Abstract

Visible and near infrared (450-1,000 nm) reflected radiance spectra of the ground cover vegetation were measured at the Mesatchee Creek porphyry copper prospect in Washington. The reflected radiance data are integrated over 400-m 2 surfaces of a forest dominated by Douglas fir with lesser amounts of western larch. Analysis of the reflected radiance data indicates that the spectra from within the pyrite halo of the deposit have anomalously high reflected radiance values at 565 nm and low reflected radiance values at 465 nm. Six flight lines were flown on each of two days. Taking one flight line from the first day's data as the control line, individual spectra with a 565 nm/465 nm reflected radiance ratio value greater than 1.7 fall predominantly within the pyrite halo. When this threshold value is applied to all flight lines from the first day, 36.8 percent of the spectra within the pyrite halo and 5.4 percent of the spectra outside the pyrite halo are classified as anomalous. The zone of mineralization is clearly defined by the cluster of anomalous spectra. There is an 87 percent probability of an anomalous spectrum lying within the mineralized zone. The same technique was applied to the second day's data. The threshold ratio value was optimized at 1.6 and the resultant probability of an anomalous spectrum lying within the mineralized zone is then 92 percent. Higher cutoffs improve the probability of an anomalous spectrum falling in the mineralized zone; but fewer spectra are classified as anomalous and the extent of the mineralized zone is not as well defined. The geobotanical anomaly correlates with the pyrite halo and is not preferentially concentrated within the high Cu soil geochemical zones. Comparison with results obtained in lodgepole pine areas at Heddleston, Montana, shows that different vegetation types manifest geobotanical anomalies in different spectral regions.

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