Abstract
Abstract from Review of the Burra Mine Project, 1980-2008: a progress report
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Review of the Burra Mine Project, 1980-2008: a progress report (.pdf 4Mb, opens in new window) Burra Mine Project and associated datasets (.zip 14Mb, large file, opens in new window) |
The first phase was initially by underground high-grade tributing, with later open cutting in the 1870s to reduce costs. This realised around 700 000 t averaging 7% Cu.
The second phase, carried out by Samin Ltd, was entirely by open cutting a resource estimated at the time as ~3.5 million tonnes grading 1.52% Cu.
The final amount of ore mined during 1970–81 was 1.89 million tonnes grading 1.71% Cu. The pre-mining grade was close to 3% Cu.
Field mapping in 1980–81 by Mines and Energy Geologists J.F. Drexel and W.S. McCallum, followed by petrology of approximately 150 rock samples during 1983–84, and reinterpretation of the thin- and polished sections during the current appraisal of the Burra Mine, has concluded that the orebody was developed by secondary remobilisation of presumably primary chalcopyrite mineralisation associated with syn-depositional and post-depositional magmatic and hydrothermal activity during sedimentation of the host Skillogalee Dolomite of Neoproterozoic age.
Secondary copper sulphides, mostly chalcocite, developed from the primary sulphides in an epigenetic environment. Primary and secondary sulphides are now scarce in the open cut, but chalcocite, bornite, covellite and chalcopyrite were present at the bottom of the pit.
Supergene enrichment developed the main copper minerals that have been mined — malachite, azurite, chrysocolla and cuprite, with lesser native copper and libethenite. These developed as fracture fills and veins in a relatively open-space environment. Gem-quality botryoidal malachite made the mine world renowned during the 1845–77 mining period. Jigsaw and crackle breccias, often copper mineralised, occur in country rock up to 200 m from the magmatic foci, and related mineralisation extends slightly beyond the open cut limits.
The Skillogalee Dolomite in the Burra area consists of a lower portion comprising dominantly pale-coloured dolomite and an upper portion of dark blue-grey dolomite. Within the lower unit, a distinctive assemblage of siltstone, limestone and dolomite that is atypical of the remainder of the Skillogalee Dolomite has been traced southward from Burra towards Robertstown during regional mapping by W.V. Preiss and is labelled ‘Nms9’ on the Burra 1:50 000 geological map.
The name ‘Kooringa Member’ has been reserved for this unit, and will be formalised in a future publication.
The geology of the Burra open cut is divided in two by the Kingston Shear. On the west side, a diapiric body intruding the Skillogalee Dolomite occupies the entire length of the pit and may extend up to 500 m to the west. On the east side, the greater part of the pit is occupied by an overall east-dipping succession of the Kooringa Member, with pale-coloured dolomites of the Skillogalee Dolomite above and below.
Petrographic examination by A. Kemp and M. Farrand indicates that the Kooringa Member contains probable volcaniclastic or air-fall tuff components developed during a period of syn-depositional magmatic activity which culminated in intrusion of a felsic porphyry, which has been dated during this current project at 797±5 Ma by PIRSA Geochronologist A. Reid. This date has led to significant reinterpretation of the timing of primary mineralisation.
Recognition that the porphyry and associated mineralisation are of similar age to the host Skillogalee Dolomite promotes large tracts of the Adelaide Geosyncline as prospective for similar mineralisation. The Burra orebody was previously thought to be much younger than the dolomite, and probably post Delamerian, which influenced previous exploration programs.
Rather than being confined to a fault-emplacement model for copper mineralisation, the Kooringa Member, which at Burra is ~150 m thick, should now be the focus as it represents a period of possibly widespread volcanism in an otherwise carbonate-dominated sedimentary environment.
Coarsely crystalline dolomitic marble and brecciated dolomitic marble carrying variable amounts of copper sulphides and pyrite were exposed at the very bottom of the open cut and in small pods along Kingston Shear. Of several possible origins considered, recrystallisation and mineralisation by hydrothermal fluids of the lower portion of the Skillogalee Dolomite is considered the most likely.
Diapiric material, which is generally unmineralised, is juxtaposed adjacent to the mineralisation at Burra by the Kingston Shear, and it too may have been tapped from lower in the Adelaidean sequence, using the Kingston Shear as its conduit to higher levels in the sedimentary pile.
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- Page Last Modified: November 8, 2011