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The role of secondary processes on the mobilization of nickel sulfides at the high-grade komatiite-hosted Ni-Cu-PGE Flying Fox deposit, Forrestania Greenstone Belt, WA

Forrestania Greenstone Belt, WA

The aim of this project is to constrain the post-depositional (secondary) controls recognised at the Flying Fox deposit - including structural, metamorphic, hydrothermal, and magmatic processes - in order to define an integrated deposit model which can be used  for exploration. 

The study focussed on the structural evolution of the Flying Fox deposit which demonstrated the importance of the interplay between volcanology, deformation, prograde metamorphism, and magmatism in controlilng the present distribution and geometry of Ni ore shoots. The effects of the secondary structural processes on the ore body has resulted in up to three phases of mechanical moblization, and segmentation of one original ore body into no less than 11 distinct ore shoots with up to 300 m offset between them. The granular ore textures and only localised exsolution of pentlandite in pyrrhotite suggests that during upper-amphibolite facies metamorphism the nickel sulfides reverted to MSS, which aided their ductile mechanical mobilization and inihibited any deformation fabrics to be preserved because the MSS annealed to polysulfide assemblages post-peak metamorphism. The intrusion of granitic magma created the atypical granite-hosted breccia ore as it entrained the nickel sulfides during dilation. The ore shoots at the Flying Fox deposit are characterized by a strong variability in nickel tenor, which is unrelated to mineralization style, and significant pyrite content, which is inconsistent with the exsolution from MSS. The PGE content of the sulfide ores are similarly variable, however the linear correlation between IPGE's and Rh, and the Pd/Ir versus Ni/Cu ratios indicate an underlying magmatic control. Bulk composition trends within the sulfide ores have been modelled as a combination of simple pyrite addition to a "primary" magmatic ore, with a component of pure S addition, possibly in the form of sulfidation of pyrrhotite. Both hydrothermal and mechanical processes are proposed to be responsible for the addition of pyrite resulting in the variability of Ni within the deposit. Currently work is being undertaken to date the granitic intrusion and define its geochemical signature in order to understand the timing and role of the granitic rocks in moblization of the nickel sulfide ores.