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Pressure - Solubility - Boiling Relationship

Solutions under high pressure (approximately 2 kbar) have a greater ability to dissolve water, and this water is expelled during the crystallization processes. The expulsion of large amounts of water under high pressure or in deep systems also leads to the expulsion of many components in the magma along with the water (Shinohara et al., 1989). The separation of aqueous fluids with less water and elements from the magma results in the enrichment of the remaining melt in metals and other elements. The crystallization of this remaining melt results in the expulsion of all components in the magma, including those in the aqueous phase and in the melt, which cannot enter the crystal phases.

Boiling within the magma is one of the factors controlling the amount and separation times of hydrothermal fluids from the magma. Boiling is particularly related to pressure as it controls the transition between magma/hydrothermal fluid formation. Two boilings occur during magmatic evolution:

  • The first boiling occurs in the magma chamber due to a pressure drop, leading to the expulsion of volatiles as their solubilities decrease.

  • The second or retrograde boiling occurs due to an increase in volatile concentrations related to increased crystallization. The increasing water content in the magma during crystallization raises the pressure in the liquid phase. When this increased pressure balances with the surrounding pressure, a second boiling occurs in the magma. The formation of this second boiling is simultaneous with the formation of a separate aqueous phase (hydrothermal solution) in the magma (Pirajno, 1992).

The metal content and enrichment in hydrothermal solutions are under the control of different magmatic differentiation and fluid release processes. Shallow magmas, compared to deeper ones, will produce more hydrothermal fluid before crystallization, so the hydrothermal fluids formed in these magmas will be released earlier and in greater amounts than those in deeper ones. One of the main reasons for this is related to the change in pressure with depth. In shallow depths, due to the reduced pressure, the formation of an aqueous phase in the magma by less dissolved volatiles will be more compared to magmas at deeper depths under high pressure. This also explains why large skarn deposits are found in shallow environments, while smaller skarn deposits are found in deeper environments.

In magmas that have undergone extensive crystallization before reaching water saturation, elements like Cu and Au present in earlier formed minerals are impoverished in the later-stage hydrothermal fluids. Conversely, lithophile elements like tin and tungsten, which are enriched during differentiation, are found in greater quantities in the last-stage hydrothermal fluids.

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