Hydrothermal systems, like magmatic systems, consist of fluids and crystalline phases in varying proportions. The transition from a magmatic to a hydrothermal system is a direct result of the decompression and crystallization events that begin with the magma's ascent to shallower depths. Therefore, the development and evolution of magmatic hydrothermal systems must be directly related to the crystallization, cooling, and emplacement processes of magma.
The fundamental difference in the relationship between the composition of magma and hydrothermal solutions lies in the physical and chemical content of the hydrothermal solution exuded from the magma as a function of its origin and crystallization process. The composition of magma provides strong evidence of an intrinsic relationship with hydrothermal fluids. Characteristics of the magma, such as composition, differentiation degree, and redox state, determine the metal content in mineralization. In reduced intrusions, ilmenite crystallizes first as the magma cools. In hydrothermal melts, Sn concentrates where it cannot enter magmatic sulfides. In oxidized intrusions, the early crystallization of magmatic sulfides enriches hydrothermal solutions in Cu and Au. The character of hydrothermal ore deposits related to acidic magmatism varies with depth. Pegmatites associated with catazonal plutons form at great depths, while epithermal deposits occur near the surface.
Generally, porphyries form at shallower depths compared to skarns; porphyry Cu-Au and Cu-Mo form shallower than magnetite skarn and scheelite skarn; hematite skarn forms shallower than magnetite skarn, and Hg-Sb veins form shallower than Pb-Zn veins (Laznicka, 1985). Ore deposits form in the upper or marginal zones of the final stages of magmatism. High-pressure, metal-rich hydrothermal fluids solidify as they move along dykes, faults, or permeable lithologies, encountering cooler temperatures and/or rocks with different chemical compositions (e.g., carbonate rocks). Different minerals crystallize at different temperatures, resulting in element zoning around intrusions, typically as follows: Mo, Cu, Pb-Zn, Au-Ag, Sb, Hg. The position of Mo, Au, and Ag in this sequence may vary depending on the deposit's characteristics. Similar zonations are observed around Sn-W deposits. Underground waters heated by intrusions can form some epithermal ore deposits, but such fluids do not play a significant role in the formation of high-temperature deposits like skarn types.
The relationship between magma and hydrothermal fluids is generally explained by two main theories:
Most ore deposits are formed by aqueous, metal-rich fluids directly exuded from magma during its emplacement and crystallization. According to a second theory, gaining acceptance in recent years,
Hydrothermal fluids, coexisting with magmatic rocks, are believed to originate from the same tectonic environment, possibly from deep sections of the crust and mantle.
In the first theory, the processes of magma emplacement and crystallization are primary, while in the second, the formation environment of magma and the events that form magma are considered significant factors. The temperature distribution and cooling rate in any magma chamber emerge as functions of various parameters of the magma and surrounding rocks. These include parameters like the temperature in the section where the magma is located, volatile component content, enthalpy (latent heat of crystallization), viscosity, thermal conductivity, specific heat, and volatile component content.
Magma cooling is explained by two models:
Conductive Cooling: Cooling occurs due to heat transfer along the boundaries where the magma chamber directly contacts the surrounding rocks.
Convective Cooling: In this model, the melt at the bottom of the magma chamber, being hotter and thus having increased volume but decreased density, moves towards the top of the chamber. Upon reaching the top, it transfers the thermal energy carried from the lower sections to the surrounding rocks at the roof of the chamber, thus cooling down. This cooling causes the magma to decrease in volume and increase in density, leading the denser magma to sink back to the bottom. As it absorbs heat from the bottom, its volume increases but density decreases, and it rises again. Thus, the magma continues to cool down by losing heat.
Pressure – Solubility – Boiling Relationship Under high pressure (approx. 2 kbar), melts can dissolve more water, and this water is expelled during crystallization processes. In high-pressure or deep systems, the expulsion of large amounts of water also leads to the release of many components in the magma along with water (Shinohara et al., 1989). The separation of less water and element-rich aqueous fluids from the magma results in the enrichment of the remaining melt in metals and other elements. The crystallization of this final melt expels all components that cannot enter the crystalline phases, including the aqueous phase and elements remaining in the melt. Boiling within the magma is one of the factors controlling the quantity of hydrothermal fluids and their separation times from the magma. Boiling is directly related to pressure, especially in controlling the transition between magma and hydrothermal fluid formation. Two boilings occur during magmatic evolution: -The first boiling occurs due to the release of volatiles from the magma due to a drop in pressure, reducing their solubility. -The second or retrograde boiling occurs due to the increase in volatile concentrations related to the increase in crystallization. The gradual increase of water 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 within the magma, simultaneously forming a separate aqueous phase (hydrothermal solution) (Pirajno, 1992). The metal content and enrichment of hydrothermal solutions are controlled by different magmatic differentiation and fluid excretion processes. Shallow magma produces more hydrothermal fluid before crystallization than deep-seated magma, resulting in the earlier and more abundant release of hydrothermal fluids in shallow magma compared to deeper ones. This also explains why large skarn deposits are found in shallow environments, while smaller skarn deposits are in deeper settings. In magmas that have undergone extensive crystallization before reaching water saturation, elements like Cu and Au, present in earlier-formed minerals, are depleted in the final phase hydrothermal fluids. In contrast, lithophile elements like tin and tungsten, enriched during differentiation, are found in significant quantities in late-stage hydrothermal fluids.