Summary:
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.
The Windarra BIF is the lowest recognisable stratigraphic unit in the Margaret sector. The overlying ultramafic rocks host both the Mt Windarra and South Windarra nickel deposits and represent the base of the first cycle of ultramafic to mafic volcanics. It is the only ultramafic cycle in the region known to host economic nickel mineralisation. Three volcanic cycles are recognised in this sector with the rocks of the uppermost being overlain by conglomerate along the western margin of the Laverton tectonic lineament.
Mine Geology
The Mt Windarra nickel occurs towards the bottom of a thick ultramafic sequence at the base of the first volcanic cycle. Granular quartz-feldspar rocks in the Mt Windarra mine area may represent metamorphosed sandstone. The Windarra (Main) banded iron formation constitutes the base of the exposed stratigraphy, and the immediate footwall to mineralisation at South Windarra. The unit is an 80m to 150m thick sequence of siliceous and banded chemical sediments, containing variable amounts of quartz, grunerite, actinolite, magnetite, albite, chlorite, pyrrhotite, pyrite and garnet, typically silicate facies at the base and changing to sulphide facies towards the top.
Mineralisation
Mineralisation at Windarra, associated with massive to disseminated sulphides (pyrite, pyrrhotite, pentlandite and chalcopyrite), is found to occur at the base of a sequence of ultramafic rocks known as komatiites with ore zones occurring as ribbon-like ore shoots around 20m thick and 50-350m in length and up to 900m down dip. At South Windarra, ore shoots were found to be around 20m in thickness, 1,300m in length and extending to around 300m down dip.
Mt Windarra
The Mt Windarra orebody consists of eight distinct, steeply dipping shoots named: A, A Hanging Wall, B, C, D, E, F and G Shoot. These shoots (at 1% nickel cut-off) vary in thickness up to 20m, have a strike length of between 50m and 350m and a down dip extent of greater than 900m.
Nickel mineralisation is associated with multiple 10 to 45m thick olivine peridotite (metamorophosed to talc-magnesite or serpentinite) flows, located at the base of the ultramafic pile, and largely restricted to the immediate area of the Mt Windarra and South Windarra mines. 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 non-massive sulphide mineralisation can be sub divided into three different textural types: matrix, blebby and disseminated type ores. Matrix type has a sulphide supported olivine cumulate texture, and occurs at the base of an olivine peridotite flow or in a transitional zone with underlying massive sulphide ore. It is rarely more than 2 m wide and generally contains between 25 and 40% sulphide. Blebby type disseminated sulphide is present in several areas of the mine, generally overlying massive sulphide and/or matrix ore, and contains between 20 and 30% sulphide. Finely disseminated sulphide mineralisation, with between 5 and 25% sulphide, is the most common style of disseminated ore and overlies the other textural variations.
The nickel tenor of sulphides in the ultramafic rocks is normally 8 to 16% nickel, and invariably higher in the disseminated ores than in the massive sulphides. The massive ore in A and B shoots rarely assays more than 8% nickel, whereas in the E-C D-G and F shoots it may assay up to 12%. The brecciated BIF hosted and remobilised stringer sulphide ores are generally of a lower and more variable tenor, typically assaying between 2 and 8% nickel.
Supergene ore, which extends from 40m (the base of oxidation at Mt Windarra) to around 80 m below surface, consists primarily of pyrite and violarite, and is often associated with elevated ore grades. Violarite and pentlandite co-exist in a transition zone, to a vertical depth of up to 180m.
Cerberus
The Cerberus deposit comprises two lenses of relatively flat lying and north-easterly plunging mineralisation. The upper unit consists of disseminated nickel sulphide whilst the lower unit typically consists of stringer to massive nickel sulphide mineralisation. Both are hosted within the Windarra Ultramafic unit. The mineralisation occurs close to but above the basal contact with the underlying Corridor Ultramafic unit.
Higher grade mineralisation has been intersected at a vertical depth of 160m below surface and potential exists in the southern up-dip position to bring mineralisation to within 100m to 120m of the surface.
Nickel grades and widths are greatest towards the centre of the komatiite lava channel and form two higher grade zones towards the top and bottom of the mineralised channel. The nickel sulphide intersections typically become thinner and lower grade towards the edges of mineralised zone. This is consistent with Kambalda-style komatiite-hosted nickel models which helps the PNL geologists to interpret the geology and target the prospective mineralised channel.
The mineralisation at Cerberus is blind at the surface and is covered by transported material. The original discovery hole (WED4) intersected the mineralisation at a depth of 700m vertically below surface. In contrast to typical exploration discoveries, PNL has drilled the mineralised lava channel from the bottom up, and has now defined the mineralisation to within 120m vertically below the surface and over a strike length of 1100m.