Integrating petrogenesis and weathering to understand regolith chemistry: examples from the Palaeoproterozoic Carson Volcanics, north Kimberley, Western Australia

The rugged topography of the Kimberley area of northern Western Australia preserves examples of the complete weathering cycle to produce regolith. With increasing weathering intensity, Palaeoproterozoic mafic volcanic rocks of the Carson Volcanics are progressively depleted in low field strength element (LFSE) and rare earth elements (REE), and enriched in high-field strength elements (HFSE), Th, Cr and chalcophile elements. This reflects the breakdown of the main rock-forming minerals, the concentration of resistate phases such as zircon, chromite, monazite and rutile, and the development of secondary clay minerals, oxides and oxyhydroxides. Some regolith samples at the base of the volcanic succession have high Cr, As and detectable Au and Pd. This regolith chemistry reflects not only the intensity of weathering, but also the less fractionated and weakly mineralized nature of flow units erupted early in the volcanic cycle. Elsewhere in the Carson Volcanics, two regolith samples with unusually high concentrations of Ba, REE and Zn are located close to regional structures, which acted as conduits along which hydrothermal fluids migrated resulting in surface alteration, probably after regolith formation. Both examples show that regolith chemistry should be interpreted in the context of both weathering and petrogenetic processes that affect bedrock composition.