage that is composed of the orderly spatial distribution of different groups of multi‐element anomaly associations. The orderly spatial distribution of multi‐element anomalies over an orefield reflects element distributions as they are changed from a dispersed “out‐of‐order” state into a concentrated “orderly” state during the mineralization of an orefield.Three different patterns of the spatial anomaly structure related to mineralization in an orefield can be concluded: (1) nested pattern; (2) eccentric pattern and; (3) peripheral pattern. There are marked differences between multi‐element anomaly patterns related and not related to mineralization. RAGSS models of orefields can be used to better understand and evaluate regional multi‐element anomalies and identify ore types.

Geological Structure and Ore Mineralogy of the Juliette Gold-Silver Deposit, Northeast Russia

Julietta is a rich epithermal gold-silver deposit of the low-sulfidation, adularia-sericite type, located in the Cretaceous Okhotsk-Chukchi volcanic-plutonic belt 250 km northeast of Magadan. The deposit was discovered in 1989 by a regional soil geochemical survey in an area previously considered barren on the basis of a regional stream-sediment survey. The deposit has not been completely explored, but presently is in the feasibility stage; proven reserves are 26 metric tons of Au (grades averaging 23 g/t) and the Au/Ag ratio is about 1:10. The deposit occurs on the periphery of a large volcanic-tectonic depression. Host andesite, andesite-basalt lava, corresponding subvolcanic bodies, and tuff are cut by Early Cretaceous quartz diorite stocks. Six vein zones occur in tensional and compression fissures. Ore shoots and smaller bonanzas comprising most of the gold reserves are located in flexures of the ore-host fissures. Ore mineralization was preceded by intense voluminous propylitization and linear seric...

Regional geochemical secondary negative anomalies and their significance

Research involving secondary negative anomalies and their application in regional stream sediment surveys has been scarce. Conventionally, negative anomalous threshold values have been calculated in the same way as positive anomalous threshold values. But the conventional methods all have drawbacks which hinder their application. In this paper, we have chosen to delineate negative (and/or positive) anomalies using contrast values in order to overcome the drawbacks. Regional stream sediment surveys at a scale of 1 : 200 000 have been carried out in western Jungger, Shanxi, Kunlun, northwestern Jiangxi, and other areas in China. The geochemical data were processed using System RESMA. On the basis of the distribution of negative and positive single-element anomalies, three possible arrangements may occur: (1) negative anomalies accompanied by positive anomalies; (2) only positive anomalies occurring with no negative anomaly nearby; (3) only negative anomalies, with no positive anomaly nearby. These situations reflect different geological settings and different mineral forming processes. Basically, two different distribution patterns of regional negative anomalies in relation to the backgrounds — low background (LB) and high background (HB) —may be observed in different geological environments: (1) regional negative anomalies are distributed only around the positive anomalies in the LB area; (2) regional negative anomalies can exist on the periphery of positive anomalies in both LB and HB areas. Two kinds of patterns for regional multi-element negative and positive anomalies reflecting different geological processes have been noted: (1) coincident positive anomalies for one group of elements and negative anomalies for another group of associated elements can be used to uniquely define ore deposition; (2) regional positive multi-element anomalies of some elements (including ore and associated elements) occurring over a deposit are accompanied by negative anomalies on the periphery of the deposits. Two regional models of negative and positive anomalies are established for Au and Cu deposits. Integration of multi-element positive and negative composited geochemical anomalies are much more useful than positive anomalies or positive composited anomalies to delineate regional structures.

Data analysis and geochemical mapping for the regional stream sediment survey of Austria

Abstract To stimulate mineral exploration, a regional stream sediment survey covering more than 40,000 km2 of Austria was initiated in 1978. By the end of 1986 about 30,000 samples (−180 μm) had been analyzed for 36 elements. During the geochemical data interpretation a variety of unsolved problems were discovered in geochemical maps and results. Some of more important factors are considered to be: 1. - the very complex geology of the survey area, 2. - the presence of many old mine workings, 3. - the resulting heterogeneity of the data, and 4. - the inherent uncertainties and pitfalls of stream sediment geochemistry in general.

Exploratory data analysis: recent advances for the interpretation of geochemical data

Single-element distributions of geochemical exploration data have been portrayed by and interpreted according to the parameters of the (log) normal distribution. Its estimators, arithmetic mean and standard deviation are sometimes used to define outliers. Practical treatment of empirical geochemical data (with examples presented from regional stream sediment data) suggests, however, that the statistical assumptions often fail to describe the real data behaviour. Few measurements or small populations, deviating extremely from the main body of data can influence the estimators significantly and at least partly mask the inherent information. To overcome these problems resulting from conventional classical statistics, Exploratory Data Analysis (EDA) with its graphical and numerical techniques, independent of distributional assumptions, was introduced and successfully applied to geochemical data derived from a regional stream sediment survey in Austria. Resistant selected order statistics (5-number summary) are explained and discussed for their usefulness in outlier detection as well as for the comparison of different data sets. Great emphasis is given to graphical data summaries using one-dimensional scatter plots, quantile plots, boxplots and density traces. EDA is also useful in geochemical mapping. This affects especially class selection, presentation of outliers and regional distributions of single elements as well as variables derived from detailed multivariate statistical analysis. Major advantages of EDA are the straightforward application of its techniques and the easily interpretable results. In addition, the geochemical contrast on maps can be enhanced considerably, resulting in a more objective outlier definition as well as a better resolution of the regional background.

Statistical evaluation of the significance of categorical field parameters in the interpretation of regional geochemical sediment data

ABSTRACT Matysek, P.F., Fletcher, W.K. and Sinclair, A.J., 1983. Statistical evaluation of the significance of categorical field parameters in the interpretation of regional geochemical sediment data. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 383–401. In an attempt to study the value and utilization of categorical data collected during regional stream-sediment surveys, we have taken data from the Canadian Uranium Reconnaissance Program in S.E. British Columbia. After initial classification of the data into six subsets on the basis of catchment geology, probability plots were constructed for each of 11 elements (Zn, Cu, Pb, Ni, Co, Fe, Mn, Mo, W, Hg and U) and used to select thresholds to reject anomalous samples. The remaining background populations then were successively subdivided into groups according to their classification with respect to four sediment characteristics (abundance of fines, sand, organic matter and sediment colour) and six environmental parameters (physiography, water flow rate, stream class, drainage pattern, bank type and contamination). After calculation of log means for each group and a pooled common standard error, differences between group means were tested for significance using Duncan's Multiple Range Test. The relative degree of confidence in the significance of difference between categorical means for any single field parameter were determined using a ratio method. Results of Duncan's Multiple Range Test, show that, many field observations can be related systematically to metal content of drainage sediments. Some elements are more susceptible than others to environmental factors and some factors influence few or many elements. For example, in sediments derived from granites there are significant relationships between bank type and concentration of 8 elements (Zn, Cu, Ni, Pb, Co, Fe, Mn and Hg). In contrast, the texture of these sediments, using estimates of fines content as an index, did not significantly affect the concentration of any of the elements studied. In general, results indicate that groups of environmental factors acting collectively are more important than any single factor in determining background metal content of drainage sediments.

Anomalous geochemical patterns from northern Sumatra: their assessment in terms of mineral exploration and regional geology

A regional stream sediment survey has been conducted in Sumatra north of 4°N. The survey area is approximately 38,000 km 2 of mountainous terrain mostly covered by dense tropical rainforest. The overall sample density achieved by the survey was one per ten km 2. However, the difficult terrain and thick vegetation blanket have meant that a uniform sample density was not obtained. The uneven sample density has presented problems with interpretation of the geochemical patterns and identification of anomalous areas for mineral exploration purposes. Two methods have been designed for identifying mineral exploration targets from the geochemical results found by the survey. The first method uses computerised probability calculations and the second a simple empirical formula. Both methods appear to give similar results. Northern Sumatra has very distinct geochemical patterns. Copper shows a linear zone of high values along the axial Barisan Mountain Range, which are derived from two main sources: the Sumatran ophiolites and copper-rich calc-alkaline intrusives. High chromium values also occur not only over the ophiolites, but also over placers derived from them. High lead values are grouped to the east of the linear copper zone and they border the oil and gas basins of the eastern coastal strip. High tin values occur west of the copper rich intrusives, but east of the present-day subduction zone off the west coast of Sumatra. The overall pattern fails to conform to the classic zonation of mineral deposits across a simple subduction system, perhaps because large transcurrent faults have sliced northern Sumatra into blocks and so brought blocks of different provenance into contact. This explanation is confirmed by the lithium distribution, which reveals two wholly distinct provinces astride the Sumatran Fault System.