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Abstract
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The Lakeh Siah iron oxide-apatite deposit occurs within the Cambrian succession of volcanic tuffs and rhyolitic rocks intercalated with dolomites, dolomitic limestones, and evaporites. Magnetite is the dominant ore mineral, accompanied by apatite, amphibole, diopside, calcite, and andradite garnet. Apatite, allanite, and titanite are the main rare earth element (REE)-bearing minerals. Concentrations of minor elements in magnetite and apatite are consistent with those in the magmatic Kiruna-type IOA deposits. The magmatic stage in the Lakeh Siah deposit is recorded in magnetite-rutile symplectites pseudomorphozing pseudobrookite and replaced by titanite-rutile ± ilmenite ± magnetite, titanite + rutile, and finally REE-rich titanite. The pseudobrookite with 28–42 mol% FeTi2O5 was either a primary magmatic mineral or a product of hemoilmenite breakdown at temperatures higher than 810 °C. The titanite-rutile-ilmenite-magnetite symplectites originated between 475 and 700 °C and oxygen fugacities 1.6–1.9 logfO2 units below the hematite-magnetite buffer. Fluorotremolite associated with diopside and fluorapatite crystallized at temperatures between 710 and 780 °C, and high oxygen fugacities, up to 6 logfO2 units above NNO buffer. The oxidizing conditions are reflected in increased ferric iron contents ( Fe3+/Fe2+ molar ratios > 1.7) in tremolite, contrasting with the negligible Fe3+ in superimposed actinolite. Hydrothermal alteration of the primary magmatic, REE-bearing fluorapatite resulted in the reprecipitation of REE-depleted fluorapatite, monazite-(Ce), and arsenian xenotime. Epidote-ferriepidote and allanite-ferriallanite with up to 26 wt% REE (0.98 apfu) were also replaced by an unidentified Y–Ce–Nd–concentrating Ca,Fe,REE-hydrosilicate, containing up to 39 wt% Y + REE. Low yttrium contents in the neoformed monazite (< 0.02 apfu) indicate crystallization temperatures below 200 °C, but the monazitexenotime thermometer based on Gd partitioning returns up to 700 °C. Bre
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