Isotopic constraints on the origin of salts in Australian playas. 1. Sulphur

Abstract

The δ 34S values of a variety of gypsum types from the moderm playa lakes of southern Australia show little relationship with underlying bedrock types or the δ 34S values of the bedrock sulphur. On a regional scale, in both Western Australia (Yilgarn Block) and South Australia, the δ 34S values of surficial gypmsum have a regular pattern over distances of 500 to 1000 km with highest values ( ≈ +21%‰) near coastlines decreasing to δ 34S values of ∼ 14% further inland. Sea-salt sulphate is the dominant source of sulphur to the lakes investigated, although the proportion decreases from ∼ 100% near coastlines to ∼ 55% in some inland areas. A secondary source of sulphate is also airborne, but derives from volatile biogenic sulphur compounds of largely marine origin. Sulphur derived from rock weathering is a minor component, except in areas where the bedrock contains abundant sulphur. Thus Lake Cadibarrawirracanna receives ∼ 10% of its sulphur from weathering of the pyritic Bulldog Shale (Cretaceous), and Lake Amadeus, possibly up to one-third of its sulphate from evaporites of the Bitter Springs Formation (Late Proterozoic). Our δ 34S measurements are the first tracers to directly establish the marine origin of components in Australian surface brines. The δ 34S analyses indicate the accession of sulphur from both sea-salt and marine biogenic sulphur and clearly support delivery of salts to the Australian landscape as aerosols following established wind patterns. The data negate the possibility of derivation of the lacustrine “seawater-like” brine chemistry from either marine transgression or weathering of connate salts from marine strata alone. The occurrence of non-marine evaporites exhibiting largely marine-like chemistry and isotopic signatures is probably common to low-latitude tectonically stable areas and will make the distinction between some marine and non-marine evaporites from the geological record more difficult. The regular pattern of δ 34S values of surficial sulphate in lakes and groundwaters in southern Australia provides an ideal baseline against which to search for anomalous δ 34S values associated with base-metal or gold mineralization.