Nature and Role of Organic Matter in Sandstone Uranium Deposits, Grants Uranium Region, New Mexico, USA

Abstract

Pore-filling organic material concentrated uranium to form sandstone-hosted primary uranium deposits in the Jurassic Morrison Formation of the Grants uranium region, New Mexico. Because this organic material is the main ore control, determination of the nature, source, and time of emplacement of the organic material is critical to understanding the primary uranium ore deposits. Petrographic observations and radiometric ages for primary ore in the host sandstones (Westwater Canyon and Jackpile Sandstone Members) require that the organic material was emplaced early in their diagenetic histories; in fact, organic material was emplaced before significant compaction. Reconstruction of the burial history of the basin indicates that Late Jurassic-Early Cretaceous time, the interval of inferred emplacement of the organic material, was probably not a time of hydrocarbon generation and migration. Nuclear magnetic resonance analyses, elemental abundances, and isotopic studies provide further constraints on the nature of the organic material, and data from these studies are consistent with a humic acid rather than an oil origin. In addition, the overall tabular shape of primary uranium orebodies and the way these orebodies are suspended in the host sandstone mimics recent deposits of humate, the precipitated form of humic acids.

The critical clue that favors a humic acid origin for ore-related organic material is provided by the nature and patterns of postdepositional alterations that are spatially and temporally related to primary ore, specifically destruction of detrital magnetite and ilmenite (iron-titanium oxide) grains. Humic acids are implicated in the destruction of iron-titanium oxide grains because these organic acids are one of the most effective natural agents in leaching of iron from these grains. Thus, timing constraints, chemical evidence, orebody geometry, and the nature and patterns of ore-related alterations favor a humic acid origin and suggest that an oil origin is highly unlikely.

Because alteration of iron-titanium oxide grains tracks the pathways of humic-bearing solutions through the host sandstones, it is possible to trace these solutions back to their source. Alteration patterns in the Westwater Canyon and Jackpile Sandstone Members suggest that the humic-bearing solutions originated as pore fluids in the smectite diagenetic mineral zone of a large alkaline-saline lake complex in the intervening Brushy Basin Member and moved into adjacent sandstones during early dewatering of the smectitic mudstones. Within the sandstones, the humic acids precipitated by the process of cation loading, in tabular layers. Uranium was concentrated within these tabular layers by complexation with the humic acids to form primary uranium orebodies.

A second episode of uranium mineralization was associated with incursion of oxidizing groundwater during the Laramide orogeny. At this time, uranium was remobilized from some primary ores and reprecipitated at a regional reduction-oxidation interface, commonly along faults and fractures. These redistributed ores yield dates that are consistently less than 10 Ma.

The two episodes of ore genesis can be related to the hydrologic history of the San Juan Basin since deposition of the Morrison Formation. Formation of primary ore can be related to the movement of compaction-driven humic-acidbearing pore waters from the Brushy Basin Member into the host sandstones early in the burial history of the Morrison Formation. Redistribution of this primary ore occurred much later in the hydrologic history of the basin, when Tertiary uplift of the basin margins resulted in recharge of the host sandstones. At this time, oxidizing groundwaters removed uranium from some primary deposits and precipitated it along reduction-oxidation boundaries not too distant from the primary deposits.