Redbank Breccias - Sandy Flat

Northern Territory, NT, Australia

Main commodities: Cu
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The transgressive, pipe like Redbank Breccias host copper deposits, the main exploited example of which is Sandy Flat, within the sediments and volcanics of the Mesoproterozoic McArthur Basin in the Northern Territory, Australia, some 600 km NNW of Mt Isa.   Approximately 40 pipes have been located over an east west interval of around 30 km.

The breccias are formed within the Tawallah Group, which is near the base of the McArthur Supergroup, close to the east-west trending Murphy Inlier of Palaeoproterozoic basement.   In the Redbank area the Tawallah Group comprises, from the base:
i). Settlement Creek Volcanics - trachytes, rhyolites and dolomites with agglomeratic breccias,
ii). Wollogorang Formation - >150 m of chocolate brown laminated siltstone, underlain by interbedded grey siltstone, pyritic bituminous shales, anhydrite beds, dolomites, dolomitic quartz sandstone and feldspathic sandstone,
iii). Gold Creek Volcanics - >225 m of trachytic to trachy-andesitic lavas and pyroclastics interlayered with dolomitic tuffs, overlain by volcanic clast breccia-conglomerates, quartz rich conglomeratic sandstone, flagstones with volcanic fragments and an upper transitional bed of feldspathic to kaolinised quartz sandstone,
iv). Masterton Sandstone - >150 m of medium to coarse grained, current bedded ortho-quartzite, locally feldspathis to kaolinitic.
In general the host sequence is only shallowly dipping.

The main Cu and Co mineralisation at Sandy Flat is contained within the Gold Creek Volcanics sequence which comprises intercalated basalts (commonly with pepperite margins) and associated pyroclastics, potassium altered trachyte (possibly altered basalt), tuffaceous sandstones and siltstones, quartz arenites, siltstones and mudstones, although volcanics predominate, mainly as more massive basalts. The host sequence is generally flat lying.

At Sandy Flat and the other Redbank 'breccia' deposits, the mineralisation is largely confined to well defined pipe-like breccia zones which appear to represent collapse structures. At Sandy Flat the'pipe' has surface dimensions of 50 x 90 m. It has fairly sharp lateral margins with generally monomictic breccias near those margins through which the stratigraphy is still intact and readily recognisable. Towards the centre of the 'pipe' the breccia becomes polymictic, with mixing of clasts from adjacent units. Marker units nearer the margins can be shown to have subsided by around 50m within the 'pipe' relative to the same beds outside. At the base of the Gold Creek Volcanics, some 200 to 250 m below the surface, the brecciation and mineralisation appears to be terminated, underlain by 'normal', un-brecciated Wollogorang Formation, composed largely of carbonates. No intrusives are observed within the Sandy Flat breccia, nor at many of the other Redbank 'Breccias', although low grade Cu mineralisation accompanies intrusives with brecciated margins elsewhere in the belt. The brecciation in the two cases need not necessarily have the same origin.

According to Wall and Heinrich (1990), some of the other breccia 'pipes' also cross-cut the Wollogorang Formation and the underlying Settlement Creek Volcanics. They further add that "... the 'pipes' are essentially zones of fracturing, veining and brecciation which pass gradually outwards into their less altered and fractured, shallow dipping hosts. Brecciation post-dates consolidation and compaction of the sediments and is commonly more or less in-situ with little evidence of large scale movement of clasts: there is only local allochthonous clast material and stratigraphic units are traceable through the breccia zones with minimal displacement. Breccia pipes occur in clusters and along broadly linear zones and are probably localised by fault jogs and fault intersections.".

Beds within the 'pipes' generally display dips towards the centre of the 'pipe', with large blocks having a downward displacement. Most 'pipes' have an upper termination within the Gold Creek Volcanics, although some terminate at lower levels (Jackson, et al., 1987).

Knutson, et al., (1979) note that in some of the 'pipes' there is a minor allochthonous breccia occurring as a narrow tabular zone within the core of the breccia pipes. This steeply dipping tabular zone comprises a mélange of dominantly sub-rounded fragments and a large matrix component. The clasts in the breccia are coarser, up to 0.5 m, and are composed of Gold Creek Volcanics, Wollogorang Formation and/or Settlement Creek Volcanics. They also describe the matrix as being the same in both the allochthonous and autochthonous breccias. The predominant matrix component is dolomite with varying combinations of calcite, quartz, chlorite, K-feldspar, apatite, celadonite, mica, hematite, rutile, clay minerals and sulphides. Traces of mineralisation are found throughout the breccia 'pipes', although the only significant Cu is found in the upper-most Wollogorang Formation and within the overlying Gold Creek Volcanics. Strongly hematised 'red rocks' consisting of fine grained K-feldspar with variable amounts of green chlorite, quartz, brown mica, dolomite, rutile, clay minerals, Fe-oxides, apatite and zircon occur adjacent to the breccia 'pipes' and as veins in the vicinity of the Wollogorang Formation to Gold Creek Volcanics contact (Knutson, et al., 1979).

The mineralisation at Sandy Flat is confined to the breccia 'pipe', with the highest grades in the most brecciated central core, decreasing outwards towards the margins. The Cu mineralisation bears a relation to the breccia composition. In general the quartzites are barren and trachytes are the most highly mineralised lithologies, while mudstones, muddy tuffs and tuffaceous sandstones are moderately mineralised within the breccia 'pipe'.

The top of the Sandy Flat 'pipe' is overlain by 6 to 8 m of quartz pebble conglomerate, regarded as probable Tertiary valley fill.   Below this there is:

* An oxide zone, which has a variable thickness, averaging around 10 m. It contains principally malachite, azurite, cuprite and chrysocolla in strongly weathered brown host rocks. Grades range from 1% (the cut-off) to 10% Cu, but average 4 to 5% Cu. A small discrete block of 5000 t of 20 to 25% Cu has been defined and separately mined from within this zone. The base of the oxide zone is marked by a sharp contact with the underlying chalcocite blanket. Chalcotrichite and native copper are found over an interval of 1 to 2m at the base of the oxide zone. A little pyro-bitumen was often present at the base of the same zone, with accompanying native copper.

* A supergene enrichment zone, which is 10 to 15 m thick, containing predominantly chalcocite with lesser covellite, digenite and traces of djurleite. The bulk of the mineralisation is between 1 and 10% Cu, averaging around 4.5% Cu. The lower contact with the primary ore is gradational and diffuse. The supergene ore is very soft and comprises a breccia mass of white, strongly kaolinitic rock (after trachyte), with a matrix largely of black sooty chalcocite and pyro-bitumen (which are often difficult to distinguish). Within the supergene zone there are frequent irregular, interconnected, remnants of chalcopyrite, surrounded by chalcocite. The chalcopyrite content increases with depth. Where the ore is harder there is more chalcopyrite and lesser pyro-bitumen. Within the supergene zone there are fine grained hard pods of flesh coloured clayey rocks with disseminated pyrite and chalcopyrite, similar to that at Running Creek.

* The primary ore, which decreases in grade downwards to the top of the Wollogorang Formation around 200m below. The ore mineralogy is dominated by chalcopyrite, with traces of bornite. The average grade is around 3% Cu. This grade probably only persists to a depth of around 150 m, based on the tonnage and pipe dimensions quoted above. Where seen in core, near the margin of the 'pipe', the primary ore is present as chalcopyrite filled fractures from 1 to 3 mm or more thick within an irregular breccia of sub-angular to sub-rounded, pink, K-feldspar altered trachyte. Within the oxidised zone the trachytes have an almost identical texture, although chalcopyrite is replaced by malachite. These malachite bearing trachytes are very similar to those seen in outcrop at Stanton-Running Creek. Mudstone bands from a few centimetres to tens of cm's thick occur within the fresh trachyte and contain sulphide mineralisation. These latter mineralised mudstones are very similar to those at Stanton-Running Creek, although they carry chalcopyrite but no siegenite. No significant Co is reported at Sandy Flat.

Pyro-bitumen is common at Sandy Flat, being more prevalent within the central, more strongly brecciated sections of the 'pipe', and decreasing towards the margins. It is most obvious in the supergene zone, but is also present in the primary ore, but to a lesser degree. The pyro-bitumen breaks down in the oxide zone and is not detectable. In the supergene zone it occurs as masses up to 25cm across of black, conchoidal fracturing, lustrous/high reflectance massive carbon. It occurs as vein like masses within the breccia, and is associated with sulphides. Locally there may be up to 30% pyro-bitumen in the central sections of the 'pipe', although overall within the mineralised zones we were told it probably constitutes 3 to 4%.

Bitumen nodules are common in the lower section of Wollogorang Formation Unit 3. Unit 3 is described as a 25m thick black, rhythmically laminated, organic and sulphide rich mudstone and dolomitic siltstone, with thin tuffs and diagenetic bituminous nodules. Higher in the Wollogorang Formation, the 55 m thick lower section of Unit 4 contains abundant gypsiferous layers within a thick bedded quartz and dolomitic sandstone (Jackson, et al., 1987).

The reserve being exploited at Sandy Flat comprised:   0.435 Mt @ 4% Cu, within a global resource of 1.63 Mt @ 2.01% Cu.
An indicated resource within the Redbank Breccia at six localities has been stated at:   4.5 Mt @ 1 to 2% Cu,
The other largest resource was The Bluff with:   1.97 Mt @ 1.66% Cu.
Sandy Flat is the second largest of the pipes, but but has a higher grade.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1995.    
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:
Ahmad, M., Dunster, J.N. and Munson, T.J.,  2013 - McArthur Basin: in Ahmad M and Munson TJ (compilers)  Geology and mineral resources of the Northern Territory Northern Territory Geological Survey   Special Publication 5 pp. 15:1-15:72
Orridge G R, Mason A A C  1975 - Redbank copper deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 339-343

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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