[HRE13-03] インドネシアおよびミャンマーにおける超苦鉄質岩の化学風化とニッケル濃集に与える地球化学的要因
キーワード：化学風化、Fe同位体、スメクタイト、鉄酸化物
Nickel (Ni) laterite deposits are formed as a result of intensive chemical weathering of ultramafic rocks in a tropical to sub-tropical climate. Whereas several factors are known to affect the Ni enrichment during the formation of Ni laterite deposit, such as annual precipitation, bedrock type, topography, and groundwater drainage, detailed geochemical processes and factors associated with the formation of Ni laterite deposit have not been well understood, in particular under different climate conditions. In this study, mineralogical and geochemical characteristics of weathering profiles were investigated in Ni laterite deposits from Indonesia and Myanmar since they have distinct climate conditions where Indonesia has higher annual precipitation and average temperature.
In Indonesia, 4 Ni laterite profiles were examined, which are developed on peridotites with difference degrees of serpentinization in Soroako and Pomalaa areas in Sulawesi Island. Among them, Petea Hill show the highest Ni enrichment in the saprolite layer in terms of both Ni content (up to 2.9 wt% as Ni) and mass transfer coefficient (τTi < ~20), indicating that Ni was greatly added from the upper layers including those already eroded away during chemical weathering, compared with other profiles developed in a similar bedrock (i.e., serpentinized harzburgite; Willson Hill). The mineral mainly hosting Ni was serpentine that is formed during weathering. Comparison of the 4 weathering profiles show a positive correlation between added amounts of Ni and iron (Fe), suggesting that mobilization of Ni is constrained by the presence of ferric (Fe3+) oxides in the upper layer (i.e., limonite). This is also supported by Fe stable isotope analysis along the weathering profiles, showing that the limonite layer in Petea Hill is more depleted in the heavy 56Fe than the saprolite layers or limonite layers in other profiles. Therefore, reductive dissolution and mobilization of Fe in limonite layers are likely key geochemical processes governing the Ni enrichment on the formation of Ni laterite deposit in Indonesia.
On the other hand, in Myanmar, no significant Fe enrichment was observed even in a weathering profile where high grade Ni ores are produced (i.e., Tagaung; up 3.2 wt% as Ni in saprolite). In weathering profiles of ultramafic rocks in Myanmar, silicon (Si) is more retained in limonite layer than those in the weathering profiles in Indonesia, may reflecting different climate conditions between Myanmar and Indonesia. Furthermore, abundant smectite was observed in saprolite layer in Tagaung as a weathering product based on X-ray diffraction (XRD) analysis. Scanning electron microscope (SEM)-Energy Dispersed Spectroscopy (EDS) analysis show that some smectite grains are highly enriched in Ni up to ~20 wt%, suggesting that smectite is an important host mineral in the saprolite layer. Thus, fixation of Ni in saprolite layer may be a more important process for the Ni enrichment and the formation of Ni laterite deposit in Myanmar relative to dissolution processes of Fe and Ni in the upper layer.
In Indonesia, 4 Ni laterite profiles were examined, which are developed on peridotites with difference degrees of serpentinization in Soroako and Pomalaa areas in Sulawesi Island. Among them, Petea Hill show the highest Ni enrichment in the saprolite layer in terms of both Ni content (up to 2.9 wt% as Ni) and mass transfer coefficient (τTi < ~20), indicating that Ni was greatly added from the upper layers including those already eroded away during chemical weathering, compared with other profiles developed in a similar bedrock (i.e., serpentinized harzburgite; Willson Hill). The mineral mainly hosting Ni was serpentine that is formed during weathering. Comparison of the 4 weathering profiles show a positive correlation between added amounts of Ni and iron (Fe), suggesting that mobilization of Ni is constrained by the presence of ferric (Fe3+) oxides in the upper layer (i.e., limonite). This is also supported by Fe stable isotope analysis along the weathering profiles, showing that the limonite layer in Petea Hill is more depleted in the heavy 56Fe than the saprolite layers or limonite layers in other profiles. Therefore, reductive dissolution and mobilization of Fe in limonite layers are likely key geochemical processes governing the Ni enrichment on the formation of Ni laterite deposit in Indonesia.
On the other hand, in Myanmar, no significant Fe enrichment was observed even in a weathering profile where high grade Ni ores are produced (i.e., Tagaung; up 3.2 wt% as Ni in saprolite). In weathering profiles of ultramafic rocks in Myanmar, silicon (Si) is more retained in limonite layer than those in the weathering profiles in Indonesia, may reflecting different climate conditions between Myanmar and Indonesia. Furthermore, abundant smectite was observed in saprolite layer in Tagaung as a weathering product based on X-ray diffraction (XRD) analysis. Scanning electron microscope (SEM)-Energy Dispersed Spectroscopy (EDS) analysis show that some smectite grains are highly enriched in Ni up to ~20 wt%, suggesting that smectite is an important host mineral in the saprolite layer. Thus, fixation of Ni in saprolite layer may be a more important process for the Ni enrichment and the formation of Ni laterite deposit in Myanmar relative to dissolution processes of Fe and Ni in the upper layer.