How Gold Deposits are Formed
As most of you already know, gold typically occurs in fractured "host" rocks (granite, schist, quartz, etc.) as veins or lodes. The Sierra Nevada Batholith (the source of the California Motherlode Region's gold) is a good example of this sort of deposit on a grand scale.
Most lodes are formed when super-heated fluids composed of various chemical constituents circulate through mineralized zones containing gold (Au), chemically bond with it, and then transport it to new locations in the earth's crust. The physical differences of various types of rock as well as the chemical composition of the fluids driven into and around them account for the vast array of various types of lode gold deposits, some of which are outlined below:
1) "Smoker" Au Deposits: These deposits occur wherever superheated waters spring up from vents ("smokers") in the ocean floor which are typically created by tectonic activity along the earth's crust. When this happens, metal-rich minerals including small amounts of gold are deposited in the zone where the superheated, auriferous water is mixed with the cold seawater it makes contact with.
2) High-Sulphur Hydrothermal Au Deposits: Hydrothermal gold deposits usually form when gold-laden water is superheated by magma (molten rock) in the shallowest layers of the earth's crust along lines of past volcanic activity. The chemical fluids in these sorts of deposits are sulphur rich, something that caused many an old timer to call these types of gold ores "sulpherets." (The gold deposits found near Summitville, CO are a good examples of high-sulphur hydrothermals.)
3) Low-Sulphur Hydrothermal Au Deposits: These deposits are very similar to the previous deposit type, with gold forming under similar conditions in similar regions. The main difference is, of course, that the mineralized chemical fluids involved are low in sulphur. (Gold ores found near Round Mountain, NV are low-sulphur hydrothermals.)
4) Microscopic or "Invisible" Au Deposits: Formed when specific chemical interactions take place between super-heated fluids and sedimentary rock, "invisible" deposits derive their name from the micron-sized gold particles that typify them. But don't let the terms microscopic or invisible fool you. These types of deposits can be very "rich" in terms of gold values (but require massive operations to mine). (The Carlin and Meikle Mines in Nevada are good examples.)
5) Uplift or Batholith Au Deposits: Typified by the extensive gold deposits of California's Motherlode Region, batholiths form when extensive fracturing of the earth's crust occurs and tremendous uplift takes place as mountain ranges rise (i.e. the Sierra Nevada Mountains). As this uplift takes place, mineralized hydrothermal fluids flow into rock fractures and cracks, usually of a quartzitic nature (these superheated fluids are created by extremely hot metamorphic rocks buried deep beneath the earth's surface).
6) Greenstone Belt Au Deposits: These are very ancient gold deposits geologically speaking (older than 2 billion years), and no longer form on the earth today. However, examples of these ancient deposits can be found in Australia, Canada, and Africa in various locations.
7) Placer Au Deposits: Over the course of millenia, fine gold, flakes, and nuggets are worn away from their source lodes by heat, cold, pressure, and erosion and begin "migrating" into low-lying areas such as benches, streambeds, gulches, washes, and the ocean floor. These easily worked deposits are called "placers" and they are the bread and butter of most recreational or small-scale miners.
So there you have it. You've just completed the basic Bedrock Dreams Geology 101 course.
(c) Jim Rocha (J.R.) 2008