MECHANISMS OF INTERNAL PERMEABILITY GENERATION DURING MAGMATIC FLUID INFILTRATION INTO TWO CONTACT METAMORPHIC AUREOLES

Comparison of the mechanisms of magmatic fluid infiltration into two contact aureoles highlights the role of mineral reactions in creating porosity and permeability. Magmatic fluid infiltration into calcareous limestone and dolostone in the Horsethief Creek contact aureole, southeastern British Columbia Canada and the Antamina Cu-Zn skarn in Ancash, Peru is recorded in the mineralogical, stable isotopic, and whole rock geochemical compositions of metasediments. In both aureoles, the lateral limit of fluid infiltration is marked by the close spatial coincidence of multiple reaction fronts. Major fluid egress pathways are manifest in large spatial separation of reaction fronts. At Horsethief Creek, lateral fluid migration extends for a kilometre from the pluton-metasediment contact. The distal limit of fluid infiltration corresponds to the predicted outer limit of thermally driven devolatilization in siliceous dolostone. Thermally driven mineral reactions generated internal permeability and enabled pervasive pore scale fluid percolation over more than a kilometre. Heat flow and compaction models suggest that the timescales of fluid flow, mineral reaction, and texture formation were on the order of tens of thousands of years. Lateral reactive fluid flow at Antamina extended for hundreds of metres and is manifest in andradite garnet skarn and bleached calcite marble. Fluid flow within skarn profoundly changed the bulk chemical composition of host marble. Fluid percolation was enabled by infiltration-driven grain scale carbonate dissolution and silicate precipitation reactions that completely replaced the metasedimentary character of the protolith. Lateral fluid infiltration beyond the skarn-marble contact was by comparison very limited. The most volumetrically significant fluid egress pathways identified at Antamina are along vertical felsic dikes. Bedding-parallel, lateral fluid flow away from the dikes extends for hundreds of metres. The strong control of bedding on lateral fluid flow pathways may reflect subtle variations in protolith composition. The record of hydrothermal fluid flow at both localities highlights the role of mineral reactions in generating internal permeability during contact metamorphism.