A geological interpretation of the Echuca magnetotelluric survey, Victoria

Dennis, Zara; Moore, David; Cull, J. P.

doi:10.1080/08120099.2011.577808

Cited by 4 publications

(2 citation statements)

References 16 publications

(20 reference statements)

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“…A number of different inversions were undertaken, testing: (a) models with and without inversion for distortion; (b) changes in the depth-weighting 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 – 17 , 18 , 19 . However, we use these data to verify the results from the three-dimensional inversion in the Supplementary Information.…”

Section: Introductionmentioning

confidence: 99%

Lower crustal resistivity signature of an orogenic gold system

Heinson

Duan

Kirkby

et al. 2021

Sci Rep

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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.

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Section: Introductionmentioning

confidence: 99%

Lower crustal resistivity signature of an orogenic gold system

Heinson

Duan

Kirkby

et al. 2021

Sci Rep

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“…MT response over the easterly Lachlan portion of the seismic transect did not find significant conductive anomalies within the crust (Dennis et al, 2011a, Dennis et al 2011b. Small, thin conductors in the mid-crust coincided with the top of ancient seafloor basalts and the anomalies were interpreted as being cause by graphitic black shales known from this level.…”

Section: Discussionmentioning

confidence: 86%

Magnetotelluric imaging zone in western Victoria of a Palaeozoic Andean margin subduction

Stepan

Heinson

Taylor³

et al. 2015

ASEG Extended Abstracts

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Lower crust and upper mantle electrical anisotropy in southeastern Australia

Dennis

Thiel

Cull

2012

Exploration Geophysics

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The dominant north-south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle.

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A geological interpretation of the Echuca magnetotelluric survey, Victoria

Cited by 4 publications

References 16 publications

Lower crustal resistivity signature of an orogenic gold system

Lower crustal resistivity signature of an orogenic gold system

Magnetotelluric imaging zone in western Victoria of a Palaeozoic Andean margin subduction

Lower crust and upper mantle electrical anisotropy in southeastern Australia

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