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 (10–104 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 (,20 V·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 (.1000 V·m) Archean crustal core from a more conductive crust and mantle to the north (typically ,500 V·m); (3) to the north of Olympic Dam, the upper-middle crust to ;20 km is quite resistive (;1000 V·m), but the lower crust is much more conductive (,100 V·m); and (4) beneath Olympic Dam, we image a low-resistivity region (,100 V·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 CO2-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.