Gold forms in hard crystalline rocks in deposits commonly known as veins. A lode typically is formed in areas where the rock containing veins has been altered in some fashion. Gold found in a lode is bound with sulphide and telluride minerals. The minerals gradually are destroyed by nature–wind or rain–leaving only the gold behind. The gold can range from grain-size pieces to nuggets.
Gold frequently is found in soil containing hard volcanic rock composed of compacted volcanic ash called tuffs. Upon examination, the soil near a gold vein will contain quartz, feldspars, feldspathoids and other light-coloured minerals. The surrounding area will be magnetic and can be found with a metal detector. Other minerals associated with a nearby vein include pyrite, arsenopyrite, pyrrhotite, galena, chalcopyrite, scheelite and stibnite. High-grade gold refers to the gold found in the veins.
Eluvial deposits are comprised of gold that has been deposited by wind or water into soil near an outcrop Streams, changes in temperature, movement of the Earth’s crust and vegetation growth all can relocate gold. According to the Arizona Outback website, the elements “reduce the rocks to gravel, sand, silt and clay and liberate the gold.”
The deposits typically are located near an eluvial placer–which is an irregular surface on a hillside below a mineral source. In other words, if a mineral mine previously was located in the area, start looking look downhill from it. A placer is a deposit left when veins have weathered and disintegrated.
Locations made up of a mixture of boulders, gravel and debris from an adjacent hillside are easy places to search for gold. Gold found in locations like this is typically coarse and highly concentrated on the bedrock.
In a process called supergene enrichment, gold is carried through the lode channel to the water table or groundwater. The gold is redeposited and enriched in the area surrounding the table in lateritic deposits. Lateritic is a type of soil rich in iron and aluminium that is found in hot, wet tropical regions. According to the Arizona Outback website, early prospectors depended on lateritic deposits to make small mines economical. The gold found in lateritic deposits typically is low grade but the deposits are large. Low-grade gold refers to the gold found on or near the surface. Large, open-pit operations are most suited to mining lateritic deposits.
Desert placers and deposits are found in the arid regions of the Southwest. Summer cloudbursts cause streams to rise quickly. The streams move gold through the dry washes and gullies. Sand and other debris are carried by lighter rain and force the gold into the washes. The next cloudburst can sweep the accumulated materials further from the vein.
The concentration and movement of gold will be erratic due to the rain, says the Arizona Outback website. Some gold even may be found at the bottom of any temporary channel that was formed during a cloudburst. Gold sometimes is found in small accumulations very close to the surface. Wind also can uncover gold by moving sand and light rocks. A surface veneer comprised of gold or other minerals will be left exposed in a fairly concentrated form.
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