YN #2. Structural geology of 700 km Norseman giant ring structure defined by Landsat. Formulating new parameters for Yilgarn exploration. Discovering the Yilgarn’s deep design?
© Bob Bingley Watchorn 21st July 2017
Interpreted Landsat image of the Norseman GRS showing structural geology and major gold and nickel occurrences.
Use of structural geology in defining giant ring structures
This paper is the second in the series describing structural geology research on the previously unreported Norseman Giant Ring Structure (GRS). The structure only exhibits some of the characteristics of an impact structure. It also has some of the features of a small mantle plume or hotspot.
Discovery of GRS through data enhancement
The Norseman GRS is enormous but had not been recognised. The Yilgarn GRS had previously been searched for on raw Landsat images without success. It was reasoned that they should be observable on Landsat as the radiometric images of Norseman GRS are very clear and radiometrics only measures radioactivity from the top 1 metre of the land surface. The Norseman GRS became visible on Landsat by data enhancement despite limited geological outcrop and woodland and semi-desert sandplain cover. The palaeochannels have a strong correlation with the rings.
The most noticeable feature of the Norseman GRS is that the annulus between the Kalgoorlie ring (C) and the Kambalda ring (D) is particularly persistent. It commences in the Fraser Orogen 300 km east of Kalgoorlie, continues in a 1000km arc and finishes back in the Fraser Orogen east of Esperance. It is also faintly visible on the east side of the Fraser Orogen above and east of Esperance.
The Norseman GRS is unaffected by the regional geology – or that should be stated as the regional geology structure is unaffected in a major way by the Norseman GRS? The regional geology is totally undisrupted by the rings. Some of the greenstone belts appear to follow the ring path especially those south of Southern Cross. The Nd dating (and those zircons perhaps did not intrude from the deepest crust) suggests that there is a much older lower crust under much of the Yilgarn Craton. The relatively undeformed rings suggest that the lower crust was, and has remained, rigid.
Geochronology and genetic models
Greenstones and mineralisation follow the rings and vary in age from >2.9 Ga in the Southern Cross domain, to <2.65 Ga in the Kalgoorlie terrane, to 1.5 – 1.2 Ga in the Fraser Orogen, suggesting that the GRS must be older than 2.9 Ga. The largest gold and nickel fields have a strong correlation with Norseman GRS.
The outpouring of huge volumes of lava onto this rigid lower crust may have been similar to the formation of the Mares on top of the Moon’s earlier crust. Hiesinger et al., 2000 states that the mares formed mainly in two peaks of 3.8 – 3.6 Ga and 3.5 – 3.2 Ga, were still forming intermittently after 2.6 Ga and were still forming as late 2.0 Ga. The period calculated for the earlier Yilgarn GRS formation is 3.2 – 3.5 Ga.
Recorded Impact debris (spherulites) in the Pilbara and Kaapvaal Cratons are 3.47 and 3.25 Ga (Bryerly et al, 2002). It is suggested that some of the Yilgarn GRSs are formed by impacts and the timing of the moon’s mares combined with the spherulite occurrence in the adjacent Pilbara and Kaapvaal Cratons give an indication of their most probable time of formation.
Research of the Vredefort, Sudbury and Chicxulub GRSs (impact structures) may also give clues as to the mechanism of the initial formation of the Yilgarn GRS and its subsequent overlying crust. Vredefort impacted on an already thick rigid Archaean craton. (Reimold and Koeberl 2014). Subsequent central uplift gives the domal structure. It is suggested that this may have been the appearance of the Norseman GRS and Watchorn GRS (dome) at about 3.4 – 3.2 Ga before the unconformable mid-crustal accretion and last out-pouring of greenstones and sedimentation formed the current upper crust.
Exploration Targeting
The giant gold mines of Kalgoorlie and St Ives and the giant Kambalda nickel field lie exactly on the dark margins of the rings and Norseman lies right in the centre of the GRS. It provides another exploration targeting parameter. More work relating these structures and the mines to the Goldfields dome and basin structure that appears to control the mining centres will be presented in future papers.
Mineralisation genesis hypotheses
The Vredefort dome caused a giant gold and uranium province which is primarily located in the median and outer rings, and the Sudbury structure has one of the world’s biggest nickel fields associated with it. How would this mineralisation be remobilized under the pressure and heat of 15 – 20km of overlying crust crustal buildup as was the case in the Yilgarn? How and where would it be deposited? The giant Sudbury nickel camp is associated with the impact. The Chicxulub impact structure exhibits similar features to the Yilgarn Craton GRSs especially the Watchorn GRS. Huge gas and oil fields in the Gulf of Mexico on the farfield rings illustrate the remobilizing power of these giant ring structures.
Structural geology in the Yilgarn
Current geoscientific research has partially defined the near surface (0-20km) local controls on the Yilgarn Craton mineralisation, but the fundamental Yilgarn Craton structural architecture and basement controls are acknowledged to be poorly defined. The giant ring structures, that are the subject of this research and these papers, have not been recorded or researched. This places a major constraint on the current research.
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| YN #2 Norseman GRS Landsat | 2.2 MB |