Occurrence and chemical composition of green garnet from the Ulsan mine,Republic of Korea.

Abstract

Green garnet in skarn zones surrounding magnetite-scheelite ore pipe at the Ulsan mine, Republic of Korea, has been investigated mainly with electron microprobe. As a result, it has been revealed that the garnet compositions in most cases fall within the field of chromian grossular, but some of them within that of aluminian uvarovite. It occurs as microscopic grains less than 100 μm across. Two distinctly different modes of its occurrence are distinguished; one is irregularly-shaped clusters filling the interstices between fine granular aggregates of the pre-existing calc-silicates, sporadically distributed within the host skarns, and some of them are intergrown intimately. with chromite; and the other is fillings of hairline fractures which cut the skarns. These skarns containing green garnet have the silicate assemblage of clinopyroxene-garnet (grandite solid-solution series) ± amphibole ± epidote ± idocrase ± biotite ± chlorite, and they are considered to have derived almost always from hornfelses which have originated from the argillaceous rocks intercalated with Upper Cretaceous volcanic sequence as thin layers and contain usually chromite in addition to green garnet. In transmitted light, the garnet is emerald green in colour and optically anisotropic. It contains appreciable amounts of chromium and is characterized by a considerable titanium content. The chromium content is quite variable by individual grains and it falls within the range from 1.90 to 16.46 weight percent Cr2O3. No significant compositional zoning within single grains is recognized, however, on rare occasions the particles consisting of core with Cr-free grandite composition and the border with chromian grossular composition may be encountered. The chemical composition of the grain with maximum chromium content is; SiO2 37.49, TiO2 0.85, Al2O3 6.36, Cr2O3 16.46, Fe2O3 2.90, MnO 0.00, MgO 0.09, CaO 35.40, Total 99.54 (all in weight percent) corresponding to the formula on the basis of O=12; (Ca3.07Mg0.01 Mn0.00)Σ3.08(Fe0.18Cr1.05Al0.61Ti0.05)Σ1.89Si3.03O12. The strongest reflections in the X-ray diffractions are; 2.98 A (60) (400), 2.66 A (100) (420), 1.932 A (40) (611, 532), and 1.592 A (55) (642). The refined cell-edge is a0=11.915 A. The garnet is considered to have formed from the components released by the breakdown of the pre-existing chromite, and pyroxene and garnet minerals, some of which have formed by the replacement of chromite by the garnet, and that the chromite grains in the host skarns are detritals in the argillaceous sediments of late Cretaceous age derived from the underlying ultramafic rocks or hypogene oxide produced during skarn formation.