Lead and zinc-rich fluid inclusions in Broken Hill-type deposits: Fractionates from sulphide-rich melts or consequences of exotic fluid infiltration?

High salinity fluid inclusions (Th=200–550°C) from the Broken Hill and Cannington “Broken Hill-type” ore deposits in Australia have been analysed by PIXE and LA-ICP-MS. They have high Pb (>1%) and Zn (>1000 ppm) and Pb/Zn ratios much higher than those of the majority of crustal brines. Laser Raman studies reveal the presence of methane and PIXE images show that Pb and Zn are respectively concentrated in a Pb-K-Cl solid, and the liquid phase. This indicates that the inclusions have very low total sulphur contents. The Pb/Zn ratios are similar to those of eutectic melts in the Gn-Sp-Po system whereas Pb/Fe ratios are variable and lower than those of such melts. If these brines originated by fractionation from synmetamorphic sulphide-rich melts, then they must been greatly modified prior to entrapment. An alternative origin involving late-to post-metamorphic interaction of externally-derived brines with pre-existing sulphide accumulations should also be considered. In either case, the unique brine chemistry would appear to relate to the large amounts of sulphides in these systems.