Geology and geochemistry of the mineralization and alteration in the Steeple Rock district, Grant County, New Mexico and Greenlee County, Arizona
The Steeple Rock district is one of several epithermal volcanic-hosted epithermal districts in southwestern New Mexico and southeastern Arizona. This area offers an excellent opportunity to examine the relationship between the distribution and timing of alteration and the formation of fissure veins in an epithermal environment. Five distinct types of vein deposits occur in the district: base with precious-metals, precious-metal, copper-silver, fluorite, and manganese. These veins are structurally controlled; hosted by Miocene to Oligocene volcanic, volcaniclastic, and intrusive rocks; and they appear to be spatially related to two types of alteration: alkali-chloride (propylitic to argillic to sericitic) and acid-sulfate (advance argillic). Mineralogic and chemical zonations, preserved textures, multiple horizons, stratigraphic relationships, and limited sulfur isotopic data are consistent with the acid-sulfate alteration being produced by acidic magmatic-hydrothermal fluids at temperatures less than 340$\sp\circ$C in a relatively shallow environment ($<$1.5 km) such as a hot springs system. The acidic fluids are produced by disproportionation of sulfate in magmatic fluids with decreasing temperature as the fluids rose toward the surface. The epithermal fissure veins are younger than the acid-sulfate alteration as evidenced by cross-cutting relationships, stratigraphic relationships, and textural relationships, and age determinations. The epithermal veins were formed by low salinity ($<$5 eq wt% NaCl), slightly acidic to neutral pH fluids at temperatures between 240 and 320$\sp\circ$C at relatively shallow depths (360-1300 m) and pressures ($<$150 bars) as evidenced by preserved textures, fluid inclusion data, mineralogy, and chemical composition. The alteration and mineralization in the Steeple Rock district occurred in response to cyclic volcanic activity and subsequent development of local hydrothermal systems in structurally-controlled areas of high heat flow. While the exact timing and duration of these events is unknown there is no evidence to support that these events were continuous; rather the alteration and mineralization were episodic, waning, and migrating from one locality to another. Economic potential for additional low-sulfidation, fissure vein deposits in the Steeple Rock district is excellent and potential also exists for the discovery of high-sulfidation, disseminated gold deposits associated with the acid-sulfate alteration. ^
McLemore, Virginia T, "Geology and geochemistry of the mineralization and alteration in the Steeple Rock district, Grant County, New Mexico and Greenlee County, Arizona" (1993). ETD Collection for University of Texas, El Paso. AAI9415227.