Year

2006

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Abstract

Leaves, twigs and bark of six different mallee species (Eucalyptus oleosa, E. gracilis, E. socialis, E. eremophila, E. transcontinentalis and E. diversifolia) and their adjacent soils have been collected from 94 sites in up to nine botanical regions spread over a wide area in southern Australia. All samples were analysed for 44 elements by INAA and ICP-MS techniques. The results of robust analysis of variance show that organs exhibit different behaviour with respect to elemental concentrations and their mobile nature. The nature of the underlying substrate (regolith or bedrock) is found to have a considerable influence on elemental composition of the plant organs. Some trace elements have higher contents from biosamples growing on substrates with powdery calcretes than either nodular or hardpan, suggesting that vegetation may more easily take up the species of trace elements that are dissolved in these regolith carbonates or it may arise from the influence of bedrock. The biological absorption coefficient (BAC), which is one of the biogeochemical parameters used in mineral exploration, was produced to characterize the absorption of chemical elements by plants from their substrate. It was concluded that the plants studied here have BAC values for trace metals higher than the average values cited in the literature and pose their eligibility for inclusion in regional biogeochemical exploration programs. Pedo- and biogeochemical patterns based on the results of exploratory data analysis (EDA) techniques and robust-class selection was applied to the analysis of data. Regional biogeochemical maps of As, Au, Co, Cr, Mn and Ni show an association of these elements coincident with known mineralisation, and delineate anomalous sites where no mineralization is reported. Several sampling sites were located directly over and adjacent to the Menninnie Dam zinc-lead prospect. The high levels of some trace elements, especially Zn at this site, are reflected in the chemical composition of all biological sample types. Concerning environmental aspects, plants contributed very little to arsenic diffusion, cycling and transfer from soil and atmosphere to the biosphere in the study areas, since comparison of washed and unwashed plant parts in several sites throughout the area displays no significant decreases in the assay values for washed samples and subtle differences between these two series of samples could be within the range of the natural variation. Generally, the Cr concentrations found in the study area do not exceed the recommended contaminated level for soils related to ecosystems (210 ppm), except for 11 of the 81 sites investigated, which are located in the Yilgarn Craton near areas of known mineralisation in ultrabasic rocks. The concentration of Co and Ni in the soil samples of this research do not exceed the recommended guideline (40 ppm and 600 ppm, respectively). This multi-elemental reconnaissance study has enabled us to get invaluable information about the natural concentrations of chemical elements in this substrate and to contribute to a baseline bio-soil geochemical survey established against which future changes can be quantified and to recognise new potential areas for mineral prospecting.

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