February 11, 2022
When visiting geologists come to New Mexico, we take them into the field, even in January. Dr. Tapani Rämö is from the Department of Geosciences and Geography, University of Helsinki and spent a day in the field in the Gallinas Mountains, Lincoln County with Dr. McLemore. Rapakivi syenite sills intrude Permian limestone and sandstone along a closed road in the Gallinas Mountains, Lincoln County, New Mexico. The Finnish word “rapakivi” means disintegrated or crumbly rock or stone and denotes the tendency of certain rapakivi granite types to disintegrate more easily than other granitic rocks of southern Finland (type area of rapakivi granite). Rapakivi texture in the Finnish rapakivi granites is characterized by ovoidal alkali feldspar megacrysts mantled by sodic plagioclase and presence of short-prismatic quartz crystals. The Gallinas Mountains syenite is devoid of quartz but displays a conspicuous population of platy alkali feldspar phenocrysts with potassic cores and plagioclase rims around them. Some of these phenocrysts bear evidence of multiple rims indicating prolonged growth histories in transient magma chambers before the final emplacement of the syenite sills. The Gallinas Mountains is an area known and under study for rare earth elements (REE) deposits, and textures like these help geologists understand the magmatic history and growth of feldspars in these igneous rocks, which are related to the REE deposits in the Gallinas Mountains. More research is underway.
- Virginia McLemore, Principal Sr. Economic Geologist, NMBGMR
- Tapani Rämö, Department of Geosciences and Geography, Geology and Geophysics Research Program, University of Helsinki, Finland
For more information on the geology and REE mineral deposits in the Gallinas Mountains see:
McLemore, Virginia T.; Kelley, Shari; Zimmerer, Matthew J.; Owen, Evan; Haft, Ethan B.; Cantrell, Tyler; Gysi, Alexander; Dietz, Haley; Cherotich, Stellah; Trivitt, Amy, 2021, Geology and mineral deposits of the Gallinas Mountains, Lincoln and Torrance Counties, New Mexico, New Mexico Bureau of Geology & Mineral Resources, Open-file Report 617, 164 p.
For more information on rapakivi textures see:
Lehtinen, Martti; Nurmi, Pekka; Rämö, O.T., 2005, Precambrian Geology of Finland, 750p. ISBN: 9780080457598.