Lithospheric Architecture and Mantle Metasomatism Linked to Iron Oxide Cu‐Au Ore Formation: Multidisciplinary Evidence from the Olympic Dam Region, South Australia

Abstract

The role of lithospheric architecture and the mantle in the genesis of iron oxide copper‐gold (IOCG) deposits is controversial. Using the example of the Precambrian Gawler Craton (South Australia), which hosts the giant Olympic Dam IOCG deposit, we integrate geophysical data (seismic tomography and magnetotellurics) with geological and geochemical data to develop a new interpretation of the lithospheric setting of these deposits. Spatially, IOCG deposits are located above the margin of a mantle lithospheric zone with anomalously high electrical conductivities (resistivity <10 ohm.m, top at ~100–150 km depth), low seismic shear‐wave velocities (horizontal component, Vsh < 4.6 km/s), and unusually high ratios of compressional‐ to shear‐wave velocities (Vp/Vsh > 1.80). The high conductivity cannot be explained by water‐bearing olivine‐rich rock alone. Relatively fertile and metasomatized peridotitic mantle with additional high‐Vp/Vs phases, for example, clinohumite, hydrous garnet, and/or phlogopite, could explain the anomalous velocity and conductivity. The top of this high‐Vp/Vsh zone marks a midlithospheric discontinuity at ~100–130 km depth that is interpreted to reflect locally orthopyroxene‐rich mantle. A sub‐Moho zone with high Vp/Vsh at ~40–80 km depth correlates spatially with primitive Nd isotope signatures and arc‐related ~1,620–1,610 Ma magmatism and is interpreted as the eclogitic root of a magmatic arc. Mafic volcanics contemporaneous with ~1,590 Ma IOCG mineralization have geochemistry suggesting derivation from subduction‐modified lithospheric mantle. We suggest that Olympic Dam formed inboard of a continental margin in a postsubduction setting, related to foundering of previously refertilized and metasomatized lithospheric mantle. Deposits formed during the switch from compression to extension, following delamination‐related uplift and exhumation.

Proxies for Basement Structure and Its Implications for Mesoproterozoic Metallogenic Provinces in the Gawler Craton

“…Our interpreted crustal architecture aligns with the mantle architecture interpreted by Skirrow et al () (Figure ), where both IOCG and CGG deposits lie along major structural breaks that connect different compartments of the mantle to the crust along long‐lasting lithospheric scars (in the sense of Heron et al, ). The oldest crustal fragment represented by Domain C is mostly rooted by the metasomatized lithospheric mantle.…”

“…The interpreted crustal framework of the Gawler Craton in contrast to the mantle lithospheric structure by Skirrow et al (). (1) Interpretations and data provided by Skirrow et al () and (2) for data from Thiel and Heinson ().…”

“…Areas that are proximal to crustal boundaries with a prolonged crustal reworking history are recognized as important sites for orogenic gold deposits to form (Huston et al, ). In contrast, the IOCG deposits lie in an inward position to the crustal domain boundaries during Mesoproterozoic tectonism, likely a back‐arc setting above a highly heterogeneous, evolved crust, forming in a postsubduction setting (Skirrow et al, ). Following this rationale, the Olympic Province is located on the upper plate of a suture zone where the lithosphere is characterized by the hydrated mantle (Skirrow et al, ), which was refertilized during plume impingement (Betts et al, ).…”

“…In contrast, the IOCG deposits lie in an inward position to the crustal domain boundaries during Mesoproterozoic tectonism, likely a back‐arc setting above a highly heterogeneous, evolved crust, forming in a postsubduction setting (Skirrow et al, ). Following this rationale, the Olympic Province is located on the upper plate of a suture zone where the lithosphere is characterized by the hydrated mantle (Skirrow et al, ), which was refertilized during plume impingement (Betts et al, ). Furthermore, our results suggest that the formation of the Mesoproterozoic IOCG deposits in the Gawler Craton could have formed similarly to mineral deposits formed in the Phanerozoic Andean chain (Rosenbaum et al, ; Sillitoe, ), a notion previously introduced by Richards and Mumin () on different grounds (e.g., overall hydrothermal alteration and isotope geochemistry).…”

“…Both provinces formed during the earliest Mesoproterozoic (approximately 1,600 to 1,570 Ma; Skirrow et al, 2007;Fraser et al, 2007) over an Archean-Palaeoproterozoic cratonic shield (Hand et al, 2007). Models for the genesis and geodynamic setting of these provinces are varied, including plumemodified orogenesis and magmatism (Betts et al, 2007(Betts et al, , 2009, the development of extensional sedimentary basins (Cherry et al, 2017;McPhie, Kamenetsky, Chambefort, et al, 2011;McPhie et al, 2016), a convergent plate-boundary settings (Direen & Lyons, 2007;Fraser et al, 2007), and postsubduction settings (Skirrow et al, 2018). Despite significant differences between these models, most suggest that the craton-scale magmatism related to the emplacement of the early Mesoproterozoic Hiltaba Suite plutons and coeval Gawler Range Volcanics (Allen et al, 2008;Betts et al, 2009) supplied the thermal energy that drove crustal-scale fluids and associated mineralization Fraser et al, 2007;Kontonikas-Charos et al, 2017;Oreskes & Einaudi, 1992;Skirrow et al, 2007).…”