Thursday, October 2, 2008: 11:20 a.m.
David W. Coulter, Ph.D.
,
Overhill Imaging and Cartography LLC, Golden, CO
Matthew A. Sares
,
Colorado Geological Survey
Phoebe L. Hauff
,
Spectral International Inc.
Douglas C. Peters
,
Peters Geosciences, Golden, CO
David A. Bird
,
Colorado Division of Reclamation, Mining and Safety
Fredrick B. Henderson III, Ph.D.
,
Hendco Services
Eric C. Prosh, Ph.D.
,
Mineral & Petroleum Resource Division, Government of Nunavut
Acid mine drainage usually is measured and characterized in systems that have been active for a relatively short period. Whereas most mines are newer than a few hundred years, natural acid drainage systems exist which have been active for much longer periods of time. East and West Red mountains in the Grizzly Peak Caldera of Colorado are just such systems. These exposed high sulfide bearing porphyries generate significant acid drainage. They are situated in high terrain and are continuously eroded to expose fresh sulfides to oxidation much as mining activity exposes sulfides through excavation. The surface and downstream impacts have been developing since the retreat of the Pinedale glaciers approximately 6000 years ago. Although some exploration activities have altered the surface morphology slightly, the patterns of acid drainage mineralogy are interpreted to be dominated by natural processes.
This study utilizes airborne hyperspectral imagery to map the surface mineralogy of these systems. The hyperspectral imagery is composed of 227 bands which finely sample the reflected light between 446 nanometers and 2441 nanometers for each pixel. It was flown with a spatial resolution of 2.5 meters. The spectroscopy of each pixel is analyzed to determine the mineralogical composition of the surface. Jarosite, goethite, hematite, illite, kaolinite and dickite are mapped. Furthermore, spectral unmixing approaches are utilized to map the relative concentrations of these minerals.
The results of the analyses show that acidic seeps and springs are identified by the concentration of jarosite relative to goethite and hematite and that these minerals are distinctly zoned. Some of the identified seeps are very subtle and not apparent in the field. The clay mineralogy also appears to reflect the presence of acidic ground waters. Although shown in the context of a natural system, the technology is applicable to mine waste monitoring and characterization.
