RESOLVING MANTLE AND CRUSTAL CONTRIBUTIONS TO ANCIENT HYDROTHERMAL FLUIDS - HE-AR ISOTOPES IN FLUID INCLUSIONS FROM DAE-HWA-W-MO MINERALIZATION, SOUTH-KOREA

Helium and argon isotopes from fluid inclusions in individual colour zones (B, C, D, and E) of a large scheelite crystal from the 88 Ma Dae Hwa W-Mo deposit, South Korea, trace the source and history of the ore fluids. A gradual decrease of the fluid He-3/He-4, He-3/Ar-36, and Ar-40/Ar-36 from the core to the edge of the scheelite reflects the progressive dilution of a magmatic fluid by meteoric water and is consistent with the previously observed decrease of delta(18)O(H2O) and fluid inclusion homogenisation temperatures (T-h) (Shelton et al., 1987). The covariation of fluid inclusion He-Ar isotope systematics with delta(18)O and T-h defines a magmatic component with He-3/He-4 = 1-2 x 10(-6), He-3/Ar-36 > 0.01, and Ar-40/Ar-36 > 1000. Anomalously high helium and argon isotope ratios in zone D fluids represents undiluted magmatic noble gases. This may reflect local variation in the magmatic gas flux or gas loss due to boiling of the hydrothermal fluids prior to mixing with magmatic gas. Helium and argon isotope systematics constrain mantle and crustal components in the hydrothermal fluids. Ar-40/He-3 (4.5 X 10(4)) are close to the mid-ocean ridge basalt value, implying that Ar-40 is mantle in origin. Radiogenic isotope ratios of the mantle endmember (Ar-40/He-4 = 0.69 +/- 0.06) are similar to contemporary geothermal fluids. The coincidence of mantle-derived He and Ar in the fluids is strong evidence that mantle melting during Late Cretaceous subduction triggered the crustal melting responsible for granite formation. Ar-40/He-4 of the meteoric fluid (0.007 +/- 0.001) is far lower than the crustal production ratio (0.2) implying an origin in crust below 200 degrees C.