New Mexico Geological Society Annual Spring Meeting — Abstracts

Areas within the southern Rio Grande Rift in which fluorite-barite deposits occur include the Bishop Cap Hills, Tortugas Mountain, the Organ Mountains, San Diego Mountain, and the southern Caballo Mountains. These deposits usually occur as fracture-fill is structures related to the development of the Rio Grande Rift. Most of the deposits are hosted by Paleozoic sedimentary rocks, but a few occur in Precambrian crystalline rocks. Replacement of carbonate rocks during mineralization was usually minor, but large-scale replacement has been observed in the southern Caballo Mountains.

Preliminary thin section analysis suggests the following paragenetic sequence for the dominant minerals found in most of the deposits studied: fluorite, barite, quartz, calcite. Galena occurs in some deposits, usually in small, local concentrations. Pyrite has also been observed, but it is less common than galena. Cathodoluminescent microscopy reveals features in quartz which may be usefull in correlating phases of mineralization among deposits in the study area.

A model for fluorite-barite mineralization in the Rio Grande Rift is proposed. Deep basin fluids, heated by a high geothermal gradient, were driven upward along rift-related structures by forced convection. Deposition of the minerals took place as these fluids mixed with shallower formation water. There were at least two phases of hydrothermal activity within this system. The first phase resulted in the deposition of fluorite and barite, while quartz and calcite were deposited during the second phase. These deposits appear to have formed during the middle to late Miocene.