The crustal geophysical signature of a world-class magmatic mineral system

Abstract

World-class magmatic mineral systems are characterised by fluid/melt originating in the deep crust and mantle. However, processes that entrain and focus fluids from a deep-source region to a kilometre-scale deposit through the crust are unclear. A magnetotelluric (MT) and reflection seismic program across the margin of the Gawler Craton, Australia yield a distinct signature for a 1590 Ma event associated with emplacement of iron-oxide copper gold uranium (IOCG-U) deposits. Two- and three-dimensional MT modelling images a 50 km wide lower-crustal region of resistivity <10 Ωm along an accreted Proterozoic belt. The least resistive (~1 Ωm) part terminates at the brittle-ductile transition at ~15 km, directly beneath a rifted sedimentary basin. Above the brittle-ductile transition, three narrow low-resistivity zones (~100 Ωm) branch to the surface. The least resistive zone is remarkably aligned with the world-class IOCG-U Olympic Dam deposit and the other two with significant known IOCG-U mineral occurrences. These zones are spatially correlated with narrow regions of low seismic reflectivity in the upper crust, and the deeper lower-crust conductor is almost seismically transparent. We argue this whole-of-crust imaging encapsulates deep mineral system and maps pathways of metalliferous fluids from crust and mantle sources to emplacement at discrete locations.

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

“…This is evident from the regional alteration and chemical footprint observed in both northern and southern sectors of the IOCG province (Kontonikas‐Charos et al, , ) and similarities in timing and composition of the hydrothermal alterations between the IOCG and CGG provinces (Bastrakov et al, ; Fraser et al, ). This fundamental role of the major structures as pathways persists through time due to reactivation associated with tectonic switches, as previously envisaged around the Olympic Dam deposit (Direen & Lyons, ; Heinson et al, , ; Korsch & Doublier, ; Stewart & Betts, , ). However, the resistivity structure could also have been affected throughout the late Mesoproterozoic or by younger events (Foden et al, ; Hall et al, ).…”

“…The collocation of the Hiltaba Suite plutons, which are hosts to the deposits in the Olympic province (Direen & Lyons, ; Skirrow et al, ), has been interpreted to be controlled by regional shear zones (McLean & Betts, ). In the vicinity of the Olympic Dam deposit, previous interpretations on deep seismic reflection data point out to the presence of a representative step in the Moho in the vicinity of the Olympic Dam deposit (Drummond et al, ; Figure S1c) and regional joint low‐resistivity and ‐impedance structures (Heinson et al, ; Wise et al, ).…”

“…The electrical conductors observed across the craton (e.g., Figures , , and ) are interpreted as zones of fluid flow along shear zones probably during the Paleoproterozoic to Mesoproterozoic, which decreased the resistivity through aqua‐carbonic fluid entrapment (Heinson et al, ; Thiel & Heinson, ) or sulfide precipitation (Heinson et al, ). The significant domain boundaries within Gawler Craton (Figures b and ) appear to have acted as agents for energy and fluid flux between different geochemical reservoirs across the craton basement along secondary structures (Bastrakov et al, ; Direen & Lyons, ; Fraser et al, ; Haynes et al, ; Kontonikas‐Charos et al, , ; Uvarova et al, ) including shallow levels in the overlying sedimentary basins and basement (Cherry et al, ; Reid & Fabris, ).…”

“…There is little consensus about the source of hydrothermal fluids associated with the IOCG Province, with genetic models proposing a crustal source (Cherry et al, 2017;McPhie, Kamenetsky, Allen, et al, 2011;McPhie, Kamenetsky, Chambefort, et al, 2011;McPhie et al, 2016), mantle/magmatic source (Creaser & Cooper, 1993;Ciobanu et al, 2013;Heinson et al, 2018;Johnson & McCulloch, 1995;Kirchenbaur et al, 2016;Kontonikas-Charos et al, 2017;McPhie, Kamenetsky, Allen, et al, 2011;Schlegel et al, 2016), and mixing of fluids from different sources Haynes et al, 1995;Huston et al, 2016;Skirrow et al, 2007Skirrow et al, , 2018Uvarova et al, 2017). The formation of the CGG deposits is not clear, as they record characteristics of both orogenic gold and intrusion-related systems (Fraser et al, 2007).…”

“…Magnetic, seismic reflection and magnetotelluric data have provided information about the near-surface controls on mineralization throughout the Gawler Craton (e.g., Direen & Lyons, 2007;Gow et al, 1994). Several crustal-scale fluid pathways responsible for fluid transport through the craton's basement and cover sequences have been identified in regional seismic reflection data (Betts et al, 2016, Drummond et al, 2006Wise et al, 2015, see Figure S1 in the supporting information), deep imaging magnetotelluric data (Curtis & Thiel, 2019;Heinson et al, 2006Heinson et al, , 2018Thiel et al, 2005;Thiel et al, 2016;Thiel & Heinson, 2010;Thiel & Heinson, 2013), and potential field data (Baines et al, 2011;Betts et al, 2003;Direen & Lyons, 2007;Stewart & Betts, 2010a, 2010b. The challenging question of how the deep crustal penetrating structures connect with the upper crustal structures and their relationship with large metallogenic provinces has been addressed with different traditional geophysical methods.…”