Journal of Economic Geology (Sep 2023)
REE-Y-Ti-Th mineralization in the albite bearing metasomatite and metasomatized rhyolite hosted in Choghart magnetite-apatite deposit, Central Iran: Interplay of evaporitic brines, hydrothermal-magmatic fluids
Abstract
Macroscopic, microscopic and geochemical studies carried out on albite-bearing metasomatites and metasomatic rhyolites hosting the magnetite-apatite deposit of Chogharat indicate the presence of three generations of albite with different concentrations of REE-Y-Ti-Th, in response to T-P reduction and chemical changes of the fluids and the ratio of fluid to rock. The geochemical analysis of the low Ca/Na fluids shows a deficiency in REE-Th mineralization in the white albites, while in the fluids with medium Ca/Na, the REE mineralization (REE>Th) has occurred in the pinkish albites. In contrast, fluids with high Ca/Na indicate Th mineralization (Th>REE) in the red albites. The stable isotopes of C-O on the paragenetic calcites show REE-Y-Ti-Th mineralization of albites due to High-T hydrothermal fluids. Otherwise, the stable O-C isotopes of the Ghoghart apatites and stable isotopes of S in the ore deposits of the BMD verify the role of evaporitic brines and fluid-rock interaction on the mineralization. The presence of calcite and titanite, associated with the calcic-amphiboles and clinopyroxenes, Ca-inclusions in the thorite structure and Ca-content of the thorites, indicate thorite mineralization from the Co32- and Ca2+ fluids due to low activity of the chlorine. According to this study, the source of metasomatism is mainly evaporitic brines with a minor amount of magmatic and related hydrothermal fluids. Mineralization is the result of interaction of the magmatic and hydrothermal fluids of the Late Ediacaran- Early Cambrian plutonic/ subvolcanic intrusions with the evaporitic brines, derived from the synchronous evaporitic sequence. Introduction Metasomatism often causes the extensive mineralogic and chemical changes of rocks (Enjvik et al., 2018). Alkaline metasomatism, as one of the most important mineralization processes, could be classified to the high-temperature and medium- to low-temperature types (Zhao, 2005). High-T and medium- to low-T alkaline metasomatism are associated with the Nb-Ta-Li-Br and U-REE mineralization, respectively. The mean array of U, Th, and REE of the associated ore deposits of alkali-metasomatism is not high, but because of their relatively high tonnage are considered important prospecting goals that usually form in the faulted area and in relation to the deep regional structures (Zhao, 2005; Cuney and Kyser, 2008). Until now different sources of metasomatic processes are suggested for the Ghoghart magnetite-apatite in the BMD (e.g. Khoshnoodi et al., 2017; Aftabi and Mohseni, 2020; Bonyadi and Sadeghi, 2020; Aftabi et al., 2021; Majidi et al., 2021). In addition, association of Ti with this type of ore deposits is less reported and associated with U(Th), REE, Ti is considered a rare phenomena in the ore deposits of this district, not reported yet. In this research, the nature of REE-Y-Ti-Th mineralizing fluids in the Ghoghart deposit would be studied. Research method After field studies, 44 samples of the Ghoghart host metasomatic rhyolites are selected for petrographic microscopy out of 63 collected samples. For complementary mineralogical studies, 24 samples of the 3-generation albite-bearing metasmatites are studied by the Scanning Electron microscopy, model LEO-1400 with 17-19kV, ray dim. 12 and 20 nA and 12 samples of different-generation albites are analyzed XRD by the Philips Xpert pro in the Iranian mineral processing center and 9 samples are analyzed by the Raman spectrometer of Senterra 2009 model of the Germany Brucker Co. with penetration depth of 2µm, spectral width of 200-3500 cm-1, wavelength of 785 nm, CCD detector and resolution> 3 cm-1 in the nano-electronic Lab of the Tehran University. Furthermore 22 samples of metasomatites with different generations of albites are analyzed ICP-MS at the Zarazma Lab, Iran. To constrain the source of mineralizing fluids, O-H isotopic analysis of 4 apatite samples are performed at the Hangaria Lab, Hungry. After petrography of fluid inclusion on the 1-generation calcite of albite-bearing metasomatites, 8 selected samples of calcite are analyzed in the Ottawa University Lab., Canada. Results The macroscopic, microscopic, and geochemical studies on the host albite-bearing metasomatites and metasomatic rhyolites of the Ghoghart magnetite-apatite deposit show extensive and intense effect of alkali-metasomatic and hydrothermal processes, associated with the sodic, sodic-calcic, potassic, and late carbonate± quartz alterations. The REE-Y-Ti-Th mineralization is related to the sodic-calcic alteration, which is shown as the 2- and the 3-generation albites. Intimate association of this mineralization with the related albites reveal their common genesis and follow the spatial pattern of the sodic, and sodic-calcic alterations. The REE and incompatible REE patterns show a common pattern of the LREE enrichment to the HREE, which is the characteristic of subduction zone magmas, in all samples. The stable O-H isotopes of the paragenetic calcite with the albites show albitization and REE-Y-Ti-Th mineralization due to the HT-hydrothermal fluids. The stable O-H isotopic study on the 1-generation apatites of the Ghoghart magnetite ore rocks display the role of evaporitic brines and rock-fluid interaction on the ore formation. Mixing of magmatic and HT-hydrothermal fluids with these evaporitic brines caused mineralization and associated alteration in the Ghoghard deposit. Moreover, stable sulfur isotopes show evaporitic brines as the main source of mineralizing fluids with minor effect of magmatic fluids. Infiltration of the hot oxide brines enriched in the REE-Y-Ti-Th, P, and Ca ± Fe, caused the weak sodic alteration (> 170 ºC) and sodic-calcic alteration (> 280ºC) of the host rocks, which was associated with extensive deposition of the magnetite-albite, low-grade magnetite-scapolite, and high-grade magnetite-actinolite. In the late stage of sodic-calcic alteration, magnetite-apatite (280-245ºC) has formed. The potassic alteration after the former aforementioned mineralization phases (> 280ºC, ~ 200ºC) and then the sericite, chlorite, and epidote assemblage is formed by the LT-hydrothermal fluids (180-250ºC) as the last stage of mineralization (Bonyadi and Sadeghi, 2020). Microtheremore data of the fluid inclusions of the calcites, related to the albite-bering metasomatites, and 1-generation apatites confirm the recorded alteration temperatures. Association of the calcite, titanite with the calcic-amphiboles and clino-pyroxene (actinolite and augite), Ca-inclusions in the thorite structure and Ca content of the thorites (CaO ~ 2.3%) indicate deposition of the thorites from the Ca2+ and Co32- bearing carbonate fluids due to the low-activity of Cl in the REE-Y-Ti-Th mineralization zones. According to this study, fluids with low-content of Ca/Na, did not cause or were associated with minor REE and Th mineralization (white 1-generation albites). The fluids with medium amount of Ca/Na caused the REE mineralization (REE>Th, 2-generation pale pink albitities) whereas the high Ca/Na content fluids was associated with the Th mineralization (Th>REE, 3-generation dark pink albites). The Th deposition occurred under the Eh and ph changes and also the fluid/rock ratio, revealed by the exsolution evidences; In fact, the increase of ph has occurred due to the expelling of CO2 from the mineralizing fluids along the fractures.This phenomena had a considerable effect on deposition of the Th-bearing carbonate complexes. The ph increase of the mineralizing fluid was associated with silica content of the fluid as the silicic acid that caused deposition of the Th-silicates. Discussion and conclusion Subduction of the Prototethys under the central Iranian crustal blocks during the late Neoproterozoic-Early Cambrian caused an extensive granitoid magmatism that was accompanied by deposition of the equivalent evaporitic-volcaniclastic deposition (Esfordi Formation) in the extensional back-arc basins (Ramezani and Tucker, 2003). The edicaran-lower Cambrian Fe-P mineralization in the BMD shows a genetic relation with these granitoids, related volcanic rocks, and associated synchronous evaporitic-volcaniclastic deposits. In fact, the intrusive and subvolcanic intrusions, as a thermal, engine caused heating of the evaporitic brines of the synchronous adjacent basins. The most important characteristic of these brines is their high Na/K and Cl/S content. These hot alkali-enriched, sulfur- oxide brines leached their passing host rocks and transferred the REE-Y-Ti-Th elements. The result was sodic and sodic-calcic metasomatism with REE-Y-Ti-Th mineralization. The results of this research constrain the mainly evaporitic brines with a minor amount of magmatic and related hydrothermal fluids as the source of metasomatism. In fact, mineralization is the result of magmatic and hydrothermal fluids of the Late Ediacaran- Early Cambrian plutonic/ subvolcanic interaction with the evaporitic brines, derived from the evaporitic sequence (Esfordi Formation). Acknowledgements The authors appreciate the Iranian Central Iron Ore Company manager and staff for their assistance during this research.
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