The Widgiemooltha Komatiite is a formation of komatiite in the Yilgarn craton of Western Australia.
Stratigraphy The stratigraphy of the Widgiemooltha Komatiite is well known to be part of the regional komatiite magmatic event also seen at the Kambalda Dome, 50 kilometres (31 mi) to the north. There are comparisons which place the Widgiemooltha Komatiite as equivalent to the Silver Lake Komatiite. The Mt Edwards Basalt is correlated regionally with the Devon Consuls Basalt of Kambalda, and the Widgeimooltha Chert correlated with the Paringa Slate. The structure of the Widgiemooltha Dome has three thrusted repetitions of the basal komatiite contact and komatiite sequence including footwall Mt Edwards Basalt and hangingwall sediments (Widgiemooltha Chert).
Widgiemooltha Dome The Widgiemooltha Komatiite is exposed around the margins of a large, 450 square kilometres (174 sq mi) granite dome. The Widgiemooltha Granite is a coarse to medium, holocrystalline equigranular granite with subordinate biotite and ferromagnesian minerals. It is mildly deformed, and is considered to have intruded concurrently with doming and uplift during late deformation. It is intrusive into the thrust repeated stratigraphy of the Widgiemooltha Komatiite and its sequence.
Lake Zot Dolerite The Lake Zot Dolerite is similar to the Defiance Dolerite and other regionally important subvolcanic doleritic sill complexes throughout the Yilgarn craton. Within the Widgiemooltha Dome area, the Lake Zot Dolerite can attain greater than 300 metres (984 ft) thickness and is intrusive into the hangingwall basalt and Widgiemooltha Chert above the Widgiemooltha Komatiite. The dolerite is coarse grained in the main, often equigranular and holocrystalline although porphyritic areas are known and chilled margins and occasional compositional variations are noted. It is often epidotised, carbonated and sodium metasomatised in proximity to major structures. It can often be reasonably strained throughout its bulk and especially in the footwall contact with the Widgiemooltha Komatiite.
Widgiemooltha Chert The Widgiemooltha Chert is a deep sea chemical sedimentary unit stratigraphically above the Widgiemooltha Komatiite. It is composed of from less than 1 metre (0 mi) to 40 metres (0 mi) of sulfidic, graphitic and siliceous cherts, often finely laminated and expressing ptygmatic folding post-sedimentation. It is considered to be an exhalative deposit formed post-magmatism. It often is intruded by the Lake Zot Dolerite. The Widgiemooltha Chert is often the host of regional shears and thrusts.
Widgiemooltha Komatiite The Widgiemooltha Komatiite is typically 30 metres (0 mi) to 300 metres (0 mi) in apparent thickness, often with several clearly defined interflow sediment intervals and A1, A2, A3 spinifex textured zones and B2 zone cumulate zones developed rhythmically throughout. Depending on area and volcanic facies, there can be anywhere from greater than 24 individual flow units to as few as three flows within the Widgiemooltha Komatiite. The komatiite is typically of high magnesian character throughout, although flow top spinifex zones may get to as low as 18% MgO. Highest magnesium contents in rocks occur within the Wannaway area, with magnesium contents of up to 45% MgO reported from adcumulate serpentinite lithologies in the basal flow. The Widgiemooltha Komatiite is host to no less than 15 individual channelised kambalda type komatiitic nickel ore deposits of which 5 have been mined, the remaining ten being of too low grade and low tenor to be economic at present. These features correlate with thickened komatiitic sequences, thick basal flows, trough like structural features and high magnesium lavas.
Mt Edwards Basalt The footwall to the Widgiemooltha Komatiite is the Mt Edwards Basalt, which is a low to medium MgO mafic extrusive rock, metamorphosed to upper greenschist facies. Mineralogy is chlorite, actinolite, rare epidote, quartz and albite. The Mt Edwards Basalt has uncommon interflow sedimentary intervals and some well developed pillow basalt flow tops which give regional facing directions. The true thickness of the Mt Edwards Basalt is unknown.
Economic importance The Widgiemooltha Komatiite is an important host for Kambalda type komatiitic nickel ore deposits. Nickel ore has been found at several locations around the Widgiemooltha Dome and has been mined from six locations, with four underground mines in operation as of 2007 and a proposed open cut mine for mid-2007. Further exceptionally high-grade gold mineralisation is present at Wattle Dam, to 6 kg (200 Oz) to the ton, within the Widgiemooltha Komatiite.
132 North From Nickel et al. (1994) The 132 North deposit was discovered by in 1968 by International Nickel Australia Limited via gossan searching and mined by Western Mining Corporation in 1981, with 900 tons of nickel metal produced. It was a small pod of low-grade nickel sulfide mineralisation hosted in a parasitic isoclinal syncline developed on the 132 Anticline on the northern flank of the Widgiemooltha Dome. The 132 North and Widgiemooltha Townsite nickel gossans are mineralogical laboratories and contain exceptionally rare minerals. The ore profile of the 132 North deposit is, from base upwards, 0.1 to 1m of massive pentlandite-pyrrhotite-pyrite-chalcopyrite sulfide, matrix sulfide containing 40-80% sulfide, and disseminated sulfide. Accessory nickeline and gersdorffite are reported. The transition zone supergene sulfide phases include; violarite, smythite, covellite, chalcocite and marcasite after pyrite. The nickel sulfides within the regolith have been oxidised into a complex series of weathering fronts including a rare expression of a carbonate front which has resulted in a host of unusual nickel carbonate, arsenate and silicate minerals such as gaspeite, atacamite, annabergite, carrboydite, glaucospaerite, hydrohonessite, kambaldaite, népouite, nullaginite, pecoraite, olivenite, otwayite, reevesite, retgersite, takovite and of course widgiemoolthalite.
… excerpt ends here. Continue reading the full article.
