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