17:15 〜 18:45
[HRE13-P12] ニッケルラテライト鉱床における低結晶質ニッケルホスト鉱物の同定
キーワード：化学風化、超苦鉄質岩、スメクタイト、ラマン分光法、低結晶質鉱物
Ni laterite deposits are formed by chemical weathering of ultramafic rocks in tropical to sub-tropical climates. Regardless of the significance in the mineral exploration and processing, host phases of Ni in the ores are not readily identifiable by conventional mineralogical analyses due to presence of amorphous and low-crystalline weathering products. In this study, we applied Scanning Electron Microscopy (SEM)-Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and Raman spectroscopy to determine the Ni-bearing minerals and their formation processes in Ni laterite deposits from Indonesia (Peter Hill in Soroako mine) [1] and Myanmar (Tagaung Taung mine) [2], which are formed under different climatic conditions.
The results of Raman spectra and SEM-EDS indicate that low-crystalline Ni-bearing minerals in saprolite layer that were not mineralogically identified in our previous study [2], were classified as either serpentine or smectite. Considering Ni contents and their abundance, the major Ni host phases were identified as serpentine and smectite in Indonesia and Myanmar, respectively. In Indonesia, serpentine tends to increase the Ni contents with decreasing (Mg+Fe+Ni)/Si ratios due to substitution of divalent ion to trivalent ion in the octahedral site of serpentine as weathering progresses toward the upper part of the saprolite layer. TEM observation indicate that the crystal structure of serpentine maintained in the upper saprolite layer although their chemical compositions (i.e., (Mg+Fe+Ni)/Si ratio = ~1.2) is far from the ideal chemical compositions ((Mg+Fe+Ni)/Si ratio = 1.5). The presence of this type of serpentine likely plays an important role in Ni enrichment in the saprolite layers in Indonesia as serpentine may fix Ni more efficiently compared to goethite, the main Ni bearing mineral in the low-grade laterite layers. On the other hand, in Myanmar, the results of Raman spectra and electron diffraction of TEM analysis revealed that smectite were low crystalline in the lower saprolite layer and coexist with serpentine in the nano-scale. Chemical compositions of the smectite were also variable. In contrast, smectite in the upper saprolite layer is crystalline and exhibit nontronite-like chemical compositions. Therefore, smectite in the Myanmar Ni laterite deposits likely becomes more crystalline with the crystal growth as weathering progresses, probably due to the supersaturation of Si with combination of Fe oxidation. These factors may have been governed by the pore water chemistry with relatively low rainfall in Tagaung Taung area compared with Indonesia. Because smectite is the important Ni host mineral in the deposit, the geochemical conditions and processes feasible for the formation of smectite is likely critical for Ni enrichment in Ni laterite deposits in Myanmar.
[1] Ito, A., Otake, T., Maulana, A., Sanematsu, K., Sufriadin, Sato, T. (2021) Resour. Geol. 71. 255-82.
[2] Murofushi, A., Otake, T., Sanematsu, K., Zay Ya, K., Ito, A., Kikuchi, R., and Sato, T. (2022) Miner. Deposita.
The results of Raman spectra and SEM-EDS indicate that low-crystalline Ni-bearing minerals in saprolite layer that were not mineralogically identified in our previous study [2], were classified as either serpentine or smectite. Considering Ni contents and their abundance, the major Ni host phases were identified as serpentine and smectite in Indonesia and Myanmar, respectively. In Indonesia, serpentine tends to increase the Ni contents with decreasing (Mg+Fe+Ni)/Si ratios due to substitution of divalent ion to trivalent ion in the octahedral site of serpentine as weathering progresses toward the upper part of the saprolite layer. TEM observation indicate that the crystal structure of serpentine maintained in the upper saprolite layer although their chemical compositions (i.e., (Mg+Fe+Ni)/Si ratio = ~1.2) is far from the ideal chemical compositions ((Mg+Fe+Ni)/Si ratio = 1.5). The presence of this type of serpentine likely plays an important role in Ni enrichment in the saprolite layers in Indonesia as serpentine may fix Ni more efficiently compared to goethite, the main Ni bearing mineral in the low-grade laterite layers. On the other hand, in Myanmar, the results of Raman spectra and electron diffraction of TEM analysis revealed that smectite were low crystalline in the lower saprolite layer and coexist with serpentine in the nano-scale. Chemical compositions of the smectite were also variable. In contrast, smectite in the upper saprolite layer is crystalline and exhibit nontronite-like chemical compositions. Therefore, smectite in the Myanmar Ni laterite deposits likely becomes more crystalline with the crystal growth as weathering progresses, probably due to the supersaturation of Si with combination of Fe oxidation. These factors may have been governed by the pore water chemistry with relatively low rainfall in Tagaung Taung area compared with Indonesia. Because smectite is the important Ni host mineral in the deposit, the geochemical conditions and processes feasible for the formation of smectite is likely critical for Ni enrichment in Ni laterite deposits in Myanmar.
[1] Ito, A., Otake, T., Maulana, A., Sanematsu, K., Sufriadin, Sato, T. (2021) Resour. Geol. 71. 255-82.
[2] Murofushi, A., Otake, T., Sanematsu, K., Zay Ya, K., Ito, A., Kikuchi, R., and Sato, T. (2022) Miner. Deposita.