 Abstract
The contiguous United States has been well explored for exposed conventional mineral deposits. Therefore, it is likely that many economically viable and strategically significant conventional undiscovered mineral deposits will be found in bedrock concealed beneath basin sediments. Mineral resource assessments must incorporate an understanding of the geometry, structure, and concealed lithology of basins in order to be accurate. This report presents an analysis of the basin geometry and structure of the San Rafael basin in southeastern Arizona. In addition, a new methodology for inferring concealed lithology is presented and applied in the San Rafael basin.
Gravity data is used to model the geometry of the basin using recent models
of sediment
density vs. depth developed in the region. This modeling indicates that the
basin has a
maximum depth of approximately 1.05 km plus or minus 0.10 km. In the southern
portion, the
basin can be modeled as an asymmetric graben faulted on the western margin. The
northern
portion of the basin is structurally more complex and may have high angle faults
on the
western, northern, and eastern margin. Near-ground closely spaced Earth’s
total
intensity magnetic field data is used to locate concealed faults within the
basin. This
data is also used to infer lithology concealed by shallow basin sediments.
Airborne
Earth’s total intensity magnetic field data is used to help infer concealed
lithology
in deep portions of the basin. The product of integrating all data and
interpretations is
a map which presents the geometry of the basin, faults and contacts concealed by
basin
sediments, and an estimate of the bedrock lithology concealed by basin sediment.
Based on basin geometry and concealed lithology, the San Rafael basin has a
high
potential for concealed mineral deposits on its western and northern margin. In
particular, a newly discovered magnetic anomaly in the northern portion of the
basin can
be modeled as a granitic intrusion with highly altered margins and may represent
a
potential mineral resource target. Based on the permeability and porosity of
upper basin
fill found in nearby basins, the San Rafael basin may contain an aquifer up to
300 meters
thick over a substantial area of the basin.
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First posted August 12, 1999
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U.S. Department of the Interior |
U.S. Geological Survey