SEQUENTIAL EXTRACTION OF SULFUR-CONTAINING MINERALS FROM HYDROTHERMALLY ALTERED VOLCANIC ROCKS FOR STABLE ISOTOPE ANALYSIS

A sequential extraction method has been developed to measure individual sulfur species that constitute major minerals of hydrothermally altered volcanic sediments and rocks. Sulfur-containing minerals, with known δ³⁴S and δ¹⁸O values, were combined into varying composite samples and analyzed using the extraction method to assess potential isotopic shifts during the extraction process.

The extraction method entailed the following extractions and their primary reagents: 1) elemental sulfur (acetone), 2) water-soluble sulfate (water), 3) reducible acid-soluble sulfate (1N HCl amended with hydroxylamine hydrochloride), 4) monosulfide (6N HCl), 5) acid-soluble sulfate (6N HCl), 6) disulfide (chromous chloride), and 7) alkaline carbonate-soluble sulfate (aqueous sodium carbonate). Elemental sulfur, monosulfide, and disulfide species were collected as silver sulfide, while all other species were collected as barium sulfate.

The recovered sulfur species exhibited average δ³⁴S values that deviated +/-0.5 per mil from their standard values excluding elemental sulfur, which deviated +/- 0.8 per mil. δ¹⁸O values for the recovered sulfur species deviated +/- 0.5 per mil from their standard values excluding barite, which deviated +/- 1.6 per mil.

The extraction methods yielded high recovery values, ranging from 89-107%, for elemental sulfur, gypsum, jarosite, anhydrite, and pyrite. The sulfur species extracted by the alkaline carbonate-soluble sulfate extraction, alunite (1-2%) and barite (19-24%), exhibited recoveries that were poor, but enough material was recovered for isotopic analyses. Under natural conditions, alunite and barite would likely be recovered together in the alkaline carbonate-soluble sulfate extraction, which would not yield individual isotopic values for either species.

The method was applied successfully to hydrothermally altered bedrock and lahar samples from Cascade volcanoes. Resulting isotope analyses were essential in resolving the origin of sulfur-containing minerals and the hydrothermal alteration history.