Gold deposits

Introduction

Discovered about 5000 years ago, gold is a precious metal that is admired and valued as much for its usefulness as its beauty. Although very heavy, gold is very soft and can easily be shaped either by flattening it into sheets (malleability), or by stretching it into wires (ductility). Gold does not tarnish (react with air) and is an efficient conductor of heat and electricity. These properties make gold prized beyond just its decorative quality.

Gold has served as a type of money in the form of coins. More recently, gold has been used for electronic circuit boards and electrical connections. Gold is even used to make reflective shields for spacecraft to repel infrared radiation from the sun.

Gold deposits

Although widely dispersed in the Earth's crust, gold is rarely found in deposits large enough to make mining it worthwhile. Gold is also found in minute (very tiny) proportions in seawater, but the cost of extracting these small amounts would be much more than the gold would be worth.

Gold deposits on land are found in two forms - primary and secondary deposits.

Primary gold deposits

Over millions of years, volcanic eruptions in various parts of the world have forced magma (molten rock) through cracks in the Earth's surface. As the magma cools and eventually hardens, it forms igneous rocks, in which solid gold can be found. See image 1

It is commonly believed that natural gold has cooled from a molten state. In fact, it is transported through the Earth's crust dissolved in warm to hot salty water. In this form it gathers wherever fractures in the rocks allow the fluids to pass. These lines of gold are known as veins. Other, larger deposits of gold are called lodes or reefs. Early miners called these rocks greenstone because of the greenish colour caused by the geological processes they had undergone.

Veins and reefs of gold-bearing quartz occur in many types of rock, including different granites, in volcanic rocks or in regions of black slate. In most cases these host rocks are not the immediate source of the gold.

Secondary gold deposits

As rocks are worn down over many millions of years, primary gold deposits that have formed gradually become exposed. Weather conditions (extremes of heat and cold) force the rock to expand and then contract, causing it to gradually break down. The gold deposits are then washed into creeks and rivers by rain. See image 2

As gold is a very heavy metal, it sinks to the bottom of creeks and riverbeds as alluvial gold. As the rivers and creeks dry up, the gold is then found near the surface. In this way gold prospectors can sometimes be successful in finding small nuggets.

As river beds dry up and then re-formed, gold deposits can be found in horizontal layers below the surface of the riverbed. Changes in the depth of the water tables and creek beds mean that gold deposits can be found at different levels. See image 3