The Study On Mineralization Of Zinc-Lead Deposits And Prospecting Direction In Northern Margin And Surrounding Area Of The Yangtze Platform, China

The metallogenic theory of marginal metallogenesis has become one of the most important research projects during recent years. Many ore deposit concentrated districts, large and super-large deposits were distributed along the continental margin, in which the metallogenesis were remarkable. The MVT type zinc-lead deposits found in the North America were typically distributed along the margin of large forelandbasin, and so were the VMS and SEDEX type zinc-lead deposits. The research on zinc-lead metallogenesis of the continental margin are very important, because of the abound ore resources, long-time activity, huge metallogenic potential, and so on. The core of the research on metallogenesis are the fluid and geological process of the continental margin, which are also the key of prospecting.The position and forecast of the ore deposits are the hotspot and front of Regional Metallogeny and Mineral Prediction at present, with the changing of prospecting objects from surface, shallow, recognizable deposits to concealed, deep, difficultly recognizable deposits. During the last 20 years, the major breakthrough on the prospecting of metal mineral resources in the world are mainly in the peripheral and deep area of the large deposit, important ore deposit concentrated districts or the district with less geological work. In addition, the major types of the deposits are mainly the types that the host rocks are volcanic rocks and sedimentary rocks during the last 10 years, and so are the porphyric deposits. The MVT and SEDEX type zinc-lead deposits are the most iomportant zinc-lead deposits all over the world now, and the Zn-Pb reserves of the deposits whose host rocks are sedimentary rocks are more than half of the world's Zn-Pb reserves. So, the prospecting directions are mainly the deposits whose host rocks are sedimentary rocks in future.Abundant lead-zinc mineral resources were distributed in the northern margin and surrounding area of the Yangtze Platform, whose types were mainly MVT and SEDEX type, and the newest zinc-lead deposits discovered were Mayuan, Zhenxun, Shennongjia bingdongshan and Muyuhe zinc-lead deposits. Ore deposits discovered in the area up to now are obviously characterized by their mineralization age, ore host strata and origin of ore deposit etc. For example, SEDEX lead zinc deposits are mainly occurred in the Zhenxun basin in the southern Qinling area and mostly reformed by later heat structural events. The lead-zinc deposits of Mayuan, southern area of Shanxi eastern-word to the western area of Hubei province are characterized by MVT mineralization series with obviously strong reformation type of lead-zinc deposits in Nanmushu, Mayuan nanmushu, and weak reformation type of lead-zinc deposits in Bingdongshan, Shennongjia. But there are many problems which need more research, such as the relationship between MVT and SEDEX zinc-lead deposits, the difference and characteristics of the MVT zinc-lead deposits in the northern margin of the Yangtze Platform and the typical MVT zinc-lead deposits, the ore-controlling factors and prospecting targets of the Zn-Pb deposits in the area, the prospecting direction, and so on.With the comprehensive data method and BP prediction method, this research systematically summarize the geological setting, ore-controlling factors, ore-forming conditions and prospecting targets based on the basic geological characteristics, taking the regional structure, sedimentary basins and mainland edge as the main research line.Lead and zinc deposits discovered in the northern margin and surrounding area of the Yangtze Platform separately occur in different structural setting with different scale, same material resources, forming different sizes of strata bound ore deposits. The mineralization system related to carbonate rocks in Sinian period and Silurian period is the main ore forming system, also suffering different later reformation. The lead and zinc ore deposits were formed in the extending period→the steady transition period→press and constringency period. The northern margin of the Yangtze Platform is placed in a platform flat setting of a stable continental margin. Large scale of thermal cycle ore forming system occurs in the ancient volcanic arc "heat basement". Mineralization is charactered by large scope, stable layer, hydrothermal filling rich in organic. Origin type of ore deposits is basically the same to the MVT. Ore deposits in Zhenan—Xunyang area are located in the passive continental margin of the Yangtze plate, i.e. the southern Zhen—Xun Paleozoic sedimentary basin. Where is tectonically charactered by stable and semi-stable natures, developed by various type of stable——semi-active sedimentary formations, and SEDEX deposits formed.The Mayuan lead-zinc ore field in the Yangtze platform occurs in carboniferous rock, and its host rock is unmetamorphosed dolomite. The ore bearing rock is breccia form dolomite controlled by strata and magma rock. The ore-forming temperature is from 85 to 210℃, which is low to ultra-low. It contains organic hot brine, accompanied by crystalline asphalt. CH₄, H₂S and CO₂ are contained in the inclusions, whose salinity is 7.6-12.0 wt. Sulphur in the ores is from sea water, and the dating by Pb isotope is 420-637 Ma, generally similar to the age of Sinian. In a word, it is similar to the MVT lead zinc deposit, compared with their characters in geology, isotope, REE and trace element.The Zhenxun Zn-Pb deposit is distributed in the strata of Silurian period, whose ore bodies is layered or like-layered. The ore-forming temperature is from 130 to 250℃, which is low to middle-low. There are lots of H₂O and CO₂, with less CH₄. The values of sulphur isotpe are all passive, and sulphur in the ores is mainly from sea water. The Pb isotope show that lead in the ores is from both the crust and mantle. The lead zinc deposits in Zhenxun ore field in Silurian is similar or consistent with the SEDEX lead zinc deposit from their occurrence of ores, ore texture, Pb isotope composition, REE composition, ore forming temperature and H-0 isotope composition of the inclusions. Therefore, the Zhenxun lead-zinc deposit is confirmed to be SEDEX deposit.The mineralization process for lead and zinc deposits discovered in the northern margin and surrounding area of the Yangtze Platform shows similarity and difference. "Heat basement"+ sedimentary are the two factors for forming the deposit. Stable platform flat and tension tidal flat on continental margin are the essential condition for the mineralization. "Heat basement" was composed of "mantle plume" of SEDEX deposit and "heat basement" of MVT deposit. The tectonic setting is normally the margin of large basin or uplift edge inside basin. Mineralized hydrothermal liquid is from the solution in sediments of strata. It shows similarity in heating form, extraction of mineral resource and transfer way etc. The host rocks in both of them are carbonate rocks, showing obvious characteristics of strata bound. The deposits were in a certain degree reformed, because of the later tectonism. The difference is the transfer path of the both mineralization hydrothermal liquid which resulted in the difference location of deposits.A vantage degree model is established for ore prospecting by using the comprehensive data method and BP prediction method. Seven prospecting targets are ascertained on the basis of quantitative calculation, which makes the intention more clearly in Zhenxun ore field and Mayuan ore field.