New Mexico Geological Society Annual Spring Meeting — Abstracts

Boiling of hydrothennal fluids is a mechanism commonly associated with deposition of epithennal ore minerals. Boiling strips the fluids of relatively insoluable gases such as H₂, He, CH₄, N₂, H₂S, and CO₂ with this gas plume migrating toward the surface to fonn a gas halo around centers of hydrothennal activity. Gas halos are known to exist around geothennal zones and presumably around extinct geothennal systems that generated many hydrothennal ore deposits. Studies at Creede CO, Copper Flat NM, and Pueblo Viejo, Dominican Republic record the presence of gas halos around ore deposits. Variations of the relative abundances of some of these gases such as CO₂, and H₂S should reflect on the location of centers for hydrothennal mineralization. In addition gas plumes may be the only geochemical evidence in rock well above an ore deposit where hydrothennal fluids did not penetrate the surface or were diluted by ground waters. Thus recognition of gas halos may prove useful in exploration for hydrothermal deposits.

The Steeple Rock district provides an opportunity to look for gas halos in an epithennaI environment on both a district-wide and local scale. Presently active exploration and good exposures of relatively undisturbed surface-mineralization make the district an ideal setting to test the hypothesis. Mineralization is mostly in the fonn of fault-controlled veins hosted by Oligocene andesite and dacite flows. Vein types range from delicate crustifonn banding of different quartz varieties and calcite to extensive brecciation with a massive matrix. In addition, areas of intense silicification and adjacent argillic alteration are present in the central part of the district. Past and current production in the district is predominantly from underground mines exploiting scattered Au-Ag-base metal deposits.

Surface samples of veins and silicified rock from throughout the Steeple Rock district have been collected with emphasis on obtaining a representative sample of all quartz/calcite generations present at each sample site. The samples are crushed and split down to about 30 grams. The sample is then crushed under high-vacuum to release the fluid inclusion volatiles which are then analyzed by a quadrupole-mass-spectrometer. H₂, He, CH₄, H₂S, and CO₂ are measured and preliminary results reveal anamolous CO₂ and H₂S gas contents. CO₂ and H₂S are known to be associated with ore deposits. Results will be presented on a two-dimensional surface map showing gas-ratio anomolies.