|Latitude & Longitude (WGS84):||31° 16' 20'' South , 121° 39' 59'' East|
|Latitude & Longitude (decimal):||-31.27240,121.66650|
|Köppen climate type:||BSk : Cold semi-arid (steppe) climate|
The Invincible gold mine was discovered in 2012, and is set to become the biggest gold mine in the St Ives area, with open pit production due to begin in 2015. The deposit is south-west of the Bahama-Santa Ana pits near the western shore of Lake Lefroy, and 8 kilometres north-west of the Lefroy Mill. A new causeway will be built across the lake bed to the mine. It is located on the Speedway Shear Zone, with potential for further deposits along the under-explored south south-east strike. The deposit has an estimated resource of 9.2 Mt at 4.5 g/t yielding 1.30 Moz. The area was formerly called LT-17 by the mining company (Goldfields Ltd).
The deposit occurs within the regional scale Speedway-Mount Morgan Shear Zones, which transect the western limb of the Kambalda-St Ives Anticline.
Gold mineralisation is found with the Black Flags Group, a steep south-west dipping package of rocks. There is a unit of mudstone/siltstone-sandstone-quartz-greywacke-conglomerate-volcaniclastic breccia. The mudstone/siltstone is 20 to 70 metres thick, which thins to the south. Overlying this is the other rocks mentioned as the Merougil Creek Beds. This contacts the west side of the Speedway Shear. The east contact is the Morgan Island Shear. There are also three un-mineralised, east-west trending, vertical dipping, Proterozoic dolerite dykes, which intrude the sedimentary package at the north and south ends of the ore body.
The Invincible ore body is truncated to the south by the north-east trending Alpha Island Fault. There is little supergene or halo development in the overlying regolith, due according to the geologists because of the absence largely of saprolite. These Tertiary sediments are 5 to 50 metres thick, and also contains a barren palaeochannel.
Gold is found in shear lodes, and extensional veins. The shear hosted deposits are confined to the mudstone/siltstone, consisting of hydrothermal quartz breccia veins, and intensely altered wallrock clasts of albite-pyrite-carbonate-biotite-pyrrhotite-chlorite-mica. Alteration zones are albite-pyrite. Gold is visible as under 1 mm irregular blebs within aggregates of biotite, as isolated grains within quartz veins, or within microfractures filled with pyrite-biotite-chlorite within the quartz breccia.
At the microscopic level, isolated gold grains are found within pyrrhotite grains, associated with pyrite, molybdenite and native tellurium.
The extensional vein hosted deposits are found within the sandstone/greywacke/conglomerate as less than 30 cm long veins, associated with albite-hematite-pyrite alteration. Visible gold is common as less than 1 mm isolated blebs within quartz veins, or associated with chlorite-biotite near the vein wall contacts.
Commodity ListThis is a list of exploitable or exploited mineral commodities recorded at this locality.
8 valid minerals.
Rock Types Recorded
Select Rock List TypeAlphabetical List Tree Diagram
Entries shown in red are rocks recorded for this region.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
0 - 2.588 Ma
|lake deposits 38492|
Age: Pleistocene (0 - 2.588 Ma)
Comments: regolith; synthesis of multiple published descriptions
Reference: Raymond, O.L., Liu, S., Gallagher, R., Zhang, W., Highet, L.M. Surface Geology of Australia 1:1 million scale dataset 2012 edition. Commonwealth of Australia (Geoscience Australia). 
|Neoarchean - Mesoarchean|
2500 - 3200 Ma
|Archean sedimentary rocks|
Age: Archean (2500 - 3200 Ma)
Comments: Yilgarn Craton
Lithology: Sedimentary rocks
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529.