NEW TECHNIQUES FOR USING TOURMALINE AS AN INDICATOR MINERAL FOR EXPLORATION: ANALYSIS BY LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) AND MULTIVARIATE STATISTICS

Tourmaline commonly forms in host rocks of ore deposits, both epigenetic and syngenetic, in various tectonic settings. Tourmaline’s complex composition is controlled by the composition of the host rock and/or ore-forming fluid; thus, it is used as an indicator mineral for ore exploration. In addition, its chemical and mechanical stability make tourmaline an ideal mineral for stream sediment exploration. Detailed studies of tourmaline textures and compositions have been used to unravel the complicated sequence of ore-forming processes and relate the tourmaline growth to these events. To adequately characterize the chemistry of tourmaline requires concentrations of both major and trace elements using multiple analytical tools (e.g., EMP, LA-ICP-MS)

An alternative technique for applying tourmaline as an ore indicator models chemical relationships in Laser-Induced Breakdown Spectroscopy (LIBS) spectra using the multivariate technique, Partial Least Squares Regression (PLSR). LIBS spectra record the atomic emission resulting from a laser-ablation plasma and contain information about nearly every element in the sample. PLSR recognizes relationships within the spectral database of all the samples studied that stem from the fundamental chemical signatures that distinguish ore-related tourmalines from those that crystallized in barren rock. These signatures are the relative concentrations of major and trace elements. For example, our pilot study analyzed 189 tourmaline samples from five igneous, metamorphic, and hydrothermal lithologies. Using this technique, hydrothermal tourmaline samples, including those from ore-deposits, were distinguished from other tourmalines with 92% accuracy. Trace elements that identify the type of tourmaline-bearing ore deposits (Pb, Cu, W, Ti, Au, Mn, Ni, Zn, Ga, Sr, and Ba) are present in the LIBS spectra and exert influence in multivariate models of hydrothermal tourmaline.

Advantages of LIBS analysis in mineral exploration are ease of operation, relatively low cost, lack of extensive sample preparation, automated analysis, and the ability to identify all significant chemical relationships. This approach has the potential to effectively enhance mineral exploration capabilities.