40Ar/39Ar ages and stable isotopes of supergene jarosite from the Baiyin VHMS ore field, NE Tibetan Plateau with paleoclimatic implications

Abstract

Supergene jarosite is widely distributed in weathering profiles derived from hypogene sulfide ores. Precise 40Ar/39Ar dating and determination of hydrogen and oxygen isotopes of supergene jarosite can not only place tight constraints on the timing and history of continental weathering and supergene enrichment of sulfide ore deposits, but also shed significant insights into paleoclimatic conditions and tectonic uplift responsible for the weathering. In this paper, we present a preliminary study of 40Ar/39Ar geochronology and stable isotope geochemistry of supergene jarosite from the early Paleozoic Zheyaoshan volcanic-hosted massive sulfide deposit of the Baiyin ore field located in the eastern portion of the Qilian Mountain along the northeastern (NE) margin of the Tibetan Plateau in an attempt to better understand the climatic conditions prevailed during the pervasive weathering in the region. Jarosite from the Zheyaoshan mining area occurs either as gravels within alluvial deposits over low-relief flat-top hills or cm-wide veins crosscutting weathered sulfide ores or their wall rocks. The gravels consist of massive jarosite aggregates with tabular crystal morphology and contain 8.21 wt%–8.31 wt% K2O on average. Two jarosite grains extracted from the gravels (08-02-2 and 08-02-5) yield well-defined 40Ar/39Ar plateau ages of 37.1 ± 0.3 and 41.2 ± 0.4 Ma (2σ), respectively. They have δD values of −133 ‰ and −156 ‰, and \({\delta ^{18}}{{\rm{O}}_{{\rm{S}}{{\rm{O}}_4}}}\) values of 2.6 ‰ and 2.5 ‰. The vein-type jarosite consists of hexagonal pyramid crystals and contains much lower K2O, averaging at 2.44 wt%–2.72 wt%. Two jarosite veins (06-16-3 and 06-16-4) yield similar plateau ages of 3.2 ± 0.1 and 3.3 ± 0.1 Ma (2σ), respectively. These jarosite veins have δD values of −158 ‰ and −160 ‰, and \({\delta ^{18}}{{\rm{O}}_{{\rm{S}}{{\rm{O}}_4}}}\) values of 2.8 ‰ and 2.5 ‰. The 40Ar/39Ar ages indicate that sulfide ores of the Zheyaoshan deposit must have been brought to the surface since the late Eocene, and subsequently subjected to prolonged oxidation and supergene enrichment. The precipitation and preservation of jarosite in weathering profiles indicate that arid–semiarid climatic conditions must have been prevailed since the late Eocene. The hydrogen and oxygen isotopes of the jarosite further point to variation of climatic conditions in the area, with the Pliocene being slightly cooler relative to the late Eocene. Given that weathering profiles overlying sulfide ore deposits are widely distributed along the Qilian Mountain, systematic studies of 40Ar/39Ar geochronology and stable isotope geochemistry of jarosite-group minerals from these profiles would provide valuable information on the timing and evolution of regional tectonic uplift, paleoclimatic changes, and supergene mineralization in this mountainous belt along the NE edge of the Tibetan Plateau.