From USGS...

Mineralization related to detachment faulting has only recently been recognized as a distinct deposit type, even though such deposits have been mined since the 1860's. These deposits have characteristic mineral assemblages, alteration patterns, ore fluid types, and structural controls that differ considerably from those of other deposit types found in the Basin and Range province of the Western United States. However, detachment-fault-related mineralization is not widely known, having been described but twice in widely circulated journals (Spencer and Welty, 1986; Roddy and others, 1988); most of the detailed studies have appeared as publications of the Arizona Geological Survey and the Arizona Geological Society.
Awareness of the unique character of these deposits has been hampered by confusion with other types of epithermal mineralization that may or may not occur near a low-angle or detachment fault, such as the Cyclopic deposit in northwest Arizona (Myers and Smith, 1986) or the Mesquite deposit in southeastern California (Manske and others, 1988). This discussion sets out the distinguishing characteristics of detachment-fault-related mineralization vis-à-vis other types of epithermal mineralization in the region and provides a justification for the new deposit model presented (K.R. Long, this volume). This deposit model is considered preliminary because this deposit type has yet to be fully investigated and has, thus far, only been recognized in a detachment-faulted terrane encompassing parts of west-central Arizona, southeastern California, and southernmost Nevada .

Epithermal gold deposits are common in Arizona. They are charaterised as being rich, but small compared to other types of mineral emplacements, such as mezothermal deposits as are most often related to porphyries (like we may have at Hay Mountain).

FWIW