Geology

Locally, the tenement area consists of granitic rocks of the 1680-1620 Ma Durlacher Supersuite, specifically the Pimbyana and Yangibana Granites, which contain rafts of sedimentary rocks including arenites, calc-silicates, and quartzites of the 1760-1680 Ma Pooranoo Metamorphics. These rocks are intruded by sills, dykes, and veins of ferrocarbonatite and ironstone, and associated fenitic alteration, which comprise the 1,300-1,280 Ma GCFS.

The Yangibana Project area has been subject to several distinct tectonothermal cycles of which the Mangaroon Orogeny (1680-1620Ma), the Mutherbukin Tectonic Event (1385-1200Ma) and the Edmundian Orogen (1030-955Ma) are the most significant. During the Mangaroon Orogeny, intense deformation and metamorphism were coincident with the intrusion of the Durlacher Supersuite. The ferrocarbonatites that occur within the project area have been dated at 1262-986Ma, dates coincident with both the Mutherbukin Tectonic Event and the Edmundian Orogeny. The Lyons River Fault, which is a deep crustal lineament that has been traced to the Mohorovicic discontinuity, may have acted as a fluid conduit during these events, resulting in the emplacement of the ferrocarbonatites and associated hydrothermal alteration over a period of several hundred million years.

The GCFS is dominated by two major southeast trending shear zones, which are cross-cut by multiple north-south to north-east trending faults. The northern shear zone, the Bald Hill Lineament, is anastomosing and variable in width, comprising multiple fault splays. Geophysical interpretation by Southern Geoscience Consultants during 2016 indicated that the Bald Hill Lineament separates an early, highly fractured granite to the north from folded Pooranoo Metamorphics to the south.

The southern shear zone, the Lyons River Fault, is less complex and relatively narrow, forming the boundary between the Edmund Group, and an early granite to the north.

The Pooranoo Metamorphics are multiply-folded along southeast trending axes. In the north-west of the area of interest these folds are generally wide with granite cores. This area of wider folding has also been intruded by a later granite.

One of the key features of the GCFS within the project area is the widespread occurrence of ironstone dykes that are spatially associated with the ferrocarbonatite intrusions. The ironstone dykes are surrounded by relatively narrow haloes of fenitic alteration, and locally associated with quartz veining. Fenitic alteration haloes are characterised by the presence of feldspars and/or Na-amphiboles and magnetite. The ironstone dykes, with dominant north, northeast and west-northwest trends, consist predominantly of goethite, hematite, and magnetite, and are locally weakly radioactive. The ferrocarbonatites and ironstones of the GCFS occur as sinuous pods and veins generally less than 10m wide, that are traceable cumulatively for up to 25 kilometres. The ironstones of the GCFS have historically been the focus of exploration activity in the area, due to enrichment in Rare Earths Elements (REE).

Rare earths mineralisation at Yangibana is predominantly hosted by monazite, a phosphate mineral. Initial petrological assessment of samples of mineralised carbonatite from Yangibana North, identified the monazite as being the Ce-rich end-member, Ce-monazite. With the acquisition of further data from other areas within the project area, significant variations in the Nd content from prospect to prospect were identified. Prospects in the Eastern Belt of mineralisation (Bald Hill, Fraser’s, Auer, and Auer North) were identified as having significantly higher Nd content than prospects within the Western Belt of mineralisation (Yangibana West to Kane’s Gossan) suggesting that monazite may vary throughout the project area from the Ce-monazite variant, toward more Nd-rich variants (Nd-monazite).

Petrology and mineralogy has identified additional rare earths-bearing phosphates, mainly bastnasite with lesser aluminium phosphates (gorceixite, florencite and rhabdophane). Fluorapatite occurs as subordinate rare earths-bearing mineral in the ironstones but is more often found in the fenite haloes.