TECTONIC-BOUNDARY TRANSVERSE DEFORMATION ZONES AND THEIR DIVERSE STRUCTURAL ROLES IN LOCALIZING TERTIARY EPIZONAL ORE DEPOSITS, WESTERN GREAT BASIN

Deformation zones transverse to tectonic boundaries in the western Great Basin have been mapped and their significance in Tertiary tectonics discussed by many investigators. Some mining districts correlate spatially with transverse fault zones, but any causative relations are poorly understood. We are conducting geologic and geophysical studies in several epithermal districts in western Nevada to investigate causative relations. There appear to be two principal categories of causative interactions. One involves zones wherein significant displacement transfer on transverse faults occurred and ore was localized within transverse and linked faults. In the Marietta and Silver Star districts, Excelsior Mountains, significant displacement occurred along parallel E-W striking reverse faults with opposing dips. WNW-ESE extension in intervening blocks localized Early Miocene magmas, and related epithermal veins occur along reverse and linked normal faults. A second category involves transverse zones that accommodated little displacement, but significantly affected local strain fields related to faults of other orientations. At Goldfield, an E-W fault zone may have caused the mechanical interaction of overlapping, en echelon NW-striking right-lateral faults and the formation of linking extensional faults. The deep intrusive source was beneath the structural pull-apart and shallow intrusions and ore are along its structural margins. In the Comstock and Silver City districts, the locus of intrusive activity was bound north and south by parallel E-W geophysical lineaments and an intervening structural basin developed that mechanically linked parallel overlapping NW-striking right-lateral strike-slip faults. Bonanza lodes were localized in the structural margins of the basin. Thus, transverse fault zones can and do affect the localization of mineralizing hydrothermal systems and ores within them.