Windimurra intrusion
Encyclopedia
The Windimurra Intrusion is a giant ultramafic-mafic intrusion emplaced within the Yilgarn craton
of Western Australia
. It is located approximately 100 kilometres south east of the town of Mount Magnet.
into the Archaean granite
-greenstone
terrain of the Murchison province within the Yilgarn Craton.
Windimurra sits astride an inferred proto-rift
zone in the Archaean basement, and is dated at 2850 +/- 10 Ma.
The intrusion is tilted to the west-northwest at about 70 degrees.
formed of cumulate layering by a process of fractional crystallization
. Individual rock types can be grouped into a troctolite
phase or series, a gabbro
phase or series and a gabbronorite
phase or series. Anorthosite
cumulates are preserved in the roof sections, most of which are sheared and faulted off. A marginal granophyre
complex exists in the roof and wall rocks, formed by advective heat transfer causing melting of the country rocks.
of South Africa
.
Exploration has focused on finding basal nickel
sulfide
deposits, although that has proved fruitless as the basalt
portions of the deposit are faulted off, and chromitite
or vanadium
deposits related to oxide cumulate layers higher up in the intrusion. Both these latter efforts have proved successful, with the opening of a major vanadium operation in the 1990s, subsequently closed.
magnetite
and ilmenite
adcumulate and mesocumulate deposit, forming a resource in excess of 120 Mt grading 5% V2O5. It is divided into a lower olivine
bearing zone ~150m thick, a middle olivine-free magnetitite zone 250 to 300m thick and an upper magnetitite-free zone 50m thick. The middle zone contains a basal 2m thick magnetitite zone (containing >70% magnetite) and podiform, lenticular magnetitite horizons above it, and is of principal economic interest.
Interstitial ore textures in some plagioclase
-bearing ilmenite zones indicate ilmenite may have been present as a melt phase. In many zones, 10-50m thick ilmenite zones may be of economic interest as vanadium contents in magnetite appear to actually be hosted in ilmenite exsolutions.
Yilgarn craton
The Yilgarn Craton is a large 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...
of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
. It is located approximately 100 kilometres south east of the town of Mount Magnet.
Setting
Windimurra is a conical sheet-like body of ultramafic rock intrudedIntrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
into the Archaean granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
-greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
terrain of the Murchison province within the Yilgarn Craton.
Windimurra sits astride an inferred proto-rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
zone in the Archaean basement, and is dated at 2850 +/- 10 Ma.
The intrusion is tilted to the west-northwest at about 70 degrees.
Lithology
Windimurra contains in excess of 13,000m of intact ultramafic stratigraphyStratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
formed of cumulate layering by a process of fractional crystallization
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
. Individual rock types can be grouped into a troctolite
Troctolite
Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a...
phase or series, a gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
phase or series and a gabbronorite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
phase or series. Anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
cumulates are preserved in the roof sections, most of which are sheared and faulted off. A marginal granophyre
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
complex exists in the roof and wall rocks, formed by advective heat transfer causing melting of the country rocks.
Economic geology
The Windimurra Intrusion has been of great interest to mineral exploration companies for decades, as it is one of the thickest and largest ultramafic intrusions in the world, though it has been fragmented and mostly removed by shearing unlike the Bushveld Igneous ComplexBushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
.
Exploration has focused on finding basal nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
deposits, although that has proved fruitless as the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
portions of the deposit are faulted off, and chromitite
Chromitite
Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa and the Stillwater igneous complex in Montana.-References:...
or vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
deposits related to oxide cumulate layers higher up in the intrusion. Both these latter efforts have proved successful, with the opening of a major vanadium operation in the 1990s, subsequently closed.
Vanadium
The Shepherd's Discordant Zone is host to a laterally extensive vanadiferousVanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
and ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
adcumulate and mesocumulate deposit, forming a resource in excess of 120 Mt grading 5% V2O5. It is divided into a lower olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
bearing zone ~150m thick, a middle olivine-free magnetitite zone 250 to 300m thick and an upper magnetitite-free zone 50m thick. The middle zone contains a basal 2m thick magnetitite zone (containing >70% magnetite) and podiform, lenticular magnetitite horizons above it, and is of principal economic interest.
Interstitial ore textures in some plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
-bearing ilmenite zones indicate ilmenite may have been present as a melt phase. In many zones, 10-50m thick ilmenite zones may be of economic interest as vanadium contents in magnetite appear to actually be hosted in ilmenite exsolutions.