2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM


MOTE, Alison Suzanne, Dept. of Geological Sciences, Jackson School of Geosciences, The Univ of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, WAWRZYNIEC, Tim F., Dept. of Earth and Planetary Sciences, The University of New Mexico, Paleomagnetism Laboratory, 141 Northrop Hall, Albuquerque, NM 87131, MELKER, Marc D., Mine Geology, AngoGold Ashanti (Colorado) Corp, P.O. Box 191, Victor, CO 80860 and KYLE, J. Richard, Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, amote@mail.utexas.edu

Fractures provide conduits for large volumes of fluid in the shallow crust and are essential in forming productive mineral deposits. Extensive fault mapping and sampling in the Cripple Creek district, Colorado reveals high-grade ore zones occur in Riedel shear geometries to the Cresson fault. Kinematic indicators preserved on the fault are interpreted to represent reactivation of an early, regionally continuous structure that became a thoroughgoing fault during structural development of the diatreme, and the fault is interpreted to be a high-permeability conduit for mineralizing fluids. Though a large structure, the Cresson fault carries extremely low gold grades, yet productive ore zones occur in secondary orientations to the structure. Throughout diatreme development, movement along the Cresson fault made it a highly permeable structure that allowed for the circulation of large volumes of fluid. With progressive shearing along the Cresson fault and greater likelihood of hydraulic fracturing as a result of pumping fluids through the system, secondary fault populations in R and R' orientations developed and acted to draw fluids from this zone of high permeability. Owing to the instability of the complexing agent carrying the Au, this change likely caused gold to precipitate, resulting in high-grade ore zones in Riedel shear geometries. An alternative interpretation can be explained by the geometry of the fault zone (e.g. Caine et al., 1996), in which large shear accommodating structures commonly develop a large amount of clay within the fault zone. This may have caused the Cresson fault to become a fluid flow barrier, acting more as an aquiclude to mineralizing fluids. Progressive right-lateral shear along the fault zone would have acted to expand the zone of deformation adjacent to the Cresson fault, while increasing the amount of clay gouge within the fault core. Secondary faults in Riedel geometries within the damage zone likely created areas of high permeability, allowing for the circulation of large volumes of fluid.