Geochemistry Of Mesozoic Magmatic Rocks And Related Cu-Au-Mo Mineralizations In The Zijinshan Ore Field Of Fujian Province

As a famous Cu-Au mine,the unique and giant Zijinshan ore field is located in Fujian Province,South China,with proven reserves of 400t Au(metal),6,300t Ag(metal),4.0 Mt Cu(metal),and 0.11 Mt Mo(metal)Mineralization is mainly affected by late Mesozoic large-scale tectono-magmatic activity,distributed around intermediate-felsic volcanic rocks and hypabyssal porphyry intrusions,and evolved into a set of porphyry-epithermal systems with various mineralization types.This system mainly includes the high-sulfidation epithermal system(Zijinshan Cu-Au deposit),the low-sulfidation epithermal system(Yueyang Ag-dominated polymetallic deposit),the porphyry system(Luoboling Cu-Mo deposit,Jintonghu Cu-Mo deposit,and Wuziqilong Cu deposit).In addition,there are some transitional type ore deposits in genetic relationships between them,such as Ermiaogou Cu-Au deposit,Longjiangting Cu deposit and Bitian Au deposit.A lot of studies including mineralogy,petrology,economic geology,geochronology,and geochemistry have been carried out previously,which helps to understand the evolution of the tectono-magmatic-metallogenic system and ore prospecting in this district.However,most of these researches focused on the Zijinshan Cu-Au deposit,and studies on other deposits such as the Luoboling Cu-Mo deposit and the Ermiaogou Cu-Au deposit are relatively rare.Reliable geochronology and geochemical data were lacked,and no detailed studies of ore deposit geochemistry were carried out.Many problems are still existed in understanding the ore-forming process of the Zijinshan ore field:(1)the relationship between the Yanshannian large-scale magmatic activity and massive metal deposition;magmatic activity was mainly divided into two main stages with?40 Ma time interval,and mineralization is closely related to the late Cretaceous magmatic activity,what is the contribution of the early stage magmatic activity?Different types of mineralization corresponding to various magmatic activities and combinations of enriched elements,what causes the difference?What are the intrinsic mechanism and mutual relationship between magmatic evolution and metallogeny?(2)Issues about ages for the formation of the metallogenic systems and the sources of ore-forming materials.Different metallogenic systems show various combinations of ore-forming elements,whether is the same magma hydrothermal system evolution results or not?What differences about the ages and sources of ore-forming materials for these deposits?(3)The relationship between Late Mesozoic tectonic movement and massive metal accumulation.What are the main controlling factors for the formation of a series of large ore deposits and a variety of metal enrichment within such small field?What kind of mechanism is corresponding to magmatic hedrothermal evolution?Combined with previous researches,three deposits(i.e.,Zijinshan,Ermiaogou,and Luoboling)were investigated in details in this study using ore microscopy,trace element analyses in pyrite,and systematic multi-isotopes(S-Pb)tracing,zircon U-Pb dating,petrogeochemistry,and Sr-Nd-Pb-Hf isotope data in rocks and ores,in an affect to constrain their ages,origins and the relationship between the magmatic evolution and Cu-Au-Mo mineralization.This study shows the following major conclusions:(1)The Zijinshan Complex was emplaced in the Late Jurassic(164-155Ma),identified as prealuminous granites,and are characterized by low REE,depleted in Ba?Nb?La?Ce?Sr?P?Ti,and enriched in Rb?Th?U?Pb relative to primitive mantle.This complex shows various initial 87Sr/86Sr ratios,negative ?Nd(t)values(-10.3 to-6.8)and negative ?Hf(t)values(-19.0 to-7.1),initial 206Pb/204Pb of 18.388?18.632,207Pb/204Pb of 15.640?15.720 and 208Pb/204Pb of 38.670?38.930.The calculated TDM2(Hf)ages range from 1.62 to 2.37 Ga.Interpretation of the elemental and isotopic data suggests that the Zijinshan Granitic Complex were most likely derived from partial melting of the crust materials including Proterozoic orthometamorphic and parametamorphic rocks,and suffered assimilation and contamination by Indosinian granitoids.The initial magmas may have generated the Zijinshan Complex through fractionation of feldspar,allanite,and apatite at a relatively low magmatic temperature.We suggest that a continental arc related to the paleo-Pacific plate subduction was the most likely geodynamic force for formation of the Jurassic Zijinshan Complex in the Cathaysia Block.(2)Similar mineralization occurs both in the Ermiaogou and the Zijinshan ore deposits,which show a close relationship with dacite and cryptoexplosive breccia.Dacites were emplaced in the Cretaceous(108 Ma to 106Ma),identified as weakly prealuminous granites,and are characterized by enriched in LILEs,LREE,while depleted in Nb,Ta,P,Ti relative to primitive mantle.These rocks show an adakitic affinity,e.g.,strongly fractionated REE pattern and high Sr/Y and La/Yb ratios,without Eu anomalies.These rocks show relatively high initial 87Sr/86Sr ratios(0.7127 to 0.7132),negative ?Nd(t)values(-6.3 to-4.9),and negative ?Hf(t)values(-4.5 to 1.7).The ore-bearing Luoboling granitoids were emplaced at?103 Ma,while the ore-barren Zhongliao granitoids were emplace at?97Ma.Both the two granitoids show similar chemical compositions,and classified as weakly prealuminous granites.They are enriched in LILEs,e.g.,Rb,Ba,Th,U and Pb,and depleted in HFSEs,e.g.,Nb,Ta,P,Ti relative to primitive mantle.These rocks show an adakitic affinity,e.g.,strongly fractionated REE pattern and high Sr/Y and La/Yb ratios,and without Eu anomalies.These granitoids show relatively uniform initial 87Sr/86Sr ratios(0.7064 to 0.7068),negative ?Nd(t)values(-4.0 to-2.6),206Pb/204Pb of 17.641?18.144,207Pb/204Pb of 15.603?15.634 and 208Pb/204Pb of 37.990?38.438,and negative ?Hf(t)values(-5.8 to 0.7).Detailed petrologic and geochemical data indicate that the dacites may have derived from partial melting of mixtures of the lower continental metamorphic rocks and mantle-derived mafic magmatic rocks.By contrast,the Luoboling and Zhongliao intrusions were derived directly from a metasomatized mantle source,and fractional crystallization processes played a key role in the formation of these granitoids.These rocks were emplaced in a back-arc extensional environment that may have caused by break-off and roll-back of the subducted paleo-Pacific slab as a corresponding geodynamic tectonic setting(3)The ages of Zijinshan-Ermiaogou Cu-Au deposits are 110 Ma to 106 Ma,which are slightly older than the Luoboling Cu-Mo deposit(105 Ma to 103 Ma).The S-Pb isotopic compositions of sulfides suggested that the Cu-Au mineralizations have multiple origins,and show a close relationship to the Zijinshan Complex granitoids and the Cretaceous sub-volcanic rocks.Dacites are closely related to the Cu-Au mineralization and have relatively low initial magma temperature,indicating the characteristics of shallow sources and the contributions of the crust materials.Fertile magma was derived from subduction metasomatic mantle source in the Luoboling deposit,and the Pb isotope compositions of sulfides suggested that the Cu-Mo mineralization were mainly related to the porphyritic rocks.Moreover,the high contents of Re in the molybdenite also suggest Cu-Mo elements may have mainly derived from a mantle source.The Zijinshan Cu-Au deposit and the Luoboling Cu-Mo deposit should be identified as to two separate magmatic-hydrothermal ore-forming systems(4)Due to the involvement of mantle-derived materials,the magmatic activity was closely associated with mineralization,and the magma shows characteristics of high oxygen fugacity and high H2O content.Crustal thickening was caused by the subduction of the Pacific plate in the Middel-Late Jurassic,and mafic magma underplated the continental crust,triggering the melting and assimilation of crustal rocks.Magmas trapped in deep crustal sills in a compressional tectonic regime evolved to intermediate-felsic compositions.Cycles of addition of fresh magma and fractionation increase the content of volatiles and metals to generate abundant metals accumulations.Meanwhile,the Zijinshan Complex was formed as a result of the compressional tectonic regime Slab break-off,foundering and roll-back of the pale-Pacific plate occurred at?110Ma,resulted in an extension setting in the Zijinshan region,thus producing large-scale volcanic-magmatic activities.Sulphide-melt phases contain large amount of siderophile and chalcophile metals and are later remelted by subducted metasomatized mantle-derived mafic magma or dissolved by exsolving volatiles,and a highly enriched melt or volatile phase was generated,which are in favor of Cu-Au mineralization in the Zijinshan deposit.Hence,a fertile mantle-derived magma through a series of fractional crystallization under an extension setting,combined with the partial melting of the upwelling asthenosphere,and forming a metal-enriched melts.Melt reduction could be triggered by magnetite crystallization;favour the partitioning of reduced sulphur species to produce highly enriched ore fluids,creating rich mineralization.