Mineralization Of The Chating Copper And Gold Deposit In Xuancheng Ore District,Anhui

The Middle-Lower Yangtze River Metallogenic Belt(MLYB),located in eastern China,is an important iron,copper and gold polymetallic metallogenic belt which developed in porphyry and skarn deposits.Based on the detailed study on the representative copper and gold deposits,many researchers consider that the Mesozoic magmatism in the MLYB are formed in an intracontinental setting.However,many issues in the MLYB are still controversial and confusing including the tectonic setting,the origin and evolution of the porphyry-related magma,the fluid exsolution process,and the mechanism of the ore deposition.Therefore,it's necessary to continue the geological study on the representative porphyry deposits in the MLYB,so that more evidence can be provided.The Xuancheng ore district is located at the Xuancheng-Nanling area of the MLYB,with east adjacent to the Tongling district,north to the Ningwu basin,and south to the Jiangnan uplift belt.With the development of prospecting and exploration in this area,many copper,iron,gold,lead and zinc deposits,including Chating copper and gold deposit have been discovered,which made Xuancheng to be a new important ore district in the MLYB.The Chating deposit is a newly found,representative,and large copper and gold in the Xuancheng district,MLYB.The drill holes show that the porphyry mineralization are much more developed than the skarn mineralization in Chating although the wall rocks are mainly carbonates,which is different from other porphyry-skarn deposits in the MLYB.Moreover,the relationship between the multiple intrusions,and breccia and the mineralization in Chating are unclear.In order to know more about the porphyry and skarn systems evolution and guide the exploration prospecting in the MLYB,the detailed researches on the genesis of Chating are necessary.In this thesis,the diagenesis and metallogenesis of Chating have been studied in detail,based on the drill hole logging and geochemical analysis,including the whole rock major and trace element,Sr-Nd isotope,mineral chemistry,fluid inclusion,and the in-situ O-S-Hf isotope.Our work provides a full picture of the Chating,which are critical to understand the genesis of the porphyry and skarn deposits better in the MLYB.The Chating deposit is associated with three stages of intrusive phases,an early stage pre-ore amphibole diorite porphyry,an ore-bearing quartz diorite porphyry and a late stage diorite porphyry,lamprophyre and andesite porphyry.The zircon U-Pb ages of the amphibole diorite porphyry(138.8±3.0 Ma)and quartz diorite porphyry(137.6±3.0 Ma)are consistent with their cross-cutting relationships and within error of the average molybdenite Re-Os age of 136.0±1.3 Ma,suggesting that the Chating porphyry mineralization belongs to the early metallogenic event in the Middle-Lower Yangtze River Metallogenic Belt.Both the amphibole diorite porphyry and quartz diorite porphyry are characterized by large ion lithophile element enrichment,depleted in high field strength elements,low Yb/Lu and Y/Yb ratios,low?Hf(t)(-8.29 to-12.02),?Nd(t)(-6.93 to-7.42),and high ⁸⁷Sr/⁸⁶Sr(i)(0.705723 to 0.705802),suggesting a source comprised of mantle-and crust-derived materials.The whole rock geochemical,mineral structural and chemical characteristics,such as amphibole,plagioclase,magnetite,biotite and zircon,revealled a full evolution history of the different intrusions in Chating:the subduction-modified lithospheric mantle and lower crust derived melts underwent melting/mixing,assimilation,storage and homogenization,forming an isotopically homogenous MASH zone at the mantle-crust boundary under a intracontinental setting.Magma derived from the MASH zone ascended to a shallow magma chamber at a depth of 5.2 to 8.6 km.The addition of mafic magma from the magma source were injected into the evolving and fractionating shallow magma chamber,causing the formation of the amphibole diorite porphyry with a small amount of exsolved fluids.With continued fractional crystallization of biotite,amphibole and plagioclase and a greater degree of wall rock assimilation,the quartz diorite porphyry was emplaced at a depth of 1.9 to 3.4 km;continued fluid exsolution from the shallow magma chamber contributed to the formation of the Chating Cu-Au deposit.Different type of alteration and mineralization are mainly found inside the quartz diorite porphyry,such as the skarn,potassic and phyllic from early to late.Very few propylitic alteration developed in Chating because of the carbonate wall rocks,instead,skarn alteration is widespread.Copper and gold mineralization are mainly hosted inside the potassic and phyllic altered quartz diorite porphyry,occurring as disseminated,vein and brecciated type ores.A large hydrothermal breccia occurs at a depth of 900 to 350m.Based on the compositions of the cement and clasts in these breccia,three different type of breccia can be identified,with an early stage quartz-cemented magmatic-hydrothermal breccia(MH),an anhydrite-cemented hydraulic hydrothermal breccia(H1),and a late stage chlorite-cemented phreatomagmatic breccia.The MH and H1breccia host an important portion of the copper and gold mineralization.Based on the mineral assemblages and vein crosscutting relationship,the alteration and mineralization in Chating can be subdivided into three stages:stage I,silicate stage,developing the skarn and potassic alteration;stage II,sericite stage,including the phyllic alteration;stage III,quartz-calcite stage.The O-S isotopic compositions for different type of anhydrite,and the geochemical characteristics of hydrothermal biotite and magnetite indicate a magmatic fluid origin for the early stage of mineralization,however,there is an increasing addition of the meteoric water into the fluids of the stage II.Thermometry of fluid inclusions and minerals chemistry indicate that the temperature,salinity and oxygen fugacity are decreasing as the fluid evolution.The changes of temperature,salinity,oxygen fugacity and the addition of meteoric water might play important roles in the ore deposition of Chating.In a summary,we concluded that the Chating is a porphyry copper and gold deposit.We also proposed possible interpretations for the genesis of the barren skarn in Chating based on the geological characteristics of skarn,the geochemistry of garnet and the O-S isotope of anhydrite:(1)the high oxygen fugacity of skarn-forming fluid prevented the deposition of coeval sulfides with skarn minerals;(2)the ore-forming fluids from the magma chamber were trapped by the massive marble inside the quartz diorite porphyry which prevented the forming of massive skarns in the contact zone between the intrusion and the carbonate wall rocks;(3)the small scale endoskarn and the high ratio of anhydrite in the exoskarn indicate that there were no enough space for the late stage mineralization.Nevertheless,the deposition of andradite and anhydrite could decrease the fluid oxygen fugacity which is beneficial to the forming of the late stage's sulfides.Based on the analysis of the metallogenic conditions including wall rock,ore-controlling structures and magma in Chating and Xuancheng area,we proposed three ore-prospecting orientations:(1)try to find skarn deposit in the contact zone between the Carboniferous carbonate and Mesozoic intrusions;(2)focus on the area where the hydrothermal breccia developed,which might be the important clue to the adjacent porphyry deposit;(3)the concealed intermediate intrusion nearby the Chating area are also possible to related to porphyry deposit.