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Gold, a chemical element; Genomes OnLine Database; Global-scale Observations of the Limb and Disk, a NASA Explorer Mission of Opportunity; GOLD (parser), an open-source parser-generator of BNF-based grammars; Graduates of the Last Decade, an Institute of Electrical and Electronics Engineers program to garner more university level student members
Placer gold deposits form via gravity, with the density of gold causing it to sink into trap sites within the river bed, or where water velocity drops, such as bends in rivers and behind boulders. Often placer deposits are found within sedimentary rocks and can be billions of years old, for instance the Witwatersrand deposits in South Africa .
Gold occurs principally as a native metal, i.e., gold itself.Sometimes it is alloyed to a greater or lesser extent with silver, which is called electrum.Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles (known as colour) embedded in rock minerals.
Only gold that is concentrated can be economically recovered. Some 400 million years ago, rocks that would be accreted onto western North America to build California lay at the bottom of a large sea. Subsea volcanoes deposited lava and minerals (including gold) onto the sea floor; sometimes enough that islands were created. [1]
Placer mining is an important source of gold, and was the main technique used in the early years of many gold rushes, including the California Gold Rush. Types of placer deposits include alluvium, eluvium, beach placers, aeolian placers and paleo-placers. [2] Placer materials must be both dense and resistant to weathering processes. To ...
The zone contains hundreds of mines and prospects, including some of the best-known historic mines of the gold-rush era. Individual gold deposits within the Mother Lode are gold-bearing quartz veins up to 15 metres (49 ft) thick and a few thousand feet long. The California Mother Lode was one of the most productive gold-producing districts in ...
The total world supply of gold in 2007 was 3,497 tonnes. [124] Gold production does not need to make up for gold demand because gold is a reusable resource. Currently, yearly gold mining produces 2% of the existing above-ground gold which is 158,000 tonnes (as of 2006). [125]
Plate depicting placer mining from the 1556 book De re metallica. Placers supplied most of the gold for a large part of the ancient world. Hydraulic mining methods such as hushing were used widely by the Romans across their empire, but especially in the gold fields of northern Spain after its conquest by Augustus in 25 BC.