How Gold Is Formed In Its Different Varieties
Gold is not only one of the oldest substances on earth
, but it is also one of the most valuable. Being a natural substance which comes out of the ground, its formation is quite interesting.
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 its formation. A popular misconception is that this natural element has cooled from a molten state.
In fact, it is transported though the crust, and dissolved in warm to hot salty water. These fluids are generated in huge volumes deep under the surface of the earth, 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, when it is dissolved, along with other elements such as silicon, iron, and sulphur, it 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 it eventually becomes insoluble and begins to crystallize, 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." As chemical weathering and erosion gradually break down the host rocks and lower the land surface, the 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 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. Interestingly enough, gold can occur in a wide variety of forms.
In massive quartz reefs, it occurs as disseminated, irregular grains, scales, plates, and veinlets with microscopic dimensions, and as larger compact, reticulated, spongy, hackly masses, or slugs. It occasionally takes forms that lend themselves to descriptive terms such as wire, nail, mustard, and paint.
While it 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 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. It crystallizes in the cubic system, and perhaps the most common variety is the eight-sided octahedron.
Other important metal-bearing minerals can also be found in the quartz reefs along with the precious metal. The presence or absence of these minerals can be used to help classify the type of 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 fields. Massive deposits of these metal sulphides may contain only small proportions, but their overall size makes them significant producers.
In the famous Golden Mile at Kalgoorlie, Western Australia, unusual minerals containing tellurium have been extremely important. The relative softness means it can be scratched by harder grains during erosion and transport.
However, its malleability often leads to particles being bent or twisted, rather than reduced in size. Grains that haven't travelled far from the reefs often preserve many of their original features, such as their basic shape or the imprints of quartz crystals.
Generally speaking, finer particles known as 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 particles to become smaller and more worn further downstream.
Now you know exactly how this beautiful, historic metal is formed. Next time a friend gets engaged, you can impress them with your knowledge about their ring!
by: Tom Selwick
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