Large-basin ground water circulation and paleo-reconstruction of circulation leading to uranium mineralization in Grand Canyon breccia pipes, Arizona
- Univ. of Wyoming, Laramie, WY (United States)
Breccia pipes - vertical collapse structures - are common in the Phanerozoic sedimentary section in the Grand Canyon region. Breccias in economically significant pipes are as great as 900 m high and 90 m in diameter. The pipes originated through collapse into paleocaverns in Mississippian carbonates. The large heights of the mineralized pipes is attributed to upward stoping resulting from progressive creation of space within the pipes through dissolution of wall rocks and soluble constituents in the breccia clasts. The paleocaves that served as nucleation sites for the pipes date from Mississippian time. Stoping appears to have been reactivated or accelerated during Triassic time as terrains to the south became uplifted. Uplift cause hydraulic gradients within aquifers in the Paleozoic section to increase significantly which enhanced ground water circulation and attendant dissolution. The most likely source for uranium in the Grand Canyon breccia pipes was eroding volcanic and Precambrian crystalline rocks in the Triassic Mogollon highlands south of the Grand Canyon region. The circulation model proposed herein assumes that uranium-rich waters originating in the highlands recharged through the exposed Redwall Limestone and circulated northward in the artesian Redwall aquifer. On reaching the Grand Canyon region, the water circulated upward into the Phanerozoic section in the breccia pipes which served as permeability pathways through thick confining strata. The pipes concentrated fluid circulation and directed it through reducing environments which caused precipitation of the uranium and associated metals yielding a number of economic uranium ore bodies. The architecture of the circulation systems in the Colorado plateau prior to incision of the Colorado river was such that hydraulic heads decreased within successively shallower aquifers. Consequently, head gradients at any location were upward in the pipes during the mineralizing stages.
- OSTI ID:
- 530590
- Journal Information:
- Mountain Geologist, Vol. 33, Issue 3; Other Information: PBD: Jul 1996
- Country of Publication:
- United States
- Language:
- English
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