Early Mesozoic Magmatic Evolution And Gold Mineralization In The Shicheng-Chong'an District,South China

The Shicheng-Chong'an Structural belt?SCSB?,located in the middle of the Wuyishan Orogenic belt?WYOB?of Southeast China,is an important prospect of gold polymetallic mineralization.Due to the strong collision between the South China plate and the surrounding plates?i.e.,the North China plate,the India plate,and Paleo Pacific plate?during the early Mesozoic period,the intracontinental region?including the WYOB?was also affected.Therefore,angular unconformity,structural deformation,metamorphism and magmatic activities related to early Mesozoic collisional orogeny are well developed in Wuyishan region.These create good conditions for metallic mineralization.However,the genetic relationships between gold mineralization and magmatic activities in the region are poor understood.The tectonic setting related to magma and metallogenic events is also controversial.These restrict the further prospecting work.In this dissertation,two typical gold deposits(i.e.,the Hebaoshan gold deposit?@11.7 t?and the Changxing gold deposit?@11.4 t?and magmatic rocks formed at early Mesozoic in the SCSB are studied in an effort to cast light on the above problems.For the magmatic rocks,field geological survey,zircon U-Pb dating,bulk rock major and trace element and Sr-Nd-Hf isotopes analysis are preformed to detect thier origin and the rock-forming tectonic setting.For the Hebaoshan and Changxing gold deposits,methods such as pyrite Re-Os dating,sericite Ar-Ar dating,fluid inclusion,in situ trace elements of pyrite,and H-O-S-Pb isotopes are used to study the timing of gold mineralization,origin of the ore-forming fluids and materials.A coupling model between gold mineralization and magmatic activities is finally proposed,which is well implied in the regional mineral resource exploration.The primary conclusions of this dissertation are as follows:?1?The early Mesozoic magmatic rock,i.e,the Zixi complex granite,is located in the north of Hebaoshan and Changxing deposits.This granite intrudes into Precambrian metamorphic rocks and is then covered by Jurassic strata.Zixi complex granite consists of a gray-white medium-fine grains biotite k-feldspar granite?ZX-G?and a slight-red medium-coarse grains k-feldspar granite?ZX-R?,without any enclaves.Zircon LA-ICP-MS U-Pb dating show that the ZX-G granite formed at 233±4 Ma,and the ZX-R granite formed at 223±1 Ma.The ZX-G granite is characterized by high-K calc-alkaline and metaluminous-weak peraluminous,with high Si O₂?73.55-77.91 wt.%?and K₂O?4.08-5.59 wt.%?contents.The rare earth elements?REE?of the ZX-G granite show a right-dipping distribution pattern with a significant negative Eu anomaly.It should be noted that the curve pattern of individual sample of ZX-G granite differs in the degree of light and heavy REE differentiation,with?La/Yb?_N=2.2-52.3.Three samples show slightly strong REE differentiation and give?La/Yb?_Nof 41.7-52.3,while the differentiation of other four samples is relatively weak??La/Yb?_N=2.2-8.57?.The trace element spider diagram shows large ion lithophile elements enrichment?LILE;e.g.,Rb,Th,U,K,and Pb?and high field strength elemente depletion?HFSE;e.g.,Ba,Sr,Nb,Ta,P,and Ti?.Element geochemical characteristics suggest that the ZX-G granite is compatible with I-type granite.The ZX-R granite shows shoshonitic and metaluminous-weak peraluminous characteristics with low Si O₂?59.08-77.91 wt.%?and high K₂O?4.87-8.02 wt.%?contents.The rare earth elements and trace elements of the ZX-R granite display a similar distribution patterns to those of the ZX-G granite but with slight negative Eu anomaly??Eu=0.65-1.19?.High K₂O+Na O,Zr+Nb+Ce+Y,Ga/Al,Zr and Nb values of analyzed samples suggest that the ZX-R granite is a A-type granite.The Sr-Nd isotopic data indicate that the ZX-G granite has slightly inhomogenous(⁸⁷Sr/⁸⁶Sr)_ivalues but homogenous?Nd?t??-14.6?-12.3?and?Hf?t??-12.7?-6.2?values.The?Nd?t?and?Hf?t?data of the ZX-R granite are also homogenous and significantly higher than those of the ZX-G granite,which are-10.6?-9.6 and-6.1?-3.5,respectively.These geochemical together with the petrographic characteristics imply that the233 Ma ZX-G granite was derived from the partial melting of ancient basement crust,with source having 85-100%metamorphic argillaceous rocks of the Mayuan Group and a small amount of amphibole.The 223Ma ZX-R granite was also generated by partial melting of ancient basement crust,but the source is characterized by 60-70%metagreywacke of the Mayuan Group and 30-40%amphibole.Further considering the temporal-spatial distribution of regional early Mesozoic magmatic rocks and metamorhism,magmatic activities in the SCSB was triggered under collision setting,which is related to intracontinental effect of collision between the South China plate,the North China plate and the India plate.Collision occurred at 243-233 Ma,and then transfer to post-collisional extension?224-204 Ma?.?2?Two largest gold deposits,i.e.,Hebaoshan and Changxing deposits are located in the north of NE-striking regional Yejia deep fault.The occurrence of gold orebodies in these two deposits are strictly controlled by the subsidiary fractures of Yejia deep fault with different strikes.Gold orebodies have been shown to be hosted by rock units of different lithology?Neoproterozoic Jiaoxi Formation metamorphic rocks in Hebaoshan deposit and Silurian granitoids in the Changxing deposit?.In the Hebaoshan gold deposit,gold mineralization is dominated by pervasively altered fracture-hosted mineralization?AFM?,with minor quartz-type mineralization?QTM?.Occasionally,the former is cut by the latter.The related wall-rock alteration is silification,sericitization,chloritization,and kaolinization.Five types of pyrite are recognized on the basis of mineral assemblages and in situ trace elements of pyrite,including three types?Py1,Py2,Py3?from the AFM and two types?Py4 and Py5?from the QTM.The native gold predominately coexists with Py2,and then occasionally with Py3 and Py5.The contents of Au in Py3 are more than 1ppm?up to 84 ppm?,while the contents of Au in other pyrite types are low?<1ppm?.Au is positively correlated with As.The comprehensive analysis show that gold is in the form of solid solution gold?invisible gold?and micro mineral inclusion gold?native gold?in the Hebaoshan deposit.The precipitation of solid solution gold is related to the lattice defect caused by the substitution of S in pyrite lattice by As,and the precipitation of micro mineral inclusion gold is caused by sulfurization and remobilization reaction.The gold orebodies in the Changxing deposit are dominated by QTM,with wall-rock alteration of silification,sericitization and chloritization.Three pyrite types?Py1,Py2,Py3?are well developed.All of these pyrite are characterized by low Au content?<1ppm?,but native gold can be observed to coexist with Py2 and minor Py3.Similar to the Hebaoshan gold deposit,gold in the Changxing deposit is also in the form of solid solution gold?invisible gold?and micro mineral inclusion gold?native gold?.However,the precipitation of micro mineral inclusion gold is triggered by fluid boiling.?3?Sericite Ar-Ar and pyrite Re-Os geochronology are applied to define the mineralization timing of the Hebaoshan gold deposit.The sericite Ar-Ar dating yield234-223Ma for the ASM and Py5 Re-Os dating reveal potentially 181-192 Ma for the QTM.The dating results suggest that the formation of gold mineralization is related to the multi-phase fluid activities during the Late Triassic-Early Jurassic.Together with previous geochronology of gold mineralization and magmatic rocks?e.g.,the233-223Ma Zixi complex granite and the 198Ma Guangze granite?in the region,there is a temporal coupling relationship between gold mineralization and magmatism.?4?Investigation of the fluid inclusions of Hebaoshan and Changxing gold deposits shows that the ore-forming fluids are H₂O-Na Cl system.The fluids related to ASM in the Hebaoshan deposit are of medium temperature?peaked at 275-215??and medium-low salinity?8-16wt%Na Cl?.The nature of fluids related to QTM in the Hebaoshan deposit is similar to that of fluids linked to ASM,but the temperature is higher?290-245?.In the Changxing gold deposit,fluids in the ore system give similar salinity to that of Hebaoshan,but the temperature is relatively low?230-200??,fluid boiling occurs at a hydrostatic pressure ca.67 bar.The H-O isotopic compositions of various types of mineralized samples from two deposits indicate that ore-forming fluids should be dominated by mixture of magmatic water and meteoric water.This conclusion is consistent with the magmatic hydrothermal genesis of all types of pyrite in two deposits indicated by high Co/Ni ratio of pyrite.Meanwhile,the low Ni content of pyrite suggests that the fluids are exsoluted from the felsic magma.The S isotopic compositions of pyrite samples from different types of mineralization in the Hebaoshan and Changxing deposits are relatively homogeneous?+2.9?+6.3‰?,overlapping with magmatic sulfur.In regard of Pb isotopes,pyrite samples from Hebaoshan and Changxing deposits all fall in the transitional area between the upper crust,orogenic belt and lower crust,indicating the complexity of Pb source.According to the linear correlation between the samples and the geochemical characteristics of the host rock,it is concluded that the Pb isotopic composition of two gold deposits is likely from the magma and the basement rock.The determined Osi from the¹⁸⁷Re/¹⁸⁸Os-¹⁸⁷Os/¹⁸⁸Os and¹⁸⁷Re vs¹⁸⁷Os^risochrons are highly imprecise?0.08±0.78 and 0.11±0.41?,which suggest that the source of the Re is uncertain.However,one possibility is that the hydrothermal fluids were enriched in Re through interaction with ancient basement rocks.The comprehensive geological and geochemical isotopic results indicate that the source of fluid and material is related to magmatic activity at Late Triassic and early Jurassic,and the contribution of the basement wall-rock to the ore-forming material can not be ignored.?5?The geochronology reveals that the Late Triassic to early Jurassic is an important gold mineralization period in the WYOB,which is accompanied by contemporaneous or slightly early magmatism related to partial melting of the basement crust under the post-collisional extensional setting.On the basis of the mineralization characteristics and the relationship between mineralization and magmatic activities,a coupling model of late Triassic-early Jurassic gold mineralization and multi-stage magmatic activities in the SCSB is proposed.