Geological and geochemical characteristics of the Baogudi Carlintype gold district(Southwest Guizhou,China) and their geological implications

The newly discovered Baogudi gold district is located in the southwestern Guizhou Province,China,where there are numerous Carlin-type gold deposits.To better understand the geological and geochemical characteristics of the Baogudi gold district,we carried out petrographic observations,elemental analyses,and fluid inclusion and isotopic composition studies.We also compared the results with those of typical Carlin-type gold deposits in southwestern Guizhou.Three mineralization stages,namely,the sedimentation diagenesis,hydrothermal(main-ore and late-ore substages),and supergene stages,were identified based on field and petrographic observations.The main-ore and late-ore stages correspond to Au and Sb mineralization,respectively,which are similar to typical Carlin-type mineralization.The mass transfer associated with alteration and mineralization shows that a significant amount of Au,As,Sb,Hg,Tl,Mo,and S were added to mineralized rocks during the main-ore stage.Remarkably,arsenic,Sb,and S were added to the mineralized rocks during the late-ore stage.Element migration indicates that the sulfidation process was responsible for ore formation.Four types of fluid inclusions were identified in ore-related quartz and fluorite.The main-ore stage fluids are characterized by an H2O–NaCl–CO2–CH4±N2system,with medium to low temperatures(180–260℃)and low salinity(0–9.08%NaCl equivalent).The late-ore stage fluids featured H2O–NaCl±CO2±CH4,with low temperature(120–200℃)and low salinity(0–7.48%Na Cl equivalent).The temperature,salinity,and CO2and CH4concentrations of ore-forming fluids decreased from the main-ore stage to the late-ore stage.The calculated δ^13C,d D,and δ^18O values of the ore-forming fluids range from-14.3 to-7.0%,-76 to-55.7%,and 4.5–15.0%,respectively.Late-ore-stage stibnite had δ^34S values ranging from-0.6 to 1.9%.These stable isotopic compositions indicate that the ore-forming fluids originated mainly from deep magmatic hydrothermal fluids,with minor contributions from strata.Collectively,the Baogudi metallogenic district has geological and geochemical characteristics that are typical of Carlin-type gold deposits in southwest Guizhou.It is likely that the Baogudi gold district,together with other Carlin-type gold deposits in southwestern Guizhou,was formed in response to a single widespread metallogenic event.     

华南巨型锑矿带的特征及其制约因素

华南巨型锑矿带位于中国南方扬子陆块与华夏陆块的接合过渡地段,锑矿床都产在沉积地层中,具有显著的层控性,常有整合的层状矿体出现,赋矿地层都显示异常高的锑丰度,充分显示了与沉积盆地演化密不可分的内在联系矿石的硫同位素受赋矿地层层位和沉积相的控制,碳同位素特征反映了深部因素与浅部过程的耦合,铅同位素则暗示研究区地层及其中锑矿化的亲缘关系和统一演化历史．锑是一个趋于在地壳浅部富集的中低温热液成矿元素,因此成熟度越高的地壳块段锑的丰度越高,锑矿产出概率也越大．华南巨型锑矿带的形成乃是扬子陆块南缘沉积盆地长期演化的结果,可能与这一岩石圈块段长期以来强烈的物质分异和再循环有关．     

Au-Sb association and fractionation in micro-disseminated gold deposits,southwestern Guizhou--geochemistry and thermodynamics

The dual geochemical character of paragenesis and fractionation between gold,arsenic and antimony in micro-disseminated gold deposits in southwestern Guizhou is discussed in terms of spatial distribution of independent deposits,lateral and vertical enrichment,mineralization stage and factor and correlation analyses.Thermodynamic calculations of solubility and speciation of gold and antimony minerals indicate that gold is transported in hydrothermal solution as Au(HS)-2 and antimony is mainly as Sb(OH)03 although HSb2S-4 may be of increasing importance in acid environment at low temperatures during the late stage of mineralization.Changes in physicochemical conditions hold the key to the association and fractionation between gold and antimony.Gold and antimony response differently to physicochemical variations because they are distinct from each other in soluble speciation and mineral solubility,leading to precipitation at different times and localities during hydrothermal evolution.     

Characteristics of the genetic mineralogy of pyrite and its significance for prospecting in the Denggezhuang gold deposit,Jiaodong Peninsula,China

This paper reports the variation rules for the typomorphic parameters of the pyrite and the gold enrichment rules of the Denggezhuang quartz vein gold deposit at a large-depth scale,providing the mineral signs for deep prospecting prediction through detailed study of the characteristics of crystal' habits,chemical composition,the thermoelectricity of pyrites,and mineralogical mapping.This paper primarily discusses the correlation between the mineralization intensity and the space-time evolution of the mineralogical parameters,clarifies the physicochemical conditions during gold mineralization,and provides information useful for deep mineralization prediction.We demonstrate that the crystal habits of the pyrites are very complex,primarily occurring as {100},{210},and their combinate form.{210} and {100}+{210} have positive correlations with gold mineralization,and {100}+{210} therefore can be useful for locating rich ore segments.The composition of pyrites is characteristically poor in S and rich in As.Their typical trace elements are composed of Mo,As,Pb,Cu,Bi,Zn,Au,Co,Se,Sb,Ag,Ni,Cr,and Te.The average contents of trace elements in pyrites from various stages show that the crystallizing temperature gradually decreased from an early stage to the metallogenic episodes.The precipitation and accumulation of Au and Ag occur primarily in the quartz-pyrite stage(III) and the polymetal minerals stage(IV).The occurrence rate of P-type pyrites(P(%)) is 83.52%.There is a larger dispersion of the thermoelectrical coefficient of pyrite(α) in the Denggezhuang gold deposit than in other deposits in the Jiaodong Peninsula.The electrical conductivity assemblage of pyrites from I to V is characterized by P≥N→PN→PN→PN→P≥N,which is usually considered beneficial for mineralization.The relative contents of As+Sb+Se+Te and Co+Ni are closely correlated to P-type and N-type average values and their occurrence rates.According to the crystallizing temperature of pyrite,the mineralization intensity,and the denudation degree,the mineralization temperature of the Denggezhuang gold deposit is in the range of 150–322℃ and is of a medium-low temperature.The orebody has already become denuded to the top-middle of the orebody,and the size of the orebody is larger than 900 m.Based on the vertical zoning of the thermoelectrical coefficients of pyrites and P-type pyrite mapping,it is suggested that the segment between ?425 and ?800 m may have exploration potential to the northeast of line 94 of the No.I2-2 orebody.     

Au-Sb association and fractionation in micro-disseminated gold deposits, southwestern Guizhou—geochemistry and thermodynamics

The dual geochemical character of paragenesis and fractionation between gold, arsenic and antimony in micro-disseminated gold deposits in southwestern Guizhou is discussed in terms of spatial distribution of independent deposits, lateral and vertical enrichment, mineralization stage and factor and correlation analyses. Thermodynamic calculations of solubility and speciation of gold and antimony minerals indicate that gold is transported in hydrothermal solution as Au(HS)^- ₂ and antimony is mainly as Sb (OH) ₃ ⁰ although HSb₂S₄ may be of increasing importance in acid environment at low temperatures during the late stage of mineralization. Changes in physicochemical conditions hold the key to the association and fractionation between gold and antimony. Gold and antimony response differently to physicochemical variations because they are distinct from each other in soluble speciation and mineral solubility, leading to precipitation at different times and localities during hydrothermal evolution.     

Fluid flow and mineralization of Youjiang Basin in the Yunnan-Guizhou-Guangxi area, China

Comprehensive studies, based on isotope geochemistry of C, H, O, S and Sr, chronology, common element and trace element geochemistry of fluid inclusions for the epithermal Au, As, Sb and Hg deposits in the Youjiang Basin and its peripheral areas, suggested that the ore fluid was the basin fluid with abundant metallic elements and the large-scale fluid flow of the same source in the late Yenshan stage was responsible for huge epithermal mineralization and silicification. The ore fluid flowed from the basin to the platform between the basin and the platform and migrated from the inter-platform basin to the isolated platform in the Youjiang Basin. The synsedimentary faults and paleokast surface acted respectively as main conduits for vertical and lateral fluid flow.     

Fluid flow and mineralization of Youjiang Basin in the Yunnan-Guizhou-Guangxi area,China

Comprehensive studies, based on isotope geochemistry of C, H, O, S and Sr, chronology, common element and trace element geochemistry of fluid inclusions for the epithermal Au, As, Sb and Hg deposits in the Youjiang Basin and its peripheral areas, suggested that the ore fluid was the basin fluid with abundant metallic elements and the large-scale fluid flow of the same source in the late Yenshan stage was responsible for huge epithermal mineralization and silicification. The ore fluid flowed from the basin to the platform between the basin and the platform and migrated from the inter-platform basin to the isolated platform in the Youjiang Basin. The synsedimentary faults and paleokast surface acted respectively as main conduits for vertical and lateral fluid flow.     

综合电磁法在矿区深部成矿机制中的应用研究——以皖南乌溪多金属矿区为例

乌溪矿区介于长江中下游多金属成矿带与华南成矿带之间,是江南造山带上的一个找矿新区.本文在这一找矿新区开展了有效的电磁方法综合勘探,试图对该区深部成矿机制进行研究.我们收集了该区的地质地球化学资料,从地质地球化学资料中分析了区域成矿背景;我们还采集了区域出露的主要岩石类型,在实验室开展物性测定,在此基础上选择了三种电磁方法开展研究区的野外测量.通过地面高精度磁测、激发极化法和可控源音频大地电磁法(CSAMT)三种地球物理方法,开展了研究区的磁化率、极化率和电阻率的分布特征研究.深入分析了地质、地球化学和地球物理三种资料与成矿的关系,相互约束,探讨成矿模型、成矿机制和成矿的可能性,推测可能的矿体赋存位置和深度范围.在地球物理研究结果基础上,结合地质和地球化学背景资料,构建了研究区的成矿动力学模型,推测了研究区成矿机制,揭示了矿区的成矿潜力.综合所得结果布设了钻孔,钻探结果揭示了研究区深部存在强烈矿化蚀变和强蚀变斑岩,初步确认为斑岩型矿床,与CSAMT剖面的解释结果基本一致,也与推测的测区内深部成矿机制相吻合.钻孔结果和地球物理结果的一致性以及对已构建的成矿动力学模型的支持,充分证明了综合电磁方法在斑岩型多金属矿床的发现和预测中的重要作用,同时斑岩型矿床的确认进一步印证了华南成矿带与俯冲作用形成的成矿带的相似性,从而推动整个华南地区的成矿地质研究.     

Fluid flow and mineralization of Youjiang Basin in the Yunnan-Guizhou-Guangxi area,China

Comprehensive studies, based on isotope geochemistry of C, H, O, S and Sr, chronology, common element and trace element geochemistry of fluid inclusions for the epithermal Au, As, Sb and Hg deposits in the Youjiang Basin and its peripheral areas, suggested that the ore fluid was the basin fluid with abundant metallic elements and the large-scale fluid flow of the same source in the late Yenshan stage was responsible for huge epithermal mineralization and silicification. The ore fluid flowed from the basin to the platform between the basin and the platform and migrated from the inter-platform basin to the isolated platform in the Youjiang Basin. The synsedimentary faults and paleokast surface acted respectively as main conduits for vertical and lateral fluid flow.

     

Post-eruptive process and products of volcanic rock alteration (transformation and synthesis of secondary products)

The conditions of transformation of the primary material of extrusive rocks and the synthesis of secondary products during post-eruptive processes have been considered using the example of three large areas of presentday volcanism and hydrothermal activity in Kamchatka, viz., the Great Tolbachik Fissure Eruption of 1975–1976 (GTFE), the Karymskii Volcanic Center (eruption of 1996 in Lake Karymskii), and the Uzon volcanic-hydrothermal system. Post-eruptive transformation of rocks causes volcanic-hydrothermal lithogenesis and low-middle-temperature mineralization in areas of present-day contrast (basalt-andesite) volcanism in zones of crustal extension. Based on the permanent presence of a wide range of acid rocks, such as dacites, rhyolites, and pumices in the areas of Holocene basalt volcanism of Kamchatka, we concluded that in conditions of pre-rift crustal extension, acid volcanism is preceded by deep-seated differentiation (similar to liquation) of primary mantle melts with separation of fluids enriched in ore-forming elements, first of all Cu, Zn, Pb, As, Sb, and Hg. During volcanogenic-hydrothermal lithogenesis in the discharge areas of those fluid systems, the destruction and replacement of primary minerals of igneous rocks and formation of complex-composition metasomatites take place. Clay minerals are major components of metasomatites in subaqueous conditions. The role of microbiota in processes of volcanogenic-hydrothermal lithogenesis and occurrences of local mineralization was studied.     

Biogenesis and binary mineralization of organism and chemism of polymetallic nodules from Pacific Ocean

The biogenic structure and biominerals in the polymetallic nodules reveal their biogenic origin. The experiment of the cation substitution capacity of synthetic 1 nm manganate suggests that chemism participates in the mineralization of the polymetallic nodules. That means the mineralization of the polymetallic nodules is two-component. After discussing the mineralization mechanism, the authors pointed out that the formation of polymetallic modules in deep-sea must meet four prerequisites: oxidative environment, very low sedimentation rate and/or hiatuses, enough nuclei, and suitable for the ultra-microbes-nodule’s builder to live. Project supported by the Deep Sea Geologic Research Project of China Oceanic Mineral Resources Research and Development Association (COMRA)     

Thermochemical sulphate reduction and Huayuan lead-zinc ore deposit in Hunan, China

In recent years, some arguments with regard to the organic mineralization of MVT lead-zinc ore deposit are focused on the thermochemical sulphate reduction in the presence of organic matter. Based on the research into the organic geochemistry and C, O, S isotopes of mineralized host rocks, mineral gas fluid inclusion and solid bitumen from Huayuan (W. Hunan, China) lead-zinc ore deposit formed in the algal limestones of Qingxudong formation, Lower Cambrian, the authors consider that a lot of organic matter occurred and participated in mineralization. The organic matter from different sources participated in the mineralization with two main forms: thermochemical sulphate reduction and thermal degradation which supplied abundant H₂S for the precipitation.     

Relationship between light rare earth deposits and mantle processes in Panxi rift,China

Many light rare earth deposits, such as Maoniuping, Dalucao, Panzhihua deposits, are collectively distributed in Panxi rift of Sichuan Province, China, and closely associated with the aegirine quartz syenite-carbonatite complex. Carbon and oxygen isotope studies demonstrate that the carbonatites in the complex are of typical igneous origin related to mantle processes. Electronic microprobe studies show that the fluid-melt inclusions found in the complex are enriched in light rare earth elements (LREE), which suggests that the magma was rich in LREE and could serve as the ore source for the regional LREE mineralization. Both the aegirine quartz syenite-carbonatite complex and the LREE mineralization found therein were derived from the mantle. The rare gas isotope analyses also support that there is a genetic association between the LREE mineralization and mantle processes.     

Relationship between light rare earth deposits and mantle processes in Panxi rift, China

Many light rare earth deposits, such as Maoniuping, Dalucao, Panzhihua deposits, are collectively distributed in Panxi rift of Sichuan Province, China, and closely associated with the aegirine quartz syenite-carbonatite complex. Carbon and oxygen isotope studies demonstrate that the carbonatites in the complex are of typical igneous origin related to mantle processes. Electronic microprobe studies show that the fluid-melt inclusions found in the complex are enriched in light rare earth elements (LREE), which suggests that the magma was rich in LREE and could serve as the ore source for the regional LREE mineralization. Both the aegirine quartz syenite-carbonatite complex and the LREE mineralization found therein were derived from the mantle. The rare gas isotope analyses also support that there is a genetic association between the LREE mineralization and mantle processes.     

Metallogenic model of the copper deposit

The copper deposits in the mid-lower reaches of the Changjiang River consist mainly of porphyry, skarn and massive sulphide deposits. According to the geological background, the types of ore deposits and the mineralization, the metallogenic model of the copper deposits is discussed. It is pointed out that the ore-forming hydrothermal fluids came mainly from magmatic water and were related to the intermediate-acid intrusions, and that the formation of skarn arid massive sulphide deposits coincided with the mineralization of porphyry deposits. Project supported by Doctorate Foundation of the State Education Commission of China.     

Large and superlarge silver deposits associated with volcanic rocks in China

This paper deals with the geology and geochemistry of the Gacun and Laochang large-sized marine volcanic rock-type Ag deposits in the Sanjiang (Tri-River) area of southwestern China and of the continental volcanic-subvolcanic rock-type Ag deposits in the Tianshan area of Xinjiang, and in the East area, China. It is considered that the marine volcanic rock-type Ag deposits occur mainly in the second-ordered volcano-sedimentary basins developed in island-arc and rift tectonic environments. The Ag deposits show an obvious zonation, with vein-network mineralization in the lower parts and hot water sedimentary rock-hosted stratified mineralization in the upper parts. From the Earth's surface downwards the ore-forming elements follows the order of As(Au))→Ag, Pb, Zn→Cu. The whole rock Rb-Sr isotopic isochron age of layered orebodies in the Gacun deposit is 204±14 Ma, indicating that the main stage of mineralization is Late Triassic in age. The continental volcanic-subvolcanic (porphyry) rock-type Ag deposits were formed later than the country rocks. The ores exhibit disseminated, veinlet disseminated, network and lumped structures. In addition, this study also deals with the geochemical characteristics of the continental volcanic-subvolcanic rock-type Ag deposits and the relations between Ag deposits or silver itself and fluorite, halogen-family elements and manganese.     

Relationship between light rare earth deposits and mantle processes in Panxi rift,China

Many light rare earth deposits, such as Maoniuping, Dalucao, Panzhihua deposits, are collectively distributed in Panxi rift of Sichuan Province, China, and closely associated with the aegirine quartz syenite-carbonatite complex. Carbon and oxygen isotope studies demonstrate that the carbonatites in the complex are of typical igneous origin related to mantle processes. Electronic microprobe studies show that the fluid-melt inclusions found in the complex are enriched in light rare earth elements (LREE), which suggests that the magma was rich in LREE and could serve as the ore source for the regional LREE mineralization. Both the aegirine quartz syenite-carbonatite complex and the LREE mineralization found therein were derived from the mantle. The rare gas isotope analyses also support that there is a genetic association between the LREE mineralization and mantle processes.

     

Evaluation of the potential risks of heavy metal contamination in rice paddy soils around an abandoned Hg mine area in Southwest China

To assess the potential ecological and health risks of trace elements(Hg,Cd,As,Mn,Sb,Pb,Cu,Ni,Cr,and Zn),a total of 138 soil samples from rice paddies were collected during the rice harvest season in the Wanshan mining area,Guizhou Province,Southwest China.Factors of the pollution load index(PLI),geo-accumulation index(I-Geo),enrichment factor(EF),and risk index(RI)were determined.High concentrations of Hg,Sb,As,Zn,Cd,Cu,and Mn were observed in the soils.The PLI,I-Geo,and EF results all showed high levels of contamination by Hg and Sb and moderate levels of contamination by As,Pb,Zn,Cu,Cd,and Mn.There was no significant contamination from Ni and Cr.The RI was very high,with Hg as the dominant pollutant,as expected,indicating that the historical large-scale Hg mining,as well as artisanal mining,has had a significant impact on the Wanshan area.Moreover,coal combustion,manganese factories,and the use of agrochemicals by the local population could also have an impact on the soil through the introduction of heavy metal loads.To address the current state of contamination,pollutant remediation and the regulation control of the anthropogenic activities in Wanshan are urgently needed.     

Large and superlarge silver deposits associated with volcanic rocks in China

This paper deals with the geology and geochemistry of the Gacun and Laochang large-sized marine volcanic rock-type Ag deposits in the Sanjiang (Tri-River) area of southwestern China and of the continental volcanic-subvolcanic rock-type Ag deposits in the Tianshan area of Xinjiang, and in the East area, China. It is considered that the marine volcanic rock-type Ag deposits occur mainly in the second-ordered volcano-sedimentary basins developed in island-arc and rift tectonic environments. The Ag deposits show an obvious zonation, with vein-network mineralization in the lower parts and hot water sedimentary rock-hosted stratified mineralization in the upper parts. From the Earth’s surface downwards the ore-forming elements follows the order of As(Au)) →Ag, Pb, Zn→Cu. The whole rock Rb-Sr isotopic isochron age of layered orebodies in the Gacun deposit is 204±14 Ma, indicating that the main stage of mineralization is Late Triassic in age. The continental volcanic-subvolcanic (porphyry) rock-type Ag deposits were formed later than the country rocks. The ores exhibit disseminated, veinlet disseminated, network and lumped structures. In addition, this study also deals with the geochemical characteristics of the continental volcanic-subvolcanic rock-type Ag deposits and the relations between Ag deposits or silver itself and fluorite, halogen-family elements and manganese.     

Antimony in iron meteorites

Antimony concentrations determined by radiochemical neutron activation analysis in 60 iron meteorites range from 0.2 ng/g to 36 μg/g. The meteorites with the highest Sb concentrations are those of the non-magmatic groups IAB and IIICD, while the lowest Sb concentrations are found in groups IVA and IVB, the groups with the lowest concentrations of the other most volatile siderophiles Ge and Ga. In all groups Sb is positively correlated with Ni. In each of the magmatic groups slopes on log Sb vs. log Ni plots decrease with increasing Ni. This decrease may reflect an increasing tendency to avoid schreibersite during the analysis of high-Ni meteorites because Sb partitions strongly into schreibersite.Schreibersite from New Westville is enriched in Cr, Ni, Ge, As, Sb and Au and depleted in Fe, Co and Ir; the content of Sb in schreibersite is 540 × higher than the bulk metal value.The Sb abundances of the iron meteorite groups are as expected from volatility trends with the exception of IAB and IIAB in which abundances appear depleted. The most likely explanation for this and the decreasing slope in the magmatic groups is that one or more Sb-rich phases were not sampled during metal analyses.