Abstract in experiments simulating the rise of hydrothermal fluids from a geothermal reservoir, water was flowed at a low rate down a temperature gradient through fractured and intact cylinders of Barre and Westerly Granite. Temperatures ranged from 80 to 105?C at the outer edges of the cylinders to 250 to 300?C along a central borehole which housed the heating coil. As a result of mineral deposition, particularly Si02 , at low temperatures in the granite samples, permeabilities were reduced 10- to 100- fold in periods of 1 to 3 weeks. Chemical analyses were made of the low-temperature fluid s discharged from the cylinders. Early-sampled fluids were supersaturated with respect to several minerals at low temperatures in the granites of the oversaturated species, Si02 showed the most rapid decrease with time, and in an experiment with low initial flow rate, the solution reached equilibrium with quartz at the low-temperature edge of the cylinder with in about 6 days. Increasing the maximum temperature of the gradient at constant confining pressure led to higher Si02 concentrations in the discharged fluids. However, increasing confining pressure along with maximum temperature resulted in lower dissolved Si02 contents, because of enhanced reaction at the reduced flow rates accompanying the pressure increase. The behavior of Si02 contrasted with most other dissolved species, which were affected by changes in temperature but not flow rate in the time of the experiments.

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