|
|
|
Geology of Gold
Australia |
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 color, 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 centimeter 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 color of gold is directly related to its purity. Crystallized 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 color 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 ton 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 favor 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 tons 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
|
|
|
|
|
