Rb mineralization mechanisms and age in the Huashi deposit northern margin of the North China Craton: Constraints from zinnwaldite geochemistry, whole-rock geochemistry, and monazite–columbite–zircon geochronology

The Huashi Rb deposit on the northern margin of the North China Craton is a super-large, granite-hosted Rb deposit containing ∼ 1.8 Mt Rb 2 O at an average grade of 0.18 wt%. The deposit is genetically linked to the Madi granitic pluton, which is dominated by fine-grained alkali-feldspar granite and porphyritic amazonite-bearing alkali-feldspar granite. In this study, we integrate whole-rock geochemistry, zinnwaldite geochemistry, and U-Pb geochronology to constrain the mechanisms and age of Rb enrichment. The Madi pluton exhibits high SiO 2 (74.22–76.23 wt%) and alkali contents, low MgO, FeO, CaO, TiO 2 , and P 2 O 5 . It is strongly enriched in Rb, Th, Ta, and Hf but markedly depleted in Ba, Sr, and Eu. The rocks have low total rare earth element contents (51.53–61.23 ppm) with light-REE enrichment and pronounced negative Eu anomalies (Eu/Eu* = 0.02–0.06), accompanied by low non-CHARAC (charge‑radius‑controlled) trace-element ratios (e.g., Zr/Hf = 8.08–10.49, Nb/Ta = 3.04–4.59). These geochemical features are indicative of extreme magmatic fractionation. Together with a weakly peraluminous affinity (A/CNK = 1.01–1.06) and a negative correlation between P 2 O 5 and SiO 2 , they collectively define the pluton as a highly fractionated I-type granite. Rubidium is primarily hosted in potassium feldspar and zinnwaldite, with secondary zinnwaldite containing higher Rb 2 O (∼1.0 wt%) than primary varieties (∼0.7 wt%). Elevated F and Rb concentrations in metasomatic secondary zinnwaldite, combined with systematic decreases in whole-rock Zr/Hf, Nb/Ta, and K/Rb ratios with increasing TE 1 , 3 values, indicates that fluid-rock interaction during the magmatic-hydrothermal transition significantly upgraded Rb concentrations. U-Pb dating of magmatic monazite and columbite yields concordant ages of 174.3 ± 1.7 Ma and 175.5 ± 3.6 Ma, respectively, constraining the magmatic stage and initial Rb enrichment. In contrast, hydrothermally altered zircon records a younger age of 163.8 ± 1.6 Ma, likely interpreted as a later independent hydrothermal event. We propose a two-stage enrichment model: (1) initial concentration into primary zinnwaldite and K-feldspar by extreme fractional crystallization during the magmatic stage (∼175–174 Ma), followed by (2) further upgrading of Rb into secondary zinnwaldite through fluid-rock interaction with exsolved F-rich hydrothermal fluids. The ∼ 175–174 Ma mineralization age highlights the Middle Jurassic as a significant period of concurrent magmatism and metallogeny in this region.