Asymmetric Extension of the lithosphere and its influence on Palaeoproterozoic Pb-Zn deposits of the Western Fold Belt, Mount Isa terrane

Peter Betts

The Palaeoproterozoic Isa Superbasin hosts several large Pb-Zn-Ag deposits and is arguably one of the richest mineral provinces in the world. This basin initially evolved during a NW-SE directed extensional event termed the Mount Isa Rift Event between ~1708 Ma and ~1653 Ma. A sag basin continued to evolve thereafter until ~1595 Ma. There is a notable difference in the locus of syn-rift sedimentation, syn-rift magmatism, and post-rift subsidence. Tectonic subsidence was focussed along the ~N-S oriented Mount Isa Rift where 3-5 km of fluvial to shallow marine clastic sediments were deposited. Magmatic provinces, along the western and north western rift flanks, mark the locus of significant sub crustal lithospheric thinning, asthenospheric upwelling, and mafic underplating, and are the locations of palaeogeographic highs during the rifting. The position of maximum sub crustal lithospheric extension is determined by the position of greatest post-rift subsidence. This occurred beneath the northern Mount Isa terrane where the thickest post-rift sequences are preserved and the depositional history is more protracted. These relations indicate the Mount Isa Rift Event involved asymmetric lithosphere extension.

This lithospheric architecture has significant implications for exploration of large sediment hosted Pb-Zn orebodies in the Western Fold Belt. These deposits spatially associated with faults active during the Mount Isa Rift Event but they are hosted within post-rift sequences of the Isa Superbasin. The spatial distribution of the deposits on the flanks of the Mount Isa Rift shifts away from conventional models for the formation of SEDEX deposits that are generally considered to form within rifts. The asymmetric extension lithosphere model provides the insight into the distribution of deposits. Mineralisation is coincident with maximum asthenospheric upwelling that provides a heat source for driving fluid flow throughout the basin. In addition, the location of maximum post rift subsidence may enhance the appropriate redox geochemical conditions for precipitation of base metals. This does not necessitate syn-depositional mineralisation. Finally, if fluid flow was gravity driven, then the asymmetric extension model for the Isa Superbasin predicts fluid flow to the deepest parts of the sag-basin in the northern Mount Isa terrane. Extensional faults may have provided the local, upper crustal control for mineralisation. Key words: Asymmetric extension, Isa Superbasin, Mount Isa Rift Event, Pb-Zn mineralisation, Mount Isa.