The Yanshanian Magmatism And Its Relations To The Cu(Mo)-Au Mineralization In Guichi District,Lower Yangtze River Metallogenic Belt

The Lower Yangtze River Metallogenic Belt(LYRMB)is located in central eastern China.It is one of the most important polymetallic metallogenic belt in China and its mineralization is closely associated with magmatism.The Guichi region of Anhui Province is one of the most important Cu-Au-Mo ore district in the LYRMB,rich in gold,silver,copper,molybdenum,sulfur and other mineral resources,and the mineralization has a close relationship with the Late Mesozoic igneous rocks.Late Mesozoic magmatic rocks are widely distributed in this region.Although a series of studies have been carried out on these late Mesozoic mgmatic rocks,however,the metallogenic relationship between magmatic rocks and the Cu-Au-polymetallic deposits is not well constrained.There is still lack of systemic petrogenesis,correspondingly deep geodynamics processes.Comparing with a study of different stages and different types magmatism and deep geological processes is necessary to rich the regional geological theory and helpful for the mineral exploration.This paper selectes the ore-bearing intrusions from the Tongshan,Anzishan,Laoshan,Tonglingpo,Baoshujian and Tongshanpai copper deposits and Zhujiachong,Zhaceqiao Au deposits and the alkaline rocks,such as Guilinzheng granite porphyry,Yangshan granite and Liwan K-feldspar granite as the research objects,and carries out integrated studies of whole-rock major and trace elements,Sr-Nd isotopes,zircon geochronology,and in-situ zircon Lu-Hf isotopes on intrusive rocks,to trace the magma sources and track the deep evolution processes.Integrated with the results of zircon U-Pb dating and previous geochronological data,these late Mesozoic magmatic rocks can be subdivided into two stages of magmatic activities.The first stage(148 to 136Ma)is characterized by high-K calc-alkaline intrusions that are closely associated with Cu-Au polymetallic ore deposits.Whereas,the second stage(130 to 125Ma)produced granites and syenites and is mainly characterized by shoshonite series that are related to Mo-Cu mineralization.The first stage ore-bearing intrusions are composed of granodiorite,monzogranite,diorite,belonging to high potassium calc-alkaline and calc-alkaline series.These rocks have relative low SiO2 but high Al2O3 and MgO,and is strongly enriched in light rare earth elements and large ion lithophile elements,but relatively depleted in high rare earth elements and high field strength elements,displays strong of Nb,Ta and Ti negative anomalies and no obviously Eu anomaly,which is characterized by adakitic rocks.The Guichi adakitic rocks could be derived from partial melting of subducted paleo-Pacific Plate,assimilated with Yangtze lower crust and remelting Meso-Neoproterozoic crust/sediments.The second stage magmatic rocks are typical A-type granites.These granites have lower MgO,CaO,Co,Sr,and higher Rb,Nb,Ta,Th,U and HREE contents,with enrichment in large-ion lithophile elements(LILE)and light rare earth elements(LREE),and slightly negative Eu anomalies,belong to the products under extension tectonic regime.The younger Nd model ages(1364?1490Ma),relative higher ?Nd(t)(-7.02?5.47)and ?Hf(t)(7.5?5.9)values indicate that the A-type granites are originated from partial melting of Mesoproterozoic-Neoproterozoic crust,with minor juvenile crust input,followed by fractional crystallization,such as plagioclase,apatite,biotite and Fe-Ti oxides.Through the REE and trace elements in zircon,the early adakitic rocks generally have high Ce4+/Ce3+ value,reflecting the high oxygen fugacity of the magma.However,the Ce4+/Ce3+ values of the late A-type granites are lower,indicating the magma with low oxygen fugacity.The early adakic rocks are enriched in Cu,Au,Mo elements,whereas the late A-type granites are mostly depleted of these elements.Thus,the high oxygen fugacity contributes to the formation of Cu,Au and Mo deposits.Combining previous research on the oxygen fugacity,we think that the magma of high oxygen fugacity is often closely related to the subduction of oceanic slab,while the late magma with low oxygen fugacity refelects the conversion of tectonic regimes,which is most likely to be caused by the lithospheric extension and thinning due to the rollback of the subducted paleo-Pacific plate.Integrating with the systemic studies,the rock association from early adaktici rocks to the late A-type granites are corresponding to continental magmatic process related to the subducted paleo-Pacific plate beneath the Lower Yangtze Block with subsequently rollback processes.After the rollback of the paleo-Pacific plate,the upwelling of asthenosphere triggers the partial melting of the Meso-Neoproterozoic rocks to produce the A-type parental magmas.The late Mesozoic magmatism in Guichi area is dominated by intrusive rocks,and controlled by regional structure.The ore-bearing intrusive rocks are distributed in the both sides of the Gaotan Fault and Jiangnan Fold and thrust belt.The main mineralization related to the late Mesozoic magmatic activity in the area can be divided into two episodes:one is the Cu-Au polymetallic mineralization related to the early adakitic rocks;the other is epithermal Mo-Cu mineralization associated with late A-type granites.In the early stage,the mineralization types include skarn type,porphyry type,hydrothermal type and contact metasomatic type.In the late stage,the epithermal mineralization is caused by the late magma overlapped with early mineralization.In the north belt of main thrust belonging to the LYRMB,the mineralization is mainly related to the late Jurassic-early Cretaceous basic-medium high-K calc-alkaline magma,in where the metallogenesis and occurrence regularity of orebody is similar to the Tongling area.Along the Dunshang-Zhangxi Fault,the mineralization is associated with the late Jurassic-early Cretaceous medium-basic and medium-acid high-K alkaline magma,but also affected by the early acid and alkaline magma.