New Mexico Geological Society Annual Spring Meeting — Abstracts

Rio Grande Rift (RGR) deposits, formerly called sedimentary-hydrothermal barite-fluorite-galena deposits (North and McLemore, 1986) or Mississippi-Valley type (MVT) deposits (Putnam et al., 1993), are found within or near continental basins along the Rio Grande Rift in central New Mexico. These deposits are low temperature, open-space fillings with little or no replacement of host rocks and they are not obviously associated with any magmatic or volcanic activity. They consist predominantly of barite, fluorite, and galena, with minor amounts of chalcopyrite and sphalerite locally. Silicification and dolomitization are locally common. The deposits are typically hosted by Paleozoic carbonate rocks as veins, breccia cement, and cavity-fillings adjacent to or within Basin and Range-related faults (e.g. Hansonburg, Salinas Peak, Rincon). Locally, RGR deposits occur along faults, fractures, contact zones, unconformities, shear zones, bedding planes, and
solution cavities in Precambrian, Paleozoic, and Tertiary rocks of varying lithologies (e. g. Zuni Mountains, Tonuco Mountain, Bear Canyon). Most RGR deposits in New Mexico are small, typically less than a few thousand tons of ore, but some deposits can be traced along strike for several thousand feet. The Hansonburg deposits are larger and consist of equal amounts of barite, fluorite, and galena; but the lead (1.3%) and zinc (0.01 %) grades are much lower than skarn and carbonate-hosted Pb-Zn replacement deposits in the state. Locally, some deposits in New Mexico contain pockets or zones of high-grade barite, fluorite, or lead-silver ore that were mined selectively in the past. No significant RGR deposits occur north of the Placitas district in north-central New Mexico. This absence corresponds to a lack of evaporites in the stratigraphic section which could provide sulfur needed to form these barite-rich deposits (McLemore and Barker, 1985). Lead-isotope data indicate that the deposits in Hansonburg, Lemitar Mountains, Salinas Peak, Caballo Mountains, and Joyita Hills formed from upper crustal sediments and/or Precambrian rocks (Ewing, 1979). Most of the RGR deposits are too small to be economic today.

RGR deposits in New Mexico formed from slightly higber temperature (95-350°C), but lower salinity (0.5-18 eq. wt% NaCl) meteroric fluids than MVT deposits. However, RGR deposits have many similarities to MVT deposits. They occur in carbonate rocks, are mineralogically simple, and have similar ore textures and alteration styles as classic MVT deposits, suggesting a similar process of formation.

Evidence suggests that RGR deposits are Tertiary in age; all RGR deposits are within or along the flanks of Tertiary rift basins. Stratigrapbic evidence in the Tonuco Mountain and Rincon districts indicates those deposits are Miocene (Seager et al., 1971; Seager and Hawley, 1973). The Gonzales deposit in the Chupadera district, Socorro County may be as young as 8 Ma according to fission track cooling dates of the host rock (Sheri Kelley, personal comm., December 1996). New ⁴⁰Ar/³⁹Ar age determinations of jarosite from the Copiapo jarosite deposit, Northern Franklin Mountains district indicates an age of 5.0 to 4.6 Ma; jarosite from Hansonburg is 6.36-5.98 Ma (Lueth and Heizler, 1997).

The RGR deposits in New Mexico were formed by low-temperature formation waters or basinal fluids that accumulated in sedimentary basins and were heated by high heat-flow, magmatic activity, or radiogenetic heat from Precambrian plutons. The warm convecting waters leached barium, sulfate, lead, silver, and other ions from source rocks such as arkosic sediments, evaporites, Precambrian rocks and mineral deposits. Fluorine may be derived from magmatic fluids. The mineralized waters were injected along open-spaces such as bedding planes, unconformitites, faults, and fractures as a result of gravity-driven flow from topographic highs, compaction of basin sediments, and/or density-driven flow produced by thermal contrasts along the basin flanks (Pearson and Gaven, 1994). Precipitation occurred as a result of cooling of the fluids, decrease in pressure, change in water chemistry, and/or mixing of the mineralized fluids with subsurface brines or meteoric water. This process could occur at any time during development of the basin and results in deposits of varying age.