Gold is found in a diverse array of deposit types, ranging from concentrations in quartz veins in igne-ous-metamorphic rocks controlled by the plate tectonic setting, to metamorphic settings, to wide areas called alluvial deposits where streams eroded primary gold sources and deposited them in places where the stream currents slowed and dropped the gold out of suspension. most of the lode gold deposits are found in quartz veins in intrusions, granites, shear zones, and deformed turbidite sequences. Basalt is a common constituent of many gold provinces, and it appears that metamorphic fluids move through the basalt and leach out the gold and related fluids, depositing them when the chemical and temperature conditions are best suited in the host rocks.
Placer gold is eroded from the lode gold sources and carried by streams and rivers, or reworked by beach processes before reaching the final site of deposition. most of the the world's placer gold is found in the Archean Witwatersrand Basin in south Africa, but many other placer gold provinces are
known from around the world. one of the most active of these, where the lode-gold sources were identified well after the placer layers, is the gold districts of Alaska.
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