Summary:
The Wonmunna’s tenements cover a large-scale anticlinal structure in the Hamersley Basin exposing rocks of the Fortescue Group. Fold keels of the Hamersley Group outcrop in the Wonmunna area within the fold structure. These folds expose BIF of the Nammuldi Formation, which host iron ore mineralisation (Trendall 1990).
The Wonmunna lies within the Hamersley Province covering an exposure of Jeerinah Formation (Fortescue Group) with fold keels of the overlying Nammuldi Formation sub cropping in the area. The area is covered by exposure of regolith (Hamersley Surface), a sub horizontal regolith surface, which is partly made up of poorly developed channel iron pisolite beds, ferruginous iron rich gravels, possibly canga, and ferruginous tertiary sediments. The regolith forms a ferruginous crust on the basement geology, which when eroded forms breakaways exposing the underlying geology. In the Wonmunna area, the Roy Hill shale, dolerite of the Jeerinah Formation and rare outcrops of the Nammuldi Formation are exposed along the breakaways (MRL 2022).
The stratigraphy of the Fortescue and Hamersley Group in the Project area consists of the Jeerinah Formation, which is made up of the Woodiana Sandstone (~60 m thick), overlain by the Warrie Member (~80 m thick) consisting of shale, a ferruginous chert and dolomite. The Warrie Member is overlain by the Roy Hill shale (~30 m thick), a carbonaceous shale. Intrusive dolerite has intruded the Jeerinah Formation to form large sills, which dominate the rock type in the WIOP area (MRL 2022).
The Nammuldi Formation is the basal BIF of the MMIF of the Hamersley Group, which appears to conformably overlie the Jeerinah Formation in the WIOP area. At the unconformity, a goethite chert outcrops at the base of the Nammuldi Formation. The BIF of the Nammuldi Formation has been mineralised to goethite, with the silica bands in the BIF being replaced by goethite and limonite (Trendall 1990).
The Hamersley Group contains five major banded iron formation (BIF) units, of which two, the Marra Mamba Iron Formation and the Brockman Iron Formation, host iron mineralisation, including most of the exploited iron ore deposits in the Pilbara Mineral Province.
There is relatively poor outcrop in the Project area. In the east alluvial/channel iron deposit (CID) cover forms a well preserved Hamersley Surface – i.e. laterite cover, and to the west more Marra Mamba Iron Formation (MMIF) outcrops. From examination of the present NMM drill and gamma logs - and with some confirmation during a limited ground examination during a brief site visit - the full sequence of MMIF appears to be present (i.e. Nammuldi Member to Mt Newman Member, together with some West Angela Member of the overlying Wittenoom Formation (Pearcey & Sergeev 2012, Slater & Kneeshaw 2012). The high grade mineralisation typically occurs within the Mt Newman Member.
Iron mineralisation of the martite-goethite (M-G) type within the Marra Mamba Iron Formation usually occurs preferentially within the Mount Newman Member only. However, enrichment can extend into the MacLeod Member and on occasion into the Nammuldi Member. The in situ ore forming process is a combination of the metasomatic, pseudomorphic replacement of magnetite (resulting in martite - M), the replacement of BIF gangue minerals (chert, silicates and carbonates) by hydrous iron oxides (goethite - G). This can often be followed by leaching which results in the development of limonite (ochreous goethite - oG). Additionally in some areas further alteration of the prime ore can be seen, typically evidenced by secondary goethite along joints / cavities / bedding planes and introduced by late stage percolating groundwaters. Mineralisation boundaries are both conformable with and transgress the stratigraphy, generally thinning down dip.
The majority of mineralisation at Wonmunna is best described as bedded goethite within Marra Mamba BIF. In outcrop, the BIF unit has maintained its primary sedimentary fabric but has been completely altered to intercalated layers of goethite and hematite with some remaining silica rich layers. The mineralisation is primarily the result of supergene iron enrichment.
In addition to bedded mineralisation, several pockets of detrital mineralisation and enriched duricrust occur along the edges of breakaways and in alluvial valleys. This material is comprised of variously mineralised clasts of the Marra Mamba BIF in a matrix of polymictic alluvial/colluvial clays and sand. This mineralisation style is volumetrically small, lower grade and of less commercial interest than the bedded style. There may be potential, however, to beneficiate the lower grade material to produce a higher-grade product. There is no plan to mine and treat any of this BIF resource at this stage.