FINGERPRINTING PALEO-GROUNDWATER SOURCES USING OXYGEN ISOTOPES OF HEMATITE CONCRETIONS FROM THE “BOILER ROOM”, MOAB AREA, UTAH

Iron oxide cements and concretions are ubiquitous throughout the Colorado Plateau and represent end-products of a hydrologic system involving removal, transport, and reprecipitation of iron by paleo-fluids. Although the occurrences, morphologies, and distributions of these features have been widely studied, the geochemical processes involved in their formation and paleo-fluid sources are debated and the depth at which they formed remains unclear.

Iron oxide (primarily hematite) cements and concretions hosted in the Navajo Sandstone from an area northwest of Moab, UT known as the “Boiler Room” were collected for microscopic, trace element, and isotopic analyses to construct a model for their formation and characterize the paleo-fluids involved. Sampled concretions span a range of morphologies from sub-vertically oriented pipes with iron oxide rims to densely-cemented iron oxide horizons (ironstones) near the top contact between the Navajo Sandstone and Carmel Formation. Pipes observed over a 3 km² area have a consistent inclination (68° ± 9 toward 199° ± 23) with hematite cement “comet tails” in the down-plunge direction, suggestive of groundwater flow during mineralization. New δ¹⁸O_VSMOW values range from -0.7 to +4.7 ‰ for sampled hematite pipes and from +2.6 to +6.7 ‰ for ironstones. Measured δ⁵⁶Fe_IRMM values range from -0.21 to 1.07 ‰.

Field observations and trace element data patterns support concurrent formation of iron oxide in pipes and ironstones from a similar groundwater source. We suggest that concretions at the Boiler Room site precipitated in a near-surface spring system with pipes serving as conduits for ascending reduced groundwater delivering dissolved iron to oxygenated surface waters in an interdunal setting. Average δ¹⁸O values in ironstone deposits are higher than in associated pipe concretions, which we interpret as fractionation due to evaporation of surface waters. Assuming precipitation temperatures between 15 and 25 °C and near neutral pH, these data indicate paleo-fluid δ¹⁸O values from -3.3 to +3.9 ‰. Iron oxide cements and concretions thus record a history of groundwater-surface water interaction and highlight the utility of using stable isotope and trace element geochemistry of secondary minerals in deciphering complex paleo-hydrological systems.