Geochemistry of the Zhibo submarine intermediate‐mafic volcanic rocks and associated iron ores, Western Tianshan, Northwest China: Implications for ore genesis
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Abstract:
The Zhibo iron deposit is hosted in Carboniferous submarine volcanic rocks in Western Tianshan, NW China. A series of magnetite‐bearing intermediate‐mafic volcanic rocks are recognized in the periphery of the Zhibo ore district. Most of these volcanic rocks formed at 314 ± 2 Ma, possess tholeiitic–calc‐alkaline affinities, and display remarkable negative Nb, Ta, and Ti anomalies on primitive mantle‐normalized incompatible element diagrams. These features, together with those of their relatively complete rock assemblages and Th/Yb versus Nb/Yb diagrams, are indicative of their formation in an active continental margin arc setting. The wide compositional spectrum of SiO 2 values ranging from 47.11 to 62.75 wt.% and lower Mg # values (55–63) of basalts suggest that the Zhibo intermediate‐mafic volcanic rocks may have experienced magmatic differentiation. Their (Th/Ta) PM > 1, (La/Nb) PM > 1, Nb/Ta (11‐16), and Th/Ce (0.06‐0.23) values suggest that the source of these intermediate‐mafic volcanic rocks was significantly contaminated by crustal materials. The magnetites in the iron ore have lower contents of Al, Mn, Ti, and V, indicating that the mineralization of magnetite in the iron ore occurred under lower temperature and higher oxygen fugacity conditions than those in the intermediate‐mafic volcanic rocks. In addition, the magnetites in the Zhibo iron ores have lower contents of compatible elements (e.g., Ti, V, Mn, Co, Cr, and Zn) than those of the magnetite in the intermediate‐mafic volcanic rocks, suggesting that the Zhibo magnetites crystallized from late‐stage, residual iron‐rich magmatic melts/magmatic‐hydrothermal fluids. In addition, the textures of the volcanic rocks suggest that iron have ever enriched in the residual melt during the magmatic stage, and the iron‐rich fragments in andesitic volcaniclastic rocks indicate that the ore‐forming material was a high‐salinity fluid‐bearing iron‐rich melt. In combination of available information, including field observations and geochemical analyses, we interpret that the Zhibo iron deposit is magmatic‐hydrothermal in origin.Keywords:
Mineral redox buffer
Felsic
Ore genesis
Ilmenite
Fractional crystallization (geology)
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Intergrown phenocrystic titaniferous magnetite-ferrian ilmenite, reduction of ferrian ilmenite by liquid rather than by oxidation of titaniferous magnetite, oxygen fugacity-temperature relationships as criteria of mode of origin
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Mineral redox buffer
Fugacity
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Ilmenite oxy-exsolution in magnetite recording different textures provides information on the Fe-Ti oxide cooling history. Magnetite-ilmenite intergrowths in carbonatites have the potential to offer important insights into the petrogenesis of the hosted ore deposits, whereas they have received limited attention. In this contribution, investigations have been conducted on different types of ilmenite oxy-exsolution hosted by magnetite within the Tiechagou carbonatite-associated iron deposit. Based on textural features, the magmatic magnetite-ilmenite intergrowths have been identified as thin lamellae (Type1), thick lamellae (Type2), fine-grained granular (Type3) and coarse-grained granular oxy-exsolutions (Type4). Major, minor and trace elements of the bulk oxide, magnetite and ilmenite were determined using EMPA and LA-ICP-MS. Bulk MgO and TiO2 generally decrease from Type1 to Type2 and Type3 to Type4, with Type1 oxides recording the highest. Bulk minor- and trace-element concentrations of all magmatic magnetite-ilmenite varieties exhibit similar trends in continental crust-normalized diagrams. Different magnetite and ilmenite groups are characterized by variable minor and trace element compositions, and have been successfully discriminated using Principal Component Analysis with the most discriminant elements of V, Cr, Ni, Co, Zn, Mn and Mg. Temperature and oxygen fugacity have been determined using WinMIgob, software developed by Yavuz (2021). Textural and chemical features of magnetite and ilmenite in the Tiechagou carbonatite combined with their re-equilibration temperature and oxygen fugacity highlight the evolution history, which is presented into two petrogenetic models. The first model records a significant decrease in temperature from 683 to 338 °C and oxygen fugacity (logfO2) from −14.58 to −34.27, which results in a progressive evolution of ilmenite oxy-exsolution from thin to thick lamellae starting at a supersolvus temperature. The second records a continuing evolution of the magnetite-ilmenite intergrowths from fine-grained to coarse-grained granular oxy-exsolutions through subsolvus oxidation, which is constrained by increase of the average oxygen fugacity (logfO2) from −28.38 to −26.93 at an intermediate temperature of 500–400 °C. Carbon in the oxidized form (e.g., CO32-) probably plays a significant role as the oxidizing agent especially when the oxygen fugacity is decreasing and/or low in carbonatite systems.
Ilmenite
Mineral redox buffer
Carbonatite
Trace element
EMPA
Columbite
Layered intrusion
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Felsic
Fractional crystallization (geology)
Silicic
Anatexis
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In order to understand the origin of differing ilmenite/armalcolite textural relationships, bulk rock composition, oxygen fugacity and cooling rate have been experimentally investigated. Results show that armalcolite and ilmenite compositions are dependent upon the liquid composition and temperature at the time of crystallization. Furthermore, the different armalcolite/ilmenite textures rely to a greater extent on f sub O2 than on cooling rate or bulk rock composition, and the Ti(3+) content of armalcolite can be used as an indicator of the approximate f sub O2 prevailing at the time of crystallization.
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Paragenesis
Mineral redox buffer
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Summary The formation of ilmenite from titanomagnetite frequently shows separate generations of production. The ilmenite products can be combined with the magnetite host to represent the intermediate stages in this development. Use of the Buddington and Lindsley (1964) geothermometer in conjunction with this method provides a number of points in the temperature and oxygen-fugacity history of a single, complex magnetiteilmenite grain. Application of the method is illustrated by an example from the Freetown layered gabbro. A titanomagnetite exsolved granular ilmenite whilst cooling and this process ceased at 930° and an oxygen fugacity of log ƒ O 2 = −11·5. Further cooling and exsolution continued until the grain reached 662 °C and log ƒ O 2 = −19·0, producing distinct ilmenite lamellae.
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Mineral redox buffer
Fugacity
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Ilmenite
Mineral redox buffer
Fugacity
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Ilmenite
Mineral redox buffer
Rutile
Metasomatism
Diopside
Phlogopite
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