Sulfur- and strontium-isotopic geochemistry of celestite, barite and gypsum from the Mesozoic basins of northeastern Mexico

Abstract

The Mesozoic sedimentary basin of northern Mexico contains large limestone-hosted stratiform "mantos" of celestite and barite that appear to have formed by carbonate replacement. We have measured the isotopic compositions of sulfur and strontium in these mantos in order to determine whether they formed by diagenetic processes and, if so, what fraction of the diagenetically released Sr is fixed in celestite deposits. S- and Sr-isotopic analyses of gypsum from layers in the limestone confirm its Cretaceous sedimentary origin. Isotopic compositions of most celestite and barite differ from values observed for the gypsum. Division of the deposits into two geologically and geochemically distinct groups shows that there is a general positive correlation between [delta] 34S and 87Sr/86Sr compositions of celestite and barite in the two systems, with celestite containing lighter S and less radiogenic Sr in both groups of deposits. Less radiogenic Sr in the celestite probably came from nearby Cretaceous limestones. More radiogenic Sr, which is found largely in the barite, probably came from basement-derived arkoses in the Cretaceous basin. Sulfur was derived from evaporites in one deposit group and from some other lighter source, perhaps coal or petroleum, in the other. These isotopic constraints, considered in the light of available solubility data, require that at least two distinct solutions existed and mixed locally during formation of the celestite and barite mantos. No more than 10% of the diagenetically-released Sr is concentrated in the celestite deposits.