Dariush ESMAEILY, Hosein Rahimpour BONA, ALI KANANIAN, Amir Esna ASHARI

Petrography and geochemistry of the Jajarm karst bauxite ore deposit, NE Iran: implications for source rock material and ore genesis

İran’ın kuzeydoğusunda bulunan Jajarm boksit yatağı İran’daki en büyük yataktır. Yatak, Triyas yaşlı Elika formasyonu ve Jura yaşlı Shemshak formasyonu arasında yüzeylemektedir. Yatak genel olarak dört farklı düzeyi (alttan üste doğru) takiben tanımlanan bir iç katman yapısı göstermektedir: (a) direkt olarak karbonat (Elika Formasyonu) tabanını üzerleyen, başlıca kil minerallerinden yapılı yaklaşık olarak 50−80 cm kalınlığındaki alt arjilikli seviye, (b) başlıca hematit, kaolinit, anatas ve diyaspor minerallerinden meydana gelen ve yaklaşık olarak 2−3 m kalınlığındaki bir boksitik kil seviyesi, (c) diyaspor, kaolinit, anatas ve hematit minerallerinden yapılı olan ve 5 m kalınlığındaki kırmızı boksit seviyesi (yüksek tenörlü ana cevher), (d) başlıca kaolinitten yapılı olan 20−50 cm kalınlığındaki üst kaolinit seviyesi ve bu seviye Shemshak formasyonu tarafından üzerlenmektedir. Detaylı petrografik çalışmalar, boksitik protolitin diyajenetik alterasyonunu göstermektedir. Çalışılmış örneklerde gözlenen boksit dokuları mikrogranüler, oolitik, pizolitik, kolloform ve mikroklastiktir. Feldspat tanelerinin yanısıra, kırılmış ve aşınmış kalıntı kuvars taneleri ile ilişkili mikrogranüler ve mikroklastik dokular, hemen hemen komple levhasal diyaspor tarafından yeri alınmıştır. Kırmızı boksitin jeokimyasal analizleri mobil elementlerin (Rb, K, Na, Sr, La, Mg, and Pb) tüketildiğini, daha az mobil elementlerin (Nb, Th, Zr, Mo, Ga, and Cr) ise zenginleştiğine işaret etmektedir. Zıt bir sonuç, boksitik kil için gözlenmektedir. Üst kaolinit seviyesi için kondrite göre normalize edilmiş NTE (Nadir Toprak Elementleri) örgüleri altlayan kırmızı boksitlerinkilere benzerdir, ve alt arjilikli seviye için gözlenen örgüler, üzerleyen arjilikli boksit seviyesindekine benzerdir. Ce, boksitik kilde bir negatif anomali ve kırmızı boksitte bir pozitif anomali gösterir. NTE ve diğer elementler arasında hesaplanmış olan korelasyon katsayıları, NTE-içeren minerallerin Ti ve Nb, kil mineralleri, ve zirkonun oksitleri olduğunu ortaya koymaktadır. Jajarm boksitlerinin var olan diyasporik mineralojik bileşiminin aksine, jeokimyasal ve mineralojik veriler, orijinal jipsitik bir bileşime işaret eder. Sonuç olarak, değişim diyagramları ve NTE örgüleri ile sağlanan kanıt ile gözlenen mineralojik ve dokusal ilişkiler, Jajarm boksiti için hem bazik hem de sedimanter kayaçlardan oluşan karışmış bir kökeni işaret etmektedir. Aslında, boksitleşme bazik köken kayaçlar üzerinde başlamış ve karstik özellikler içerisinde yerleşim ve yeniden işlenmesi süresince devam etmiştir.

Jajarm (KD İran) karst boksit cevher yatağının petrografisi ve jeokimyası: kaynak kaya materyali ve cevher kökeni için göstergeler

The Jajarm bauxite deposit, northeast Iran, is the largest such deposit in Iran. The deposit is sandwiched between the Triassic Elika formation and the Jurassic Shemshak formation, housed within karstic features developed within the former unit. The deposit generally shows an internal layering defined by the following four distinct horizons (from bottom to top): (a) a lower argillaceous horizon, approximately 50–80 cm thick, is mainly composed of clay minerals that directly overlies the carbonate footwall (Elika formation); (b) a bauxitic clay layer approximately 2–3 m thick that consists mainly of hematite, kaolinite, anatase, and diaspore; (c) a red bauxite layer (the main high-grade ore), about 5 m thick and composed of diaspore, kaolinite, anatase, and hematite; and (d) an upper kaolinitic layer that is 20– 50 cm thick, composed mainly of kaolinite, and overlain by the Shemshak formation. Detailed petrographic studies reveal diagenetic alteration of the bauxitic protolith. The main observed bauxite textures are microgranular, oolitic, pisolitic, fluidal-collomorphic, and microclastic. Microgranular and microclastic textures associated with the residual fractured and corroded quartz grains, as well as feldspar grains are almost completely replaced by platy diaspore. Geochemical analyses of the red bauxite reveal enrichment of less mobile elements (Nb, Th, Zr, Mo, Ga, and Cr) and depletion of mobile elements (Rb, K, Na, Sr, La, Mg, and Pb); the opposite result is obtained for the bauxitic clay. Chondrite-normalized REE (Rare Earth Element) patterns for the upper kaolinite layer are similar to those for the underlying red bauxite, and the patterns obtained for the lower argillaceous layer are similar to those for the overlying argillaceous bauxite horizon. Ce shows a positive anomaly in the red bauxite and a negative anomaly in the bauxitic clay. The correlation coefficients calculated between REE and other elements demonstrate that the likely REE-bearing minerals are oxides of Ti and Nb, clay minerals, and zircon. In contrast to the present diasporic mineralogical composition of the Jajarm bauxite, the geochemical and mineralogical data indicate an original gibbsitic composition. Finally, the observed mineralogical and textural evidence, combined with the evidence provided by variation diagrams and REE patterns, indicates a mixed origin for the Jajarm bauxite from both basic igneous and sedimentary rocks. In fact, bauxitization was initiated on basic source rocks and continued during reworking and replacement within the karstic features.

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