Multiscale physical–chemical analysis of the impact of fracture networks on weathering: Application to nickel redistribution in the formation of Ni-laterite ores, New Caledonia

Abstract

Supergene processes related to downward water migration and dissolution of primary minerals in the New Caledonian peridotites result in Ni enrichment at the bottom of the lateritic profile near bedrock. However, the combined effects of relief, lateral fluid flow, and complex inherited discontinuity networks filled by serpentine generate a more complex Ni distribution than a simple flat horizon. In surficial environments, fluid flow and mass transfer are not limited to the vertical scale; they can also be related to lateral fluid circulation and local drainage along damage zones in the vicinity of faults. In this study, we present new structural, petrophysical and geochemical data for core samples to shed light on the impact of a fracture network on the alteration and progression of the Ni-enrichment front during weathering. The analysis of element redistribution by XRF mapping coupled with fracture extraction from computed tomography (CT) establishes that the serpentine fracture network comprises sets of sub-orthogonal fractures whose orientations are potentially determined by pre-existing magmatic foliation. The progression of the olivine dissolution front leads to the redistribution and concentration of Ni into the serpentine fracture network as secondary Mg-Ni talc-like phases. The increase in porosity from < 1 % in the bedrock to > 10 % in the saprock is mainly driven by the formation of nanopores. Nanoporosity may control the development of the weathering front and the redistribution of Ni by small-scale diffusion. Structural and geochemical characteristics identified at the sample scale are analogous to those observed on the larger scale of the outcrop and deposit. Scale invariance in the structural organization and geochemical evolution indicates the potential upscaling of the processes involved in the redistribution of Ni.