The Formation,Change And Preservation Of Granite-Related Hydrothermal Deposits,Southwest Fujian:Contraints From Late Mesozoic Granites

Southwest Fujian is located in the conjugation district of the Wuyishan and Nanling terranes,southeastern South China Block.Widespread magmatic intrusion and concomitant polymetallic hydrothermal deposits occurred in this area during Late Mesozoic,such as Makeng iron deposit,Zijinshan Cu-Au deposit.However,with improvement of ore prospecting and exploration in recent years,the burial depth of newly discovered deposits has increased and no major breakthrough has been made in prospecting and exploration of large polymetallic hydrothermal deposits.The regional prospecting potential is not only related to the metallogenic dynamic background and metallogenic process,but also closely related to the change and preservation after the formation of the ore deposit.This thesis firstly takes the late Mesozoic granite as the research object,and discusses the metallogenic geological background using zircon U-Pb dating,Lu-Hf isotope analysis and whole-rock geochemistry.Then zircon and apatite fission track thermochronology of bedrock is used to explore the exhumation history and its dominant factor of Late Mesozoic-Cenozoic basin-and-range system landscape.Finally,the time-temperature path of thermal history modeling is used to quantitate erosive thickness and elucidate the change and preservation of ore deposits,in order to evaluate the prospecting potential of hydrothermal deposits in Southwest Fujian.To be specific,the main research contents and conclusions obtained in the dissertation are as follows:（1）Metallogenic geological background of Late Mesozoic magmatic-hydrothermal deposit in Southwest FujianThe zircon U-Pb ages reveal that Southwest Fujian has experienced three periods of magmatism:The Late Jurassic magmatism（Stage 1:～164-161 Ma）includes the Hetian,Xikou and Gutian plutons;The Early Cretaceous magmatism（Stage 2:～143-142 Ma）includes Caixi,Yongfu and Dayang-Juzhou plutons;The late Early Cretaceous magmatism（Stage 3:～120-118 Ma）is granitic dykes interspersed in the edge of the previously emplaced granites.Whole-rock major element analysis shows high silicon（SiO₂>65%）,high potassium（K₂O>4.55%）,high aluminum（Al₂O₃:13.53-16.29%）,and low calcium（Ca O:0.482-2.1%）,and indicates a metaluminous-peraluminous high-K calc-alkaline series.Whole-rock trace elements show lowΣREE and enrichment of LREE,high（La/Yb）_N,negativeΔEu and relative enriched in large ion lithophilic elements（LILE）,light rare earth elements（LREE）,depleted in high field strength elements（HFSE）,indicating that the parent magma originated from crustal material.The low Fe O*/Mg O,low Zr+Nb+Ce+Y and the positive correlation between La and La/Sm indicate that the magma is mainly controlled by the partial melting of crustal materials.Magmatic rocks of Stage 1（Hetian and Xikou plutons）and stage 2（Caixi pluton）show lowε_Hf（t）value（Median:-7.1,-6.6,respectively）and ancient T_DM^Cages consistent with the age of Cathaysian basement,indicating that the parental magma generated from partial melting of the Neoproterozoic lower crust.Magmatic rocks of Stage 2 with highε_Hf（t）value suggest magma source was partial melting of juvenile material in the Jurassic thickened lower crust.Hf isotopic composition of granites of Stage 3 indicates its parent magma has experienced extensive crust-mantle mixing.（2）Late Mesozoic-Cenozoic tectonothermal histry in Southwest FujianThe Late Mesozoic-Cenozoic Basin-Basin system landscape of southeastern South China Block is of great significance to the evolution of tectonism and geomorphy in Eastern China.However,its formation timing still unclear.The fission track dating and thermal history modeling reconstruct the exhumation and cooling history of Basin-and-Range system in Southwest Fujian.The time-temperature paths of thermal history modeling reveal two episodes of rapid cooling events at～110-85 Ma and～40-25 Ma.The first episode rapid cooling event during～110-85 Ma is considered to be the cause of the formation of the Basin-and-Range system in back-arc extensional setting,and the normal fault at the margin of the Cretaceous basin mainly control the exhumation of range generally occupied by granites on its footwall.The second episode cooling event during～40-25 Ma is consistent with the youngest AFT ages（～25-20Ma）and probably indicates headwater incision towards drainage divide with the intensification of the East Asian Monsoon climate.The sharp discrepancy in apatite fission track ages across the Zhenghe-Dapu Fault zone implies that the collapse of the late Mesozoic coastal magmatic arc in the early Eocene（51.4-45.6 Ma）.Its tectonic mechanism was attributed to the northward propagation of extension related to the rollback of the Paleo-Pacific Plate,forming the eastward-dipping topography.（3）Changes and preservation of magmatic-hydrothermal deposits in Southwest Fujian and their constraints on prospecting potentialThe spatial mapping of apatite fission track ages in the Southwest Fujian shows that the distribution of mid-high temperature hydrothermal deposits in central and southeastern units is more preferred to low apatite fission track ages.This pattern indicates that the changes of low degree of denudation isconducive to the preservation of magmatic-hydrothermal deposits.The maximum erosional thickness of Makeng-Zhongjia deposit in southeastern unit is estimated to be 2.34-3.46 km,which is close to the estimated ore-forming depth by previous study of～2.9-3.5 km.This indicates this region experienced extensive erosion and low potential of the deep prospecting.The erosion thickness of the Zhudi porphyry molybdenum deposit in the central unit is about 2.01 km,which is obviously less than the maximum ore-forming depth of the porphyry-type deposit,indicating a great potential for deep prospecting.The uplift of the Zijinshan ore field in the northwest unit is～1.23-1.53 km,which is less than the ore-forming depth of the Zijinshan Cu-Au deposit（1.2-2.1 km）and Luobuling Cu-Mo deposit（1.2-3.2 km）,indicating high potential to explore deep ore bodies.Finally,two metallogenic prospective area are indentified in Zijingshan（Ⅰ）and Gutian（Ⅱ）according to the metallogenic geological background and exhumation history.