STRATIFORM ORE DEPOSITS IN SANMEN(ZHEJIANG) VOLCANO-TECTONO-DEPRESSION AND RELATION WITH ITS EVOLUTION
Zhu Shaohua, Rui Xinjian Gu Qiaogen, Feng Yicheng, Zhang Chunsun ( IGMR, Nanjing )
Sanmen volcano-tectono-depression is located in the north of the volcanic are in the coastal margin of Zhejiang-Fujian-Kuangdong provinces. In the depression there are over 50 mineralizations of iron, lead, zinc, copper, manganese, silver, molybdenum and sulphur. They are mainly volcano-sedimentary origin with late hydrothermal modification, such as Longchushan lead-zinc deposit in Linghai and Lijin iron-lead-zinc deposit in Sanmen. They are strictly stratabound. Orebodies are situated in the layer with definite rock assemblages. The composition and texture-structure of ores possess characteristics both of volcano-sedimentary and hydrothermal imprints. Wallrock alterations are mainly silicification, skarnization, chloritization etc.. showing vertical zoning. The formation of ore deposits can be subdivided into 3 stages, i. e. volcano-sedimentary stage, volcano-hydrometamorphic stage and volcano-hydrometa-somatic stage, thin mineralization temperatures are correspondingly 100~13℃, 200~400℃ and 230℃. The δ(34)S=+4~3‰ are considered that sulphur might come from upper mantle and lower crust with contamination of crustal substances. (206)Pb/(204)Pb= 18.292~18.435, (207)Pb/(204)Pb= 15.569~15.702, (208)Pb/(204) Pb = 38.586~38.974. The modal age is 207~266 Ma. Oxygen isotope suggests a lot of meteoric water participated in the mineralization fluid. Formation and evolution of the depression can be subdivided mainly into 4 stages: the first stage isto form the basement i. e. Xianrenqiao and Qiushuishan volcanic clomes; next is the formation of volcano-tectono-depression in NNE strike between the two domes and followed by the deposition of thick volcanic series; in the third stage the caldera and intrusion in it are formed in the depression, and at the end, the caldera is reformed by aregional NW shearing zone. In each stage of the evolution there are special geological conditions accompanied with unique ore deposits, forming a set of metallogenetic sequence in this subaerial volcanic area.