Ni-bearing smectites in the Wingellina laterite deposit (Western Australia) at nanoscale: TEM-HRTEM evidences of the formation mechanisms

Ni-bearing clays represent the main ore minerals in many laterite districts. Although the world-class Wingellina deposit (Western Australia) is an oxide-dominated ore, about 30% of the total Ni resources are hosted in the saprolitic section of the tenure, making the Ni-bearing clays a strategic target to increase the future ore reserves. In this study we present a detailed micro- to nanotextural TEM-HRTEM investigation of Ni-bearing smectites, which at Wingellina represent the main mineralogical species in the phyllosilicates assemblage. Ni-bearing smectites belong mainly to the dioctahedral (i.e. montmorillonite and nontronite) and to the trioctahedral (saponite and ferrosaponite) groups. The nanoscale TEM-HRTEM imaging indicates that Ni-bearing clays occur either as a replacement of the former clay suite (i.e. polygonal serpentine and chlorite) or as porous clay aggregates (i.e. PCA). Most of PCA was found closely associated with nanometric Co-bearing Mn-oxy-hydroxides. This finding not only provides a better understanding of the early stage formation of Mn-oxy-hydroxides in laterite systems, but is also important to better constrain the Co distribution and targeting within unconventional laterite ore facies. The nanoscale HRTEM imaging coupled with SAED allowed detecting minor Ni-bearing chlorite amounts, which were not found through previous XRPD-based techniques. Moreover, the paragenetic association of chlorite with primary serpentine suggests a pre-lateritic formation for chlorite. The detection of Ni-bearing trioctahedral smectite as alteration product of primary chlorite is of relevant importance, as it fills a gap in the knowledge of ore-formation processes occurring in smectite-endowed laterites.