Biogeochemistry of Pb-Zn gossans, northwest Queensland, Australia: Implications for mineral exploration and mine site rehabilitation

Abstract

The concentration and distribution of metals were studied in metallophytes, growing on and in the vicinity of Pb-Zn gossans, NW Queensland. The study investigated the accumulation of metals in plant species and assessed their potential use as indicators in geobotanical and biogeochemical prospecting and as metal excluders in mine site rehabilitation. Plant species colonising the gossans tolerate high concentrations of metals. Total mean metal concentrations of soils ranged from minima of 14 ppm Cu, 28 ppm Pb and 34 ppm Zn in background areas to maxima of 660 ppm Cu, 12000 ppm Pb and 2100 ppm Zn over mineralised soils. Over the gossans, the grass species 'Eriachne mucronata forma', 'Enneapogon lindleyanus' and 'Paraneurachne muelleri' replace the characteristic grass 'Triodia molesta' where the soils have high Pb and Zn concentrations. Of the 16 plant species identified, 3 of them, 'Hybanthus aurantiacus', 'Clerodendrum tomentosum' and 'Bulbostylis barbata', were confined to the gossan sites. 'B. barbata' appears to be of particular use in geobotanical prospecting as it indicates base metal mineralisation in the region. The biogeochemical analyses indicate significant enrichment of Cd, Pb and Zn in the tissue of all plant species, with the abundance of Cd, Pb and Zn in dried vegetation from the gossans being up to one order of magnitude above background. In particular, the uptake of Cd, Pb and Zn by 'Sida' sp., 'T. molesta', 'Cleome viscosa' and 'Eriachne mucronata forma' increases linearly with DTPA-extractable soil metal concentrations. The biogeochemistry of these plants provides the best anomaly definition of the exposed gossans. Furthermore, the analysis of roots demonstrate that the plant species 'T. molesta', 'Eriachne mucronata forma' and 'P. muelleri' allow the transport of Cd, Pb and Zn from the roots to the above-ground biomass. Hence, the species best suited for biogeochemical prospecting for base metals in the region and semi-arid inland northern Australia are 'Eriachne mucronata' and the genus 'Triodia'. The analyses also reveal differences in the ability of each species to accumulate metals. Among all gossan plants, 'Eremophila latrobei' displays distinctly low Pb concentrations and low correlations with soil DTPA extractable Pb. This plant is the most efficient in excluding Pb from its biomass, making it most suitable for the revegetation of Pb contaminated soils. This study demonstrates that biogeochemical examinations of gossans can reveal indicator and excluder plants, which are of potential use in mineral exploration as well as mined land reclamation.