Abstracts concerning the geology of the Steeple Rock district, New Mexico and Arizona

(Also see my summary of Steeple Rock district)

McLemore, V. T., 2000, Alteration and epithermal mineralization in the Steeple Rock district, Grant County, New Mexico and Greenlee County, Arizona; in Geology and Ore Deposits 2000, The Great Basin and Beyond: Geological Society of Nevada, Abstracts with programs, p. 63.

ABSTRACT

 The Steeple Rock district in the Summit Mountains in southwestern New Mexico and southeastern Arizona 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 occur in the district: base metals (± gold-silver), gold-silver, copper-silver, fluorite, and manganese. The epithermal 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) as evidenced by preserved textures, fluid inclusion data, mineralogy, and chemical composition. 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 in the district 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) as evidenced by mineral and chemical zonations, preserved textures, multiple horizons, stratigraphic relationships, and limited sulfur isotopic data. These acidic fluids 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 epithermal mineralization, but the exact timing and duration of these events is speculative. There is no evidence to suggest that the alteration and epithermal mineralization were continuous; instead events were episodic, waning, and migrating from one locality to another in response to variations in local structurally controlled areas of high heat flow. The Steeple Rock district is one example of the early development of a geothermal system with both neutral pH and acid-sulfate alteration occurring at approximately 31-28 Ma, followed by younger epithermal vein mineralization at about 28-18 Ma.

---

McLemore, V. T., 2000, Alteration and epithermal mineralization in the Steeple Rock district, Grant County, New Mexico and Greenlee County, Arizona; in Cluer, J. K., Price, J. G., Struhsacker, E. M., Hardyman, R. F. and Morris, C. L. eds., Geology and Ore Deposits 2000, The Great Basin and Beyond: Geological Society of Nevada, Symposium Proceedings, in press.

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 high-heat flow. The Steeple Rock district is one example of the early development of a geothermal system with both neutral pH and acid-sulfate alteration occurring at approximately 31-28 Ma, followed by younger epithermal vein mineralization at about 28-18 Ma.

KEYWORDS: acid-sulfate alteration, epithermal veins, New Mexico, Arizona

---

McLemore, V. T., McIntosh, W. C. and Applet, R., 2000, Volcanic stratigraphy, geochemistry, and structure of the Steeple Rock district, Summit Mountains, Grant County, New Mexico and Greenlee County, Arizona; in Lawton, T. F., McMillan, N. J., McLemore, V. T., Austin, G., and Barker, J. M. eds., Southwest Passage, A Trip through the Phanerozoic: New Mexico Geological Society Guidebook 51.

ABSTRACT

Rocks exposed in the Steeple Rock district consist of a sequence of Oligocene to Miocene (34-18? Ma) andesite, basaltic andesite, and dacitic lavas interbedded with sandstones, volcanic breccias, and rhyolite ignimbrites. This sequence is locally intruded by intermediate to silicic plugs, dikes, and domes (33 and 28-18 Ma), some of which are associated with epithermal vein formation, brecciation, and faulting. The ignimbrites in the district are outflow sheets that were erupted from calderas in the Mogollon-Datil and Boot Heel volcanic fields. Extensional deformation of the volcanic rocks in the district produced a series of half-grabens and horsts with district-wide, northeastward dips of bedding planes and foliation. The stratigraphic nomenclature of two units in the area (Summit Mountain and Dark Thunder Canyon formations) is formalized in this report. The Summit Mountain Formation is named after Summit Mountain in the northern part of the area. It is approximately 240 m thick at the type locality and includes intrusive andesite and volcaniclastic sedimentary rocks, but the base of the unit is faulted. The Dark Thunder Canyon Formation consists of multiple gray to brown to purple to red porphyritic amygdaloidal andesitic to basaltic andesite lava flows with interbedded <28 Ma ignimbrites and volcaniclastic sandstones and is nearly 800 m thick at the type locality. Geochemical data suggest that pre-28 Ma (Summit Mountain and Bloodgood Canyon Tuff) and 28-20 Ma (Dark Thunder Canyon Formation and <28 Ma ignimbrites) volcanic rocks represent predominantly lithosphere-derived magmas, with increasing amounts of asthenosphere-derived magmas from 28-20 Ma.