The carbonate-hosted silver-lead deposits of the ediacara mineral field, South Australia: petrological, fluid inclusion and sulphur isotope studies
Titel:
The carbonate-hosted silver-lead deposits of the ediacara mineral field, South Australia: petrological, fluid inclusion and sulphur isotope studies
Auteur:
Drew, G. J. Both, R. A.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 31 (1984) nr. 2 pagina's 177-201
Jaar:
1984-06
Inhoud:
The Ediacara mineral field is situated 30 km W of Beltana on the western margins of the Flinders Ranges, South Australia, and consists of silver-lead and copper deposits in lower Cambrian carbonate rocks that contain anomalous base-metal contents throughout the Adelaide Geosyncline. The lower Cambrian rocks, which consist of the basal Parachilna Formation and overlying Ajax Limestone, rest disconformably on the Precambrian, and at Ediacara occupy a shallow N-S elongate syncline near the hinge zone of the Adelaide Geosyncline. The main primary ore minerals of the silver-lead mineralization are galena and pyrite, with very minor chalcopyrite and sphalerite, and rare tetrahedrite and pearceite. The gangue consists mainly of silica (both chalcedony and quartz), with minor dolomite and rare barite. The mineralization is stratabound and occurs in conformable zones, the lowest of which commences about 30-50 m above the base of the Cambrian sequence. The host to the silver-lead mineralization, the Ajax Limestone, can be subdivided into three units which represent a set of lithologies, structures and organic traces indicative of a shallow near-shore carbonate environment. The silver-lead mineralization is mainly present in sandy and laminated dolomites which were deposited in an environment ranging from sub-tidal to bar and channel and tidal flat, respectively. Four types of mineralization have been recognized; disseminated sulphides of syngenetic and/or diagenetic origin and epigenetic concentrations along stylolites, in veins and as breccia fillings. Post-depositional solution activity has affected a large proportion of the carbonate sequence. The effects of this activity range from stylolites through stylobreccias to solution collapse breccias. The epigenetic concentrations of mineralizations have apparently been formed by the remobilization of the disseminated sulphides during solution activity. The ore and gangue minerals of the epigenetic mineralization display both euhedral forms and distinct colloform banding, and framboidal textures have also been observed in both pyrite and galena. There is evidence of repeated episodic precipitation and no simple paragenetic sequence can be recognized. Fluid inclusions in silica and dolomite associated with the epigenetic mineralization have homogenization temperatures of 159 to 199°C and freezing temperatures that indicate the fluids to be saline brines containing NaCl with CaCl2 and/or MgCl2. Sulphur isotope analyses show a range of 834S values from -12.5 to +8.6 per mil, with no evidence of significant differences between the four types of mineralization. The data suggest deposition of the disseminated sulphides as a result of biological reduction of seawater sulphate in a system partially open with respect to sulphate supply. Subsequent remobilization of sulphides apparently involved little or no sulphur isotope fractionation. The Ediacara silver-lead deposits have many features in common with Mississippi Valley-type lead-zinc deposits and appear to have similarities in terms of genesis, in that the epigenetic mineralization has been formed as a result of post-depositional solution activity during diagenesis in a sedimentary basin. The scale of transport of the metals deposited as the epigenetic mineralization at Ediacara appears, however, to have been very much less than that of the metals in other Mississippi Valley-type deposits.
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