PACROFI VI - Electronic Program

Tracing mantle volatiles in ancient hydrothermal fluids: He-Ar isotopes in fluids inclusions from Dae Hwa W-Mo deposit, S. Korea.

F.M. Stuart1, P.G. Burnard2, R.P. Taylor3 and G. Turner2

  1. Isotope Geosciences Unit, S.U.R.R.C., East Kilbride G75 0QF, UK
  2. Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK
  3. Department of Earth Sciences, Carleton University, Ottawa, K1S 5B6, Canada

Introduction
While the rare gases have proved powerful tracers of the source and interaction histories of contemporary fluids, their use in tracing ancient, deep fluids has largely been overlooked due to potentially major problems of diffusive loss from, and addition of in situ components to, fluid inclusions. These problems can be overcome by extracting inclusion-hosted volatiles by in vacuo crushing minerals with low ionic porosites and radio-element contents. This opens the way for using He, Ne and Ar isotopes to provide information which has been hitherto unavailable about, for example, the heat source and magmatic volatile contribution to hydrothermal fluids. We demonstrate this using a detailed study of the aqueous hydrothermal fluids trapped in gangue and ore minerals from the granite-related Dae Hwa W-Mo deposit in S. Korea.

Mantle He and heat in an ancient hydrothermal system
Fluids from the complete paragenesis at Dae Hwa record 3He/4He which can be considered as mixtures between mantle-derived He (1 x 10-5) and radiogenic He produced by U-Th decay in the crust (3 x 10-8). This provides unequivocal evidence that mantle melting was the heat source for granite formation and the subsequent mineralisation. While the fluid inclusion composition, temperature and chemistry reflects a progressive dilution of magmatic fluids by meteoric water [1], inclusion-hosted He isotopes show that the addition of juvenile volatiles into the hydrothermal system has been independent of evolution of the hydrothermal system (Fig. 1). Mantle-derived He dominates the fluids hosted by pyrite, chalcopyrite and scheelite (3He/4He ~ 5 x 10-6) from the iron sulphide-scheelite stage (230-320oC). Hotter fluids (320-400oC) from the earlier wolframite-molybdenite stage have a significantly greater radiogenic crustal contribution (1.5-5 x 10-7). Correcting the measured 3He/4He using the previously identified coupling of 3He and 40Ar in fluids from iron sulphide-scheelite stage minerals [2] it appears that the low 3He/4He of the early fluids is not due to the liberation of crustal radiogenic He during heating associated with granite intrusion. It is more likely that this represents a time lag between the peak of hydrothermal activity (which is probably temporarily related to granite intrusion) and the maximum flux of mantle-derived volatiles into the hydrothermal system. This may be due to different transport mechanisms for He and heat, perhaps related to fracture propagation into a deep reservoir of magmatic volatiles.


Figure 1. Schematic representation of the 3He/4He (normalised to air ratio: Ra) of inclusion fluids from Dae Hwa arranged in paragenetic order.

Short-timescale variation in input of magmatic volatiles
A detailed study of fluids from individual zones of a large scheelite crystal demonstrates that the addition of magmatic (mantle-derived) volatiles to the hydrothermal system is episodic and un-related to changes in fluid composition. The progressive dilution of the magmatic fluid by meteoric waters, as described by decreasing d18Ofluid (+4 to -4 o/oo) and fluid inclusion homogenisation temperatures (305-240oC) across the crystal, is not reflected in the He and Ar isotope systematics. A sharp 3He/4He increase is recorded by fluids from zone D (Fig. 2) which is correlated with Ar isotopes, but coincides with barely detectable shifts in d18Ofluid and fluid temperatures. Spatial and temporal changes in He isotopes are seen in contemporary geothermal fluids [3] which may be related to deep magmatic activity or fluid boiling. Evidence for the existence of a vapour phase in the hydrothermal fluids is clear, though it is no more so in zone D than elsewhere. It cannot be ruled out that the pulses of magmatic volatiles originate by episodic fracturing at depth.


Figure 2. Variation of rare gas, d18O and fluid inclusion temperatures across zoned scheelite SCH-3.

References

  1. Shelton et al. (1987). Economic Geology, 32, 471-481.
  2. Stuart et al. (1995). Geochimica Cosmochimica Acta, 59, 4663-4673.
  3. Sorey et al. (1993). J. Geophys. Res., 98, 15871-15889.