Genesis Of Carbonaceous Material And Its Association With Gold Mineralization In The Bumo Deposit,Hainan Province

Carbonaceous material(CM),which existed in many orogenic gold deposits,such as Macraes deposit in New Zealand,Cosmo Howley in Australia,and the Syama,Obuashi and Inata deposits in West Africa,which are important mineralization indicators of orogenic gold deposits.The Gezhen gold-bearing ductile shear zone,Hainan Island in southern China is an important gold-polymetallic mineralization zone on the island.Similar to most other gold deposits in eastern China,the gold ore bodies in this area are basically endowed in the metamorphic sedimentary rocks of the Mesoproterozoic strata.Previously found in the field survey work,in Gezhen gold-bearing ductile shear zone area of the major gold deposits are found in a large number of carbonaceous material outcrops,but the gold deposit mineralization process and carbonaceous material formation mechanism and other understanding has not yet been studied in depth.Hence,clarifying the process between carbonaceous material and gold mineralization is essential.The Bumo gold deposit is located in the southern part of the Gezhen gold-bearing ductile shear zone,and is one of the largest gold deposits in the area.Recent studies have classified the Bumo gold deposit as an orogenic gold deposit modified by shear zone tectonics and hydrothermal superposition.Meanwhile,a large amount of carbonaceous material has been found in the deposit.However,the presence of carbonaceous material is seldom described and analyzed in previous reports.In older mineralization model,carbonaceous material is usually regarded as a component of the surrounding rocks,and is not considered to be involved in the gold mineralization process of the Bumo gold deposit.Although a lot of research has been done on the Bumo gold deposit and the wall rocks,but no attention has been paid to the possible contribution of carbonaceous material to mineralization.Hence,correct understanding of the possible influence of carbonaceous material on the mineralization process in Bumo gold deposit is of great significance to elucidate the relationship between gold mineralization in the Gezhen gold-bearing ductile shear zone.In this paper,based on the fact that carbonaceous material is closely related to gold mineralization in space,systematic mineralogical,petrographic,XRD diffraction analysis,EMPA analysis,Laser Raman thermometry and carbon isotope studies are carried out on carbonaceous material,ore and surrounding rocks in the deposit,and the results of these studies are used to clarify a series of scientific questions about carbonaceous material in the Bumo gold deposit.According to field and petrographic characteristics in this paper.The results show that there are two types of carbonaceous material has been identified in the deposit,which are referred to as layered CM and veinlet CM,respectively.layered CM,in the form of gray-black shale,is several meters thick and is exposed in the lower plate of the ore body,with close spatial connection to both the surrounding rocks and the ore body.It is noteworthy that the gold grade increases significantly as the thick layered CM near the vein thins.In addition,gold-bearing sulphides are typically distributed along the boundaries of the quartz veins and layered CM.In contrast,veinlet CM is interspersed in fine veins through quartz grains and hydrothermally altered rocks in large quantities,with micron to centimeter diameters and small grains of sulfide present near the fine veins.Meanwhile,EMPA analysis data show that thus sulfides which total contained large amounts of gold in their own lattice,and their contents range from 100ppm to 600ppm.Scanning electron microscopy(SEM)analysis shows that layered CM has a scale-like structure,gray-black color,and high degree of crystallization.In contrast,the structure of the vein-like carbonaceous material shows a rounded granular structure,gray in color,and irregularly disordered distribution.The temperature calculation based on Raman spectroscopy shows that the temperature range of the veinlet CM is between 190? and 350?,which is the same temperature range as the mineralizing fluid.The formation temperature of layered CM is between 400-550?,which is consistent with the temperature range of metamorphic phase of surrounding rocks.Carbon isotope analysis(-16%to-26%)and XRD diffraction(d₍₀₀₂₎=0.0035-0.3375)results for the layered CM indicate that these layered CM originated from organic debris of the ancient ocean and underwent a late regional high temperature metamorphic process.In contrast,veinlet CM is a product of the mixing of hydrocarbon gas in the deep mantle and metamorphic mixed fluids at the right temperature and pressure in the middle and late stages of metallogenesis,and its formation period is much later than that of the layered CM.Based on the results,this paper classifies layered CM as metamorphic origin CM,which is a high temperature-high crystallinity product(400-600?),formed by marine organic matter through diagenesis and metamorphism,and belongs to metamorphic diagenesis.The formed temperature of veinlet CM is similar to the mineralization temperature(180-350?),formed during the mineralization period,and the veinlet CM is coeval with gold-sulfide particles,which belongs to the inorganic genesis.Meanwhile,both two types of CMs are closely associated with gold mineralization in the metallogenic process of Bumo gold deposit.As for the possible ways of the involvement of both CMs through gold mineralization process in orogenic gold deposits,this paper further investigates that veinlet CM can provide sufficient reducing sulfides for mineralization through redox reactions at high to medium temperatures(<350?)leading to the disruption of the equilibrium of mineralizing fluids and then gold precipitation.On the other hand,layered CM can act as an effective reducing agent by destabilizing the gold complexes in the mineralized hydrothermal fluid leading to rapid precipitation of large amounts of gold,and the layered CM is physically softer and can also provide suitable host space for the mineralized fluid.