Deformation-driven, regional-scale metasomatism in the Hamersley Basin, Western Australia

Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000km(2) Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite-pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc, in subsets of the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-Fe2O3, successfully reproduces observed metamorphic mineral assemblages, giving conditions of similar to 210 degrees C, 2kbar in the north and 335 degrees C, 3.2kbar in the south. Superimposed on this metamorphic gradient, regional-scale metasomatism in the Fortescue Group progressively produces a suite of prehnite-bearing and pumpellyite-quartz/epidote-quartz-dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260-280 degrees C, 2.5-3kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional-scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.