Reaction mechanisms, physical conditions, and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from south-central Maine, USA

Abstract

Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X _{CO 2} = 0−0.13; ln([K⁺]/[H⁺ ]) = ∼10.0; ln([Ca²⁺]/[H⁺]²)=∼9.1; ln([Na⁺]/[H⁺]) = ∼ 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm³ altered rock) of CO₂, H₂O, H⁺, and K⁺ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm³ altered rock) of Mg²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Na⁺, and Ti⁴⁺. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm³ rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm³ fluid/1,000cm³ altered rock to participate in the hydrothermal event.