Summary:
The Iron Bridge mineralisation lies within the Pincunah Member of the Soanesville Group, which is part of the Pilbara Super Group in the East Pilbara Terrane.
Regionally the rock sequence is dominated by mafic to andesitic volcanics and volcaniclastics, BIF’s and terrigenous clastic sequences intruded by Archaean granitoids. In the project area the rocks have been tightly folded, having a general strike of north-south with a steep sub-vertical dip.
The Pincunah Member forms an overturned north-south striking ridge through the North Star area of the deposit, dipping steeply to the west. Several vertical faults have been interpreted to rotate the Pincunah Member to north-west, parallel to the Mount Yule Granite complex in the north-west region of the project area.
Iron mineralisation at Iron Bridge occurs primarily within the Pincunah Member which outcrops across the entire project tenement. It is often masked by cemented caps of ferruginous weathered BIF, silcrete and detritus. A surficial weathered zone forms in upper (20–60 m) horizons dominated by maghemite and kenomagnetite, hematite and goethite after magnetite.
The hanging wall of the Pincunah Member is the Kangaroo Caves Formation and is composed of BIF bands interlayered with laminated, micro to mesobanded shales and chert. BIF units are up to 15 m in thickness and occur most commonly immediately adjacent to the Pincunah Member contact, while the remainder of the formation is dominated by shales and cherts.
The footwall of the Pincunah Member consists of a sequence of shale and sandstone layers, termed the Eastern Shale and Quartzite, passing upwards into the sandstones and conglomerates of the Corboy Formation. Outcrop of the contact is marked by a high-relief zone of silicification before transitioning into white interbedded shale and sandstone beds with fine scale mesobanding and laminations.
Primary mineralisation occurs as disseminated grains and aggregates of magnetite micro-bands with gangue phases at concentrations of 30–60% magnetite (by volume). Lower grade mineralisation occurs within gangue dominant micro-bands with similar texture to primary mineralisation with a range of 1– 30% magnetite.
Secondary magnetite occurrences are observed in association with quartz and stilpnomelane as well as monomineralic cross-cutting veins and generally forms coarser sub-euhedral grains.
The main mineralised BIF zones within the Pincunah Member are described as having a relatively limited mineral suite, with magnetite mineralisation overprinting the original lithology. Primary mineralisation occurs as disseminated grains and aggregates of magnetite micro-bands with subordinate gangue phases at concentrations of 30% to 60% magnetite (by volume). Lower grade mineralisation occurs within gangue dominant micro-bands with similar texture to primary mineralisation with a range of 1% to 30% magnetite.
The main zones of mineralisation at North Star, Eastern Limb, Glacier Valley, West Star and South Star is the Pincunah Member, which is comprised of sedimentary BIF with magnetite mineralisation, and dips steeply to the west at approximately 70-80°.
Mineralisation within the Pincunah Member has been geologically defined into Western, Middle and Eastern units according to assay results and geophysical logging. Each unit is interpreted to be conformable within the Pincunah Member and adjacent stratigraphy and are observable along the entire strike of the project area. The Western and Eastern units comprise the high-grade mineralisation domains, while the Middle zone is typically lower grade and often barren.
North Star comprises three distinct mineralisation style areas, North, Central and South, which are separated by interpreted fault zones. Eastern Limb runs sub-parallel and to the east of North Star.
The Northern part of North Star extends approximately 2.4 km in strike length, 200–400 m across strike and has been modelled to a vertical depth of approximately 600 m.
The Central part of North Star extends approximately 1.9 km in strike length, 400 m across strike and has been modelled to a vertical depth of approximately 600 m.
The Southern part of North Star extends approximately 1 km in strike length, 200 m across strike and has been modelled to a vertical depth of approximately 600 m.
Eastern Limb lies approximately 400 m east of North Star and is 2.3 km in strike length, 200–400 m in width and is modelled to a vertical depth of approximately 200 m.
Glacier Valley extends approximately 3.4 km in strike length, 200–300 m across strike and has been modelled to a vertical depth of approximately 600 m.
West Star is approximately 3.5 km in strike length overall, of which 1.8 km strike length has been modelled. The mineralisation is typically 150–200 m across the strike and has been limited to a depth extent of approximately 200 m.
South Star lies immediately south of Glacier Valley and extends approximately 5.2 km in strike length, 400 m across strike and has been modelled to a vertical depth of approximately 500 m.