Western Australia, WA, Australia

Main commodities: Au
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The Youanmi gold deposit is located 125 km SE of Mount Magnet. 85 km SW of Sandstone and 550 km NE of Perth in the Youanmi Greenstone Belt. It lies within the East Murchison Mineral Field and Murchison Domain of the Youanmi Terrane of the Archaean Yilgarn Craton in Western Australia (#Location: 28° 36' 18"S, 118° 50' 17"E).

The structure of the Youanmi area is dominated by the NNE-trending Youanmi Fault Zone which forms the boundary between the Southern Cross (to the east) and Murchison (to the west) domains of the Youanmi Terrane, and offsets the Youanmi Greenstone Belt which occurs in both domains. The bulk of the gold mineralisation at Youanmi is hosted within NNW splays off the Youanmi Fault. These splays diverge from the Youanmi Fault where it intersects the informally named Youanmi Granite to the SE of Youanmi, and trend sub-parallel to its SW contact with the 'greenstone' sequence.

Youanmi is the most significant of a cluster of gold occurrences that are distributed over a NW-SE elongated area of ~3 x 2 km, straddling the granite-greenstone contact, and terminated to the SE by the Youanmi Fault.

The area has produced an estimated 21 t of gold at an average grade of 5.47 g/t Au since discovery in 1901 with most of the gold produced from the Youanmi Mine with additional gold sourced from Youangarra, Penny West, Columbia–Magenta and Currans.

The NW trending Youanmi greenstone belt is mainly composed of metamorphosed tholeiitic basalts and intrusives, minor banded iron formation and abundant felsic pyroclastics.   These are overlain <2 km to the west by gabbro, pyroxenite, anorthosite and magnetite rich rocks of the layered Youanmi Intrusion.   Outcrops of gniess are either uplifted basement or early kinematic diapiric granitoid domes formed from the greenstones.

The ore occurs in a zone of shearing in the greenstones adjacent to an intrusive granitoid dome.   The contact zone is arcute averaging 340° and dipping at 50 to 70° W.   Gold bearing sericite-quartz-sulphide shear zones are sub-parallel to the granitoid contact within both greenstone and granitoid.   Within the pit weathering has reduced most rocks to clay and goethite to a depth of 80 m.   The host mafic rocks contain ubiquitous magnetite.   Interbeds (at 20 to 30 m intervals) of interflow tuffaceous sediments are more competent and more strained and thus contain much of the gold.   BIF (quartz-magnetite) horizons are rare, but where present in shears the magnetite is pseudomorphed by pyrite.   The granitoids in the mine area are coarse grained quartz-plagioclase-microcline-biotite admellite (quartz monzonite).

Gold mineralistion and alteration are localised along ductile/brittle shear zones which form anastomosing networks that occupy a zone that extends from 20 m within the adamellite to 100 m into the greenstone, and has a 1 km strike length.   They are sericite-quartz mylonites with abundant sulphides, chlorite and carbonate and vary from 1 to 20 m thick, averaging 3 to 4 m.   Gold occurs within pyrite and arsenopyrite which comprise 15% of the mylonite. The shear zones are often enclosed by chlorite-quartz-sericite altered metasediments from 1 to 5 m on either side, with low sulphides and low gold.

The production history of the Youanmi mine (Munro, 1990) includes:
    Underground production to 300 m to 1942 - 0.7 Mt @ 11.6 g/t Au for 8.1 t of recovered Au,
    Open pit production to 100 m, 1986-89 - 2.03 Mt @ 2 g/t Au for 4.0 t of recovered Au,

Remaining reserves in 1990 were:
    Proven+probable reserve: 0.13 Mt @ 3.1 g/t Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1990.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Munro K D  1990 - Youanmi Gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 279-282

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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