Summary:
The tenements held by Macarthur are known to be prospective for iron as well as nickel and gold mineralisation. The iron mineralisation is related to the extensive BIF units that occur throughout the tenements.
The mineralisation at Moonshine and Moonshine North deposits is associated with primary magnetite
mineralization hosted by banded iron formation (BIF). The multiple BIF units steeply dip 75° to 85° to
the west and strikes approximately 320° and 335° respectively the units have an average thickness of
15m, over a strike length of 17 km. The BIF units frequently outcrop.
The parts of the north-northwest trending Yerilgee greenstone belt covered by the Project tenements comprise a layered succession of Archean rocks. At the interpreted base of the succession is a sequence of high-magnesium basalt flows more than 1 km thick overlain by komatiitic ultramafic volcanic rocks with narrow interflow BIFs and in some cases, other sedimentary rocks (Svensson, 2012). Further high- magnesium basalt lavas with occasional interflow BIFs overlain, possibly unconformably, by sedimentary rocks (cherty, silicified, pyritic and graphitic) are interpreted to form the top of this sequence. In places, gabbroic sills interpreted to be co-magmatic with the upper highmagnesium basalts, have been intruded into the lower mafic and ultramafic lavas. The elongated lens shaped Yerilgee belt is bounded by major north-northwest trending fault/shear zones.
The Archaean sequence has been intensely folded. At least five possibly sinistral fault zones of similar but slightly more north-westerly trend are interpreted within the widest part of the belt and are believed to successively repeat the layered succession. Two northerly trending sinistral faults obliquely crosscut the belt in this area.
A number of large synclinal fold structures have been identified. These appear to be located adjacent to the eastern margins of the fault blocks. These folds have north-westerly and north-north-westerly trending axes and were mapped in detail (Greenfield, 2001) show plunges at 30–60° in the same direction. In general, the fold axes are steeply dipping. The folding appears to have been contemporaneous with faulting. In plan, the movement on the fault planes was sinistral but in a true sense is believed also to have been reverse faulting with the direction of movement on the western down-throw sides of the fault planes being inclined at 30–60° towards the east-northeast. The synclines and anticlines are considered to be drag fold structures.
The most recent notable tectonic event was approximately 2.6 billion years ago and appears to have dilated the north-northwest trending shear zones, generating north-northeast trending and conjugate northeast to easterly trending structures. These brittle fractures have in many places been intruded by granitic dykes or quartz veins. The Project tenements cover about 60 km of the greenstone belts strike length but because of fault repeats, they are estimated to cover more than 150 km of BIF sequence strike length.
The iron ore mineralisation consists of secondary pisolite mineralization, primary magnetite mineralization associated with un-oxidized BIF and ultramafic rocks, and goethite-hematite mineralization associated with oxidized BIF.
The hematite/goethite units exist largely as a supergene product. Weathering has resulted in the leaching of the majority of the silica from the BIFs, thus producing a rock rich in iron and low in silica. These enriched bands vary from 1 to 30 metres in true thickness and are largely steeply dipping by 70° to 90°.
The magnetite mineralisation is associated with primary magnetite hosted by BIF. The multiple BIF units steeply dip 75–85° to the west and strike approximately 320° and 335° respectively. The units have an average thickness of 15 m, over a strike length of 17 km.
The outcropping geology of the project area is comprised of a combination of unaltered silica rich BIFs and altered, enriched haematite/goethite BIFs. Weathering has resulted in the leaching of the majority of the silica from the BIFs, thus producing a rock rich in iron and low in silica, near surface. Below the depth of oxidation (generally between 45 m and 90 m from surface), the BIF units are comprised almost entirely of ferrous/ferric Fe(II,III) iron, silica and small amounts of alumina with occasional incipient iron sulphides (predominantly pyrite).
The iron grades are generally normally distributed, as opposed to log-normally for the altered haematite/goethite BIF, with grades consistently between 20% Fe and 40% Fe. Macarthur believes the majority of the underlying BIF units have experienced minimal metamorphism beyond their original formation. A notable exception to this is a pocket of high-grade magnetite mineralisation, up to 15 m true thickness, continuous along strike for >200 m, and >60% Fe, located in the Moonshine North deposit. This pocket of high-grade magnetite mineralisation is interpreted to be the result of structural and geothermal alteration of the primary BIF fabric.
Logged komatiite and ultramafic units are typically thin (<10 m true thickness) and strongly weathered near the surface and are only identifiable at depth through drilling. The ultramafics are usually found proximal to the hanging wall of the BIF units.
Serpentinised high-magnesium basalts form the bulk of the geology at Lake Giles, forming thick, continuous, fine to medium grained granular units, occasionally cut by minor quartz veins and hosting sulphidic shales, locally including several metres of massive iron sulphides. Mafic intersections of interest have been occasionally investigated for gold mineralisation, but no specific targeting for gold has been recorded.
Structural deformation within the main BIF packages is generally weak, forming gentle kink banding and box folding, although some sections are interpreted as showing intense recumbent folding with sub- vertical axial planes, such as the southern edge of Moonshine. The larger BIF bodies at Moonshine and Moonshine North have relatively consistent thickness and dip to depths of over 250 m from surface as tested by a number of drillholes, increasing confidence that the remainder of the BIF ridges at Moonshine behave in a similar way and are not truncated at depth by synclines or other structural mechanisms.
The rocks of the Lake Giles Iron Project have been logged into six different weathering classifications:
• Complete – All clay with no remnant rock texture;
• Extreme – Largely clay with some remnant rock texture;
• Strong – Rock texture moderately preserved, significant presence of fines, often weak;
• Moderate – Rock texture fully preserved, all minerals show weathering;
• Partial – Oxidation limited to the most unstable minerals only (e.g., sulphides); and
• Fresh – No oxidation of any minerals.
The magnetite is present in the fresh BIFs along with high quantities of silica. This is the primary unaltered form of BIFs at site and in general has not been subject to any significant later iron enrichment.