Gold is one of the heaviest metals known to man, being exceeded in weight only by the platinum group of metals. In its natural state gold is found either as native metal or in a combination with the element tellurium forming gold tellurides (sylvanite, calaverite, petrite). It can also occur in sulphide minerals.
The native minerals may be up to 99.8% pure gold, but more commonly is 85.95% pure, containing small amounts of impurities: generally silver, copper and platinum. Gold is most commonly found in alluvial (water lain) or eluvial (residual) deposits and is associated with gold-bearing sulphide ore bodies. It also occurs as veins in massive quartz. Free gold found in alluvial or eluvial deposits ranges in sizes from microscopic particles to large nuggets. The largest found in Western Australia was the "Golden Eagle" which was found at Larkinville in 1931. When found it weighed 35.326kg. Gold is frequently obtained by special treatment of sulphide ores, such as pyrite (iron sulphide) and arsenopyrite (iron arsenic sulphide). The gold occurs in these minerals as minute segregations.
Primary gold
Primary gold occurs in crystalline rocks, which have commonly been affected by heat and/or pressure. In Western Australia these rocks are very ancient, often being more than 2,500 million years old. Much of the state is underlain by ancient granitic rocks within which are found remnants of metamorphic rocks known as greenstones. In the Yilgarn block, where most of the States gold has been mined, the greenstone belts extend in a NNW-SSE direction. They range in width from a few kilometres to a few tens of kilometres and in length up to several tens of kilometres. Gold deposits occur generally within, or near to the edge of, greenstone belts. Occasionally they are found in adjacent granitic rocks.
Gold tellurides
Tellurides were first discovered at Kalgoorlie in 1896. Specimens were sent to the Paris World Exposition in 1900 and may indeed be some of the specimens included in the gold display at the WA Museum Kalgoorlie-Boulder. Tellurides tend to occur in veinlets and small segregations filling cracks which cross the line of gold-bearing bodies. On weathering, tellurides liberate free mustard gold.
Tellurides were deposited at a late period of ore formation at relatively very low temperatures, around 150°C.
Geological Survey of WA Gold Collection
During the early part of this century the Geological Survey of WA had an active policy of acquiring fine gold specimens. Many of these had been sent by various goldmines to exhibitions in Europe, such as the Wembley Exhibition in London and the Paris Exposition, and were subsequently acquired by the Geological Survey for its collection. The specimens displayed from this collection at the WA Museum in Hannan Street, Kalgoorlie-Boulder illustrate a variety of forms of gold: nuggets, flat sheets, leaves, veins and crystals.
They are all from the Western Australian Goldfields. The most spectacular leaf gold specimens are from Queen Margaret goldmine, Bulong. Other specimens are from Coolgardie, Mulgabbie, Cue, Norseman, Kalgoorlie, Mt Egerton and Meekatharra.
The WA Museum wishes to thank the Director of the Geological Survey of Western Australia for the loan of these specimens for display.
Kalgoorlie Consolidated Gold Mines Pty Ltd has developed two open pit mines on its leases in the Goldfields area. The Mt Percy open pit on the Northern leases has ceased operations but did until recently produce up to a million tonnes per annum (tpa) of free milling oxide and sulphide ores. The other mine, the Super Pit, is essentially an aggregation of the Judd, South, Paringa, Croesus/Eclipse, Central, Brownhill, Drysdale and Horseshoe pits into one continuous large scale operation. When completed it will be some five kilometers long, 2 kilometres wide and up to 450 metres deep. In its expected life of 20 years, it will produce more than eight million ounces of gold. The concept of open pit mining is the bulk mining of high tonnage, lower grade ore deposits, processed by high capacity treatment plants to achieve low cost gold production.
Open pit mining depends on four major factors to make it profitable:
- The ability to accurately delineate and evaluate the ore body.
- The ability to mine and haul the maximum amount payable ore with the minimum amount of waste by the shortest route to the processing plant.
- The ability to extract the gold from the ore efficiently.
- The ability to carry out the entire operation safety.
At the onset of mining, a pit shape is designed with the aid of sophisticated computer equipment.
Factors taken into account include:
- The slope of the pit walls for wall stability.
- The provision of berms (steps) so that rock slides can be controlled.
- The width and height of benches (layers) which are to be formed as the ore is removed.
- Blast design and its effect on the rock.
- The width and route of the haul road.
Currently mining is carried out on 10 metre high benches using hydraulic excavators with bucket capacities of 13-20m³ to load 130 to 200 tonne haul trucks.
The use of such large equipment allows for economies of scale, and enables the economic recovery of low grade ore that once was considered waste. Open pit mining on the Golden Mile is complicated by the presence of stope voids, shafts, drives and cross-cuts of workings from original underground mines that date back to 1893. There are more than 2000 kilometres of old mine headings under this area. To maintain control of costs and efficiency, open pit mining requires close attention to geology, planning, scheduling of earthmoving equipment, drill and blast technology and safety. Through constant control they can reduce costs and improve the extraction of ore from the ground in the most efficient and safest manner.
TROY MEASURE
24 Grains (gr) = 1 Pennyweight
20 Pennyweights (dwt) = 1 Ounce
12 Ounces (oz) = 1 Pound (obsolete)
Metric measure
- Grades are expressed in grams per tonne - g/t(1g/t = 1ppm).
- Quantities of gold are quoted in grams or kilograms.
- Troy ounces are used internationally and persist locally for pricing and investment gold sales.
- Alluvial grades are quoted as grams per cubic metre - g/m³.
Gold ore grades prior to 1973
Grades were normally quoted in pennyweights per ton (2240lbs avoirdupois) - dwt per ton. - n.b. usually rounded to nearest pennyweight but may be pennyweights and grains or decimal pennyweights. Certain companies during some periods used the short ton (2000lbs avoirdupois) [e.g. Hill 50 Gold, Lake View and Star]. Grades in shillings (usually shillings per short ton) refer to a gold price of 4-4s-11½d (see note).Therefore divide by 4s/3d (4.25 shillings) to obtain grade in pennyweights.
Grades are written - 2oz.13dwt.21gr. or 2.13.21. Alluvial grades may be quoted in pennyweights per cubic yard (dwt.per.cu.yd.) but quite often were converted into monetary value using the gold price at the time (e.g. pence per yard). American grades are normally quoted in decimal troy ounces per short ton.
Conversion factors
1 pennyweight (dwt) = 1.55 174g
1 ounce (oz) Troy = 31.103 48g
1 long ton (avoirdupois) = 1.016 047t
1 short ton (2000 lbs) = 0.907 1847t
1 foot (ft) = 0.3048m
1dwt/ton = 1.530 612 g/t
1 dwt/short ton = 1.714 286g/t
1 dwt/cubic yard = 2.034 906g/m3
1/-per short ton = 0.403 361g/t
Gold bars
London Market Standard - minimum 350oz - maximum 430oz
Perth Mint - 400oz+ 10oz
80 bars = 1 metric tonne approx.
The Perth Mint also casts bars (from ½oz to 50oz) for the small investor.
Western Australia's development is paved with gold. The rushes of 1880s and 1890s transformed WA from a struggling backwater into a thriving State. The influx of diggers who joined the gold rush increased Western Australia's population from 46,000 in 1890 to 180,000 in 1900.
Today, Western Australia is experiencing another gold boom. Modern mining and processing techniques have seen output jump from about 15 tonnes in 1980 to 180 tonnes in 1992 (an increase of 1200%). The previous record was 60 tonnes in the early 1900s.
Australia - the world’s third biggest gold producer - relies on WA for more than 70% of the nation’s gold. In 1992, the gold industry, which employed 10,000 people, was worth $2.7billion.
The formation of gold deposits
Most of the gold deposits in Western Australia were formed during the Archaen age more than 2,500 million years ago. Notable exceptions are the Telfer deposits formed 1000 million years ago during the Proterozoic age.
Under the influence of changing temperatures and pressures the precipitate gold is concentrated in rock fractures.
Some deposits have subsequently been discovered in the form of alluvial nuggets, free gold in quartz veins or alloys, combined with silver, copper, platinum and tellurium.
Most gold mined today is so fine that it cannot be seen with the naked eye. The presence of gold particles can only be identified by chemical testing.
Where is the gold produced?
The States most significant goldfields have been developed in the Kalgoorlie, Murchison, Pilbara and Boddington areas. In 1992, there were 45 major mining companies in Western Australia. The ore is mined and smelted to more than 75% gold and often over 90% at or near each site.
