Geological Characteristics and Genesis of the Greisen-Hosted Nb-Ta Mineralization in the Qidashan Iron Deposit, Liaoning Province, China, and Its Implications

The newly identified greisen-hosted Nb-Ta mineralization in the Qidashan iron deposit, Liaoning Province, China, offers a unique opportunity to explore how hydrothermal processes contribute to the enrichment of critical metals. In this study, an integrated analytical approach of petrographic observation and scanning electron microscopy–energy-dispersive spectrometer (SEM-EDS), electron probe microanalyzer (EPMA), and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U-Pb dating of columbite-group minerals (CGMs) were employed to systematically decipher the paragenetic sequence, micro-structure, elemental composition and mineralization age of CGMs, aiming at the genesis of greisen-hosted Nb-Ta mineralization. The mineralization is characterized by the abundant occurrence of CGMs. Three generations of CGMs and two mineralization stages are distinguished: stage I contains CGM Is and CGM IIs, with Nb2O5 ranging from 25.7 to 69.56 wt.% and Ta2O5 from 5.8 to 52.5 wt.%; stage II contains CGM IIIs, with Nb2O5 between 59.5 and 71.5 wt.% and Ta2O5 between 3.5 and 16.2 wt.%. CGM Is consist of euhedral, homogeneous crystals of more than 100 μm, exhibit low Ta/(Nb + Ta) ratios (0.05–0.06) and high Mn/(Fe + Mn) ratios (0.19–0.26), and belong to columbite-Fe. CGM IIs generally overgrow on CGM Is with hydrothermal overprinting textures, and show significant compositional gaps compared to CGM Is, exhibiting higher Ta/(Nb + Ta) ratios (0.13–0.55) and restricted Mn/(Fe + Mn) ratios (0.15–0.18), with some belonging to columbite-Fe and others to tantalite-Fe, which reveals a transition from magma to “hydrosilicate fluid”. CGM IIIs are mainly anhedral and homogeneous, with a grain size of less than 50 μm. However, some CGM IIIs overgrow on CGM IIs and/or CGM Is with patchy textures indicative of subsequent hydrothermal overprinting of hydrosilicate fluid, forming a coarse-grain size over 100 μm. CGM IIIs are characterized by lower Ta/(Nb + Ta) ratios (0.03–0.14) and variable Mn/(Fe + Mn) ratios (0.08–0.26), and they belong to columbite-Fe. LA-ICP-MS U-Pb dating yields weighted mean 206Pb/238U ages of 2646 ± 15 Ma for stage I and 2500 ± 28 Ma for stage II, indicating two-stage Nb-Ta mineralization. The early mineralization may correlate with the partial melting of volcanic–sedimentary rocks due to the geothermal anomalies associated with ~2.7 Ga submarine volcanism, and the late mineralization formed by the magmatic hydrothermal activities related to emplacement of the Qidashan granite in 2.5 Ga. We therefore propose that the two-stage greisen-hosted Nb-Ta mineralization probably widely occurred in these sedimentary–metamorphic iron deposits in the Anshan–Benxi area and even in the northern edge of the North China Craton, and it may provide new insights for evaluating the Nb-Ta resource potential in similar Algoma-type iron deposits globally.