(2) Hydrothermal processes
Hot aqueous solutions have played a part in the formation of many different
types of mineral and ore deposit, for example veins, stockworks of various
types, volcanic-exhalative deposits and others. Such fluids are usually called
hydrothermal solutions. Hydrothermal ore deposits are small compared with
most geological features – the largest are only a few km
2
in volume. They
may appear to be random accidents with little or no control over the position
or geological environment in which they occur within the crust. Deposits can
be classified into families and individual family members occur more
frequently in some areas of the crust than others. Moreover, their variation in
occurrence and the large number of minerals known in nature, hydrothermal
deposits displays a chemical consistency that is best expressed by the limited
and repetitive ranges of minerals, mostly sulphides and oxides.
Sources of the solutions and their contents
Minerals and rocks suggest that there are five sources of subsurface
hydrothermal waters:
1. surface water, including water, commonly referred to as meteoric
water (ɚɬɦɨɫɮɟɪɧɚɹ ɜɨɞɚ) ;
2. ocean (sea) water;
3. formation and deeply penetrating meteoric water;
4. metamorphic water (ɦɟɬɚɦɨɪɮɢɱɟɫɤɚɹ ɜɨɞɚ);
5. magmatic water (ɜɵɞɟɥɢɜɲɚɹɫɹ ɢɡ ɦɚɝɦɵ ɩɪɢ ɟɟ ɨɯɥɚɠɞɟɧɢɹ).
Differences in source rocks and differences in leaching conditions will
produce different concentrations of economic metals in the reacting solutions.
Very scarce elements, such as tin, mercury and silver may require pre-
enrichment. Because of the spatial relationship (ɩɪɨɫɬɪɚɧɫɬɜɟɧɧɨɟ
ɫɨɨɬɧɨɲɟɧɢɟ) that exists between many hydrothermal deposits and igneous
rocks indicate that consolidating magmas are the source of many, if not all,
hydrothermal solutions. The solutions are considered to be low temperature
residual fluids left over after pegmatite crystallization and containing the base
metals and other incompatible (ɧɟɫɦɟɲɢɜɚɟɦɵɣ) elements. This model
derives not only the water, the metals and other elements from a hot body of