• Mir Ali Asghar Mokhtari

      Articles written in Journal of Earth System Science

    • Genesis of iron-apatite ores in Posht-e-Badam Block (central Iran) using REE geochemistry

      Mir Ali Asghar Mokhtari Ghader Hossein Zadeh Mohamad Hashem Emami

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      Rare earth elements in apatites of different ore types show characteristic patterns which are related to different modes of formation of the ores. Most of the apatite-bearing iron ores are associated with alkaline magmas with LREE/HREE fractionation varying from moderate to steep.

      Iron-apatite deposits in Posht-e-Badam Block (Central Iran) have a high concentration of REE (more than 1000 ppm up to 2.5%), and show a strong LREE/HREE ratio with a pronounced negative Eu anomaly. This REE pattern is typical of magmatic apatite and quiet distinct from sedimentary apatites (phosphorites) which have a low REE contents and Ce negative anomalies. On the other hand, they are comparable to the REE patterns of apatites in Kiruna-type iron ores in different parts of the world.

      The REE patterns of apatites, iron-apatite ores and iron ores are similar and only have different REE contents. This similarity indicates a genetic relation for these rocks. Most of the iron-apatite deposits in Central Iran have similar REE patterns too, which in turn show a genetic relation for all of these deposits. This similarity indicates a similar origin and processes in their genesis. There are some small intrusions around some of the iron-apatite deposits that are petrographically identified as syenite and gabbro. These intrusions also have REE patterns similar to that of iron-apatite ores. This demonstrates a genetic relation between these intrusions and iron-apatite ores.

      The REE patterns of apatites in different deposits of Posht-e-Badam Block iron-apatite ores show an affinity to alkaline to sub-alkaline magmas and rifting environment. The alkaline host rocks of Central Iran iron-apatite ores are clearly related to an extensional setting where rifting was important (SSE–NNW fault lines). A probable source for this large scale ore forming processes is relatively low partial melting of mantle rocks. The ores have originated by magmatic differentiation as a late phase in the volcanic cycle forming sub-surface injections or surface flows. These ores have formed during magmatism as immiscible liquids (silicate and Fe-P-rich magmatic liquids) which separated from strongly differentiated magmas aided by a large volatile and alkali element content. Separation of an iron oxide melt and the ensuing hydrothermal processes dominated by alkali metasomatism were both involved to different degrees in the formation of Posht-e-Badam Block iron-apatite deposits. We proposed that the separation of an iron oxide melt and the ensuing hydrothermal processes dominated by alkali metasomatism were both involved to different degrees in the formation of Posht-e-Badam Block iron-apatite deposits.

    • Geochemical constraints on the origin and tectonic setting of Chargar intrusions in the Alborz orogenic belt, NW Iran


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      Chargar area is located in the southern border of Tarom subzone within the Alborz magmatic belt of NW Iran. Two types of intrusions, mainly present in the southern part of the area, have been identified. These bodies intruded into the Karaj Formation volcanic and volcaniclastic rocks. Type-I intrusions occur in the south and include two magmatic bodies: (a) Gabbro-pyroxene quartz monzodiorite-quartz monzodiorite and (b) quartz syenite. Type-II crops out in the west and has a gabbro – gabbro-diorite composition. Geochemically, Chargar intrusive rocks belong to the high-K calc-alkaline to shoshonitic series and are classified as I-type and metaluminous granitoids. These intrusive rocks show insignificant different distribution patterns of trace elements and REEs, but generally they are characterized by highly enriched large ion lithophile elements (e.g., K and Ba) and depleted Nb, similar to that of continental arc magmatism. Type-I intrusions and Type-II mafic intrusion were originated from mantle source with amphibole-bearing spinel peridotite composition. These intrusions formed in a continental arc to post-collisional tectonic setting.

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