Hot water acts as a concentrating, transporting, and depositing agent in hydrothermal mineral deposits. Out of all the deposit classes, they are the most numerous.
Since pure water is a poor solvent for most ore minerals, hydrothermal deposits are never created from pure water. It would be more accurate to refer to them as products of hydrothermal solutions since they are instead generated by hot brines.
Brines, particularly those containing sodium-calcium chloride, are excellent solvents for a wide range of sulphides and oxide ore minerals. In addition, they can dissolve and move native metals like silver and gold. A hydrothermal solution’s water can originate from various sources.
It can originate at the Earth’s surface as rainwater or seawater and then trickle down to great depths through fractures and porous rocks, where it will be heated, react with nearby rocks, and become a hydrothermal solution. Alternatively, it can be released by a crystallising magma or expelled from a mass of rock undergoing metamorphism.
The final compositions of all hydrothermal solutions tend to converge due to reactions between the solutions and the rocks they encounter, regardless of the source and beginning composition of the water. Hydrothermal solutions are brines of sodium and calcium chloride that also contain minor amounts of numerous other chemical elements, magnesium, and potassium salts.
The concentration of dissolved solids in the solutions varies from a few percent to up to fifty percent by weight. Current hydrothermal solutions can be investigated in hot springs, oil-field brines, submarine springs along the mid-ocean ridge, and subterranean brine reservoirs like those in the Imperial Valley of California, Turkmenistan’s Cheleken Peninsula on the eastern side of the Caspian Sea, and others.
Fluid inclusions are small samples of solution trapped in crystal defects by a growing mineral that can be used to study fossil hydrothermal solutions. Hydrothermal solutions are very abundant in the Earth’s crust because they can develop from a variety of mechanisms.
In contrast, hydrothermal mineral deposits are uncommon and very small in relation to other geologic features. This makes it clear that most solutions eventually blend in with the hydrosphere, leaving behind minimal evidence of their previous existence.
The reason behind the formation of mineral deposits in restricted spaces or tiny volumes of porous rock by certain solutions is that they deposit their dissolved loads there due to a specific process.
