Mineralogy and geochemistry of the Dongmyeong tungsten mine, Korea

Abstract

Skarn of the Dongmyeong tungsten mine developed at the contact between a Paleozoic limestone and a Jurassic granodiorite. Early, massive skarn is marked by diopsidic clinopyroxene and wollastonite within recrystallized limestone at the skarn front and salitic clinopyroxene within granodiorite. Later massive skarn has clinopyroxenes with salite to ferrosalite compositions and garnets with grossular-rich compositions. Most scheelite deposition occurred at the end of the early skarn stage. Late skarn formed narrow areas of hedenbergitic clinopyroxene, anisotropic garnet, wollastonite and/or quartz along fractures in limestone. Retrograde hydrous alteration is represented by breakdown of clinopyroxene and formation of calcic amphibole, calcite and quartz, accompanied by minor scheelite deposition. In the waning portions of the late skarn stage, base-metal sulfides were superimposed upon previous tungsten mineralization. Whole-rock and mineral compositions indicate that the major element chemistry of the metasomatic skarn-forming fluids was controlled largely by the granodiorite intrusion. Phase equilibria and fluid inclusions indicate that skarn evolution was the result of interaction of water-rich fluids (X CO 2 ≈ 0.1) with original lithologies at ≈ kb with declining temperature (endoskarn, ≈630°; massive skarn, ≈580°–490°C; vein skarn, ≈500°–420°C; hydrothermal sulfides, ≈300°C.