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Yilgarn craton

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The Yilgarn Craton is a huge craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts.

It is composed primarily of approximately 2,800 million year old (2.8 Ga) granites and greenstone belts. 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 Yilgarn is divided into four main provinces of Archaean rocks under 3.0 Ga, with several older gneiss terranes sandwiched at the edges. The main provinces are the greenschist metamorphic grade Murchison Province, Southern Cross Province and Eastern Goldfields Province, together with the high metamorphic grade Western Gneiss Belt.

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 3.2Ga 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; regional metamorphism and deformation as well as the emplacement of the vast majority of the craton's endowment in gold mineralisation.

The accretion events occurred in several phases, probably by accretion of continental fragments separated by pauses in subduction, with renewed activity occurring episodically.

Contents

Western Gneiss Terrane

The Western Gneiss Terrane is a series of polydeformed high-grade early Archaean metamorphic belts, comprised predominantly of feldspathic lecocratic granulite gneisses, which represent some of the oldest crustal fragments on Earth. 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 is comprised 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, comprosed primarily of micaceous quartzite, quartz-feldspar-biotite-garnet gneiss, andalusite and sillimanite schists, banded iron formation and other exoitics, 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 c. 2.70 to 2.55 Ga greenstone metavolcanic belts have been deposited and into which the voluminous Archaean TTG (trondhjemite-tonalite-granodiorite) suite and TTD (trondhjemite-tonalite-diorite) suite granites were emplaced.

Murchison Province

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 provvinces 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.

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

Sedimentary Basin Cover

The Yilgarn Craton is partially covered by onlapping sedimentary basins of Palaeozoic and Phanerozoic age in the east and north-east. 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.

Regolith

The Yilgarn craton is believed to have remained at or above sea level for a considerable length of time. 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, 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:

Economic geology

The Yilgarn 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 of the Yilgarn craton, 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.

Gold

The Yilgarn Craton gold endowment is considered to be a process of a prolonged period of cratonic development during a series of orogenic episodes beginning at about ~2.9Ga and culminating in ~2.67Ga. These events saw the assembly of the Yilgarn Craton from several 'proto-cratons' or unconsolidated terranes of perhaps older earlier-formed granite-gneiss, probably of similar nature to the Narryer Gneiss Terrane. These have been mostly destroyed by the voluminous tonalite-trondhjemite-granodiorite (TTG) magmatism of c. 2.75-2.85Ga, which saw vast quantities of essentially uniform igneous-derived granitoids intruded into the existing greenstone belts, thus forming the cratonising event.

These granites now form pillow-like flatly-dipping to steeply dipping sheath-like margins to the greenstone terranes, and may have contributed to the gold mineralisation either during the metamorphic decarbonation-dehydration reactions or as heat engines to drive thermal convection and hydrothermal fluid flow.

The greenstone-granite terranes of the Yilgarn Craton have subsequently been affected by several later metamorphic events and deformations, which have now overprinted the craton with zones of steeply-dipping foliation and vertically thrust-offset fault blocks. These later events tend not to cause mineralisation, instead causing structural disruption of the gold lodes.

Nickel-PGE Deposits

Nickel and PGE's in the Yilgarn Craton are associated primarily with an hypothesised plume event which occurred at or around the Kambalda Dome at c 2.85Ga. This resulted in a voluminous thoeliitic basalt event, with widespread deposition of thick sequences of mafic volcanic rocks. This may represent the broad, voluminous plume head erupting through a cratonic margin which separated a slightly older western section of the Yilgarn from a younger eastern section.

Closer to the centre of the plume, the plume tail erupted much hotter, and caused voluminous ultramafic and komatiite volcanism. This was submarine volcanism and occurred over the top of sulfidic sediments, cherts and silts. These sediments tended to melt from the komatitic lava, which was at temperatures in excess of 1600°C, causing sulfur saturation. This caused precipitation of molten nickel-iron-copper sulfides.

Today, these deposits are heavily sheared and mechanically disrupted, often occurring many metres away from their original position in a lava palaeo-channel. Massive nickel sulfide lodes are mined at Kambalda, and disseminated nickel sulfides are mined at the Black Swan-Silver Swan deposit and at Leinster and Mount Keith.

Nickel laterite has also been formed on top of significant accumulations of forsteritic ultramafics, such as at Murrin-murrin and in Ravensthorpe, during prolonged tropical weathering.

Partial list of ore deposits and mines

See also

References

Hammond, R.L. & Nisbett B.W., 1992. Towards a Structural and Tectonic Framework for the central Norseman-Wiluna Greenstone Belt, Western Australia. in, The Archaean: Terrains, Processes and Metallogeny, University of Western Australia, Publication 22, pp. 39-49.

Swager, C.P., Witt W.K., Griffin A.L., Ahmat A.L., Hunter W.M., McGoldrick P.J. & Wyche, S., 1992. Late Archaean Granite-Greenstones of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. in, The Archaean: Terrains, Processes and Metallogeny, University of Western Australia, Publication 22, pp. 39-49.

 

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