STATISTICAL STUDY OF TRACE-ELEMENTS IN THE SOLUBLE ORGANIC AND AMORPHOUS FE-MN PHASES OF SURFICIAL SEDIMENTS, SUDBURY BASIN .1. MULTIVARIATE AND SPATIAL-ANALYSIS

Samples of humus and B- and C-horizon till collected from the north an d east margins of the Sudbury Basin were analyzed for about 30 trace e lements. The data consist of two sets of analyses, one being a convent ional ''near total'' analysis following an aqua regia digestion, the o ther being a ''partial'' analysis from a selective leach designed to e xtract the metals bound to the soluble organic fraction of humus and t o the amorphous Fe and Mn oxides of B- and C-horizon till. Statistical processing, using both standard approaches and some newly developed m ethods of spatial analysis, were applied to evaluate spatial geochemic al patterns and their relationship to geological factors such as proxi mity to known mineral occurrences. In particular, the information cont ent of the partial analyses was evaluated to determine whether the par tial analyses reveal interpretable patterns not detected in the ''near total'' analyses. From a visual appraisal of dotplots of Ni and Cu in till, it is difficult to determine whether the partial data add anyth ing not already seen in the ''total'' data. However, when residuals ob tained by removing that part of the partial variation explained by the ''total'' variation are mapped, distinctive patterns are produced. Th ese patterns are enhanced by applying an adaptive filtering technique using a spatial U-statistic. Multielement groups (''total'' data, 30 e lements in till) determined by principal components analysis show that the first two components are due to Ni-Co-Cr-Cu-Zn and Pd-Au-Pt-Sb as sociations. A multifractal area-concentration model fitted to componen t scores allows multielement anomalies to be mapped, The first compone nt is shown to be spatially associated with Ni-Cu deposits, the second with Au and Ni-Cu-Au deposits. Analysis of variance, with ''total'', partial and residual data taken in turn as the dependent (''response'' ) variable, shows that both Ni and Cu in till are significantly ''expl ained'' by geological factors (underlying rock type, proximity to cont acts and to mineralization). Neither the ''total'' nor the partial ana lyses on their own show significant associations with these geological factors for either element in C-horizon till. This demonstrates the v alue of selective leach analyses, where they are carried out in additi on to the ''total'' analyses, for this dataset. A multivariate analysi s of variance shows that if all three surfical media (humus, B- and C- horizon till) are used, the selective extraction data are not so advan tageous, although still adding some predictive capability, because add itional ''explanatory'' information comes from multimedia rather than multiphase sources.