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

Restoration of fault displacements on a section through the Lunnon Shoot is made in accordance with the general importance, noted by others, of flexur al slip and ductile flow in the growth of the Kambalda Anticline. Coupled w ith the interpretation here that 'normal' and 'reverse' faults at Kambalda may simply be opposite walls of the same dilated fracture (one movement ins tead of two) this restoration allows the prism of anomalous stratigraphic s equence confined by faults and hosting the ores to be interpreted structura lly-metamorphically (-metasomatically) rather than magmatically. The moveme nt picture may be synoptically described as 'boudinage on radial fractures or axial-plane cleavage', the structure being close to that for which the t erm 'boudinage' was originally coined, A model is proposed whereby flatteni ng and commensurate pullapart due to tangential longitudinal strain between the footwall Lunnon Basalt and the overlying Upper (ultramafic) Sequence i s focused within the contact zone occupied by the Lower (ultramafic-sedimen t) Sequence (the ore sequence). Sulfur-halogen-rich volatiles expressed fro m the shales and ductile interlayered ultramafics are mobilised intraformat ionally commensurate with flattening, and are juxtaposed in the gaps create d 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 s ilicate nickel) in the sites that are now ore, supports the view that metam orphism has been important in the formation of the orebody, and provides sc ope for interpreting the entire orebody as having been structurally-metamor phically-metasomatically emplaced. A boudinage model is arguably simpler th an magmatic models, accounting not only for all the features supporting mag matic models but also for features that magmatic models cannot explain adeq uately. As the Lunnon Shoot is typical of Kambalda ores, and Kambalda is th e type for stratiform ultramafic-hosted nickel deposits in Archaean greenst one belts worldwide, the currently widely accepted magmatic model could use fully be reappraised.