GEOCHEMISTRY OF DEEP FORMATION WATERS IN THE CANNING BASIN, WESTERN-AUSTRALIA, AND THEIR RELATIONSHIP TO ZN-PB MINERALIZATION

The Canning Basin contains several Mississippi Valley-type Zn-Pb sulph ide prospects and deposits in Devonian carbonate reef complexes on the northern edge of the Fitzroy Trough, and in Ordovician and Silurian m arine sequences on the northern margin of the Willara Sub-basin. This study uses the ionic composition and delta D, delta(18)O, delta(34)S, Sr-87/Sr-86 isotopic data on present-day deep formation waters to dete rmine their origin and possible relationship to the Zn-Pb mineralizing palaeofluids. The present-day Canning Basin formation waters have sal inity ranging from typically less than 5000 mg/L up to 250 000 mg/L lo cally. The brines are mixtures of highly saline water, formed by seawa ter which evaporated beyond halite saturation (bittern water), with me teoric water ranging in salinity from low (<5000 mg/L) to hypersaline water (up to about 50000 mg/L) formed by re-solution of halite a nd ca icium sulphate minerals. The original marine chemical composition of t he bittern-dominated brines was changed to that of a Na-Ca-Cl water by addition of Ca and removal of Mg and SO4, initially by bacterial sulp hate reduction and later by dolomitization of carbonate. Other reactio ns with terrigenous components of the sediment have provided additiona l Ca and Sr, including a small proportion of Sr-87-rich material. The delta(34)S values of the bittern-containing waters are within the rang e over which marine sulphate has fluctuated from the Ordovician to the Holocene, although one of the hypersaline waters has a value of +6.8 parts per thousand, indicating SO4 of non-marine origin. The pH of the bittern-containing waters is low (about 5) and they contain significa nt concentrations of dissolved Fe (up to 120 mg/L). The Canning Basin bitterns appear similar in origin and chemical composition to highly s aline marine brines in the Mississippi Salt Dome Basin, USA, which are known to be either metal or sulphide-rich depending on the organic co ntent of the host rock. In the Canning Basin, mixing of the bittern wa ter with the various types of meteoric water has resulted in decreases in salinity, Na, Ca, Mg, K, Sr, Li and Fe, and increases in HCO3, SO4 and pH. Mixing of the bitterns with other types of metalliferous flui ds and/or with sulphate-containing hypersaline meteoric waters formed from the same marine evaporite sequence should produce ore-precipitati ng fluids which are relatively hot and saline, and the resulting ore d eposit should be of high grade and contain abundant sulphate minerals. In the southern Canning Basin, this type of mixing and the correspond ing style of ore deposit is evident in the evaporite-associated areas of Zn-Pb mineralization near the Admiral Bay Fault. If the bitterns mi x with low salinity HCO3-waters in near-surface environments, then the ore-precipitating fluids should have relatively low salinities and ca rbonate minerals would precipitate during later stages of mixing. In t he Lennard Shelf, the present-day formation waters, the style of the Z n-Pb deposits, and range of salinity and temperature of the ore-formin g palaeofluids are consistent with this type of mixing.