Summary
The Payette National Forest (PNF), in west-central Idaho, is geologically diverse
and contains a wide variety of mineral resources. Mineral deposit types are grouped into
locatable, leasable, and salable categories. The PNF has substantial past production and
identified resources of locatable commodities, including gold, silver, copper, zinc,
tungsten, antimony, mercury, and opal. Minor lignitic coal is the only leasable mineral
resource known to be present in the PNF. Resources of salable commodities in the PNF
include sand-and-gravel, basalt for crushed-rock aggregate, and minor gypsum.
Locatable mineral resources are geographically divided between eastern and western
parts of the PNF. The western PNF lies west of the Riggins-to-Cascade highway (US 95 -
Idaho 55), and the eastern PNF is east of that highway. The western and eastern parts of
the PNF are geologically distinctive and have different types of locatable mineral deposits,
so their locatable mineral resources are described separately. Within the western and eastern
parts of the PNF, locatable deposit types generally are described in order of decreasing
geologic age.
An expert panel delineated tracts considered geologically permissive and (or)
favorable for the occurrence of undiscovered mineral deposits of types that are known to be
present within or near the PNF. The panel also estimated probabilities for undiscovered
deposits, and used numerical simulation, based on tonnage-grade distribution models, to
derive estimates of in-situ metals contained. These estimates are summarized in terms of
mean and median measures of central tendency. Most grade and tonnage distributions
appear to be log-normal, with the median lower than the mean. Inasmuch as the mean is
influenced by the largest deposits in the model tonnage-grade distribution, the median
provides a lower measure of central tendency and a more conservative estimation of
undiscovered resources.
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First posted May 27, 2003
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