Dispersion of gold and other metals by trees, gravels and soils near Boddington Gold Deposit, Western Australia

Abstract

South west Western Australia is host to some of the world's largest mineral deposits including bauxite, Ta and Au. The giant Boddington Gold Mine exploits one such deposit and is located in forested areas south east of Perth. Exploration for Au in this area has concentrated on sampling surficial Fe-rich lateritic residuum and its degradation products and was how Boddington was originally discovered; it has a multi-element signature. Despite its location in forest there has been little biogeochemistry undertaken in this area or consideration of whether trees (including eucalypts) are agents of dispersion of metals in this terrain. Eucalyptus trees have been shown to be responsible for forming anomalies above gold deposits in semi-arid areas of Australia and the location of Boddington in a humid environment provided an important comparison in which to investigate this possibility.The Golden Triangle Au prospect near Boddington Gold Deposit is located on the flank of a small lateritic hill. Limited shallow drilling has identified sub-economic mineralisation with some metre composite cuttings grading >1ppm Au. A selection of different trees and shrubs, organic soil and ferruginous pisoliths were collected over a surface traverse of 800m at 50m intervals from across the prospect. Samples were analysed for major elements, Au and several pathfinder metals. Additional foliage samples were collected from the same trees over mineralisation and background to test for sample heterogeneity.Golden Triangle vegetation samples were generally anomalous in Au and pathfinder elements (e.g. Ag, Bi, W and Sn). For example, Banksia and Macrozamia had elemental anomalies in Bi and Ag located directly above or down slope of mineralisation although for Eucalyptus the anomaly over mineralisation itself was poorly defined. Topsoil containing organic matter overlying the ferruginous pisoliths was particularly anomalous in Au (mean of 47ppb against of background of <5ppb). Pathfinder element anomalies were present in all sample media (soil, vegetation and pisoliths) although the specific elements and tenor of the anomaly varied between sample media.The results demonstrate a biotic (plant) influence on the dispersion of metallic elements at Golden Triangle prospect. This process of dispersion may serve to not only disperse and dilute metals in the regolith but potentially create a larger exploration target to detect the presence of the mineralisation. The anomalous metal content of the pisoliths themselves may be due to an earlier biotic mechanism of metal mobilisation from the deeper regolith or be solely a result of inorganic processes as previously thought. The sampling of deep-rooted trees and soil beneath them may assist in the discovery of mineral deposits in areas where there is transported material or leached regolith.