Correlation of Plumridge Giant-Ring-Structure with Seismic Traverse 12 GA T1. Understanding the 3D Structure of Giant Ring Structures and their Associated Mineralisation.
© Robert Bingley Watchorn 31st August 2017
Plumbridge Summary
This research paper describes the Plumridge giant-ring-structure (GRS) morphology in plan using radiometrics and in cross-section using seismic traverse 12GA T1. Independent co-incident evidence is presented that the rings and the seismic arcuate structures most likely had a common mode of formation. The morphology of Plumridge GRS in both plan and section is identical to that exhibited by an impact. If a large meteorite impacted it would have been before 2.72 Ga when the last cycle of Archaean crust was formed.
The suggestion that the Tropicana mineralisation consists of remobilised underlying Archaean Gruyere-type mineralisation is mooted and discussed.
Past Research
The Yamarna rings were only described recently (Watchorn YY #1, 2017) and so research on them is embryonic. However, they share many features with the Watchorn, Cauden and Norseman giant ring structures (Watchorn YC #1 and 2, YN #1 and 2 and YW #1 – #4).
It may seem that the research references on this giant-ring-structure is selective but as the GRS were not previously recognised these papers are the only ones describing research on them. This is astounding considering their importance and the extent of geoscientific research on the Yilgarn Craton. This will normalise soon!
Plumbridge geochronology
The surface outcrop of the Fraser Orogen is Proterozoic but the relatively undeformed radiometrics rings crossing it suggest that the lower crust is Archaean and has remained rigid throughout the development of the overlying younger crust from 2.75 Ga through 2.5 Ga (Watchorn YY #1, 2017). The continuity of the Plumridge giant-ring-structure is surprising (especially in the east) considering the later deformation and overlying later stratigraphy.
Relationship of rings to Tropicana gold mine
It is suggested that the original, now buried, giant ring structure was the locus of Archaean mineralisation and that this mineralisation was remobilised through the overlying Proterozoic crust to form the mineralised Tropicana position.
Also discussed – is the age difference of 130 Ma between the Gruyere and Tropicana mines unusual? This age difference between the Gruyere and Tropicana mines (Doyle, M. et al. 2015) suggests that the rings are the focus for gold mineralisation and fluid flow over a longer time period than is normally attributed to the Yilgarn NNW shear structures. Is this a wrong assumption?
There is a large age disparity between the earliest and last gold mines in most goldfields in the Yilgarn Craton – especially the giant ones. What makes the Gruyere and Tropicana age disparity interesting is that it occurs over the very obvious, exposed Archaean/Proterozoic boundary. However, as that boundary is an arbitrary geological division it is perhaps no different to the generally obscured intra-Archaean boundaries.
The Yilgarn Craton gold mineralisation appears to be related to (and usually postdates) the last episode of deformation in each terrane area (McCuaig et al 2010). However, in areas close to terrane boundaries both older and younger mineralisation may be present. Does this mean there are two gold episodes associated with the last deformation in both terranes? Or are there two gold episodes associated with two episodes in the lower Terrane?
The Yamarna GRS’ size suggests that they should extend to great depth but they also extend to the surface and thus may channel to the surface the later fluids associated with the initial formation of the Frazer Orogen (2.5 Ga).
If there is mineralisation on the Yamarna GRS at depth under the Fraser Orogen, then this mineralisation may have been channeled through the Fraser Orogen to form the Tropicana gold deposit. Gold bearing fluid may be remobilised and travel perhaps 10 – 15 kms from the middle crustal impact gold source to the upper crust. It is suggested that this could be the mechanism for an original gold source forming mines of varying ages at different levels of the crust.
The correlation of the rings in plan to those in the seismic section.
Examination of the seismic cross-section 12 GA T1 suggests that there are >50 km deep arcuate structures that penetrate through the mantle, totally seismically fracturing the mantle where it passes through (see image above).
The upper crust has undergone very strong deformation. Despite this there appears to have been minimal deformation of these lower arcuate structures suggesting the lower crust was rigid and did not undergo deformation after the formation of the giant ring structures. It is presumed that this rigidity seen in cross-section would also extend in plan. These structures (or intrusive filled structures) terminate at the estimated top of the Archaean middle crust but extend through the Proterozoic crust surface as large but much weaker, sharper structures or fractures. These fractures overprint the arcuate structures and are thus later.
There is the strong possibility that the Tropicana mineralisation traveled up these huge later structures. The chemical similarity of the Gruyere and Tropicana mineralisation may support this assumption.
Is the Plumridge giant-ring-structure an impact structure?
There is a high degree of correlation between the surface radiometric rings and the deep seismic arcuate structures. This is independent co-incident evidence that the rings and the arcuate structures most likely had a common mode of formation. This mode of formation is suggested to be as the result of an large meteor impact, as the morphology in both plan and section is identical to that exhibited by a meteorite impact.
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