Structural Controls on Sulfide Mobilization at the High-Grade Flying Fox Ni-Cu-PGE Sulfide Deposit, Forrestania Greenstone Belt, Western Australia

Abstract

The Flying Fox komatiite-hosted Ni deposit is located in the Forrestania greenstone belt of the Yilgarn craton in Western Australia. The host volcanosedimentary succession has been metamorphosed to amphibolite facies and deformation has sheared and dismembered the orebody, creating 11 distinct ore shoots. The ore shoots are hosted at the base of a komatiite sequence structurally overlying deformed metasedimentary rocks. The Ni sulfide orebody has undergone six phases of deformation: (1) D 1aReg : E-W compression resulted in tilting of the stratigraphy, (2) D 1bFF : continued E-W compression resulted in coaxial flattening along the footwall sedimentary rock-sulfide/komatiite contact and the basalt-hanging-wall sedimentary rock contact, (3) D 2FF : ENE-WSW compression resulted in noncoaxial shearing of the D 1bFF structures, (4) D 3FF : ESE-WNW compression resulted in shallow-angle thrusts, (5) D 4FF : during E-W extension, normal ductile-brittle faults formed by the pooling and dilation of granitic magma along the F 2 structural contact between the footwall sedimentary and komatiite rocks, and (6) D 5FF : intrusion of a Proterozoic dolerite dike during N-S extension accompanied by late-stage brittle normal faulting. During deformation the Ni sulfides reverted to monosulfide solid solution (MSS) and were mechanically relocated along the footwall sedimentary rock-komatiite contact, offset into the footwall sedimentary rocks, and entrained in the granitic magma, creating atypical granite-hosted sulfides. The Ni sulfides were mobilized up to 5 m away from the ultramafic rocks during the D 1 and D 2 events; however, subsequent dismemberment of the original orebody into separate ore shoots along flat-lying faults during the D 3 event led to offsets of up to 300 m. The detailed structural analysis of the Flying Fox deposit has shown that restoration of the lithostratigraphy along known faults can be used to define the volcanic stratigraphy, including primary Ni ore horizons. This information can be used as a guide to the location of concealed extensions to known ore shoots or completely unknown orebodies at depth or along strike.