Mineral chemistry of tourmalines from the Bottino mining district, Apuane Alps (Italy)

At Bottino (SW Apuane Alps, Italy), stratabound tourmalinites containing up to 80 % tourmaline by volume and associated Pb-Ag(Zn) vein mineralization are hosted within a low-grade, multiply deformed Palaeozoic basement. Chemical compositions of tourmalines from tourmalinites and host rocks fall between the end-members of the schorl-dravite series and a proton- and alkali-deficient end-member, R (sub 1-x) (super +) R (sub 3) (super 3+) R (sub 6) (super 3+) (BO (sub 3) ) (sub 3) Si (sub 6) O (sub 18) O (sub 3-x) (OH) (sub 1+x) . The extent to which proton-loss substitution (R (super 2+) + OH (super -) = R (super 3+) + O (super 2-) and alkali-defect substitution (R (super +) + R (super 2+) = R (super 3+) + vacancy) affects the tourmaline composition varies within the different lithologies, and is higher for tourmalines from tourmalinites than for tourmalines from host rocks. Among the factors that may have controlled the extent of such coupled substitutions, temperature and degree of saturation in volatiles are likely to have been more effective than bulk chemistry. Tourmalines from Bottino essentially preserve primary pre-metamorphic features. In particular, some tourmaline crystals show sharp, concentrical colour zones, interpreted to represent primary growth structures. Post-growth chemical exchange with fluids of probable metamorphic origin have led to the local development of discordant palecoloured tourmaline rims. Microprobe analyses reveal that growth and discordant zonings mainly depend upon Fe/(Fe+Mg) and Na/(Na+Ca) variations, respectively. Colour zoning of the crystals appears to be related to Ti content rather than to Fe/(Fe+Mg) variations. Except for (rare) discordant zoning, the effect of greenschist-facies metamorphism on tourmaline composition appears limited, as shown by the lack of systematic cation partitioning between coexisting tourmaline and metamorphic chlorite.