Feldspathic quartzite and metapelite of Middle Proterozic age north of Gouverneur, N.Y., contain abundant dravite-uvite (magnesian tourmaline). These rocks, more than 1,000 feet thick, are regionally metamorphosed to the upper amphibolite facies, are pyritic, and locally contain porphyroblastic scapolite. The rocks are underlain by talctremolite schist and a thick sequence of calcitic and dolomitic marble, and are overlain by quartz, calc-silicate carbonate rock, and a pyroxene scapolite unit. All rocks are part of the metasedimentary Grenville Complex.

Individual tourmaline-rich layers in the quartzite are as thick as 3 cm and contain as much as 50 percent finegrained, brownish-green dravite. Subjacent talc schist contains fine-grained amber dravite, and silicated marble locally contains brown porphyroblasts of uvite. Gneissic granite, pegmatite, and migmatitic segregations cutting the tourmaline-bearing quartzite, gneisses, and schists are rich in black schorl.

Tourmalines from the tourmaline-rich quartzite and other metasedimentary and metaigneous rocks were analyzed by means of an electron microprobe. The compositions of tourmalines in the North Gouverneur area are clearly a function of the bulk composition of the rock. Tourmaline compositions in the quartzite have a wide range in FeO/(Fe+MgO) ratios, from 0.15 to 0.58, which are distinct from the ratios for tourmalines in granitic and pegmatitic rocks that range from 0.55 to 0.75. Tourmalines in the area have Na2O contents that range from 0.85 to 4.25 weight percent. Aluminum in all tourmalines ranges widely from about 25 to 37 weight percent AI 2O3 . The most heterogeneous tourmaline compositions are in the quartzites, although most compositions cluster in the dravite-uvite solid solution series. Substitutions involving Na, Ca, Mg, Fe, and Al result in the following compositional schemes: Na+Fe = Al+vacancy, and Na = Ca. Such coupled substitutions probably represent valid constraints for tourmalines from North Gouverneur.

The abundance of magnesian tourmaline in the metasedimentary rocks of the North Gouverneur area is comparable to that of tourmaline-rich rocks associated with sedimenthosted massive sulfide deposits for example, Sullivan Mine, British Columbia. A recently described large lead-zinc deposit at Dugald River, Australia, is associated with tourmaline- and scapolite-bearing rocks similar in many respects to those north of Gouverneur. Compared with known tourmaline compositions in the Sullivan Mine area, tourmalines from North Gouverneur have comparable FeO and MgO values. We suggest that other tourmaline components such as a low TiO2 content, variable Na/(Na + Ca + K) ratios, and the antipathetic correlation of Na and Ca might also be useful in discriminating tourmalines associated with ore deposits.

Ore bodies at the nearby Balmat-Edwards mining district are in marbles associated with rocks of evaporitic origin. The boron- and scapolite-rich rocks under consideration in this study possibly also are related to an evaporite-producing environment. Similar rocks that have an evaporite origin are associated with stratabound sulfide deposits at many places.