U.S.
Geological Survey
Open-File Report 01-501
Assessment
Method for Epithermal Gold Deposits in
Northeast Washington State using Weights-of-Evidence GIS Modeling
By
ABSTRACT The weights-of-evidence
analysis, a quantitative mineral resource mapping tool, is used to delineate
favorable areas for epithermal gold deposits and to predict future exploration
activity of the mineral industry for similar deposits in a four-county
area (222 x 277 km), including the Okanogan and Colville National Forests
of northeastern Washington. Modeling is applied in six steps: (1) building
a spatial digital database, (2) extracting predictive evidence for a particular
deposit, based on an exploration model, (3) calculating relative weights
for each predictive map, (4) combining the geologic evidence maps to predict
the location of undiscovered mineral resources and (5) measuring the intensity
of recent exploration activity by use of mining claims on federal lands,
and (6) combining mineral resource and exploration activity into an assessment
model of future mining activity. The analysis
is accomplished on a personal computer using ArcView GIS platform with
Spatial Analyst and Weights-of-Evidence software. In accord with the descriptive
model for epithermal gold deposits, digital geologic evidential themes
assembled include lithologic map units, thrust faults, normal faults,
and igneous dikes. Similarly, geochemical evidential themes include placer
gold deposits and gold and silver analyses from stream sediment (silt)
samples from National Forest lands. Fifty mines, prospects, or occurrences
of epithermal gold deposits, the training set, define the appropriate
a really-associated terrane. The areal (or spatial) correlation of each
evidential theme with the training set yield predictor theme maps for
lithology, placer sites and normal faults. The weights-of-evidence analysis
disqualified the thrust fault, dike, and gold and silver silt analyses
evidential themes because they lacked spatial To develop
a model to predict future exploration activity, the locations of lode
mining claims were summarized for 1980, 1985, 1990, and 1996. Land parcels
containing historic claims were identified either as those with mining
claims present in 1980 or valid claims present in 1985. Current claim
parcels were identified as those containing valid lode claims in either
1990 or 1996. A consistent parcel contains both historic and current claims. The epithermal gold and mining claim activity models were combined into an assessment (or mineral resource-activity) model to assist in land use decisions by providing a prediction of mineral exploration activity on federal land in the next decade. Ranks in the assessment model are: (1) no activity, (2) low activity, (3) low to moderate activity, (4) moderate activity and (5) high activity. |
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