Significance of ferruginous pisoliths and interface sampling for gold exploration in the covered terrains

Abstract

The deeply weathered Yamarna Terrane is the easternmost, least explored Terrane of the Yilgarn Craton in Western Australia. Investigation of the landscape evolution, mineralogy and geochemistry of the transported cover showed that the near surface ferruginous regolith can provide a significant tool for exploration under cover. Three types of ferruginous materials were identified and examined in three gold prospects: Smokebush, Toppin Hill and Santana. The ferruginous materials include: 1) lateritic residuum and its detrital ferruginous clasts of underlying or nearby bedrock; 2) authigenic pisoliths and nodules of Permian glacial sediments (PPS); and 3) authigenic pisoliths of aeolian sand (PAS) overlying the other two. Our findings show that As is the main pathfinder element for Au mineralization. Arsenic and Au are elevated and form near surface anomalies in lateritic residuum, detrital ferruginous clasts, PAS, <75 μm and <2 μm soil size fractions at Smokebush. At Toppin Hill, Au migrated directly from the mineralization through the Permian cover and became enriched at the surface in the PPS above mineralization. Laser ablation ICP-MS mapping shows Au in the PPS as nano-grains, microcrystalline aggregates and veinlet fillings. This is supported by partial extraction analyses, where Au was extracted mainly by K-iodide and K-cyanide, indicating that it is dispersed in particulate and soluble forms. The absence of an As anomaly in transported cover over the Toppin Hill prospect compared to Smokebush is related to the absence of arsenopyrite in the Toppin Hill Au mineralization. Arsenic, which was extracted mainly by 0.1 M tetra‑sodium pyrophosphates, is associated with organic compounds and has a different fractionation pathway from Au near surface. This is also shown by an As anomaly in the Eucalyptus foliage over the Smokebush Au mineralization. At the Santana prospect, there are no Au and/or As anomalies in the PPS over the mineralization. The PPS at Santana are reworked from barren Permian cover and could be deposited with ferruginous clasts from different sources, as shown by textural and mineralogical variations between the grains and matrix. Furthermore, there is no evidence of hydromorphic dispersion of Au and As from the underlying mineralization. Thus, the PPS at Santana cannot be used to vector towards the mineralization. However, sampling at the unconformity (interface sampling) is more effective in locating the mineralization at Santana.This study shows that ferruginous pisoliths and clasts in the cover are potential sampling media for Au exploration in the Yamarna Terrane. The exploration methods used in this study can be also applied in similar covered terrains not only in Australia but also in areas with complex weathering histories in the tropics and sub-tropics.