New Mexico Geological Society Annual Spring Meeting — Abstracts

The epithermal vein deposits of western North America, which are found throughout a region of some 800,000 km sq, occur in structures produced by regional tectonism. Veins of the early Tertiary, including those of Paleocene through early Oligocene age occur in generally east-west trending, vertical to subvertical strike-slip faults of minor offset. The host structures of this age comprise swarms consisting of many short, discontinuous faults. Individual veins within these systems are less than 2 km in length and commonly are of greatest extent in the vertical direction. Fault-veins of the early Tertiary are apparently related to the Mesozoic through Laramide compressive deformation of western North America.

In the western U.S., vein deposits of mid to late Oligocene age are rare but those surviving examples are poorly developed. Typical mid-late Oligocene veins would be the sheeted lodes and fracture intersection deposits at Cripple Creek, Colorado. These bodies are characterized by replacement textures, assay walls and the absence of vein breccias. They appear to have been localized in fracture zones of local extent produced by tectonism of the gentlest degree. Epithermal veins of Miocene age in western North America are more widespread and have been much more productive than those of the early and mid Tertiary. Miocene epithermal veins occur in a great variety of structures. Within an area encompassing southwestern and southern Nevada, the low desert area of southeastern California and Arizona and stretching down around the Gulf of California to about Mazatlan, epithermal vein deposits of Miocene age occur in left and right-lateral strike-slip faults, low-angle normal faults, listric normal faults and decollement zones. Peripheral to this area on the north and east, epithermal veins of Miocene age are localized in generally north trending normal faults having dip angles of 65 to 70 degrees. Throughout western North America epithermal veins of Miocene age are distinguished by great horizontal length and continuity. They occur in the wide variety of structures that opened in response to the tensional deformation of western North America which has been ongoing since about 25 my ago.

While it is currently fashionable to relate epithermal veins to caldera development this study has shown little if any relationship between ore-controlling structures and fractures produced by caldera subsidence.