New Mexico Geological Society Annual Spring Meeting — Abstracts

Jarosite, KFe₃(SO₄)₂(OH)₆, and other K-bearing minerals in the alunite group, can be dated by K/Ar and ⁴⁰Ar/³⁹Ar techniques and contain OH and SO₄ groups that can provide four stable isotope analyses: δD, δ¹⁸O_OH, δ¹⁸O_OH, and δ³⁴S. Age dating and analysis of these isotopic parameters can provide information on the genesis and natural destruction of the deposits by weathering, This information in turn can provide insight into the climatic, geomorphic, and tectonic evolution of an area, as well as the hydrologic controls on present day water quality. A number ofjarosite occurrences in the Rio Grande Rift have been studied and preliminary results are reported here.

Geochronologic and stable isotope studies of supergene jarosite are underway at the copper porphyry deposits in the Organ and Jarilla Mountains on the southeast margin of the rift. These studies are patterned after the work at Creede, Colorado, by Rye et al. (1993). Jarosite and alunite can be used to define the hydrogeochemical environment of supergene jarosite and alunite mineralization, record the age of paleowater tables, reconstruct climate, geomorphic, and tectonic history of the area. Sulfur isotopes trace sulfur to precursor sulfide sources, Hydrogen and oxygen isotopes can be used to infer climate changes during the course ofsupergene oxidation. The ages and distribution of jarosite and alunite in time and space define periods of uplift related to tectonic events.

An extensive geochronological and stable isotope study ofjarosite at the Copiapo Mine, Dona Ana County, New Mexico has defined a new mode of occurrence -hydrothermal jarosite related to sour gas from sedimentary basins. Mineralization is hosted by a fault cutting Pennsylvanian-age limestone and consists of a replacement body with sequential deposition of halloysite, fluorite, prosopite, gypsum, hematite, jarosite and natrojarosite. ⁴⁰Ar/³⁹Ar age dates on hydrothermal jarosite and natrojarosite display a narrow range of apparent ages from 5.0 ± 0,12 to 4.5 ± 0.16 Ma with the natrojarosite consistently younger. Low sulfur isotope values for the jarosites (δ³⁴S = -16 to -24) indicate a basin-derived source of H₂S with subsequent oxidation during mineralization. Hydrogen (δD_smow = -64 to -96) and oxygen isotope (δ¹⁸O = -4 to -10) values of parent waters indicate an exchanged meteoric water origin typical of basin derived brines similar to the deep saline ground waters found in the basins today. Calculated temperature of formation, from the fractionation of ¹⁸O between the SO₄ and OH sites in many of the jarosites, is approximately 130° C.

Analysis of jarosite from the Hansonburg deposits, Socorro County, New Mexico, reveals a range of apparent ages from 7.9 ± 0.85 Ma to 1.63 ± 0.06 Ma. Each specific age of jarosite mineralization contains a unique mineral paragenesis and distinct stable isotope signatures. Both hydrothermal and supergene jarosites are present. Hansonburg, Copiapo and many other jarosite-bearing deposits along the rift contain abundant hydrothermal fluorite suggesting that they have fundamentally similar origins controlled by tectonic events, climate and the evolution of brines and sour gas in sedimentary basins.