Metallogenic And Model Of Gold Deposits In Bengbu Uplift,Anhui Province,China

Jiaodong area is located within the southeastern margin of the North China Craton(NNC)and is the largest gold producing region in China with reserves of>5000t gold.The world-class giant gold province has attracted global attention and extensive studies for decades which play an important guiding role in mineral exploration in this area.The Bengbu uplift is located in the North China Craton,the west of Tan-Lu Fault Zone.It is considered that the current position of the Bengbu uplift and Jiaodong gold province was caused by the intense sinistral movement of the Tan-Lu Fault Zone,and the two regions have similar metallogenic backgrounds.Therefore,many geologists have been explored the“Jiaondong type”gold deposit in Bengbu uplift.But only medium and small quartz-vein type gold deposits have so far been found in the Bengbu uplift.The newly discovered Jiangshan gold deposit in 2012,a large fracture-disseminated-type gold deposit,is the largest gold deposit in the Bengbu uplift,indicating that this area is a good prospecting prospect area.In this work,the detail study on geological characteristics,mineralization age,isotopic geochemistry and ore-forming fluids features were used to determine the ore-forming process of the Jiangshan gold deposit.And the study of geological characteristics and geochemistry of quartz-vein type gold deposit(Zhongjiashan gold deposit,Dafanshan gold deposit,Hekou gold deposit,Fangan gold deposit and Rongdu gold deposit)in Bengbu uplift were conducted to compare the mineralization between the fractures-disseminated type and quartz-vein type gold deposit and to build the gold metallogenic model.By the contrast of gold mineralization background and gold metallogenic between Bengbu uplift and Jiaodong area to determine similarities and differences of gold mineralization and discuss the prospective of ore-exploration in Bengbu uplift.Jiangshan gold deposit is hosted in the Archean Wuhe Group metamorphic rocks.The zircon U-Pb ages show the crystallization age of leptite is 2496±19Ma and a late metamorphic age is 2452±47Ma,whereas the metamorphic age of amphibolite is1824±11Ma.The zircon?Hf(t)values(1.81?8.30)and t DM2(2545?2849)of leptite indicated that the ore-hosting rock source is remelting of juvenile crust materials.In contrast to reginal stratigraphic lithology and age,it can be inferred that the host unit is not part of Zhangzili Formation but probably belongs to the Xigudui Fromation of Wuhe Groupe.Jiangshan gold deposit is controlled by eastern part of NNE-trending Guangou-Xiajijia fault,ore bodies occur in the footwall of the fault.Ore-bearing zones are comprised three different types of good ore:veinlets-style,disseminated-style and quartz vein-type,The Jiangshan gold deposit can be clearly divided into three hydrothermal stages based on the crosscutting relationships of ore sample and mineral assemblages:quartz-pyrite-muscovite stage(Stage I),quartz-pyrite vein stage(Stage II)and quartz-pyrite veinlets stage(Stage III).The visible gold appears as native gold and electrum,and the invisible gold occurs as mostly lattice-bound gold and minor nanoparticles based on the TIMA analysis and LA-ICP-MS pyrite trace elements analyses.It is determined that the quartz-pyrite veinlets stage is the main gold stage in Jiangshan gold deposit.The zircon ages of pre-ore and post ore dike are 128.3±1.7Ma and 121.2±1.4Ma,respectively.The zircon?_Hf(t)values(-13.4?-24.02)and t _DM2(2545?2849)of the dikes indicated the magmatic source is the partial melting of ancient crust material.And the pyrite Rb-Sr isochron age(117±5.7Ma)and the ages of pre-and post-ore dikes showing the mineralization age of Jiangshan gold deposit was determined to be about 120Ma.Fluid inclusions microthermometry,oxygen isotope thermometer and quartz Ti thermometer suggest that temperature and pressure of the three hydrothermal stages were constrained as 390?,300?,280?and 104Mpa,79Mpa,9Mpa,respectively.The initial ore-forming fluids were CO₂-rich,low density,and low salinity H₂O-Na Cl-CO₂homogeneous fluids.Fluid decompression,caused by tectonic events,led to significant gold deposition in quartz pyrite veinlets stage based on the evolution of fluids and petrographic features.In-situ S-O-C isotope analyses were conducted on pyrite,quartz and ankerite of the three ore-forming stages,the values of sulfur and oxygen isotope are relatively concentrated in 5?9‰(?³⁴S)and 5?9(?¹⁸O),suggesting a common ore-forming fluids source.And the values of ?¹³C in the three ore-forming stages are-10?-5‰,-4?-1‰and-3?0‰respectively.The geological and geochemical characteristics of the deposit suggesting that the ore-forming fluids and material(Au and S)are interpreted to derived from devolatilization of mantle materials.The fluids of Stage II and Stage III suffered interaction with ore-hosting rocks,which did not provide gold and sulfur.Quartz-vein type gold deposits are distributed in the east of Bengbu uplift and all controlled by the subsidiary second-or third-order faults of Tan-Lu Fault Zone.In this type of gold deposit,the quartz veins occurred on filling the fault zone with dips>60°generally.According to the previous research,the age of quartz-vein types gold deposit is 120±10Ma,The ore-forming fluids were CO₂-rich,low density,and low salinity H₂O-Na Cl-CO₂homogeneous fluids.The in-situ geochemical analyses were conducted on quartz-vein type gold deposits in this study.The values of sulfur isotope of Zhognjiashan,Dagongshan and Rongdu gold deposit are concentrated in 6?9‰,and that of Hekou and Fangan deposit is ranging in 3?5‰,and that of pyrite in ore-hosting rocks range from 0?5‰.The values of oxygen isotope in quartz-vein types gold deposits have a narrow range from 6 to 9‰,and some values in Hekou and Fanan gold deposit are<2‰.The?¹⁴C values of Zhongjiashan deposit range from-8 to-4‰,and that of Dagongshan and Rongdu deposit range from-7 to 0‰,and that of Hekou and Fangan deposit range-2 to 2‰.The S-O-C isotopic data are considered that the ore-forming fluids and material(Au and S)are interpreted to derived from devolatilization of mantle materials and interaction with ore-hosting rocks,and the sulfur in ore-hosting rocks were added in ore-forming fluids in Hekou and Fang deposit.According to this work and previous work,it is confirmed that the gold mineralization age of fracture-disseminated type and quartz vein type gold deposit in Bengbu uplift is 120±10Ma.The ore-forming fluids belong to medium-low temperature,medium-low salinity and H₂O-Na Cl-CO₂±CH₄system.The two types of gold deposits distributed in brittle-ductile shear zone,the quartz vein type gold deposits occur in high angle fault zone(>60°)and the low angle fault zone(<40°)controls the formation of fracture-disseminated type gold deposit.The values of sulfur isotope and carbon isotope in fracture-disseminated type gold deposit is similar with that in quartz vein type gold deposit(?³⁴S=3?10‰,?¹³C=-10?2‰).And the values of oxygen isotope of fluids in the two types of gold deposit also show similar variation between 4?10‰,and some oxygen isotopic data in quartz vein type deposit are lower than 2‰.Base on the S-O-C isotopic data and previous work,suggesting that the ore-forming fluids of gold deposit in Bengbu uplift have a similar source,The ore-forming fluids and Au and S are interpreted to derived from devolatilization of mantle materials and interaction with ore-hosting rock materials.We classify the Bengbu uplift gold deposits as orogenic gold deposit according to deposit geology,ore-forming fluids features,gold deposition mechanism,the sources of ore-forming fluids and materials.According to the regional geology between Bengbu uplift and Jiaodong area,suggesting that the two districts have similar mineralization background except on the scale of Yanshanian magmatic rocks.Besides,the two regions show the similar mineralization process based on the detailed comparison of the deposit geology.The main difference of gold deposits in the two regions is the ore-host rocks.We think that the mineralization in the two districts occurred in simultaneous and had similar gold metallogenic events.The Jiaodogn gold deposits are hosted in the brittle-ductile shear zones which developed at different lithologic contact zone and controlled by the several faults in Jiaodong.Jiangshan gold deposit in Benbug uplift occurs in Linhuaiguan-Lianggang fault,which controls the brittle-ductile shear zone developed in the contact area between leptite and gneisses.The ore-controlling factors of Jiangshan gold deposit is similar with Jiaodong gold deposit.It is considered that the brittle-ductile shear zone controlled by Linghuaiguan-Lianggang fault can be a preferential area for gold ore exploration.