A preliminary geochemical study of the Redrock anorthosite and granite, Burro Mountains, southwestern New Mexico
— Virginia T. McLemore and Christopher McKee


Approximately two dozen small anorthosite bodies are scattered throughout a northeast-trending zone in a Proterozoic granitic terrain near Redrock in the Burro Mountains, Grant County, New Mexico. Compositions of these chemically and texturally heterogeneous bodies range from anorthosite and gabbroic anorthosite to fine-grained hybrid anorthosite to quartz anorthosite near the contacts. The anorthosites are tan to white, fine- to coarse-grained, and consist of predominantly untwinned plagioclase with accessory hornblende, chlorite, and minor quartz, K-feldspar, biotite, magnetite-ilmenite, sphene, apatite and zircon. Preliminary geochemical studies of one body indicate that the rock is characterized by high CaO (9-10%), high MgO (1- 5%), high Al2O3 (13-20%), low SiO2 (46-51%) and low K20 (1-2%). 

The Precambrian granitic rocks in the Burro Mountains have been named the Redrock granite near Redrock and the Burro Mountain granite elsewhere in the range. The Redmck granite is similar in composition and appearance to the Burro granite. It is orange to pink, fine- to medium-grained and consists of equal amounts of quartz, microcline and twinned plagioclase. Geochemically, the Redrock granite is similar to other high-Si and high-K Proterozoic granites in New Mexico. It is characterized by high K2O (3-6%), intermediate to high SiO2 (73-76%), low CaO (0.2-0.7%) and low MgO (0.2-1.5%).

Petrographic textures and geochemical data are consistent with a magmatic origin of the anorthosite bodies. A larger body of anorthosite could be in the subsurface. Although mineral deposits have not yet been found associated with the Redrock anorthosites, anorthosites worldwide are known to contain much of the world's resources of titanium with vanadium and iron as potential by-products. Subsurface drilling is required to properly evaluate the mineral resource potential of the Redrock anorthosites.

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Recommended Citation:

  1. McLemore, Virginia T.; McKee, Christopher, 1988, A preliminary geochemical study of the Redrock anorthosite and granite, Burro Mountains, southwestern New Mexico, in: Cretaceous and Laramide tectonic evolution of southwestern New Mexico, Mack, G. H.; Lawton, T. F.; Lucas, S. G., New Mexico Geological Society, Guidebook, 39th Field Conference, pp. 83-88. https://doi.org/10.56577/FFC-39.83

[see guidebook]