Carbonaceous Slate Research Articles

The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China

The Huangjindong gold deposit in northeastern Hunan is one of the most representative gold deposits in the Jiangnan Orogenic Belt. The orebodies are mainly hosted in the Neoproterozoic Lengjiaxi Group, which comprises carbonaceous slates. Abundant carbonaceous material (CM) can be found in the host rocks and ore-bearing quartz veins, but its geological characteristics and genesis, as well as its association with gold mineralization, are still unclear. Systematic petrographic observation demonstrated two types of CM in host rocks and ores, i.e., CM1 and CM2. Among them, CM1 is the predominant type and mainly occurs in the layered carbonaceous slates, while CM2 is mostly present in quartz veins and mineralized host rocks. Laser Raman spectroscopic analyses of CM1 were performed at higher temperatures (376–504 °C), and CM2 was generated at similar temperatures (255–435 °C) to gold mineralization. Combined with previous studies, we can conclude that CM1 was produced by Neoproterozoic to early Paleozoic metamorphism before gold mineralization, while CM2 is of hydrothermal origin. Geochemical modeling indicates that CM1 could promote gold precipitation through reduction, as well as facilitate structure deformation and metal absorption as previously proposed. However, hydrothermal CM2 is favorable for gold mineralization because it triggers sulfidation, similar to other Fe-bearing minerals (such as siderite) in the host rocks. Consequently, both types of CM in the Huangjindong deposit are favorable for gold mineralization and carbonaceous slates could be important gold-bearing units for future ore prospecting in the Jiangnan Orogen as well as other places in South China.

Experimental and Numerical Investigations on the Slate Shearing Mechanical Behavior

Multi-scale assessment of shear behavior in the tunnel carbonaceous slate is critical for evaluating the stability of the surrounding rock. In this study, direct shear tests were conducted on carbonaceous slates from the Muzhailing Tunnel, considering five bedding dip angles (β) and four normal stresses (σn). The micro-mechanism was also examined by combining acoustic emission (AE) and energy rate with PFC2D Version 5.0 (particle flow code 2D Version 5.0 software) numerical simulations. The results showed a linear relationship between peak shear stress and normal stress, with the rate of increase inversely related to β. Cohesion increased linearly with β, while internal friction angle and AE activity decreased; the energy release rate is 3.92 × 108 aJ/s at 0° and 1.93 × 108 aJ/s at 90°. Shearing along the preset fracture plane was the main failure mode. Increased normal stress led to lateral cracks perpendicular to or intersecting the shear plane. Cracks along the bedding plane formed a broad shear band with concentrated compressive force, and inclined bedding was accompanied by a dense tension chain along the bedding plane.

Compensation mechanics application of NPR anchor cable to large deformation tunnel in soft rock

NPR anchor cable is a new type of support material with negative Poisson's ratio effect, which is widely used in mine support because of its superb compensating mechanical effect. In order to study more deeply the support effect of NPR anchor cable in soft rock large deformation tunnel, indoor test, numerical simulation and field monitoring were used to study the strong weathering carbonaceous slate tunnel in Min County. The study shows that NPR anchor cable has extraordinary compensating mechanical behavior for soft rock large deformation tunnel, which can control the deformation of tunnel surrounding rock below 300 mm and keep the constant resistance value around 350 kN, which has obvious effect on the control of broken rock. To provide a basis for other research on support for large deformation tunnels in soft rock.

Effects of foliation angle on mechanical characteristics of carbonaceous slate using uniaxial compression tests

Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (β). As β increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when β ≤ 45°. The AE events are active during the crack compaction phase and remain calm during the linear elastic deformation phase when β > 45°. Upon failure, the energy release accounts for the highest proportion (67%) when β = 45°, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when β = 60°, which is six times greater than that at β = 0°. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts / duration time) and high RA (rise time / amplitude) increases initially, and then decreases when β > 60°. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when β ≥ 60° compared to other cases. The failure behaviors correspond well with field observations.

Long-Term Stability Analysis and Structural Safety Assessment of Tunnel Surrounding Rock in Carbonaceous Slate Formation

Long-Term Stability Analysis and Structural Safety Assessment of Tunnel Surrounding Rock in Carbonaceous Slate Formation

Study on large deformation mechanism and control technology of layered carbonaceous slate in deep tunnels

Study on large deformation mechanism and control technology of layered carbonaceous slate in deep tunnels

Microscopic deformation mechanism and characteristics of carbonaceous slate during the creep process

Carbonaceous slate exhibits a significant creep deformation that seriously affects the construction and operation of underground projects. To investigate the microstructural changes characteristics and reveal the microscopic deformation mechanism of the carbonaceous slate during the creep process, multiple methods were performed, including the triaxial creep test, SEM and MIP. The following conclusions were drawn: The rock samples underwent three stages during the creep test: microporosity closure at a low-stress level, material densification at an intermediate stress level, and microcracks emerging and expanding to failure at the high stress. The creep deformation was particularly significant in the first and third processes. The lamellar particles are compressed or bent under stress in parallel and vertical directions, showing the anisotropic properties of deformation. The deformation of the rock sample is related to the angle between the bedding and the orientation of major principal stress, and the effect of the anisotropy decreases with the increased stress level. The sprouting and expansion of microfractures occur at high-stress levels, showing pressure dissolution of mineral particles, migration of very fine particles, and cement damage between lamellar particles. Finally, the horizontal samples formed a combined rupture surface composed of the laminar surface and the fracture surface intersecting it, showing brittle damage, while the vertical samples formed a fracture surface parallel to the laminar surface, showing a ductile damage pattern. Those results could provide the basis for a further understanding of the mechanical properties of carbonaceous slate and the improvement of its creep model and parameters. It was significant for the stability analysis and deformation prediction of engineering structures using numerical simulation.

Experimental Study on Creep Characteristics and Damage Model of Carbonaceous Slate After Freeze-Thaw

Experimental Study on Creep Characteristics and Damage Model of Carbonaceous Slate After Freeze-Thaw

A case study on the recovery of unevenly embedded particle size in high-carbon chalcopyrite using an alkyne-based thioester collector: Flotation processing and adsorption mechanism

The difference in chalcopyrite's primary ore-hosting rocks (dolomite and carbonaceous slate) in the Democratic Republic of the Congo results in an extremely uneven grain size distribution. Additionally, the presence of 2.21% organic carbon in the gangue impacts flotation efficiency. To address these challenges, ore properties were analyzed using the Mineral Liberation Analyzer (MLA), X-Ray Diffractometer (XRD), and microscopy. Flotation process was modified to incorporate a "middlings regrinding" processing, utilizing PDEC (an alkyne-based thioester collector, prop-2-yn-1-yl diethylcarbamodithioate) as the collector for experimental studies. Density Functional Theory (DFT) calculations elucidated the interaction mechanism of PDEC on chalcopyrite's surface. The MLA analysis indicates that chalcopyrite is mainly found in medium to fine grains, with the presence of fine-grained copper minerals smaller than 0.04mm accounting for 16.29% of the sample. This implies that these minerals require fine grinding for effective separation. Despite interference from organic carbon, PDEC demonstrates remarkable selectivity and efficiency in chalcopyrite flotation. By employing the "middlings regrinding" flotation method, a concentrate with a Cu content of 26.79% and a recovery of 87.88% was achieved, representing an increase of 0.17% in Cu grade and 4.09% in recovery rate compared to the conventional flotation process. DFT analysis demonstrates that the S 3p orbitals in carbon-sulfur double bond of PDEC and the C 2p orbitals in its acetylene group significantly affect its collection efficiency, engaging in hybridization with the Fe 3d orbitals on the surface of chalcopyrite, thereby facilitating a robust bonding interaction.

The genetic link between lode and disseminated gold ores and relationship between hypozonal and mesozonal deposits: A case study of the Yanzigou orogenic gold deposit, northwestern Yangtze Craton, China

The process controlling the formation of orogenic lode and disseminated gold ores is the basis for understanding the complexity of the orogenic gold system. The mesozonal Yanzigou gold deposit, located in the Danba ore district of the northwestern Yangtze Craton, is hosted by Devonian carbonaceous slate with coexisting fault-hosted lode and strata-bound disseminated gold ores. Three types of pyrite (PyI-q, PyII-q and Pyd) were formed during two stages of hydrothermal evolution. PyII-q in quartz-polymetallic veins and the rims of Pyd in disseminated ores have the highest Au and As contents, contrasting to distal sedimentary pyrite that have low Au and As contents. This implies that the Devonian host sequences are incapable of providing ore elements to the ore fluid. Pyd and associated quartz have intermediate isotopic values (δ34S = -1.7∼+0.8 ‰; δ18OH2O = 10.6 ∼ 12.7 ‰) between the quartz-vein ores (δ34S = 8.5 ∼ 10.7 ‰; δ18OH2O = 9.0 ∼ 10.1 ‰) and wall-rocks (δ34S ≈ −5‰; δ18OH2O ≈ 12 ‰), implicating a critical role of fluid-rock reaction in the formation of the disseminated gold ores and preservation of original ore fluid characteristics in the lode gold ores. The 40Ar/39Ar dating of hydrothermal muscovite from auriferous quartz veins at Yanzigou yields a total fusion age of 118.4 ± 0.4 Ma, later than ∼ 185 Ma for the nearby hypozonal Danba gold deposit. However, both deposits have common geological, geochemical, and isotopic signatures, suggesting a multi-stage orogenic gold mineralization on the Mesozoic dome margins associated with episodic reactivation of metasomatized mantle lithosphere during the continued crustal uplift.

Fluid evolution and genesis of the Zhuanghuhe Au–Sb mineralization in the Duobaoshan orefield, Northeast China: Evidence from fluid inclusions and H–O–C isotopes

The newly discovered Zhuanghuhe Au–Sb occurrence is located 20 km north of the famous Duobaoshan Cu–Mo deposit, it is the first gold–antimony deposit discovered in the Duobaoshan orefield. The hydrothermal quartz vein-type orebodies occur in the Middle Ordovician Duobaoshan Formation and are controlled by NNW and ENE compressional faults. Four mineralization stages are identified: stage I quartz + pyrite + arsenopyrite, stage II quartz + polymetallic sulfides, stage III quartz + stibnite, and stage IV quartz + calcite. Fluid inclusions (FIs) in quartz and calcite veins include six types: types I (two-phase aqueous), IIa (aqueous–carbonic; the volume of the carbon phase <50 %), IIb (aqueous–carbonic; the volume of the carbon phase >50 %, with more CO2 than CH4), IIc (aqueous–carbonic; the volume of the carbon phase >50 %, with more CH4 than CO2), IIIa (carbonic, with more CO2 than CH4) and IIIb (carbonic, with more CH4 than CO2). FIs of stage I are types I, IIa, IIb and IIIa, with homogenization temperatures of 279–311 °C and salinities of 4.87–11.84 wt%, indicating a medium-temperature low-salinity immiscible NaCl–H2O–CO2 fluid system. Stage II contains all FI types with final homogenization temperatures of 233–288 °C and salinities of 4.94–8.67 wt%, indicating a medium- to low-temperature low-salinity NaCl–H2O–CO2–CH4 system. FIs of stage III contain types I and IIa with homogenization temperatures of 193–240 °C and salinities of 3.85–6.63 wt%, belong to a low-temperature low-salinity NaCl–H2O–CO2 ± CH4 system. FIs of stage IV contain only type I with homogenization temperatures of 158–212 °C and salinities of 2.56–4.01 wt%, indicating a NaCl–H2O system. The H–O–C isotope data show that the NaCl–H2O–CO2 fluids of stage I (δD = −98 to −105.7 ‰, δOH2O = 5.7 to 6.9 ‰, δ13CCO2 = −11.9 to −11.1 ‰) were derived from a magmatic system, the fluids of stage II (δD = −85.7 ‰, δOH2O = 2.8 ‰, δ13CCO2 = −14.2 ‰, δ13CCH4 = −32.5 to −31.9 ‰) were characterized by the addition of CH4, which might have come from the reaction between the original ore-forming fluids and carbonaceous slate of the Duobaoshan Formation. The fluids in stage III (δD = −90.2 to −89.6 ‰, δOH2O = −9.1 to −6.1 ‰) and stage IV (δD = −110.6 to −96.3 ‰, δOH2O = −13.0 to −12.6 ‰) were characterized by mixing of meteoric water. According to the geology, fluid inclusion and H-O-C stable isotope evidences, the initial magma-derived NaCl-H2O-CO2 fluids were immiscible in stage I and then the process of mixing with CH4-bearing fluids in stage II, resulting in the gold precipitation; the precipitation of antimony was mainly related to the temperature drop and the mixing of meteoric water in stage III.

Experimental study on multi-scale damage and deterioration mechanism of carbonaceous slate under freeze-thaw cycles

Experimental study on multi-scale damage and deterioration mechanism of carbonaceous slate under freeze-thaw cycles

Metallogenic Model for Pb-Zn Deposits in Clastic Rocks of the Dahai Mining Area, Northeast Yunnan: Evidence from H-O-S-Sr-Pb Isotopes

The Dahai Pb-Zn mining area is located in the northwestern Pb-Zn district in northeastern Yunnan Province in the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic triangle (SYGT), east of the Xiaojiang fault. Numerous Pb-Zn deposits (spots) occur in clastic rocks in this area. In this study, the Maliping, Laoyingqing, and Jinniuchang Pb-Zn deposits, representative clastic rocks in the Dahai mining area, were selected as research objects. The results of H-O-S-Sr-Pb isotope analyses show that the three deposits mainly formed through the mixing of a basinal brine with a hydrothermal fluid derived from deep within the underlying (deformed) basement, and brines leached organic matter from wall rocks. The δ34S values range from −2.62–30.30‰. The S isotope results show two different sources of reduced S: one in the Laoyingqing deposit derived from the S reduction generated by the pyrolysis of sulfur-bearing organic matter in the carbonaceous slate of the Kunyang Group, and the second in the Maliping and Jinniuchang deposits derived from the S reduction generated by the thermochemical sulfur reduction (TSR) of seawater sulfate in the Lower Cambrian Yuhucun Formation and Sinian Dengying Formation. The Pb isotope results show that the Pb sources of the three deposits are derived from basement rocks (Kunyang Group) with a small portion derived from Devonian–Permian carbonate rocks and Dengying Formation dolomite, both of which have undergone homogenization during mineralization. The Sr content varied from 0.09629 to 0.2523 × 10−6, and the study shows that the main source of Sr is a mixture of ore-forming fluid flowing through basement rocks (Kunyang Group) and through sedimentary cover. However, most of the Sr in the Maliping deposit is derived from marine carbonate, and in the Laoyingqing deposit, it is provided by basement rocks (Kunyang Group). Based on a comparative study of the deposits, the Pb-Zn deposits in the clastic rocks of the Dahai mining area and the SYGT belong to the same metallogenic system and were formed under the same metallogenic geological background. Finally, a unified metallogenic model of the two types of fluid migration and mixed mineralization of the Pb-Zn deposit in clastic rocks of the Dahai mining area is proposed. The metallogenic model provides a basis for the study of the Pb-Zn metallogenic system and guidance for deep and peripheral prospecting in this area.

Hydrothermal Graphite as a Trigger for High-Temperature Orogenic Gold Mineralization at Haoyaoerhudong, Northern China

Abstract Carbonaceous materials are a key factor controlling mineralization processes in many world-class gold deposits. Haoyaoerhudong is the largest carbonaceous metasediment-hosted gold deposit on the north margin of the North China craton. Gold-bearing orebodies are hosted in carbonaceous slates and schists belonging to Mesoproterozoic rift-related successions. Typical hydrothermal minerals are pyrrhotite, quartz, biotite, graphite, apatite, titanite, and native gold. The ore mineralogy, combined with microthermometry and Raman spectra on fluid inclusions, has demonstrated three stages of hydrothermal activity: (I) quartz-biotite ± sulfide stage associated with gold mineralization (315°–510°C; ~4.8 wt % NaCl equiv; H2O-NaCl-CO2 ± CH4 ± N2 system); (II) quartz-sulfide stage, including quartz-sulfide stringers (IIa, 250°–334°C; ~5.4 wt % NaCl equiv; H2O-NaCl-CH4 ± CO2 ± N2 system) and fractured quartz-sulfide ores (IIb, 234°–308°C; ~4.1 wt % NaCl equiv; H2O-NaCl-N2 ± CH4 system); and (III) post-ore quartz-calcite stage (70°–219°C; ~4.8 wt % NaCl equiv; H2O-NaCl system). The molar ratios of CO2 and CH4 progressively decreased from stage I to II, consistent with the occurrence of graphite in alteration zones. Microscopic observation and Raman spectra suggest that the fine-grained graphite from altered schist (Gr-1/2) and coarse-sized graphite from gold-bearing veins (Gr-3/4) are of high crystallinity and exhibit characteristics indicating a hydrothermal origin. The δ13C values of graphite, varying from −27.1 to −26.0‰ Vienna-Pee Dee Belemnite (V-PDB), suggest that the carbon was of biogenetic origin. Apatite Sr isotopes (87Sr/86Sr: 0.708293–0.708842) and titanite Nd isotopes (εNdt: –11.76 to –14.84) also indicate contributions from carbonaceous rocks during mineralization. Thermodynamic modeling demonstrates that graphite may have precipitated at Haoyaoerhudong due to cooling and reduction of the H2O-CO2-CH4 fluids at high temperatures. Graphite precipitation would significantly consume CO2 and effectively destabilize Au bisulfide complexes, facilitating the codeposition of pyrrhotite, graphite, and native gold at high temperatures (≥379°C). We infer that deposition of hydrothermal graphite is a crucial process for mesothermal-hypothermal mineralization in sediment-hosted orogenic gold deposits.

LA–ICP–MS U–Pb geochronology and geological significance of two types of monazite in the Xinjiazui gold deposit, northwestern margin of Yangtze Block, China

The newly discovered Xinjiazui gold deposit situates on the northwestern margin of the Yangtze Block, China. Previously, the absolute age of gold metallogenesis in this area has yet to be well defined due to the lack of suitable dating minerals, significantly hindering the study of ore genesis and metallogenesis of gold deposits. This study presents high precision in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) U–Pb ages of monazite to address this issue. Two types of monazite have been found in this deposit, with Type 1 developed in the sedimentary diagenetic stage and Type 2 formed in the ore-forming stage. Type 1 monazite collected from carbonaceous slate is symbiotic with framboidal pyrite in a skeletal texture. It exhibits a high total amount of rare earth elements (REE) and low Y, Th, and U contents, indicating a low-grade metamorphic origin. In contrast, Type 2 monazite from the auriferous quartz vein is intergrown with hydrothermal mineral assemblages including free gold, Au-bearing pyrite and arsenopyrite, ankerite and sericite, suggesting a hydrothermal origin. The monazite (Type 1) LA–ICP–MS 206Pb/238U age is 460.8 ± 7.0 Ma, representing the sedimentary age of the host rock. A combination of lithologic associations and geochemical characteristics indicates that the Ordovician strata (O2ch) are important surrounding rocks in the Xinjiazui gold deposit. Additionally, the forming ages of monazite symbiotic with free gold in the auriferous quartz–sulfide veins range from 206.4 to 207.4 Ma, representing the gold mineralization age of the Xinjiazui deposit. Field geological observation shows that the vertical vein wall of columnar quartz crystals grows inward in the auriferous quartz–sulfide vein geodes and the euhedral granular sulfides develop in geodes. Therefore, this study proposes that the tectonic system changed from compression to extension in the late orogenic period with the closure of Paleo-Tethys and gold mineralization occurred in the Back-Longmenshan tectonic belt.

Investigation of Damage and Creep for Bedding's Carbonaceous Slate with Chemical Erosion Effect.

Tunnel projects in the southwestern mountainous area of China are in full swing. According to the tunnel burial depth, structural characteristics, and chemical erosion environments of the Lixiang railway tunnel, carbonaceous slate specimens obtained in the field were taken to experimentally investigate the physical, mechanical, and creep characteristics of the bedding's slate specimens after chemical erosion. The results of scanning electron microscopy (SEM) indicate that chemical erosion leads to internal damage in the carbonaceous slate specimens, and the internal damages are increasing with the increase of erosion days. Moreover, the specimens' ultrasonic test (UT) results prove that specimens with smaller bedding angles suffer a more serious erosion and induce more internal cracks. Under conventional triaxial compression conditions with 40 MPa of confining pressures, the conventional triaxial compressive strength (σs) decreases with the decrease of the bedding angle and the increase of erosion days, and the failure modes of the specimens are mainly controlled by oblique shear fractures and accompanied by the occurrence of slip dislocation fractures between the bedding inclination. Under creep conditions with 40 MPa of confining pressures, the final deformations of specimens are increasing with the increase of erosion days, which means the longer the erosion days, the greater the deformations. The failure modes of the specimens under creep conditions are controlled by shear fractures, and for the specimen with a 60° bedding angle and long-term erosion, there are block separations and many cavities along the shear planes. Therefore, more attention should be paid to prevent serious failure of the surrounding rock if the surrounding rock has a bedding angle of 60° or suffers long-term erosion.

Research on large deformation control technology of highly weathered carbonaceous slate tunnel

Abstract In order to further explore the large deformation control technology of high ground stress soft rock tunnel, this study takes the highly weathered carbonaceous shale section of Muzhailing tunnel as the research background, the large deformation control effect of scheme 1 “three-step method” and scheme 2 “pilot tunnel expansion method” are compared and analysed by field test and numerical simulation. The results show that the numerical simulation results are consistent with the field test, and the deformation trend of the tunnel is the same. The maximum deformation of the tunnel occurs at the middle step. From the perspective of tunnel deformation, control effect is reduced by about 10% compared with scheme 1, and the deformation of both schemes does not exceed the reserved deformation (400 mm). From the perspective of construction efficiency, the construction efficiency of scheme 2 is 23.07% lower than that of scheme 1. Taking into account the deformation control effect and construction efficiency, it is recommended that the three-step method should be adopted in the construction of this section, and the research results can provide a reference for the construction of the carbonaceous slate section of the tunnel.

Field test on deformation control of weathered carbonaceous slate tunnel in high geostress

The problems of large deformation often occur in strongly weathered carbonaceous slate tunnels with high geostress. In order to control large deformation in the process of tunnel construction, an advanced drilling measure and an advanced pilot tunnel measure are proposed, then relevant field tests are carried out to verify their results. Compared with conventional support measures, the advanced pilot tunnel measure has a better control effect on horizontal convergence and vault settlement, which are reduced by 29.70% and 75.81%, respectively. In addition, the advanced drilling measure provides more advantages in controlling the maximum surrounding rock stress and the maximum steel frame stress, which are reduced by 77.44% and 23.12%, respectively. The results show that the large deformation of a strongly weathered carbonaceous slate tunnel in high geostress cannot be effectively solved by conventional support measures, but the two advanced stress-control measures can effectively reduce deformation of the surrounding rock of the tunnel and decrease the stress of the tunnel's steel frame and surrounding rock.

Genesis of the world-class Dachang gold deposit, northern Qinghai-Tibet Plateau: A multiproxy approach

Dachang is a world-class gold deposit that occurs in the Bayan Har Terrane, northern Qinghai-Tibet Plateau, with Au resources of 121 t at an average grade of 3.0 g/t. Although this deposit was investigated in some previous studies, its genesis remains debated. The deposit is controlled by the Gande-Maduo anticlinorium and thrust fault zone, and the orebodies occur in subsidiary interlayer faults or fractures in thin-bedded, stratatoid, lenticular and vein forms. The host strata belong to the Lower Triassic Bayan Har Shan Group consisting of metasandstone, metasiltstone, slate, and carbonaceous slate interlayered with phyllite. The mineralization of Dachang can be divided into the following four stages. Stage 1 gold-sulfide-quartz veins, stage 2 gold-sulfide-carbonate-quartz veins, stage 3 stibnite-quartz veins and stage 4 quartz-carbonate veins. There are two types of ores: altered rocks and quartz veins. The main ore minerals are pyrite, arsenopyrite, stibnite and native gold, with trace amounts of tetrahedrite, chalcopyrite, galena and sphalerite. The wallrock alteration comprises mainly sulfidation, silicification, carbonatization, and sericitization. The element association is As, Sb, Au, Te, Bi, Ag, W and Pb in order of enrichment factors, whereas Cu, Mo, and Sn are depleted relative to their crustal abundances.In this study, we identified three types of primary fluid inclusions in quartz from veins of the four mineralization stages as follows: type 1 liquid-dominated two-phase aqueous inclusions; type 2 aqueous-carbonic inclusions, including aqueous-dominated (type 2a) and carbonic-dominated inclusions (type 2b); and type 3 two-phase hydrocarbon inclusions. Microthermometric measurements of fluid inclusion assemblages (FIAs) and bulk fluid inclusion analyses show that the ore-forming fluids had similar low-salinity compositions (1.7 to 6.9 wt% NaCl equiv.) in the H2O-CO2-N2 system with trace amounts of hydrocarbons. The mineralization temperatures range from 150 to 200 °C, and estimated depths are approximately 5 to 6.5 km. Fluid/wallrock reactions and immiscibility or boiling during mineralization in response to pressure fluctuations in a brittle faults were the main triggers for gold precipitation at Dachang.The hydrogen, oxygen, carbon and He-Ar isotopic compositions indicate that the ore-forming fluids originated from metamorphic devolatilization of immediate- and deep-level country rocks and no meteoric water or mantle volatiles were involved. The organic carbon proportions account for 42 to 53% of the total carbon in ore-forming fluid, and carbon fractions from sedimentary carbonates range from 47 to 58%. The gold mineralization followed immediately after the regional metamorphism (240–225 Ma) and occurred when the host rocks had exhumated into a brittle environment. The Dachang deposit is an epizonal orogenic gold deposit.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.