Lower crustal resistivity signature of an orogenic gold system

Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.

Orogenic gold deposits provide a significant source of the world's gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and southeastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world's foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS 2 ) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS − ligands for Au, and CO 2 . We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO 2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world's richest orogenic gold provinces.
Orogenic gold deposits in upper crustal settings are a significant source of the world's gold resources, but their origin depth is contentious [1][2][3][4][5]6,7,8 The 440 Ma orogenic belt in south eastern Australia is one of the largest global gold provinces and had been widely researched in terms of its structural and geodynamic setting 9,10-12 but the 3D crustal architecture is poorly constrained 13 . To provide new insight on the crustal structure beneath this gold province, we have compiled 30 years of broadband and long-period MT and GDS surveys across western Victoria and south-eastern South Australia 20,21 . Since 2013, high-quality long-period MT data have been collected in the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) with site spacing of approximately 55 km 22,23 that covers all of South Australia and Victoria. After removal of poor-quality sites, 123 long-period MT and GDS sites, 252 broadband MT (mostly with GDS data) and 40 long-period GDS sites were identified as a composite database, covering an area approximately 800 km east-west and 500 km north-south, as shown in Fig. 1.
Three-dimensional inversion of all long-period MT (123 sites) and long-period GDS (40 sites) data, including bathymetry and topography, produced a model that fits the full tensor data to an RMS misfit of 2.1, with error floors of 5% for each impedance element and 0.02 for magnetic transfer functions. A number of different inversions were undertaken, testing: (a) models with and without inversion for distortion; (b) changes in the depthweighting of smoothness parameters; (c) varying levels of horizontal and vertical regularization; and (d) models with and without near-surface (top 3 km and top 500 m) smoothing. Broadband MT data were not included in the three-dimensional inversion as they have quite a different spatial and frequency sampling [14][15][16][17]18,19 . However, we use these data to verify the results from the three-dimensional inversion in the Supplementary Information. Such low crustal resistivities in silicate minerals are unlikely to be solely due to temperature, even if minerals are significantly hydrated 25 , so another mechanism to enhance conduction is required 26 . Such additional conduction is generally argued to be graphite 26,27 , magnetite and sulphides 25,28 , or sometimes free fluids 29,30 , which are secondary overprints of the primary crustal materials 31 . Of these competing mechanisms, magnetite and sulphides are generally of negligible volume and are isolated in fresh xenolith granulites 25 , and are unlikely to interconnect over tens of kilometres. Similarly, aqueous fluids are unlikely to be stable and connected over long geological time scales 32 .Thus, the mostly commonly proposed mechanism for widespread electrical conduction at lower crustal depths is graphite 31,27 .
Large et al. 33 argue that Au deposits in orogenic settings are originally sourced from marine sediments in deep ocean environments that host C, FeS 2 and potentially colloidal Au. When buried to mid to lower crust depths, and with additional rapid heating from the upper mantle 6,7,9 such oceanic sediments rich in C and FeS 2 generate significant amounts of free aqueous sulphur (HS − ,S 2− ) in a relatively short time frame that acts as the ligand for Au through the following relationship 7 .
For the case of the Victorian gold province the additional heat source at 440 Ma has been suggested to be due to slab break-off and subsequent mantle upwelling that allowed a rapid introduction of mantle heat into the crust 10 . This mechanism is thought to have occurred along many hundreds of kilometres of a mega-subduction www.nature.com/scientificreports/ zone off the eastern margin of Gondwana, explaining the widespread occurrences of world-class orogenic and instruction-related gold deposits that were formed simultaneously in the Lachlan Orogen at 440 Ma 10 .
Organic carbon in sediment may be metamorphosed to flake graphite at amphibolite grade conditions at depths of ~ 20-30 km and temperatures ~ 550 °C 34 . In addition, mobilised CO 2 may be precipitated as graphite 35,36 either at grain boundaries 27 or along more permeable zones 37,38 through retrograde hydration reactions where the host-rock oxygen fugacity (fO 2 rock ) is below the upper fO 2 limit of graphite 39 . We argue, therefore, that the low resistivity imaged ~ 30 km depth may be due to graphite. At 20 km depth, the zone of lower resistivity is narrowed and major deposits appear to align along the western margin of the resistivity anomaly, suggesting that the pathway of Au deposits to the surface is controlled by variations in permeability that are expressed as gradients in electrical properties. The resistive western flank may represent a permeability boundary that is structurally aligned with the Heathcote Fault Zone 24,40 . Deposits to the west of the low-resistivity zone at 30 km depth, particularly in the Stawell area, are porphyritic and instruction-related rather than orogenic 11,42,44 . Figure 2 shows an additional zone of low resistivity of < 300 Ωm centred around the town of Mount Gambier. It is most evident as a separate region in the 20 km depth slice; at 30 km depth the region has slightly lower resistivity (minimum 100 Ωm), but the inherent smoothing of the three-dimensional modelling with increasing depth merges these features. We argue that the cause of this lower resistivity at crustal depths may be due to hotter temperatures associated with the Newer Volcanic Province [43][44][45][46][47][48][49][50] . The most recent volancism (~ 4.5 ka) at Mount Schank, 10 km south of Mount Gambier (volcanism 5 ka), indicates that higher crustal temperatures are still present. For the 20 and 30 km depth slices, an order of magnitude decrease in resistivity (1000 to 100 Ωm) can be explained by thermal anomalies of 100 °C 25 .
At the base of the lithosphere, at a depth of ~ 150 km, there is a resistivity gradient of > 1000 Ωm to 100 Ωm from the southern Delamerian Orogen to the Lachlan Orogen. This trend is also seen in an eastward reduction of P and S-wave velocities 13 suggesting that there is a step in lithospheric thickness due to higher temperatures beneath the Lachlan orogenic belts.
Reflection seismic profiles 06GA-V1 to V3 were collected across the western Victorian goldfields in 2006 12,24 , as shown in Fig. 2b. In Fig. 3 we show a cross-section from the resistivity model along the seismic lines 06GA-V1 to V3 with a structural interpretation derived from the seismic data 24,40 . The resistivity model is a smoothed representation of the geology, but clearly shows that the low-resistivity region (< 20 Ωm) lies near the boundary between the Bendigo Zone and the Selwyn Block in Fig. 3. This low-resistivity region extends from a depth of about 20 km to the seismically defined Moho, and is centred on the west-dipping listric Heathcote Fault Zone, which bounds the Proterozoic Selwyn Block 47 . The east-dipping Moyston Fault that is recognized as the boundary between the Delamerian and Lachlan Orogens 24 has a less pronounced electrical signature, but delineates the western extent of the low-resistivity region in the lower crust.
The most electrically conductive zone in Fig. 3 is in a broad region of shearing where oceanic mafic crustal elements are highly faulted and stacked above and below the Heathcote Fault Zone 23,24 , representing a zone of enhanced transient permeability during tectonism. The lower resistivity observed along the Heathcote Fault Zone may represent a zone of enhanced graphite deposition from CO 2 -rich fluids 27,37,38 evolved from tectonically thickened carbon-rich sediments.

Conclusions
We conclude that the south-eastern Australia orogenic gold deposits have a deep crustal origin. Such gold deposits are spatially correlated with a broad region of lower crust (> 20 km depth) with electrical resistivity of less than 20 Ωm. We argue that this footprint of the source is due to the presence of graphite derived from carbon and pyrite-rich source sediments, from direct metamorphism to flake graphite and precipitated graphite through retrograde hydration reactions of CO 2 released from the sediments.

Methods
MT and GDS responses used in the inversion were rotated to 305° (clockwise from geographic N), in line with the 3D mesh orientation. The orientation was primarily chosen to parallel the orientation of the continental shelf and slope to the south. Data were resampled to five per decade over a bandwidth from 10 to 10,000 s, for a total of 16 periods. Error floors of 5% for all tensor impedances and 0.02 for magnetic transfer functions were assigned. Static distortion matrices were also determined from the inversion.
Cell width in the core area is 5 km and the core extended beyond sites by 30 km (6 cells). Lateral padding of 500 km was included, with a growth factor of 1.3. Vertical spacing starts from 100 m at the topographic level, increasing by a factor of 1.06 per cell down to 10 km depth, 1.04 per cell to 100 km, and finally 1.   www.nature.com/scientificreports/ and Geoscience Australia using Australian Government NCRIS Capability AuScope and Geoscience Australia instruments. The AusLAMP South Australia data were collected using AuScope instruments by the Geological Survey of South Australia, the University of Adelaide and Geoscience Australia, with acquisition funded from the Geological Survey of South Australia, Geoscience Australia and AuScope. The authors acknowledge the many people from Geoscience Australia, Geological Survey of South Australia, Geological Survey of Victoria and the University of Adelaide involved in collecting the data and the support provided by individuals and communities to access the country, especially in remote and rural Australia. The authors acknowledge the traditional custodians of the land on which the data in this paper were collected. Alison Kirkby and Jingming Duan publish with the permission of the CEO, Geoscience Australia. Stephan Thiel and Kate Robertson publish with the permission of the CEO, Geological Survey of South Australia. We thank Dr Chris Lawley and an anonymous reviewer for the constructive comments that have significantly improved the manuscript.