1988 Volume 38 Issue 209 Pages 263-278
Chemical compositions and characteristics of zoning patterns in sphalerite from 16 Besshi-type, skarn, vein, and Kuroko deposits were examined by electron microprobe. Compositional zoning of FeS in sphalerite varies according to the deposit type. Ore textures and compositional zoning of minerals are generally complex. The remarkable zoning heterogeneity in sphalerite which has not been metamorphosed suggests these features are primary. Sphalerite from metamorphosed Besshi-type deposits is characterized by a distinct lack of compositional zoning.
If a deposit has been thermally metamorphosed, originally zoned sphalerite may have been compositionally homogenized by the diffusional migration of atoms. The average grain size of sphalerite from strongly metamorphosed deposits is less than 1 mm.If the mean displacement of cations in sphalerite is assumed to be 1 mm at annealing temperatures of 300° and 250°C, calculated times of homogenization based on experimentally determined diffusivities are 23 and 470 million years, respectively. Sanbagawa metamorphic rocks range in age from 60 to 100 Ma; therefore, Besshi-type deposits could have been subjected to temperatures over 300°C for several tens of million years.
In skarn deposits, it is feasible that post-depositional diffusion in the cooling stage of plutonic intrusion related to mineralization generates gently curved zoning patterns of iron and manganese in the margins of sphalerite crystals. However, the compositional inhomogeneity of sphalerites from some other types of deposits appears to be precluded because of weak post-depositional annealing activity.
