Watchorn GRS size, genesis and mineralisation
Watchorn GRS is examined using seismic tomography in plan and section to help determine its size, genesis and mineralisation potential. Seismic tomography offers the best approach for investigating continental scale variations in the properties of the upper mantle. However, few detailed structural geology seismic tomographic images are published. Seismic tomography records the finer structures, as well as the broader ones shown on the image, in a similar manner to a medical tomographic scan.
Seismic tomographic research
Australian cratonic structure is revealed by seismic tomography. The seismic tomography plan and cross-section suggests the cratonic roots and these structures extend to at least 250 km. This is not surprising as the surface GRS are up to 700 km diameter. Some of the major structures in the local surface seismic sections (especially the steeper arcs and flatter structures) correlate with deeper structures in the seismic tomographic sections.
Watchorn GRS impact structure credentials
Impact structures have the same morphology as these seismic tomographic structures. The features on the 01 AGS NY 1 seismic line and the seismic tomographic cross-sections are analogous to features on the Chicxulub Impact seismic section and based on this data it is certain that Watchorn GRS is an impact structure. There is strong prima facies evidence for this assertion.
Prima facies evidence for Watchorn GRS to be an impact is observed in ELV 071 diamond drill-core from near Leonora. The core is Archaean and shows strong, pervasive 45˚ alteration, structure and veining. The core has shock metamorphic breccia xenoliths derived from an Archaean impact. The Watchorn GRS is the only documented giant ring structure in the area and ELV 071 lies on the strong, well documented Leonora ring. ELV 071 shock metamorphic xenoliths have micro-textural characteristics of quartz and plagioclase diagnostic of an impact origin. This is prima facies evidence of impact origin and points to Watchorn GRS as an impact structure.
The seismic structures suggest that the deep lithosphere was formed by mega-impacts over a protracted period and buried deeply. This means these impacts were earlier than the oldest surface rocks on Earth of 3.5 – 3.8 Ga. The Hadean/Archaean cratons most likely mode of formation is during the Late Heavy Bombardment (LHB) which occurred from 4.1 – 3.8 Ga.
Mineralising systems and Exploration targeting
The extreme destructive event accompanying mega-impacts forms giant structures which are the prime structures up which stress is relieved and up which intrusive rocks and mineralising fluids will travel.
The ring structures are the only mantle tapping structures observed on plan and the seismic and seismic tomographic sections and appear to focus the mineralisation as >80% of the gold and nickel mineralisation in the Yilgarn correlates with these rings. Gold mineralisation in the Yilgarn is specifically related to the antiformal deformation of the felsic crust underlying the greenstones.
This seismic research has defined surface ring structures and mantle tapping structures that come from the Asthenosphere, through the lower felsic crust to the surface. It is suggested that these antiformal structures extend right around the ring perimeter and define exploration targets in favourable structural/lithological intersection areas. An exploration targeting plan is included encompassing these parameters.
Ongoing Research
Further research on Seismic Tomography will include; Continue seismic tomographic structural research over shallower depths using tomographic plans for 200, 150, 100 and 50 km depths. Continue seismic tomographic sections structural research for cross-sections 24˚S and 16˚S. Construct a broad 3D structural plan of the Australian plate giant ring structures and linear structures. Continue to relate these to surface geology, mineralisation and exploration targeting.
For more detail and great illustrations open the file below —–
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| aa YW #6 Watchorn GRS correlation with seismic tomography on 30 South section aa.pdf | 1 MB |