ALBERT

Description: Lithospheric cratons withstand significant tectonic events over time due to their strong rheology and buoyancy. The Archean-Proterozoic Gawler Craton, South Australia, experienced heightened tectonic activity between 2.0 and 1.4 billion years ago, leading to a metasomatised mantle signature and world-class mineral systems including iron-oxide copper gold (IOCG) deposits and gold deposits. While the various deposit styles are often hundreds kilometers apart, vast arrays of magnetotelluric (MT) data point to a common primary architecture, in which the deposits are ultimately linked to a common metasomatised mantle source. Along the eastern margin of the Gawler Craton, the IOCG deposits have been extensively imaged with MT, including the super-giant Olympic Dam deposit. Here, we present new magnetotelluric data across a 12.5 km array and two closely spaced profiles totalling ~ 400 new broadband MT stations, across the western margin of the Gawler craton. The new data reduces the 50 km site spacing from the existing AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) by a factor of four to increase the resolution of the resistivity structure from the lower to upper crust. The scale reduction will constrain the magma and fluid pathways in between Archean lithologies present in the crust beneath the survey area and inform the genesis of the prevailing gold and nickel-copper deposits throughout the western Gawler craton. The 3D models show unprecedented insight into fossilised pathways of Proterozoic fluid flux along the craton margin.

Description: Isotropic three-dimensional (3-D) inversion has become a standard tool in the interpretation of magnetotelluric (MT) data. 3-D anisotropic inversion codes are under development, yet the number of unknowns increases by a factor of 6 rendering the problem extremely ill-posed. The presence of anisotropy is usually inferred from (i) spurious sequences of conductive and resistive bodies or (ii) comparison with two-dimensional anisotropic modelling approaches. Here, we investigate the 3-D structure of the Gawler Craton down to ∼250 km depth using 282 sites of the AusLAMP array located in the southern half of South Australia. Inversions of the MT impedance as phase tensors and real and imaginary parts result in diverging structures at depths 〉 70 km. We demonstrate that a unifying model that explains all data types similarly well is suggestive of an anisotropic resistivity structure at the base of the Gawler Craton lithosphere at depths of 120–210 km. Depth location and orientation of the anisotropy agree well with results from the analysis of seismic receiver functions. We suggest that electric anisotropy in the Gawler Craton is a result of lattice-preferred orientation of olivine crystals and metasomatic processes with macroscopic preferential orientation. Our results illustrate that inversion of phase tensor data is superior for the direct imaging of anisotropic resistivity contrasts in otherwise isotropic resistivity models; inversion models obtained with impedances may miss such structures. “Comparable” overall RMS misfits are often meaningless when comparing inversion results for various data types since sensitivities differ between data types. Reliable inversion results consistent with the entire data set can only be recovered if data fits are assessed systematically for all data representations. We also discuss the influence of error settings for phase tensors on the inversion. Our study also revealed that, if persistent across large areas, (i) parallel orientation of phase tensor major axes, (ii) constantly high phase tensor maximum phases or (iii) diverging directions of phase tensor major axes and induction arrows are suggestive of anisotropic structures and corresponding hypotheses should be evaluated.