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| Titel: |
Geochemistry of deep formation waters in the Canning Basin, Western Australia, and their relationship to Zn-Pb mineralization |
| Auteur: |
Ferguson, J. Etminan, H. Ghassemi, F. |
| Verschenen in: |
Australian journal of earth sciences |
| Paginering: | Jaargang 40 (1993) nr. 5 pagina's 471-483 |
| Jaar: |
1993-10 |
| Inhoud: |
The Canning Basin contains several Mississippi Valley-type Zn-Pb sulphide prospects and deposits in Devonian carbonate reef complexes on the northern edge of the Fitzroy Trough, and in Ordovician and Silurian marine sequences on the northern margin of the Willara Sub-basin. This study uses the ionic composition and 5D, δ18O, δ34S, 87Sr/86Sr isotopic data on present-day deep formation waters to determine their origin and possible relationship to the Zn-Pb mineralizing palaeofluids. The present-day Canning Basin formation waters have salinity ranging from typically less than 5000 mg/L up to 250 000 mg/L locally. The brines are mixtures of highly saline water, formed by seawater which evaporated beyond halite saturation (bittern water), with meteoric water ranging in salinity from low (<5000 mg/L) to hypersaline water (up to about 50 000 mg/L) formed by re-solution of halite and calcium sulphate minerals. The original marine chemical composition of the 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 sulphate reduction and later by dolomitization of carbonate. Other reactions with terrigenous components of the sediment have provided additional Ca and Sr, including a small proportion of 87Sr-rich material. The δ34S values of the bittern-containing waters are within the range over which marine sulphate has fluctuated from the Ordovician to the Holocene, although one of the hypersaline waters has a value of +6.8%, indicating SO4 of non-marine origin. The pH of the bittern-containing waters is low (about 5) and they contain significant concentrations of dissolved Fe (up to 120 mg/L). The Canning Basin bitterns appear similar in origin and chemical composition to highly saline marine brines in the Mississippi Salt Dome Basin, USA, which are known to be either metal or sulphide-rich depending on the organic content of the host rock. In the Canning Basin, mixing of the bittern water 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 fluids and/or with sulphate-containing hypersaline meteoric waters formed from the same marine evaporite sequence should produce ore-precipitating fluids which are relatively hot and saline, and the resulting ore deposit should be of high grade and contain abundant sulphate minerals. In the southern Canning Basin, this type of mixing and the corresponding style of ore deposit is evident in the evaporite-associated areas of Zn-Pb mineralization near the Admiral Bay Fault. If the bitterns mix with low salinity HCO3-waters in near-surface environments, then the ore-precipitating fluids should have relatively low salinities and carbonate minerals would precipitate during later stages of mixing. In the Lennard Shelf, the present-day formation waters, the style of the Zn-Pb deposits, and range of salinity and temperature of the ore-forming palaeofluids are consistent with this type of mixing. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
Details van artikel 2 van 7 gevonden artikelen
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