Mineralogy, geochemistry and depositional environment of phosphates in the Pabdeh Formation, Khormuj anticline, SW of Iran
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Abstract Phosphate deposits are found in the Khormuj anticline at the end of the Folded Zagros Zone of Iran and are enriched in REE and trace metals. Field survey, petrography, X-ray diffraction and whole-rock geochemistry were used to determine the petrogenesis of these phosphate deposits and evaluate the mechanisms of trace metal enrichment. Khormuj anticline phosphate layers are hosted by carbonate rocks of the Pabdeh Formation (Lower Paleocene-Oligocene). The phosphatic layers are composed of phosphorus grainstone–packstone with microfossils and contains green glauconite. Whole-rock compositions of phosphates indicate a minimal detrital component and enrichment in U and HREE. These elements are not enriched in the limestone units that overlie and underlie the phosphate layers. Overall, the textures and trace element compositions of phosphate layers are interpreted to represent accumulation on a basin margin carbonate ramp, in the reduced and suboxic-to-anoxic zone, with low detrital input but occasional high-energy erosional events. Upwelling process played a fundamental role in the deposition of the sandy glauconite-bearing phosphate layers. Phosphate mineralization has syngenetic, diagenetic, and epigenetic components. Positive correlations between P 2 O 5 and REE, U and other trace elements suggest that cation substitution into carbonate fluorapatite and not ion adsorption is the dominant mechanism for metal enrichment in these phosphates. REE patterns in these phosphate layers show strong negative Ce anomalies, positive Eu and Y anomalies and high La/Yb ratios (> 10). Yttrium versus (La/Nd) N ratios are in the seawater range and have been affected by diagenesis process. These elevated ratios suggest that the phosphates are relatively enriched in both the LREE and HREE. This enrichment is related to their marine origin, and weathering had no effect on the phosphate horizons. This research show that marine phosphates have high potential to preferentially fractionate the HREE and U and could represent a future source of these metals.Keywords:
Glauconite
Phosphate minerals
Phosphorite
Grainstone
Degradation and subsequent calcitization of phosphate grains from the upper part of the Late Cretaceous Amman Formation at southeast Irbid in northern part of Jordan have been petrographically verified. A number of calcitization patterns were observed. The variety in calcitization patterns reflects the replacing nature of calcite. Filamentous schizophytes are assisted, through borings, in early degradation of bone fragments into phosphomicrite fillings and dark envelopes (haloes) around grains. Calcitization process is more likely attributed to the availability of calcite, restriction of apatite formation, nature and composition of phosphate grains, varying pH of sediment and pore-fluids and physico-chemical conditions of the depositional site. Diagenetic replacement of phosphate grains by calcite as a result of calcitization process resulted in poorer grade of phosphate deposits where the P2O5 is less than 15.2%.
Phosphorite
Phosphate minerals
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Dolomitization
Grainstone
Dolostone
Outcrop
Shoal
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Three times during the deposition of the Lower and Middle Cambrian Carrara Formation, shallow-water carbonate banks developed in southern Nevada and southeastern California. Each bank attained a minimum width of 150 km normal to the depositional strike. Eastward the banks were bordered by terrigenous clays, silts, and sands from the stable craton, and westward they were bordered by siliceous lime muds lacking shallow-water depositional features. The position of rock units with respect to 5 trilobite faunules within the Carrara suggests that carbonate deposition waxed and waned subsequent to eastward and westward migrations of areas of detrital sedimentation. During each of the 3 episodes of carbonate deposition, a sequence of 4 lithologic changes is repeated as follows: (1) upward fining and thinning of terrigenous clastic deposits; (2) the beginning of carbonate deposition, first as scattered skeletal grainstones, later as oolitic grainstone, oncolite packstone, and lime mudstone accumulating as a shallow subtidal bank; (3) the deposition of loferites, bird's-eye and mudcracked limestones, cryptalgal laminites, and stromatolites as low carbonate islands on the western half of the banks; and (4) the relatively abrupt termination of carbonate deposition with renewed sedimentation of detrital clay and silt. Each sequence is repeated farther east during succeeding episodes of carbonate sedimentation. End_of_Article - Last_Page 782------------
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The carbonate-evaporite sequences of the Upper Jurassic Arab and overlying Hith formations in the southern and southwestern Arabian Gulf form many supergiant and giant fields that produce from the Arab Formation and are excellent examples of a classic reservoir/seal relationship. The present-day sabkha depositional setting that extends along most of the southern and southwestern coasts of the Arabian Gulf provides an analog to these Upper Jurassic sedimentary rocks. In fact, sabkha-related diagenesis of original grain-supported sediments in the Arab and Hith formations has resulted in five distinct lithofacies that characterize the reservoir/seal relationship: (1) oolitic/peloidal grainstone, (2) dolomitic grainstone, (3) dolomitic mudstone, (4) dolomitized grainstone, an (5) massive anhydrite. Interparticle porosity in grainstones and dolomitic grainstones and intercrystalline porosity in dolomitized rocks provide the highest porosity in the study area. These sediments accumulated in four types of depositional settings: (1) supratidal sabkhas, (2) intertidal mud flats and stromatolitic flats, (3) shallow subtidal lagoons, and (4) shallow open-marine shelves. The diagenetic history of the Arab and Hith formations in the southern and southwestern Arabian Gulf suggests that the anhydrite and much of the dolomitization are a result of penecontemporaneous sabkha diagenesis. The character and timing of the paragenetic events are responsible for the excellent porosity of the Arab Formation and the lack of porosity in the massive anhydrites of the Hith, which together result in the prolific hydrocarbon sequences of these formations.
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ABSTRACT The Jurassic upper Smackover Formation in Lincoln Parish, Louisiana, consists mainly of nonskeletal grainstones, argillaceous, anhydritic dolomitic mudstones to wackestones, and bioturbated to graded siliciclastics. Two shoaling-upward cycles, the A and B limes of local stratigraphers, were identified. These grainstone deposits accumulated as tidal bars on east-west trending paleo highs (salt structures) which parallel regional depositional strike. The salt ridges originated during middle Smackover deposition along the northern rim of the north Louisiana salt basin and produced a slope-break on the regional Smackover ramp. Between the salt ridges in the study area is an interridge trough or withdrawal basin. The strike-trending trough was filled episodically with mixtures of grain carbonates, sandstones, siltstones, and shales. The siliciclastics include fining-upward litharenites and graded, alternating grainstone-sandstone bedsets. The textural grading probably reflects strong but intermittent wave drift or storm surge currents that moved from east to west along the flanks of the salt highs. The finer grained rocks occur in the withdrawal basin center. Diagenetic products observed in thin sections indicate that the Smackover carbonates were exposed to early marine, meteoric phreatic, and subsurface diagenesis. Marine diagenesis included micritization and isopachous aragonite rim cementation. Fresh-water diagenesis included leaching of allochems and cements and precipitation of blocky calcite rim and mosaic cements. Subsurface diagenesis included precipitation of pore-filling and replacement anhydrite, silicification, and stylolitization. The distribution of early diagenetic effects basically follows the trend of the salt ridges.
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Lacustrine carbonate lithofacies in the Hot Spring limestone vary systematically at meter to decameter scales and record paleobathymetry, limnologic conditions, and paleogeographic influences. This unit accumulated during the late Miocene in a lake system in an extensional basin complex, closely associated with lava flows and volcaniclastics. Sedimentology and sequence stratigraphy enable understanding and prediction of the occurrence, distribution, and character of lacustrine carbonates. Occurrence of lacustrine carbonates is a function of lake-basin type, in this case a volcanically mediated balanced-filled lake basin that contains various lithofacies: microbialite, grainstone, packstone, wackestone, and carbonate mudstone. Distribution of lithofacies is strongly controlled by depositional subenvironment and gradient (through water depth, bottom energy, circulation, and accommodation). Internal character of microbialites (i.e., type, size, porosity, vertical and horizontal permeability, associated lithotypes, diagenesis) is influenced by stratal position (at the parasequence scale) and location along the depositional profile. Depositional reservoir quality characteristics, such as microbialite porosity and thickness, grainstone size and sorting, and overall carbonate continuity and connectivity peak in the medial sublittoral zone. Conversely, secondary diagenetic effects, such as dissolution, carbonate and quartz cement, and possible authigenic clays, are highest in the updip lake-plain–littoral zones and are less common lakeward. Thus, the medial sublittoral zone has the optimum potential, where primary depositional characteristics are best developed, and negative secondary diagenetic effects are minimal. These strata share many attributes with the Cretaceous presalt systems of the south Atlantic and can provide insights about controls on potential reservoir character, distribution, and connectivity for exploration and development.
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