CAN KIMBERLITE GARNETS BE RECOGNIZED BY LIBS ANALYSIS?

A common approach used in diamond exploration is to trace mantle-derived ‘pathfinder’ minerals to their source. Garnet is particularly important in this regard as pyrope, the Mg-Al variety of garnet (Mg₃Al₂Si₃O₁₂) that is exceptionally rare in upper crustal rocks, is transported to the Earth’s surface in kimberlites and alkaline volcanic rocks produced in deep mantle sources. Kimberlite garnets associated with diamonds tend to be depleted in Ca and Ti, but enriched in Mg and Cr. A two-part study was undertaken to assess the potential of using analysis of garnet by laser-induced breakdown spectroscopy (LIBS) to distinguish garnet from kimberlites. Initially, from the analysis of some 400 samples from more than 130 locations worldwide, we documented that the common garnets can be readily differentiated on the basis of their broadband LIBS spectra through partial least squares discriminant analysis and then observed that pyrope from kimberlites could be distinguished. Overall success for discrimination of common garnet types was >90%. Significant misclassification was observed only for the Fe-Al almandines, Ca-Fe andradites and Ca-Al grossulars that form a complete solid solution series and which commonly occur in intermediate compositions. Kimberlite pyrope from South Africa was discerned from other pyropes based on differences in the intensity of emission lines for Mg, Ca, Al, Mn, Fe, Ti, Cr, and Na. Different diamond-bearing kimberlites from within South Africa could likewise be differentiated at high success, with the same seven elements responsible for discrimination.