Gold has been found all over the world on every continent except for Antarctica (probably because it’s all covered in ice!). The two main places that miners or gold enthusiasts find gold are called placer deposits and quartz veins.

A placer deposit of gold forms when erosion causes pieces of gold to separate from the rest of a rock that it was originally found in, and gets carried into a stream. Many pieces of gold get carried into streams this way and eventually the gold will settle out of the water column, as it’s quite heavy, into the bed of the river. These form large deposits of gold in river beds in which people can then use the technique of gold panning to sift through the river sediment and find the gold pieces.

The second type of gold deposit is found in quartz-rich veins. Quartz is one of the most common minerals in the Earth’s crust and this mineral can become concentrated in veins within rocks. As these rocks get compressed and folded, which can happen as a result of plate tectonics, these quartz veins get squeezed and heated, forcing the quartz into a fluid that can flow through the rest of the rock and dissolve any gold within the rock. This fluid later will solidify once it cools down and form gold-rich quartz veins! So, gold is very often associated with quartz-rich veins within rocks, keep an eye out!

Gold is found in lode and placer deposits. Lodes are concentrated deposits in igneous and metamorphic rocks left by hydrothermal fluids. When lodes are eroded, heavier minerals including gold can be deposited in places such as stream bends and beaches, while the lighter sediment is washed away. These accumulations of heavy minerals are known as placers. In the United States, most deposits are located in the southwest and in Alaska.

There is some discussion of this on ScienceLine already, but here is other information.

Gold can be found throughout the crust of the earth and in the oceans, though the concentrations are typically too low to be of interest or value. Portions of the crust with higher concentrations of gold are called deposits, of which there are four types Lode, Intrusive, Placer, and Laterite.

These deposits are formed by two fundamental processes (see first link and here). The first two involve magma transporting gold from within the mantle to nearer the surface and result in gold embedded within solid rock. The latter two are more like piles of gold which collect after deposits of the former two types are eroded. Rivers transport the gold particles and nuggets until they become caught somewhere, gradually accumulating. Eventually the waterway dries up and the collections remain as deposits. The water of Earth's oceans contain vast amounts of dissolved gold ( estimated at 20 million tons, compared to a 2017 global production of 3,200 tons), but the concentrations are so low ( 1 gram per 100 million tons of seawater) that it is not economically feasible to extract.

Gold is also found throughout the universe. This is believed to be due to the formation and distribution of gold during supernovae and merging of neutron stars. This gold, in cosmic dust, then collects into the various rocky bodies of the universe. While this includes planets such as Earth, the gold present during formation would have sunk to the core (since gold has a relatively high density compared to most elements in the crust.) The gold which can actually be mined seems to have been delivered by a meteorite bombardment some time after formation.

[Some practical instructions on finding gold deposits.]

Gold is deposited by hot water that dissolves it, most often along fault planes. It is fairly common in granite-dominated areas.