FORMATION OF SUPERGENE FERRUGINOUS PISOLITIC (‘LATERITE') GOLD DEPOSITS IN WESTERN AUSTRALIA: NEW INSIGHTS INTO MECHANISMS OF ANOMALY FORMATION

Lateritic Au deposits form where intense weathering under humid tropical to subtropical conditions has led to the development of a thick, lateritic regolith. They are mostly small (<10 Mt) and of low grade (1-5 g/t), but were very important in reviving the gold industry in Australia, Brazil and West Africa. Supergene Au mineralization in a weathering profile is broadly mushroom shaped and is composed of two facies: saprolitic and lateritic. The saprolitic ore corresponds to oxidized mineralization between the weathering front and the mottled zone. The lateritic ore is comprised of a dispersion halo in ferruginous pisoliths and retains its ‘mushroom’ distribution, even where buried. In ore-grade laterite deposits, gold mineralization is much larger (several kilometres long) in pisoliths than in saprolite and is thought to have formed by residual concentration by chemical wasting. We integrate pisolith formation, landscape history, macro and microanalysis from several deposits to show that Au in pisoliths is not residual but has undergone several stages of mobilization and reprecipitation to form these deposits. Pisolitic units contain detrital and authigenic pisoliths and consist of hematite, goethite, maghemite, kaolinite and quartz. Goethite and kaolinite, introduced later as a result of changes in environmental conditions, occur as 1-3 mm thick cortices on most pisoliths. Individual pisoliths are highly variable in Au and As. Gold is more abundant in magnetic (maghemite-rich) pisoliths than in non-magnetic pisoliths. Despite Fe oxides forming the major constituent of cores of pisoliths, secondary Au is associated with recently introduced organic-clay-rich material in the cortices and cracks and cavities in cores. Mechanical dispersion of older mineralized and unmineralized pisoliths and saprolite fragments into paleovalleys, followed by reweathering in the presence of abundant organic material in wetter environments redistributed Au and As. Secondary Au often consists of nano-particulate spheres and rods forming sheets implying biological processes played a role in their formation. In contrast, As is strongly associated with several generations of goethite and hematite in cortices and cores indicating precipitation under different environmental conditions