Tectono-Magmatic Evolution And Ore Genesis Of Gold Deposit In Northern Helanshan

The northern part of Helanshan is located in the west of the continental collision belt,namely Khondalite Belt,along which Yinshan Block collided with Ordos Block in Paleoproterozoic.To the west of Helanshan is Alxa Terrane,and to the north is the gold and polymetallic metallogenic belt in northwestern margin of North China Craton（NCC）.Many gold deposits or ore occurrences,including Niutougou-Lianggen and Shulonggou gold deposits,have been discovered in this area.And there are also widespread granitoids and mafic dikes,and minor ultramafic rocks.In this study,attention has been paid on the representative gold deposits and different igneous units after sufficient field work.Based on high-precision geochronological data,multi-episodic magmatism have been identified,and combined with whole-rock composition and Sr-Nd isotopes,and zircon Lu-Hf isotopes,their sources,petrogenesis,and related tectonic setting have been discussed.With comprehensive studies of geological characteristics of representative deposits,fluid inclusions and H-O-S-Pb isotopic compositions,the sources of ore-forming fluid and materials have been discussed,and ore-forming process of gold deposits is proposed.And then Niutougou-Lianggen is compared with two other representative gold deposits from neighboring area.The main results and conclusions are summarized in below:（1）The mineralization type of Niutougou-Lianggen gold deposit consists of structurally fractured metasomatic type and quartz vein type,respectively;the latter can be further divided into gently dipping and steeply dipping quartz vein subtypes according to their field occurrences.In Shulonggou deposit,it is composed of predominantly metasomatic type and partly quartz vein type.All orebodies from these two deposits are hosted in the Paleoproterozoic Helanshan metamorphic complex and migmatite.In Niutougou-Lianggen,the distribution of all orebodies is apparently fault-controlled,and they are controlled by main fault F₁ with the strike of NNW to NNE direction and its secondary faults.In contrast,this fault-controlled character is not obvious for Shulonggou deposit.In combination with previous ones,this study focus on the sources of ore-forming fluid and materials for three mineralization types from Niutougou-Lianggen deposit.Fluid inclusions in the quartz from the structurally fractured metasomatic type are mainly liquid aqueous inclusions,whereas those from quartz type are predominantly by two-phase of liquid-rich inclusions.The ore-forming fluids of various mineralization types are medium-low temperatures（160–210°C）in H₂O-Na Cl fluid systems with medium-low salinities of 6–12 wt.%Na Cl eqv..The results show that the structurally fractured metasomatic type and steeply dipping quartz vein type have similarδD_H2O values of-75.2–-89.3‰and-87.0–-93.8‰,respectively,while the gently dipping quartz vein has relatively higherδD_H2O values of-45.7–-67.7‰.Their calculatedδ¹⁸O_H2O values decrease sequentially from structurally fractured metasomatic type（3.7–4.4‰）,through gently dipping quartz vein type（1.9–3.3‰）,to steeply dipping quartz vein type（0.5–0.9‰）.The structurally fractured metasomatic type and steeply dipping quartz vein type have positiveδ³⁴S values of 1.3–6.9‰and 2.2–5.8‰,respectively;whereas the gently dipping quartz vein type have distinctly negativeδ³⁴S values with-5.1–2.6‰.Apparently the gently dipping quartz vein type has differentδD_H2O andδ³⁴S compositions compared to the other two types,which may be related to the relatively increase in oxygen fugacity of ore-forming fluid due to either difference of its ore-controlling structures or its deposition site away from the ore-transmitting structure（F₁ fault）.Sulfides from three mineralization types are enriched in the Th-radiogenic Pb,with comparatively homogeneous ²⁰⁶Pb/²⁰⁴Pb（16.467–17.994）and²⁰⁷Pb/²⁰⁴Pb（15.382–15.582）compositions but a wide range of ²⁰⁸Pb/²⁰⁴Pb（37.413–42.345）.In general,H-O isotopic compositions of fluid inclusions in quartz from three different mineralization type indicate their ore-forming fluids are composed of mixed fluids between magmatic fluid and paleometeoric water,with the proportion of the latter consistently increase from structurally fractured metasomatic type,through gently dipping quartz vein type,to steeply dipping quartz vein type.And the S-Pb isotopes imply their ore-forming materials were derived from both the underlying magma and their country rocks with high Th/U ratios.Taking the regional tectono-magmatic evolution into consideration,we suggest that the ore-forming process of Niutougou gold deposit includes four stages,including basement formation in Paleoproterozoic,rift-basin formation stage in the Late Paleozoic,regional compressive thrusting stage in the Late Jurassic,and magmatic-hydrothermal stage in the Late Jurassic to Early Cretaceous.（2）Representative granitic rocks are selected for zircon LA-ICP-MS U-Pb geochronology study,and it shows zircon crystals from all these rocks show clear core-rim texture,with zircon core of magmatic origin,and rim of metamorphic origin.The results suggest that biotite monzonite granite has emplacement age of 2037±10Ma,with metamorphic age of 1959±48 Ma;Porphyritic monzonite granite and alkali feldspar granite have same emplacement ages of 1951±17 Ma and 1953±17 Ma,respectively,with metamorphic age of 290³00 Ma;porphyritic biotite granite have emplacement age of 1902±22 Ma,with metamorphic age of 1356±57 Ma.To better constrain the metamorphic age of porphyritic biotite granite,the SIMS depth profiling method is adopted,and yields a metamorphic age of 1317±18Ma,confirming an unconstrained 1.32 Ga tectono-thermal event occurred in this region.Porphyritic biotite granite has high Si O₂,Al₂O₃ and K₂O content,and low Na₂O and Ca O content,and show peraluminous and high-K calc-alkaline to shoshonitic feature.It is enriched in LREE and LILE,depleted in HREE and HFSE,similar to Helanshan Group.Its whole-rock Nd model ages and zircon Hf model ages are similar to the detrital zircon Hf model age ranges of Helanshan Group,indicating their genetic connection.Zircon saturation temperatures of porphyritic biotite granite vary from 750°C to 800°C,same to P-T conditions of post-peak depression metamorphic stage of Helanshan Group.And granite has distinct negative Eu anomaly,and high Rb/Ba and Rb/Sr ratios,and large variations of HREE concentration and（La/Yb）_N ratios,which is consistent with the presence of K-feldspar and plagioclase in the residues during post-peak decompression stage,with proceeding biotite breakdown and the progressive replacement of garnet.This suggests porphyritic biotite granite was derived from the partial melting of Helanshan Group during post-peak decompression stage.On the other hand,most Paleoproterozoic S-type granites from Helanshan area is similar to the melts generated by metagreywackes,in conjunction with geochemical composition of khondalites,implying the khondalite protolith is low-maturity greywackes and minor pelites,and arguing for an active continental margin of north Ordos Block.（3）Zircon or baddeleyite U-Pb dating of intermediate to ultramafic rocks shows that there are two generations of diabase dikes,with emplacement ages of 1965±26Ma and 1867±23 Ma respectively,and that the crystallization age of pyroxenite is1905±10 Ma,similar to porphyritic biotite granite,and the emplacement age of dioriteMg O content,and belongs to calc-alkaline series.It has high Cr,Co and Ni,and is enriched in LREE,Th,U,Pb,depleted in HREE,Nb,Ta and Ti,similar to the composition of subcontinental lithosphere metasomatized subduction melt.Its high Zr/Nb and low Nb/La ratios are similar to arc basalts.Pyroxenite shows cumulatic texture,and has low Mg^#and Mg/Si ratios,resembling ultramafic cumulates from Dabie Orogen.All that the enrichment of Th,U and Pb,homogeneous Hf isotope composition,and young Hf model ages suggest source region of pyroxenite was metasomatized by subduction fluid and melt.In combination with variable Nd model ages andε_Nd（t）values,pyroxenite should be ultramafic cumulates that are derived from subcontinental mantle modified by subduction melt,and formed near crust-mantle boundary with assimilation of ancient crust.The diabase has high Si O₂,Na₂O,K₂O,Fe₂O₃^T,but low Mg O content,ranging from calcalkalic to tholeiitic composition.It is enriched in LREE,La and Pb,depleted in HREE,Nb,Ta,Zr and Hf,and has high Zr/Nb and low Nb/La ratios,similar to continental arc basalt.Relatively high（Hf/Sm）_PM but low（Ta/La）_PM ratios imply that diabase was generated by partial melting of subcontinental mantle modified by subduction fluid.Large variation ofε_Nd（t）andεHf（t）values of diabase indicate it experienced AFC process with olivine,clinopyroxene and plagioclase being dominant crystalizing phase.（4）Prolonged,episodic magmatism of 2.3².0 Ga,1.96¹.94 Ga,1.92¹.90 Ga,1.87¹.84 Ga and 1.78 Ga are identified in Helanshan and adjacent regions,corresponding to pre-collision,continental collision,exhumation,post-collision and post-orogeny stages,respectively.During 2.3².0 Ga period,ancient oceanic slab subducted beneath Ordos Block,and induced intermediate to felsic arc magmas that were derived from depleted mantle or juvenile lower crust as revealed zircon Hf isotopes.From 1.96 Ga to 1.94 Ga,the ancient ocean closed and Yinshan Block collided with Ordos Blocks.Magmatism over this period consists of dominant crust-derived magmas and minor mafic rocks,and those S-type granites are the partial melt of khondalites induced by underplating of mantle-derived melt.In 1.92¹.90 Ga,the exhumation of subducted crust initiated which might be related to the break-off of oceanic crust,and minor S-type granites formed that were derived from the partial melting of khondalites during post-peak depression metamorphism.From 1.87 Ga to1.84 Ga,intensely magmatism occurred in this area including abundant mafic dikes,which more likely be related orogenic collapse,and thus marked the end of continental orogeny and the formation of unified Western Block.From continental collision to orogenic collapse,the magmatism continued about 100 Myr,which is comparable to continental orogenesis duration of North Qaidam（ca.75 Myr）,Dabie-Sulu（ca.85 Myr）and Variscan orogen（ca.85 Myr）.The Western Block then evolved into post orogenic stage,and experienced multi-stage rifting events.（5）From the Paleozoic to the Mesozoic,complex and prolonged tectonic and magma evolution process is recorded in the northwest NCC,accompanied by multi-stage gold and polymetallic mineralization,including Hadamengou（386 Ma）,Zhulazhaga（282 Ma）,Changshanhao（246²56 Ma）and Saiyinwusu（249 Ma）gold deposits,which is influenced by the tectonic evolution of Paleo-Asian Ocean and subsequent continental collision.The Niutougou-Lianggen is compared with two other representative gold deposits（Hadamengou and Zhulazhaga）,and the results show that although there are some distinctions among these tree deposits in metallogenic geological features and ore-forming material sources,comparability still exist in host rock type,ore-controlling structure and ore-forming fluid.The host rocks of three deposits are all Precambrian basement;ore-forming fluids are mainly composed of magmatic fluid.Considering the geological characteristics of these deposits and their ore isotope composition,the Precambrian basement may play an important role in gold mineralization in northwest NCC.On the one hand,because of its high Au background value,the Precambrian basement as one of the important source layers may provide part of ore-forming materials during gold mineralization.On the other hand,the Precambrian basement normally experiences earlier high-grade metamorphism including prograde dehydration reaction,and thus its rock mechanical property is relatively rigid,and is prone to fracture during late multi-stage tectonic activity.Those fractures are the ideal spaces for transmitting and depositing gold mine.And the dominant magmatic hydrothermal fluid indicates that contemporaneous magma also contribute to the gold mineralization process.