Silver Deposits And Their Metallogenic Geodynamics Setting In Yanshan Area

Yanshan area lies on the middle part of North China platform, with longitude114°～120°and latitude40°00’～42°20’N, including North and East Hebei province and also West Liaoning province, which is one segment of west pacific ring. The researches on this area are very high, but there are many disputations or lack of metallogeny of single deposit or regional metallogenic regularities of large area, the study on metallogeny is not deep enough to appraise and ore-hunt of regional silver resources to certain degree.Based on practical job and the achievements of former scholars, the metallogenic regularities of silver ploy-metallic deposits are summarized by new thoughts in this thesis, including theories, practical illustration, exploration and summaries. The regional metallogenic model and the coupled relationship between metallogeny and geodynamics setting are brought out first time, which is of great help to unify the different opinions on metallogenic theories and acquaints, and also valuable to guide the ore-hunting and the study of fundamental geology.The thesis is carried out as the following:firstly, based on the field investigation combined with regional geological, geophysical and geochemical data, the metallogenic regularity and spatial-temporal distribution characteristics of silver, poly-metallic deposits are determined. Evidences, features and related theories are provided to confirm the metallogenic theories and also the constrict mechanism of geodynamics. Secondly, Fengning silver deposit is analyzed in detail, combined with the other silver poly-metallic deposit, i.e. Xiangguang in Zhuolu county, Caijiaying in Zhangbei city, Bajiazi in Jianchang county of Liaoning province, the geological and geochemical metallogeny characteristics and its evolution are discussed, and determine the major metallogenic features and their characteristics of terrestrial volcanic silver deposits. Thirdly, based on the metallogeny and the known typical deposits data, the regional metallogenic model is constructed, and the regional prospective targets are appraised in attempt.The achievements obtained by detailed and systematic researches are as fellowing:1. The metallogenic terrain in Yanshan area can be summarized into two type, terrestrial trough structural magmatic belt and upwarped structural magmatic belt. The typical deposits occurred in the former belt are Bajiazi, Pengjiagou, Xiangguang etc, whose common characteristics are Mn-bearing. The typical deposits in the latter belts are Caijiaying, Dajing and Jinjiagou etc, with elements combination of Gu、Pb、Zn、 Sn etc, also Mn-Ag. The occurrence of most silver poly-metallic deposits are controlled by Ⅲ、Ⅳ faults belts, especially the NE-NNE faults. The ore bodies are filled in the sub-faults of regional deep tectonics, or on or near the contact parts between the upper parts of sub-volcanic bodies and country rocks. The ore hosted strata are mainly Mesozoic terrestrial acid to neutral epithermal or super epithermal intrusion bodies, Jurassic volcanic clastic sediments and Paleozoic marine carbonates etc. The rock series occurred in ore deposits are lava, volcanic clastic rocks, sedimentary tuff, tuff-bearing sediments and various sub-volcanic rocks.2. The shape of silver ore bodies is complicated, with major type of vein, mesh vein, lens or bed, the mineral composite and hydrothermal alteration is of moderate to low hydrothermal gneiss. The gangue minerals is mainly fluorite, while the ore mineral is mainly of Fe-Mn type. The metallogenic physic-chemical parameters and geochemical information indicate that the deposits are formed under conditions of middle to low temperature epithermal or super epithermal environments. The ore forming materials are of various sources, the fluids are mixture of major precipitation with few magmatic and formation fluids.3. The cryptoexplosive mechanism is only a structural dynamics, and of dynamic source and provide volume-extension. The direct metallogenic environment is epithermal evolutionary fluid. The hydrothermal fluid can be divided into two type: SiO2-rich fluid which is formed after cryptoexplosion and acted as cementation of granite breccia, with little pyrite precipitation; ore-bearing fluid is mixed with lots of groundwater, which led to the precipitation of galena, sphalerite, pyrite, gold and silver and their sulfides. The major metallogenic material come from the deep parts of the earth, the primary fluid is intimately related to deep re-molten magmatism, and the final ore forming fluid is the mixture of the former and epithermal precipitation. The faults occurred after cryptoexplosive breccia or dissolution of groundwater were used by latter hydrothermal activity, and finally the silver poly-metallic deposits were formed.The strength of mineralization indicated by the color of fluorite is promoted first time, i.e. the darker the color of fluorite, the stronger the mineralization, which can provide new thoughts for ore-hunting.4. The terrestrial volcanic silver deposits in the studying area are controlled by the geodynamic setting of regional Mesozoic collision, transition of continental dynamic mechanism and also the mantle plume.5. The regional metallogenic regularity and model of terrestrial volcanic silver deposits are determined and ore hunting targets are promoted. There are vast perspective of large or super-large terrestrial volcanic silver deposits in Yanshan area, even the whole North China platform. The Mesozoic and Cenozoic sedimentary rebuilt bedded silver poly-metallic deposits should be sought in upper layers of volcanic sedimentary basins, while imbedded Mesozoic sub-volcanic silver deposits’ should be sought in floor rock series.The thesis is done on the basis of the achievements of former scholars, there must be some errors or shortcomings on opinions, methods or some certain conclusions, and any suggestions or criticisms are welcomed.