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[1] The discovery of sulfate-rich layered deposits with hematite spherules at the landing site of the Opportunity rover is consistent with mineral deposition in an aqueous environment. We evaluate conditions responsible for the formation of a jarosite-goethite-gypsum assemblage with speciation calculations. The results show that the assemblage could have precipitated from acidic solutions formed through near-surface aqueous oxidation of pyrite. Our hypothesis is that regional heating in the Meridiani Planum caused a release of sulfide-rich hydrothermal waters, leading to formation of pyrite-rich regional deposits in a depression. Aqueous oxidation of these deposits by atmospheric O2 created an acidic environment that allowed formation of sulfates and goethite. Partial neutralization of the solution caused further goethite precipitation and conversion of jarosite to goethite, leading to formation of goethite concretions. Subsequent dehydration of goethite to coarse-grained hematite would also have been facilitated by regional heating.