Metallogenic Model And Prognosis Of The Longtoushan Gold Field, The Guangxi Zhuang Autonomous Region, China

The Dayaoshan metallogenetic belt is one of the important noble metal metallogenetic belts with the largest gold geochemical anomaly and about one hundred of gold deposits(or spot). In recent years, there are more new gold deposits discovered following breakthrough on research and exploration work, such as Wandao gold deposit. Meanwhile, other discovery on polymetallic deposits also indicates promising prospecting potential. The Longtoushan gold deposit in the Dayaoshan metallogenetic belt has been chosen to be the study object. Based on the field geological survey, the Longtoushan gold deposit has been studied through methods of ore deposit, geochemistry, stable isotope and chronology. Through three-year study, several results have been made as following:1. About the metallogenetic setting, the study has confirmed that the Cambrian strata in the region and mining area is the important source bed for gold with high background value. The orefield tectonic shows NNW(near SN) tensional-shear fracture is the leading fracture structure to control the ore-forming. Structure at the different scales controls variously on diagenesis and mineralization that deep fracture controls rock body and orefield, whereas anticline controls the occurrence of ore deposit, and small fracture controls the distribution of ore body.2. Through the systematic work on lithology, lithofacies, alteration and mineralization have been zoned in the Longtoushan rock body, meanwhile three mineralization epochs and six mineralization stages have been divided. Tourmaline related to mineralization is rich in iron but poor in magnesian, whereas tourmaline in rock body concentrates near the end member of rich magnesian through identification under microscope and electron probe analysis, so tourmaline rich in iron may one of the typomorphic mineral for prospecting.3. Through Re-Os dating of molybdenite, SHRIMP U-Pb dating of zircon and Rb-Sr dating of quartz fluid inclusion, diagenetic age of Pingtianshan and Shiziwei rock body and age of molybdenite, magmatic rock and ore-bearing vein have been determined. Thus the relationship between diagenesis and mineralization has been confirmed, and the order of intrusion is clear. Major rock body in the mining area intruded during Late Yanshanian, and the Longtoushan gold deposit and Pingtianshan molybdenum deposit formed closely relating to magmatism, so they are the same time products by tectonic-magmatism-mineralization event in Yanshanian.4. Based on the study about Sr, Nd and Hf isotope geochemistry and rock body geochemistry, diagenetic material source has been discussed that magmatic rock in the mining area mainly formed by anatexis of Cambrian strata. Characteristics of S, Pb and C, H, O isotope indicates metallogenetic material is mainly from old strata with few from other sources, so the ore-bearing fluid has multiple sources. The ore-bearing fluid in hydrothermal ore deposit such as Longtoushan gold deposit is from rock body with few rainwater at the late stage, whereas fluid in the fracture zone type gold deposit is mainly from meteoric water.5. Fluid inclusion has been studied from the Longtoushan gold deposit and other ore deposits and rock body in this orefield. The ore-forming fluid in Longtoushan is at the middle temperature(320-180℃), middle salinity(30-40wt%NaCl) and middle density (0.9～1.10g/cm3). So the mineralization depth has been calculated among 427-1152m. Fluid inclusions in quartz from the Longtoushan gold deposit, Pingtianshan granite rock body and Shiziwei granite porphyry rock boby have the similar characteristics that is NaCl-KCl-H2O system. Whereas fluid inclusion from the fracture zone type gold deposit in the periphery Cambrian strata indicates the epithermal feature.6. According to core logging and related analysis, Au, Ag and Cu have the vertical zoning with "upper gold and lower copper" zonation, so prospecting for copper ore body in the deep will be one promising aspect in the Longtoushan gold deposit. In the periphery, polymetallic mineralization occurrences in the Pingtianshan rock body are important clue for prospecting, and the Cambrian fracture zone type gold deposit is also promising.7. Combined with above study, the metallogenetic mode of the Longtoushan gold deposit has been built, and the orientation and criteria for ore prospecting have been suggested.