Alteration and Epithermal Mineralization in the Steeple Rock District, Grant County, New Mexico and Greenlee County, Arizona

ABSTRACT
The Steeple Rock district offers an excellent opportunity to
examine the relationship between the distribution and timing of the
alteration and the formation of fissure veins in an epithermal environment.
Five distinct types of epithermal veins are found in the district:
base metals (±Au-Ag), gold-silver, copper-silver, fluorite, and
manganese. The veins were formed by low salinity fluids (<5 eq. wt.% NaCl), slightly acidic to neutral pH fluids at temperatures between 240¡C and 340¡C at relatively shallow depths (360-1300 m) and low pressures (<1500 bars). These veins are structurally controlled, hosted by Oligocene to Miocene volcanic and intrusive rocks, and are spatially associated with two types of alteration: neutral pH (alkali chloride or propylitic to argillic to sericitic) and acid sulfate (advanced argillic). Neutral pH alteration is the most pervasive type of alteration and occurred in three stages: regional pre-mineralization, local syn-mineralization, and regional post-mineralization. The mineral assemblage and geochemistry indicate that the neutral pH alteration was formed by near neutral pH, alkali-chloride fluids at low to moderate temperatures (<300¡C). Localized acid-sulfate alteration was formed by acidic magmatic-hydrothermal fluids at temperatures less than 340¡C at relatively shallow depths (<1.5 km) that were produced by the disproportionation of magmatic sulfur as a result of decreasing temperature as plumes of magmatic fluids rose towards the surface. Crosscutting relationships, stratigraphic position, and a few age determinations indicate that the acid-sulfate alteration preceded the mineralization, but the exact timing and duration of these events is speculative. Alteration and mineralization were episodic, waning, and migrating from one locality to another in response to variations in local structurally-controlled areas of highheat flow. The Steeple Rock district is one example of the early development of a geothermal system with both neutral pH and acidsulfate alteration occurring at approximately 31-28 Ma, followed by younger epithermal vein mineralization at about 28-18 Ma.