Abstract

Heat and mass transport processes related to the Mayflower stock in the Park City district, Utah, have been simulated using calculations based on geological observations and numerical methods which approximate convective and conductive heat transfer in permeable media. Permeability and flow porosity values for the Mayflower stock were estimated on the basis of a planar fracture model and data on the abundances and apertures of continuous fractures. Estimated permeabilities ranged from 4 X 10 (super -9) to 10 (super -7) cm 2 and were hypothesized to represent initial permeabilities at the onset of the hydrothermal fluid circulation. An estimated fluid mass of 10 13 kg/km strike length of the stock circulated through the upper 1.5 km of the Mayflower stock in 1.8 X 10 5 years of cooling, thereby reducing the thermal anomaly to 0.3 of its initial value. Temperatures decreased rapidly in the permeable portions of the stock, as a result of convective transfer of heat, but remained at 350 degrees to 250 degrees C in the upper 1.5 km for approximately 7 X 10 4 years subsequent to fracturing of the stock. Fluids in the host rocks flowed toward and often into the stock from distances about 5 km away from the stock side contact. Irreversible mass transfer between these circulating fluids and the Mayflower stock altered the stock to mineral assemblages which reflect the chemical composition of the rocks through which the fluids circulated, the pressure and temperature conditions along the flow paths, and initial composition of the fluids.Simulation of the heat transport processes in the Mayflower system provides an initial approximation of the temperature, pressure, and fluid fluxes that may have been realized in the natural system. These data allow the mineral content of the altered Mayflower rocks to be predicted from mass transfer computations. Irreversible mass transfer reactions between the unaltered Mayflower rocks and solution compositions derived initially from fluid inclusion data were computed at discrete temperatures over the interval from 300 degrees to 150 degrees C. The alteration processes in the Mayflower stock were thus simulated by a sequence of isothermal reactions over the cooling history of the stock. The mineral content of the altered igneous rocks exposed in the Mayflower mine was determined by least-square treatment of bulk chemical compositions of rocks and mineral phases and was used to test the validity of coupling existing theoretical models of mass and heat transfer. The assumed solution compositions, prevalent temperature and pressure during the hydrothermal processes, and the estimated mass of fluids that circulated through the upper 1.5 km of the Mayflower stock as it cooled predicted masses of the mineral assemblages similar to those measured in the altered Mayflower igneous rocks.The determined mineral modes disclose two broad zones over the north-south cross section of the mine: a lateral zone characterized by an increase of K-feldspar (3-15 wt %), kaolinite (0-10 wt %), and quartz (12-24 wt %), and a decrease of andesine (55-25 wt %) toward the main veins; and a vertical zone characterized by higher concentrations of K-feldspar, kaolinite, anhydrite, and pyrite below the 2,400-ft mine level, and calcite-quartz and biotite above. Gains and losses for elemental components indicate an overall loss (in grams of components per cm 3 of rock) of Si (0.009), Al (0.050), Na (0.032), and Ca (0.011) and an overall gain of Mg (0.026), K (0.020), S (0.038), SO 3 (0.037), and CO 2 (0.007). A relatively small gain of Fe (0.006) is the result of decrease in bulk density caused by the dissolution of igneous mafic minerals to produce pyrite, as evidenced by the shift from an annitic to a phlogopitic biotite during the hydrothermal event.This analysis of the Mayflower hydrothermal system suggests that the original igneous minerals were altered by acid-sulfate, Na-K-rich solutions at moderate temperatures, < 400 degrees C, and pressures, < or = 1 kb. These solutions added large masses of Mg, K, S, and C to the stock and, concomitantly, altered the original igneous minerals. In order to account for the observed masses and compositions of alteration products, fluid fluxes on the order of 10 (super -7) g/cm 2 s are required for at least 2 X 10 5 years. This large mass ( approximately 10 10 g, km 2 of area) of hydrothermal fluid was evidently derived from a variety of environments within and around the stock.

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