Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea

The metallogenic process of gold deposits is typically characterized by multi-stage mineralization and complex tectonic evolution. Precise determination of metallogenic age is thus critical yet challenging for establishing ore-forming models and tectonic evolutionary frameworks. The Koka gold deposit in Eritrea represents the largest gold discovery to date in the area, though its metallogenic age and tectonic evolution remain debated. This study employs in situ micro-analysis techniques to investigate major/trace elements and U-Pb geochronology of hydrothermal monazite coexisting with gold mineralization, providing new constraints on the metallogenic timeline and tectonic setting. Petrographic observations reveal well-crystallized monazite with structural associations to pyrite and native gold, indicating near-contemporaneous formation. Trace element geochemistry shows peak formation temperatures of 270–340 °C for monazite, consistent with fluid inclusion data. Genetic diagrams confirm a hydrothermal origin, enabling metallogenic age determination. Monazite Tera–Wasserburg lower intercept ages and weighted mean 208Pb/233Th ages yield 586 ± 8.7 Ma and 589 ± 2.3 Ma, respectively, overlapping error ranges with published sericite 40Ar/39Ar ages. This confirms Ediacaran gold mineralization, unrelated to the Koka granite (851 ± 2 Ma). Statistical analysis of reliable age data reveals a three-stage tectonic evolution model: (1) 1000–875 Ma, Rodinia supercontinental rifting, with depleted mantle-derived mafic oceanic crust formation and Mozambique Ocean spreading; (2) 875–630 Ma, subduction-driven crustal accretion and Koka granite emplacement; and (3) 630–570 Ma, post-collision crustal/lithospheric remelting, with mixed metamorphic–magmatic fluids and meteoric water input driving gold precipitation.