Alkali-deficient tourmaline from the Sullivan Pb-Zn-Ag deposit, British Columbia

Alkali-deficient tourmalines are found in albitized rocks from the hanging-wall of the Sullivan Pb-Zn-Ag deposit (British Columbia, Canada). They approximate the Mg-equivalent of foitite with an idealized formula □(Mg2Al)Al6Si6O18(BO3)3(OH)4. Major chemical substitutions in the tourmalines are the alkali-defect type [Na*(x) + Mg*(Y) = □(x) + Al(Y)] and the uvite type [Na*(x) + Al(Y) = Ca(x) + Mg*(Y)], where Na* = Na + K, Mg* = Mg + Fe + Mn. The occurrence of these alkali-deficient tourmalines reflects a unique geochemical environment that is either alkali-depleted overall or one in which the alkalis preferentially partitioned into coexisting minerals (e.g. albite).

Some of the alkali-deficient tourmalines have unusually high Mn contents (up to 1.5 wt.% MnO) compared to other Sullivan tourmalines. Manganese has a strong preference for incorporation into coexisting garnet and carbonate at Sullivan, thus many tourmalines in Mn-rich rocks are poor in Mn (