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GEOLOGY

The Windarra region forms part of the Mt Margaret Goldfield. Mafic and ultramafics, metavolcanics and intrusives form important members of the Windarra greenstone belt. A major granitoid pluton has intruded the stratigraphy and has locally stoped out the main BIF. Mafic-ultramafic and BIF xenoliths thought to be stratigraphically equivalent to the Windarra sequence occur within the granites in the region.

 

Bedrock consists of granite or granite gneiss, enclosed by north to northwest trending belts of metavolcanics, metasediments and intrusive rocks. Mafic dykes with an east-west strike are abundant in the region and cross-cut the greenstone, granite and granite gneisses. Regional trends are predominantly north-west but the main BIF horizon traces the regional Mt Margaret Anticline to South Windarra where the trend is more east-west.

 

Economic nickel mineralisation in the Mt Windarra area is hosted at the base of the Windarra Ultramafics, a 100–300m thick sequence of ultramafic (komatiite) lava flows, overlain by basalts. The Windarra Ultramafics host four significant nickel deposits, two of which have previously been mined, the Mt Windarra underground mine and the South Windarra open-pit and underground mine. The third discovery was at Woodline Well which has a small near surface oxide deposit which may contain a deeper sulphide extension. The latest and most significant discovery made by Poseidon Nickel was at Cerberus.

 

Nickel mineralisation at Mt Windarra is restricted to the sulphide zones at the base of the olivine cumulate ultramafic sequence. Massive sulphides form the dominant ore type and the non-massive sulphide mineralisation can be sub-divided into three different textural types: matrix (25-40% sulphide), blebby (20-30% sulphide) and disseminated (between 5-25% sulphide).

The nickel tenor of sulphides in the ultramafic rocks is normally 8 to 16%, and invariably higher in the disseminated ores than the massive sulphides. The massive ore in A and B shoots rarely assays more than 8% nickel, whereas in the E-C-D and F shoots it may assay up to 12%.

 

In the primary ore, pyrrhotite, pentlandite, pyrite and chalcopyrite are the most common sulphide phases, in decreasing order of abundance. The pyrrhotite to pentlandite ratio varies from 1:1 in disseminated ore, to up to 8:1 in the matrix hosted ore. The average nickel to copper ratio is 9:1 for most ore types, though may be as low as 4:1 in the copper rich basal matrix hosted ore and remobilised massive sulphide stringers.

 

The Mt Windarra orebody consists of nine distinct, steeply dipping shoots named: A, A Hanging Wall, B, C, D, E, F, G and H Shoots as depicted below. These shoots vary from 2m up to 20m in thickness; have a strike length of between 50m and 350m and a down dip extent of greater than 900m.

 

The diagram below shows the Mt Windarra Nickel Deposit depicting the remaining lodes that have not yet been mined out and form the JORC (2012) compliant Mineral Resource.

 

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