Study Of Mineralization Process And Mechanism In Baguamiao Super Large Gold Deposit, Fengtai Ore Region, Shaanxi, China

The west Qinling is an important origin of Au resources in China, abundant of Au deposit is hosted within the sediments and epimetamorphic rocks in this area. For example, the Baguamiao gold deposit is a typical one hosted in the Fengxian-Taibai ("Fengtai" for short) ore cluster. Although a lot of prospecting and research works on Baguamiao gold deposit have been done, the metallogenic age, the metallogenic mechanism and many other scientific problems are needed to be solved.This paper aims to describe the geological characteristics of the typical deposit in detail and do further studies of the characteristics, source and evolution of the ore-forming fluids, and the mineralization time of the ore cluster. Focused on those scientific issues, we got some main understanding as follows:1)The occurrence of the Baguamiao gold deposit is controlled by the brittle-ductile shear zone.The ore bodies are hosted by stripped marlstone with iron dolomitic silty phyllite interlayer. Ore types are Au-bearing quartz veins and altered wall-rocks. There are four mineralization stages: Ⅰ,NWW-trending nearly straight quartz-ankerite stage; Ⅱ, NWW-trending corrugated quartz-ankerite-sulfide stage; Ⅲ, NE-trending straight quartz- sulfide- ankerite stage;Ⅳ, NW or NE-trending tensile crack quartz vein stage. The original fluid of Baguamiao Au deposit is metamorphic hydrothermal characterized with low salinity and high CO2, the latter has a magmatic hydrothermal characterized with high temperature and salinity added, and mixed with some deposition fluid system.Therefore, the CO2-H2O-CaCl2(NaCl) ore-forming fluid with the development of more CO2-rich inclusion, and contains more CH4, N2. The salinity of fluid changes greatly, up to 21.44 wt.% NaCl eq., give priority to with low salinity. The uniform temperature peak of four mineralization stages changed from stage Ⅰ(300～350℃) evolution to Ⅱ(350～400℃) by slightly increases, then gradually decreased from Ⅲ(250～300℃) to Ⅳ (200～300℃),the corresponding metallogenic pressure also gradually reduced.From early to late stage, the Eh value present a trend with gradually reduced to slight rise in latest stage, changes from -0.35～-0.32 and -0.34～-0.21 to -0.08～-0.15,-0.13～-0.18.The sulfur fugacity present a trend with gradually reduced, the value changes from -11.25～-6.54 and -9.31～-13.53 to -15.72～-14.56 and -16.46～-14.98.The oxygen fugacity changes from -28.61～-27.32 and -30.12～-27.38 to-39.35～-36.77 and -35.45～-32.38. The value of oxygen fugacity and sulfur fugacity reduce soon from stage Ⅱ to Ⅲ,it can effectively change the physical and chemical conditions of ore-forming fluids. The value of oxygen fugacity rise slightly in stage Ⅳ,which adverse to gold mineralization.2) The ore-forming stage Ⅰ of Baguamiao gold deposit is at intense extrusion state, and has the characteristic of press-shear deformation. The ore-fluid system is compactly closed and the volume of quartz veins and altered wall-rocks deplete (△V=-0.01～-0.33).With the progressive of mineralization, the structure field transforms from extrusion to extension.The structure field of stage Ⅱ is extension, the tectonism transforms gradually from ductile deformation to brittle deformation.The ore-fluid system changes from closed to open, and in the mineralization process of stage Ⅱ the ore-forming system is mainly open, with △V=0.24～0.45.In stage Ⅲ,the structure field is in extrusion, and the brittle deformation is in the leading role. The ore-forming process is mainly open with △V= 0.03～0.5,indicating that a large number of matter from outside enter into the system.In stage Ⅳ, matter outside entered into the ore-forming system decreases, with △V= 0.03～0.08, the structure field of this stage is weaken.Differential stress from stage Ⅰ (62.98-93.20Mpa), and stage Ⅱ (99.36-122.96Mpa) evaluates to stage ；(24.75～53.72Mpa) rapid drawdown, causing phase separation in ore-forming fluid and developed with tensile fracture. In stage Ⅳ, the differential stress is low with 22.85～57.95Mpa, and in this stage brittle deformation developed.3)There are five stage pyrite and pyrrhotite in Baguamiao gold deposit, that are PyO to Py4, and PoO to Po4, corresponding to sedimentation-diagenesis, and 4 ore-forming stage;three types chlorite are closely related with aregillaceous rock and formed in reducing environment. Based on the sequence of crystallization of pyrite and pyrrhotite and the metasomatism between them, and the precipitation environment of chlorite and its relationship with the gold mineralization, with the addition of the occurrence regularity of native gold, we conclude that the upper and lower temperature limit of gold precipitation are 345℃ and 225℃ respectively. The best temperature for gold precipiting form ore-forming fluids is about 250℃ and the benifical environment is from fainry acid to close to neutral.4) The spotted structure develops at phyllite, or argillaceous stripes of marlstone in Baguamiao gold deposit, the content of quartz in wall-rocks has a negative correlation with gold mineralization, the deformation extent of spot, the density of spot, while the last has a positive correlation with the gold mineralization, relatively. The progressive of tectonism strengthens the deformation extent of spots, and then the grade of Au increases. Therefore the more intensity of wall-rock, the gold mineralization will be stronger.5) The ages constrained by the Sm-Nd dating of ankerite, dolomite and calcite in ore-forming stage Ⅱ and Ⅲ are 209.3±4.2 Ma and 208.1±3.1Ma, respecitively, which are spatially temporally and coupled to the polymetallic mineralization of the Fengtai ore cluster, and all these ore mineralization is the product of multi-stage epigenetic hydrothermal mineralization, driven by the late Triassic regional tectono-magmatic acivities of West Qinling.6) The gold in the metallogenic fluid in Baguamiao gold deposit transportes as Au-Cl complex [AuCy and Au-S complex [Au(HS)2]- in ore-forming Stage Ⅰ, and the ratio of the two complexs is 1:1. In ore-forming stage Ⅱ,Ⅲ, and Ⅳ, gold transports mostly as Au-S complex [Au (HS)2]-, barely transports as Au-Cl complex [AuCl2]-. The mechanism of gold precipitation is caused quite possibly by phase separation resulted in drastic loss of pressure in fluid, water-rock reaction, fluid mixing action, and change of physical-chemical conditions of ore-forming fluid, such as pH, Eh, sulfur fugacity and oxygen fugacity, caused by mechanisim above mentioned are the main factors causing the precipitation of gold. Multi-factor working together will efficiently promote the precipitation of gold.7) In the orogenic process of west Qinling at Triassic, dehydration occurred when formation lithology in deep developed ductile deformation and metamorphism in ore concentrated area, and then formed metamorphic hydrothermal with low salinity and rich CO2.These fluid extracted metal elements like Au and As changed into ore forming fluid, Au transformed as [AuCl2]- and [Au(HS)2]-.When ore forming fluid raised up along ductile-brittle shear belt, magmatic hydrothermal and sedimentary system fluid entered into, and then changed the equilibrium of and [Au(HS)2]-,Au will precipitate out.