Sedimentology and geochemistry of Upper Jurassic (Iran) and Precambrian (Tasmania) carbonates

This thesis is a study of the origin and diagenetic history of the Upper Jurassic carbonates of the Mozduran Formation (Iran) and the Neoproterozoic carbonates of the Renison Mine (Tasmania, Australia). Therefore, this thesis is structured into two parts: Part 1: The Upper Jurassic Mozduran Formation, from north eastern Iran, comprises mainly thick sequences of bedded limestones, dolomitic limestones, dolomites, mudrocks, and locally thick gypsiferous layers. Nine microfacies have been recognized and these were interpreted as representing deposition in environments that include supratidal, intertidal, and shallow to relatively deeper subtidal zones. The algal-rich biota, diverse skeletal and non-skeletal grains, evaporites and early diagenetic dolomites present in the shallowest part of the basin, are similar to those of modern subtropical shallow marine carbonates. Other petrographic evidence, such as acicular to fibrous isopachous cements, abundant deformed and spalled ooids, and shattered micritic envelopes, suggest an original aragonite mineralogy. Carbonate samples from the relatively deeper part of the basin contain predominantly calcitic skeletons, radial calcite ooids, bladed and radial fibrous marine calcite cements. The most striking features of these carbonates are the absence of bedded evaporites, stromatolites, and gastropods, decreasing detrital quartz content, and abundance of chert and carbonate muds. Sr/Na covariance plots with respect to Mn, and ˜í¬•18O and ˜í¬•13C equilibrium lines also support aragonite and a mixture of calcite-aragonite mineralogy in the shallowest and relatively deeper parts of the basin respectively. Petrographic studies indicate that the Mozduran carbonates were subjected to a complex diagenetic history, including meteoric and burial cementation, early and late dolomitization, micritization, boring, and physical and chemical compaction. Part 2: Dolomites and associated clastic sediments of the Neoproterozoic, at Renison in western Tasmania, host significant stratabound replacement tin deposits. Dolomite samples outside and within the mineralized area were selected for analysis. The sedimentary textures and elemental and isotopic compositions of dolomites outside of the Renison mine area, clearly indicate dolomicrites probably formed by either direct precipitation or during very early diagenesis, with Mg2+ being supplied by seawater. The calculated paleotemperature of seawater during the Neoproterozoic indicates that seawater temperature was around 8 ¬¨¬± 4¬¨‚àû C (Irwin equation) or 12 ¬¨¬± 4¬¨‚àû C (Land equation). This cool to cold water marine origin is further supported by the presence of diamictites in the Renison sequences and glacial erratics in the lithostratigraphic correlative. Sedimentological features of the Renison dolomites indicate a marine shallow intertidal to supratidal environment. The carbon isotope chemostratigraphy of the least-altered Renison dolomites, gives an age range of between 570-820 Ma (Cryogenian to Neoproterozoic III). Petrographic, major and minor elements, together with ˜í¬•18O and ˜í¬•13C studies, indicate that dolomites, within the mineralized area, have been altered mainly by diagenetic and hydrothermal alteration. The variations in intensity of alteration are mainly dependent on proximity to fractures, faults or mineralization. Dolomite within the mineralized area is divided into five different types, based on crystal size and mode of origin. Dolomite samples from the mineralized area contain more generations of variable luminescence, reflecting a more complex fluid history in the vicinity of the Renison mine area. Multiple cross-cutting veins, brecciation, crack-seal textures, recrystallization, silicification, deformation and oblitration of original texture, is more pronounced in these dolomites than in the dolomites from the unmineralized area. Less recystallized dolomites, from the mineralized area, generally have more Ca and Mg, less Mn and Fe, and heavier ˜í¬•18O and ˜í¬•13C values than strongly recrystallized samples. The range of Ca, Mg, Mg/Ca, Sr, Fe, Mn and ˜í¬•18O and ˜í¬•13C values from the least to most altered dolomites, suggest that magmatic-meteoric hydrothermal fluid infiltration has occurred. The ˜í¬•18O and ˜í¬•13C values of the most altered dolomites indicate that temperatures were up to 350¬¨‚àû C for magmatically derived hydrothermal fluids. Isotherms on a ˜í¬•18O and ˜í¬•13C covariance plot illustrate that isotopic variation in the Renison carbonates is a result of changing temperatures and water/rock interactions. Water/rock ratios were predicted to be as high as -6 (open system), close to the carbonate replacement orebodies, and decreased with declining temperatures away from mineralization.