DETERMINING HOST-ROCK ENVIRONMENT OF TOURMALINITES USING LASER-INDUCED BREAKDOWN SPECTROSCOPY

Tourmalinites, rocks containing >15% tourmaline, require local enrichments of boron and their formation primarily results from metamorphism of boron-rich units or from the infiltration of boron-bearing fluids. Tourmalinites are found commonly in stratabound layers and along fracture zones in metasedimentary and metavolcanic sequences and are frequently associated with ore-deposits. Because the compositions of tourmalines commonly reflect the host-rock environment, signatures found in tourmalinites may be useful for determining the environment of formation.

Chemical signatures from tourmalines in 70 tourmalinite samples were obtained using Laser-Induced Breakdown Spectroscopy (LIBS). Tourmalinite samples, from known source rocks and disparate geographic localities, were characterized as from meta-evaporitic, metasomatic/hydrothermal, or submarine volcanic host-rock environments and as ore-bearing or ore-barren. Up to 64 LIBS shots per sample were averaged to produce a single spectrum for each sample. All spectra were analyzed using PCA and PLSR to evaluate grouping according to lithologic type and ore association. These data were then compared with tourmaline spectra obtained from other host-rock environments, such as silicic igneous, metapelitic, metacarbonate, and metamafic rock types, to evaluate similarities and differences relative to tourmalinite host-rocks.

Tourmaline spectra from meta-evaporitic tourmalinites are distinct from the metasomatic/hydrothermal and submarine volcanic groups. Ore-related tourmalinites show slight separation from those that are barren. Elements influencing host-rock environment and ore-related groupings the strongest are Mn, Cu, Ti, and Li. Tourmalinite spectra show similarities with tourmalines from silicic igneous and metapelitic host-rock types. Thus, tourmalinite LIBS data appear to: distinguish meta-evaporitic host-rock environments from other tourmalinite environments; differentiate ore-related from ore-barren; and reflect compositional similarity to tourmalines from silicic igneous and metapelitic host-rocks, which suggests that they inherit some chemical signature of their host-rock. Tourmalinite analyses by LIBS may fingerprint tourmalines that are a useful prospecting guide for ore-bearing formations.