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.
Highlights
World-class magmatic ore systems are often characterised by fluids/melts that are derived from deep lithosphere, mostly located at the margins of ancient craton[1,2,3,4,5,6]
The cover thickness is least around the Olympic Dam deposit at about 300 m7, but thickens significantly in a deep-rifted basin to the north-east, with sediment thickness >2 km
A low-resistivity
Summary
World-class magmatic ore systems are often characterised by fluids/melts that are derived from deep lithosphere, mostly located at the margins of ancient craton[1,2,3,4,5,6]. Most of the magmatic ore metals have been dated at 1590 Ma, and associated with the widespread Mesoproterozoic Hiltaba volcanism and silicic-dominated Gawler Range Volcanic large igneous province[8,9,10,11,12], but there is evidence of post Hiltaba U mineralization dating to 1100 Ma13. Since these events, the lithosphere appears to have experienced little subsequent deformation so that the crust has remained largely undisturbed for over a billion years[14,15]. Additional long-period MT sites were acquired in an approximate grid formation with 10 km spacing in 2009–2010, and a total of 110 broadband MT sites (periods of 10−3–103 s) along the seismic transect and an adjacent transect with site spacing of 1–2 km in 2015–2016
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