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Volatiles found in varying amounts in nearly all wall rocks and magmas figure prominently in the magmatic differentiation process of assimiliation. These volatiles include CO2, SO2, O2, Cl2, and particularly H2O.

Water is available in wall rocks of the mid-crust, both as free water and within the hydrated minerals commonly found at depths. Some assimilated water enters hydration reactions with predominantly anhydrous melt components, but most water simply accumulates in the ever-shrinking, residual silicate melt. When a melt has taken on sufficient water, that magma will develop a water-saturated silicate fraction and a separate water-based fluid phase.

Under some conditions, water-saturated silicate fractions can release a whitish fine-grained vein-filling slurry of quartz and feldspars termed aplite. The water-based phase easily assimilates trace elements that do not accommodate well into most silicate crystals. These trace elements include beryllium, lithium, niobium, tantalum, tin, uranium, thorium, tungsten, zirconium and the rare earths. Ore deposits can form when hot, pressurized, mineral-laden hydrous fluid permeates fractured country rock and cools into veins of pegmatite—an very coarse-grained igneous rock containing megacrysts of quartz, feldspars, and sometimes, highly prized minerals. Pegmatite and aplite dikes and veins are common around intrusions.

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