xeyla

Originally Posted by waldronate

A lot of economically important minerals (lead, copper, silver, gold, tin, zinc, etc.) are associated with hydrothermal systems, as are many types of gems (beryls like emeralds, oxide gems like sapphires, and silica gems such opals are commonly formed in hydrothermal systems; diamonds are not).

Hydrothermal systems come in a number of forms, but the basic idea is that hot water dissolves out soluble things like the common metals and sulfur in the hot areas and deposits them in cooler areas.

What often happens is that an intrusion of melted material enters the bottom of a stack of rock and slowly begins to cool. As the melt cools, it fractionates into different components. The first things to freeze out are minerals such as quartz and feldspar (the primary component of granite). Water lowers the melting point of rock, so the water-associated things will tend to crystalize out last. The water-holding material percolates and infiltrates into surrounding rocks, metamorphosing them into other minerals and depositing a lot of the things that are dissolved (there is a lot of silica in solution, so often things like quartz veins with minerals in them are formed). This sort of intrusive process tends to form veins of minerals around the edges of granitic systems rather than bulk deposits. As the sulfides oxidize at the surface, they become oxides. Most sulfide ores need to be converted to oxides in order to extract the metal content, so the oxidation saves a step.

Another common type of hydrothermal system forms bulk sulfide ores on the ocean floor. We've probably all seen pictures of black smokers; these are examples of water percolating down, hitting hot rocks, dissolving things, and moving back up to the ocean floor, where the cool temperatures let things precipitate back out. Lots of economically important metal sulfides such as copper get set up this way. A deposit can be refined to very high purity through a few iterations of this process and the deposits can eventually be raised above the surface by tectonic forces.

In addition to the primary deposition, there are secondary deposits such as placer deposits. These deposits form as rivers erode the mineral-bearing rocks and the action of the flowing water separates the dense materials such as gold, gems, or native copper from the lighter minerals. These dense materials tend to deposit where the water flow slows down such as the inside bends of rivers or where a canyon exits a mountain range. The gold deposits of South Africa, for example, are actually an ancient and buried placer deposit. Many of the surface diamond and sapphire deposits are also secondary deposits, where the dense gems were worn out the places were they formed and moved downriver.

Diamonds are formed at great pressure and temperature, and brought to the surface as part of magma bodies. The diamonds may erode out or the primary deposit and collect in placer deposits for mining, but miners may also choose to mine the primary kimberlite (a kind of frozen magma) deposit.

An amusing side note on bitumen deposits is that the economically important bitumen is often found associated with a far more fluid fraction that is hazardous to have around and reduces the value of the bitumen. These days we refer to that stuff as petroleum (lit. "rock oil") and it's more important than the bitumen.