HIGH TEMPERATURE (> 600O C) DEVELOPMENT OF FRACTURE PERMEABILITY IN THE ALTA, UTAH STOCK: CONSTRAINTS FROM THERMAL AND ISOTOPIC EVIDENCE PRESERVED IN THE CONTACT AUREOLE

The temperature and d¹⁸O profiles recorded in carbonate wall rocks of the S. Alta, Utah contact aureole can be used to constrain limits to the permeability of both the Alta stock and carbonate wall rocks during prograde hydrothermal metamorphism with numerical models of heat and mass transport. The model simulations of the thermal and d¹⁸O profiles recorded in the carbonates limit the permeability of the stock (constant over temperature and time) to values between 1.5x10^-16 m² and 1x10^-15 m², and limit the permeability of the carbonate wall rock to values between 7x10^-17 m² and 1x10^-15 m². Models in which the permeability of the stock increased linearly with decreasing temperature from 750^o C to 300^o C require final values of stock permeability at 300^o C between 2.5x10^-15 m² and 4x10^-14 m² and values of carbonate permeability between 6x10^-17 m² and 4x10^-16 m².

Estimates of permeability for the Alta stock using measured density (avg. = 5 veins/m) and width (avg. = 0.65 mm) of hydrothermal veins in the stock and a parallel plate model range from 4x10^-12 to 2x10^-9 m². These estimates are two to six orders of magnitude greater than the model estimates of both time-constant permeability and the final permeability (at 300^o C) of the stock. This discrepancy suggests either that the effective hydraulic aperture of the veins was only a small percentage (£ 2-7%) of their measured width or that only a small percentage (£ 1%) of these veins were continuous on a sufficiently large scale to provide active conduits for fluid flow at any given time during the cooling history of the stock. Modeling results also show that the stock must obtain significant permeability (>1x10^-15 m²) by no lower than 610^o C in order to satisfy the thermal profile in the south aureole. This temperature (610^o C) is significantly higher than temperatures usually assigned to the brittle-ductile transition, and to the development of significant fracture permeability in cooling intrusions (350 ± 50^o C).

If the behavior of the Alta stock is representative, field-based measurements of hydrothermal vein systems will likely overestimate the permeability of a cooling stock by up to several orders of magnitude. Discrepancies of this magnitude would induce significant errors in models of heat and mass transport for hydrothermal systems associated with granitic igneous intrusions.