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In the Grants mineral belt, greenish-gray lacustrine claystones and mudstones of the Brushy Basin Member and K
shales of the Morrison Formation may have served as "source rocks" for the humate in uranium-bearing sandstones of the Westwater Canyon Member and Poison Canyon sandstone (of economic usage). This hypothesis provides a convenient, local source for the humic substances associated with tabular ore, as the K shales are interspersed with, and the Brushy Basin Member immediately overlies, the ore-bearing sandstones. The reduced nature of these claystones and mudstones suggests that organic matter incorporated during sedimentation would have been initially preserved in the pore waters of lake-bottom sediments. The diagenetic alteration of volcanic ash contained in these sediments would have raised the pH of the pore water, causing solution of humic substances. Subsequent compaction would ha e forced the alkaline, humic-rich pore fluids out of the claystones and mudstones, into the nearby sandstones.
The two stages of ore in the Grants mineral belt--the primary tabular or trend ore and the secondary redistributed or stacked ore--can be related to the hydrology of a compacting basin. During early burial, formation water moves generally upward and laterally toward basin margins. Meanwhile, fresh meteoric water flows downdip from recharge areas that flank the basin. A similar early-burial hydrology is envisioned for the formation of tabular ore in the San Juan basin. In this case, humic substances, derived from pore waters expelled from the greenish-gray mudstones and claystones, moved into sandstone beds that served as escape conduits for formation water. When these alkaline formation waters encountered fresh meteoric water, the pH was lowered, facilitating organo-clay reactions tha resulted in precipitation of humate. The meteoric water also delivered uranium to the humate masses; the uranium probably had been leached by ground water moving through the sandstone beds.
Sometime after compaction ceased, the basin achieved hydrologic unity, and water flowed from areas of recharge to areas of discharge deeper in the basin. Redistribution of ore into stacked or roll-type orebodies probably occurred during this time, destroying some preexisting tabular humate ores. Redox mechanisms were involved in this redistribution of uranium into rolls but not in the formation of the earlier tabular ores. Location of tabular ores within reduced ground and the association with humate indicates that uranium in these orebodies was fixed by organic materials, which does not necessarily involve redox processes.
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