Ni and Fe isotope fractionation during weathering and the formation of Ni laterite ore deposits in the Philippines

Congxi Zhu, The University of Western Ontario

Abstract

Weathering processes on Ni-rich ultramafic rocks in tropical areas produce laterites that become exploitable for Ni mining. To better understand these processes, samples were collected stratigraphically with water samples from several Ni mines in the Philippines and studied for Ni and Fe isotope compositions. This study found that Ni isotope fractionation takes place during the formation of Ni-enriched minerals (goethite), when light Ni isotopes are preferentially incorporated into new minerals formed, leaving heavy Ni isotopes in groundwater. The Δ⁶⁰Ni_{Limonite-Bedrock} is up to -0.19 ± 0.32‰. Even though Fe is partitioned with redox state change during these reactions, Fe isotope fractionation was not detected within our analytical precision. In complement, iron meteorites were analyzed for their δ⁵⁷Fe to shed light on the origin of Lovina ataxite. This study supports other reports that Lovina has instead a terrestrial origin.