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.

Test your knowledge of mineral deposits with this quiz on pegmatites, greisens, and skarns. Explore the stages of granitoid magma ascent and crystallization, from the magmatic stage to the pegmatitic stage. Ideal for geology enthusiasts and students.