Geochemistry, geochronology, and tectonic implications of jarosite mineralization in the northern Franklin Mountains, Dona Ana County, New Mexico
— Virgil W. Lueth, Philip C. Goodell, Matthew T. Heizler, and Lisa Peters

Hypogene jarosite, K₂Fe₆(SO₄)₄(OH)₁₂, has recently been identified at the Copiapo jarosite mine and Schneider claims in the Webb Gap area of the northern Franklin Mountains, south-central New Mexico. Coarse grain sizes, mineral paragenesis, and co-precipitation with barite, galena, and fluorite establish a hypogene origin. Jarosite at the Copiapo deposit is localized in an east-dipping, low-angle normal fault. Mineralization occurs in a paragenetic sequence of: (1) hydrated halloysite, (2) hematite-gypsum, (3) jarositefluorite, and (4) natrojarosite. Jarosite occurs in a quartz-breccia vein at the Schneider claims with fluoritebarite- galena. ⁴⁰Ar/³⁹Ar analyses yield geologically significant apparent ages of 4.68 ± 0.13 and 4.72 ± 0.28 Ma for jarosite and natrojarosite, respectively, from the Copiapo mine and are interpreted to indicate that mineralization occurred at 4.69 ± 0.12 Ma. An age of 3.25 ± 0.29 Ma was determined for jarosite from the Schneider claims that dates the age of galena-barite-fluorite mineralization. These ages partially constrain the age of faulting in the northern Franklin Mountains and slickensides in the jarosite orebody indicate more recent reactivation and rotation on the host faults.