Source and genesis of sulphate and phosphate-sulphate minerals in a quartz-sandstone cave environment

摘要

Gypsum (CaSO_4?2H_2O), alunite (KAl_3(SO_4)_2(OH)_6), and rare phosphate-sulphate sanjuanite Al_2(PO_4)(SO_4)(OH) 9(H_2O) and rossiantonite (Al_3(PO_4)(SO_4) 2(OH)_2(H_2O)_(14)) have recently been identified as secondary mineral deposits in different quartz-sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz-sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho-sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz-sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non-sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H2S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co-occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz-sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.