Contributions from source enrichment and late hydrothermal activity to the formation of the No. 782 REE-Nb-Zr deposit, NE China

Abstract

Granite-type rare metal and rare earth element deposits (GRMEDs) have garnered significant attention from both academia and industry owing to their strategic importance in supplying critical metal resources. Nevertheless, the critical factors controlling the formation of GRMEDs remain empirically ambiguous. In the No. 782 REE-Nb-Zr deposit, NE China, three distinct periods of granites have been recognized: pre-mineralization monzogranite, syn-mineralization granitic complex comprising biotite granite-muscovitized granite-albitized granite, and post-mineralization syenogranite. Systematic petro-genetic investigations of these intrusive phases provide crucial insights into element super-enrichment mechanisms inherent to GRMEDs systems. Zircon UPb geochronological constraints reveal successive emplacement ages of 488 ± 4 Ma for monzogranite, 458 ± 4 Ma for the mineralized complex, and 451 ± 4 Ma for syenogranite. These granites exhibit diagnostic A-type affinities manifested by elevated Zr + Ce + Nb + Y concentrations (330–544 ppm), enhanced 10,000 × Ga/Al ratios (2.93–3.40), and pronounced negative Eu anomalies (Eu/Eu* = 0.15–0.29), consistent with magmatic rocks formed under extensional tectonics. Zircon grains from the monzogranite, biotite granite and syenogranite show positive ε_Hf(t) values of 4.2 ± 0.93 ‰ (1σ, n = 19), 3.6 ± 0.63 ‰ (1σ, n = 10) and 5.4 ± 0.82 ‰ (1σ, n = 10), respectively, coupled with elevated δ¹⁸O values of 7.72 ± 0.80 ‰ (1σ, n = 15), 7.45 ± 1.23 ‰ (1σ, n = 10) and 7.06 ± 0.74 ‰ (1σ, n = 20), respectively, indicative of derivation from juvenile lower crustal sources. Our geochemical data reveal distinct variations in ore-forming elements: the monzogranite shows significant depletion, contrasting with pronounced enrichment in the biotite granite and syenogranite. This result suggests that the monzogranite was likely derived from a crustal source depleted in ore-forming elements, while the enrichment of ore-forming elements in the granitic complex and syenogranite may be attributed to the incorporation of fluorine-rich volatile components from mantle sources. Furthermore, the granitic complex displays distinct HREE and HFSE distribution patterns, where biotite granite shows moderate HREE-Nb enrichment, albitized granite demonstrates significant enrichment (∑REE up to 2850 ppm), while muscovitized granite exhibits relative depletion. This differential distribution implies that element remobilization during muscovitization and subsequent precipitation through albitization constituted the dominant mechanism for rare earth super-enrichment. We therefore propose that the formation of GRMEDs requires two essential and interdependent conditions: (1) the generation of fertile magma through lower crustal metasomatism by mantle-derived volatiles, and (2) subsequent metal extraction and reconcentration via hydrothermal activity during the late magmatic evolutionary stages.