Polygenetic titanites constraining the genesis of Neoproterozoic leucocratic-dyke-hosted U mineralization at the western margin of the Yangtze Block

Accurately constraining the age and source of primary U mineralization is commonly difficult as diverse U minerals are affected by α-recoils and subjected to overprints of multi-stage fluids. Titanite has a good potential in revealing rock-forming and metallogenic information, and widely occurs in magmatic, metamorphic, and hydrothermal U deposits. However, the textural and chemical characteristics of titanite from these U deposits are poorly documented and little is known about the relationship between titanite and U mineralization. At the western margin of the Yangtze Block, a series of U occurrences and uraninite-predominant ores are hosted in the leucocratic dykes, ranging from fine-grained (albitised) monzogranite to pegmatitic quartz vein. The genesis of U mineralization remains unclear largely because of the uncertainties as to the timing of mineralization and the origin of metals. Here, we present comprehensive UPb and SmNd isotopic data, and major and trace element compositions for metamorphic (Ttn-I) and magmatic/hydrothermal (Ttn-II) titanites from the leucocratic dykes and/or amphibolitic wall rocks. Ttn-I is in direct contact with metamorphic hornblende and plagioclase or occurs as xenoliths hosted in the leucocratic dykes, and is thus inferred to result from amphibolite facies metamorphism. Chemically, Ttn-I is characterized by low Fe/Al ratios, LREE-depleted patterns and weakly negative to positive Eu anomalies. By contrast, magmatic/hydrothermal Ttn-II crystals intergrown with uraninite in the leucocratic dykes generally display flatter REE patterns, more negative Eu anomalies, higher Fe/Al ratios, and U, Nb, REE and Y contents, which could be applied as an indicator for U exploration. LA-ICP-MS UPb dating on Ttn-I yielded ages of ∼860 Ma for amphibolite facies metamorphism. Instead, UPb dating on syn-ore Ttn-II yielded ages of ∼786–779 Ma for U mineralization, which is coeval with the emplacement of leucocratic dykes. Uraninite and intergrown Ttn-II have uniform, negative εNd(t) values (−5.7 to −15.5), indicating the initial melts were predominantly sourced from the old crustal materials (TDM2 = ∼1.9–2.7 Ga). Considering the occurrences as dykes, feldspar-quartz-dominant lithologies, and wide textural and compositional variations of leucocratic dykes, we infer that low-degree partial melting of U-rich protolith is the main factor controlling U concentration in the melt. Variable REE patterns and ThO2 contents in different uraninite types s