Mechanism And Indications For Deposits Prospecting Of Lithocaps In Luzong Basin,Anhui,Eastern China

The Middle-Lower Yangtze River Metallogenic Belt(MLYB)is an important polymetallic area in eastern China,which the deeply study on the geology,metallogenetic regularity,and metallogenic scale has obtained the relatively recognized theory research achievements.A number of ridges and depressions have been formed by long-term tectonism,magmatism,and metallogenesis,and iron oxide-apatite,porphyry-skarn,and hydrothermal vein copper,iron and gold polymetallic deposits have been developed.in the MLYB.The Luzong Mesozoic continental volcanic basin is located in depression area of the MLYB,in the northern Yangtze plate,and in the southern section of the Tanlu fault,and is rich in metal minerals such as iron oxide-apatite deposits,vein-type copper-leadzinc deposits and non-metallic deposits such as alunite.Among the non-metallic deposits,the largest Fanshan alunite deposit in the northwestern of the basin constitutes a typical lithocap,the detailed and systematic work including the mechanism of Fanshan lithocap and relationship with metallic deposits needs to be solved.Therefore,this thesis selects the Fanshan lithocap in Luzong basin as the research object to study systematically the geology,geochemistry,genesis,and prospecting indicator of Fanshan lithocap,based on the comprehensive collection and sorting of previous research results,through a large number of field geological surveys,sample collection and analysis testing work,and comprehensive application of alteration lithology,mineralogy,isotope geochronology,fluid inclusion geochemistry,isotope geochemistry,in situ high-precision trace element analysis and other methods.The Fanshan lithocap is hosted by the volcanic rocks of Zhuanqiao Formation.The mineral assemblages analyzed by short-wave infrared spectroscopy,scanning electron microscopy and X-ray diffraction show horizontal zoning characteristics from the Dafanshan alunite mining area to the southwest and south by developing silicification,Fe-containing mineralization(pyrite),advanced argillic alteration,and argillic alteration.The silicification is mainly vuggy quartz developing in the Dafanshan alunite mining area and massive quartz developing in Niutoushan area because of the lithology of tuff.Fecontaining mineralization including pyrite,hematite and goethite is widely distributed in Dafanshan area.The minerals of advanced argillic alteration are alunite,kaolinite,dickite,pyrophyllite,nacrite,and distribute in Dafanshan area.Argillic alteration is characterized by the mineral assemblage of quartz,kaolinite,illite/smectite,illite and jarosite,which is mainly developed in the southeast area far from the Dafanshan mining area.There are three stages including hydrothermal early stage,hydrothermal late stage and supergene stage after detailed mineralogy analysis,alunite exists in every stage.IA1 alunite which occurred in andesite and IA2 alunite which occurred in tuff are the products of hydrothermal fluid replacing felsic minerals in the hydrothermal early stage,distributing in the surface and underground of Dafanshan alunite mining area.IB alunite filling the open space is distributed in the surface of the Dafanshan mining area in the hydrothermal late stage.II alunite formed by oxidation during the surface stage is widely distributed but with less contents.The altered rocks in Fanshanlithocap are divided into three types including siliceous altered rock,alunite altered rock,clay altered rock according to the alunite content and the geochemistry of the whole rock,which correspond to the above-mentioned Niutoushan silicification,the Dafanshan advanced argillic alteration and peripheral argillic alteration.The characteristics of element content in these three altered rocks are gradually not obvious,which indicate that the degree of water-rock reaction is gradually weakened,and the acidity of the fluid is gradually neutralized by the surrounding rocks.The results of stable isotopic composition of alunite and pyrite indicate that the hydrothermal alunite in Fanshan lithocap is mainly formed in a magmatic hydrothermal environment at 180 ? 220 ?,and the fluids are mainly magmatic water mixing with a small amount of meteoric water.The Fanshan lithocap formed at 131 Ma by IA alunite 40Ar-39 Ar dating and was superposedby supergene oxidation at 33Ma(rutile in-situ LAICP MS U-Pb dating).The Fanshan lithocap is formed under the background of the lithospheric thinning and extensional structure and is the product of the second magmatism in the MLYB.The content of Na,Ca,Sr,and Ba in alunite gradually decreases from hydrothermal early to late stage according to the electronic microprobe analysis and laser analysis,indicating that both surrounding rock and temperature are the influencing factors,and temperature plays a key role.The gradual decrease of LREE and U content and the gradual increase of Cl content in p XRF analysis indicate that although the fluid is relatively rich in Cl during the hydrothermal alteration process,the elements are gradually depleted in fluid.Combination with the fluid inclusion results that the altered fluid displayed the characteristics of original fluid,depletion of LREE suggests the hydrothermal altered fluid forming Fanshan lithocap is simple in activity and without multiple fluid components.During the process of hydrothermal fluid derived from magma undergoing SO2 disproportionation to form vuggy quartz and alunite in the near surface,the fluid evolved from weakly acidity,high temperature through strong oxidation and acidity,temperature decreasing to neutral environment and low temperature.Low temperature conditions are not conducive to the dissolution of metal elements in the complex,causing the oreforming complexes to precipitate at a deeper location,with the potential deposit being at the bottom of Fanshan lithocap.The detailed comparisons of mineral composition,fluid environment and sulfur isotope indicate that Fanshan lithocap is not genetically associated with iron oxide-apatite deposits system in Luzong basin.There are many similarities in the tectonic setting,geology,mineral geochemistry,and fluid characteristics between Fanshan lithocap and typical fertile lithocap in the world such as Zijinshan high – sulfidation copper-gold deposit and Lepanto lithocap in Philippine,indicating that there is a high-sulfidation epithermal metallogenic system in the Luzong Basin,and the magma associated with Fanshan lithocap has a large prospecting potential.The spatially indicative characteristics of hydrothermal alteration centers or mineralization directions from 1480 nm peak of the short-wave infrared spectrum of the alunite,geochemistry of whole rock,and geochemistry of alunite in Fanshan lithocap suggest that the precious metal mineralization might be located in the deep part of the Dafanshan alunite deposit,whereas copper mineralization might be located in the northeastern of Dafanshan alunite deposit.The diagram of Ca + Sr + Ba-Na /(Na + K)could be used to judge the spatial position(such as hydrothermal channel or horizontal position)of alunite in lithocap,or the fertility or barren of lithocap after detailed analysis and verification of hydrothermal alunite geochemical data.Combined with the geology and mineralogical characteristics of other alunite deposits in the Luzong Basin,the diagram provides directions for exploring porphyry-epithermal deposits in the areas of Bajiatan – Wudingshan – Jingbian – Mopanshan – Shimenan,Fanmushan,Qianpu in Luzong basin.