New Mexico Geological Society Annual Spring Meeting — Abstracts

The Steeple Rock district is in the Summit Mountains in southwestern New Mexico and southeastern Arizona. An estimated $10 million worth of gold, silver, copper, lead, and zinc have been produced from epithermal veins from 1880 to 1986; mostly from the Carlisle and Center mine in the central portion of the district. Host rocks consist of multiple sequences of Oligocene to Miocene andesite, dacite, and rhyolite flows and ash flow tuffs that were intruded by Miocene rhyolite dikes and plugs. Subsequent Basin and Range faults have offset the sequence and provided conduits for the epithermal veins. Preliminary data suggest that there are two distinct types of ore deposits in the district: base-metal veins with gold and silver along the east-west trending Carlisle fault (Carlisle and Center mines) and silver-gold veins with some base metals along the northwest-trending faults (East Camp). Both types are characterized by epithermal features such as comb structures, colloform banding, brecciation, replacement textures, and vug fillings. The base-metal veins contain gold, electrum, unidentified silver minerals, sphalerite, galena, and chalcopyrite in a gangue of quartz, pyrite, chlorite(?), fluorite, iron oxides, and clay minerals. This mineralization occurs in an intensively altered area consisting of bleached and Iiesegang banded rock composed of quartz, opal, alunite, pyrophyllite, jarosite, diaspore, kaolinite, and other clay minerals (XRD studies). A vertical zonation may be present (illite at the base to pyrophyllite to alunite to quartzopal); more detailed studies are under way. This alteration is suggestive of alunitekaolinite epithermal mineralization, but it is uncertain if this alteration is associated with the base-metal mineralization along the Carlisle fault. The second type of mineralization which occurs along the northwest-trending faults consists of gold, unidentified silver minerals, and rare chalcopyrite, galena, and sphalerite in a gangue of quartz, pyrite, calcite, barite, and fluorite. Some veins are associated with manganese oxides and hematite. These veins are associated with various degrees of sericitic, argillic, and silicic alteration, but the alteration is not as intense as at the Carlisle-Center mines. This alteration is suggestive of adularia-sericite epithermal mineralization, although sericite has not yet been positively identified. Both types of mineralization have a similar paragenetic sequence; although more detailed studies are in progress. Preliminary observations suggest that there are at least three stages of quartz deposition. The alteration probably occurred during the first stage and the predominant base and precious metal mineralization occurred during the second stage. Alteration, vein textures, mineralogy, paragenesis, and metal zonation are consistent with an epithermal origin. However, a number of questions remain concerning the nature and origin of the veins and associated alteration.