Middle-Late Jurassic Cu-Pb-Zn-bearing And W-bearing Granitoids And Their Skarn Mineralization In The Nanling Range,South China

Granitoids and their related mineralization is always the highly focused topic for geologists all over the world.The Nanling Range in South China is one of the most famous metallogenic belts around the world,especially for its large-scale W-Sn mineralization.Middle-Late Jurassic is the most important epoch for granitic magmatism and associated mineralization in this region.Although numerous studies have been carried out on the petrogenesis and metallogenesis of the Middle-Late Jurassic ore-bearing granitoids in the Nanling Range,there are still great controversies and problems remaining to be solved,especially on the Cu-Pb-Zn-bearing and W-bearing granitoids and their skarn mineralization.Four main scientific problems are proposed as follows:(1)What are the petrogenetic differences and links between the Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granitoids in the Nanling Range?(2)How do the skarn deposits structurally connect to the Middle-Late Jurassic ore-bearing granitoids in the Nanling Range?(3)What are the genetic links between different mineralization types and the formation mechanism of complex zonation in the Middle-Late Jurassic Cu-Pb-Zn skarn deposits in the Nanling Range?(4)What are the key factors controlling the occurrence of unusual magnesian skarn W mineralization during Late Jurassic in the Nanling Range?The Tongshanling-Weijia area in the western part of the Nanling Range is selected as the study area.The main research objects of this Ph.D.thesis are the Tongshanling Cu-Pb-Zn and Weijia W skarn deposits.The South China Block is a major continental component with a complex history of tectonic evolution in East Asia.It was constructed through a Neoproterozoic(1.0-0.8 Ga)amalgamation of the Yangtze and Cathaysia Blocks.In Phanerozoic,the South China Block mainly experienced three tectonothermal events,i.e.,the Early Paleozoic(460-390 Ma)intracontinental orogeny,the Early Mesozoic(240-200 Ma)intracontinental compressional deformation,and the Late Mesozoic(Jurassic to Cretaceous)palaeo-Pacific plate subduction.As a consequence,the corresponding multiple-aged(i.e.,Neoproterozoic,Early Paleozoic,Triassic,Jurassic,and Cretaceous)granitoids and associated polymetallic mineral deposits are widely developed in South China.In the Nanling Range of South China,the most extensive Middle-Late Jurassic(165-150 Ma)ore-bearing granitoids can be divided into W-bearing,Sn-bearing,Nb-Ta-bearing,and Cu-Pb-Zn-bearing granitoids.Skarn is one of the most important mineralization types for the Middle-Late Jurassic polymetallic mineral deposits especially the Cu-Pb-Zn and W deposits in the Nanling RangeThe Tongshanling-Weijia area is situated in southern Hunan Province.The exposed strata in this area include Ordovician to Triassic systems except for the absence of Silurian system and Upper Permian to Lower Triassic series and are dominated by Devonian and Carboniferous systems.The Middle Devonian Qiziqiao Formation,the Upper Devonian Shetianqiao and Xikuangshan Formations,and the Shidengzi Member of the Upper Carboniferous Datang Stage are the main ore-bearing horizons in the Tongshanling-Weijia area.The structural framework of this area is south-north-to southwest-northeast-trending on the whole.The Tongshanling granodiorite and the Weijia granite are the dominant magmatic rocks in this area,which are genetically related to Cu-Pb-Zn and W mineralization,respectively.The Tongshanling Cu-Pb-Zn deposit,Jiangyong Pb-Zn-Ag deposit,and Yulong Mo deposit are distributed around the Tongshanling intrusion and constitute the Tongshanling Cu-Mo-Pb-Zn-Ag ore district.The Weijia W deposit is located 15 km to the northeast of the Tongshanling polymetallic ore districtThe Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granitoids in the Nanling Range are distinctly different in mineralogy and geochemistry.The Cu-Pb-Zn-bearing granitoids are dominated by weakly fractionated metaluminous I-type amphibole-bearing granodiorites,which have higher CaO/(Na2O+K2O)ratios,light/heavy rare earth element(LREE/HREE)ratios,and ?Eu values,lower Rb/Sr ratios,and weak Ba,Sr,P,and Ti depletions.The W-bearing granites are mainly highly differentiated peraluminous S-type granites,which have lower CaO/(Na2O+K2O)ratios,LREE/HREE ratios,and ?Eu values,higher Rb/Sr ratios,and strong Ba,Sr,P,and Ti depletions.The Cu-Pb-Zn-bearing and W-bearing granitoids were formed predominantly between 155.2 Ma and 167.0 Ma(peak value:160.6 Ma)and between 151.1 Ma and 161.8 Ma(peak value:155.5 Ma),respectively.There is a time gap of about 5 Ma between the two different types of ore-bearing granitoids.Based on detailed geochronological and geochemical studies of both the Tongshanling Cu-Pb-Zn-bearing and Weijia W-bearing granitoids and combined with a comparison between the Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granitoids in the Nanling Range,a genetic model of the two different types of ore-bearing granitoids has been proposed.The underplated basaltic magmas induced by the subduction of the palaeo-Pacific plate provided heat to firstly cause partial melting of the mafic amphibolitic basement in the lower crust,resulting in the formation of Cu-Pb-Zn mineralization related granodioritic magmas.With the development of basaltic magma underplating,the muscovite-rich metasedimentary basement in the upper-middle crust was then partially melted to generate W-bearing granitic magmas.The non-simultaneous partial melting of one source followed by the other brought about a time gap of about 5 Ma between the Cu-Pb-Zn-bearing and W-bearing granitoids.The microgranular enclaves in the Tongshanling granodiorite have dioritic compositions with a mineralogy dominated by plagioclase,amphibole,and biotite.Abundant residual materials,such as mafic mineral clots,inherited and metamorphic zircon,and Ca-rich core plagioclase,occur in the enclaves and are in favor of a restite origin.The amphibole-rich clots are considered as vestiges of residual pyroxene-rich precursors from the source.Three types of amphibole,i.e.,magmatic,metamorphic,and magma reworked metamorphic amphibole,have been recognized in the granodiorite and its enclaves according to their different occurrences,textures,and compositions.The zonal amphibole-rich clots exhibit increasing Al and decreasing Si contents from interior amphibole to exterior amphibole and also from core to rim in amphibole grains,and the outer parts of enclave magmatic zircon have higher ThO2+UO2 contents and lower Zr/Hf ratios than the inner parts,showing the process of magma reworking of the restite enclaves.Based on the textural and compositional evidence,these microgranular enclaves are thought to be reworked restite enclaves.This is also supported by thermobarometric calculation.A model illustrating the formation process of reworked restite enclave has been proposed.Combined with previous experimental studies of partial melting,the Tongshanling granodiorite is deduced to be derived from dehydration melting of amphibolite in the mafic lower crust.The fertile amphibolitic source is beneficial to the formation of Cu-Pb-Zn-bearing granodiorites in the Nanling RangeMagma emplacement-induced structural control on skarn formation at the Tongshanling Cu-Pb-Zn deposit was studied by structural analysis.Raman spectroscopy of carbonaceous material(RSCM)thermometry,and electron backscatter diffraction(EBSD)mapping.In the Tongshanling area,the regional normal faults were in all probability formed during the Late Triassic to Early Jurassic decompression and are not related to the emplacement of the Middle-Late Jurassic Tongshanling granodiorite.The emplacement of the Tongshanling granodiorite induced strong marbleization and deformation of the surrounding carbonate rocks.The wall-rock foliation cuts the bedding and is parallel to the intrusion boundary with a gradually decreased density outward from the contact zone.RSCM thermometry shows an outward trend of gradually decreased metamorphic temperature from ca.620? to ca.300?EBSD mapping reveals that the deformed calcite in the contact zone has strong shape preferred orientation(SPO)and crystallographic preferred orientation(CPO).The exoskarn and sulfide-quartz veins of the Tongshanling Cu-Pb-Zn deposit strike and dip consistently with the foliation of the deformed wall rocks.Combined with the RSCM and EBSD results,it is concluded that the exoskarn and sulfide-quartz veins were controlled by the magma emplacement-induced wall-rock deformation which significantly increased wall-rock permeability to promote the infiltration of magmatic fluids along fractures.The Tongshanling Cu-Pb-Zn deposit shows a well-developed outward zonation from proximal endoskarn through proximal exoskarn to sulfide-quartz veins and then to distal skarn,additionally with a few late Pb-Zn sulfide-quartz veins and carbonate replacement Pb-Zn sulfide veins distributed in the proximal part.The Jiangyong Pb-Zn-Ag deposit and Yulong Mo deposit are dominated by carbonate replacement and skarn mineralization,respectively.Garnet U-Pb dating yields a 207Pb/235U-206Pb/238U concordia age of 162.0 ± 3.7 Ma with a weighted average 206Pb/238U age of 162.4 ± 4.2 Ma for the proximal exoskarn in the Tongshanling Cu-Pb-Zn deposit.Molybdenite Re-Os dating of the Tongshanling Cu-Pb-Zn deposit and Yulong Mo deposit yields 187Re-187Os isochron ages of 161.8 ± 1.7 Ma and 160.0± 5.8 Ma with weighted average model ages of 161.9 ± 1.1 Ma and 160.1±0.8 Ma,respectively.U-Pb dating of the hydrothermal titanite in the altered granodiorite yields lower intercept ages of 155.5 ± 3.1 Ma and 155.6 ± 3.1 Ma in the Wetherill and Tera-Wasserburg U-Pb concordia plots,respectively,with a weighted average 206Pb/238U age of 154.4 ± 1.9 Ma S,Pb,and H-O isotopic studies reveal that the ore-forming materials and fluids of the Tongshanling polymetallic ore district were derived from the Tongshanling intrusion.The Cu and Zn were most probably released from the mafic amphibolitic lower crust by partial melting,whereas,the Pb was extracted from the upper crust by the ascending granodioritic magma.Based on the geological,geochronological,and isotopic geochemical studies,it is concluded that the different mineralization types and ore deposits in the Tongshanling ore district are genetically linked together and are the productions of evolution and zonation of the same skarn system associated with the Tongshanling granodioritic intrusion.A comparison of the Middle-Late Jurassic Cu-Pb-Zn and W deposits in the Nanling Range further supports the petrogenetic model of non-simultaneous partial melting illustrating the origins of the Cu-Pb-Zn-bearing and W-bearing granitoidsThe Weijia scheelite skarn deposit comprises dominant magnesian skarn and subordinate calcic skarn with abundant fluorite and is genetically related to a highly fractionated porphyritic granite with a felsitic to fine-grained matrix.The magnesian skarn generally occurs as stockwork veinlets in dolostone with a mineralogy dominated by serpentine and phlogopite.Wollastonite,garnet,and pyroxene are the main calcic skarn minerals.Scheelite is mostly distributed in the magnesian and calcic skarns as disseminated grains.Biotite fluorimetry indicates that the Weijia granite was crystallized from a F-rich magma.The high fluorine activity of granitic melts results in a low magmatic viscosity to enable the prolonged crystal fractionation and tungsten enrichment of the granitic magma to a relatively low temperature,finally with the porphyritic Weijia granite formed under a water-saturated condition.During the magmatic to hydrothermal evolution.F-rich hydrosaline melts were firstly separated from magma by liquid immiscibility followed by the exsolution of F-poor hydrothermal fluids.Both of the F-rich hydrosaline melts and F-poor hydrothermal fluids have transported tungsten from the magma into the wall rocks for subsequent skarn mineralization.RSCM thermometry indicates a distinctly lower temperature of magnesian skarnization than calcic skarnization.The relatively low temperature and high fluorine activity during magnesian skarnization are not favorable for the development of anhydrous prograde skarn minerals,such as forsterite and spinel,and lead to the formation of the special F-rich garnet.As the shared precipitant of fluorine and tungsten,calcium could be the critical factor to induce the positive correlation between the WO3 and CaF2 grades of skarn ores.The higher WO3 grades of the calcic skarn than those of the magnesian skarn are controlled by the higher calcium activity during skarnization in limestone than that in dolostone.Main conclusions of this Ph.D.thesis are summarized as follows:(1)The Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granitoids in the Nanling Range were derived from non-simultaneous partial melting of the mafic amphibolitic basement and the muscovite-rich metasedimentary basement,respectively;(2)The microgranular enclaves of the Tongshanling granodiorite are reworked restite enclaves derived from partial melting of the mafic amphibolitic source;(3)The exoskarn and sulfide-quartz veins in the Tongshanling Cu-Pb-Zn deposit are structurally controlled by the magma emplacement-induced wall-rock deformation;(4)The different mineralization types and ore deposits in the Tongshanling polymetallic ore district are genetically linked together and are the productions of evolution and zonation of the same skarn system associated with the Tongshanling granodioritic intrusion;(5)The key factors controlling the occurrence of unusual magnesian skarn W mineralization during Late Jurassic in the Nanling Range mainly include an enriched source,a fluorine-rich magma,a strong crystal fractionation,and a fluorine-rich hydrosaline melt.