LARS Tech Report Number



Gossans in the Mt. Bross area of the Alma mining district, Colorado, were characterized by use of iron oxide mineralogy and spectral reflectance as a possible aid to mineral exploration using remotely sensed data. Mine site gossan samples produced by the weathering of lead-zinc replacement deposits were found to be composed primarily of goethite, and nonmine samples produced by the weathering of pyrite in the local country rock were found to be composed primarily of jarosite. A three stage genetic model was proposed to explain the observed iron oxide assemblages, whereby goethite precipitated early and was followed by jarosite and hematite.

Bidirectional reflectance factor was measured on undisturbed sample surfaces with an Exotech 20C spectroradiometer from 0.5 µm to 2.35 µm. A pressed barium sulfate powder reference was used for calibration. The samples were organized into two groups. The first contained goethite as the major oxide. The spectra showed a 0.65 µm shoulder, broad 0.94 µm absorption, and low reflectance factor in the visible and near infrared (13 percent at 0.75 µm). The other group was mainly hematite; the spectra showed a weak 0.65 µm shoulder, sharp 0.85 um absorption, and high reflectance factor (37 percent at 0.75 µm.) It appeared that hematite, although a minor constituent in the nonmine gossans when compared to jarosite, was spectrally dominant.

Exploration programs, using spectral reflectance studies to characterize gossan types, should be undertaken with care because iron oxide genesis is influenced by a number of physio-chemical factors that can produce similar mineralogies and spectral characteristics from different parent sulfide assemblages. In the Alma district, the observed spectral differences between gossans appeared to be indirectly a function of wall rock chemistry and not parent sulfide assemblage.

Date of this Version

January 1983