Magnetotelluric evidence for a deep-crustal mineralizing system beneath the Olympic Dam iron oxide copper-gold deposit, southern Australia

Abstract

The iron oxide copper-gold Olympic Dam deposit, situated along the margin of the Proterozoic Gawler craton, South Australia, is the world's largest uranium deposit and sixth-largest copper deposit; it also contains significant reserves of gold, silver, and rare earth elements. Gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses Olympic Dam and the major crustal boundaries. In this paper we present results from 58 long-period (10-10 4 s) MT sites, with site spacing of 5-10 km. A two-dimensional inversion of MT data from 33 sites to a depth of 100 km shows four notable features: (1) sedimentary cover sequences with low resistivity (Ͻ20 ⍀·m) thicken to 10 km toward the northern cover sequences of the Adelaide Rift Complex; (2) a northeast-dipping crustal boundary separates a highly resistive (Ͼ1000 ⍀ ·m) Archean crustal core from a more conductive crust and mantle to the north (typically Ͻ500 ⍀·m);(3) to the north of Olympic Dam, the upper-middle crust to ϳ20 km is quite resistive (ϳ1000 ⍀·m), but the lower crust is much more conductive (Ͻ100 ⍀ ·m); and (4) beneath Olympic Dam, we image a low-resistivity region (Ͻ100 ⍀·m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the lowresistivity and low-reflectivity region beneath Olympic Dam may be due to the upward movement of CO 2 -bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

“…Le Pape et al 2012;Kühn et al 2014), characterizing the geoelectrical subsurface in a large variety of regions and for both academic and commercial applications (e.g. Heinson et al 2006;Falgàs et Complementing MT profiles with T and H data 1819 Campanyà et al 2011;La Terra & Menezes 2012;Ogaya et al 2014;Piña-Varas et al 2014). However, limitations of the MT method still exist and there is ample room for improvement.…”

New government geophysics from the Onshore Energy Security Program

“…The MT may be imaging a deep fluid alteration system at a major crustal boundary, suggesting the area could be prospective for iron-oxide-copper-gold deposits. Large conductivity anomalies have also been detected at depth beneath the giant Olympic Dam copper-gold-uranium mine (Heinson et al, 2006). The Moho layer is evident on the southern end of the seismic sections as the boundary between an upper reflective zone and a lower non-reflective zone at about 16s TWT (~50km).…”

Evolving 3D lithospheric resistivity models across southern Australia derived from AusLAMP MT

“…Carapateena, Punt Hill). This conductor is known to connect to the surface at these deposits (Soeffky et al, 2016;Heinson et al, 2006). At a depth of 50 km this conductor extends around to link the Olympic Domain to the Mount Woods Inlier, (host to the Prominent Hill Cu-AuAg mine), the Mabel Creek Ridge and the NE-SW Karari Shear Zone, almost reaching the coast just to the east of the Coompana Block.…”

“…Regions of anomalous conductivity may underlie regions of mineralogical potential such as that which occurs along the IOCG belt in the Olympic Domain (Heinson et al 2006) and the Beverley Uranium mine (Thiel et al 2016b). Finalising of models in the coming months will further illuminate conductive regions that can be targeted for future infill surveys at higher resolution.…”