Windimurra intrusion

The Windimurra Intrusion is a giant ultramafic-mafic intrusion emplaced within the Yilgarn craton of Western Australia. It is located approximately 100 kilometres south east of the town of Mount Magnet.

etting

Windimurra is a conical sheet-like body of ultramafic rock intruded into the Archaean granite-greenstone terrain of the Murchison province within the Yilgarn Craton. Windimurra sits astride an inferred proto-rift zone in the Archaean basement, and is dated at 2850 +/- 10 Ma.

The intrusion is tilted to the west-northwest at about 70 degrees.

Lithology

Windimurra contains in excess of 13,000m of intact ultramafic stratigraphy formed of cumulate layering by a process of fractional crystallization. Individual rock types can be grouped into a troctolite phase or series, a gabbro phase or series and a gabbronorite phase or series. Anorthosite cumulates are preserved in the roof sections, most of which are sheared and faulted off. A marginal granophyre complex exists in the roof and wall rocks, formed by advective heat transfer causing melting of the country rocks.

Economic geology

The Windimurra Intrusion has been of great interest to mineral exploration companies for decades, as it is one of the thickest and largest ultramafic intrusions in the world, though it has been fragmented and mostly removed by shearing unlike the Bushveld Igneous Complex of South Africa.

Exploration has focused on finding basal nickel sulfide deposits, although that has proved fruitless as the basalt portions of the deposit are faulted off, and chromitite or vanadium deposits related to oxide cumulate layers higher up in the intrusion. Both these latter efforts have proved successful, with the opening of a major vanadium operation in the 1990's, subsequently closed.

Vanadium

The Shepherd's Discordant Zone is host to a laterally extensive vanadiferous magnetite and ilmenite adcumulate and mesocumulate deposit, forming a resource in excess of 120 Mt grading 5% V₂O₅. It is divided into a lower olivine bearing zone ~150m thick, a middle olivine-free magnetitite zone 250 to 300m thick and an upper magnetitite-free zone 50m thick. The middle zone contains a basal 2m thick magnetitite zone (containing >70% magnetite) and podiform, lenticular magnetitite horizons above it, and is of principal economic interest.

Interstitial ore textures in some plagioclase-bearing ilmenite zones indicate ilmenite may have been present as a melt phase. In many zones, 10-50m thick ilmenite zones may be of economic interest as vanadium contents in magnetite appear to actually be hosted in ilmenite exsolutions.

References

Hoatson, D.M., 1998. "Platinum-group element mineralisation in Australian Precambrian layered mafic-ultramafic intrusions." AGSO Journal of Australian Geology & Geophysics, "17(4)", pp. 139-151.