Spectral Methodology for the Rapid Discrimination of Kimberlite Indicator Minerals using Reflected Light in the Visible Spectral Region

Exploration methods for potentially diamond bearing kimberlites involve the analysis for specific transition metals within mantle related indicator minerals using electron microprobe. The chemistry of each indicator mineral is characterized primarily on the basis of the transition metal geochemistry. These transition metals included within the grains crystal lattice produce absorption bands at specific wavelengths in the visible spectral region. Reflection spectra of kimberlite indicator minerals (pyrope garnet, chrome diopside) as well as genetically related minerals (almandine, uvarovite) have been examined. Laboratory spectra of each grain have been measured in 5 orientations in the 380 - 900 nm range. Preliminary data has suggested that: 1) First series transition metals (Fe, Mn, Cr, etc.) are evident within the spectrum of each grain as absorption bands each varying in depth, width and slope according to molecular proportions. 2) The absorption band at 690nm can be used to determine the approximate molar chromium contents of the pyrope, almandine and uvarovite phases of garnet. 3) Spectra in all 5 orientations are consistent in band depth, width and slope indicating that only one or two measurements are required for accurate results. 4) Measurements in each orientation take only a few seconds which allows for rapid identification of relevant kimberlite indicator minerals. The composition of each grain will be determined using electron microprobe and micro XRD equipment. The results will then be compared to absorption band depths and widths to determine how much of a transition metal such as chromium is necessary to intensify the spectral feature.