Platinum-group element and gold enrichment in soils monitored by chromium stable isotopes during weathering of ultramafic rocks

2018

Article référencé dans le Web of Science WOS:000447407300006

Rivera J., Reich M., Schoenberg R., Gonzalez-Jimenez J. M., Barra F., Aiglsperger T., Proenza J. A., Carretier Sébastien

Chemical Geology, 2018, 499, p. 84-99 ISSN 0009-2541

Weathering of ultramafic rocks can lead to the formation of soil profiles with high contents of Fe, Ni, Co, platinum-group elements (PGE) and gold. Traditionally, these metal-rich soils are known as "laterites" and are formed under tropical climates and stable tectonic conditions. However, little is known about their possible development in cold/humid regions, and the factors governing PGE and gold mobility and enrichment under these weathering conditions are poorly constrained. In this study, five soil profiles developed on serpentinized, chromite-bearing ultramafic rocks at La Cabana, located in the Coastal Range of south-central Chile (38 degrees S) were studied by combining major and trace element geochemistry with chromium stable isotope data. The results show that the soils developed at La Cabana have higher PGE and Au contents than the parent serpentinite rock, with Sigma PGE and Au reaching up to 160 ppb and 29 ppb in a limonitic soil horizon and clay saprolite, respectively. Most soil samples have slightly negative delta Cr-53/52(SRM979) values, within a range of -0.089 +/- 0.012 parts per thousand to -0.320 +/- 0.013 parts per thousand (average of -0.178 parts per thousand), and are in agreement with previous data reported for modern soils. A noteworthy relation between delta Cr-53/52 data and PGE + Au contents is observed in the studied soil horizons, where isotopically lighter values of delta Cr-53/52 match the higher contents of PGE and gold. These results show that pedogenetic processes operating at the cold and humid La Cabana area are capable of increasing the total PGE and Au contents of certain soil horizons. Such processes are complex and multivariate but are primarily modulated by chromite dissolution and the formation of secondary phases such as clay minerals and oxy-hydroxide phases in the soil. These findings provide evidence that important weathering and PGE + Au supergene accumulation are not only restricted to tropical latitudes, and that the chromium isotope system is a useful proxy to track surface redox process and noble metal enrichment during pedogenesis.

Géologie et formations superficielles [064] ; Pédologie [068]

CHILI

Fonds IRD [F B010074152]

fdi:010074152