hydrothermal solutions

hydrothermal solutions Hydrothermal means, simply, hot water, but in geological parlance the term is restricted to hot subsurface waters. If hot subsurface water contains dissolved materials—as most heated subsurface waters do—it is said to be a hydrothermal solution. Research has shown that hydrothermal solutions have the capacity to dissolve, transport, and redistribute minerals within the Earth's crust, and, under suitable circumstances, to scavenge and concentrate valuable metals, such as gold, silver, copper zinc, and tin into rich mineral deposits. Indeed, most mineral deposits mined for their metallic contents were formed through the agency of hydrothermal solutions and are referred to as hydrothermal deposits.

Origins of hydrothermal solutions

The water in a hydrothermal solution can come from any of four principal sources, and is commonly a mix from more than one source. The four kinds of water are groundwater, formation water, metamorphic water, and magmatic water.(1) Groundwaters are meteoric in origin and can be either fresh or saline (as in sea water). When groundwater comes in contact with hot igneous rock, or penetrates deeply enough to be heated by the geothermal gradient, a hydrothermal solution is the result.(2) Formation waters are waters trapped between particles of buried sediment, within the strata of a sedimentary basin. Compaction slowly expels formation water, which then rises and mixes with groundwater.(3) Metamorphic waters are formed by the dehydration of hydrous minerals such as clays, micas, and amphiboles as a result of high temperature and high pressure deep within the crust.(4) Magmatic waters are formed from the vapors released by a magma that is cooling and crystallizing to an igneous rock.There are two sources for the metals and other constituents dissolved in hydrothermal solutions: the trace amounts of minerals present in all common rocks, and magmas.

When a magma crystallizes, most of the chemical elements present are incorporated into minerals such as feldspar, mica, amphibole, and pyroxene. Some trace elements do not easily fit into the atomic structures of the crystallizing minerals and become concentrated in the magmatic vapour that is released by the magma. As such vapours cool, hydrothermal solutions of magmatic origin are formed; they are commonly enriched in copper, lead, zinc, gold, silver, tin, tungsten, and molybdenum (Table 1).

Table 1 Compositions of some hydrothermal solutions

	Amounts (parts per million)
Element	1	2	3
1. Salton Sea Geothermal field, California. After Muffler, L. J. P. and White, D. E. (1969) Bulletin of the Geological Society of America, 80, 157–82.
2. Well in aquifer No. 9, Cheleken Pennisula, USSR, now Kazakhstan. After Lebedev, L. M. and Nikitina, I. B. (1968) Doklady of the Academy of Sciences of the USSR, Earth sciences sections. [English translation] American Geological Institute, Washington. 183, 180–2.
3. Oilfield brine, Gaddis Farms D–1 well, lower Rodessa reservoir, central Mississippi, 11 000 ft. After Carpenter, A. B., Trout, M. L., and Pickett, E. E. (1974) Economic Geology, 69, 1191–206.
Cl	155 000	157 000	158 200
Na	50 400	76 140	59 500
Ca	28 000	19 708	36 400
K	17 500	409	538
Sr	400	636	1 110
Ba	235	Not det.	61
Li	215	8	–
Rb	135	1.0	–
Cs	14	–	–
Mg	54	3 080	1 730
B	390	–	–
Br	120	526	870
I	18	32	–
F	15	–	–
NH4+	409	–	39
HCO3−	>150	32	–
H2S	16	0	–
SO42−	5	309	310
Fe	2 290	14	298
Mn	1 400	46.5	–
Zn	540	3.0	300
Pb	102	9.2	80
Cu	8	1.4	–
Temperature	320 °C	>80 °C

Any heated water, regardless of origin, will react chemically with the rocks with which it is in contact. If a heated water is even slightly saline, it is an effective solvent for any metals present in trace amounts in all sediments and rocks. By reaction, a hot, saline solution of groundwater, formation, or metamorphic origin will slowly evolve into a hydrothermal solution that contains metals of the same kinds as those carried by solutions of magmatic origin.

Compositions of hydrothermal solutions

Many hydrothermal solutions have been encountered by drillers seeking oil or other commodities; three examples are shown in Table 1. The solutions encountered are all chloride-rich, and this explains how significant amounts of heavy metals can be carried in solution. Minerals such as galena (PbS), sphalerite (ZnS), and metallic gold (Au) have exceedingly low solubilities in water. However, in saline solutions, chloride complexes such as (AuCl₄)²⁻, and (PbCl₄)²⁻ can form and thereby raise the solubility even though sulphur ions (S²⁻) might also be present in solution.

Deposition from hydrothermal solutions

The origins of hydrothermal solutions, the sources of the dissolved metallic constituents, and the means of solution and transport of heavy metals in solution, despite the very low solubility of metallic sulphur minerals, have become clear as a result of research in the second half of the twentieth century. Less clear, however, are the mechanisms whereby solutions deposit their dissolved loads and form mineral deposits.

Hydrothermal solutions range in temperature from about 120 °C to as high as 600 °C; and the higher the temperature, the more material the solution can dissolve. When a hydrothermal solution cools, therefore, minerals will be precipitated from the solution. However, the chemical nature of a solution—the acidity and its capacity to serve as an oxidizing or reducing agent—is also affected by temperature. Because the stabilities of chloride complexes are dependent on acidity and the reduction—oxidation capacity of a solution, the chemical effects of cooling on the solubility of a hydrothermal solution may be more pronounced than simply the temperature effect of cooling.

Other factors beside temperature change can influence the chemical nature of a solution, and thereby cause precipitation. For example, an acid solution that reacts with a limestone will become less acid; a metal-rich solution may mix with another water mass, such as a brine containing dissolved hydrogen sulfide (H₂S); or a solution might be reduced by organic matter in a sedimentary rock. All such reactions, of which the three cited are selected examples among a large number of possible reactions, can cause precipitation, and the formation of a mineral-rich deposit. It is difficult in most cases to determine exactly which reactions were most important in the formation of a specific hydrothermal deposit.

Brian J. Skinner