Metallogenic Characteristics And Prospective Prediction Of Dahaoshan Gold Deposit In The North Jiangxi Province

Dahaoshan-Lishan gold deposits are located in a junction part between Jiangnan Orogen,middle-lower Yangtze River Valley area and Yangtze Block.The northern part of gold orefield is Cu-Fe-Au metallogenic belt in middle-lower Yangtze River Valley area.The southeastern part is Cu-W metallogenic belt in the northeast of Jiangxi province.The southwest part is Jiurui ore-concentrated area.Xiushui Formation of Shuangqiaoshan Group is sub-greenschist to greenschist facies metamorphosed basement in the Neoproterozoic.The overlying Sinian Formations has an unconformable or fault contact with the underlay Xiushui Formation.Guilingjia and Fangkengwu gold deposits were discovered after Dahaoshan and Lishan gold deposits.Gold deposits were controlled by NNE and NE trending faults and meta-sedimentary sequences together.Most of gold deposits were occurred in the Sinian Liantuo Formation near the unconformable boundaries rather in the metamorphosed basement of Shuangqiaoshan Group.In the past,few researches were carried out.The limited gold resource reserve restricts the enterprise development.In this thesis,Dahaoshan and Lishan gold deposits were selected as a research object to study the geological characteristics,ore-forming age,geological setting,geodynamic background,ore-forming fluid and material source on the basis of previous works,geological survey and sampling,and geochemical analysis.Then,the metallogenic model and prediction model were set up and used to make a mineral resources assessment in a regional scale and orefield scale.According the research objectives and contents,the following works were carried out:?1?the previous research work,geological data of study area were collected from National Geological Archives of China and bibliographic data base.?2?Geological survey and sampling.The joint profile of geology and geochemistry were surveyed in the key localities.In order to study the attitude of ore bodies,ore-controlling structure,mineralization type,alteration and genesis,the footrill and drill were observed and recorded.?3?Indoor data and samples processing.It includes geological mapping,production of slices,mineral identification,EPMA analysis,and inclusion thermometry.The above works were used to study the ore texture and structure,gold occurrence,metallogenic temperature and pressure.?4?The stable isotope H-O,S were analyzed to study the fluid and material sources.?5?The research of geochronology and metallogenetic geodynamical setting.LA-ICP-MS method was used to detect the zircon U-Pb age.The ore-forming age was restricted by the spatial association between intrusion and the gold deposits.The main and trace element and Hf isotope were employed to explore the intrusion genesis,material source and geodynamical setting.?6?The research about metallogenic model and ontology-driven mineral prediction model.The logistical relationship between was analyzed based on the metallogenic model and prediction model to create the predictive variables of mineral prospecting.?7?In the scale of orefield,CSAMT,high precision magnetic survey,primary halos and typomorphic characteristics of mineralogy were used to assess the potential of mineral deposits in depth.According the above research works,the following conclusion can be drawn:?1?Dahaoshan and Lishan gold deposits are located in the northeast margin of Jiangnan Orogen,Yangtze block.It is one part of gold metallogenic belt of Jiangnan Orogen.In the Neoproterozoic,the Yangtze block and Cathaysia block had a double-sided subduction,which results in the formation of Jiangnan Orogen.The study area developed a series of meta-sedimentary sequence in this period.In Mesozoic,the subduction from Pacific Ocean to Eurasia bought a strong magma activities and crust thickening.The gold element contained in the meta-sedimentary of Jiangnan Orogen reworked and formed the gold under the influence of heat produced magma activities.?2?The lode gold ore bodies are occurred in the secondary faults of NE and NNE trending regional faults.The ore bodies in Dahaoshan orefield are located in the faults of Liantuo Formation and the structure between Liantuo Formation and granite porphyry vein.No.1 and No.5 ore bodied are distributed in the hanging wall and footwall of granite porphyry vein,respectively.No.2 ore body are located in the south of No.1 and No.5 ore bodied in the fault of Liantuo formation.NE and NNE trending fault provided a favourable pathway and space for the ore forming.Granite porphyry vein and silicrete layer gave a favorable shielding for ore-forming fluid.Lishan gold ore bodied are located in the NEE secondary fault of NE trending faults.The later NNE and NNW trending faults cut the ore bodied into sidestep shape.?3?Alteration was dominated by medium-low temperature alteration types.It contains silicification,chloritization,carbonatation and sericitization.Besides,epidote,limonite and jarosite also could be found.Alteration zonation in the horizontal is not obvious,which can be divided into quartz vein-silicification zone-chloritization from ore body to wall rock.?4?The types of ore were dominated by altered type and qiartze type.Ore texture contains idiomorphic-hypidiomorphic granular texture,metasomatic relict texture,poikilitic texture,stockwork texture,cataclastic texture,granular texture,skeletal texture,mosaic texture.Metallic mineral mainly contains pyrite,and a small quantity of copper,galena and sphalerite.Gold mainly occurred in the crushing gap of pyrite and other sulfide in the form of natural gold.Pyrite and other sulfides have a low gold content.According the ore texture and structure,the ore-forming processes can be divided into quartz-pyrite stage,quartz-sulfide stage and quartz-calcite stage.?5?Dahaoshan gold deposits is similar with gold in Jiangnan Orogen in the terms of homogenization temperature,salinity and H-O isotope.The homogenization temperature of quartz fluid inclusion in metallogenic epoch is 131³22.9?,whilst the homogenization temperature before mineralization is 99.6²65.3?.The salinity of metallogenic fluid is 0.9²1.6?NaCl%with a mean of 11.8?NaCl%.The mineralization depth is 1.7².18Km,which indicates a shallow mineralization depth.?D ranges from-98 to 48‰,?¹⁸O_waterater ranges from 202 to 6.2‰.The plots of H and O isotope are near the magma water and metamorphic water.?³⁴S of pyrite ranges from-2.8 to 5.8‰with a mean of-0.0183‰,which is similar with Shuangqiaoshan Group in term of the S isotope composition.The isotope composition characteristics of S and H-O indicate ore-forming fluid is metamorphic water mixed with magma water,material comes from metamorphic basement.?6?The intrusive rocks in the orefield district are late Jurassic-early cretaceous granodiorite-porphyry with a U-Pb age of 140.07±0.7Ma.These intrusive granites belong to a series of quasialuminous-peraluminous and alc alkaline-high potassium calc-alkaline.Rb,Th and K have an enrichment,while Nb,Ta P,Ti high field strength elements have a relative depletion.LREE is relative enriched,HREE is relative depleted.The REE distribution pattern shows an obvious right deviation shape and has a high Sr/Y?43.46⁵4.13?,which indicates that intrusive rock has some characteristics of adakite rock.¹⁷⁶Hf/¹⁷⁷Hf of zircon ranges from 0.28261 to 0.28269?Hf?t?ranges from-6.29 to-6.19.Depleted mantle model ages range from 1171 to 1355Ma.Based on the geochronology and geochemical data,it was concluded that granodiorite porphyry was produced in a crustal thickening setting caused by subduction of the Pacific plate.As the increase of temperature and pressure,the adakite magma was generated by melting of the lower crust in the anhydrous area of the subduction mantle wedge.?7?Lamprophyre in the orefield belongs to ultramafic lamprophyre with a U-Pb age of 128.8±1.6Ma.REE distribution pattern shows a right deviation shape,which is similar with the curst REE distribution pattern.The element concentration of HREE of Gd-Lu is similar with upper earth crust and higher than low earth crust.The pattern of Nb,P and Ti displays a“V”shape.⁷⁶Hf/¹⁷⁷Hf of zircon ranges from 0.28251 to 0.28260,?Hf?t?ranges from-9.29 to-6.19.Depleted mantle model ages range from1397 to1594Ma,which is similar with granite.Lamprophyre usually is produced in an extension setting.Lamprophyre origins from the amphibolite-garnet-leached magma,which is remelting residue of basaltic low crust that produced the adakitic magma.The residue magma invaded into the exiting formations and produced the lamprophyre vein in ca.129 Ma.?8?According to the spatial relationship between granodiorite porphyry veins and ore bodies,the metallogenic age is after the age of granodiorite porphyry.Lamprophyre generally was produced in the lithospheric thinning environment caused by geodynamic transform form flat subduction of the Pacific plate to parallel strike-slip.During this period,some NS trending strike faults were generated and cut the ore bodies.Therefore,the gold ore age is about 140¹29Ma.?9?In this thesis,the geophysical method was used to interpret the scale of Pacific plate and the association between orogenic gold and Pacific plate subduction.Geochemical studies have shown that subduction of the Pacific plate to the Eurasian continent has resulted in Mesozoic metal mineralization in South China.The interpretation of satellite magnetic data shows that there is a high magnetic anomaly zone in the mainland of China.The orogenic gold deposits in the Jiangnan orogenic belt and the North China are distributed on both sides of the high magnetic anomaly zone.The p-wave profile of the earthquake proves that the high magnetic anomaly zone is the leading position of the subduction of the Pacific plate.?10?Based on the metallogenic model and prospecting model,the ontology-driven mineral prediction method has been introduced.The mineral prediction variables extracted from geological map,geochemical data and geophysical data were on basis of ontology-driven mineral prediction method.Then,the cosine similarity method was applied to make a prospective mapping of gold deposits in a regional scale.According to the cosine similarity,a first-level prospecting area?A1?,two secondary prospect area?B1 and B2?and three three-level prospecting area?C1,C2,C3?were delineated.?11?There is a co-existence pattern of front halo elements and tail halo element in the primary halo zonation of Dahaoshan and Lishan gold deposits.Combined with the typomorphic characteristics of pyrite,it was concluded that there is a high potential in depth to find concealed gold ore bodies.The results of high precision magnetic survey and CSAMT indicate that there is concealed intrusion below-600 m.Based on the theories of metallogenic system and spectrum of ore deposit,the vein gold mine is regarded as distal end of the ore-forming system of intrusion granite,thus,there is the possibility to discover high-temperature magmatic hydrothermal deposit in depth.