A wide range of people work in the gold sector, including technical and other professionals, operators and administrative personnel. In addition, there are many employed in services and other industries which support the gold sector. Today, operations are highly mechanised and the ever-increasing use of technology is a feature of the Australian gold sector.
In general, there are three key, specialist professions most closely associated with the gold sector, although a large number of other professionals and personnel are also essential. Geologists explore for ore deposits, or focus on assessing known prospects and deposits. Mining engineers determine and manage the methods used to mine ore. Metallurgists are responsible for processing ore and recovering the gold.
Exploration is termed “brownfields,” if it is undertaken within or nearby known gold bearing ground and old gold mining areas. It is termed “greenfields,” if it is undertaken in areas where gold has not previously been known to occur. A fundamental knowledge of the geology of an area is the first requirement for basic exploration. Then many types of chemical and geophysical techniques are used to identify potential locations suitable for undertaking a more intensive search. Geologists also use modelling, whereby a knowledge of other prospects and other deposits is used in order to compare a new location with known occurrences. Relatively shallow drilling may be undertaken as a next step, after a prospect is identified and this might be followed by deeper drilling, if it is warranted by the results. If sufficiently encouraging, then a drilling program may be commenced, better to define the extent and grade of the mineralisation. Where appropriate, further detailed drilling is undertaken to enable ore reserves and resources to be calculated, according to defined parameters.
While existing gold producers undertake considerable exploration, in Australia there are a host of companies that are “explorers,” hunting hopefully to find gold and other minerals which they may develop, or alternatively sell to others at a profit.
Ultimately feasibility studies may be carried out, with finally a bankable feasibility study being completed. This is used to gain finance for development of a mine by way of equity and debt, to obtain the necessary permits and consents, and to negotiate contracts for mine and plant development, provision of energy and a host of other services.
Today, gold is mined from open pits and from underground. At the start of the Third or Modern Boom, the new operations were almost always developed as open pits. Many of these were only mined as deep as the base of the oxidised ores, as these were often easier to process and also softer than the underlying fresh rock which usually contained sulphides. Many were located near older workings which dated from the 1890s and earlier 1900s. Others resulted from successful “greenfields” exploration, in locations where gold had been previously unknown. The open pits were developed with benches and haul roads, so that ore and waste could be mined and trucked from the pit. The ore was sent to stockpiles for processing and the waste was dumped on heaps which formed low hills that were then planted with native vegetation. Often the pits had relatively steep walls as many were designed only to last for a limited time, given the size of the orebody. In open pits, for every tonne of ore mined about five tonnes of waste material was mined, on average.
By the 2000s, although some open pits had grown considerably, more and more operations had begun to mine ore from underground. Near surface oxide ores were less common and producers were mining fresh ore, often from ore bodies which extended much deeper below the initial oxide zone. The majority of such underground mines are developed by driving declines, that is descending “tunnels,” often spiral, which trucks can drive up and down. The orebody itself is then accessed from drives and crosscuts off the main decline, or from draw-points below the zone being extracted. Only a few underground mines have the more traditional shafts with headframes. A number of mining techniques are now used underground and over time, the dimensions of the underground workings have increased substantially in the larger orebodies, to take advantage of economies of scale. In underground operations, less waste is mined compared with that of open pits and furthermore, some of the waste is returned underground to fill the mined voids and stabilise the ground. Whether ore is trucked from the open pits or from underground, it is dumped on stockpiles for later processing.
Today the gold sector in Australia has a mix of open pit and underground mines. Operations are usually highly mechanised. One of the largest open pits is the Super Pit in Kalgoorlie, which is approximately 3km long, 1 km wide and over 500m deep. It is planned also to begin underground mining there shortly.
The main processing method which was so important in enabling the Third Boom, is termed “carbon in pulp” or “carbon-in-leach” (“CIP” or “CIL.”)
The ore is first crushed by jaw and/or gyratory crushers. It is then usually ground in a SAG (Semi-Autogenous Grinding) Mill which is a huge rotating barrel – essentially a much larger diameter, shorter length, version of a ball mill. Centrifuges, jigs or spirals are then used to separate as much free gold as possible by means of gravity. Next, the ground ore, mixed with water, is pumped to the first of a series of large tanks where very dilute sodium cyanide solution is added. The pulp, containing around 50 percent ore, is stirred mechanically and is pumped sequentially to a series of tanks, as the gold is leached, that is, dissolved by the cyanide. At the same time, activated carbon, often as lentil-sized chips of burnt coconut shell, is added counter current to the train of the leach tanks. The carbon ADSORBS (not absorbs) the gold and is then screened off and stripped chemically, to obtain a solution which contains the gold. This solution is treated by electrolysis to recover the gold, which is then melted in a furnace with fluxes, cast into impure gold doré bars and sent to a mint for refining.
In a few cases in Australia where the ore is not free-milling, it is roasted or ultra-fine ground prior to cyaniding using the CIP/CIL process. Also, a few treatment plants use bio-oxidation prior to cyaniding using the CIP/CIL process, whereby specialised bacteria are used to digest the sulphides in the ore and liberate the gold from the sulphides.
Much of the refractory (difficult to treat) ore is processed by crushing and grinding and then obtaining a flotation concentrate, by agitation in flotation cells with specialised chemicals. The gold is present in the concentrates, together with a mix of sulphides and these are sent elsewhere for smelting, then refining. Gold ore which contains copper minerals is often refractory and the gold is usually recovered within a concentrate which is then smelted and refined.
Gold producers rarely market gold. The impure gold doré needs to be refined before being sold commercially, either to be used in jewellery and other manufactured goods (such as computers) or to be securely stockpiled as a store of wealth. Therefore the refineries, rather than the gold mining companies, are usually the marketers.