| Zincian spinels {Zn,Fe,Mg)Al(,2)O(,4)} are common constituents of metamorphosed base metal sulfide deposits and of some aluminous metasediments, marbles and pegmatites. In and near metamorphosed sulfide deposits, zincian spinels have formed by desulfidation and oxidation reactions of the type: (i) xZnS + (1 - x)FeS + Al(,2)SiO(,5) + 1/2 O(,2) = xZnAl(,2)O(,4) + (1 - x)FeAl(,2)O(,4) + SiO(,2) + 1/2 S(,2); (ii) Fe(,3)Al(,2)Si(,3)O(,12) + xZnS + S(,2) = xZnAl(,2)O(,4) + (1 - x)FeAl(,2)O(,4) + (2 + x)FeS + 3SiO(,2) + O(,2); and (iii) xZnS + (1 - x)FeS + Al(,2)O(,3) + 1/2 O(,2) = xZnAl(,2)O(,4) + (1 - x)FeAl(,2)O(,4) + 1/2 S(,2). Experimental reversals of reaction (iii), using hydrothermal techniques, have been obtained at 2 and 5 kb at various temperatures using pyrrhotite-pyrite-magnetite as a buffer. Reactions (i), (ii) and (iii) have been located in fO(,2) - fS(,2) - X space by thermodynamic calculations in which the ZnAl(,2)O(,4) - FeAl(,2)O(,4) join is assumed to be ideal as is suggested by octahedral site preference energies. The calculations vary markedly depending on the choice of free energy of formation of hercynite. Experimental results, theoretical calculations and natural compositions are most consistent when extrapolated free energy data of Pillary et al. (1960) and McLean and Ward (1966) are used.;Spinels associated with metamorphosed massive sulfide deposits, marbles and aluminous metasediments are enriched in the gahnite (ZnAl(,2)O(,4)), spinel (MgAl(,2)O(,4)) and hercynite (FeAl(,2)O(,4)) component, respectively, whereas those associated with pegmatites are depleted in the spinel component. Such variability in composition with environment constitutes a potential exploration guide for metamorphosed sulfide deposits. The composition of spinels in pegmatites, marbles and aluminous metasediments is controlled largely by that of the host rocks. In metamorphosed sulfide deposits, P, T, fO(,2) and fS(,2) are the primary factors determining composition.;Zincian spinels that are compositionally zoned are most often observed in metamorphosed massive sulfide deposits. An increase in gahnite component, a decrease in hercynite component and, to a lesser extent, a decrease in spinel component from center to edge is the most common compositional pattern in zincian spinels. Although this trend can be formed by growth of the spinel during falling temperatures alone, it is enhanced by the partitioning of zinc and iron primarily between members of the Zn-Fe-S system and spinels as the latter grew by desulfidation of sphalerite during metamorphism. |
