A numerical modelling approach to fluid flow in extensional environments: Implications for genesis of large microplaty hematite ores

Numerical modelling of deformation and fluid flow on a regional scale was utilised to test recent models for deep (≥ 5 km) penetration of surface fluids involved in genesis of Whaleback style microplaty hematite ores in the Pilbara, Western Australia. Current models suggest they formed in the waning stages of the ca. 2300 Ma Ophthalmian Orogeny. This study uses a finite difference continuum modelling code, Fast Lagrangian Analysis of Continua (FLAC), to test whether deep penetration of surface fluids is mechanically feasible in relation to ore formation. Several "conceptual models" were tested and model boundary conditions were conducive to an extensional collapse of the mountain range. During extensional deformation, downward fluid flow is evident along a subvertical fault for reasonable strain rates and topographic elevation. Throughout deformation, fluids move progressively deeper into the model. Deeper seated fluid is forced upwards from the base of the model due to perturbations in hydraulic head and pore pressure. Both fluids mix at intermediate levels and this mixing process becomes progressively deeper within the model as extension takes place. Fluid mixing is apparent at the banded iron formation (BIF) and fault boundaries, as is lateral fluid migration along the BIF layers. This study supports the hypothesis that downward flow of meteoric fluid may have played a role in the evolution of the microplaty hematite ores. However, unlike the model of Morris [Morris. R.C., 1985. Genesis of iron ore in banded iron formation by supergene and supergene-metamorphic processes - a conceptual model. In: Wolff, K.H. (Ed.), Handbook of Strata-Bound and Stratiform Ore Deposits, vol. 13, Elsevier, Amsterdam, pp. 73-235] in which downward penetration is essentially superficial, the association of fluid flow with extension may have allowed deep fluid penetration (≥ 5 km) and potential fluid mixing, as proposed by Powell et al. [Geology 27 (1999) 175] and Taylor et al. [Econ. Geol. 96 (2001) 837].