Exploring Mineral Deposits

Granitoid Magma Crystallization Stages

• The ascent and crystallization of granitoid magma involves four main stages: magmatic, pegmatitic, pneumatolytic, and hydrothermal.
• The magmatic stage occurs at temperatures typically above 700°C and involves super-solidus equilibrium between crystals and magma.
• The pegmatitic stage occurs at temperatures typically between 600-700°C and involves near-solidus equilibrium between crystals and volatile-saturated magma, with the possible presence of a supercritical aqueous fluid.
• The pneumatolytic stage occurs at temperatures around 600°C and involves sub-solidus equilibrium between crystals and supercritical fluid, leading to the formation of skarns and greisens.
• The hydrothermal stage involves equilibrium between crystals and sub-critical aqueous liquid/vapour, typically at depths ranging from 1 cm to several meters.
• The igneous rocks involved are typically of granitic bulk composition, with compositions near the minimum melting point in the Qz-Fsp-H2O system.
• Fluxes, such as F, P, Li, B, and H2O, behave as lowering agents for the solidus of the granitoid magma.
• Many granophile elements are noted as fluxes, contributing to the reduction of the solidus temperature of the granitoid magma.
• Pegmatites, greisens, and skarns are formed during different stages of the crystallization process of granitoid magma.
• The pegmatitic stage can extend to much lower temperatures, around 400°C, and involves near-solidus equilibrium between crystals and volatile-saturated magma.
• The pneumatolytic stage involves the formation of skarns and greisens through sub-solidus equilibrium between crystals and supercritical fluid, typically at around 600°C.
• The hydrothermal stage involves equilibrium between crystals and sub-critical aqueous liquid/vapour, occurring at various depths.