Genesis Of Dongyang Epithermal Gold Deposits In Fujian Province

The Dehua Dongyang gold deposit in Fujian Province is one of the most important epithermal gold deposits in the Golden Triangle of Fujian Province in China.Geologists pay more attentions to its metallogenesis,ore genesis and prospect since it was discovered as a large size at the1990s.On the basis of previous studies,this paper has carried out a systematic studies of geological,fluid geology,chronology,element and isotope geochemistry.The results show that:1.The main outcropped geological bodies in the mining area are the sedimentary rocks of Changlin Formation and volcanic-subvolcanic rocks of Nanyuan Formation,which are closely associated with dacite porphyry and alkali feldspar fine-grained rock dikes.The main structures are EN trend faults and secondary near WN trend faults,and the ore bodies are mainly hosted by secondary near WN trend faults.2.The deposit is composed of three low-angle gold mineralization belts?No.I,II,III?and one high-angle gold orebody?No.IV?.Most of the deposits occur in rhyolite porphyry of the second member of the Nanyuan Formation in Late Jurassic,which occurs in veins and is controlled by volcanic faults.The ore minerals are mainly pyrite,secondly chalcopyrite,sphalerite,galena,natural gold,etc.Surrounding rock alteration includes kaolinization,pyrophyllite,silicification,propylitization and carbonation,with a certain zonation.Orebody-centered outward sequential development:silicification–pyrophyllite-propylitization.The mineralization process can be divided into four stages from early to late:?1?gray silicified quartz stage,?2?gray quartz-pyrite stage,?3?gray quartz-polymetallic sulphides stage,?4?milky white quartz-calcite stage.3.The study of chronology reveals that zircon U-Pb dating results from rhyolite porphyry?host rock?,gold-bearing quartz vein,quartz diorite porphyry associated with ore bodies and dacite porphyry in the second member of the Nanyuan Formation of the Late Jurassic are 160.32?+0.8 Ma?,158.3?+0.9 Ma?,156.56?+0.8 Ma?and 154.14?+0.6Ma?.It represents that the acid volcanic eruption occurred in the early stage of the Late Jurassic?160 Ma?,and that the ore-bearing hydrothermal solution was located later than158 Ma.The acidic hypabyssal porphyry emplacement?157 Ma,154 Ma?was formed in the extension process of the Yanshanian Paleo-Pacific plate subducted to the Eurasian plate.4.Fluid inclusions mainly consist of pure liquid inclusions?type I?,liquid-rich inclusions?type??and three phases of CO₂?type??.Combining with the temperature measurement results of inclusions and laser Raman analysis results,it is concluded that the deposit is mainly composed of NaCl,CO₂ and H₂O and contains a small amount of C2H6 ore-forming multi-phase fluid system,in which the early stage ore-forming fluid is medium-high temperature fluid dominated by NaCl-CO₂-H₂,while the main component is NaCl-CO₂-H₂.The ore-forming stage is a medium-temperature ore-bearing fluid dominated by NaCl,CO₂ and H₂O.The late stage is a pure NaCl-H₂O system.Combined with the characteristics of H,O,S and Pb isotopes,the early stage of ore-forming fluid has the participation of magmatic water,and the later stage has the continuous addition of atmospheric precipitation.The ore-forming material should come from the deep lower crust.5.The geochemical characteristics of rhyolite porphyry,quartz diorite porphyry and dacite porphyry in the mining area show that they are deficient high field strength elements?HFSE?and enriched large ion lithophile elements?LILE?.They belong to high silica,peraluminous high potassium calc-alkaline rocks or potassium basalts series.They have the element geochemical characteristics of volcanic arc granite or active continental margin rocks,and have the characteristics of S,Pb isotopes.It shows that the ore-forming material or thermal source produced by magmatic fluid occurs in the ancient lower crust.From the point of view of compatibility and incompatibility of ore-forming elements in the magmatic evolution process of continental crust source,it is considered that the magma of this period may provide Au,Ag and As ore-forming fluids.Comparing with the typical low-sulfur epithermal gold deposits at home and abroad,the Dongyang gold deposit is considered to be a low-sulfur epithermal gold deposit.The diagenetic and mineralization geological process underwent partial melting of Paleoproterozoic basement crust source material during the subduction and extension of the Mesozoic Pacific plate and rapid upwelling of ore-bearing magmatic hydrothermal fluid under the action of intense compression and extension.After a series of fluid evolution?water-rock reaction,ore-forming fluid boiling,fluid mixing?,the ore-forming hydrothermal fluid is finally enriched.