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


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Adabi, MH 1997 , 'Sedimentology and geochemistry of Upper Jurassic (Iran) and Precambrian (Tasmania) carbonates', PhD thesis, University of Tasmania.

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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
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 δ18O and δ13C 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° C (Irwin
equation) or 12 ± 4° 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 δ18O and δ13C
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 δ18O
and δ13C values than strongly recrystallized samples. The range of Ca, Mg, Mg/Ca,
Sr, Fe, Mn and δ18O and δ13C values from the least to most altered dolomites,
suggest that magmatic-meteoric hydrothermal fluid infiltration has occurred.
The δ18O and δ13C values of the most altered dolomites indicate that
temperatures were up to 350° C for magmatically derived hydrothermal fluids.
Isotherms on a δ18O and δ13C 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.

Item Type: Thesis - PhD
Authors/Creators:Adabi, MH
Keywords: Rocks, Carbonate, Rocks, Carbonate, Sedimentation and deposition, Sedimentation and deposition, Geochemistry, Geochemistry
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Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references

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