Mineralogy and chemistry of Ni phyllosilicates in the Wingellina Ni Co laterite deposit (Western Australia): alteration processes of mafic to ultramafic lithologies

Ni-phyllosilicates are the main phases in the saprolite zone of the Wingellina Ni-Co laterite deposit (Western Australia). In this study, we analyzed the mineralogy and chemistry of the fine-clay fraction of the saprolite from two different zones of the deposits. Our results highlight that the mineralogy, chemistry and general paragenesis of clays reflect the heterogeneity of the underlying bedrock, as well as the different alteration history recorded in two drill cores. The mineralogy of the saprolite zone lying on the gabbro bedrock is dominated by dioctahedral smectite clays (montmorillonite), which developed directly from the weathering of pyroxenes. On the contrary, the alteration of olivine resulted in the precipitation of Fe-(oxy)-hydroxides. The early stage alteration of the ultramafic unit (peridotite) involved the serpentinization of the ferromagnesian minerals. This process was followed by a Fe-(oxy)-hydroxides/quartz and smectite precipitation stage, with the latter mineral belonging both to the tri- and di-octahedral types (i.e. saponite- and nontronite-like). The last local alteration process consisted in the formation of Ni-rich mixed-layers minerals, such as talc/smectite and chlorite/smectite. The genesis of chlorite/smectite could be potentially due to the low temperature diagenetic process that occurred afterward the main lateritization phases.