Boudinage on radial fractures: An alternative to magmatic models for the emplacement of nickel ores, Lunnon Shoot, Kambalda, Western Australia

Abstract

Restoration of fault displacements on a section through the Lunnon Shoot is made in accordance with the general importance, noted by others, of flexural slip and ductile flow in the growth of the Kambalda Anticline. Coupled with the interpretation here that ‘normal’ and ‘reverse’ faults at Kambalda may simply be opposite walls of the same dilated fracture (one movement instead of two) this restoration allows the prism of anomalous stratigraphic sequence confined by faults and hosting the ores to be interpreted structurally‐metamorphically (‐metasomatically) rather than magmatically. The movement picture may be synoptically described as ‘boudinage on radial fractures or axial‐plane cleavage’, the structure being close to that for which the term ‘boudinage’ was originally coined. A model is proposed whereby flattening and commensurate pullapart due to tangential longitudinal strain between the footwall Lunnon Basalt and the overlying Upper (ultramafic) Sequence is focused within the contact zone occupied by the Lower (ultramafic‐sediment) Sequence (the ore sequence). Sulfur‐halogen‐rich volatiles expressed from the shales and ductile interlayered ultramafics are mobilised intraformationally commensurate with flattening, and are juxtaposed in the gaps created as the sediment units are pulled apart over the tightening anticline (the ‘zones of missing sediments'—ore zones). A boudinage model that allows for the juxtaposition of ore constituents (sulfur‐halogen‐rich volatiles and silicate nickel) in the sites that are now ore, supports the view that metamorphism has been important in the formation of the orebody, and provides scope for interpreting the entire orebody as having been structurally‐metamorphically‐metasomatically emplaced. A boudinage model is arguably simpler than magmatic models, accounting not only for all the features supporting magmatic models but also for features that magmatic models cannot explain adequately. As the Lunnon Shoot is typical of Kambalda ores, and Kambalda is the type for stratiform ultramafic‐hosted nickel deposits in Archaean greenstone belts worldwide, the currently widely accepted magmatic model could usefully be reappraised.