关键词: 矿床成因, 花岗斑岩, 成矿时代, 地球化学, Sr-Nd同位素, 丰宁兴源萤石矿床

Abstract:

The Fengning Xingyuan superlarge fluorite deposit in Hebei Province is located in the western section of the north Hebei-west Liaoning fluorite metallogenic belt. While previous studies have established fundamental geological characteristics of the deposit, but the formation age, metallogenic dynamics background, material sources and genetic classification of the deposit are still unclear. This study integrates zircon U-Pb dating, rock geochemical analysis and Sr-Nd isotope studies of the mineralised granite porphyry, building upon comprehensive field investigations. The results show that fluorite mineralization occurs within granite porphyry veins and their hanging wall/footwall of fracture zones, exhibiting vein, reticulate-vein, breccia, and banded structures, and accompanied by pervasive silicification and sericitization of surrounding rocks. Zircon U-Pb dating yields an Early Cretaceous age (（137.9±3.4） Ma) for the granite porphyry, which displays distinctive geochemical features of high silica (w(Si2O)=72.48%-75.70%), high alkalinity (w(Na2O+K2O)=5.28%-10.23%), calcium-poor (w(CaO)=0.35%-2.52%), magnesium (w(MgO)=0.10%-1.47%), belonging to the peraluminous, calcium alkali-potassium basalt series. The trace element spider diagrams show enrichment in Rb, Th and Zr but depletion in Sr, Ba, Nb and Ta, and the relative enrichment of light rare earth elements and the strong Eu-negative anomalies, toghther indicating A-type granite characteristics. Integrated with regional tectonic evolution, these features suggest the granite formed during large-scale extension and thinning of the North China craton. The Sr-Nd isotopes show that the granite porphyry likely originated through partial melting of crustal source materials. The geological and geochemical characteristics of the deposit suggest that the Ca in the mineralizing fluids originates from the hydro-rock interaction between the hydrothermal fluid and the volcanic host rocks/Ca-rich strata, while the F derives principally from the magma, and the deposit constitutes a medium-low-temperature magmatic-hydrothermal fluorite deposit genetically related to the granite porphyry.

Key words: deposit genesis, granite porphyry, age of mineralisation, geochemistry, Sr-Nd isotopes, Xingyuan fluorite deposit, Fengning