Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (~2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of △33S varies from -0.89‰ to 1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero △33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative △33S value, however, Superior-type BIFs which are far away from the volcanic center have positive △33S value. The δ 34S varies in a large range from -22.0‰ to 11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10-2―10-3 of the present atmospheric level (PAL).     

鞍山北东部韧性变形带中BIF型铁矿的转变

本文以鞍山地区樱桃园——眼前山一带为例,讨论BIF型铁矿在韧性变形带中的转变,旨在阐明中低温韧性变形带中BIF型铁矿的矿物、化学成份和流体性质的演变;矿石层中组构的变化,特别是该类铁矿特征的条带状构造的演变;韧性变形带中变质与变形的关系及其对铁矿石质量的影响。韧性变形带中铁矿围岩的变化将一起讨论,以便对照说明。     

鞍─本地区条带状铁建造中硅铁矿物分异反馈及生长动力学演化

自组织过程中矿物的竞争性颗粒生长模型与颗粒生长动力学演化方程是鞍─本地区条带状铁建造（ＢＩＦ）在变质分异作用过程中硅铁分异形成硅质矿物条带与铁矿物条带的生长反馈系统及矿物竞争性颗粒生长的动力学演化过程，竞争状态的表面扩张担负起了Ｏｓｔｗａｌｄ成熟律作用，使系统向减少颗粒数但加大颗粒体积的状态演化，导致了矿物相的成层分异。     

坦桑尼亚太古宙绿岩带BIF型金矿地质-地球化学特征——以马黑加金矿床为例

坦桑尼亚马黑加金矿是典型的BIF型金矿,位于坦桑尼亚克拉通的太古宙卡哈马绿岩带,是隐伏的条带状含铁建造型(BIF)金矿床。该矿床受地层、构造双重控制。地层强烈褶皱并发生剪切,含矿岩性为上尼安萨(Nyanzian)群上下段接触带附近的中基性火山岩。其主要化探异常元素组合为Au-As-Sb-Ag-Hg-Pb-Ba,其中Au,As,Sb在含矿的条带状磁铁石英岩中富集明显,是深部寻找金盲矿体的主要指示元素。     

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt-hosted gold mineralization at the Tau deposit of the Mupane gold mine,Botswana

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite-rich zones associated with biotite-chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold-thiocomplexes in the ore-forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two-stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ³⁴S of gold-bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi-processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.     

岩石流变学原理在构造成矿研究中的应用——以BIF型富铁矿床为例

根据岩石流变学的基本原理与研究进展及在成矿学中的应用,初步讨论了构造成矿与岩石流变在研究内容、研究目标等方面的一致性问题,并以世界上变质条带状铁建造(BIF)型富铁矿床为例,综合分析了构造变形和岩石流变在该类矿床形成过程中的地位与作用。由于岩石流变行为存在差异,褶皱变形及伴随的韧性剪切和高温塑性流动是高品位BIF型富铁矿床形成的关键。文章指出,深入开展特色和重要矿床的构造物理化学研究将是实现成矿学与岩石流变学有机结合的重要方向,并有可能为创新成矿地质理论、发现一批大型、超大型矿床提供重要依据。     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造(BIF)有关,可分为2种类型:1深成—表生矿床;2表生—富集矿床,主要分布在尤恩米(Youanmi)地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

新太古代清原绿岩带下甸子BIF铁矿地质特征及含黄铁矿条带BIF的成因探讨

新太古代清原绿岩带位于华北克拉通北缘,该绿岩带中发育独特的VMS和BIF(Algoma型)组合,其中下甸子铁矿是此类BIF的典型代表。下甸子BIF赋存于绿岩带南天门组下部,围岩及夹层为斜长角闪岩及少量石榴云母片岩。矿体夹层斜长角闪岩中锆石的SIMS年代学分析获得了2497.8±7.4Ma的变质年龄,而原位氧同位素分析显示变质锆石的δ18O值为5.3‰~6.2‰,与现代地幔基本一致,表明在变质过程中其锆石的氧同位素体系保持稳定。BIF矿石类型主要有硅酸盐型和含黄铁矿条带型两种,前者的矿物组合为石英、磁铁矿和铁阳起石,而后者的矿物组合为石英、磁铁矿、阳起石、黄铁矿和少量方解石。大部分矿石的Al_2O_3、TiO_2和HFSE(如,Zr、Hf、Th、U等)含量极低,说明其未受到碎屑物质混染,PAAS标准化后,两类矿石稀土元素显示与海水类似的特征,即La、Y的正异常和LREE相对于HREE的亏损;同时,显著的正Eu异常指示成矿过程中有海底高温热液组分的参与;此外,所有样品均无明显的Ce异常,表明其沉淀自还原的海水中。通过与华北地区其他Algoma型BIF对比发现,下甸子两类矿石均具有较高的CaO/(CaO+MgO)值以及接近球粒陨石的Y/Ho值,表明其可能沉淀环境与海底热液喷口较近,且热液组分(以高温热液为主,可能有少量低温热液)的贡献较大。相比于硅酸盐型矿石,含黄铁矿条带型矿石的HREE含量较低、Eu正异常和LREE含量偏高,这可能与其沉淀过程中海底的局部热液的脉动式活动有关,其中黄铁矿条带可能为热液喷流沉积成因。BIF围岩斜长角闪岩的地球化学特征分析显示,其原岩玄武质岩石的岩浆可能来自亏损地幔,但在上升过程中受到了少量陆壳物质的混染,结合前人对清原绿岩带表壳岩系和TTG的年代学及地球化学研究,推测下甸子BIF可能形成于晚太古代洋陆俯冲过程中的火山弧或弧后盆地环境中。     

山西五台金岗库矿床成矿作用研究

金岗库矿床位于华北克拉通中部造山带,具有典型的VMS与BIF共生特征。本文对金岗库矿床的地质与地球化学特征进行系统研究,探讨金岗库硫化物矿石与磁铁石英岩的共生特点与成矿动力学模式。研究表明,硫化物矿体受地层及岩性控制,多呈扁豆、层状—似层状赋存于五台绿岩带金岗库组的磁铁石英岩、斜长角闪岩、斜长片岩和云母石英片岩中。矿石中金属矿物组合为黄铁矿—黄铜矿—磁黄铁矿—磁铁矿,矿石主要呈半自形—他形粒状结构和块状、条带状构造,围岩蚀变为绿泥石化和绢云母化。斜长角闪岩的原岩恢复,表明斜长角闪岩的原岩为拉斑玄武岩,可能形成于岛弧环境。LA-ICP-MS锆石U-Pb定年显示变基性火山岩的原岩形成于2 500 Ma,代表了金岗库矿床的成矿年龄。变质流体体系的成分模式为H2O-NaCl-CO2-CH4±N2±H2,变质峰期为中高温(322℃~473℃)、低盐度(2.2%~6.74%)的热液流体,并叠加少量中高温(290℃~470℃)、高盐度(37.4%~55.79%)的岩浆热液流体;峰后阶段为中低温(225℃~302℃)、中低盐度(4.03%~11.81%)的热液流体。金岗库矿床赋存的磁铁石英岩和硫化物矿体紧密共生,具有相同的成矿时代、物质来源和变质变形历史。综合以上研究认为金岗库矿床的成因类型为海相火山喷流沉积—变质热液流体叠加改造型。     

A Textural and Mineralogical Study of the Shanzhuang Banded Iron Formation, Southeastern Margin of the North China Craton: Implications for the Overprint History of Hydrothermal Alteration and Supergenesis after Mineralization

The newly discovered Shanzhuang BIF is hosted in the Shancaoyu Formation of the Taishan Group within the Eastern Block, southeastern margin of the North China Craton. The ores can be subdivided into three types in terms of mineral assemblages, corresponding to three types (I, II, III). The element concentration of the type I magnetite is similar to that of the type II magnetite, while the type III magnetite is similar to that of the schist. In general, magnetite and hematite grains from the ores show high concentrations of Mn (1317, 1162 ppm), Co (787, 1023 ppm), Al (2224, 2435 ppm) and Ti (540, 300 ppm), Whereas magnetite is depleted in Si (420 ppm) and hematite enriched in Si (1690 ppm). Detailed petrographic and mineral chemical analysis of magnetite, hematite, amphibole/hornblende and pyroxene, reveals that almost all the minerals occur as subhedral-anhedral grains with pits and fractures, and the BIF is recrystallized to metamorphic assemblages of high amphibolite facies. Hornblende is highly enriched in Fe, Mg and Ca, but depleted in K and Na, mostly belonging to magnesiohornblende. In addition, the ratios of Mg/(Mg+Fe2+), Fe3+/(Fe3++Fe2+), Si/(Si+Ti+Al) and Al/Si are 0.48–0.64, 0.17–0.36, 0.79–0.88 and 0.14–0.27, respectively. It is suggested that hornblende is neither a typical magmatic origin nor a typical metamorphic. Pyroxene has the characteristics of high Ca and Fe, but low Ti and Al, with end-member components En, Wo and Fs in the ranges of 25.22–28.64 wt%, 43.71–46.40 wt% and 24.51–27.62 wt%, respectively, belonging to clinopyroxene, and mostly diopside, might be formed during the prograde metamorphism in the absence of H2O. The carbonate such as dolomite-ankerite series is probably a precursor mineral of the BIF deposit. Mass mineral chemical and structural characteristics indicate that the Shanzhuang iron deposit has been subjected to varying degrees of oxidized hydrothermal superimposed reformation, metamorphism, and supergenesis after mineralization, during which some elements have been migrated in some degree.