SIPVADE: A New Computer Programme with Seventeen Statistical Tests for Outlier Detection in Evaluation of International Geochemical Reference Materials and its Application to Whin Sill Dolerite WS-E from England and Soil-5 from Peru

A new computer programme was written in programming language TURBOC, which enables us to apply a procedure involving seventeen statistical tests (a total of sixty five single or multiple outlier versions of these tests) for outlier detection in univariate sample at a high confidence level of 99% (significance level α= 0.01). The outlying observations should be evaluated first for technical reasons and then rejected manually from the data base until no more outliers are detected and the final statistical parameters are computed from the remaining data. This programme has been used successfully to process two reference material data bases: WS-E from England and Soil-5 from Peru. The final mean values for WS-E are more reliable (characterized by smaller standard deviations and narrower confidence limits) than those obtained earlier using a different statistical approach. The application of a large number of statistical tests to Soil-5 also resulted in smaller standard deviation values for most elements than the method involving a limited number of such tests. For WS-E, some laboratories seem to have produced multiple data that were detected as statistical outliers. A close analysis of the distribution of outliers as a function of laboratory, country and analytical method leads to a technical justification for these outlying observations, probably in terms of inadequate QA/QC practices. Use of geochemical criteria indicates that the new mean values in WS-E might be closer to the "true" concentrations. This procedure of outlier detection and elimination is therefore recommended in the study of the existing RM.     

Relative Efficiency of Single-Outlier Discordancy Tests for Processing Geochemical Data on Reference Materials and Application to Instrumental Calibrations by a Weighted Least-Squares Linear Regression Model

Numerous studies report geochemical data on reference materials (RMs) processed by outlier-based methods that use univariate discordancy tests. However, the relative efficiency of the discordancy tests is not precisely known. We used an extensive geochemical database for thirty-five RMs from four countries (Canada, Japan, South Africa and USA) to empirically evaluate the performance of nine single-outlier tests with thirteen test variants. It appears that the kurtosis test (N15) is the most powerful test for detecting discordant outliers in such geochemical RM databases and is closely followed by the Grubbs type tests (N1 and N4) and the skewness test (N14). The Dixon-type tests (N7, N8, N9 and N10) as well as the Grubbs type test (N2) depicted smaller global relative efficiency criterion values for the detection of outlying observations in this extensive database. Upper discordant outliers were more common than the lower discordant outliers, implying that positively skewed inter-laboratory geochemical datasets are more frequent than negatively skewed ones and that the median, a robust central tendency indicator, is likely to be biased especially for small-sized samples. Our outlier-based procedure should be useful for objectively identifying discordant outliers in many fields of science and engineering and for interpreting them accordingly. After processing these databases by single-outlier discordancy tests and obtaining reliable estimates of central tendency and dispersion parameters of the geochemical data for the RMs in our database, we used these statistical data to apply a weighted least-squares linear regression (WLR) model for the major element determinations by X-ray fluorescence spectrometry and compared the WLR results with an ordinary least-squares linear regression model. An advantage in using our outlier procedure and the new concentration values and uncertainty estimates for these RMs was clearly established.     

National-Scale Geochemical Mapping Projects in China

Regional, national and global scale geochemical mapping projects have been carried out in China since the late 1970s, due to the development of cost‐effective, low detection limit analytical methods. These projects have provided a huge mass of high‐quality, informative and comparable data for mineral resource exploration and are now making contributions to environmental assessment. In this paper, four national‐scale geochemical mapping projects are described. (1) The Regional Geochemistry‐National Reconnaissance Project (RGNR project), which is China's largest national geochemical mapping project, has covered 6 million km² of upland regions since 1978. Generally, stream sediment samples were collected at a density of 1/km² and four samples were composited into one sample and analysed for thirty‐nine elements. (2) The deep‐penetrating geochemical mapping project (DEEPMAP Project) has been conducted since 1994 in covered terrains, including sedimentary basins, at a density of 1 sample per 100 km² with thirty to seventy elements determined per sample. In the past 10 years, an area of approximately 800 000 km² has been covered and this project has played an important role in finding sandstone‐type uranium deposits in basins. (3) The seventy‐six geochemical element mapping project (76 GEM project) has been carried out since 1999 and involved the collection of stream sediment samples from the RGNR project targets which were analysed for seventy‐six elements. Samples from each 1:50 000 map sheet were composited into one analytical sample (approximately one composite sample per 400 km²). Approximately 1 million km² have been surveyed to date. (4) The multi‐purpose eco‐geochemical mapping project has been conducted since 1999 in Quaternary plain areas for environmental and agricultural applications. Surface soils (depths from 0–20 cm) were collected at a density of one sample per km², and four samples were composited into one for analysis. Deep soils (from a depth of 150 to 200 cm) were collected at a density of one sample per 4 km² and four samples were composited into one analytical sample. All the composite samples were analysed for fifty‐four elements.     

Multivariate statistical techniques applied to pisolitic laterite geochemistry at Golden Grove, Western Australia

Multivariate statistical procedures are applied to pisolitic laterite geochemistry in a study of the Golden Grove massive sulphide district. The objective is to optimize identification of geochemical anomalies caused by base metal mineral deposits.The statistical approach used in this paper depends upon geochemical data for appropriate reference groups (or training sets) being available. The target group consists of orientation data from pisolitic laterite about the Gossan Hill Cu-Zn massive sulphide deposit. A group representing background sequence was selected by combining three subareas in a geochemically quiet part of the prospective acid volcano-sedimentary sequence.A multi-element allocation procedure was set up using data from the reference groups. The exploration samples are then allocated, one sample at a time, to either one of the reference group categories, using the probability of group membership. A map showing the relative probability values for each sample site is the final product for interpretation, aided by ancillary use of an index of typicality.The allocation procedures were carried out using different element combinations, these being based on a procedure for subset selection to give maximum separation of reference groups, and on geochemical insight. Whilst many versions of the allocation procedure gave positive identification of the anomaly related to the blind Scuddles Cu-Zn deposit, allocation using only Cu, Pb, Zn and Ag did not. The results emphasize the importance of pathfinder elements in geochemical studies in weathered terrain.The allocation procedure using the most appropriate element combinations provided more positive identification of the main areas of known mineralization than had the previously used empirically derived methods of Smith and Perdrix (1983). The formal allocation procedure has the following additional advantages: results are not markedly affected by a very high value for any single element since robust procedures are incorporated into the analysis; better discrimination appears to be possible for weaker anomalies; separation of target from background can be optimized by formal calculations instead of by trial and error; and better suppression of background variation results.     

Soil-5, A New IAEA CErtifiEd Reference Material for Trace Element Determinations*

60 geochemical laboratories from the world over, contributing in all 3687 bits of chemical data on 60 elements, haue willingly participated in the study of SOIL-5 as a reference material. The preparation of SOIL-5 in 45 kg and homogeneity teats are described; in addition, all the data compiled are presented. In evaluating the compiled data, the use of four concurrent tests for outlier-rejection at the same level of significance, has been applied. After proper statistical treatment, “recommended values” could be established for 31 elements, most of them trace elements and “information values” for 29 elements. 23 of the recommended values could be given with a “relatively high” degree confidence whereas the other 18 recommended Values with a “reasonable” degree of confidence. An attempt is made to compare the analytical methods employed; the existence of possible sy3tematic differences is pointed ont.     

The detection and correction of outlying determinations that may occur during geochemical analysis

‘Wild’, ‘rogue’ or outlying determinations occur periodically during geochemical analysis. Existing tests in the literature for the detection of such determinations within a set of replicate measurements are often misleading. This account describes the chances of detecting outliers and the extent to which correction may be made for their presence in sample sizes of three to seven replicate measurements. A systematic procedure for monitoring data for outliers is outlined. The problem of outliers becomes more important as instrumental methods of analysis become faster and more highly automated; a state in which it becomes increasingly difficult for the analyst to examine every determination. The recommended procedure is easily adapted to such analytical systems.     

地球化学普查样品实验室分析异常点抽检改进方法

在地球化学样品多元素分析过程中,为了实现地球化学异常点抽查和整批测试质量监控的双重目的,本文提出了一种基于地球化学异常统计的多区间分元素组异常点抽样方法。该方法主要包括:(1)按照正态分布或对数正态分布假设,对各元素,将该元素的测量值集,划分为离群低值区间、累积频率分布区间(有多个区间)和离群高值区间,统计每个样品中各元素测量值所属区间;(2)将测量元素分为几个元素组,统计各元素组中每个样品的总异常元素数、高异常元素数和低异常元素数;(3)对Au元素组,设置各个区间的抽检比例,随机抽取异常点抽检样品;对其他元素组,按照"总异常元素数"为第一顺序、"高异常元素数"为第二顺序和"低异常元素数"为第三顺序,按降序排序所有样品,顺序抽取异常点抽检样品;(4)必要时进行分析批补充抽检样品。该方法兼顾了地球化学异常和实验室测试"异常"的抽查;同时考虑非异常点抽查和分析批平衡抽检,有效解决了有限抽样数量和抽样代表性不足两者之间的矛盾;并开发了EXCEL应用程序,应用于实验室地球化学样品分析异常点抽检取得了满意效果。     

Inductively Coupled Plasma Determination Of Nine Rare-Earth Elements In Sixty International Geochemical Reference Samples

Data for nine rare-earth elements (REE) determined by inductively coupled plasma spectrometry (ICP-AES) in sixty international geochemical reference samples (igneous and sedimentary rocks, soils and sediments) are presented. The method is based on acid digestion of the sample and cation-exchange separation from matrix elements. Good agreement is generally achieved between the results of this work and the recent compilation values. The present paper includes analytical data on several reference materials for which existing data are rather scarce or even inexistent     

New Reference Material of Dunite Rock, NGRI-UMR: Preparation and Evaluation

A dunite reference material has been prepared as part of the reference materials programme at the National Geophysical Research Institute, Hyderabad, India. The scarcity of reference materials for undertaking the chemical analysis of ultramafic rocks for major and trace elements justified the preparation of such a rock reference material. A 300 kg sample of starting material was collected from an active quarry near the town of Salem in Tamil Nadu, southern India. The sample was homogenized and an invitation for round robin participation was published in Geostandards Newsletter (October 1995). A total of twenty seven international laboratories participated in the study, providing information on the total element concentration of major, minor, trace and rare earth elements. Following data compilation, outlier rejection, and statistical analysis, proposed and indicative total element concentrations are reported for twenty four elements.     

New Series of Soil Geochemical Reference Materials (GSS 10-16) from the Main Overburden Region in China

Seven soil geochemical certified reference materials (CRMs) GSS 10-16 recently prepared by the Institute of Geophysical and Geochemical Exploration (IGGE), are mainly used for sample analysis in geochemical surveys of the overburden region in China. Fifteen Chinese institutes and central laboratories, which operated at a high level of analytical performance analysed these samples. More than ten reliable analytical methods based on different principles of measurement were adopted, of which ICP-MS, ICP-AES and NAA were taken as the primary analytical methods. In total, 23715 determinations were carried out, 5660 average data sets were obtained, seventy two elements and components were determined and certified values of sixty nine elements and components were assigned. Rhenium concentrations were measured by isotope dilution ICP-MS in GSS-10, GSS-13, GSS-15 and GSS-16. Certified values of elements in GSS 10-16 have good precision and for 90% of these values relative uncertainties are less than 10%.     

A Preliminary Study on the Preparation of Four Pacific Ocean Polymetallic Nodule and Sediment Reference Materials: GSPN-2, GSPN-3, GSMS-2 and GSMS-3

The research project for the preparation of two polymetallic nodule (GSPN-2, GSPN-3) and two marine sediment (GSMS-2, GSMS-3) reference materials was completed in 1996. The samples were collected from the Eastern and Central Pacific Ocean and prepared to less than 74 μm fine powder. The homogeneity of the materials was tested by X-ray fluorescence spectrometry. The relative standard deviations (RSD %) of measurements of selected elements in samples taken from thirty bottles selected at random were less than 0.5%. Thirty four laboratories worldwide participated in the collaborative analysis programme. The number of elements or constituents analysed was seventy nine for nodules and eighty one for sediments. Among those, sixty three elements were certified in each sample. The sum of major elements or components is 99.7% for GSPN-2, 99.5% for GSPN-3, 99.8% for GSMS-2 and 99.8% for GSMS-3.     

多元地球化学异常识别的核马氏距离方法

地球化学数据满足多元正态分布时,马氏距离是一种有效识别多元地球化学异常的综合指标。然而,由于地质系统的复杂性、成矿作用的多期多阶段性以及控矿因素的多重性常常导致多元地球化异常临界面是非线性的和模糊的,用马氏距离定义的平滑超椭球面不能准确表示这种复杂曲面。核函数能够将地球化学样品集非线性变换至特征空间,背景样品的映像集合在特征空间中构成一种流型,异常样品的映像则零散分布于流型的边缘及外围。计算和比较样品映像到样品映像总体的核马氏距离,可以识别异常样品。把该方法应用于白山地区多元地球化学异常识别,用核马氏距离、马氏距离和主成分得分识别金-银、金-银-砷-铋-汞、金-银-铜-铅-锌-锑-钴、金-银-铜-铅-锌-砷-锑-铋-汞-钴4种组合模式的多元地球化学异常。研究结果表明：复合核函数马氏距离的多元地球化学异常识别效果优于其他方法。     

Factor analysis applied to regional geochemical data: problems and possibilities

A large regional geochemical data set of C-horizon podzol samples from a 188,000 km² area in the European Arctic, analysed for more than 50 elements, was used to test the influence of different variants of factor analysis on the results extracted. Due to the nature of regional geochemical data (neither normal nor log-normal, strongly skewed, often multi-modal data distributions), the simplest methods of factor analysis with the least statistical assumptions perform best. As a result of this test it can generally be suggested to use principal factor analysis with an orthogonal rotation for such data. Selecting the number of factors to extract is difficult, however, the scree plot provides some useful help. For the test data, a low number of extracted factors gave the most informative results. Deleting or adding just 1 element in the input matrix can drastically change the results of factor analysis. Given that selection of elements is often rather based on availability of analytical packages (or detection limits) than on geochemical reasoning this is a disturbing result. Factor analysis revealed the most interesting data structures when a low number of variables were entered. A graphical presentation of the loadings and a simple, automated mapping technique allows extraction of the most interesting results of different factor analyses in one glance. Results presented here underline the importance of careful univariate data analysis prior to entering factor analysis. Outliers should be removed from the dataset and different populations present in the data should be treated separately. Factor analysis can be used to explore a large data set for hidden multivariate data structures.     

Variogram models for regional groundwater geochemical data

Four variogram models for regional groundwater geochemical data are presented. These models were developed from an empirical study of the sample variograms for more than 10 elements in groundwaters from two geologic regions in the Plainview quandrangle, Texas. A procedure is given for the estimation of the variogram in the isotropic and anisotropic case. The variograms were found useful for quantifying the differences in spatial variability for elements within a geologic unit and for elements in different geologic units. Additionally, the variogram analysis enables assessment of the assumption of statistical independence of regional samples which is commonly used in many statistical procedures. The estimated variograms are used in computation of kriged estimates for the Plainview quadrangle data. The results indicate that an inverse distance weighting model was superior for prediction than simple kriging with the particular variograms used.     

The microcomputer in geochemical exploration

A microcomputer system can be a powerful interpretive tool in the hands of the explorationist, particularly for his geochemical surveys. Data entry and plotting of site parameters and/or trace-element values on 18 cm × 26 cm maps, on site, can be carried out at reasonable cost with available technology. Plotting characteristics of some microcomputer systems are the equal to those of mainframes.All geochemical data, even for surveys of 38 elements with up to four sample media, can be displayed by a large plotter as ten single-element plots (18 cm × 26 cm) on each of four mapsheets. Plotting 2000 points for ten elements requires five hours and can be scheduled for overnight operation. Elements showing meaningful variations can be replotted subsequently at large scales using character and/or symbol formats. Examples of geochemical plots are given for soil- and stream-sediment surveys and for drill-core lithogeochemical studies.The microcomputer can calculate statistics; however, large numbers of such calculations are slow. Transfer of data to a mainframe computer allows access to fast, sophisticated statistical software. The mainframe can also provide high-quality laser-printed paper copy and can be used for fail-safe archiving.     

Importance of Skewness and Kurtosis Statistical Tests for Outlier Detection and Elimination in Evaluation of Geochemical Reference Materials

A common characteristic of concentration data compilations for geochemical reference materials (GRM) is a skewed frequency distribution because of aberrant analytical data. Rejection of outlying results usually is required to obtain a better estimate of mean concentration values. The present work describes the application of an approach, based on skewness and kurtosis statistical tests, to establish reliable concentration values in study of rare-earth element compilations for GRM. Frequency histograms of initial concentrations for these elements differ significantly from a normal distribution, as deduced from their skewness and kurtosis coefficients. The statistical procedure shows that rejection of outliers yields normal distributions which can be used to provide mean concentrations with smaller standard deviations for a large number of cases, although for many elements our %RSD are similar (within 5%) to literature values. This procedure has been applied to derive new concentration data for rare-earth elements in 26 GRM, which are compared with earlier compilations to show that the statistical procedure proposed here provides mean values with greater quality value. Although most present mean concentrations are similar (within 5%) to those reported in previous literature, some of them are significantly different (with differences of up to 40%) in a few GRM.     

Determination of Thirteen Rare-Earth Elements by High-Performance Liquid Chromatography in Thirty and of K, Rb, Cs, Sr and Ba by Isotope Dilution Mass Spectrometry in Eighteen International Geochemical Reference Samples

This study reports the results of thirteen rare-earth elements in thirty geochemical reference samples. Three alkali (K, Rb, Cs) and two alkaline earth (Sr, Ba) elements were also determined in eighteen reference samples. The analytical procedure involved is based on spiked samples and later measurement of the rare-earth elements by high-performance liquid chromatography and the alkalies and alkaline earths by thermo-quartz mass spectrometric isotope dilution (THQ-MSID). Whenever possible, the results are compared with literature values.     

Integrated approach to determine background concentrations of chemical elements in soils

Analysis and calculation techniques of geochemical background concentrations of chemical elements in various systems is of paramount importance for applied geochemistry. Herein we assume the geochemical background as the average of natural variations in the concentrations of chemical elements determined at a territory that highly probable does not host any natural and/or anthropogenic sources of contaminating chemical elements. In the context of determining geochemical background, our research was focused on determining the concentrations of heavy metals in soils in the city of Yerevan with the application of an integrated approach. Comparison of the obtained background values with the mean concentrations of elements in the upper continental crust yields representative data, and the application of various statistical tests (±3σ, ±2σ, and boxplots) is proved to equally efficient.