TEM STUDY OF INDIUM-BEARING AND COPPER-BEARING GROWTH-BANDED SPHALERITE

An investigation by transmission electron microscopy of (Cu, In)-rich bands in sphalerite from base-metal veins in West Shropshire, England, revealed these elements to be preferentially substituted near the inn er margins of the (Fe,Cd)-rich layers within the zoned grains. The iro n-rich bands, furthermore, comprise several iron-rich and iron-poor su b-bands, the former containing oriented rows of ''chalcopyritic'' part icles, probably formed by exsolution. Opaque zones of sphalerite devel oped adjacent to late chalcopyrite are enriched in copper but contain no microscopic particles. High-resolution microscopy of the opaque sph alerite reveals the presence of Guinier-Preston (GP) zones representin g the early development of chalcopyrite. Equilibrium precipitation in response to changes in solution chlorinity, temperature and f(O2) cann ot satisfactorily explain the observed banding. Instead, the repetitiv e chemical separation of (Cu,In)- and (Fe,Cd)-rich bands is ascribed t o the preferential irreversible adsorption of these ions over differin g ranges of pH during cyclic changes of solution pH. The pattern of ba nding caused by such a mechanism is consistent with that expected from a consideration of the first hydrolysis constants of the trace elemen t cations.