Summary:
The Sanjiv Ridge (Corunna Downs) Project comprises five separate deposits: Split Rock, Shark Gully, Glen Herring, Razorback and Runway.
The project area is located within the Kelly and Coongan greenstone belts of the East Pilbara Craton, between the Shaw and Corunna Downs granitoid complexes (Figure 2). The deposits are located in the Cleaverville Formation of the Gorge Creek Group. The Cleaverville Formation hosted high-grade (+60% Fe) deposits at Goldsworthy, Shay Gap, Yarrie and Nimingarra that were mined as early as 1966 but are now closed.
The Cleaverville Formation comprises metamorphosed BIF and cherty BIF together with some interbedded siltstone and chert units and outcrops as a northeast trending plateau extending for some 20 km (Hermawan, 2016). To the north, the Cleaverville Formation and Warrawoona Group are unconformably overlain by the Mount Roe Basalt (the basal unit of the Fortescue Group) and sandstones with beds of conglomerate of Croydon Group, Lalla Rookh Sandstone.
Mineralisation
The majority of the mineralisation at Corunna Downs is gœthitic near surface and has probably been formed by supergene processes. This is reflected in the chemistry, which indicates that an overall grade of between 55% Fe and 57% Fe is dominant in the area. The deposits have a hard-cap (hydrated zone) dominated by vitreous gœthite extending to approximately 30 m in some places.
There are some indications that a higher grade, metamorphic mineralisation (hypogene or hydrothermal) may be present. At the Runway deposit, zones of hydrothermal breccia have been identified within the BIF, which may indicate that hypogene alteration of the protolith has occurred. An examination of the chemistry of the deposits indicates that this has not resulted in major post-mineralisation alteration of gœthitic mineralisation to hæmatite (as has occurred elsewhere).
Zones of hæmatitic mineralisation were identified at Split Rock (e.g. CDRC0061, grading 61.1% Fe, 0.130% P and 5.6% LOI from 14 m to 48 m) and Glen Herring (e.g. CDRC0366, grading 62.7% Fe, 0.078% P and 7.8% LOI from 64 m to 104 m). However, these zones were rare, and apart from a few isolated grades above 60% Fe, most were lower grade.
The structure of the Sanjiv Ridge (Corunna Downs) deposits is complex, with four deformation phases recognised (Hermawan, 2016). These are termed D1 to D4, respectively:
• D1: North to north-northeast trending, tight steeply plunging folds associated with bedding plane parallel shear zones. This is the dominant deformation structure across Corunna Downs.
• D2: Sub-vertical tight southeast-trending Z folds.
• D3: Reactivation of a series of northeast-southwest trending faults which offset the earlier Warrawoona Group volcanics and Euro Basalt of the Kelly Group. Believed to relate to the mineralisation.
• D4: Major north-south faulting that offset the D3 structures. One of these offsets the mineralisation at Runway and a second fault offsets the Cleaverville Formation in the north, separating the Glen Herring deposit from the rest of the deposits.