Alternative interpretation of sulphur isotope ratios in the McArthur lead-zinc-silver deposit

SMITH and Croxford 1 have reported important new measurements of variations among the isotopes of sulphur in the enigmatic stratiform sulphide deposit at McArthur River, Northern Territory, Australia. The data place severe constraints on processes capable of forming the deposit, and Smith and Croxford interpret them to indicate a syn-sedi-mentary origin with sulphur supplied from two separate sources—one for pyrite and one for galena and sphalerite. We suggest that Smith and Croxford have misinterpreted several important aspects of their data and that they have thereby been led to erroneous conclusions. The isotopic data do not really support their preferred models of mineralisation. Rather, they support the kind of single-sulphur source model demonstrated for many other stratiform base-metal sulphide deposits, in which dissolution of pre-existing pyrite by a circulating, metal-rich but sulphur-poor brine is followed by deposition of base-metal sulphides. Brines of the kind capable of forming such deposits have been reported from the Salton Sea geothermal field 2 , from the Cheleken region in Russia 3 , in oil fields 4 and in fluid inclusions 5 . The most complete elucidation of such a model is that by Brown 6 for the White Pine deposit in Michigan but the applicability of the model to many deposits has been pointed out by White 7 and others.