Evolution of the Starra and Selwyn high-strain zones, Eastern fold belt, Mount Isa Inlier; implications for Au-Cu mineralization

Abstract

The Starra (magnetite) ironstone-hosted Au-Cu deposit in the Eastern fold belt, Mt. Isa inlier, is localized by the development of the Starra high-strain zone. The associated eastern (barren, hematite-rich) ironstones occur in the adjacent Selwyn high-strain zone. Integrated macro-, meso-, and microstructural analysis of rocks from both of these high-strain zones and the surrounding country rocks reveals that they have undergone the same sequence of successive shallow and steeply oriented deformation events. All of these rocks preserve evidence for four main ductile deformation events, D 1 to D 4 (D 1 D 2 , and D 4 equate with regional D 1 , D 2 , and D 3 , respectively, as observed by workers in the Mount Isa region) followed by two weaker events (D 5 -D 6 ), which exhibit both ductile and brittle character.D 1 produced a shallowly dipping, layer-parallel foliation. D 2 is the most pervasive event and produced upright, tight to isoclinal F 2 folds with an intense, steeply southeast-dipping axial plane foliation (S 2 ) trending northeast-southwest. D 3 rotated initially upright F 2 folds (onto their western limb) by top-to-the-west shear and formed crenulations with shallowly dipping, east-west-striking axial planes. The fourth deformation event (D 4 ) produced a steeply east-dipping foliation (S 4 ), which is similar in orientation to S 2 . Consequently, differentiation of S 2 from S 4 is difficult, except where S 2 was rotated by D 3 . S 4 , when observed, is generally developed as a broad crenulation cleavage. Microstructural analysis of 115 spatially oriented thin sections from the vicinity of the Starra orebodies demonstrates that the dominant fabric is a strongly differentiated composite S (sub 2-4) foliation rather than simply S 2 , as had been thought previously.The Starra and Selwyn high-strain zones resulted from a significant increase in strain during D 4 and preserve a predominant shear sense of west-side-up. The western (mineralized) and eastern (unmineralized) ironstones resulted from intense iron metasomatism and were emplaced as en echelon north-northeast-striking lenses late syn-/post-D 4 . The rocks between the high-strain zones young consistently eastward, hence duplication of the ironstones by folding is precluded. Au-Cu mineralization within the Starra orebodies was emplaced during and after iron metasomatism late syn- to post-D 4 .