Fracture analysis, hydrodynamic properties and mineral abundance in altered igneous wall rocks of the Mayflower Mine, Park City District, Utah

A 670-meter vertical section of the igneous wall rocks of the Mayflower Mine, Park City District, Utah, was studied with the purpose of documenting quantitatively some fluid flow parameters and the mineral abundance in the altered igneous rocks. Fracture abundance and fracture aperture determinations allowed an estimation of the permeability and flow porosity of the igneous rocks by using a parallel plate model for a fractured medium. Values for the permeability fell in the range of 0.1 to 10 darcies, whereas flow porosity values ranged from 0.05 to 0.4%. Total porosity was calculated from bulk and grain densities of the rocks and analyzed in a model in which flow, diffusion and residual porosities are its major contributing parts. Residual porosity accounted for most of the total porosity of the fractured igneous rocks. Mapping of the fractures revealed two prominent fracture directions, N50°E/80°NW and N50°W/80°SW, which apparently constitute a conjugate set of shear fractures supposed to have been formed soon after the stocks crystallized (solidus temperature) and under a high confining pressure. Mineralogical compositions of the rocks were determined by a computer technique which requires chemical composition of the rocks and mineral phases as input data. The rocks were analyzed for Si, Al, Fe (total iron as Fe++), Mg, Ca, Na, K, Mn and Ti with X-ray flourescence instrumentation, and for sulfide sulfur, sulfate sulfur and carbon dioxide with LECO equipment. The mineral phases were analyzed for the same components, except for the last three, with an electron microprobe. Chemically simple phases were considered to be stoichiometric. The resulting mineral abundances disclosed two broad zonings over the sampled N-S cross-section of the mine: one lateral, characterized by both an increase of the K-feldspar, kaolinite and quartz abundances and a decrease of andesine, biotite and chlorite towards the main veins; and the other vertical, characterized by major deposition of K-feldspar, anhydrite, pyrite and kaolinite below the 2,200' level; and calcite, quartz and biotite more abundantly precipitated above. Mineral and component gains and losses were also computed and agreed with the abundance of reactant and product minerals. Estimated initial solution composition was derived from available fluid inclusion data and constraints imposed by equilibrium relationships of the alteration assemblages at 300°C. The interpretation of the major alteration mineral constitutents was based upon a physico- chemical model in which H+ ion consumption was the major chemical change of the solution composition and was also based upon a sequence of isothermal conditions for the wall rocks. Activity diagrams depicting chemical equilibrium among the major alteration phases at 100°C, 200°C, and 300°C and 1 bar total pressure were used for such an interpretation.