Graduate Thesis Or Dissertation
 

Reactive transport modeling in a discrete fracture : applications to the formation of sericitic hydrothermal alteration at Butte, Montana

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  • A model for the spatial and temporal evolution of the gray sericitic (GS) and sericite with remnant biotite (SBr) hydrothermal alteration selvages within the Butte Quartz Monzonite (BQM) of the porphyry copper deposit at Butte, Montana, is presented. The model provides a mathematical description for the location and advance of multiple mineral reaction fronts during diffusive solute transport under quasi-stationary state conditions. A method for scaling initial mineral masses was developed to efficiently model the system for sufficient geologic time. Simulations show that a reducing, highly acidic, and low salinity fluid can produce the GS and SBr alteration selvages up to 5 cm wide during the time span of approximately one hundred years or less. Hydrothermal alteration causes little change in the porosity and pore diffusivity of the BQM. Mineral precipitation and dissolution act as sources and sinks for the solutes and impose additional concentration gradients on the solution, which changes solute transport. The hydrothermal alteration zones are characterized by a sequence of reaction fronts. The biotite dissolution front occurs closest to the fracture and marks the transition between the GS and SBr zone. The plagioclase dissolution front occurs farthest into the matrix and marks the edge of unaltered BQM. From the mathematical description for multiple alteration fronts, it follows that once the sequence of reaction fronts is fully established, it must remain constant and the widths of reaction zones expand proportional to the square root of time. The relative widths of reaction zones into the wall-rock remain approximately constant to each other through time. The widths of reaction zones decrease along the fracture, because the rate of diffusion of reactive solutes in the rock matrix is lower than the rate of advection of the reactive solutes in the fracture.
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