GEOCHEMISTRY OF SANDSTONE BLEACHING AND (U-Th)/He GEOCHRONOLOGY OF IRON OXIDE DEPOSITS IN THE PARADOX BASIN

Abstract

Within the Colorado Plateau, many oxidized Paleozoic and Mesozoic sandstones have been bleached by migrating hydrocarbons and other reducing fluids. These fluids reduced iron and other elements in the red sandstones; in some cases, reprecipitating them as pyrite and in other cases dissolving and mobilizing them over large distances to form concentrated deposits. We aimed to simulate and characterize the geochemistry of sandstone bleaching by reducing fluids, specifically water and oil. XRF and ICP-MS analyses were used to understand the element mobility associated with these fluid-rock reactions. Our geochemical analyses suggest the mobilization of all major elements (especially Ca, Na, Mg, Fe, Ti, and Mn) in the most altered rock aliquots, consistent with dissolution of albite, carbonates, and the dissolution of hematite coating which causes the red discoloration of bleached sandstones. Our mass balance calculations show that 88.9% of the mass of major elements removed from the rock are missing and not accounted for in the analyzed fluids. The colloidal phases precipitated in our mixture could be the oversaturated and enriched phases that host the missing elements. We also performed (U-Th)/He geochronology of the Fe-oxide deposits of the Upper Triassic Shinarump Member to constrain the timing of paleo-fluid migrations. We obtained ages ranging from 23 Ma to 153Ma,with ~3-5% (2σ) uncertainties. Based on the distribution of our ages and eU concentrations, we theorized that there were 1 or 2 formation ages of the Fe-oxide and that the wide distribution of the ages is caused by U mobilization. These theorized ages - ~87 Ma and 144 Ma – coincide with a period of rapid subsidence (~90-70 Ma), the onset of the Sevier Orogeny (~140 Ma), and a Late Jurassic early-phase movement of the Moab Fault.