Two types of jarosite in the early Cambrian sedimentary rocks: Insights for genesis and transformation of jarosite on Mars

Abstract

Jarosite is a common sulfate mineral on Mars. However, there are limited exposures of jarosite in ancient (pre-Cenozoic) sedimentary rocks on Earth, and it has limited our understanding of genesis of jarosite and analogous exposures on Mars. Sulfates- and sulfides-enriched black shales in the early Cambrian could provide more information on the genesis and transformation of jarosite. According to detailed field investigations, petrology, and mineralogy studies of the black shales from the northern unit of the South Qinling (SQL) in China, two types of jarosite have been identified. Type I jarosite is associated with limonite and occurs as veins and spots, in contrast to type II jarosite intergrown with apatite and that develops as concretions and flakes. In combination with a paragenetic sequence of quartz, barite, K-feldspar, calcite, apatite, pyrite, limonite and clays (mainly illite) in the black shales, the type II should be produced earlier than the type I jarosite. Energy-dispersive X-ray spectrometry analysis show atomic ratios of Fe/Al from type II jarosite range mainly from 1.0 to 2.5 and much smaller than that of type I jarosite. In addition, backscattered electron images and microprobe X-ray maps suggest that type II jarosite enriched vanadium and other transition elements could be produced with apatite simultaneously. It is very likely that type II jarosite is syngenetic and the other jarosite is produced by weathering of pyrite. The close affinity between type II jarosite and apatite is ascribed to “iron pump and shuttle” in this scenario. Compared with this study, jarosite on Mars may not only been produced from oxidized pyrite during diagenesis, but could also have been produced directly in syngenetic stage.