Geology And Geochemistry Of Typical Ore Deposits From The Qinling Orogen And Their Indications For The Key Orogenic Events

Orogenic belt,as the most complex tectonic zone of lithospheric plates subduction-accretionary and collision,not only produces various petrological-tectonic assemblages but also prompts metallogenic materials enrichment,forming numerous large-super large deposits and ore-centralization areas.Therefore,the studies on continental orogenic belts and their related tectonic-magmatic-fluid-metallogenic activities have been the frontier issues of geosciences.The Qinling Orogen situated in central China marks the tectonic boundary between the North China Block(NCB)and Yangtze Block.It is considered a multistage continental orogen that was built through ocean-continent intermingling from Late Archaean to Mesoproterozoic,northward subduction-collision main orogeneses among the NCB,South Qinling Microblock and Yangtze Block along the Shangdan suture zone and Mianlue suture zone,which developed in modern plate tectonic system from the Late Proterozoic to Middle Triassic,and finally intracontinental superimposed reformation in post orogenic stage.This orogeny records an integrated and complex geological history,including continental rifting,opening of ocean basins and their subsequent destruction at convergent margins,continental growth,continental collision,and intracontinental tectonics,and also carries abundant mineral resources.With taking the advantage of evolution complexity,this dissertation starts off from the coupling of metallogenic system and orogenesis,focuses on the three pivotal Qinling orogenic events,including Caledonian subduction orogeny,Indosinian collisional orogeny and Yanshanian intracontinental orogeny,and selects representative Tongyu VHMS-type Cu deposit in the North Qinling Orogen,Wenquan porphyry-type Mo deposit in the Western Qinling Orogen and Xiaohekou skarn-type Cu deposit in the South Qinling Orogen to conduct systematic studies,in order to discuss the constraints of different orogenic events on the crust-mantle interaction and exchange,ore source supply,ore-forming fluids transport,metal endowment and deposits location,and to reveal the coupled-relationships among tectonic-magma-fluid-metallogenesis caused by different Qinling orogenic events.The petrogenic and metallogenic models were set up based on formation and evolution of the Qinling Orogen.Guided by plate tectonics and metallogenic system theories,the dissertation uses continental dynamics approach and comparative ore geology thought,incorporating ore formation regularities of different tectonic settings,different eras and different genetic types deposits into the evolving processes of the Qinling composite orogen.Based on carefully studies of geological features,the dissertation conducts multiple disciplines of petrography(or mineragraphy),chronology,petrogeochemistry,deposit isotope and fluid inclusion geochemistry as well as analytical test methods to research the tectonic-magma-fluid-metallogenesis for the Tongyu VHMS Cu deposit,Wenquan porphyry Mo deposit and Xiaohekou skarn Cu deposit,to discuss their petrogenesis-mineralization ages,material sources,ore geneses,metallogenic regularities and metallogenetic geodynamical settings,to clarify the restriction of ocean-continent subduction orogeny,continent collisional orogeny and intracontinental orogeny on fluid transport,ore source supply,ore enrichment and deposit location.The main conclusions are as follows:1.During the Late Ordovician to Early Silurian,northward subduction of the Paleo-Qinling oceanic crust induced dehydration of the slab and melting of the overlaying sediments,which generated a large amount of high fO2 aqueous hydrothermal fluids.The fluids rose into mantle wedge,activated and extracted Cu and other ore-forming elements,then triggered partial melting of the mantle wedge,which consequently produced the ore-bearing hydrothermal fluids.The magma and ore-forming fluid upwelled and erupted in surface,finally formed the Tongyu VHMS-type copper deposit.At the meantime,the partial melts derived from the subducted mafic oceanic slab entered and reacted with the mantle wedge and produced the adakitic magma which intruded as the Meigou granodiorite.The Tongyu copper deposit occurs in the metamorphic intermediate-mafic volcanic rocks of the Xieyuguan Group.Orebodies are lenticular and stratoid,generally parallel with the strata,and continuously distributed with diverse thicknesses.Host rock alterations in the Tongyu deposit are pervasive with marked zoning,and mineralization is spatially associated with hydrothennal alteration.Alteration types in center are dominated by actinolitization and diopsidization,accompanied by garnetization,epidotization and chloritization.From the center to the periphery area,alteration is weakening and replaced by silication and sericitization.Ore sulfides are dominated by chalcopyrite,pyrite and pyrrhotite,and subordinate metallic minerals include sphalerite,magnetite and stibnite,etc.The Meigou granodiorite intruded the south wing of anticline in the Tongyu deposit.There's only sporadic mineralization in it without industrial orebody.Pyrite,chalcopyrite and sphalerite yield two sets of Re-Os isochron ages of 448±33 Ma and 390±19 Ma,respectively.The former isochron age is identical with the U-Pb ages of volcanic host rocks(445±2.0 Ma?437±4.2 Ma)and Meigou pluton(442±2.2 Ma?441 ±2.0 Ma),suggesting the volcanic eruption,magmatic intrusion and volcano-sedimentary mineralization occurred simultaneously in the O3-S1.The later age probably represents another hydrothermal metallogenesis that caused by structural reworking and overprinting,when the Shangdan ocean basin closed.The volcanic host rocks from Tongyu Cu deposit display a differentiation character from basalt to andesite,to dacite and to rhyolite,which constitutes an integrated island arc tholeiite suite,with the similar trace element characteristics to typical island arc basalt.The Meigou granodiorite is characterized by high Mg#values,enrichment in Ni and Cr,intensive LREE-HREE fractionation,unobvious Eu anomalies,high Sr content,and low Y and Yb contents,showing an island arc adakitic geochemical signature.Systematic trace elements and Sr-Nd-Pb-Hf isotope geochemistry studies reveal that both the volcanic host rocks and Meigou pluton were produced by a same oceanic subduction event in island arc setting,but their parent magma derived from different source regions.The volcanic host rock belongs to normal island arc lava,which was derived from partial melting of mantle wedge with contamination of lower crustal materials,while the Meigou pluton is island arc granitoid with adakitic nature generated by partial melting of the mafic Paleo-Qinling oceanic slab and interacted with mantle wedge peridotite.The studies on S-Pb isotope consistently show that the ore-forming materials and volcanic host rocks share a common source,reflecting a mixed mantle-crust source,and the Meigou pluton might has no contribution to mineralization.Ore sulfides exhibit similar chondrite-normalized REE patterns and REE eigenvalues,such as(La/Yb)N,LREE/HREE,?Eu,?Ce and Y/Ho to the volcanic host rocks,rather than Meigou pluton,further suggesting that ore-forming materials were sourced from the volcanic host rocks.2.At the Late Triassic,the Wenquan Mo deposit was formed in the transition tectonic regime of syn-collision compression to post-collision extension during the continental collision orogeny between NCB and Yangtze Block along the Mianlue suture.The break-off of the subducted Yangtze lithosphere initiated an upwelling thermal pulse,which in turn initiated partial melting of the Neoproterozoic SCLM and the Neo-Mesoproterozoic lower crust.The processes finally resulted in the generation of mafic and granitic magmas,respectively.The mafic magma intruded the lower crust and interacted with granitic magma along with exchange of materials and energy,which gave birth to the middle-high fO2,water-rich,Mo-bearing granitic pluton containing MMEs.Ore-forming fluids were generated during the late aqueous and volatile-rich phase of magma crystallization stage,and subsequently filled in the joints and cracks to form the Wenquan Mo deposit.The Wenquan Mo deposit is hosted in the Wenquan porphyry.The structures developed in Wenquan granitoids such as faults and joint fissures control the distribution of Mo orebodies.From the interior to exterior of the deposit alterations can be divided into the K-feldspathization,quartz-sericite and propylitization alteration zones.Hydrothermal mineralizaiton of the Wenquan Mo deposit can be subdivided into three stages from early to late:? quartz-biotite-K-feldspar veins,? quartz-polymetallic sulfides veins and ?sulfide-calcite veins.Petrography,element geochemistry,zircon U-Pb chronology and Lu-Hf isotope researches reflex that the MMEs in the Wenquan granite could be interpreted as magmatic mixture origin.The granite yielded the zircon U-Pb ages of 219±2.4 Ma and 221 ± 1.3 Ma,as the same as the ages of 217±2.0 Ma and 218±2.5 Ma obtained for the MMEs.These ages are indistinguishable with the molybdenite Re-Os isochron age of 219 ± 5.2 Ma,which indicate that the magmatic mixture processes of the Wenquan pluton and the Mo mineralization contemporaneously occurred in the T3.The Wenquan magma derived from a mixture origin of the Meso-and Neoproterozoic crust-derived component and Neoproterozoic SCLM-derived materials.S-Pb isotope data indicates that the ore-forming materials were mainly derived from a hybrid magma of lower crust and mantle and incorporated by magmatic-hydrothermal fluid differentiated from the Triassic magmatic system.The fO2 for the Wenquan Mo-bearing porphyry is uneven with AFMQ-10.9 to +6.5(average of-4.1),which accords with those igneous in intraplate or collisional orogenic settings that fO2 values lower than ?FMQ-1.But it is much higher than those of adjacent contemporaneous Mo-barren "Five Golden Flowers" granitic group,which confirms the affinity between Mo mineralization and oxidized melt system and implies the Wenquan porphyry has relatively high fO2 characteristics at its tectonic location and epoch.The initial magmatic-hydrothermal ore-forming fluid at the Wenquan deposit is compositionally H2O-NaCl-CO2 system.The early stage fluids are characterized by relatively high temperature,high salinity,high fO2,CO2-rich and NaCl-poor,suggesting that the Wenquan deposit is likely to be a collisional-type porphyry Mo deposit.The ore-forming fluids in the middle stage have nature of medium temperature,medium salinity,low fO2 and high fS2.The dramatically decreasing percentage of PC-and C-types FIs,indicating that CO2 escaped,possibly resulted from the earlier fluid immiscibility and phase separation.These changes of ore-fluids facilitated the deposition of sulfides,forming the middle stage quartz-molybdenite stockworks.The fluids in late stage got more CO2-poor,dilute and further cooling,which were likely sourced from meteoric water.Mineralization at the Wequan Mo deposit began at a depth of?8.6 Km in early stage,and the mineralization of middle stage occurred at depth of 5.4 Km?6.1 Km,which are obviously deeper than those formed in arc settings and tie in with the fact that the collisional thickening between the NCB and SCB in the Triassic.3.During the Late Jurassic to Early Cretaceous,the Qinling Orogen occurred in the tectonic dynamic transition stage from Indosinian EW-trending tectonic system to Yanshanian NS-trending tectonic system,accompanied with NS-trending squeezing action weakening of the intracontinental subduction.Controlled by the extension tectonic stress field,thinning of lithosphere and lifting of asthenosphere triggered mantle materials and heat flow upwelling,which provided enough energy to cause melting of the thickened lower crust and lithospheric mantle,and induce intense crust-mantle interaction,subsequently forming ore-bearing granitic magma.When the magma rised and emplaced in the Tongyusi Formation along tectonic weakness,the ore-forming fluids exsolved from the magma,and finally forming the Xiaohekou skarn-type deposit.Orebodies in the Xiaohekou deposit are stratiform,lentoid and veined,and hosted in the skarn of contact zones between the corcovadite and carbonate(Tongyusi Formation).Hydrothermal ore-forming events at the Xiaohekou deposit can be divided into four stages.These include:? anhydrous-skarn stage,? hydrous-skarn stage,? quartz-sulfides stage and ? carbonate-quartz stage.The Xiaohekou granitic pluton yielded zircon U-Pb age of 141 ± 1.3 Ma?138±2.0 Ma,which is coeval with other granitoids and metallogeneses in the Zhashui-Shanyang ore cluster district,indicating that the Xiaohekou skarn and copper mineralization occurred in 141 Ma?138 Ma.The corcovadite belongs to high-K calc-alkaline?-type granite and is relatively enriched in LILE(eg.U,K,Ba and Pb),depleted in HFSE(eg.Nb,Ta,P and Ti),with Sr/Y>20 and a moderate negative Eu anomaly,suggesting the magma was sourced from a thickened lower crust or root of continental orogenic belt.The Xiaohekou pluton has the characteristics of post-collisional granitoids,which could formed in post-collision or intraplate setting after orogeny.Pb-Hf isotope compositions show that the diagenetic materials came from lower crust with much mantle input.Co,Ni and As contents of pyrite,as well as TiO2,Al2O3,MgO and MnO contents of magnetite are indicative of magmatic-hydrothermal(skarn-type)genesis for the Xiaohekou Cu deposit.Electron probe analyses show that the skarn mineral assemblages are dominated by andradite-grossular and diopside-hedenbergite series,and are consistent with the garnet and pyroxene of Cu-Fe-Mo skarn deposits worldwide,showing that the Xiaohekou Cu deposit is a typical calcium skarn-type deposit.Ore S-Pb isotope data shows magmatic-hydrothermal source for the ore-forming materials and fluids,which were derived from the skarn-related Xiaohekou granitic pluton.The skarn stage from early to late formed grossularite,grossularite-andradite alternation and diopside-pure andradite.The exannulate grossularite(Grta)reflects a low fO2,acid reducing environment in early skarn stage that against Cu mineralization.Along with the increase of fO2,ore-forming fluids evolved from acid to alkalescence and FeOT content increased,forming the grossularite-andradite alternate series(Grtc)in an oscillatory physical-chemistry condition(multiple boiling).In late skarn stage,the system was relatively stable high fO2 and alkaline that decided the environment in favour of Cu mineralization,finally forming stable diopside-pure andradite.Detailed fluid inclusions studies show that ore-forming fluids in the Xiaohekou Cu deposit are NaCl-H2O nature.The NaCl-daughter bearing inclusions are commonly present in stage ? diopside and ? actinolite.The ore-forming fluids are high temperature,high salinity and high fO2,recording multiple boiling conditions.As the Xiaohekou granitc pluton cooled and crystallized,the initial fluids are magma-hydrothermal transitional fluids that have strong ability for extracting and carrying metal and are the key contributors to mineralization.Accompanied by diopside,andradite and magnetite deposition,temperature and fO2 in the system began decreasing,which could speed up sulfides discharge.The fluids of stage ? are represented by inclusions with middle-high temperature and middle-low salinity.These fluids yield wide range salinities and double peaks of homogenization temperatures,indicating that the ore stage fluids originated from the mixing of magma-hydrothermal with meteoric water flowing.Fluids cooling and dilute,with lg(fO2)(SO42-)decreasing during this stage attended the pyrite,chalcopyrite and molybdenite deposition.Thus,fluid inclusions data suggest that metal precipitation of the Xiaohekou deposit resulted from the fluid boiling and inflow of meteoric water.With more flowing of meteoric water,fluids in stage ? are characterized by low temperature and low salinity and mineralization went to end.Estimates of trapping pressure decrease from 350 bar?580 bar in stage ? to 6 bar?190 bar in stage ? and metallogenetic depth decreases from initial?2.3 Km to 0.06 Km?0.76 Km accordingly,suggesting that the Xiaohekou deposit has experienced regional rapid uplift and erosion during its formation.This may be related to the J3-K1 regime transition from crust thickening of intracontinental subduction to extensional crust thinning of the Qinling Orogen,which might provide a favorable environment for fluids decompressional boiling and sulfides deposition.The magmatic JO2 for the Xiaohekou granite concentrates between AFMQ-2 to +6(average of AFMQ+1.6),with Ce4+/Ce3+ ratios from 150 to 600(average of 445),which are similar to the magma fO2 of typical porphyry-skarn Cu-Mo deposits worldwide,indicating the Xiaohekou magma has relatively high JO2 signature and has the potential to form large-medium copper deposit.