Application of multi-element geochemistry in the weathered environment: Controls, considerations and implications for exploration

Abstract

Understanding the mobility of individual elements in the oxidised environment is important for the accurate interpretation of geochemical data from regolith and weathered bedrock samples. In the weathered environment, immobile element geochemistry can reflect primary lithological and in situ mineralisation signatures, whereas mobile elements can help establish the degree and extent of weathering. Three exploration case studies are presented here to demonstrate the application of multi-element geochemistry in the weathered environment: (1) An example from the Shorty Creek Project, Alaska (Freegold Ventures), highlights the importance of reviewing and understanding pathfinder elements in soil, weathered bedrock and fresh basement. Here, commodity elements Cu and Zn are highly mobile and relatively depleted in the soils and weathered zone over Cu-Au mineralisation, whereas Au, As, Bi and Sb are less mobile and highly anomalous in the oxidised bedrock and associated soils. (2) The Hermosa Deposit, Arizona (South32), where downhole geochemistry can help map the oxide-fresh rock boundary at the deposit scale. Subtle depletion in mobile elements allows the weathered zone to be identified in altered rhyolites, and Zn:S and Pb:S ratios in the mineralised zone helps distinguish Zn and Pb oxides from Zn and Pb sulphides. (3) A review of residual soils over a Ni-Cu-PGE prospect in Northern Queensland, where immobile elements are reliable discriminants of primary lithologies in highly weathered environments. Zirconium, Y, Th and Nb can be used to distinguish felsic from mafic bedrock, and variations in Cr, V, Al, Fe and Sc in soils confidently identify blind, compositionally distinct mafic-ultramafic bodies. This paper also highlights the importance of collecting high quality, multi-element geochemistry with low detection limits at every stage of exploration.