The genesis of the Sangdong tungsten deposit, the Republic of Korea

Scheelite mineralization at Sangdong is probably related to a late Cretaceous episode of igneous activity following the Daebo Orogeny in the Korean peninsula. It occurs. in stratabound skarns that replace Cambrian limestones of the Myobong Slate or the overlying Pungchon Limestone, and also in a number of associated quartz veins. These sediments in the Sangdong area overlie the Jangsan Quartzite and Precambrian schists and lie in the southern limb of the Hambaeg Syncline, striking N 70-80¬¨‚àûE and nipping 25-35¬¨‚àûNW. The M1(6 m thick) and F.(0.3 to 0.8 m thick) orebodies are stratiform, extend about 1.2 km along strike and down dip, and grade laterally to interbedded limestones in the Myobong Formation. H1 orebody has an irregular shape, bounded on the bottom by the Myobong slate and on top by Pungchon limestone, with thickness varying from 10 to 100 m. The M1 and F. orebodies show a generalized zonal distri bution of the major component minerals: a central quartz-mica zone is surrounded by an amphibole-rich zone, which in turn is enveloped by a pyroxene-garnet zone. Scheelite is highly concentrated in the quartz-mica zone where the grades-average 6 % WO3. No zonation and little mica is observed in the H1 orebody. Relics of pyroxene-garnet skarn in the quartz-mica zone, relict blocks of limestone in the pyroxene-garnet skarn zone, and small-scale veins and fractures with quartz-mica skarns rimmed by amphibole and the pyroxene garnet skarn, demonstrate the sequential metasomatic replacement of lime stone. Early garnet-pyroxene¬¨¬±wollastonite skarns are replaced by late pyroxene-garnet skarns, then amphibole skarn, and finally, mica skarn. Geochemical analyses, fluid inclusion and stable isotope data assist in defining fluid composition and source, and P - T conditions. ˜í¬•180, ˜í¬•34s and ˜í¬•13c of ore minerals indicate a derivation from magmatic fluids, and the high temperatures indicated by the mineralogy and fluid inclusions (up to 600c) strongly suggest a magmatic heat source. Though drilling has not encountered granite to a depth of 500 m below the mine, a granitoid pluton is inferred within a kilometre of the deposit. Tectonic fracturing may have caused a change from lithostatic to hydrostatic pressure(from 800 to 300-400 bars), while precipitating pyroxene-garnet at 350-500¬¨‚àû0, and the fluid.boiled. Later in pyroxene garnet growth, lithostatic pressure was restored, possibly because of fracture sealing by mineral precipitation. In the M1 and F. ore zones, hydrous skarns formed mainly during the non-boiling phase, growing outwards from a central feeder zone marked by a concentration of footwall veins. The skarn assemblages appear to approach equilibrium with the hydrothermal fluid, justifying the use of phase equilibria. Fluid inclusion evidence indicates that the major solution species were NaCl, KCl, CaC12 and MgCl, with an average salinity of about 1 m NaCl. There was also localized development of co2-rich and more saline fluids. The f02- fs2 conditions were close to the pyrrhotite-magnetite-pyrite point with f02 increasing slightly_ towards the central core, and 5s::s = 2.5 x 10-3 m. Part of the mica zone is muscovite-rich and probably lower temperature( 350¬¨‚àûc); the pH range estimated for this assemblage, assuming mK+=0.1, is between 4.2 and 5.9. Under these conditions, tungsten was transported mainly as HW0-4 and at pH = 4, the maximum aCa++ in he fluid to maintain 2 ppm W in solution in equilibrium with scheelite is 10-7. at 350¬¨‚àû0. Double diffusive mechanisms may have maintained steep temperature and chemical gradients at the margins of the skarn zones, with the central, hot, dilute magmatic plume in contact with an outer envelope of hot, saline groundwater. Local and occasional mixing, or influx, of groundwater may have given rise to the saline fluids identified locally in fluid inclusions. The quartz veins appear to have formed in similar P-T-X conditions, with wolframite- and molybdenite-bearing veins forming at higher temperatures tnan sulphide-bearing veins. Late fluid circulation involved formation of calcite and local hematite, with ˜í¬•13ctvalues indicating non-magmatic fluid.