Subsurface dissolution of heavy minerals, Frio Formation sandstones of the ancestral Rio Grande Province, South Texas

Abstract

Oligocene sandstones (Frio Formation) of South Texas, deposited by the ancestral Rio Grande, contain a heavy mineral assemblage that is markedly altered, even in the shallowest sample examined (approximately 1 km depth). The weight fraction of detrital heavy minerals in the modern Rio Grande is approximately 1% whereas subsurface Frio sandstones contain as little as 0.1% (or less) heavy minerals. Modern Rio Grande sand contains a diverse and unstable heavy mineral assemblage dominated by pyroxenes, amphiboles, and an assortment of iron-titanium oxides; minor components include apatite, garnet, sphene, epidote, and zircon. Heavy minerals in subsurface Oligocene sandstones include abundant apatite, zircon, and titanium oxide among detrital components; minor iron-rich garnet also survives in the subsurface to depths of about 3 km. Other very stable, but also very minor, heavy minerals include tourmaline, chromite, spinel, and monazite. Locally, the heavy mineral fraction in the Frio is dominated by authigenic components such as siderite, pyrite, or barite. Authigenic sphalerite is also observed among the heavy minerals. Feldspars and rock fragments suggest that sediment deposited by the ancestral Rio Grande was more strongly dominated by volcanic detritus than is the modern river. It is reasonable to infer that the initial heavy mineral assemblage of the Frio Formation was likewise more unstable than that which appears today in the Rio Grande. Thus, heavy mineral loss determined by comparing subsurface Frio sandstones with the sand in the modern Rio Grande is conservative. Comparison of Sr and Nd concentrations in modern Rio Grande sands and subsurface Frio sandstones suggests that heavy mineral destruction in the subsurface does not cause major perturbations of whole rock isotopic systems involving these elements.