NUMERICAL SIMULATION OF FLUID FLOW WITH ENHANCED PERMEABILITY RELATED TO CALC-SILICATE MINERAL REACTIONS AROUND PLUTONS

Mineral assemblages in contact-metamorphic aureoles are the products of the interplay between heat and fluid flow induced my intrusion of magma. Intrusion of magma into calcareous shales causes metamorphic reactions that produce CO₂, which then becomes part of the hydrodynamic system. Flow of reactive H₂O-CO₂ fluids and metamorphic assemblages in an aureole of a granite pluton were simulated using a modified version of the SUTRA code that allows calculation of metamorphic reactions in a transient P-T-XCO₂^fluid field. Three different permeability structures of the aureole were considered: a) homogeneous, b) layered and c) reaction-enhanced. Reactions were assumed to occur only in horizontal calcareous shale beds. The granite intrusion had a laccolith shape. Magmatic, metamorphic, and sedimentary fluid sources were considered. The rate of reactions was assumed to be related to their ΔG's at the prevailing P-T-XCO₂^fluid conditions at each node at any given time of the simulation, an Arrhenian rate constant, and grain size.

Results show that after magma intrusion, overall fluid flow is away from the intrusion as the hydrostatic head in the inner aureole becomes elevated with temperature increase. In all cases, fluid composition in inner aureole evolves rapidly toward high XCO₂^fluid as metamorphic reactions begin before water is exsolved out of the pluton. Only after tremolite and diopside-forming reactions come to completion and local fluid pressure drops, infiltration of H₂O from the pluton becomes significant and can drive production of minerals such as wollastonite and vesuvianite. In the case of homogeneous permeability structure, fluid flux and composition are dispersed, while in layered permeability, highest fluid flux is confined to high permeability layers and fluid composition remains discrete. When permeability increase is related to reaction progress, the highest fluid flux is confined to inner aureole. The reaction-enhanced permeability model is in agreement with field and geochemical observations that suggest confinement of reactive fluid flow largely to inner calc-silicate contact aureoles.