Metallogenesis Of Pb-Zn Deposits And Enrichment Regularity Of Dispersed Elements Cd, Ga, And Ge In SW Yangtze Block

The southwestern Yangtze block, located in the combined zone of Tethys and circum-Pacific tectonic domains, has undergone a complex geologic-tectonic evolutionary history since late Archean. Among various kinds of mineral resources in this area, Pb-Zn deposits are characterized by enrichment in dispersed elements such as cadmium (Cd), gallium (Ga) and germanium (Ge). A case study on some typical Pb-Zn deposits enriched in Cd, Ga, and Ge from SW Yangtze block (Huize and Fule in Yunnan, Tanbaoshan and Daliangzi in Sichuan, and Shanshulin and Niujiaotang in Guizhou) was undertaken during this study. Based on systematic field investigations, multiple methods including chemical analysis, ICP-MS and electron microprobe (EMP) are used to study ore deposit geochemistry and occurrence state of dispersed elements. In addition, the geologic, tectonic and geochemical conditions of Pb-Zn mineralization, the occurrence state and enrichment regularity of Cd, Ga, and Ge in SW Yangtze block are summarized in this dissertation.Regional strata and host rocks of the Pb-Zn deposits in SW Yangtze block have high contents of Zn, Pb, and Cd, which is fundamental for the Pb-Zn mineralization and the enrichment of dispersed elements. Element geochemical study shows that the contents of Zn, Pb, and Cd in the regional strata and host rocks are typically higher than those of similar lithologies of the earth's crust.Sulfur in the deposits was chiefly derived from host rocks. Sulfur isotopic compositions of sulfides in the typical ore deposits hosted in different strata are on the whole consistent with those of marine sulfates in corresponding host strata. Lead isotopic model ages suggest that the mineralization ages of the studied Pb-Zn deposits are later than or close to the ages of the host strata. The ore lead in the deposits mainly came from the upper crust.According to the lead isotopic model ages and geologic characteristics of the ore deposits, two main types of Pb-Zn deposits can be classified in SW Yangtze block, i.e., the Mississippi valley type (MVT) and the sedimentary exhalative (SEDEX) type. Metallogenetic ages of the MVT deposits, in which the orebodies occur as veins or stockworks, are ca. 200-300 Ma younger than the ages of corresponding host strata. By contrast, the metallogenetic ages of SEDEX deposits, in which the orebodies are stratiform or layer-like, are slightly younger than or close to the ages of corresponding host rocks.In primary ore minerals like sphalerite, galena, pyrite and chalcopyrite, Cd, Ga and Ge occur mainly as isomorphic forms. Trace amounts of independent minerals of cadmium, such as greenockite, were locally observed in the oxidized zones of the deposits.Cd contents in the studied Pb-Zn deposits are abnormally high, whereas Ga and Ge contents are relatively low. Cd and Ga are mainly enriched in sphalerite. Concentrations of Cd and Ga in the MVT deposits are much higher than those in SEDEX deposits...