Abstract

At Mamandur, southern India, zincian spinel is associated with Zn-Pb-Cu sulphide ores, metamorphosed to the granulite facies. The Zn-spinel (X Zn = 0.44-0.82) occurs in assemblages of: (a) cordierite + biotite + sillimanite + sphalerite + pyrite and/or pyrrhotite; (b) garnet + biotite + sphalerite + pyrite and/or pyrrhotite; (c) hornblende + biotite + sphalerite + pyrite and/or pyrrhotite; and (d) it also occurs in highly sheared quartz veins with sphalerite + pyrite/pyrrhotite. Two texturally and compositionally distinct modes of occurrence of Zn-spinel have been recorded from the metamorphic assemblages: one shows a polygonal, granular (equilibrium) fabric with Zn-spinel grains in textural equilibrium with cordierite in the granulite facies assemblage; the other which is involved in the formation of coronas in garnet- and hornblende-bearing assemblage of a younger paragenesis (presumably belonging to a late retrograde stage), shows distinct compositional zoning with depletion of Zn and enrichment in Fe and Mg from the rim inwards. A third mode of occurrence is as perfectly euhedral grains embedded in a quartzose matrix in quartz veins, often with sphalerite moulded over the Zn-spinel grains. From textural evidence and consideration of stoichiometric balance, Zn-spinel occurring in association with cordierite is suggested to have formed by the prograde reactions: 0.46 biotite (Mg-rich) + 1.80 sillimanite + 3.01 quartz + 0.11 sphalerite = cordierite + 0.20 Zn-spinel + 0.80 K-feldspar + 0.06 pyrite + 4(OH,F) or 0.48 biotite (Mg-rich) + 2.08 sillimanite + 2.72 quartz + 0.26 sphalerite = cordierite + 0.50 Zn-spinel + 0.83 K-feldspar + 0.13 S 2 + 4(OH,F) and, the Zn-spinels occurring in association with garnet and hornblende by the retrograde reactions: garnet + 4.53 Al 2 SiO 5 + 4.17 sphalerite = 5.38 Zn (-Fe)-spinel + 0.22 Ca-feldspar + 0.92 pyrite + 7.06 quartz + 1.23 S 2 and hornblende + 26.93 Al 2 SiO 5 + 20.17 sphalerite + 0.88 pyrite = 25.93 Zn-spinel + 2 Ca-feldspar + 56.81 quartz + 11.32 S 2 + 2(OH,F) Zn-spinel in the sheared quartz vein may be of (meta-) hydrothermal origin. Formation of sphalerite through sulphurization of gahnite is ruled out on the evidence of the perfectly euhedral nature of the spinel and the absence of any breakdown product from the spinel.

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