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Murdie Murdie

South Australia, SA, Australia

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The Murdie Murdie magnetite-rich mineralised system is located within the salt flats of Lake Torrens, ~7 km east of the southern end of Andamooka Island, 107 km SE of Olympic Dam, 40 km NE of Carrapateena, and ~500 km NNW of Adelaide in northern South Australia (#Location: 30° 27' 04"E, 137° 45' 47"E).

Murdie Murdie lies within the Olympic IOCG Province, which incorporates all of the significant known IOCG mineralised systems of the Gawler craton, distributed within Palaeo- to Mesoproterozoic rocks along the eastern edge of the currently preserved craton.

No economic mineralisation has been discovered at Murdie Murdie, which on current known information represents the early high-temperature magnetite-K feldspar-calcsilicate alteration stage that precedes Cu-Au-U mineralisation seen in many IOCG systems of the province. The mineralisation encountered has only a weak overprint by a hematitic assemblage, unlike the main Cu-Au-U systems of the province.

Vertical diamond drill hole MRD1 (Western Mining Corporation, 1982) targeted a complex, elongate, double-peaked magnetic anomaly and an overlapping, broad, sub-circular, 7 mgal gravity feature centred ~ 1 km east of the main magnetic peak (Paterson and Muir, 1986). The gravity high has a pronounced peak and a diameter of ~10 km, lying on the northern extremity of a broad ridge that trends SW to SSW towards the Carrapateena arm of Lake Torrens. The magnetic peak targetted is toward the southern tip of a regional SE to SSE-trending magnetic trend that follows the axis of Lake Torrens. In the vicinity of Murdie Murdie, the magnetic ridge intensifies and has a tail that curves to the SSW, following the gravity response, to form a 7 km long boomerang shaped feature with the main ~1000 nT peak in the hinge zone (data in Paterson and Muir, 1986).

Basement is overlain by ~20 m of gypsiferous clay and lake sediments, ~100 m of strongly oxidised and decomposed Neoproterozoic(?) sediments and then by the Neoproterozoic Arcoona Quartzite, Corraberra Sandstone, Tregolana Shale, Nuccaleena Dolomite and Whyalla Sandstone, before passing through a basal breccia at the unconformity with altered Mesoproterozoic host rocks at a depth of ~800 m.

Mineralisation in drill hole MRD1 is characterised by abundant magnetite, developed as replacement zones and veins within altered, fine-grained, quartzo-feldspathic metasedimentary rock, siliceous in parts, most likely belonging to the Wallaroo Group. These rocks were originally logged as fine-grained massive pink felsic to intermediate volcanics (Paterson and Muir, 1986), but following petrographic examination have been reclassified as metasedimentary.

In drill hole MRD1, the interval from the unconformity at 806.8 to 832.8 m was occupied by fine-grained, quartzo-feldspathic metasedimentary rocks, strongly veined by magnetite, diopside, amphibole, microcline, carbonate and minor pink barite. In the upper palaeoregolith, there are gypsum veins which contain specular hematite and minor chalcopyrite.

From 832.8 m, this zone of veined metasediments passes across a sharp boundary, into a 48.2 m thick zone of coarse-grained euhedral magnetite, intergrown with euhedral to anhedral pink microcline up to a few cm in size that replaced albite, accompanied by pink subhedral crystals of apatite. This assemblage persists to a depth of 881 m where it has a sharp lower boundary. The magnetite is associated with green, fine-grained, diopside, and needle-like to columnar crystals of colourless to pale green amphibole (tremolite-actinolite) up to 5 mm long. Carbonate, quartz, pyrite and chalcopyrite are interstitial to these minerals, with some quartz and carbonate infilling fractures in the coarse-grained minerals. These are all subsequently crosscut by veins of calcite and gypsum containing minor apatite, microcline and euhedral pyrite. This late carbonate veining leads to brecciation of earlier vein material. There is also a strong local alteration of microcline to chlorite and lesser sericite (Hinde, 1983).

Two main stages of veining are indicated, namely an initial prograde stage producing a medium- to coarse-grained assemblage including apatite, diopside, titanite, magnetite, pyrite (minor), microcline, amphibole and minor chalcopyrite galena and sphalerite. This phase was followed by a second stage altering the former assemblage and infilling by those of lower temperature formation, specifically, chlorite/sericite, hematite, quartz, carbonate and gypsum (Hinde, 1983). Paterson and Muir (1986) note that this coarse-grained magnetite- rich zone may represent either total replacement of the host metasedimentary rock or a zone of intense overlapping veining.

From 881 the drill hole passed out into fine-grained pink quartzo-feldspathic metasedimentary rock with strong veining by diopside, amphibole and carbonate, but no magnetite, crosscut by carbonate veins with minor fluorite and barite, without further change to the end of hole at 918 m (Hinde, 1983; Paterson and Muir, 1986).

The host fine-grained, quartzo-feldspathic metasedimentary rock largely consists of microcline and albite (initially the dominant feldspar, prior to being replaced by the former), with low but variable quartz (due to silicification), with minor chlorite and disseminated hematite/magnetite throughout (Hinde, 1983).

No significant Cu-Au assays are associated with alteration. Copper mineralisation is very weak, occurring as euhedra and patches of pyrite and chalcopyrite associated with magnetite in the zone of coarse magnetite, diopside and amphibole, and in late calcite/gypsum veins. Copper values within the metasedimentary rocks vary from 6 to 70, averaging 26 ppm, while in the magnetite-rich zone, it ranges from 28 to 440, averaging 117 ppm. U3O8 varies from 4 to 6 and 4 to 10 ppm in the metasediments and magnetite zone respectively. All gold values were below the limit of detection, 0.05 ppm. The metasediments and magnetite zone respectively average 0.29 and 0.15% F; 2101 and 372 ppm Ba; 4.45 and 35.2% Fe; 50 and 60 ppm La; 90 and 130 ppm Ce; 25 and 77 ppm Pb; 170 and 324 ppm Zn (Hinde, 1983).

Dating of hydrothermal titanite in this alteration gave an age of 1576±5 Ma (SHRIMP U-Pb), interpreted as the age of magnetite-bearing hydrothermal alteration (Skirrow et al., 2007).

The most recent source geological information used to prepare this summary was dated: 2007.    
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
 References to this deposit in the PGC Literature Collection:
Bastrakov E N, Skirrow R G and Davidson G J,  2007 - Fluid Evolution and Origins of Iron Oxide Cu-Au Prospects in the Olympic Dam District, Gawler Craton, South Australia: in    Econ. Geol.   v102 pp 1415-1440


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