STRUCTURAL CONTROLS ON EARLY TO MIDDLE MIOCENE EPITHERMAL FLUID FLOW IN A STRIKE-SLIP CONTRACTIONAL STEPOVER, EASTERN EXCELSIOR MOUNTAINS, NEVADA

In the eastern Excelsior Mountains, Nevada, NW-striking, right-lateral strike-slip faults step left, contractionally, and form a several square kilometer "pop-up" structure. The pop-up is a lozenge-shaped, reverse-fault bounded, asymmetric uplifted block (Thunder Mt.) between underlapping strike-slip faults and forms an approximately 30-degree restraining stepover. Paleomagnetic data are permissive that there could be as much as 15-degrees of clockwise rotation in the pop-up (J. Geissman, per. comm., 2005). The stepover is within a zone of regional east-west transverse faults that have been intermittently active since at least the Jurassic.

Epithermal hydrothermal activity related to a 20-15 Ma volcanic field formed veins and alteration along the strike-slip and reverse faults. Advanced-argillic and argillic alteration is widespread at the eastern end of the pop-up structure along and between overlapping strike-slip faults. Fluids flow predominantly along NW-striking faults, but also coalesced in ground-water saturated, high-permeability rocks to form siliceous manto deposits. Banded quartz-adularia veins occur generally continuously along the opposing, pop-up bounding reverse faults. In general, the economically most productive structures are midway along the pop-up bounding reverse faults. Stoping was more extensive on the northern (Camp Douglas) side of the pop-up where the reverse faults dip more shallowly than on the south (Silver Dyke) side. The most substantial ore bodies appear to have formed where clockwise vertical-axis rotations were likely greatest and, therefore, the zones of greatest hydraulic conductivity at the time of deformation. Within the reverse zones and their hanging walls and footwalls, veins occur in two orientations, parallel to reverse faults, and along NNE to NE fractures.