Study On Metallogenesis, Metallogenic

Dingjiashan Pb-Zn ore district in central Fujian province is located in Southeast tiwa region, Zhemin geodome system, Shaoguang tiwa series. Geological conditions in study area are very complex because of heavily influenced from many types of sedimentary formation, many times of structural activity, magmatism and metamorphism.There are two views on metallogenesis, one is contact metasomatic metamorphism ore deposit and the other one is massive sulfide ore deposit related to bimodal volcanic rocks (VMS). The focus of contention is original rock types of wall rock, metallogenesis type, mineralogenetic epoch, metallogenic material sources, controlling factors, and so on.Main wall rock in Dingjiashan Pb-Zn ore district is metamorphic rock composed by garnet, diopside, and epidote in Neoproterozoic Mamianshan formation Longbeixi group upper. Synthesis studies is used to recover the original rock of wall rock inclouding metamorphic rock attitude, mineral association, texture and structure, characteristic of distribution, petrochemistry, petrography, REE chemistry, and so on. The result confirms that wall rock of Pb-Zn ore body in study area are pararocks, original rock of them are calcareous rocks, like silty mudstones, calcium shale siltstone, shale calcium siltstone, marlstone and so on, but not bimodal volcanic rocks mentioned in previous research. These rocks are neritic (littoral) shelf facies-neritic platform facies.The result of Mineral mapping reveals that essential rock-forming minerals in wall rock and essential metallic minerals in ore body are all cut stratums, enrichment points of them distribute along Northeast and Northwest. The overall disposition of minerals volume percentage contour lines is northeast, which is consistent with the trend of Yanshanian granite in Upper Jurassic Changlin group. It confirms that the heat source of diagenesis and metallogenesis is Yanshanian calc alkalic-alkalic remelted granite. It intruded into the junction of NE fault and NW fault (the junction of ore body Ⅲ-1and Ⅲ-2) in the central of Dingjiashan Pb-Zn district and flowed to SW along NE fault. Rb-Sr isochron of fluid inclusion in quartz indicates the mineralogenetic epoch is (146.15±3.95) Ma, it was late Jurassic and third stage of early Yanshanian. It is found that there are many metallogenic material sources through systematic research on isotope like Pb, S, H and O, REE composition of granite, ore body and wall rock. Pb and Zn is supplied by Yanshanian granite and Neoproterozoic Mamianshan formation Longbeixi group. S is supplied by Yanshanian granite. Ore-forming fluid is composed by magmatic water and atmospheric water. Ore-forming temperature is200℃～350℃, which is confirmed by gas-liquid inclusions homogenization temperature, the result of sphalerite-galena geothermometer and pyrrhotite genetic mineralogy. Ore-forming pressure get from sphalerite (hexagonal pyrrhotite+pyrite) geobarometer is0.197～0.409GPa, which is close to0.2～0.3Gpa, the pressure of Yanshanian granite get from Q-Ab-Or temperature and pressure three phase diagram. There are many differences between Dingjiashan Pb-Zn ore district and VMS and SEDEX ore deposits. The metamorphism which is directly bound up with metallogenesis is contact metasomatic metamorphism between Yanshanian granite and the regional metamorphiced calcareous rock in Neoproterozoic Mamianshan formation Longbeixi group upper.There are three control factors to ore body, Yanshanian calc alkalic-alkalic remelted granite, calcareous rock in Neoproterozoic Mamianshan formation Longbeixi group upper and many kinds of structures like fault, anticline and angular unconformity. Yanshanian granite and calcareous rock in Neoproterozoic Mamianshan formation Longbeixi group upper supply material to contact metasomatic metamorphism, like Pb, Zn, S and a part of ore-forming fluid. Structures open up roads to granite and ore-forming fluid. Finally, these structures become to the spaces for ore bodies. NE faults are the most important ore-controlling structures.It is entirely different between VMS, SEDEX type ore deposits to Dingjiashan Pb-Zn ore district metallogenic system elements. Dingjiashan Pb-Zn ore district is contact metasomatic metamorphism metallogenic system related to Yanshanian granite, which belongs to compression structure metallogenic system. Ore bodies in Neoproterozoic Mamianshan formation Longbeixi group upper, Upper Jurassic Changlin group and angular unconformity between them are products of one metallogenic system but on different Mineralization network points.