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

Secondary iron oxide concretions and cements are ubiquitous throughout the Colorado Plateau and represent a history of fluid-rock interaction that begins with the removal of iron from hematite-cemented sandstones and culminates in the formation of deposits exhibiting a range of different morphologies. Although their occurrences and distributions have been widely studied, the geochemical processes involved in their formation remain unclear and debated. Several models have been proposed to explain how these features form. Proposed models include subsurface mixing of reduced and oxidizing groundwater, microbially-mediated alteration of initially precipitated siderite to hematite, and replacement of early diagenetic calcite concretions by iron oxides. Alternatively, the morphology and stratigraphic association of some iron oxide occurrences suggest these might be related to near-surface soil forming phenomena involving leaching by organic acids and localized accumulation of iron in underlying soil horizons (i.e. podzolization).

Hematite concretions and cements hosted in the Navajo Sandstone from an area northwest of Moab, UT known as the “Boiler Room” were collected for microscopic and isotopic analysis with the aim of using their δ¹⁸O values to discern between hypothesized paleo-fluid sources and evaluate the proposed formation models. Concretions comprise several different morphologies including sub-vertically oriented “pipes”, columns, and spheres. Near the top contact between the Navajo Sandstone and Carmel Formation, densely-cemented iron oxide horizons (“ferricretes”) are common. Pipes observed over a 3 km² area have a consistent inclination (68 ± 9 toward 199 ± 23) with hematite cement “halos” in the down-plunge direction, suggestive of groundwater flow during mineralization. New δ¹⁸O values from hematite pipes range from -0.1 to 2.0 ‰ (VSMOW). Assuming precipitation temperatures between 15 and 50 °C, these data indicate paleo-fluid δ¹⁸O values from -7.4 and -1.3 ‰. These preliminary values can be explained by multiple processes, such as mixing between basin brine and oxidizing groundwater, or perhaps infiltration of Jurassic meteoric water. However, future work incorporating microscopy and δ⁵⁶Fe measurements will enable comparison between the competing formation models.