ALKALI-DEFICIENT TOURMALINE FROM THE SULLIVAN PB-ZN-AG DEPOSIT, BRITISH-COLUMBIA

Alkali-deficient tourmalines are found in albitized rocks from the han ging-wall of the Sullivan Pb-Zn-Ag deposit (British Columbia, Canada). They approximate the Mg-equivalent of foitite with an idealized formu la square(Mg2Al)Al6Si6O18(BO3)(3)(OH)(4). Major chemical substitutions in the tourmalines are the alkali-defect type [Na((X)) + Mg*((Y)) = square((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 envir onment that is either alkali-depleted overall or one in which the alka lis preferentially partitioned into coexisting minerals (e.g. albite). Some of the alkali-deficient tourmalines have unusually high Mn conte nts (up to 1.5 wt.% MnO) compared to other Sullivan tourmalines. Manga nese has a strong preference for incorporation into coexisting garnet and carbonate at Sullivan, thus many tourmalines in Mn-rich rocks are poor in Mn (<0.2 wt.% MnO). It appears that the dominant controls over the occurrence of Mn-rich tourmalines at Sullivan are the local avail ability of kin and the lack of other coexisting minerals that may pref erentially incorporate Mn into their structures.