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Open File Report 59

Strategies and methods for the interpretation of geochemical data. Discussion paper applied to laterite geochemistry

Grunsky, E.C.

This paper summarizes a systematic approach to analyzing the geochemical data collected over lateritic terrains in the Yilgarn Block of Western Australia.

Critical steps in the sequence of data analysis are:

  1. preliminary data analysis,
  2. exploratory multivariate data analysis, and
  3. specific multivariate data analysis and modeled multivariate analysis.

For the evaluation of geochemical data in laterites of the Yilgarn Block the following sequence of investigation is recommended:

1. Preliminary Data Analysis

  • The use of histograms, box & whisker plots, Q-Q plots, scatter plot matrix, data ranking;
  • Preparation of summary statistical tables;
  • Maps of elements with each sample ranked into percentile ranges;
  • Elimination of gross outliers
  • Investigate outliers for each element: analytical error or atypical abundance;
  • Adjust data for censored values;
  • Transformation of data based upon samples below the 95th-98th percentile;
  • Scatterplot matrix for transformed data
  • Threshold selection after transformation

2. Exploratory Multivariate Data Analysis

  • Robust estimates to compute means and covariances to enhance the detection of outliers;
  • Application of dimension reducing techniques such as principal components analysis;
  • The use of methods to delineate structure in the data (cluster analysis, multi-dimensional scaling, non-linear mapping, and projection pursuit);
  • The use of chi-squared plots applied to transformed data to isolate outliers based upon all of the elements of interest; maps of large Mahalanobis distances (>95th percentile) may identify anomalous areas.

3. Specific Multivariate Data Analysis and Modeled Multivariate Analysis

  • Calculation of empirical indices specifically tailored to areas in which multi-element associations are understood.
  • Multiple regression applied to areas where a linear model of the multi-element association can be computed with good results (i.e. high R2 coefficients). Residuals can be examined for the potential of being associated with mineral deposits.
  • The establishment of background and target groups that characterized the geochemical variation of the regional geochemistry and the mineral deposits.
  • Analysis of variance and canonical variate analysis to test the statistical uniqueness of the groups.
  • The use of all possible subsets to compare reference groups with each other and determine which group of elements enhance the group separations.
  • The application of allocation-typicality procedures to test unknown samples from a regional exploration; programme. Each sample is assigned a probability of belonging to one of the reference groups. Maps of typicality or posterior probability can be made to indicate group membership.

This approach of systematically analyzing the laterite geochemistry forms the basis of an effective exploration programme strategy in geochemistry.

Last updated: Thursday, January 06, 2000 10:57 AM


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