Ni-PHYLLOSILICATE ORES FROM Ni-LATERITE DEPOSITS (Ni-SERPENTINE AND GARNIERITE): STRUCTURE, CHEMISTRY AND ORE FORMING PROCESSES UNRAVELED BY EPMA, HRTEM, RAMANSPECTROSCOPY, XANES AND EXAFS

摘要

Ni-Co-laterites are regolith deposits formed by weathering of ultramafic rocks under temperatureand rainfall conditions of tropical latitudes . During weathering, the most soluble elements areleached from the protolith, and the least mobile elements accumulate in successive horizons of thelateritic profile: the lower, saprolite horizon (Ni-serpentine) and the upper, limonite horizon(Ni-adsorbed Fe-oxyhydroxides) [1]. In the Falcondo mining district (Dominican Republic), as inmany other hydrous silicate type Ni-laterite deposits worldwide, the main Ni ores area) Ni-serpentine that forms most of the bulk saprolite horizon (< 1 wt% NiO), and b) garnierites,which occur as vein infillings at the lowermost part of the profile (2 - 40 wt% NiO) . Garnieritesare green, fine grained and poorly crystalline mixtures of Ni-Mg-phyllosilicates. This group ofminerals had been extensively studied during the 1970s and gained interest recently, not onlybecause they are significant Ni ores but because they are the natural analogues of synthetic catalystsinvolving Ni and phyllosilicate substrates. However, their characterisation, classification andnomenclature is still challenging . The aim of this study is to characterise structural, texturaland chemical variations of different garnierite types and Ni-serpentine, in order to shed light ontothe supergene processes that lead to the formation of these minerals.