Formula omitted] analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa

Abstract

Supergene alunite (KAl 3(SO 4) 2(OH) 6) and jarosite (KFe 3(SO 4) 2(OH) 6) are often precipitated during the oxidation of sulfide-bearing rocks by meteoric solutions. Dating of these phases by the 40Ar 39Ar method allows us to time the progression of the oxidation front during chemical weathering. Finescale laser-heating 40Ar 39Ar dating of hypogene alunite and supergene jarosites allows us to precisely and accurately time hydrothermal alteration and subsequent supergene oxidation in Goldfield, Nevada. The results indicate that pervasive weathering occurred in the western USA during the Late Miocene (~ 10 Ma ago). Similar application of this technique to the study of weathering and laterite formation in West Africa indicates that the last pervasive oxidation event recorded in the weathering profile in this area also occurred in the Miocene (~ 13 Ma ago). The occurrence of a pervasive Mid to Late Miocene oxidation event recorded in these weathering profiles in the western USA and Africa, and also previously measured in Brazil and Chile, indicates that climatic conditions at that time were conducive to worldwide development of deep weathering sequences. Subsequent weathering processes have not been as pervasive as the Late Miocene event, indicating a general climatic transition to cooler, drier climates in most of the areas studied. Our results indicate that deep weathering profiles reflect past climatic conditions and may not be directly linked to the present climates.