Zinc-lead skarns of the Yeonhwa-Ulchin District, South Korea

Abstract

The Yeonhwa I, Yeonhwa II, and Ulchin mines, producing 13.5 million metric tons of ore in the period 1962 to 1981, account for 80 percent of South Korea's Zn and Pb production. These skarn deposits occur in basal Cambrian limestone (locally dolomitic) overlying slate, quartzite, and Precambrian granitic basement, and they are the result of hydrothermal activity associated with felsic calc-alkaline plutonism of Late Cretaceous to early Tertiary age.Skarn at Yeonhwa I consists of pipes of pyroxene (+ or - garnet) at depth branching to veins of rhodocrosite-pyrite near the surface in an area of sparse quartz-porphyry dikes. In contrast, skarns at Yeonhwa II and Ulchin display higher garnet/pyroxene ratios and occur as stratabound lenses and irregular masses at or near contacts with larger bodies of quartz monzonite. Exoskarns are zoned in the sequence garnet-pyroxene-(pyroxenoid)-marble relative to slate-limestone contacts or fissures in limestone. The lack of zoning relative to igneous contacts and the presence of porphyry-limestone contacts without skarn indicate that skarn-forming fluids did not originate from magma at the present level of exposure.Electron microprobe analyses reveal that calc-silicates are notably enriched in Mn and Fe. Garnets are intermediate grandite-andradite with 1 to 5 mole percent spessartine. Calcic rhodonite is characteristic of endoskarns and garnet zones, whereas a more calcic pyroxenoid, manganoan bustamite, occurs in pyroxene skarn. Pyroxenes are manganoan salite-ferrosalite; average Fe/Mn ratio (and mole % hedenbergite) in pyroxene decreases from 5.7 (65%) at Ulchin, to 3.5 (59%) at Yeonhwa II, to 1.6 (30%) at Yeonhwa I. This ratio also decreases with distance from garnet zones and correlates with an increase in Mg, suggesting a progressive depletion of Fe in solution due to precipitation of iron-rich garnet near channelways, and a progressive enrichment in Mg in the fluid as it approached equilibrium with dolomitic wall rocks. Based on this and the geologic setting, Ulchin is the most proximal and Yeonhwa II the most distal skarn in terms of distance from their sources of fluids.Pyrrhotite, accompanied by sphalerite, lesser galena, and minor chalcopyrite, are concentrated in pyroxene zones variably altered to chlorite, Mn-rich clays, calcite, quartz, and fluorite. Pyroxene compositions indicate that the maximum oxidation-sulfidation state that could have been imposed on the ore fluid by early skarn was located near the pyrrhotite-pyrite-magnetite buffer. Only the hedenbergitic skarn at Ulchin lay below this buffer and could have controlled the precipitation of pyrrhotite rather than pyrite. Ulchin also contains the district's highest grades of Pb + Zn in skarn, suggesting that reduction of aqueous sulfate by ferrous iron was an important factor in ore deposition. Later ore fluids, and fluids that reached beyond skarn in the most distal deposit, envolved to higher sulfidation states, as demonstrated by pyrite-rhodochrosite veins that cut skarn and pyrrhotite ore, and by the near-surface pyrite-sphalerite-galena-rhodocrosite veins in limestone overlying skarn pipes at Yeonhwa I.