Superior Mining’s Redcastle gold project is located 65 kms east of Leonora in the Eastern Goldfield of Western Australia in the Kurnalp Terrane. The Eastern Goldfield province occupies the eastern third of the Yilgarn Craton covering an area 900 km north-south by 400 km east-west and is a typical Archaean granite-greenstone terrain “characterised by large areas of granitoid and, generally narrow, linear to arcuate belts of greenstone” (Griffin, 1990). The greenstones are more abundant than in other provinces of the craton, and are generally deformed and metamorphosed to greenschist facies. The greenstones comprise thick mafic to ultramafic volcanic sequences and include sedimentary sequences with inter-bedded shales, chert and banded iron formations. Granitoid intrusions have disrupted the greenstone sequences. The province is richly mineralised in gold and nickel.
The Yilgarn Craton appears to have been assembled between about 2.94 and 2.63 billion years ago by the accretion of a multitude of formerly present blocks or terranes of existing continental crust, most of which formed between 3.2 billion and 2.8 billion years ago.
This accretion event is recorded by widespread granite and granodiorite intrusions, which comprise over 70% of the Yilgarn craton; voluminous tholeiitic basalt and komatiite volcanism; (Chen et al, 2003) regional metamorphism and deformation as well as the emplacement of the vast majority of the craton's endowment in gold mineralisation.
These accretion events occurred in several phases, probably by accretion of continental fragments separated by pauses in subduction, with renewed activity occurring episodically.
The Yilgarn Craton is primarily composed of approximately 2.8 billion year old granite-gneiss metamorphic terrane (the Southwestern Province and Western Gneiss Belt), and three granite-greenstone terranes (the North-East Goldfields, the Southern Cross and the greenschist metamorphic Murchison Provinces). Some greenstone belts and granites are as old as 3.1 to 2.9 billion years, and some are younger, at about 2.75 billion to 2.65 billion years old.
The craton is one of the distinct physiographic provinces of the West Australian Shield physiographic division, which comprises the Stirling-Mount Barren Block, Darling Hills, and Recherche Shelf sections.
The Yilgarn Craton is host to around 30% of the world's economically demonstrably recoverable reserves (EDR) of gold.
Major gold deposits occur at Kalgoorlie, Kambalda, Mount Magnet, Boddington, Laverton and Wiluna, and are hosted in greenstone belts. These form linear belts of mafic, ultramafic and felsic volcanics, intercalated with sedimentary sequences, and have been multiply deformed and metamorphosed. The mode of occurrence of the gold mineralisation tends to be small to medium-sized structurally controlled lodes, shears, and quartz veins.
A key feature beneath many of the region's gold deposits are granite-cored domes at a range of scales. These provided an architecture that focussed metal-rich fluids into the upper crust depositional sites. Debate continues on whether oxidized magmatic fluids mixed with a reduced (mantle-deep crustal fluid) at the depositional site.
Signatures of the mantle are found in many (large) deposits, including melts from a metasomatised mantle wedge (enriched Mafic-type granites/porphyries and syenites) as well as lamphrophyres. Debate continues whether these mantle rocks were a fluid and/or metal source, or simply reflect a favourable pathway.
Regional Geological Setting
The geological setting of the Redcastle project area comprises low-strain greenstones and granitoids within the Kurnalpi Terrane, and is bounded by the Keith-Kilkenny Lineament to the west and the Laverton Tectonic zone to the east.
The Redcastle project is located in a belt of west-northwest-trending basalts and differentiated dolerite intrusives. The sequence dips between 45º and 60º to the northeast. The basalt sequence occurs mainly to the south of the prospect. The area surrounding the deposit has been intensely lateritized with some of the original laterite surface preserved as small mesas. Faults occur within the immediate prospect area and are commonly marked by quartz veins. Previous photo interpretation by Loosemore (1983) did not identify any regional scale faults near the prospect.
The immediate area surrounding the Redcastle deposit is intensely lateritized with most of the old workings occurring approximately 10 to 15 m below remnants of the laterite surface. As a consequence, outcrop is poor and mostly unrecognizable. Some geology is discernible from the dumps and shallow underground workings.
Local Geological Setting
The local geology was originally described by Jeffrey (2007). The Redcastle Project lies within the Murrin-Margaret geological sector of Hallberg (1985). This is a terrane of low strain greenstones and granitoids bounded by the Keith-Kilkenny lineament to the west and the Laverton Tectonic Zone to the east. There are three main lithological entities within the mapped portion of the project area:
- a greenstone sequence of tholeiite basalts with minor interflow sediments is intruded by gabbro and dolerite sills. The dolerite sills have pyroxenitic or gabbroic bases and quartzrich or more feldspar-rich tops;
- the greenstones have been intruded by weakly porphyritic tonalite and dacite porphyry. The late stage dacite porphyry is confined to the periphery of the tonalite bodies which it intrudes, and the greenstones close to the tonalite contact. Regional metamorphism is lower greenschist facies; and
- tonalite and dacite have been intruded early in the structural evolution of the greenstone belt. They are foliated and folded with the greenstone sequence. Early isoclinal folds have been refolded into broad north trending open folds with an amplitude of about four kilometres.
Three structural domains have been recognized:
- a northern domain of gabbro, dolerite and basalt, folded into a south plunging anticline;
- a central domain of tonalite with sheets of dolerite, quartz dolerite and basalt, dominated by 300º/120º striking, 40 to 60 º north dipping shear zones (thrusts) up to 100m wide; and
- southern domain of basalt and dolerite cropping out as a folded isoclinal anticline with a
- dolerite core, and with east trending quartz filled fault and shear zones, up to 15m wide, and poorly outcropping east-west structures.
In addition, six fault and shear directions have been recognized:
- 320º to 330º trending, steeply west dipping fault/shear zones up to 60m wide;
- 300º trending 40º to 60º north dipping thrusts that host the gold mineralization at the historic Redcastle Mining Centre. Five major thrusts are recognizable that can be traced along strike for up to 4.5 km;
- east trending faults up to 15m wide and steeply dipping. They are often accompanied by dacite porphyry and foliated tonalite and are host to narrow gold bearing quartz veins. Four such faults have been mapped with individual faults traceable for up to one kilometre strike length;
- north-trending faults up to 15m wide which dip steeply to the west. They displace the 300º trending thrusts and can host gold mineralization. Two major faults and several smaller faults have been mapped by others in the past;
- a single 075º trending, non-exposed fault traceable for six kilometres. This fault, the ATB fault sinistrally displaces the 300º trending thrusts and probably the north-trending and 330º trending sets as well. It appears to be a late fault but may be displaced the late faults; and north-northeast trending steeply dipping late faults. Minor gold mineralization is associated with these faults where they cut favourable lithologies.
Post-laterite palaeochannel sediments are preserved as remnants on the tops and slopes of isolated hills. Palaeochannel sediments unconformably overlie laterite duricrust, pallid zone clays and upper saprolite. The ferruginous conglomerate is composed of goethite and maghemite. Erosion of the palaeochannel conglomerates has resulted in detrital talus of magnetic ironstones around the isolated hills.
Gold mineralisation is hosted in quartz veined faults and shears where they transect favourable mafic lithologies, in particular dolerite and, more specifically, quartz dolerite, and are controlled by numerous northwest trending generally northeast-dipping fault zones. The intersections of early structures by later cross-cutting faults where mafic rocks are present are important sites for gold deposition, as are quartz stockworks in tonalite. The area has been subjected to intense lateritisation which is preserved as mesas and in breakaways.
Considerable gold remobilization has occurred with depletion of gold from the laterite surface to a depth of approximately 8 to 15 m. Supergene enrichment has occurred near the base of the pallid zone resulting in development of nuggets (as seen from the numerous "patches") and rich supergene gold as previously mined. Much of the early mining selectively targeted material containing these supergene enriched quartz veins, which were mined from between approximately 10 to 20 m below the Tertiary laterite surface; or down to 11 m below the present surface.