New Mexico Geological Society Annual Spring Meeting — Abstracts

Breccia pipes are a common host of many precious metals, base metal and rare earth elements (REE) mineral deposits because they provide conduits for fluid flow and open spaces for mineral precipitation, hence are a focus area for exploration.

The Gallinas Mountains district in Lincoln County, New Mexico produced copper, lead, silver, fluorite, iron, REE (as Bastnäsite) and gold from 1902 to 1980, but no production has been reported from the breccia pipes. However, some magmatic-hydrothermal breccia pipes in the Gallinas Mountains host high concentrations of fluorite-REE and gold.

Previous studies have described the occurrence of REE in breccia pipes, but the controls for their transportation and deposition are still unclear. The purpose of this research is to characterize the magmatic-hydrothermal breccia pipes in order to understand the geochemical and physical conditions of deposition of REE and gold in the breccia pipes found in the Gallinas Mountains.

A total of 66 samples were subjected to various analysis, chemically, the breccia pipes exhibit light REE-enriched chondrite-normalized patterns, some host high concentrations of REE (80,500 ppm total REE) and Au (175 ppb).

These breccia pipe can be classified into three groups, group one has low concentration of REE and Au and contains mostly unaltered primary magmatic minerals, group two has low concentration REE and elevated concentration of Au, most sample in this group are slightly altered with some evidence of hydrothermal fluids being introduced into the system leading to some alterations minerals. The third group host high concentrations of REE, they are filled with hematite, fluorite, calcite and REE bearing minerals, mostly bastnäsite. In conclusion, the magmatic breccia pipe was formed followed by a younger carbonate-rich hydrothermal fluids precipitated F-REE and Au along the edges of some breccia pipes.

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