SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.     

Hydrothermal Mineralization on the Mesoproterozoic Passive Continental Margins of China： A Case Study of the Langshan-Zha＇ertaishan Belt, Inner Mongolia, China

Most ore-forming characteristics of the Langshan-Zha‘ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha‘ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical zonation of sulfides; (5) The Cu/(Pb Zn Cu) ratio of the large and thick Pb Zn Cu orebodies gradually decreases from bottom to top; and (6) barite is interbedded with pyrites and sometimes with sphalerite. However, some characteristics such as the Co/Ni radio of the pyrites, the volcanism, for example, of the Langshan-Zha‘ertalshan metallogenic belt, are different from those of the typical SEDEX deposits of the world. The meta-basic volcanic rock in Huogeqi, the sodic bimodal volcanic rocks in the Dongshengmiao and potassic bimodal-volcanic rocks with blastoporphyfitic and blasto-glomeroporphyritic texture as well as blasto-amygdaloidal structure in the Tanyaokou deposits have been discovered in the only ore-bearing second formation of the Langshan Group in the past 10 years. The metallogeny of some deposits hosted in the Langshan Group is closely related to syn-sedimentary volcanism based on the following facts: most of the lead isotopes in sphalerite, galena, pyrite, pyrrhotite and chalcopyrite plot on both sides of the line for the mantle or between the lines for the mantle and lower crust in the lead isotope composition diagram; cobalt content of some pyrites samples is much higher than the nickel content (Co/Ni= 11.91-12.19). Some volcanic blocks and debris have been picked out from some pyritic and pyrrhotitic ores. All Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying metamorphic volcanic rocks in the only ore-bearing second formation. In the Jiashengpan deposit that lacks syn-sedimentary volcanic rocks in the host succession only Pb and Zn ores occur without Cu ore, but in the Dongshengmiao, Tanyaokou and Huogeqi deposits with syn-sedimentary volcanic rocks in the host succession Cu ores occur. This indicates a relatively higher ore-forming temperature. The process of synsedimentary volcanic eruption directly supplied some ore-forming elements, and resulted in secular geothermal anomaly favorable for the circulation of a submarine convective hydrothermal system, which accounts for the precipitation of deep mineralizing fluids exhaling into anoxidic basins along the syn-sedimentary fault system in the Langshan-Zha‘ertai rift. The Dongshengmiao, Tanyaokou, and Huogeqi deposits hosted in the Langshan Group appear to be a transitional type of mineral deposit between SEDEX and VMS-types but with a bias towards SEDEX, while the Jiashengpan deposit hosted in the Zha‘ertai Group is of a characteristic SEDEX type. This evidence, together with other new discoveries of Mesoproterozoic volcanic rocks and the features of lithogeny and metallogeny of the Bayun Obo deposit in the neighborhood emphasize the diversity, complexity and uniqueness of the Mesoproterozoic Langshan-Zha‘ertal-Bayun Obo ore belt.     

Sensitive High-Resolution Ion Microprobe U-Pb Zircon and 40Ar-39Ar Muscovite Ages of the Yinshan Deposit in the Northeast Jiangxi Province, South China

The Yinshan deposit in the Jiangnan tectonic belt in South China consists of Pb‐Zn‐Ag and Cu‐Au ore bodies. This deposit contains approximately 83 Mt of the Cu‐Au ores at 0.52% Cu and 0.8 g/t Au, and 84 Mt of the Pb‐Zn‐Ag ores at 1.25% Pb, 1.02% Zn and 33.3 g/t Ag. It is hosted by low‐grade metamorphosed sedimentary rocks and mafic volcanic rocks of the lower Mesoproterozoic Shuangqiaoshan Group, and continental volcanic rocks of the Jurassic Erhuling Group and dacitic subvolcanic rocks. The ore bodies mainly consist of veinlets of sulfide minerals and sulfide‐disseminated rocks, which are divided into Cu‐Au and Pb‐Zn‐Ag ore bodies. The Cu‐Au ore bodies occur in the area close to a dacite porphyry stock (No. 3 stock), whereas Pb‐Zn‐Ag bodies occur in areas distal from the No. 3 stock. Muscovite is the main alteration mineral associated with the Cu‐Au ore bodies, and muscovite and chlorite are associated with the Pb‐Zn‐Ag ores. A zircon sensitive high‐resolution ion microprobe U‐Pb age from the No. 3 dacite stock suggests it was emplaced in Early Jurassic. Three ⁴⁰Ar‐³⁹Ar incremental‐heating mineral ages from muscovite, which are related to Cu‐Au and Pb‐Zn‐Ag mineralization, yielded 179–175 Ma. These muscovite ages indicate that Cu‐Au mineralization occurred at 178.2±1.4 Ma (2σ), and Pb‐Zn‐Ag mineralization at 175.4±1.2 Ma (2σ) and 175.3±1.1 Ma (2σ), which supports a restricted period for the mineralization. The Early Jurassic ages for the mineralization at Yinshan are similar to that of the porphyry Cu mineralization at Dexing in Jiangnan tectonic belt, and suggest that the polymetallic mineralization occurred in a regional transcompressional tectonic regime.     

扬子地台西缘铅锌矿床分布规律及矿源层探讨

为探索川滇黔相邻区铅锌矿床之成因规律,提升成矿理论认识及预测找矿效果,通过对区内铅锌矿床分布规律研究得出如下认识:1)发现矿床(点)之集群分布趋势,据此将成矿区域划分为3个矿集区;2)统计发现,震旦系和石炭系具有较高的成矿机率(51.57%),灯影组和摆佐组汇聚了区域80.98%的金属量;3)构造单元分级控制了成矿单元展布,而矿集区与二级构造单元之间具有不完全的对等性,矿集区Ⅰ、Ⅱ由康滇地轴和龙门山拗陷及二者向上扬子区域跨越地带联合控制;4)根据菱(赤)铁矿与铅锌矿空间耦合,以及菱(赤)铁矿伴生铅锌元素、铅锌矿物含量均较高等现象,论证了在盆地演化早期,古陆边缘拗陷带(或海盆)内之次级单元代表了浅海环境之低能较深水凹(断)陷或海湾环境,沉积了古生界志留系兰多维列统特列奇阶至下石炭统德坞阶和中元古界下昆阳群(会理群)两套含铁建造,形成了区域Pb、Zn成矿金属元素的初始富集,并于成岩-后生期经热液流体循环改造而成矿,含铁建造提供了成矿的主要矿质来源;5)本区成矿物质硫源-膏盐层主要赋存于灯影组和摆佐组下伏地层以及寒武系多个层位;6)矿源层、硫源共同决定了矿集区以及层控的形成机制,并成为控制其分布的决定性因素。     

西秦岭寨上金矿床稀土元素和微量元素特征

寨上金矿的金矿石稀土元素特征与容矿岩石相似,与侵入岩(二长花岗岩)的差别较明显,说明寨上金矿床成矿与地层建造具有成因联系.矿化带中明显富集Au,Hg,As,Sb,Pb,Zn黄铁矿中Co/Ni平均值为0.27～1.5,矿体的Co/Ni平均值为0.33～0.40,说明成矿温度不高.从赋矿岩石到金矿体Co,Ni值有增加的趋势,说明从沉积成岩到成矿,成矿元素及微量元素有进一步的富集.     

Sulfur Isotope Variations of Metallic Ore Deposits in the Gyeongsang Basin,South Korea

Many metallic ore deposits of the Late Cretaceous to Early Tertiary periods are distributed in the Gyeongsang Basin. Previous and newly analyzed sulfur isotope data of 309 sulfide samples from 56 ore deposits were reviewed to discuss the genetic characteristics in relation to granitoid rocks. The metallogenic provinces of the Gyeongsang Basin are divided into the Au–Ag(–Cu–Pb–Zn) province in the western basin where the sedimentary rocks of the Shindong and Hayang groups are distributed, Pb–Zn(–Au–Ag–Cu), Cu–Pb–Zn(–Au–Ag), and Fe–W(–Mo) province in the central basin where the volcanic rocks of the Yucheon Group are dominant, and Cu(–Mo–W–Fe) province in the southeastern basin where both sedimentary rocks of the Hayang Group and Tertiary volcanic rocks are present. Average sulfur isotope compositions of the ore deposits show high tendencies ranging from 2.2 to 11.7‰ (average 5.4‰) in the Pb–Zn(–Au–Ag–Cu) province, ?0.7 to 11.5‰ (average 4.6‰) in the Cu–Pb–Zn(–Au–Ag) province, and 3.7 to 11.4‰ (average 7.5‰) in the Fe–W(–Mo) province in relation to magnetite‐series granitoids, whereas they are low in the Au–Ag(–Cu–Pb–Zn) province in relation to ilmenite‐series granitoids, ranging from ?2.9 to 5.7‰ (average 1.7‰). In the Cu(–Mo–W–Fe) province δ³⁴S values are intermediate ranging from 0.3 to 7.7‰ (average 3.6‰) and locally high δ³⁴S values are likely attributable to sulfur derived from the Tertiary volcanic rocks during hydrothermal alteration through faults commonly developed in this region. Magma originated by the partial melting of the ³⁴S‐enriched oceanic plate intruded into the volcanic rocks and formed magnetite‐series granitoids in the central basin, which contributed to high δ³⁴S values of the metallic deposits. Conversely, ilmenite‐series granitoids were formed by assimilation of sedimentary rocks rich in organic sulfur that influenced the low δ³⁴S values of the deposits in the western and southeastern provinces.     

Stratabound phyllosilicate zones associated with syntectonic copper orebodies at Mt Isa,Queensland

Three distinctive stratabound phyllosilicate zones are present at the margins of the syntectonic Mt Isa Cu orebodies and their host rock, the ‘silica‐dolomite’, in northwestern Queensland. The zones show close spatial relationships with adjacent stratiform Pb‐Zn ores within the overall host, the dolomitic Urquhart Shale. The Pb‐Zn orebodies may be either sedimentary‐exhalative or, as suggested recently, late diagenetic in origin, whereas the Cu ores were formed during the third regional deformation event. Talc‐stilpnomelane layers within the footwall of the silica‐dolomite, and biotite‐rich layers at the margins and adjacent to the silica‐dolomite are present in sideritic intervals within the dolomitic sequence. These Fe‐carbonate‐rich layers correlate with the Pb‐Zn orebodies, but have a much greater lateral extent. Chloritic layers occur along the hanging wall contacts of several Pb‐Zn orebodies with overlying silica‐dolomite lobes. Microstructural and petrographic studies suggest that the phyllosilicates grew at the silicification stage during early syn‐D3 alteration that also formed the silica‐dolomite and its Cu orebodies. The stratabound distribution of talc, stilpnomelane and biotite is explained by the chemical control of the sideritic beds on silicification. The origin of the Fe‐carbonates is not known and may be either sedimentary or diagenetic. The chlorite zones may have developed because of more extensive fluid‐rock interactions controlled by higher permeabilities along the brittle shale and ductile galena layering within the Pb‐Zn orebodies.     

Particulate metal speciation in surficial sediments from Loch Dee, southwest Scotland, U.K.

The geochemical partitioning of ten elements in stratified Holocene sediments from Loch Dee, southwest Scotland, has been established by use of a five-stage sequential extraction procedure. Samples from below 15 cm sediment depth show minimal evidence of modification by anthropogenic contamination or active diagenesis and hold Fe, Mg, Cu, Cd, Co, Pb, and Ni primarily in detrital silicates or organic complexes, while Mn, Ca, and Zn reside largely in adsorbed and reducible oxide phases. In the uppermost ca 15 cm of sediment, enhanced total concentrations of Zn, Cu, and Pb reflect increased atmospheric deposition during the postindustrial period. Of these metals, only Pb displays any notable adjustment of partitioning in the enriched zone, showing disproportionate accumulation in labile oxides and organic-Pb phases. The lack of Pb and Zn carbonates in the contaminated horizon may reflect inherent thermodynamic instability under the acid surface and pore-water conditions of Loch Dee. Increments to total Mn and Co in the surficial ca 5 cm of sediment are attributable to the accumulation of secondary oxides and adsorbed species, consistent with precipitation from the interstitial pore-waters across a sedimentary redox front. The presence of metals such as Zn and Cd in soluble or acid-volatile phases in the interfacial sediment has implications for the future management of the Loch Dee basin, with leaching into the overlying waters likely, given the continuation of current trends of lake acidification.     

Metallogeny of the Baiyangping Lead‐Zinc Polymetallic Ore Concentration Area,Northern Lanping Basin of Yunnan Province,China

The Lanping Basin in the Nujiang‐Lancangjiang‐Jinshajiang (the Sanjiang) area of northeastern margin of the Tibetan Plateau is an important part of eastern Tethyan metallogenic domain. This basin hosts a number of large unique sediment‐hosted Pb‐Zn polymetallic deposits or ore districts, such as the Baiyangping ore concentration area which is one of the representative ore district. The Baiyangping ore concentration area can be divided into the east and west ore belts, which were formed in a folded tectogene of the India‐Asia continental collisional setting and was controlled by a large reverse fault. Field observations reveal that the Mesozoic and Cenozoic sedimentary strata were outcropped in the mining area, and that the orebodies are obviously controlled by faults and hosted in sandstone and carbonate rocks. However, the ore‐forming elements in the east ore belt are mainly Pb‐Zn‐Sr‐Ag, while Pb‐Zn‐Ag‐Cu‐Co elements are dominant in the west ore belt. Comparative analysis of the C‐O‐Sr‐S‐Pb isotopic compositions suggest that both ore belts had a homogeneous carbon source, and the carbon in hydrothermal calcite is derived from the dissolution of carbonate rock strata; the ore‐forming fluids were originated from formation water and precipitate water, which belonged to basin brine fluid system; sulfur was from organic thermal chemical sulfate reduction and biological sulfate reduction; the metal mineralization material was from sedimentary strata and basement, but the difference of the material source of the basement and the strata and the superimposed mineralization of the west ore belt resulted in the difference of metallogenic elements between the eastern and western metallogenic belts. The Pb‐Zn mineralization age of both ore belts was contemporary and formed in the same metallogenetic event. Both thrust formed at the same time and occurred at the Early Oligocene, which is consistent with the age constrained by field geological relationship.     

沉积岩型层状铜矿床研究进展

沉积岩型层状铜矿床是全球第二重要的铜矿类型,重要性仅次于斑岩型铜矿床。并且,它们常伴生一定规模的钴、银、铅、锌、铀、金、铂族元素等其他金属资源。其矿体通常呈层状、似层状发育在沉积盆地的还原性岩石或地层中。大多数沉积岩型层状铜矿床形成于围岩的成岩作用或者成岩晚阶段,但也经常会受到成矿后变质作用、变形作用的改造,发生成矿物质的活化一再沉淀。原生成矿作用的发生通常要经历成矿流体(低温、中—高盐度、含硫)在矿体下盘的红层中持续、长期的循环,萃取铜等金属元素,随后沿着盆地边界断裂迁移至盆地还原性地层中或者被迁移的还原性物质(石油、天然气)还原而发生铜等成矿物质的沉淀。超大规模的层状铜矿化可能对应地球地质历史时期特殊的地质事件和地质条件,其中包括超大陆裂解、炎热干旱的古气候、大氧化事件以及冰期和富镁的海洋等。     

Sediment-hosted Pb-Zn Deposits in Southwest Sanjiang Tethys and Kangdian Area on the Western Margin of Yangtze Craton

<正>The western margin of Yangtze Craton is known as a significant sediment-hosted base-metal aggregate cluster,especially for Pb-Zn deposits in China,e.g.Jinding,Daliangzi,Tianbaoshan, Kuangshanchang and Qinlinchang deposits.In comparison with the classic MVT deposits in the world, based on the basic geology of the sediment-hosted Pb-Zn deposits,this paper focuses on temporal-spatial distribution of this deposit to further discuss its large scale mineralization and tectonic evolution history.In the SW Sanjiang Thethys,Jinding deposit is typically thrust fault-controlled and hosted mainly in the sandstones and breccia-bearing sandstones,whereas MVT-type deposits are controlled by lithology and faulting/fracturing with a strong preference for carbonate-hosted rocks.Most importantly,Jinding Pb-Zn deposit differs from the other types of sediment-hosted Pb-Zn deposits in which it was formed in a strongly deformed foreland basin within a continental collision zone.In the Kangdian area,the sediment-hosted Pb-Zn deposits were formed in the extensional basin on the side of the continental orogenic belt along the Yangtze Craton.Compared with classic MVT deposits,the Pb-Zn deposits in the Kangdian area belong to MVT deposits.This paper is significant not only for interpretation of the genesis of sediment-hosted Pb-Zn deposits but also for exploiting large base metal deposits in large sedimentary target areas.     

Isotopes and Geochronology of the Meixian-type Pb-Zn-(Ag)Deposits,Central Fujian Rift,South China:Implications for Geological Events

Central Fujian Rift is another new and important volcanogenic massive sulfide Pb-Zn polymetallic metallogenetic belt. In order to find out the material genesis and mineralization period of Meixian-type Pb-Zn-Ag deposits, S and Pb isotope analysis and isotope geochronology of ores and wall rocks for five major deposits are discussed. It is concluded that the composition of sulfur isotope from sulfide ore vary slightly in different deposits and the mean value is close to zero with the 834S ranging from -3.5‰ to ＋5.6‰ averaging at ＋2.0‰, which indicates that the sulfur might originate from magma or possibly erupted directly from volcano or was leached from ore-hosted volcanic rock. The lead from ores in most deposits displays radioactive genesis character （206pb/204pb〉18.140, 207Pb/204pb〉15.584, 208pb/204pb〉38.569） and lead isotope values of ores are higher than those of wall rocks, which indicates that the lead was likely leached from the ore-hosted volcanic rocks. Based on isotope data, two significant Pb-Zn metallogenesis are delineated, which are Mid- and Late-Proterozoic sedimentary exhalative metailogenesis （The single zircon U-Pb, Sm-Nd isochronal and Ar-Ar dating ages of ore- hosted wall rocks are calculated to be among 933-1788 Ma.） and Yanshanian magmatic hydrothermal superimposed and alternated metallogenesis （intrusive SHRIMP zircon U-Pb and Rb-Sr isochronal ages between 127-154 Ma）.     

鄂尔多斯盆地中东部马五段碳酸盐岩气藏富气主控因素——以陕200井区为例

靖边气田是中国最早发现的海相碳酸盐岩气藏之一,目前勘探范围仍在不断扩大。在回顾历年勘探和研究的基础上,以靖边气田陕200井区为例,总结和探讨了鄂尔多斯盆地中东部奥陶系碳酸盐岩储层富气的主控因素,以期为进一步勘探开发提供合理建议。研究表明:构造演变是靖边气田成藏的前提,决定了气田的分布范围和产气层位;沟槽的分布位置和切割深度对马五气藏的富集程度有着重要影响,鼻状构造不是主控因素。沉积环境是形成有利储层岩石类型的先决条件,潮间带云坪、潮上带膏云坪和含膏云坪微相是有利沉积微相。建设性的成岩作用是形成有利储集空间的重要保证,研究区多期溶蚀、白云化等建设性成岩作用对储层质量的改进较大,其中多期溶蚀作用是形成优质储集空间的主要成因。裂缝是天然气富集的重要因素,但裂缝发育程度因层位而不同。孔、洞、缝及沟槽的有效配置受构造、沉积、成岩作用的控制,每个层段的储集性能和天然气的富气程度不同。     

A Preliminary Review of Metallogenic Regularity of Silver Deposits in China

China has widely distributed silver deposits,and is rich in silver resources.Although silver deposits are mainly associated with Pb-Zn deposits,a number of independent silver deposits have also been discovered in recent years.Silver deposits include different types,such as submarine volcanism and continental volcanism related type,intrusion related type,and sedimentary related type.This study summarized the metallogenic regularity of China’s silver deposits systematically based mainly on the data from 490 silver deposits.It is shown that submarine volcanic sedimentary type,continental volcanic or sub-volcanic type,skarn type,hydrothermal type(including vein type and stratabound type),sedimentary metamorphic type,sedimentary type and regolith type should be regarded as the most important prediction types of silver deposit.A total of 32 silver mineralization belts and 111 silver concentration areas have been delineated.The map of "Spatial distribution of silver mineralization belts in China" and other series of maps finished in this study may provide a theoretical basis for the evaluation and prognosis of silver resources potential in China.     

Rb-Sr and Sm-Nd Isochron Ages of the Dongmozhazhua and Mohailaheng Pb-Zn Ore Deposits in the Yushu area,southern Qinghai and Their Geological Implications

Located on the northeast margin of the Qiangtang terrane between the Jinshajiang suture zone and Bangonghu-Nujiang suture zone, the Dongmozhazhua and Mohailaheng Pb-Zn deposits in the Yushu area of Qinghai Province are representative Pb-Zn deposits of the Pb-Zn-Cu polymetallic mineralization belt in the northern part of the Nujiang-Lancangjiang-Jinshajiang area, which are in the front belt of the Yushu thrust nappe system. The formed environments of these two deposits are different from those of sediment-hosted base metal deposits elsewhere in the world. The authors hold that they were formed during the Indian-Asian continental collision and developed within the foldthrust belt combined with thrust and strike-slip-related Cenozoic basins in the interior of the collisional zone. Studying on the metallogenic epochs of these two deposits is helpful to the understanding of ore-forming regularity of the regional Pb-Zn-Cu mineralization belt and also to the search for new deposits in this region. The age of the Dongmozhazhua deposit has been determined by the Rb-Sr isochron method for sphalerite residues, whereas the age of the Mohailaheng deposit has been determined by the Rb-Sr isochron method for sphalerite residues and the Sm-Nd isochron method for fluorite. The age of the Dongmozhazhua deposit is 35.0±0.0 Ma((87Sr/86Sr)0=0.708807) for sphalerite residues. The age of the Mohailaheng deposit is 32.2±0.4 Ma((87Sr/86Sr)0=0.708514) for sphalerite residues and 31.8±0.3 Ma((143Nd/144Nd)0=0.512362) for fluorite with an average of 32.0 Ma. Together with the regional geological setting during mineralization, a possible tectonic model for metallogeny of the Dongmozhazhua and Mohailaheng Pb-Zn deposits has been established. These two ages are close to the ages of the Pb-Zn deposits in the Lanping and Tuotuohe basins, indicating that it is possible that the narrow 1000-kilometer-long belt controlled by a thrust nappe system on the eastern and northern margins of the Tibetan plateau could be a giant Pb-Zn mineralized belt.     

Simulation of sedimentary Ore-Forming processes: Concentration of Pb and Zn from brines into organic and Fe-bearing carbonate sediments

An experimental sedimentary system comprising a tank of 4 m³ capacity equipped for monitoring chemical, mineralogical, and biological changes has been used to investigate the mechanisms by which Pb and Zn may be removed from solution in sulphide-deficient brines and concentrated in sediments. In the experimental system, Pb and Zn together with ferric hydroxides (probably lepidocrosite), organic matter, and a variety of calcium and magnesium carbonate phases, were deposited from an aerobic, highly saline, Pb and Zn-rich brine supporting a vigorous growth of the green alga Chlorococcus sp. The resultant organic and Febearing carbonate sediments contained Pb up to 0.5% and Zn up to 1.0%. Overall concentration factors compared with the overlying brine were in the range 200 to 300. Pb was removed from solution mainly by coprecipitation with carbonate phases; the Pb content of the two major carbonate phases decreasing in the order aragonite to monohydrocalcite. Zn was deposited in association with the Fe-bearing minerals. Complexing of Pb and Zn by organic matter, and the direct precipitation of Pb and Zn carbonates and/or hydroxides made, at most, a secondary contribution to the overall concentration process.     

论华南喷流—沉积块状硫化物矿床

现代海底喷流－沉积硫化物矿床的发现极大地推动了海底热液成矿理论的发展,也大大地提高了对古代海底喷流块充化物矿术的研究水平。本文指出喷流－沉积是重要的成矿作用,提出喷流－沉积矿床是华南Ｃｕ、Ｐｂ、Ｚｎ、Ｓｎ、Ａｇ、Ａｕ等矿产资源的重要来源,形成了一批超大型矿床,并将华南许多曾被认为属夕卡岩矿床重新确认为喷流－沉积岩床。文章还论述了华南喷流－沉积块状硫化物矿床的特征、分类、时空分布及其成矿特点等问题,提出断裂拗陷带型喷流－沉积块状硫化物矿床是华南具有特色的类型,而陆相断陷盆地中喷流－沉积矿床值得进一步深入研究。     

Organic geochemistry in Precambrian research

Cherts, shales, and carbonates containing small amounts of organic matter occur throughout the Precambrian sedimentary record. The oldest known organic-rich sediments have been dated at > 3,000 million years. Knowledge of the composition, origin, and fate of the organic content of these rocks relates to many facets of Precambrian earth history. Apart from its primary paleobiological significance, organic geochemical research is also concerned with problems in atmospheric and hydrospheric evolution, sedimentation, diagenesis, metamorphism, and ore genesis.Precambrian organic geochemistry to date has rested on the premise that ancient life process can be profitably studied at a chemical level, as well as by examination of the morphological fossils of conventional paleontology. A decade of intensive research on selected Precambrian carbonaceous and bituminous sediments has been made possible by the refinement of analytical techniques (notably gas chromatography and mass spectrometry) which are now capable of detecting and characterizing the minute quantities of complex organic mixtures found in geological materials. It has resulted in the development of criteria which allow discrimination between indigenous and adventitious organic matter. A diverse array of stable organic compounds has been isolated from Precambrian sedimentary rocks. The compounds, termed chemical fossils, are thought to be derived from ancestral microorganisms and as such have contributed to the formulation of the biological marker concept. Despite reports of possible abiotic occurrences, a biological origin for most of the organic matter preserved in Precambrian rocks (whether sedimentary or metamorphic) now appears likely.The possibility of secondary emplacement from migrating formation fluids means that an indigenous chemical fossil need not have been syngenetic with the deposition of its host sediment. No conclusive test of Precambrian age as yet exists for these compounds. The bulk of the organic material (kerogen) in Precambrian rocks is insoluble in organic and aqueous solvents. For this reason, kerogen is almost certainly both indigenous and syngenetic, but clarification of its chemical structure must await the development of new analytical procedures. The nature and extent of the modification that the soluble and insoluble organic fractions have undergone during late diagenesis and incipient metamorphism is still virtually unknown. Correlation of chemical fossils with specific biogenic precursor compounds, or a particular type of primitive organism, therefore remains equivocal. So also comparison of the organic geochemical facies of one sediment with that of another may be complicated by differences in their thermal history and mineralogy.Recent interest in the possible involvement of living and decayed microorganisms in the genesis of certain Precambrian Fe, U, Au, Cu, Pb, and Zn metal deposits signals the beginning of an important new phase in organic geochemical studies.     

Terrestrial geochemistry of Cd,Bi, Tl,Pb, Zn and Rb

About 2000 common magmatic, metamorphic and sedimentary rocks and rockforming minerals contained in 465 individual samples have been analyzed for 6 trace metals and potassium with high precision, mainly by combined distillation and AAS methods. Estimates of average abundances in the continental crust are: 98 ppb Cd. 82 ppb Bi. 490 ppb Tl, 14.8 ppm Pb, 77 ppm Zn and 98 ppm Rb (K/Rb: 223). These averages are close to the mean concentrations of the 6 elements in sedimentary and in low to medium grade metamorphic rocks. In relation to the upper mantle the earth's crust has very effectively accumulated Rb, Pb, Tl (and Bi). Cd and Zn are equally distributed between the upper and lower crust. Bi, Tl, Rb, Pb and K are accumulated in the upper relative to the lower continental crust by factors between 3.5 and 1.4. This is mainly due to higher concentrations in granites and lower abundances in granulites relative to gneisses and schists. The five metals form large ions with bulk coefficients less than one for the partition between metamorphic rocks and anatectic granitic melts. The major hosts of Rb, Tl, Pb and Bi in rocks are minerals with 8- to 12-coordinated sites such as mica, K-feldspar, plagioclase etc. (except for some preference of Bi for sphene and apatite). As examples of significant correlations those of Pb with Tl, K, Bi and Rb in mafic rocks and of Bi with K, Rb, Tl and Pb in sedimentary rocks can be reported. In granites and gneisses hydroxyl containing Fe²⁺-Mg-silicates are major host minerals for Zn and Cd. Except in some carbonate rocks Cd has no preference for Ca minerals.     

Timing of mineralization and deformation,Peak Au mine,Cobar, New South Wales∗

The ⁴⁰Ar/³⁹Ar dating of alteration muscovite from the Peak Au mine in the Early Devonian Cobar Basin, New South Wales, has distinguished two major episodes of mineralization. Veined (Pb‐Zn‐Cu‐Au) mineralization was broadly synchronous with cleavage formation during the post‐inversion, shortening deformation of the basin sedimentary rocks, and replacive Ag‐Pb‐Zn mineralization significantly postdates the latter event. Veined base metals (Pb‐Zn‐Cu) and Au associated with silicification were coeval with three stages of cleavage formation (D₁, D₂ and D₃) after basin inversion. The Cu‐Au phase of mineralization at the Peak Au mine which was broadly contemporaneous with the culmination of the cleavage‐forming events (D₃) and with the local development of high‐strain zones occurred at 401.5 ± 1.0 Ma (⁴⁰Ar/³⁹Ar on muscovite). This date is essentially coeval with known fossil constraints on the age of basin formation, and indicates that basin inversion and deformation rapidly followed sedimentation. In contrast, replacive Ag‐Pb‐Zn mineralization occurred at 384.0 ± 1.4 Ma (⁴⁰Ar/³⁹Ar on muscovite) during an extended period of relaxation characterized by normal faulting (D₄) which followed the shortening deformation. This mineralization was associated with desilicification and chlorite‐muscovite replacement assemblages which cross‐cut the cleavages, and which may have been broadly contemporaneous with the deposition of part of the Mulga Downs Group which unconformably overlies the Cobar Supergroup. Rhyolite exposed in the core of the Peak Au mine largely contains inherited zircons that range in age from ～430–1500 Ma. A few euhedral zircons have an age of ～430 Ma and this is interpreted as a maximum date for the rock. Zircons from a syn‐D₃ chlorite‐muscovite replacement zone within the deposit have ²⁰⁶Pb/²³⁸U ages of ～410–650 Ma and are apparently inherited.