Study on breccia body characteristics and element migration during the metallogenetic process of Qiubudong silver deposit in Pingshan County, Hebei Province

The Qiubudong silver deposit is a cryptoexplosive breccia type deposit. In order to find the indicators of proximity to the ore body, the characteristics of the breccia body and the geochemical characteristics of alteration elements in the host rock are studied in this paper. The statistical analyses of the geology and lithology of the breccia body, characteristics of the breccia (e.g., shape, size, composition) and cement indicate that: ① the shape of breccia changes from angular to subangular to subcircular as the depth increases; ② the size of breccia decreases from the edge to the center in the surface, and the spatial change of breccia size is not obvious in the borehole; ③ the composition of breccia changes from leptite to quartz porphyry, and then become complicated as the depth increases, which indicates that the formation processes of breccia body are dominated by gas explosion (rock powder cementation), liquid explosion (polymetallic sulfide cementation) and magmatic explosion (magma cementation) from the surface to the depth. The geochemical characteristics of elements in the alteration process of host rocks indicate that the introduced components are SiO₂, CaO, MgO, and the migrated components are K₂O, Na₂O, Al₂O₃, Fe₂O₃, FeO, Sr, Ba, etc. The contents of ore-forming elements such as Au, Ag, Cu, Zn, Sb and Pb increased significantly. Considering that Fe, Cu, Pb and Zn existed in the sulfide, REEs showed that the ore forming environment gradually changed from oxidation condition to reduction condition. The comprehensive characteristics reveal that the deposit has undergone at least three stages of fluid boiling processes, and silver unloading mineralization occurs on the basis of sericitization as well as pyritization based on cryptoexplosive breccia formation. Therefore, the occurrence of these two types of alteration is a favorable indicator of proximity to the ore body.