| Abstract: | This report tests the usefulness of airborne time-domain electromagnetic (TEM) data on
three mineral-resource-related issues: (1) to test whether known mineral deposits at or
near the surface display any signal in the TEM data; 2) determine whether TEM data can
be used to locate bedrock concealed by basin fill; and (3) if the exposed mineral deposits
display a signal in the TEM data, to determine if whether deposits are recognizable at
depth in outcropping bedrock or in bedrock concealed beneath basin fill. Because Earth‘s
total-intensity magnetic field data are also acquired with the TEM data, these data are
included in the analysis.
The Cottonwood Canyon area in Santa Cruz County, Arizona, contains several
polymetallic vein deposits, including those of the Tyndall, Salero, and Wrightson mining
districts, all of which have had significant mineral production. Polymetallic vein deposits,
which generally consist of veins of disseminated metallic minerals, commonly exhibit a
response to electrical geophysical methods. Also, on the basis of other studies, the
conditions that produced the polymetallic vein mineralization in the region are believed
to extend offshore into the bedrock concealed by basin fill.
The polymetallic vein deposits of the Cottonwood Canyon area all display a geophysical
signature in the TEM data. These deposits occur in bedrock that has, in general, a very
low resistivity. The polymetallic vein deposits are associated with high-conductivity
regions that extend from deep in the bedrock to the surface. These high-conductivity
regions can be quite narrow (100 m) or quite wide (1 km); most are relatively narrow.
Every known mineral deposit or prospect is associated with a high-conductivity feature.
High-conductivity regions can also occur without an association with known mineral
deposits.
Airborne TEM data appear to be able to locate the basin fill/bedrock contact beneath
basin fill. The basin fill (both dry and saturated) is generally more conductive than the
bedrock. The low conductivity of the bedrock ensures that no signal from the bedrock is
received from under the basin fill in most places. Therefore, in most areas of basin fill,
the bedrock is interpreted to be the bottom of the signal in the conductivity-depth
transforms (CDTs). Although the CDTs locate the basin-fill/bedrock contact, on the basis
of a comparison with a recent report, they mostly overstate its depth by a factor of at least
2.
Because CDTs based on airborne TEM data do not see into bedrock below basin fill, it is
impossible to detect mineralization in the bedrock concealed by such fill. Although the
geophysical signature of polymetallic vein deposits is seen at depth in outcropping
bedrock, the existence of this geophysical signal does not guarantee the occurrence of
polymetallic vein deposits. |
| Genre: | USGS Numbered Series |
| ProdID: | 50476 |
| Citation Author: | Bultman, Mark W. |
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| Citation Edition: | Version 1.0 |
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| Citation Language: | ENGLISH |
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| Citation Phsyical Description: | p. 49, illus. incl. sects., 1 table, geol. sketch maps, 22 refs |
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| Citation Series: | Open-File Report |
| Citation Series Code: | OFR |
| Citation Series Number: | 2002-18 |
| Citation Search Results Text: | Time-domain electromagnetic signatures of polymetallic vein deposits in Cottonwood Canyon area, Santa Cruz County, Arizona; 2002; OFR; 2002-18; Bultman, Mark W. |
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| Citation Year: | 2002 |
| Type: | citation/reference |
| Text: | Time-domain electromagnetic signatures of polymetallic vein deposits in Cottonwood Canyon area, Santa Cruz County, Arizona; 2002; OFR; 2002-18; Bultman, Mark W. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/usgs_thumb.jpg |
| URL (INDEX PAGE): | http://geopubs.wr.usgs.gov/open-file/of02-18/ |
| Date Other: | Tue, 1 Apr 2003 00:00 -0600 |
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