In 1848, word that gold had been found in California on John Sutter's property triggered the first gold rush of modern times. In the stream of Sutter's saw mill near Sacramento now California's state capital gold nuggets were discovered. Thousands of adventurers and fortune hunters from all over the world converged on California, each seeking his own personal Eldorado.
In those days, prospecting techniques were still promitive. Usually trusting to frivolous luck, the prospectors tended at first to scour the beds of shallow rivers and streams, washing the sand in pans, whirling them in a circular motion. If they were lucky, a glimmer of gold dust, perhaps even a few larger grains, could be seen in the pan when everything else had been washed away. When a substantial deposit was located away from waterways, the first consideration was to obtain a water supply. Gold bearing sand and gravel was then shoveled into a narrow channel with built in wooden slats and the water was played over it. The gold was retained by the slats as the water carried other substances away. Through this laborious method, California's annual gold production amounted to ninety five tons two years after the beginning of the gold rush. That was half the total world production at the time. By 1863, prospectors with financial backing had introduced more sophisticated methods to get at the gold hydraulic techniques in which the rock was sluiced away by high pressure jets of water.
In the 1850's, some of the California prospectors made their way to Australia, to join the gold rush there. They brought with them the experience and methods of their California days. Australia soon was to become, for a short time, the greatest gold producer in the world, and then gold was also discovered in New Zealand. Much of it was found in river deltas from which it was extracted with the help of dredges.
Rich gold deposits had also been unearthed in Russia. In the 1830s, the newly invented centrifuge was widely used there to separate gold from virgin rock. And, as the nineteenth century drew to a close, gold was also found in the inhospitable region straddling the Alaska-Canada border. This led to the most dramatic gold rush in history. In 1896, during a serious economic depression, thousands braved the harsh elements to seek elusive gold in the Far North.
But the richest yields of all were to be found elsewhere. In 1886, in Witwatersrand, near present-day Johannesburg in South Africa, gold was found in exhilaratingly large quantities. The gold-bearing lodes were on the surface of a forty-mile-wide bowl of countryside. But, for the most part, relatively thin layers of conglomerate extended underground and reached straight down to depths of up to 10,000 feet. These conglomerates were probably the result of the collapse of a mountain range some two billion years ago. It is likely that what had been surface rock was pressed together over millions of years and squeezed down into the depths of the Earth. The gold is in the form of fine particles pressed into the rock. The gold-laden "reefs" extend underground for more than seventy miles.
By the end of the nineteenth century, some fourteen percent of the world's gold production came from South Africa. By 1916, that figure had been raised to forty-two percent of the world's gold production, and reached about fifty percent by 1930. During the 1960s, South Africa produced seventy percent of the world's gold. Even when new rich gold sources were found, the country managed to produce a large portion of the world's total. In 1974, for example, 1338 tons of gold were produced worldwide. About fifty-four percent (729 tons) came from South Africa. The country's total production to date amounts to about 20,000 tons. The best know gold fields Of South Africa, Southwest Africa (Namibia) and Rhodesia are believed to contain some 18,000 tons of gold. That is considered to be about sixty percent of the total recoverable gold resources in the world.
New Methods for Gold Extraction
South Africa's spectacular success as a gold producer was intimately linked with the introduction of the cyanide separation process for treating ore and with generally improved prospecting techniques.
The new separation method was first used in South Africa in 1890. Finely ground gold ore was treated in a solution of sodium cyanide into which air had been blown. Gold entered into a complex compound with the sodium cyanide and was later separated by precipitation with another metal, such as zinc. This technique proved very efficient. Even the finest gold particles could be extracted from the ore. In South Africa, the method is used either alone or in tandem with the amalgamation process. South African gold tends to be very pure.
During the early years of the Witwatersrand fields, yields of twenty grams of gold per ton were the order of the day. Now, however, the yield in some mines has sunk to five, and even two, grams per ton. But when the price of gold soared, after it was allowed to float in 1968, a new inducement for technological advance was provided. Research into new and more effective mining and processing techniques was encouraged, with the higher gold price justifying increased development costs.
Until about fifty years ago, the search for new deposits was conducted without the full benefit of scientific method and equipment. Finds were usually made by chance, sometimes the result of the persistence of a lucky prospector proceeding merely on a hunch. Only in a few isolated instances was gold discovered through the efforts and experience of trained professional prospectors.
Not until the 1940's and 1950's were truly modern scientific prospecting techniques and mechanisms developed. These included instruments for measuring the force of gravity on the earth's surface, highly sensitive meters which indicate gravity variations. Analysis of these variations allows for an assessment of the structure of the earth's crust which helps t determine the kind and density of rock layers.
Various other magnetic and electric techniques are also now employed. Magnetic measurements determine local variations in the earth's magnetic field. This method is used to indicate the presence of iron sulphur compounds which sometimes contain gold. Airplanes equipped with the proper instruments can carry out magnetic investigation over regions which would otherwise be, for practical pre discovery .purposes, inaccessible.
Other devices are used to measure electrical conductivity on the earth's surface, with reference to both direct and alternating current. Telltale differences permit conclusions to be drawn about the mineral structure of the surface.
With these and other techniques, large areas of South Africa were meticulously surveyed. Various surveys indicated that an area about 150 miles south-west of Johannesburg might contain substantial gold deposits. The mining rights were acquired by the Anglo-American Corporation, South Africa's largest gold mining company, which had conducted the survey. A borehole was sunk on a farm near the town of Welkom and gold-bearing quartz was located at a depth of about 4,500 feet. Further drillings indicated that one of the richest lodes in the world had been discovered. The company now operates several mines in the area. South African gold is often found coupled with uranium oxide and prospectors often use Geiger counters in their search. The method has a drawback, however. Test drillings are required to make room for the meters which measure radioactive presence.
A more recent prospecting technique is a spin-off of space technology. Pictures taken by earth-circling satellites are scanned for hints of geological structures similar to those in which gold has already been found. So far this method has had limited success.
The second most productive gold fields in the world are those of the Soviet Union, discovered by geologists in the Soviet Republic of Uzbekistan, in the Kyzylkum Desert. This is the only large opencast gold mine so far discovered. Workable gold-arsenic beds were recently discovered in Sweden.
Gold of the Chemists
In his professional capacity, a chemist thinks of gold simply as a metal, like other metals. It has a place in the periodic table of elements. Its atomic number is seventy-nine. Its atomic nucleus has seventy-nine positively charged particles protons with a similar number of negatively charged electrons surrounding them. The atomic weight of gold, in relation to carbon 12, is 196.967. In chemical compounds, gold appears with both univalent and trivalent auro compounds, the latter being more stable.
There is only one natural gold isotope gold 197. Its atomic nucleus contains 79 protons and 118 neutrons. However, by using nuclear reactors, atomic physicists have succeeded in producing twenty-six artificial gold isotopes. They are classified according to the number of their nuclear particles. Apart from Au 180, Au 182 and Au 184, various isotopes between Au 177 and Au 204 can be created.
Precious metals are remarkably stable chemically. Neither hydrogen, nor oxygen, nor nitrogen can be dissolved in gold. Hot sulphur vapor will not corrode it. Oxygen cannot form spontaneous oxides with gold at high temperatures. Auro- and aurio-oxides can, however, be produced indirectly. Gold is resistant to water and most acid bases. Only aqua regia, a particularly potent mixture of hydrochloric and nitric acids, and other acids which produce nascent chlorine, can corrode gold (producing gold chloride.) Thus gold, particularly in powder form, dissolves in hydrochloric acid in its gaseous state. Gold may also be dissolved in liquid alkaline cyanide with which it forms complex salt compounds. In addition, gold joins with liquid mercury. This chemical response can be used to separate the metal from surrounding rock.
Gold is not easy to produce in its chemically pure form. However resistant it may be to chemical compounds, it frequently produces mixed crystals with other metals. The density of gold has so far not been established with total accuracy because such impurities are dispersed only with the greatest difficulty. The most reliable values are 19.369 g/cm3 at zero degrees centigrade and 19.297 g/cm3 at twenty degrees centigrade. Both values were established by X-ray crystallography. Accordingly, gold is the fifth heaviest of the natural chemical elements a kilogram of gold corresponds to a sphere with a diameter of no more than forty-two millimeters.
The precise melting point of gold has been accurately measured: it holds the highest value on the international temperature scale used for calibration of temperature measuring instruments 1063 degrees centigrade or, on the absolute scale, 1336.16 degrees Kelvin. Like most other metals, gold shrinks as it cools to room temperature by about two percent. That means when cast, gold is easily freed from its mold. At 2966 degrees centigrade, molten gold vaporizes.
With regard to electricity and heat conductivity, gold trails silver by about thirty percent. In these respects, silver ranks highest among metals.