Stable isotope studies of the Dae Hwa tungsten-molybdenum mine, Republic of Korea; evidence of progressive meteoric water interaction in a tungsten-bearing hydrothermal system

Abstract

The Dae Hwa W-Mo mine is located approximately 100 km southeast of Seoul within the Precambrian metamorphic belt of the southern Korean peninsula. The major ore minerals, wolframite, scheelite, and molybdenite, together with minor cassiterite, chalcopyrite, and bismuthinite, occur within fissure-filling quartz veins contained within Precambrian granitic gneiss adjacent to a granitic stock of Cretaceous age. K-Ar dates of 88 + or - 2 m.y. for vein muscovite suggest a Late Cretaceous age for W-Mo mineralization.The ore mineral paragenesis can be divided into three distinct stages: a molybdenite-wolframite stage (400 degrees -315 degrees C), an iron sulfide-scheelite stage (315 degrees -230 degrees C), and a late carbonate stage (230 degrees -150 degrees C). The delta 18 O values of vein minerals are: quartz, 10.6 to 11.4 per mil; muscovite, 8.2 per mil; cassiterite, 2.9 per mil; wolframite, 4.0 to 4.3 per mil; color-zoned scheelite, -7.3 to 2.2 per mil (cores = -0.8-+2.2ppm, edges = -7.3 - -2.7ppm); siderite, 11.8 per mil; dolomite, 7.7 to 10.0 per mil; calcite, 5.7 to 9.3 per mil. There is a nearly monotonic decrease in calculated delta 18 O values of hydrothermal waters with decreasing temperature in the Dae Hwa hydrothermal system, from values of approximately 6 per mil for quartz-muscovite-molybdenite-cassiterite-wolframite deposition, to approximately 3.5 to 0.0 per mil for iron sulfide-early scheelite mineralization, to -3.5 to -7.0 per mil for late scheelite-carbonate deposition. We believe the decrease of delta 18 O water values with paragenetic time and decreasing temperature represents the progressively increasing importance of meteoric water interaction in the Dae Hwa tungsten-molybdenum hydrothermal system.The measured and calculated ranges of delta D values of inclusion fluids and hydrous minerals are: -48 to -78 per mil for quartz-muscovite-molybdenite-wolframite deposition, -60 to -67 per mil for early scheelite deposition, and -71 to -101 per mil for late scheelite-carbonate deposition. The hydrogen and oxygen isotope compositions of hydrothermal fluids at Dae Hwa are consistent with progressive mixing of a magmatic or highly exchanged meteoric water (exchanged with granitic rocks over a series of temperatures at variable, but low water to rock ratios) with an unexchanged meteoric water (delta D = -140ppm). The stable isotope composition of the tungsten-molybdenum-mineralizing fluid is controlled predominately by the host granitic rocks.Sulfur isotope analyses of sulfide vein minerals suggest an igneous source of sulfur with a delta 34 S (sub Sigma S) value near 3.0 per mil. Together with the hydrogen and oxygen isotope data, they indicate that deposition of molybdenite, wolframite, and scheelite occurred during a period of declining temperatures from 400 degrees to 230 degrees C in response to inundation of an original magmatic system with low-temperature waters of meteoric origin.Differences in mineralogy, paragenesis, and delta 18 O values of hydrothermal fluids between W-Mo (scheelite rich) and W-base metal (wolframite rich) vein-type deposits may reflect differences in their postwolframite deposition interactions with meteoric waters. The hydrothermal system at Dae Hwa may have experienced more pervasive fracturing and a higher degree of meteoric water interaction, resulting in remobilization of tungsten and a higher scheelite to wolframite ratio than lower water to rock ratio systems such as Pasto Bueno and San Cristobal, Peru, and Panasqueira, Portugal.