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Victorian deep lead gold - Ballarat-Buninyong, Berry, Chiltern, Duke and Main, Pitfield-Smythesdale, Cathcart, Homebush |
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Victoria, Vic, Australia |
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Au
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Super Porphyry Cu and Au
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A series of preserved Palaeochannel alluvial/placer gold deposits known as deep leads have historically produced substantial quantities of gold from central Victoria, Australia. The largest of these were the Ballarat-Buninyong, Berry, Duke and Main, Pitfield-Smythesdale, Cathcart, Homebush and Chiltern deep leads, all of which individually yielded more than 3 tonnes of gold.
The bulk of the deep leads occupied a northward flowing network of drainage channels in an ~150 km diameter area defined by Ballarat in the south, Ararat and Stawell to the west and Bendigo to the NW. These four centres are all important lode gold fields. Other significant lode gold fields in this area include Berriga, Mt Egerton, Beaufort, Creswick, Dalesford, Clunes, Avoca, Maryborough, Chewton, Maldon and Fosterville, all of which produced (or have resources of) >15 t of gold from either lode or alluvial sources. The Chiltern deep lead was an outlier, 200 km to the ENE just south of the Murray River, but lies near or downstream of the Chiltern, Beechworth and Harrietville gold fields that each produced >15 t of Au.
The productive deep leads of central Victoria are estimated to have a cumulative channel length of around 750 km. The actual total recorded production from the Victorian deep leads is estimated at 165 tonnes of gold, mostly between the mid 1880's and the early 1900's. Most were mined by underground methods and due to intense water problems only a small proportion of the potential resources could be exploited.
The mineralised channels are generally broad, with flat bottoms and steep sides. Basal cobble beds occupy much of the width of channels which vary in width from 30 to 60 m in upper reaches, 100 to 400 m in the middle profile and up to 1000 m downstream. Channels generally have a uniform stratigraphy over their width and length, commencing typically with a 0.5 to 5 m thick 'Basal Wash' of well rounded, quartz rich pebble to cobble to boulder conglomerate, overlain by 10 to 60 m of coarse to medium to fine grained sand with sulphide and lignite lenses in the lower sections. These are in turn followed by 5 to 40 m of stiff kaolinitic white to orange clay with silty lenses. Many of the deep leads are preserved by a capping of Tertiary basalt which is 20 to 60 m thick. Where the basalt is present all units with the exception of the 'Basal Wash' are reduced in thickness.
Where present appreciable gold is generally concentrated in the basal 2 m of the 'Basal Wash' unit (generally representing 99% of the gold in the profile), particularly at the bedrock interface. Although coarse gold and nuggets are known, most is less than 1 mm in diameter with 500 to 1500 particles per gram. Gold grade is very variable in detail, but remarkably consistent over 10 to 40 km intervals of a channel. Grades varied from 8 to 34 grams per m2 (in plan), averaging 10 to 15 g/m2. These grades can persist for tens of kilometres from an identified source. Gold grains are corroded and it is suggested mineralisation may have been transported in and deposited chemically from solution.
The larger leads and their historic production were (Whiting and Bowen, 1976):
Ballarat-Buninyong - 62.2 t of Au - to the south of Ballarat;
Berry - 52.9 t of Au - north of Ballarat;
Chiltern - 27.0 t of Au - in NE Victoria;
Duke and Main - 9.0 t of Au - in a channel that flowed east, halfway between Ararat and Bendigo;
Pitfield-Smythesdale - 5.6 t of Au - SW of Ballarat;
Cathcart - 5 t of Au - near Ararat;
Avoca - 5.1 t of Au - 40 km ENE of Ararat;
Homebush - 3.2 t of Au - in a tributary flowing north towards Duke and Main;
Loddon - west of Bendigo, downstream from Duke and Main;
Moolort - SW of Bendigo in a major channel flowing north from Ballarat;
Huntly - north of Bendigo;
Greenock - equidistant from Ararat, Ballarat and Bendigo;
Ascot-Clunes - north of Ballarat, near Clunes;
Preston Hills - west of Ballarat;
Snake Valley - between Preston Hills and Ballarat;
Phillips and Hughes (1998) list production from the source goldfield, rather than by deep lead complex, as follows:
Ballarat - 70 t of Au;
Creswick - 54 t of Au - 10 km north of Ballarat;
Maryborough - 29 t of Au - 55 km north of Ballarat and 45 km SE of Bendigo;
Chiltern - 45 t of Au - in NE Victoria;
Berriga - 12 t of Au - 25 km WSW of Ballarat;
Clunes - 10 t of Au - 15 km WNW of Ballarat;
Beaufort - 8 t of Au - between Ballarat and Ararat;
Ararat - 5 t of Au - 25 km SE of Stawell;
Avoca - 5 t of Au - 40 km ENE of Ararat;
Daylesford - 3 t of Au - 40 km ENE of Ararat;
TOTAL deep lead production - ~300 t of Au.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
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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Holdgate, G.R., Wallace, M.W., Gallagher, S.J., Witten, R.B., Stats, B. and Wagstaff, B.E., 2006 - Cenozoic fault control on ‘deep lead’ palaeoriver systems, Central Highlands, Victoria: in Australian J. of Earth Sciences v.53, pp. 445-468.
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Phillips, G.N. and Hughes, M.J., 1998 - Victorian gold province: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp. 495-506.
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Swensson C G 1990 - Early Tertiary deep lead gold deposits of Victoria: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1637-1641
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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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