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Macraes Flat - Frasers, Round Hill |
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South Island, New Zealand |
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Au
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Super Porphyry Cu and Au
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The Macraes Flat gold deposits are within the Otago Schist Belt of the South Island of New Zealand, just over 50 km NNE of Dunedin.
The 120 km wide, generally NW trending, arcuate Otago Schist Belt extends across the island for 280 km from the major NE-SW trending Alpine fault near the north-west margin, to the south-east Pacific coast. It is composed of regionally metamorphosed and deformed, Palaeozoic, mainly quartzo-feldspathic rocks of prehnite-pumpellyite grade near the margin to upper greenschist facies in the Macraes Flat district. To the west of, but not within the Otago Schist Belt the lower grade sediments are intruded regionally by the north-south trending, 10 000 sq. km, 158 to 132 Ma Jurassic to Cretaceous Median Batholith.
In the Macraes Flat district, ductile deformation produced large recumbent folds, while more brittle features are associated with regional uplift, including low angle thrust zones. These features result from Mesozoic compression, Cretaceous to middle Tertiary extension and a later period of upper Tertiary to Recent compression.
There are no indications of igneous activity in the Macreas Flat area.
Gold deposits in the district are spatially related to the Hyde-Macraes shear zone, a low angle NW-SE trending duplex thrust system. The shear zone is occuppied by an up to 100m wide carbonaceous intershear pelite which hosts the gold, separating hangingwall and footwall psammites. Well developed shears are found near both the hangingwall and footwall of this pelite.
A number of vein types are recognised, the dominant ore bearing types being the hangingwall shear, flat, lateral ramp and stockwork veins. These veins all cut the regional schistosity. The stockwork veins are the oldest part of the hydrotherma system, followed by flat and ramp veins. The hangingwall shear veins formed early but have been influenced by late deformation.
Gold mineralisation exhibits a strong correlation with quartz veining and is rarely seen in the adjacent wall rocks. Gold in stockwork veins averages 6 g/t, while ramp veins irregularly carry up to 20 g/t Au and are more auriferous than the flat veins (2 to 4 g/t Au). Minor late quartz-carbonate veining cuts all of the above and post date mineralisation.
Total mine production to 2000 from the various Macraes Flat deposits, which are distributed over a trend length of around 10 km, was: 24 Mt @ 1.73 g/t Au for near 42 tonnes of contained gold.
Reserves in 2000, mainly from the Frasers deposit, totalled: 41.8 Mt @ 1.65 g/t Au for 69 tonnes of contained gold.
The total field production+reserve therefor is 65.8 Mt @ 1.68 g/t Au for 111 tonnes of gold. (de Ronde, et al., 2000).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2000.
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.
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Selected References:
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Allibone, A., Jones, P., Blakemore, H., Craw, D., MacKenzie, D. and Moore, J., 2018 - Structural Setting of Gold Mineralization within the Hyde-Macraes Shear Zone, Southern New Zealand: in Econ. Geol. v.113, pp. 347-375.
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Bierlein F P, Christie A B, Smith P K, 2004 - A comparison of orogenic gold mineralisation in central Victoria (AUS), western South Island (NZ) and Nova Scotia (CAN): implications for variations in the endowment of Palaeozoic metamorphic terrains: in Ore Geology Reviews v25 pp 125-168
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Cave, B.J., Pitcairn, I.K., Craw, D., Large, R.R., Thompson, J.M. and Johnson, S.C., 2017 - A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand: in Mineralium Deposita v.52, pp. 515-537.
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Craw D, Windle S J, Angus P V 1999 - Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand: in Mineralium Deposita v34 pp 382 - 394
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Craw D, MacKenzie D J and Petrie B, 2004 - Disseminated gold mineralisation in a schist-hosted mesothermal deposit, Macraes mine, Otago, New Zealand: in Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne, pp 135-141
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de Ronde C E, Faure K, Bray C J, Whitford D J 2000 - Round Hill shear zone-hosted Gold deposit, Macraes Flat, Otago, New Zealand: evidence of a magmatic ore fluid: in Econ. Geol. v95 pp 1025-1048
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Goodwin, N.R.J., Burgess, R., Craw, D., Teagle, D.A.H. and Ballentine, C.J., 2017 - Noble gases fingerprint a metasedimentary fluid source in the Macraes orogenic gold deposit, New Zealand: in Mineralium Deposita v.52, pp. 197-209.
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Henne A and Craw D, 2012 - Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand: in Mineralium Deposita v.47 pp. 781-797
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Mortensen J K, Craw D, MacKenzie DJ, Gabites JE and Ullrich T, 2010 - Age and Origin of Orogenic Gold Mineralization in the Otago Schist Belt, South Island, New Zealand: Constraints from Lead Isotope and 40Ar/39Ar Dating Studies : in Econ. Geol. v105 pp 777-793
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Petrie B S, Craw D and Ryan C G 2005 - Geological controls on refractory ore in an orogenic gold deposit, Macraes mine, New Zealand: in Mineralium Deposita v40 pp 45-58
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Upton P , Begbie M and Craw D, 2008 - Numerical modelling of mechanical controls on coeval steep and shallow dipping auriferous quartz vein formation in a thrust zone, Macraes mine, New Zealand: in Mineralium Deposita v43 pp 23-35
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Upton P and Craw D, 2008 - Modelling the role of graphite in development of a mineralised mid-crustal shear zone, Macraes mine, New Zealand: in Earth and Planetary Science Letters v266 pp 245-255
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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