Oak Dam East
South Australia, SA, Australia
Cu Au U
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The Oak Dam East iron oxide copper-gold-uranium deposit is located approximately ~57 km SSE of Olympic Dam, 40 km NNW of Carrapateena, 20 km NE of Emmie Bluff and ~500 km NNW of Adelaide in northern South Australia.
Oak Dam East, along with Carrapateena, Olympic Dam, Prominent Hill, Moonta-Wallaroo and Hillside, and all of the other significant known IOCG mineralised systems of the Gawler craton are hosted within Palaeo- to Mesoproterozoic rocks and are distributed along the eastern edge of the currently preserved craton to define the Olympic IOCG Province.
See the Carrapateena record for a summary of the geological setting of the Olympic IOCG Province.
The deposit underlies >500 m of cover, which comprises the Neoproterozoic Arcoona Quartzite and Tregolana Shale, and the post 1424 Ma Mesoproterozoic Pandurra Formation. Although absent now, a Gawler Range Volcanics (GRV) succession (~1591 Ma: Fanning et al., 1988) is interpreted to have unconformably overlain basement in the area prior to being removed by erosion that unroofed and stripped sections of the orebody and its enclosing rocks during the early Mesoproterozoic. By 1424 Ma, the deposit was overlain above a sharp unconformity by unaltered and unmineralised sandstones of the Pandurra Formation (Davidson et al., 2007). This interpretation is supported by the presence at Oak Dam West (4 km to the west) of intensely hematitised, disaggregated, feldspathic, porphyritic GRV-like volcanic rocks at the unconformity surface, overlain by hematitic conglomerate. Below this unconformity, the remnant Oak Dam East ironstone formed a distinct elongate basement high with an elevation of several hundred metres over a strike length of ~1.5 km, before the deposition of the Pandurra Formation.
The altered and mineralised schistose metagranitic basement below the unconformity has been interpreted to belong to the 1860 to 1845 Ma Donington Suite (Creaser, 1989). The basement is also characterised by rare 0.5 to 5 m thick dykes of undeformed coarse to pegmatitic alkali granite and minor, altered medium-grained diorite ut by specularite-quartz veins along the trace of a basement fault.
The Oak Dam East deposit comprises a large, sub-horizontal body of hematite and hematite matrix breccia ~200 m thick, which overlies an areally less extensive magnetite dominated core near the base of the system. Davidson et al. (2007) considered that the Oak Dam East breccia complex was formed at a shallow depth, following regional albite-calc silicate±magnetite alteration caused by hypersaline, 400 to 500°C fluids. Large-scale brecciation was an early feature, with progressive breccia-fill and replacement comprising the following paragenetic stages:
i). hematite and goethite (hematite stage I);
ii). magnetite-apatite-quartz±actinolite (magnetite stage). The hematite l and magnetite stages are evident in thin section, where a mesh of elongate hematite crystals are completely replaced and overgrown by magnetite which pseudomorphs hematite;
iii). hematite-goethite-quartz-chalcedony-pyrite-chlorite-monazite (hematite stage II). Tabular magnetite (which pseudomorphs hematite l), is in turn partially replaced and corroded by hematite stage II; and
iv). chalcopyrite-pitchblende-illite-hydromuscovite-florencite-carrolite (Cu- U-[Au] stage).
The deposit is divided into an upper sulphide-free, and lower sulphidic zone, the boundary being within the thick hematite rich upper layer. Davidson et al. (2007) outline observations and evidence that indicates the upper sulphide free to sulphidic zone interface reflects the limits of, chemical weathering below the implied Mesoproterozoic regolith suface prior to deposition of the Pandurra Formation. Alternatively, the same observations may suggest the interface represents a reaction front between a hot ascending and a descending cooler fluid during ore deposition. Below this interface, disseminated and vein chalcopyrite occurs in the upper parts of the sulphidic zone, while disseminated and vein pyrite occurs at depth.
In more detail, the alteration/mineralisation zoning is composed of:
i). an upper, sulphide-deficient, hematite-dominated breccia, which varies from 110 m thick in the centre to 80 m on the margins. The dominant primary feature in this zone is irregularly shaped relict zones of colloform banded hematite (typically 0.2 to 0.5 m diameter) in a matrix of replacive, massive, blue, steely hematite, most of
which is assigned to hematite stage II. Clasts are entirely converted to hematite, with only zones of skeletal quartz fans that may be tens of cm across representing unconverted protolith. The upper 10 m of the ironstone below the unconformity is vughy and consists of massive to brecciated, weathered, brown to red and black hematite-goethite mixtures and brown caliche within the vugs;
ii). sulphide-bearing hematite-dominated breccia, which carries >0.1% Cu and is 60 to 80 m thick. This zone contains the main sub-horizontal, 10 to 70 m thick copper-uranium mineralised sheet, the top surface of which is generally found at the upper contact of the sulphidic zone. This mineralised sheet has an upper, variably developed low-grade transition to the underlying >1% Cu layer, which passes down in turn into a variable thickness 0.1 to 1% Cu zone. Mineralisation occurs as sulphides in the hematitic breccia matrix. A distinctive feature of the upper sections of the sulphide-bearing zone are domains of banded quartz and chalcedony with minor specular hematite, which developed during both the magnetite and hematite II stages;
iii). sulphide bearing, magnetite-dominated breccia, forms an irregular, 600 m long tongue that is 30 to 80 m thick, passing down into disseminated hematite (Davidson et al., 2007). Clasts within the breccia include red metagranite replaced by magnetite-apatite-amphibole (variably altered to chlorite), and colloform, banded magnetite. The breccia matrix is composed of semi-massive magnetite and infilling quartz. Martite surrounds fractures, veins and discrete vugs. Veins are filled by later hematite±pyrite±chalcopyrite. Some magnetite grains in the matrix are tabular and elongate, replacing hematite l (i.e., "mushketovite").
Breccia clasts in the basement vary from <1 mm to 5 m in diameter, but are typically ~20 cm across. Fragments of metagranite, schist, and minor felsic volcanic rocks become more
abundant ~100 m below the main high grade mineralised sheet, comprising 50 to 60 percent of the breccia. Many fragments have been partly replaced by hematite but have preserved a relict granoblastic to schistose texture, while in others the only preserved relicts are ribboned or equant metamorphic quartz.
Mineralisation is zoned vertically from pyrite at depth to shallower chalcopyrite. High-grade pitchblende occurs centrally within the chalcopyrite zone in the sulphide-bearing hematite-dominated breccia (e.g., 10 m of 0.46% Cu and 3.7 kg/t U3O8), straddling the boundary between a sericite-illite assemblage and underlying iron-magnesium-chlorite alteration. Chalcopyrite occurs intergrown with prismatic hematite, predominantly as crosscutting fine veinlets, patches, and infill within martite-lined interstices. Chalcopyrite commonly replaces pyrite, while very minor amounts of secondary Cu minerals (e.g., covellite) are associated with chalcopyrite dissolution. Carrolite (Cu(Co, Ni)2S4) and sphalerite occur along the contacts between chalcopyrite and colloform hematite. Within the main mineralised zone, uranium occurs as colloform pitchblende (UO2) in vuggy zones associated with chalcopyrite and hematite. This pitchblende is intimately intergrown with florencite (CeAl3(PO4)2(OH)6), monazite, and xenotime. The latter phases also contain very fine disseminated cubes of uraninite, <1 to 3µm in diameter. Fluorite is absent at Oak Dam East.
Davidson et al. (2007) speculate that Oak Dam East represents the remnant outer wing of a much larger, now eroded breccia complex with a core to the west. This core they suggest is partially represented by Oak Dam West, which has upper sections consisting of very thick, graded, resedimented hematitic breccia, similar to some upper sections of the Olympic Dam Breccia Complex.
The deposit is estimated to contain a resource of ~560 Mt @ 41 to 56% Fe, and 300 Mt @ 0.2% Cu.
Best intersections include: 63 m @ 0.3% Cu, 0.7 kg/t U3O8, including 5 m @ 7.1 kg/t U3O8.
This description is summarised and paraphrased from sections of Davidson 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.
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References to this deposit in the PGC Literature Collection:
Davidson G J, Paterson H, Meffre S and Berry R F, 2007 - Characteristics and Origin of the Oak Dam East Breccia-Hosted, Iron Oxide Cu-U-(Au) Deposit: Olympic Dam Region, Gawler Craton, South Australia: in Econ. Geol. v102 pp 1471-1498|
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