Parent rock and climatic evolution control on the genesis of Ni-bearing clays in Ni-Co laterites: New inferences from the Wingellina deposit (Western Australia)

Oxide-type Ni-laterites seldom contain significant Ni-phyllosilicates reserves. This study is focused on the mineralogy of Ni-clays mainly occurring in the saprolite zone of the Wingellina oxide-type Ni-Co laterite deposit. Despite most of the Wingellina reserves occur in the oxide zone, the deposit also has a well-developed high Ni grade saprolite horizon. This study presents a mineralogical and chemical characterization of the Ni-phyllosilicates of the Wingellina saprolite ore, aimed to understand the role of diverse bedrocks (gabbro and serpentinite) and the influence of climate on the formation of different clay types. The main phyllosilicate in the gabbro-derived saprolite consists of dioctahedral Al- and Ni-montmorillonite, originated from the weathering of pyroxene. The genesis of Al- and Ni-rich montmorillonite from pyroxene can be explained by an open system alterative process, in which Al mainly derives from the dissolution of plagioclase. In the serpentinite-derived saprolite, the alteration pathway followed a multistage evolution: (i) lateritic alteration of serpentine I into Ni-serpentine II and tri- and dioctahedral smectites (saponite and nontronite); (ii) late precipitation of interstratified Ni-clays (chlorite-smectite and talc-smectite) as a replacement of former phyllosilicates and as neoformed minerals. The latest phenomenon may be related to the late Miocene shift from humid-and-acid (tropical climate) to saline-and-alkaline conditions (arid to semi-arid climate), and represents an example of post-lateritization processes, rarely observed in laterites located on stable cratons. The aridity-driven precipitation of Ni-rich interstratified clays in the Wingellina laterite can be seen as a new model for explaining the evolution of Ni-laterites in stable cratonic settings experiencing post-lateritization aridity.