The Choghart iron oxide-apatite (IOA) deposit is located 124 km southeast of Yazd, in the Bafq district within the Central Iranian microcontinent.The Choghart deposit is hosted by the rhyolitic rocks of the Early Cambrian volcano-sedimentary sequence (the Esfordi formation).Both host rocks and the orebodies are crosscut by diabase dykes.Tectonically, the Choghart rhyolites represent the continental margin setting and the Choghart diabase dykes formed in the back-arc basin environment, respectively, indicating that the evolution of the Bafq district is associated with subduction of Palaeotethys oceanic crust beneath the Central Iranian microcontinent followed by formation of continental arc related granitoids and rhyolites and then formation of backarc basin diabase dykes.Similar to the other subduction-related rhyolites, the Choghart rhyolite is enriched in Th and LREE compared to Ta, Nb, and HREE.The main host minerals of Th and REE in the Th-REE mineralization zone are thorite and sphene.Albitization is the most important alteration aspect related to Th-REE mineralization (mainly Th, La, Ce, Nd, and Y).In addition to albite, Th-REE mineralization is associated with actinolite, augite, diopside, minor microcline and orthoclase, plus magnetite, calcite, pyrite, rutile, and minor amounts of chalcopyrite.The negative Eu anomaly in Th mineralization zone, as well as the paragenetic occurrence of magnetite, pyrite and chalcopyrite with thorite suggest that Th-REE mineralization formed in relatively reduced condition.The presence of paragenetic calcite accompanied by thorite and sphene in the Th-REE mineralization zone indicates that Th and REE were likely transported by the carbonate complexes in the mineralizing fluids.The similarity between the chondrite-normalized REE patterns of the host rhyolite and the Th-REE mineralization zone suggests that post-magmatic driven fluids of continental margin rhyolitic magma played an important role in Th-REE mineralization.
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.
Abstract The Kangan anticline in the Folded Zagros Zone contains phosphate deposits enriched in trace metals. Field observations, petrography, X-Ray Diffraction, Scanning Electron Microscopy, and whole-rock geochemistry are used to determine the petrogenesis of this phosphate deposit, evaluate the mechanisms of deposition, and assess the implications for trace metal enrichment. Phosphatic layers are grainstone–packstone with microfossils and contain green glauconite. Carbonate rocks of the Early–Middle Eocene Pabdeh Formation host the phosphate units. Glauconite, calcite, and fluorapatite are the primary minerals of the marine sedimentary phosphate deposit in the Kangan anticline. Whole-rock compositions of phosphate layers indicate negligible clastic components and show enrichment in U and HREE. Limestone and pelagic limestone units in the Pabdeh Formation do not display enrichment of these elements. Carbonate fluorapatite is the host mineral for REEs and uranium. Cation substitution into carbonate fluorapatite is considered to be the main mechanism of trace element enrichment due to positive correlations between P2O5 and trace metals; ion adsorption did not play a crucial role in the metal enrichment in these phosphates. Strong negative Ce anomalies, slight positive Eu anomalies and low ΣLREE/ΣHREE ratios of phosphate layers indicate enrichment of the HREE relative to their marine origin. The depositional environment of the phosphate units is interpreted as a basin margin carbonate ramp in the reduced and suboxic-to-anoxic zone that had low detrital input but occasionally high-energy erosional events. Ocean upwelling had an essential role in depositing the sandy glauconite-bearing phosphate layers.
The Choghart iron oxide–apatite (IOA) deposit is one of the largest iron mines of the Bafq district in Central Iran. This deposit is hosted by the Early Cambrian rhyolites, and diabase dikes crosscut both host rocks and the ore bodies. The Choghart rhyolites erupted in a continental arc setting, while the Choghart diabase dikes formed in a back‐arc basin environment. Thorite, minor titanite, and REE‐oxide are the main hosts of Th and REEs. The mineralogy and geochemistry support that the Th–REE mineralization formed at relatively reduced conditions. The presence of calcite accompanied by thorite and titanite suggests that Th and REE probably migrated as carbonate complexes in the mineralizing hydrothermal fluids. Microthermometric data of calcite associated with thorite indicate that the salinity of the ore‐forming fluids varies from 20 to 30 wt% NaCl equivalent with temperature estimates between 300 and 370°C. The narrow range of homogenization temperature and low salinities of the fluid inclusions imply the presence of only one fluid phase in the Th–REE mineralization. The calcite δ 13 C PDB (−3.9 to −4.1‰) and δ 18 O SMOW (6.6–7.0‰) support a magmatic source for the ore‐forming fluids. Actinolite δD (5.55–6.72‰) and δ 18 O (−100.8 to −82.4‰) also suggest a magmatic source of the ore‐forming fluids. Microthermometric data imply that fluid–rock interaction and cooling were critical factors triggering the thorite precipitation at Choghart IOA deposit. Stable isotopic data indicate that post‐magmatic fluids derived from rhyolitic magmas played a significant role in the Th–REE mineralization.
The Bafq district, well-known in the world, is the most important iron province in Central Iran. The Early Cambrian volcano-sedimentary sequence in this district, hosts a lot of mineral deposits such as Kiruna-type iron oxide-apatite (IOA), Fe-Mn exhalative and Pb-Zn SEDEX types. Th-U-REE mineralisation in the Bafq IOA deposits occurs in a variety of element associations, (i) REE-P mineralisation as apatite hosting REE-mineral inclusions, (ii) Th-REE mineralisation as Th-REE minerals (e.g. REE-bearing thorite and titanite), (iii) thorium mineralisation as thorium minerals (e.g. thorite and huttonite), (iv) U-REE mineralisation as U-REE minerals (e.g. uraninite and cleveite) and (v) REE-U-Th mineralisation as REE-U-Th minerals (e.g. davidite and allanite). Two main types of apatite are recognised: apatite with inclusions (dominantly monazite, xenotime and allanite) and inclusion poor/free apatite. The presence of calcite paragenesis with Th-REE, Th and U-Th-REE mineralisation indicates transportation of Th and REEs by carbonate complexes in post-magmatic alkaline fluids.
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