Yilgarn Craton

The Yilgarn Craton appears to have been assembled between ~2.94 and 2.63 Ga by the accretion of a multitude of formerly present blocks or terranes of existing continental crust, most of which formed between 3.2 Ga and 2.8 Ga.

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;[3] 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 craton is primarily composed of approximately 2.8 billion year old (~2.8 Ga) granite-gneiss metamorphic terrain (the Southwestern Province and Western Gneiss Belt), and three granite-greenstone terrains (the North-East Goldfields, the Southern Cross and the greenschist metamorphic Murchison Provinces). Some greenstone belts and granites are as old as 3.1-2.9 Ga, and some are younger, at ~2.75-2.65 Ga.

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 Western Gneiss Terrane is a series of polydeformed high-grade early Archaean metamorphic belts, composed predominantly of feldspathic leucocratic granulite gneisses, which represent some of the oldest crustal fragments on Earth.[4] The Western Gneiss Terrane is distinct from the remainder of the Yilgarn Craton in that the latter has a predominance of metavolcanic rocks, both felsic and mafic, whereas the former consists of high-grade metasediments and gneisses of unknown protolith.

The Western Gneiss Terrane is exposed along the western half of the northern margin of the Yilgarn Craton as the Narryer Gneiss Terrane, a composite of heavily polydeformed feldspathic metagranite and metasedimentary amphibolite-grade gneisses and migmatites, dated at greater than 3.3 Ga and up to 3.8 Ga in age, flanked by the Murgoo Gneiss Terrane (2.95 Ga), as well as sheets of 2.75 Ga to 2.6 Ga granite, obducted ophiolite sheets (the Trillbar Complex) and some 2.4 Ga to 2.0 Ga Proterozoic gneiss belts.

On the western edge of the Yilgarn Craton, partially covered by Phanerozoic sedimentary basins and in faulted contact with the 2.7 Ga to 2.55 Ga Yilgarn tectonic domains, lies the Jumperding Gneiss Complex of 2.75 to 2.65 Ga age, composed primarily of micaceous quartzite, quartz-feldspar-biotite-garnet gneiss, andalusite and sillimanite schists, banded iron formation and other exotics, intruded by minor masses of porphyritic granite.

Detrital zircons in the Jumperding Gneiss Complex range in age from 3267 ± 30 Ma to 3341 ± 100 Ma, with metamorphic overgrowth dated at 3180 Ma.

On the southwest of the Yilgarn Craton the Balingup Gneiss Complex is situated inboard from the Early Proterozoic Leeuwin Complex of metamorphic rocks. The Balingup Complex consists primarily of metasedimentary paragneiss, granite orthogneiss, with minor layers of calc-silicate, ultramafic and ortho-amphibolite gneiss. The metamorphic grade is considered to be peak granulite facies, but the majority has preserved peak amphibolite facies assemblages.

In total, the Western Gneiss Terrane sub-blocks represent an earlier substrate upon which the majority of the Yilgarn Craton's about 2.70 to 2.55 Ga greenstone metavolcanic belts have been deposited and into which the voluminous Archaean trondhjemite-tonalite-granodiorite suite and trondhjemite-tonalite-diorite suite granites were emplaced.

The Murchison Province is exposed in the western and northern third of the Yilgarn Craton. The Province is bounded by major transcrustal structures which separate it from the surrounding tectonic provinces of the craton and the Western Gneiss Belt.

The Murchison Province Stratigraphy, after Watkins (1990), is divided into six basic structural-stratigraphic components - two greenstone belt metavolcanic-metasedimentary sequences and four suites of granitoids.

• Luke Creek Group metavolcanics
• Mount Farmer Group
• Early granodiorite-monzogranite intrusive suite (now pegmatite-banded orthogneiss)
• Monzogranite Suite (now folded, metagranite)
• Two post-tectonic differentiated suites of granitoid rocks

The structural framework in the northeastern Yilgarn craton was largely shaped by transpression that led to the development of folds, reverse faults, sinistral strike-slip movement on NNW-trending regional shear zones, followed by regional folding and shortening. The later occurred in overlapping tectonic processes. The first deformation event is poorly understood but appears to have involved N-S thrusting.

The Murchison Province contains the Yarrabubba crater, which is the oldest dated meteorite impact crater, at 2229 ± 5 Ma. The crater is heavily eroded and no surface expression remains of the original structure. The primary trace is an elliptical aero-magnetic anomaly, measuring approximately 20km by 11km, as well as the presence of shock-recrystallised minerals. This impact may have ended the Huronian glaciation by climate forcing with a significant pulse of greenhouse gases, as the age broadly overlaps with the youngest glacial deposits.[2]

The Southern Cross Province lies in the central area of the Yilgarn craton. The Marda–Diemals greenstone belt in the Southern Cross Terrane can be divided into three layers: the lower greenstone belt (ca. 3.0 Ga) characterized by mafic volcanic rock and banded iron formation, a felsic-intermediate volcanism layer, and an upper sedimentary layer (ca. 2.73 Ga) of calc-alkaline volcanic (Marda Complex) and clastic sedimentary rocks (Diemals Formation).[5]

East–West orogeny (ca. 2730–2680 Ma) occurred in two stages; an earlier folding phase and a late phase that resulted in deposition and deformation of the Diemals Formation. Subsequent orogeny (ca. 2680–2655 Ma) resulted in shear zones and arcuate structures.

The lithostratigraphy of the Marda–Diemals greenstone belt are similar to the northern Murchison Terrane, but has older greenstones and deformation events than the southern Eastern Goldfields Terrane. This indicates that the Eastern Goldfields Terrane may have accreted to an older Murchison–Southern Cross granite–greenstone nucleus.

The Archaean Norseman-Wiluna Greenstone Belt[6] in the Eastern Goldfield Province contains most of Australia's lode gold deposits, including the famous Kalgoorlie Golden Mile containing the Super Pit.

These gold deposits are generally of large tonnage and are confined to the volcanic-intrusive-sedimentary sequences of the greenstone belts and not the granites. There is a pattern of gold distribution along the Archean Boulder-Lefroy shear zone.

Extrusive komatiites (ultramafic volcanic rocks) occur along the Norseman-Wiluna Greenstone Belt. A change from volcanic-dominated to plutonic-dominated magmatism occurred in the Norseman-Wiluna Greenstone Belt approximately 2685–2675 Ma. Voluminous high-Ca granite intrusions occurred 2670–2655 Ma.[7] Much of the gold was deposited between 2650–2630 Ma, with much of this associated with strike-slip reactivation of earlier faults (normal and reverse).

An earlier gold event 2660-2655 Ma was associated with major extension (normal faulting and granite doming) resulting in the formation of late basins and the intrusion of mantle-derived magmas (syenites and Mafic-type granites/porphyries) and tight anticlockwise PTt paths.

The Yilgarn Craton is bound on all sides by younger terranes of various ages, but predominantly of Proterozoic age. The boundaries between the various flanking terranes provide considerable evidence of the post-Archaean events which have involved the Yilgarn Craton.

The Yilgarn Craton is partially covered by onlapping sedimentary basins of Palaeozoic and Phanerozoic age in the east and north-east, including the Canning Basin. It is bounded on the western edge by the Darling Scarp and Darling Fault which separate the Yilgarn Craton from the Perth Basin to the west, and is covered by several remnant sedimentary basins of Jurassic age such as the Collie Sub-Basin.

The Yilgarn Craton also has a considerable Tertiary and younger sedimentary veneer of palaeochannel deposits derived from prolonged erosion, sedimentation and redeposition of older cover sequences and regolith as well as the Archaean basement itself.

Recognised Tertiary cover sequences include the Bremer Basin, Officer Basin and others.

The Yilgarn craton is believed to have remained at or above sea level for a considerable length of time.[10] Some of the Yilgarn regolith is the oldest in the world, recording weathering events as early as the Cretaceous Period. This has been created by the generally subtropical latitudes and conditions of the Yilgarn craton, with minimal to no glaciation and generally flat topographical relief resulting in comparatively minor erosion.

The regolith is extremely deeply weathered, in some areas completely converted to saprolite up to 100 metres below surface. This is considered to have been produced during Caenozoic to Palaeocene tropical conditions, as evidenced by mottled duricrust which records fossilised tree roots, some over 60 million years old. Previous weathering events have been recorded in magnetically remnant ferruginous laterite of a Jurassic age, at about 180 Ma.

The regolith of the Yilgarn impacts directly on the flora and fauna, as some of the soil is essentially fossilised. Much of the groundwater of the Yilgarn is hypersaline, with some being supersaturated in salt. This renders swathes of land barren, with significant salt lakes, and high saline water tables. The origin of this salt is thought to be from precipitation of sea salt carried over the Australian landmass for the past several dozen million years, and the high evaporation rate leaving the salt behind.

The greenstone belts of the Yilgarn Craton include:

• Southern Cross Greenstone Belt
• Norseman-Wiluna Belt
• Duketon Belt
• Gullewa Greenstone Belt

The Yilgarn Craton is Australia's premier mineral province. It attracts more than half of Australia's minerals exploration expenditure, and produces two thirds of all gold and most of the nickel mined in Australia. The craton contains some 30% of the world's known gold reserves, about 20% of the world's nickel reserves, 80% of the world's tantalum reserves, considerable iron ore, copper, zinc and minor lead reserves. The craton contains significant platinum, vanadium, hard-rock titanium and considerable iron ore resources.

Mining is conducted mostly in the greenstone belts around mining centres such as Kalgoorlie, Kambalda, Norseman, Meekatharra and Wiluna, and minor centres such as Laverton, Leinster, Leonora and Southern Cross.

Ore concentrates or finished product are transported by rail or road to Perth, Fremantle, Esperance, Albany or Geraldton.

• Archaean – Second eon of the geologic timescale
• Australian Shield – Large part of the continent of Australia
• Centralian Superbasin – Sedimentary basin in Australia
• Gawler Craton – Province of the larger West Australian Shield in central South Australia
• Geology of Australia – Overview of the geology of Australia
• Ore genesis – How the various types of mineral deposits form within the Earth's crust
• Perth Basin – Sedimentary basin in Western Australia
• Pilbara Craton – Old and stable part of the continental lithosphere located in Pilbara, Western Australia
• Western Plateau – Australian drainage region