Metallogenic characteristics and relationships of ore deposits in the north and south lead-zinc zones in the Xicheng area, Gansu

Abstract：SEDEX lead-zinc deposits in the Xicheng(Xihe-Chenxian) area, Gansu, have long been divided into the reworked sedimentary type of the south ore belt and the metamorphic sedimentary type of the north ore belt. The two types show appreciable differences in host horizon, sequence of host strata and geological characteristics. Comparing the geological and geochemical characteristics of the two ore belts, the authors find that the two ore belts have much more similarities than differences. The sequence of the host strata in the Changba-Lijiagou deposit in the north ore belt is inverted, with limestone lying below and schist above, being similar to that in the south ore belt. In the Xicheng basin, there are several nearly E-W-trending lead-zinc ore belts, and the north ore belt is only one of them. The differences of the two ore belts are mostly manifested in the manner of late-stage reworking. The reworking of the Changba-Lijiagou deposit occurred in a relatively “dry and enclosed” system and thus the intact striped-banded sedimentary character may be found. By contrast, the reworking of many lead-zinc deposits in the south ore belt occurred after the in-situ ore-forming material was transported and precipitated and concentratied in a structural saddle in an “allochthonous”, “wet” (through hydrothermal processes), relatively open environment, with some exotic material introduced into it. The cause for the difference in the manner of reworking is the difference in the metamorphic grades related to acid intrusive magmatism in the study area in the Indosinian period.