Oak Dam - Oak Dam East, Oak Dam West

South Australia, SA, Australia

Main commodities: Cu Au U
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The Oak Dam iron oxide copper-gold-uranium mineralised system is located ~60 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. It comprises the Oak Dam East and Oak Dam West deposits which are 4 km apart.

Oak Dam, 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 Olympic IOCG Province and Carrapateena records for a summary of the tectonic, geologic and geophysical setting of the deposit.

Oak Dam East

The Oak Dam East 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 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.

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 U
3O8, including 5 m @ 7.1 kg/t U3O8.

This description is summarised and paraphrased from sections of Davidson et al. (2007).

Oak Dam West

Davidson et al. (2007) speculated 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.

Past drilling (pre-2007) was centred on a north-south elongated, ovoid, ~30 Mgal above background gravity anomaly. Altered upper basement in AD4, a drill hole near the centre of the gravity high, comprises barren, iron rich hematite breccias and sedimentary rocks that carry <0.03% Cu. A further hole, AD7, ~700 m to the east of AD4, intersected iron rich conglomerates and breccias at the expected base of post mineral cover in fault contact with moderately brecciated and sericite altered granite and is also barren (<0.01% Cu), whilst AD13 ~1 km NW of AD4 was still in post mineral cover when terminated at 852 m.

Recent Developments
  On 27 November, 2018, BHP released drilling results from Oak Dam West which included results from 4 diamond drill holes testing an area of ~400 to 500 m radius centred on the gravity anomaly described above. These drill holes intersected chalcocite, bornite and chalcopyrite mineralisation, which occur primarily as disseminations, and is associated with iron oxide rich breccias.
  The mineralisation reported is overlain by between 700 and 950 m of consolidated post-mineral, sedimentary rocks of Meso- to Neoproterozoic and Lower Palaeozoic age.
  The two 2018 drill holes on the southern 'slope' of the gravity high, returned the following intersections:
    • AD22 - 124.5 m @ 0.52% Cu, 0.48 g/t Au. 0.085 kg/t U, 3.37 g/t Ag from a depth of 936 m; and
    • AD24 - 406 m @ 0.66% Cu, 0.35 g/t Au, 0.266 kg/t U and 2.09 g/t Ag from a depth of 848 m.
  The two intervals were ~220 m apart, and ~400 to 500 m south and SW of AD4 respectively, and the centre of the gravity high. The hosts rocks intersected in these holes were iron oxide and siderite altered, brecciated granites, with chalcopyrite dominant mineralisation.
  Two drill holes on the northern 'slope' of the gravity high returned intersections of:
    • AD23 - 425.7 m @ 3.04% Cu, 0.59 g/t Au. 0.346 kg/t U, 6.03 g/t Ag from a depth of 1063 m; and
    • AD25 - 77 m @ 2.11% Cu, 0.54 g/t Au. 0.327 kg/t U, 2.94 g/t Ag from a depth of 1193 m.
  These two interval are ~350 m apart and 300 to 400 m south and SW of AD4 respectively. AD23 included 180 m @ 6.07% Cu, 0.92 g/t Au, 0.401 kg/t U and 12.77 g/t Ag from a depth of 1070 m.

  AD26 - drilled in 2019, crossing close to AD23, intersected 205 m @ 2.05% Cu, 3.8 g/t Au, including 44 m @ 5.77% Cu, 14g/t of gold, 1.2 g/t Ag and 643 ppm U.
  Holes AD23 and AD25 passed downward from pervasively altered iron oxide rich breccias and fine grained sedimentary clastic rocks, which were generally barren of copper mineralisation, before cutting massive hematite breccias with high grade chalcocite-bornite mineralisation. Both holes end in moderate to weakly hematite altered, brecciated granites dominated by chalcopyrite mineralisation in AD23 and pyrite mineralisation in AD25.
  The 2019 BHP release indicates an upward expanding cone shaped alteration pattern cut by the AD 23 and 25 holes, grading from an upper core of barren hematite-quartz altered breccias and sedimentary units, passing outward and downward into a shell of mineralised massive hematite hydrothermal breccia, then to more weakly mineralised Fe-altered granite breccias grading out and down to poorly mineralised to barren granite. The AD 22 and 24 pair of holes is apparently confined to the outer, lower grade, Fe-altered granite breccia shell.

The most recent source geological information used to prepare this summary was dated: 2018.     Record last updated: 28/11/2018
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
   Selected References:
BHP Group  2018 - BHP copper exploration program update: in    BHP Group Press Release, 27 November, 2018    7p.
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
Katona, L.,  2020 - Geophysical signatures of IOCG prospects in the Olympic Cu-Au Province: in    Mesa Journal   v.92, pp. 26-37.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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