Geochronology, Ore Geochemistry And Genesis Of The Banpo Antimony Deposit,Guizhou Province, China

The Banpo antimony deposit, located between the southeastern margin of the Yangtze Platform and the South China fold belt, is a representative large one in the South China Metallogenic Belt of Antimony. It has favorable metallogenic background and geological condition for mineralization. In addition, it is the only proved super-large one in the Dushan antimony Orefield, occupies a decisive position in the antimony resources of Guizhou Province. The Banpo antimony deposit has made great contribution to the development of the antimony industry and local economy of Guizhou. However, the Banpo antimony deposit is running out of resource now. Thus, it’s urgent to develop new metallegenic theories to find more deposits of such kind at deeper and peripheral area to achieve breakthroughs of prospecting. Many researchers have studied on the deposit type, metallogenic condition, source and evolution of ore-forming metallogenic material and fluids, genesis and metallogenitic regulation, however, there are still many scientific problems demanding prompt solution, which include the metallogenic chronology and geodynamical setting, detailed mineralization process and metallogenic regularity, deep structure and prospecting prediction of the deposit.Due to many scientific problems exist in the studying of mineralization theory and metallogenic prognosis demanding prompt solution, in this study, we present systematic researches on regional metallogenic background, deposit geology, ore-forming age, and ore deposit geochemistry to discuss geodynamic background, major ore-controlling factors, metallogenic condition, and source and evolution of ore-forming metallogenic material and fluid of the Banpo antimony deposit. Moreover, we have revealed the processes of the activation, migration and mineralization of metallogenic matter, and established a metallogenic pattern for Banpo antimony deposit based on system summary the research results of the solubility and the numerical simulation of the tectonic stress field, temperature field and fluid migration of stibnite,. The study has important significance for enriching and improving the metallogenic theory for the Baopo antimony deposit and even for the South China Metallogenic Belt of Antimony, and providing theoretical support for mineral resource predication in the deep and periphery and obtaining breakthroughs of prospecting. The major results of the thesis are outlined as follows:1. Metallogenic epoch and geodynamic framework of the Banpo antimony deposit have been determined. Metallogenic epoch of the Banpo antimony deposit is dated at130.5±3.0Ma (MSWD=0.43) by calcite Sm-Nd isochron method, which is consistent with previously published metallogenic age of the Banian Antimony Deposit in the Dushan antimony Orefield. Thus, deposits in the Dushan antimony orefield, including the Banpo antimony deposit, were probably mineralized within the same period of125-130Ma which is consistent with the late Yanshanian mineralization ages of the South China Metallogenic Belt of Antimony (125-130Ma). Metallogenic geodynamic background of the Banpo antimony deposit is likely to be extensional environment of circum-Pacific subduction zones.2. The major ore-controlling factors were ascertained based on detail study on geology of the Banpo antimony deposit. The deposit is mostly controlled by lithofacies paleogeography, stratum lithology and structure. The lithofacies palaeogeography environment is littoral facies, shallow platform, facies, neritic shelf facies, and shallow sea basin caused by ancient faulting. The major hosting sequences of the Banpo antimony deposit is the Lower Devonian Danlin Formation composed of thick-medium thick Layered fine grain quartz sandstone intercalated by thin layers of sandy mudstone shale and, sandstone and argillaceous sandstone. The Banpo antimony deposit is mainly controlled by box-shaped anticline and fracture, and the fracture exhibits progressively ore-controlled features:regional level1fractures are ore-guide structures, which control the distribution of ore field; regional level Ⅱ fractures are ore-distribution structures, which control the distribution of ore deposits (spots); and regional level Ⅲ fractures are ore-containing structures, which directly control the distribution, morphology, scale and occurrence of the ore bodies.3. Based on systematic geochemistry study, we have determined the ore-forming physiochemical condition and source of ore-forming metallogenic material and fluid of the Banpo antimony deposit. The metallogenic conditions were weak acid and reducing, and the ore-forming fluid is Ca2+-SO42-type that riches in H2O and CO2, thus the Banpo antimony deposit is a typical low temperature, low salinity deposit. The metallogenic material was mainly derived from regional basement rocks, and the mineralization fluid was mantle-derived fluid that leached the country rocks. The sulfur showing characters of reductive marine sulfate was mainly from the host rocks; the CO2was mainly derived from the carbonate rocks, and the H2O predominantly comes from natural rainfall with partly from the magmatic water and metamorphic water. 4. Detail ore-forming processes of the Banpo antimony deposit was reconstructed based on analysis of Stibnite solubility and numerical simulation of Structure-Energy-Fluid Migration. With enrichment of Sb, all strata and rocks in the Banpo mining area are potential source for the Sb-rich ore-forming fluid. The tectonic stress action produced huge heat energy which remobilized the ore-forming elements to form Sb-S complex or hydrated antimony sulfide, thereafter, the stress releasing, temperature decreasing and solution acidizing leaded to the precipitation of the ore-forming elements.5. A probable metallogenic model has been builded based on all the available data. In Cambrian, the folded basement rich Sb and some other ore-forming elements was unlifted caused by major geological events such as Wuling and Caledonian movement that took place in the area. During Devonian, sedimentary environments of the mining area are littoral facies-shallow sea basin facies-neritic shelf facies-shallow sea basin facies, and the weathered material sources from the basement rocks was discontinuously deposited. During the Yanshanian period (125-130Ma), the circum-Pacific subduction cased a regional lager-scale fluid transportation and migrated Sb and some other ore-forming elements from the basement and stratas to form ore-forming fluid. The ore-forming fluid then migrated along the fault zones and interlayer fractures under tectonic stress. Finally, the temperature decreased and addition of SiO2and CO2resulted in fluid acidizing, the ore-forming fluid then precipitated in the fault zones and interlayer fractures and their vicinity.