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Platinum-group element and gold enrichment in soils monitored by chromium stable isotopes during weathering of ultramafic rocks
Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Santiago, Chile.
Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Santiago, Chile.
Department of Geosciences, University of Tübingen, Tübingen, Germany.
Departmento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Spain.
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2018 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 499, p. 84-99Article in journal (Refereed) Published
Abstract [en]

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 Cabaña, located in the Coastal Range of south-central Chile (38° S) were studied by combining major and trace element geochemistry with chromium stable isotope data. The results show that the soils developed at La Cabaña have higher PGE and Au contents than the parent serpentinite rock, with ∑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 δ53/52CrSRM979 values, within a range of −0.089 ± 0.012‰ to −0.320 ± 0.013‰ (average of −0.178‰), and are in agreement with previous data reported for modern soils. A noteworthy relation between δ53/52Cr data and PGE + Au contents is observed in the studied soil horizons, where isotopically lighter values of δ53/52Cr match the higher contents of PGE and gold. These results show that pedogenetic processes operating at the cold and humid La Cabaña 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.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 499, p. 84-99
Keywords [en]
Supergene, Noble metals, Chromium isotopes, Weathering, Ultramafic rocks, Soil formation
National Category
Geochemistry Geology Other Earth and Related Environmental Sciences
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-70987DOI: 10.1016/j.chemgeo.2018.09.008ISI: 000447407300006Scopus ID: 2-s2.0-85054175601OAI: oai:DiVA.org:ltu-70987DiVA, id: diva2:1250992
Note

Validerad;2018;Nivå 2;2018-11-01 (svasva)

Available from: 2018-09-25 Created: 2018-09-25 Last updated: 2018-11-21Bibliographically approved

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Aiglsperger, Thomas

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