Late Mesozoic Magmatic Evolution And Polymetallic Mineralization In Fanchang Basin

The doctoral dissertation is centered on the diagenesis and metallogenesis of late Mesozoic intrusive rocks within the Fanchang volcanic basin(FVB)of the MiddleLower Yangtze Metallogenic Belt(MLYMB),a significant copper-iron polymetallic belt in China.The FVB is geographically situated between the ore-cluster districts of Tongling and Ningwu in the MLYMB,and the current research on the diagenesis and metallogenesis of intrusive rocks,primarily consisting of Mesozoic granite suites in this region,is relatively inadequate.Therefore,this study aims to conduct an in-depth investigation of the granite suites and their primary metallogenic types.Petrographic observations,chemical analysis of intrusive rocks,LA-ICP-MS analysis of magmatic zircons,zircon LA-ICP-MS U-Pb geochronology,and wholerock Sr-Nd isotope analysis,were applied to a variety of intrusive rocks,namely Baimashan,Xiangxingdi,Zhuhouling,Chichong,Suishan,Puyingshan,and Zishanling.The rocks known as monzonite,quartz diorite,granite,and syenite exhibit high-K calcalkaline and shoshonite characteristics,with elevated levels of Rb,Th,U,Ce,Pb,Nd,Sm,and Gd,and reduced levels of Ba,Nb,Ta,Sr,P,and Ti.Additionally,these rocks display LREE enrichments and a flattened right-dip of HREE.The majority of these intrusions were formed during the 128～121 Ma period,with two exceptions:Baimashan quartz dioritic porphyrite(138.5±4.9 Ma)and Puyingshan diorite(132.1±3.8 Ma).Interestingly,ages of inherited zircons primarily manifest in three intervals:2675.9～2211.1 Ma,2057.1～1862.0 Ma,and 990.7～661.1 Ma.This observation suggests the presence of diachronic magmatic sources and the potential existence of late-Archean and Proterozoic basements.Furthermore,the intrusive rocks within the FVB exhibit a significant increase in crystallization temperatures,ranging from 802 ℃ to 931 ℃,and a corresponding decrease in(Ce4+/Ce3+)zircon values,from 192.34 to 24.84,over the period from 132 Ma to 125 Ma.This trend is likely attributable to an intensified magma assimilation process with the carbonaceous wall rocks.The(87Sr/86Sr)i values range from 0.70718 to 0.72642,with an average of 0.70970,while the εNd(t)values range from 14.0 to 2.2,with an average of 7.1.The intrusive rocks present in the FVB exhibit features that are indicative of both enriched mantle and lower crust.While the process of magma mixing may not be readily apparent during the evolution of these rocks,there is a discernible assimilation process that suggests the possible presence of late-Archean and Proterozoic bases in the deeper layers.The primary minerals that crystallize within this magma system include monoclinic pyroxene,hornblende,potassium feldspar,and plagioclase.The Late Mesozoic magmatic rocks in the region may have been influenced by the dynamic Mesozoic reactivation of the Neoproterozoic enriched mantle.Additionally,it is postulated that in addition to the A-type granites of Suishan,Zhuhouling and Fushan,Binjiang rocks,there are also some I-type granitoids with earlier diagenesis ages,namely,Chichong and Puyingshan rocks.Both A-type and I-type granites lack an adakitic feature.The study defines A-type granites in the region as probably having originated from the partial melting of early I-type granite source area.I-type granitoids are predominantly found within the Jurassic and Cretaceous volcanic strata range,whereas A-type granitoids are extensively distributed throughout the volcanic basin.Ore-forming fluid,and ore-forming age of the Xiaoyangchong skarn-type zinc deposit and Songyuan skarn-type pyrite deposit have been investigated,and a comparison with the porphyrite-type iron ore in Ningwu Basin has been presented.The results of fluid inclusion experiments indicate that the transparent minerals,quartz and calcite,in the two deposits are primarily composed of gas-liquid two-phase inclusions.During the quartz-sulfide stage,the fluid inclusions exhibit a range of homogenization temperatures spanning from 150 to 380 ℃,with corresponding salinity values ranging from 3.6 to 22.0 wt%NaCleq.Additionally,the fluid density ranges from 0.70 to 1.00 g/cm3,which suggests a mineralization depth of 0.50 to 2.00 km,indicative of shallow mineralization.Furthermore,electron probe analysis reveals that andradite is the primary mineral present in the two deposits,with grossularite being a secondary mineral.The amalgamation of the trace element variation trend in the garnet composition belt serves as evidence that the garnet was formed in an oxidized environment.Furthermore,after conducting a comprehensive analysis and discussion of H,O,S isotopes,as well as the multi-genetic types of magnetite,it can be inferred that the mineralization process from skarn stage to quartz-sulfide stage involved not only the mixing of magmatic water and meteoric water but also deep brine.In comparison to the porphyrite type iron ore found in the Ningwu volcanic Basin,the skarn in the FVB exhibits a lower formation temperature for magnetite(approximately 300-400℃).Additionally,the Re-Os age of pyrites formed during the primary mineralization stage of the Xiaoyangchong zinc deposit,as determined through general Os correction,is 125.7Ma,which is later than that of the porphyrite type iron ore found in the Ningwu region.Furthermore,the Baimashan iron-sulfur polymetallic deposit represents a prototypical crypto-explosive breccia deposit within the FVB.Through an extensive petrographic investigation,it has been determined that magnetite solely belongs to a singular metallogenic stage(Mag,0.5～1 mm),whereas pyrite can be categorized into Py Ⅰ(0.5-1 mm)and Py Ⅱ(<0.1 mm).By integrating LA-ICP-MS analysis,encompassing both point and mapping analyses,as well as sulfur isotopes of pyrite,the genesis of the deposit and the progression of ore-forming fluid have been elucidated.The time-resolved signal diagram of LA-ICP-MS point analysis,element correlation diagrams,and LA-ICP-MS mapping results provide detailed evidence that the majority of trace elements in magnetite and pyrite exist in solid solution within the mineral structure.Nonetheless,the elements Co,Ni,As,Sb,Ti,Sn,and W in magnetite and Ag,Bi,T1,and Sb in pyrite exhibit partial incorporation into metal sulfide micro-inclusions.Moreover,the LA-ICP-MS findings indicate a notable enrichment of Co,Ni,and As in Py Ⅰ,whereas Cu,Pb,Zn,Ag,Sb,and T1 are enriched in Py Ⅱ.Notably,the Co/Ni ratios of Py Ⅰ and Py Ⅱ exhibit a similar trend and temperature range variation.Additionally,the Co/Ni mapping image’s cross-section provides a detailed account of the temperature variation during pyrite formation.The discriminant diagram provides evidence that magnetite has a hydrothermal origin,while pyrite is likely formed in a hydrothermal environment with lower temperatures.Analysis of sulfur isotopes reveals that the δ34S values of pyrite range from+4.9 ‰ to+9.2 ‰,suggesting that the ore-forming fluids are a mixture of magmatic hydrothermal fluids,brines associated with gypsum strata,and meteoric water.Furthermore,examination of the mineral microstructure and LAICP-MS mapping indicates that post-metallogenic hydrothermal may have altered the distribution of trace elements in Py Ⅰ.In-depth analysis reveals that the enrichment range of Si,Mn,and Zn is comparatively narrower,measuring approximately 50 μm,while Cu,Ag,Sb,Pb,and Bi exhibit a wider enrichment range of approximately 250μm.While,the depletion of Co,Ni,and As is primarily concentrated within the intersection area of fractures,spanning approximately 500 μm.Consequently,it is recommended that mineral exploration in this region should not only prioritize skarn and hydrothermal iron and zinc deposits,but also consider the potential for skarn and porphyry copper deposits associated with Ⅰ-type granites.