Master of Science
Planetary Science and Exploration
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 Δ60NiLimonite-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.
Summary for Lay Audience
Nickel (Ni) is a metal that is widely used, most notably in the alloy industry. Lateritic ore deposits host 60% to 70% of the world Ni resources, and they provide 40% of world Ni production. Nickel is very enriched in ultramafic rocks, and weathering processes on ultramafic rocks in tropical areas lead to high grade concentrations of Ni, which becomes valuable for the mining industry. To better understand laterite formation and assess the potential of Ni isotopes as tracers of weathering processes, Ni and Fe isotope compositions of bedrock, saprolite, limonite, topsoil, and mineralization collected stratigraphically, as well as water samples from four Ni mines in Zambales and Palawan ophiolite zones in the Philippines, were studied. Nickel is primarily hosted in olivine and pyroxene before being leached into the groundwater. This study found that the replacement of Fe and Mg in the Fe- and Mg-oxide minerals by Ni might be the controlling mechanisms of Ni fractionation during weathering processes in tropical area laterite profiles. Light Ni isotopes are preferentially incorporated into the newly-formed minerals, leaving heavy Ni isotopes in water. The heavy Ni-rich groundwater can result to heavy Ni isotope compositions in mineralized samples. In addition, Fe isotopes show very little isotopic variation among rocks existing in five different zones, which might be a result of low mobilization of Fe3+. In complement, iron meteorites were analyzed for their Fe isotopic composition to shed new light on the origin of the Lovina ataxite ungrouped meteorite. Based on its chemical composition and heavy Fe isotope composition compared to iron meteorites analyzed so far, this study supports previous findings that Lovina has a terrestrial origin but remains enigmatic in its formation process.
Zhu, Congxi, "Ni and Fe isotope fractionation during weathering and the formation of Ni laterite ore deposits in the Philippines" (2019). Electronic Thesis and Dissertation Repository. 6593.
Available for download on Friday, September 17, 2021