COMPOSITIONAL ZONING IN ZINCIAN SPINEL

Tincian spilel (7-n,Fe, Mg)Al2Oa has been oramined for compositional 26ning. The samples come from fifty-two widely scattered localities, in metamorphosed massivesulfide deposits, pegmatites, aluminous metasediments, metamorphosed oxide-silicate deposits, a marble, and a syenite. Most zoned crystals of zincian spinel exhibit an enrichment in Zn, witl depletion in Fe and Mg, from ttre core to tie rim. This pattern is more common in samples from metamorphosed massive-sulfide deposits than any other gsological settings; it results primarily from partitioning Zn and Fe between their sulfides and zincian spinel, by sulfurization and oxidation reactions, during retrograde metamorphism. In some of these deposits and some aluminous metasediments, zoning in zincian spinel is due to partirioning of Zn, Fe and Mg between zincian spinel and adjacent ferromagnesian silicates. Zincian spinel from the Broken Hill metamorphosed massive-sulfide deposit laftqaA) varies up to 30 mole tlo Zn!J^2Oa,20 moie go FeAl2Oa and 7 mole 7o MCAI2O4. Spinel fiom otler localities, however, shows a less pronounced compositional variation. The uncommou opposite trend noted in zincian spinel, i.e., incrase in Fe and Mg with decrease in Zn from core to rirn, may be due to growth during prograde metamorphism.