⁴⁰Ar/1b³⁹ Ar laser microprobe study of fluids in different colour zones of a hydrothermal scheelite crystal from the Dae Hwa WMo mine, South Korea

Laser probe analyses of Ar isotopes in fluid inclusions in five neutron-irradiated zones from a single scheelite crystal indicate the presence of two fluid components: (1) a Cl-rich hydrothermal fluid containing excess ⁴⁰Ar; and (2) a meteoric fluid containing dissolved Ar with a ⁴⁰Ar ³⁶Ar ratio similar to the atmospheric value, and low Cl content. Core to rim variations indicate increasing dilution of the hydrothermal fluid with the meteoric component; however, the expected decrease in ⁴⁰Ar ³⁶Ar ratio is not recorded. At present the explanation of this is not clear; however, two possibilities are that either the dilution was accompanied by a complex process of boiling and fluid mixing, or that the ⁴⁰Ar ³⁶Ar ratio of the hydrothermal fluid increased with time due to leaching or diffusive loss of ⁴⁰Ar from country rock minerals within the hydrothermal system, without significant addition of Cl.