Origin of barite-fluorite-galena deposits in the southern Rio Grande rift, New Mexico
Virginia T. McLemore, Thomas H. Giordano, Virgil W. Lueth, and James C. Witcher
Rio Grande rift (RGR) deposits, formerly called sedimentary-hydrothermal barite-fluorite-galena deposits or Mississippi Valley-type (MVT) deposits, are found within the RGR in central New Mexico. RGR deposits are characterized by a unique regional geologic setting, simple mineralogy dominated by fluorite and/or barite, low temperature of formation (<200-250°C), deposition as open-space fillings with little or no replacement of host rocks, common paragenetic sequence, and no obvious direct association with any magmatic or volcanic activity. These deposits occur along faults, fractures, contact zones, unconformities, shear zones, bedding planes, and solution cavities in diverse Precambrian, Paleozoic, and Tertiary rocks. Age determinations and field relationships suggest that RGR deposits formed during the last 12 Ma coincident with the later stages of rifting in southern New Mexico. Low-temperature formation waters or basinal fluids that accumulated in sedimentary rift basins and were heated by high heat-flow or magmatic activity formed RGR deposits in New Mexico. The warm convecting waters leached barium, sulfate, lead, silver, and other ions from source rocks. Mixing of magmatic and meteoric fluids may have occurred locally, thereby providing fluorine and other constituents. The mineralized fluids were injected along open-spaces as a result of gravity-driven flow from topographic highs and/or compaction of basin sediments. Although density-driven flow produced by thermal contrasts along the basin flanks could also mobilize fluids, this process is not likely to occur in the RGR. 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. Numerous factors, such as surface topography, development of rift structural architecture, erosion and tectonic elimination of aquitards, unroofing of carbonate aquifers by karsting, climate, rates of extension, basin sedimentation, subsidence, compaction, overpressuring, and crustal magmatism, may have played roles in the location, types, and intensity of RGR deposit evolution at any particular site. As a result, deposits of different ages, size, temperature of formation, and accessory mineralogy are a common feature of the RGR deposits.
- McLemore, Virginia T.; Giordano, Thomas H.; Lueth, Virgil W.; Witcher, James C., 1998, Origin of barite-fluorite-galena deposits in the southern Rio Grande rift, New Mexico, in: Las Cruces Country II, Mack, G. H.; Austin, G. S.; Barker, J. M., New Mexico Geological Society, Guidebook, 49th Field Conference, pp. 251-263.