Genesis of phyllosilicates in the Wingellina Ni-Co laterite deposit (Western Australia)

Wingellina is a large Ni-Co laterite deposit, located in Western Australia. The laterite pro le formed from the weathering of the Mesoproterozoic ma c and ultrama c intrusions of the Giles Complex, mainly consisting of peridotite and gabbro. The exploitable Ni-Co resources of the deposit are represented by Ni-Co-bearing Fe/ Mn-oxy-hydroxides mostly occurring in the more sur cial limonite horizon (probable Reserve = 168 Mt @ 0.98% Ni and 0.08% Co). This makes the Wingellina deposit an unusual “oxide-type” laterite. The deeper saprolite horizon has a lower tonnage, but contains higher Ni concentrations (up to ~3 wt%). We have studied the mineralogy of the Ni-phyllosilicates from saprolite samples of the Wingellina deposit collected from two different pro les, overlying a gabbro and a serpentinite (former peridotite) bedrock, respectively. From mineralogical and petrographic analyses, we determined that the initial alteration stage of the gabbro was characterized by the formation of nontronite and montmorillonite, at the expenses of olivine and pyroxene, respectively. The congruent dissolution of plagioclase played an important role, supplying the Al required for montmorillonite formation. The following stage was represented by the alteration of montmorillonite, resulting in the precipitation of kaolinite. This paragenetic sequence is in agreement with a typical lateritization pathway. The alteration of the serpentinite followed instead a multistage route. During the initial lateritization stage, the original serpentine was altered to a Ni-rich srp II, as well as to tri- and dioctahedral smectites (saponite and nontronite). The chemistry of smectites suggests that the saponite-to-nontronite conversion developed through a progressive increase of the Fe content within trioctahedral smectite. In a following stage, in the upper section of the serpentinite-derived saprolite, disordered talc-smectite and chlorite-smectite interstrati ed clays formed at the expenses of the former serpentines and smectites. This process is not typical of humid lateritic environments, but common under arid climates characterized by circulation of saline and alkaline uids. In the Wingellina area, the onset of arid climate during the Late Miocene-Pliocene (Anand and Paine, 2002) likely promoted the formation of these interstrati ed clays. Taking into the nature and the late stage formation of disordered interstrati ed clays in the Wingellina saprolite, the evaporative-driven clays authigenesis can be considered as a counterpart of the per descensum process that explains the garnierite formation in active margins.