MULTIPLE FLUID INTERACTIONS RECORDED IN TOURMALINE FROM THE DOROTHY CHINA CLAY PIT, CORNWALL, UK

Tourmaline (tur) can record geochemical signatures of its host rock and environment of formation, including multiple generations of fluid phase interactions. Fluid-mineral interactions in tur physically manifest as intricate chemical-zoning patterns, dissolution features, and recrystallization textures. These processes are highlighted in a tur sample from the Dorothy China Clay Pit (CCP), St. Austell, Cornwall, U.K. The sample is composed of elaborately zoned tur (85%), with crystals ranging in size from 1mm – 2 cm, within a quartz matrix (15%). Chemical and textural characterization of tur grains was carried out primarily through EPMA and BSE analyses.

A large euhedral crystal oriented along the c axis displays zoning from the base to the tip, which is replaced by feathery zoning and fibrous tur, and surrounded by an oscillatory zoned rim. A late homogeneous tur overgrows these crystals and appears to also partially replace the matrix quartz. A lengthwise traverse of 275 analytical points reveals a wide variety of tur species within the single grain, reflective of the fine scale compositional heterogeneity. The core is composed of Li-bearing species, rossmanite, elbaite, and fluor-elbaite which are partially replaced by Fe-richer species fluor-schorl, oxy-schorl, schorl and foitite. The outer rim predominantly consists of Fe-bearing oxy-dravite. Such zoning and replacement textures suggest at least four distinct generations (gen) of tur growth. Gen 1 Li-rich tur, Gen 2 schorl, Gen 3 oxy-dravite with Gen 4 overgrowing the previous tur generations and quartz. The extreme compositional variation and fine-scale zoning result from multiple stages of fluid interactions in an open system. When new fluids infiltrated, tur dissolved and reprecipitated to attain equilibrium. Likely the original elbaite crystallized in a hydrothermal environment. Gen 2 tur formed by infiltrating fluids richer in Fe, B, Na, and F which removed Li and Al. Gen 3 fluids had lower Na and higher Mg. The final fluids, richer in Fe, B, Na, Al, and F with low Si, resulted in Gen 4 Fe-rich schorl overprinting the original matrix phase, retaining its habit. This study provides a detailed chemical characterization of tur from the Dorothy CCP deposit, underscoring its utility as a geochemical recorder of hydrothermal activity reflected by multi-generational self-cannibalization of tur grains.