Study On Occurrence State And Enrichment Mechanism Of Dispersed Elements In MVT Deposites--A Case Study For The Tianbaoshan And Daliangzi Pb-Zn Deposits In Sichuan Province

In many Mississippi valley type (MVT) Pb-Zn deposits, dispersed elements such as Cd, Ge, and Ga may reach to economically significant enrichment. Taking the Tianbaoshan and Daliangzi Pb-Zn deposits as examples, this thesis discusses the occurrence state and enrichment mechanism of dispersed elements in the MVT Pb-Zn deposits.The Tianbaoshan and Daliangzi Pb-Zn deposits, strictly controlled by "formations, phases and structures", are typical MVT Pb-Zn deposits. They also belong to the epigenetic hydrothermal strata-bound deposit.Studies on geology of the deposits and geochemistry of trace elements, REEs, isotopes, ore-forming fluids show that the ore-forming materials came from varied sources. Ore-forming materials of the Tianbaoshan deposit was chiefly derived from different sedimentary rocks of the upper crust and orogen, while those of the Daliangzi deposit mainly from the Sinian Dengying Formation. Meteoric waters were the major source of the ore-forming solution. Sulfide sulfur came from marine sulfates that were reduced by bacteria in the Sinian Dengying Formation.Based on observation of many polished and thin sections and analyses of electron microprobe, the author suggests that dispersed elements in both the Tianbaoshan and the Daliangzi deposits occur as isomorphism. Cd chiefly occurs in sphalerite with moderate to light colors, while Ga and Ge are mainly enriched in galena. Predominant amounts of the dispersed elements occur at the middle and late stages of the hydrothermal ore-forming epoch.Dispersed elements like Cd, Ge, and Ga were transported in the form of chlorine complexes or adsorbed by chlorine complexes of Pb and Zn. Thermal degradation of organic materials resulted in the generation of CH4 which further initiated the reduction of sulfates and the precipitation of sulfides. Under favorable physico-chemical conditions, dispersed elements such as Cd, Ge, and Ga that were co-transported with Pb and Zn may have replaced corresponding ions and accumulated as the mode of isomorphism.