Abstract The present study focuses on three outcrops of Upper Permian to Lower Jurassic karst bauxite deposits in the Alborz zone, north of Iran. The deposits occur as sharp semi-continuous lenses within the Upper Permian Ruteh Formation (Gheshlagh) and the Upper Triassic Elika Formation (Siahrudbar and Separdeh) carbonate rocks. Textural characteristics such as pelitomorphic matrix, local ooidic-pisoidic, fluidal, and clastic textures indicate authigenic-allothigenic origins. Mineralogically, kaolinite, boehmite, goethite, and hematite in the Gheshlagh, diaspore, hematite, and kaolinite in the Siahrudbar, and kaolinite, hematite, goethite, diaspore, siderite, and chamosite in the Separdeh deposits are the main phases. The distribution patterns of major elements normalized to the upper continental crust (UCC) depict enrichments of Al, Fe, and Ti in all three deposits. The three deposits are variably enriched in transition trace elements (Cr, Ni, Co, V), U, Th, and high field strength elements (Ga, Hf, Zr, Nb, Y) relative to UCC. The studied bauxites have average concentrations of rare earth elements (ΣREE, La-Lu) between 305.97 and 373.69 ppm. Geochemical data reveal progressive enrichment of REE and an intense (LREE/HREE)N fractionation toward the lower parts of the profile in the Separdeh deposit due to the alkaline conditions of the carbonate bedrock, and irregular distribution patterns at the Gheshlagh and Siahrudbar deposits due to water level and pH fluctuations. The positive Ce anomalies at Separdeh and Siahrudbar suggest oxidizing conditions in the upper parts of the profiles. At Gheshlagh, the negative to nil Ce anomalies reflect low intensity of oxidation. Based on previous geochemical analysis data, the Upper Permian basalts in the Gheshlagh, and the Upper Triassic basalts in the Separdeh and Siahrudbar areas were the most likely source materials. The local uplift of the Permian-Upper Triassic series led to the formation of karstic topography filled with materials derived from weathering of the basaltic rocks under humid tropical paleoclimate. The initial weathering converted the source rocks to kaolinitic soils characterized by the enrichment of Al, Fe, and Ti. Then, they were transferred to the karstic depressions and sinkholes and underwent in situ leaching and desilicification bringing about a relatively higher ore grade. The overall field and petrographic evidence demonstrate that these deposits were formed in shallow and swampy environments. In the eastern part of the Alborz zone (at Siahrudbar and Gheshlagh), better drainage conditions have enhanced the quality of ores.

中文翻译：

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Alborz 带（伊朗北部）岩溶铝土矿矿床的地球化学和矿物学特征：对形成条件、痕量和稀土元素的行为以及亲本亲和力的影响

摘要 本研究的重点是伊朗北部 Alborz 区上二叠统至下侏罗统岩溶铝土矿的三个露头。沉积物在上二叠统 Ruteh 组 (Gheshlagh) 和上三叠统 Elika 组 (Siahrudbar 和 Separdeh) 碳酸盐岩中以尖锐的半连续透镜形式出现。质构特征，如泥质基质、局部卵形-海藻质、流体和碎屑纹理表明自生-异质起源。矿物学上，Gheshlagh 的高岭石、勃姆石、针铁矿和赤铁矿，Siahrudbar 的水铝石、赤铁矿和高岭石，以及 Separdeh 矿床的高岭石、赤铁矿、针铁矿、水铝石、菱铁矿和菱铁矿是主要相。归一化到上大陆地壳 (UCC) 的主要元素的分布模式描绘了 Al、Fe、和 Ti 在所有三个矿床中。相对于 UCC，这三个矿床不同程度地富含过渡微量元素（Cr、Ni、Co、V）、U、Th 和高场强元素（Ga、Hf、Zr、Nb、Y）。所研究的铝土矿的稀土元素（ΣREE、La-Lu）的平均浓度介于 305.97 和 373.69 ppm 之间。地球化学数据显示，由于碳酸盐基岩的碱性条件，在 Separdeh 矿床剖面的下部逐渐富集 REE 和强烈的 (LREE/HREE)N 分馏，以及由于 Gheshlagh 和 Siahrudbar 矿床的不规则分布模式水位和 pH 值波动。Separdeh 和 Siahrudbar 的正 Ce 异常表明剖面上部的氧化条件。在 Gheshlagh，从负到零的 Ce 异常反映了低氧化强度。根据之前的地球化学分析数据，Gheshlagh 的上二叠统玄武岩和 Separdeh 和 Siahrudbar 地区的上三叠统玄武岩是最可能的源材料。二叠纪-上三叠统的局部隆升导致岩溶地形的形成，其中充满了来自潮湿热带古气候下玄武岩风化的物质。最初的风化作用使烃源岩转变为以富集铝、铁和钛为特征的高岭土。然后，将它们转移到岩溶洼地和落水洞中，进行原位浸出和脱硅，使矿石品位相对较高。总体现场和岩相证据表明，这些沉积物形成于浅水和沼泽环境中。