Nature and origin of dolomite-hosted Th-U-HREE mineralization in the Tethyan Metallogenic Belt, Biga Peninsula, Northwestern Türkiye

The Biga Peninsula in northwestern Türkiye, a metallogenic province in the Tethyan Metallogenic Belt, hosts economically significant gold-copper deposits and thorium-uranium-rare earth elements (Th-U-REE) occurrences, including both light REE (LREE) and heavy REE (HREE) styles of mineralization. This contribution focuses on the nature and origins of Th-U-REE mineralization in the Arıklı district, Biga Peninsula, where three zones of Th-U-REE mineralization (A, B, and C) each extends only a few meters along faults in Cenozoic volcanic rocks. Zones A and C of U-Th-LREE mineralization are hosted by fluorapatite-sulfide-hematite and dolomite-fluorapatite assemblages and have U-Th-REE concentrations up to 1,152 ppm and 2,221 ppm, respectively, with fluorapatite as the principal U-Th-REE-bearing mineral. In contrast, Zone B of Th-U-HREE mineralization is hosted by a dolomite-dominant assemblage, with Th, U, and total HREE + Y concentrations up to 25,600 ppm, 4,100 ppm, and 3,535 ppm, respectively, with cheralite (CaTh(PO₄)₂) as the principal ore mineral. Four stages of dolomite have been identified in the main mineralization Zone B: fine to medium-crystalline dolomite (D1; anhedral), pervasive medium-grained dolomite (D2; subhedral to anhedral), dolomite rhombs (D3), and void-/vein-filling coarse-crystalline dolomite (D4). Two stages of void-/vein-filling cheralite coincide with dolomite D3 and D4. The first stage cheralite shows intense hydrothermal modification, resulting in a disturbed U-Th-Pb isotopic system. Secondary cheralite of minimal hydrothermal disturbance yielded ²⁰⁶Pb-²³⁸U age of 18.8 ± 1.6 Ma (2σ, MSWD = 1.3) and ²⁰⁸Pb-²³²Th age of 18.15 ± 0.08 Ma (2σ, MSWD = 2.3). Carbon and oxygen isotope compositions differ significantly from those of primary igneous carbonatites but are close to sedimentary rocks, indicating that all dolomites have a hydrothermal origin with sedimentary sources. These results collectively suggest that Early Miocene hydrothermal fluids most likely originated from mantle-derived alkaline magmatism and interacted with carbonate sedimentary rocks, facilitating the mobilization, enrichment, and precipitation of dolomite-hosted Th, U, and HREE mineralization in fault zones.