F. Stuart1, A. Boyce1, A. Foxford2,3, D. Polya2 and A. Fallick1
Where high altitude meteoric recharge fluids can be ruled out, it has been proposed that low dD hydrothermal fluids (<-90 o/oo) result from fluid interaction with organic compounds in sediments. The highly organo-philic nature of I leads to enrichments in hydrothermal fluids  which allows the link between low dD and organic-fluid interaction to be tested. Hydrothermal inclusion fluids from the Panasqueira Sn-W deposit are one example of where low dD hydrothermal fluids are ascribed to organic waters . We demonstrate that low dD fluids at Panasqueira are reflected in differences in halogen ratios which provide constraints on the source and history of the fluids which are unavailable from conventional stable isotopes.
Fluid inclusions and stable isotopes
A vug quartz crystal (Pa66) from the Main Sulphide Stage of mineralisation display growth zones that represent a time series of hydrothermal fluids. Fluid inclusions suggest that the zoning does not reflect a significant change in fluid composition. Primary fluid inclusions dominate, they are two phase (liquid and vapour) aqueous inclusions which have rather constant salinites (7.3-8.7 wt.%) and homogenisation temperatures (254-260oC). Moreover, interzonal d18Ofluid variation is small (3.7-4.4 o/oo) and is consistent with only small changes in fluid temperature. However, replicate measurement of dDfluid exhibit variation from typically magmatic values in the earliest fluids (-65 o/oo) to low values (-110 o/oo) in the latest fluids (Fig.1.) This range cannot be due to subtle changes of fluid temperature but must reflect changing fluid composition.
Ar isotope systematics
40Ar excesses (40Ar*) are present in fluids from all zones. A striking increase of 40Ar/36Ar with dD is due to an increased contribution of 36Ar to the low dD fluids. This is likely due to simply to atmospheric contamination. Fluids from all zones display a limited range of 40Ar*/Cl (9 - 19 x 10-6). This rules out boiling as a source of the interzonal dD variation.
Cl/Br (216 - 400) are lower, and I/Br (0.036 - 0.078) are slightly higher, than other magmato-hydrothermal fluids . The data define a positive correlation on a Cl/Br v. I/Br plot (Fig. 2) which can be interpreted as a mixture between two fluids. The halogen (Cl/Br > 400; I/Br > 0.08) and dD values (-60 to -70 0/00) of the Br-poor competent are consistent with a magmatic-hydrothermal source for this fluid. Thus the low dD fluid is enriched in Br relative to Cl and I relative to the magmatic fluid. Cl/Br lower than the seawater value (600) are generated by surface evaporation which is vigourous enough to induce halite precipitation. Due to the absorption of I on organic material, seawater has a low I/Br (0.0005). Thus the observed trend is consistent with a residual brine origin for the low dD fluid and there is no evidence for significant I-enrichment that might reflect an organic source. Permo-Triassic evaporites are regionally abundant and 40Ar39Ar dating of the mineralisation is underway to ascertain whether the evaporites are chronologically significant.
If the halogen ratio variation does indeed reflect fluid mixing, the constancy of the fluid temperature and d18Ofluid requires a rather fortuitous similarity in the compositions of the two fluids. Further work is being undertaken to test this.
Figure 1. The distribution of fluid temperature, salinity and stable isotope compositions across zoned quartz (Pa66). Note the large change in dDfluid that is unrelated to fluid temperature and d18O.
Figure 2. Halogen ratio variation in zones of Pa66 quartz. The zones charcterised by low dD fluids (A, B & C) are enriched in Br compared to the magmatic fluids.