A geological and geochemical study of a sedimentary-hosted turquoise deposit at the Iron Mask mine, Orogrande, New Mexico
Josh C. Crook and Virgil W. Lueth
Turquoise occurs in veins up to 8 cm thick in a shale unit of the Pennsylvanian Gobbler Formation at the Iron Mask claim in the Orogrande mining district, Otero County, New Mexico. Previous investigators have proposed three models for turquoise genesis: hydrothermal (magmatic-related), contact metasomatic, and supergene (weathering-related). The geologic setting and mineral assemblages at the Iron Mask claim suggest a supergene origin of turquoise. The presence of other oxidized copper minerals with turquoise supports the inference of Cu+2 mobilization derived from widespread porphyry-type copper mineralization in the district and subsequent introduction into the host shale. The ubiquitous presence of gypsum, which commonly occurs with turquoise, indicates that sulfuric acid solutions were abundant. X-ray fluorescence analysis of the shale indicates it is enriched in phosphate (up to 3 times average), which consists of apatite with minor xenotime. Dissolution textures exhibited by these minerals and depletion of phosphate in altered portions of the shale suggest they are the primary source of phosphate for turquoise genesis. Alteration within the shale units is confined to faults, fractures and along particular bedding planes, and characterized by the development of kaolinite, a feature characteristic of sulfuric acid-induced alteration. Coexisting supergene alunite and jarosite are found within the turquoise, indicating that the minerals precipitated from acid solutions at or near the surface of the phreatic zone. The presence of goethite in the mineral assemblage also indicates the solutions were cold, less than 100°C.
- Crook, Josh C.; Lueth, Virgil W., 2014, A geological and geochemical study of a sedimentary-hosted turquoise deposit at the Iron Mask mine, Orogrande, New Mexico, in: Geology of the Sacramento Mountains region, Rawling, Geoffrey; McLemore, Virginia T.; Timmons, Stacy; Dunbar, Nelia, New Mexico Geological Society, Guidebook, 65th Field Conference, pp. 227-233.