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Geology of gold


Properties of gold

Gold has had an inestimable effect on human history. It has been crafted, mined, worshipped, plundered, fought over and traded for thousands of years. Today, the search for gold is as eager as ever, despite the vast stocks stored away in underground bunkers. So why has gold held this fascination for humanity?

Its initial attraction is its colour, an eye-catching and characteristic bright yellow with a soft metallic glint. Gold’s pleasant ‘feel’, a combination of its density (19.3 grams per cubic centimetre when pure) and coldness, cannot be duplicated by any other metal. Furthermore, gold can be hammered into very thin sheets or leaves, drawn into wire, cast, carved, polished, heated without tarnishing and easily combined (alloyed) with other metals.

Gold also conducts heat and electricity, reflects light and is untouched by nearly all acids, a property which led alchemists to christen it the noble metal. This combination of properties makes gold very stable in its natural metallic form, and also gives it many uses in electronics, ornaments,jewellery and advanced technology.

The colour of gold is directly related to its purity. Crystallised gold and silver have the same atomic structure and their atoms are nearly identical in size, so that natural alloys of gold and silver are common. Pure, or 24-carat gold, is the brightest yellow, but as the amount of silver increases the colour becomes paler. Pale gold containing more than 20 per cent silver (corresponding to about 20-carat gold or less) has been called electrum. Trace amounts of copper, iron and palladium can also substitute in gold. Man-made alloys of gold with rhodium, iridium or palladium, intended to give gold greater hardness when used in jewellery, have been given names such as ‘white gold’. The carat scale is commonly used in jewellery, while in mining, an alternative scale uses ‘fineness’ of gold, where a figure of 1000 corresponds to pure or 24-carat gold.


Formation of gold

Because gold is very stable over a wide range of conditions, it is very widespread in the earth’s crust. While its overall concentration is very low (about 5 milligrams per tonne of rock), rich concentrations of gold, forming ore deposits, are known throughout the world. The well-known saying amongst prospectors that "gold is where you find it" suggests its occurrence is unpredictable, but it is now known that certain geological environments favour gold’s formation.

A popular misconception is that natural gold has cooled from a molten state. In fact, gold is transported though the Earth’s crust dissolved in warm to hot salty water. These fluids are generated in huge volumes deep in the Earth’s crust as water-bearing minerals dehydrate during metamorphism. Any gold present in the rocks being heated and squeezed is sweated out and goes into solution as complex ions. In this form, dissolved gold, along with other elements such as silicon, iron and sulphur, migrates wherever fractures in the rocks allow the fluids to pass. This direction is generally upwards, to cooler regions at lower pressures nearer the Earth’s surface. Under these conditions, the gold eventually becomes insoluble and begins to crystallise, most often enveloped by masses of white silicon dioxide, known as quartz. This association of gold and quartz forms one of the most common types of "primary gold deposits".

Veins and reefs of gold-bearing quartz can occur in many types of rock, for example around granites, in volcanic rocks or in regions of black slate, but in most cases these host rocks are not the immediate source of the gold.

Gold deposits have formed at many different times during Earth’s history. For example, those in Western Australia are believed to have formed about 2400 million years ago, during a period of intense metamorphism and intrusion of igneous rocks. The gold-bearing quartz reefs in Victoria are significantly younger, about 400 million years, but also owe their origin to a period of intense metamorphism in the region.

As chemical weathering and erosion gradually break down the host rocks and lower the land surface, the quartz and gold veins are eventually exposed to the atmosphere. The veins provide far more resistance to chemical attack than the surrounding rocks, so that mechanical weathering is required to fragment the quartz, thereby releasing the gold. Because they are relatively heavy, particles of gold are more difficult to move and so become naturally concentrated in the soil or in adjacent gullies or streambeds. These concentrations are known as alluvial or placer deposits and have yielded incredible riches on some goldfields, such as those in California and central Victoria.

Alluvial deposits take many forms, including sands and gravels in the beds of modern-day streams, in old river valleys buried under lava flows or perched on hilltops due to uplift of the land surface. The terms shallow and deep leads are used in Victoria for gold-bearing gravels covered by younger sedimentary layers or lava flows. These were especially important in the Ballarat district. Because of its resistance to chemical attack, gold can be recycled from one type of alluvial deposit to another.


Types of gold

For such an apparently simple element, the mineralogy of gold is quite complex. To begin with, gold can occur in a wide variety of forms. In massive quartz reefs, gold occurs as disseminated, irregular grains, scales, plates and veinlets with microscopic dimensions, and as larger compact, reticulated, spongy or hackly masses or slugs.

Gold occasionally takes forms that lend themselves to descriptive terms such as wire gold, nail gold, mustard gold and paint gold. While all gold has a crystalline structure, distinct crystals showing well-formed faces are relatively rare. They require special conditions to form, in particular space in which to grow. Hence crystals of gold are found in cavities in quartz reefs or in softer minerals such as iron oxides where they have been able to push aside the enclosing material as they grew. Gold crystallises in the cubic system, and perhaps the most common variety is the eight-sided octahedron.

Possibly the best surviving Australian specimen is a group of sharp, branching, octahedral crystals from Matlock, in the Woods Point goldfield of Victoria. A superb 715 gram (23 troy ounce) mass of crystals known as the Latrobe Nugget was found in the Heathcote district in Victoria and obtained by the British Museum in 1858.

Other important metal-bearing minerals can also be found in the quartz reefs with the gold. The presence or absence of these minerals can be used to help classify the type of gold field. The most common and widespread are pyrite and arsenopyrite, two minerals containing iron and sulphur. This assemblage is distinctive in many of the Victorian goldfields. Sulphides of lead, zinc, silver, bismuth and antimony also occur and may be locally abundant in some gold fields. Massive deposits of these metal sulphides may contain only small proportions of gold, but their overall size makes them significant producers. In the famous Golden Mile at Kalgoorlie, Western Australia, unusual minerals containing tellurium combined with gold have been extremely important.

The relative softness of gold means it can be scratched by harder grains during erosion and transport. However, gold’s malleability often leads to particles being bent or twisted, rather than reduced in size. Gold grains that haven’t travelled far from the quartz reefs often preserve many of their original features, such as their basic shape or the imprints of quartz crystals. Generally speaking, finer gold particles known as gold dust have been carried further from their source reefs, possibly by fast-flowing streams. A common observation by early Victorian diggers working alluvial deposits along streambeds was for the gold particles to become smaller and more worn further downstream. This was especially evident in the Woolshed Valley in the Beechworth district in northeastern Victoria, where Reedy Creek flowed vigorously through a steep-sided gorge cut in granite.


Australia’s gold bearing potential

Australia produces about 300 tonnes of gold annually, currently ranking it about fourth in the world. Exploration for gold continues to be a major activity for Australian mining companies, with target areas extending to the South Pacific and Southeast Asian regions. Whether a discovery is ever mined depends greatly on the size and nature of the deposit and its geographic position, the concentration of gold in the rocks (known as the grade) and, perhaps most importantly, on the international price of gold. Despite some rich deposits being brought into production, none have changed the course of a nation like the fabulous finds of the 1850s.


Credits

By Dr Bill Birch, Senior Curator, Geosciences, Museum Victoria



 
 

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