The source of pedogenic carbonate associated with gold-calcrete anomalies in the western Gawler Craton, South Australia

Ionic gold in calcrete revealed by LA-ICP-MS, SXRF and XANES

“…Manceau et al, 2000Manceau et al, , 2004Singh et al, 2000;Furnare et al, 2005;Walker et al, 2005;Li et al, 2008). The presence of ionic Au in regolith materials (including soil) has been suggested by recent studies in mineral exploration geochemistry that indicate a significant proportion of naturally occurring Au in soil (particularly in calcareous types) is soluble in water and other mild lixiviants such as 0.1 M KI Lintern et al, 2005). Furthermore, this soluble Au is being exploited by commercial geochemical laboratories using partial extraction techniques to vector towards buried ore deposits.…”

“…Carbon isotopes indicate that much of the carbonate anion in calcrete that contains Au is derived from vegetation via root respiration (Lintern et al, 2005). This strong link between biota and pedogenic calcrete formation is supported by calcareous rhizoliths, some hundreds of mm long, commonly found in semi-arid soils, including those at Bounty (Fig.…”

Hematite/Maghemite trace element geochemistry in base metal exploration

“…Calcrete has been shown to be a reliable and highly sensitive medium for detecting Au anomalies at ppb level and the application has led to a number of significant recent discoveries (e.g. the Challenger gold deposit in South Australia, Lintern et al, 2006).…”

Terrestrial Evaporites

“…The extensive modern playa systems of western and southern Australia, despite their terrestrial setting, have 87 Sr/ 86 Sr ratios indicative of a marine solute origin (Chivas et al, 1987;McArthur et al, 1989) as do extensive regional calcrete deposits (Quade et al, 1995;Lintern et al, 2006), as discussed in following sections. However, some playa gypsum samples from the Yilgarn Craton of Western Australia, also demonstrate calcium isotope ratios ( 40 Ca/ 42 Ca) indicative of Precambrian calcium (a minor amount of the 40 Ca is radiogenic, a progeny of 40 K decay), thereby indicating a component of bedrock calcium from the Archaean basement (Nelson and McCulloch, 1989).…”