The petrography, stratigraphy and structure of the Osborne Mine sequence: evidence for the origin of banded iron-formation-hosted Cu-Au deposits in the Soldiers Cap group, northwest Queensland

Two stratabound banded iron-formations (BIFs) host epigenetic Cu-Au mineralisation in multiply deformed metasediments of the Osborne mine, within the Cloncurry-Selwyn zone of the Mount Isa Eastern Succession. The Proterozoic host rocks have been subdivided into seven conformable units that are cut by amphibolites and pegmatites, affected by four phases of ductile deformation (Dx-D3), amphibolite facies metamorphism and later alteration involving the formation of sulphide minerals. Evidence for the first deformation, labelled "Dx" because of its uncertain age and extent, is restricted to straight inclusion trails (Sx) within some porphyroblasts of garnet, cordierite and tourmaline. Di is labelled to conform with existing nomenclature involving the regional 1595 Ma Isan Orogeny, which resulted in a lineation in amphibolites (Li) and a widespread bedding-parallel foliation in metasediments (Si), which is also parallel to the axial surfaces of small asymmetrical Group 1 folds. Peak metamorphic mineral assemblages, which developed during D1, include garnet-cordierite-sillimanite in metapelitic gneisses, suggesting metamorphic conditions of T=746+/-136° C and P=5.1+/-1.0 kbar. Refolding of So and Si during D2 resulted in the localized development of small symmetrical Group 2 folds, and D3 produced sets of near-parallel Group 3 kink bands with curvi-planar S2 axial surfaces. Post folding Au-Cu minerals, quartz, pyrite and magnetite formed by late replacement (post D3) of lenses of brecciated BIF. Brecciation is most intensely developed near contacts between BIFs and quartzo-feldspathic units, and formed as a result of strain incompatibility during post D3 brittle deformation. Narrow retrograde shear zones displace and thus postdate all metasedimentary units, intrusions and ore-bearing breccias. The relative displacements of marker units suggest that the net movement along the majority of shears was normal and non-rotational. The BIFs are inferred to be metamorphosed syn-sedimentary units that were affected by all stages of deformation (Dx-D3), and later Au-Cu mineralisation. A sedimentary origin for the BIFs is consistent with local and regional features, such as the stratigraphic restriction of BIFs to the Soldiers Cap Group, the conformable nature and lateral continuity of the BIF units, and the delicate bedding-parallel laminations of quartz-magnetite and quartz-magnetile-cummingtonitc.