庐枞盆地龙桥铁矿床中菱铁矿的地质特征和成因意义

龙桥铁矿床是庐枞火山岩盆地中的一个大型的铁矿床,多年来对其矿床成因的认识存在较大的争论.文章在野外地质研究工作的基础上,通过对矿床中菱铁矿的岩矿分析鉴定和电子探针测试,确定了矿床纹层状矿石中的菱铁矿为沉积成因.通过对菱铁矿的产出特征分析,并结合龙桥铁矿床的部分地质地球化学研究成果,认为在该矿床形成过程中,早期沉积形成了纹层状的菱铁矿层,在燕山期的岩浆热事件中,部分沉积菱铁矿被交代形成了磁铁矿和具有残余骸晶结构等一系列矿石交代组构特征的矿物.纹层状矿石既具有沉积特征,也具有热液改造特征,证实了矿床的形成存在早期(三叠纪)的沉积成矿(菱铁矿)作用和晚期(燕山期)的热液成矿(磁铁矿)作用.菱铁矿的研究为进一步确定龙桥铁矿床的成因提供了新的佐证.     

安徽庐江龙桥铁矿层新资料及成矿作用多阶段演化模式

根据龙桥铁矿层位岩性，地球化学特征及其与南京地区中三叠世周冲村期含膏岩系的对比，结合含矿层位的展布与区域构造及火山岩盆地的构造分析。论证龙桥铁矿含矿层位是中三叠世东马鞍山期（周冲村期）萨布哈－泻湖沉积环境下，含铁沉积（菱铁矿，铁白云石，磁铁矿及赤铁）矿源层。经燕夺浆侵入的热变质和热液叠加构造，成矿作用经历多阶段演化。     

安徽新桥矿床矿相学与Fe同位素特征及其对矿床成因的制约

对安徽新桥矿床进行系统的野外地质调查和矿相学研究发现,层状矿体中的胶状黄铁矿交代矽卡岩磁铁矿矿体,为探讨层状硫化物矿床是早期沉积成因还是岩浆热液成因提供了新的地质约束。对铜陵矿集区内的新桥矿床层状菱铁矿矿体和凤凰山矽卡岩型矿体中的菱铁矿开展了Fe同位素组成的对比研究,结果显示：新桥矿床菱铁矿与典型低温热液脉型矿床和沉积铁矿中的菱铁矿在Fe同位素组成特征上有所不同,而与凤凰山矽卡岩型矿床中的菱铁矿更为接近;新桥矿床中胶状黄铁矿和菱铁矿相对于磁铁矿富集Fe的轻同位素,表明磁铁矿不是过去认为的由胶状黄铁矿和菱铁矿矿胚层经热液改造形成,而是与典型的岩浆热液有关。新桥矿区层状硫化物矿体和矽卡岩型矿体中,近岩体矽卡岩和最早形成的金属矿物磁铁矿比岩体更为富集Fe的轻同位素,而赋矿围岩比岩体更为富集Fe的重同位素。同时,不同矿化阶段形成的含铁矿物和不同空间位置的硫化物中的Fe同位素组成呈现出时空分带现象,Fe同位素组成的时空演化特征与流体出溶、流体演化非常一致,并且符合同位素分馏的基本理论,表明层状硫化物矿体和矽卡岩型矿体具有相同的成矿物质来源,为同一流体体系演化的产物。新桥矿区岩相学的研究结果和Fe同位素组成特征均表明,新桥层状硫化物矿床不是海西期喷流沉积成矿作用的产物,而是燕山期热液成矿作用的产物,为一个典型的热液成因矿床。     

Examples and genetic significance of the formation of iron oxides in the Nigerian banded iron-formations

Ore microscopic studies reveal two main parageneses in the banded iron-formations of Nigeria. In the low-grade metamorphic schist belts of northern Nigeria, a magnetitic paragenesis comprising magnetite, silicates (grunerite and garnet), and quartz is developed. Magnetite which sometimes contains carbonate inclusions is markedly martitized. In contrast, the higher-grade metamorphic terrains of central Nigeria exhibit a different paragenesis consisting of hematite (including specularite) and quartz. Here, minerals of the magnetitic paragenesis only occur as relics. The protolith of these banded iron-formation occurrences envisioned as carbonate-containing sediments, with high concentrations of Fe and Si, and lower contents of Ca, Mg, Al (and also Mn where they are associated with gondite) underwent both submarine weathering and metamorphic changes in their evolution. During submarine weathering, sheet silicates and porphyroblasts of Fe-Mn-(Mg-Ca)-carbonate solid solutions, were formed. At the outset of a regional metamorphic episode, grunerite, garnet and porphyroblastic magnetite were developed. Magnetite formed at the expense of carbonate and sheetsilicates but was later martitized under post-metamorphic conditions. In the course of a later heterogeneous tectono-metamorphic event, martitized magnetite was transformed as follows: under low-grade metamorphism, as observed in the northern Nigerian schist belts, recrystallization into coarse-grained martite occurred, while at the higher grades of metamorphism in central Nigeria, recrystallization into hematite and, ultimately, specularite, took place. This relationship between magnetite and hematite has also been observed in many other banded iron-formations from different parts of the world, thus underscoring its widespread significance. Magnetite crystallizes first at the expense of carbonate and silicate minerals and hematite is subsequently derived from it directly or generally through martitization. This metamorphic phenomenon contradicts the common assumption that magnetite and hematite in banded iron-formations are invariably the products of direct precipitation from solution, in response to changes in environmental Eh/pH or different (reducing/oxidizing) diagenetic alterations of precipitated ferric hydroxide.     

Le minerai de fer d'Hajigak (Afghanistan). Position stratigraphique,cadre géologique et type du gisement

Age and grade of metamorphism of the considerable iron deposit of Hajigak (Central Afghanistan) have been controversial until now. A recent field study on the eastern part of the district produced new data concerning the geological environment and paragenesis of the ore. The iron beds are situated in the Upper Kalu Formation (= Awband Suite) of lower Paleozoïc, may be Silurian to Lower Devonian age. Basic lavas and tuffs, converted into green-schists, always are present near the ore bodies; and straight relationship is evident between basic rocks and iron ore. We consider the succession of events at Hajigak to be as follows: — at the bottom of a Paleozoïc sedimentary basin, an andesitic submarine volcanism brought along tuffs, lavas, and sericitic and chloritic sediments of exhalative origin; in the same time, hematite and magnetite precipitated. — afterwards gabbro and dolerite sills were intruded; as a result, iron oxides were remobilized, filling fractures across gabbro boundaries, and replacing neighbouring dolomites. — at last, but before Upper Devonian, Lower Paleozoïc rocks of Hajigak and Turkman area were affected by two kinds of metamorphism: the first one, of green-schist facies, is contemporaneous with magmatic activity; the second one, sericitic dynamo-metamorphism, appeared when the folding took place, mainly in the eastern region (Khesh and Zerok areas). In conclusion, the Hajigak iron ore appears to be a submarine — exhalative deposit, synchronous with a Lower Paleozoïc basic volcanic activity (but non ophiolitic).     

Physicochemical formation conditions of banded iron formations and high-grade iron ores in the region of the Kursk Magnetic Anomaly: Evidence from isotopic data

The oxygen and carbon isotopic compositions of minerals from banded iron formations (BIFs) and high-grade ore in the region of the Kursk Magnetic Anomaly (KMA) were determined in order to estimate the temperature of regional metamorphism and the nature of rock-and ore-forming solutions. Magnetite and hematite of primary sedimentary or diagenetic origin have δ¹⁸O within the range from +2 to 6‰. During metamorphism, primary iron oxides, silicates, and carbonates were involved in thermal dissociation and other reactions to form magnetite with δ¹⁸O = +6 to +11‰. As follows from a low δ¹⁸O_av = ?3.5‰ of mushketovite (magnetite pseudomorphs after hematite) in high-grade ore, this mineral was formed as a product of hematite reduction by organic matter. The comparison of δ¹⁸O of iron oxides, siderite, and quartz from BIFs formed at different stages of the evolution of the Kursk protogeosyncline revealed specific sedimentation (diagenesis) conditions and metamorphism of the BIFs belonging to the Kursk and Oskol groups. BIF of the Oskol Group is distinguished by a high δ¹⁸O of magnetite compared to other Proterozoic BIFs. Martite ore differs from host BIF by a low δ¹⁸O = ?0.2 to ?5.9‰. This implies that oxygen from infiltration water was incorporated into the magnetite lattice during the martite formation. Surface water penetrated to a significant depth through tectonic faults and fractures.     

沉积铁矿形成过程中的生物作用

    沉积赤铁矿和层控黄铁矿、菱铁矿的成矿过程中都离不开生物的作用。不同情况下，其表现形式不尽相同。生物的活动形成赤铁矿受层石、毒球状黄铁矿以及有机质的还原作用由赤铁矿转变为菱铁矿。
    冀西北宣龙地区铁质叠层石和铁质核形石同心纹层中普遍含有微体古植物化石，属于低等蓝藻类，形态以丝状体为主，部分为羽状体，它们直接地参与了铁的成矿作用。
        

繁昌桃冲铁矿成因探讨

The problems of the formation conditions for stratiform skarns and the genesis of the Taochong iron deposits are dealt with in this paper. Following is a summary of this discussion: 1. Stratiform skarns in this area occur in carbonate rocks of the Upper and Middle Carboniferous Period and the lower part of the Permian Qixia Group. No outcropping or concealed igneous bodies have ever been found, let alone any indications of an igneous contact zone or a corresponding zonality from "dry" skarn to "wet" skarn. The mineral facies and zonation of the skarns depend predominantly on the properties of the initial host rocks, and the development of skarns seems to have had much to do with chemical potential of silicon in these host rocks. As a result of the reaction of iron-bearing carbonates with siliceous materials in the rocks, iron-bearing silicates were formed, which in turn were transformed by pneumato-hydrothermal processes of the later stage. The stratiform skarns of this area, therefore, probably fall into the category of stratabound skarns subjected to transformation of thermometamorphism. 2. The iron deposits bear undisputable stratabound characteristics. The positions of ore-bearing beds and the petrological features as well as the mineral associations all point to a sedimentary ore-forming process of iron-carbonate (siderite). The presumption of siderite ore source is supported by the following facts: (l) Remnants of sedimentary siderite which survived the metamorphism have recently been observed in magnetite ore from neighbouring Xinqiao mining area. Siderite can have as many as 12.07% Fe⁺⁺ and, after corrosion, shows oolitic texture. (2) The ore is mainly of calcite/ dolomite- magnetite type. Mineral associations are quite simple and sulfides are rarely seen. (3) A comparison of the analytical data suggests that the content of organic carbon in iron ore decreases due to oxidation caused by metamorphism but is still higher than that in magnetite of contact- metasomatic skarn. (4) The paleogeographic reconstruction shows that this area was once an ancient underwater uplift favorable for the precipitation of iron carbonates. After its formation, the siderate bed underwent thermodynamic metamorphism and was hence decomposed into magnetite, which was then subjected to the superimposed transformation by subsequent hydrothermal fluids, leading to the partial activation and migration of iron matter and thus the formation of such ore as hematite (specularite) at shallow depth of the Changlongshan mining area. In brief, this deposit has a complex genesis: it experienced sedimentation, thermal metamorphism and transformation by hydrothermal fluids.     

内蒙白云鄂博铁矿床中磁铁矿和赤铁矿的氧同位素组成

白云鄂博矿床分布在内蒙地轴北部边缘的过渡带。含矿岩系为元古代海相沉积碳酸盐、碎屑岩建造,主要由石英岩、白云岩和板岩组成,其中白云岩是矿体围岩。 矿床受东西向向斜构造控制。向斜以北为一大背斜构造,沿轴部被断层破坏,出露有古老的片麻岩和片岩。向斜以南的背斜构造轴部有海西期黑云母花岗岩侵入,使背斜构造轴部遭受破坏。     

繁昌桃冲铁矿成因探讨

桃冲铁矿,开采历史悠久,由于其品位富,以平炉富矿为主要矿石类型,受到了重视。有关矿床的成因也一直被人们所注意。自三十年代提出火成接触变质——热液成因的观点以来(谢家荣、程裕淇1935),人们习惯于将矿床划归于矽卡岩型。作者通过野外调查和初步研究之后,对本区铁矿的成因产生了疑问。本文就现有资料的分析,对层状矽卡岩的形成条件和铁矿的成因,做如下讨论。     

滇西北衙多金属矿田矿床成因类型及其与富碱斑岩关系初探

北衙金多金属矿田是与金沙江-哀牢山新生代富碱斑岩有关的成矿作用的典型代表之一,近年来在矿产勘查方面又有重大突破,金已达到超大型矿床,伴生铁、铜、银、铅、锌也达到了大-中型矿床规模。本文基于野外观察与室内研究,结合前人研究成果,对北衙多金属矿的成因类型,富碱斑岩与成矿作用的关系及成矿机制进行了系统总结,对与成矿相关的富碱斑岩进行了主量元素及锆石LA-ICP-MS的测试,探讨了铁矿的成因。研究表明,矿田原生金属矿床可分为:斑岩型铜金矿化,夕卡岩型铁、金、铜、铅、锌矿化,爆破角砾岩筒中的铁、金、铅、锌矿化以及热液型金、银、铅、锌矿化。其中夕卡岩型和热液型矿床是该区最主要的成矿类型。新生代富碱斑岩(石英正长斑岩)的年龄分别34.92±0.66Ma和36.24±0.63Ma。属于钾质碱性岩系列。它不仅为含矿流体的上升提供了动力和热能,而且还是成矿物质和成矿流体的主要来源,因此形成以斑岩体为中心,由斑岩型、夕卡岩型、热液型等矿床构成的一个连续的成矿系统。钾质碱性岩及矿床是在碰撞造山走滑构造系统深部壳幔相互作用的产物。本区岩体接触带中发育大量由菱铁矿和磁铁矿组成的铁矿体,其中大部分的磁铁矿是一种具有赤铁矿的板状晶或聚片双晶假象的穆磁铁矿。对磁铁矿和菱铁矿形成条件的分析表明,磁铁矿和菱铁矿主要是在碱性环境下交代含铁夕卡岩矿物形成的。当热液中H+的浓度降低时,赤铁矿被还原为磁铁矿,但仍保留了赤铁矿的晶形,于是成为穆磁铁矿。由此推测,本区成矿作用是在成矿流体及夕卡岩化交代作用长时间反复持续进行的条件下发生的,这可能是本区得以形成巨量金属堆积的重要原因之一。     

BIF-hosted iron mineral system: A review

The BIF-hosted iron ore system represents the world's largest and highest grade iron ore districts and deposits. BIF, the precursor to low- and high-grade BIF hosted iron ore, consists of Archean and Paleoproterozoic Algoma-type BIF (e.g., Serra Norte iron ore district in the Carajás Mineral Province), Proterozoic Lake Superior-type BIF (e.g., deposits in the Hamersley Province and craton), and Neoproterozoic Rapitan-type BIF (e.g., the Urucum iron ore district).The BIF-hosted iron ore system is structurally controlled, mostly via km-scale normal and strike-slips fault systems, which allow large volumes of ascending and descending hydrothermal fluids to circulate during Archean or Proterozoic deformation or early extensional events. Structures are also (passively) accessed via downward flowing supergene fluids during Cenozoic times.At the depositional site the transformation of BIF to low- and high-grade iron ore is controlled by: (1) structural permeability, (2) hypogene alteration caused by ascending deep fluids (largely magmatic or basinal brines), and descending ancient meteoric water, and (3) supergene enrichment via weathering processes. Hematite- and magnetite-based iron ores include a combination of microplaty hematite–martite, microplaty hematite with little or no goethite, martite–goethite, granoblastic hematite, specular hematite and magnetite, magnetite–martite, magnetite-specular hematite and magnetite–amphibole, respectively. Goethite ores with variable amounts of hematite and magnetite are mainly encountered in the weathering zone.In most large deposits, three major hypogene and one supergene ore stages are observed: (1) silica leaching and formation of magnetite and locally carbonate, (2) oxidation of magnetite to hematite (martitisation), further dissolution of quartz and formation of carbonate, (3) further martitisation, replacement of Fe silicates by hematite, new microplaty hematite and specular hematite formation and dissolution of carbonates, and (4) replacement of magnetite and any remaining carbonate by goethite and magnetite and formation of fibrous quartz and clay minerals.Hypogene alteration of BIF and surrounding country rocks is characterised by: (1) changes in the oxide mineralogy and textures, (2) development of distinct vertical and lateral distal, intermediate and proximal alteration zones defined by distinct oxide–silicate–carbonate assemblages, and (3) mass negative reactions such as de-silicification and de-carbonatisation, which significantly increase the porosity of high-grade iron ore, or lead to volume reduction by textural collapse or layer-compaction. Supergene alteration, up to depths of 200 m, is characterised by leaching of hypogene silica and carbonates, and dissolution precipitation of the iron oxyhydroxides.Carbonates in ore stages 2 and 3 are sourced from external fluids with respect to BIF. In the case of basin-related deposits, carbon is interpreted to be derived from deposits underlying carbonate sequences, whereas in the case of greenstone belt deposits carbonate is interpreted to be of magmatic origin. There is only limited mass balance analyses conducted, but those provide evidence for variable mobilization of Fe and depletion of SiO₂. In the high-grade ore zone a volume reduction of up to 25% is observed.Mass balance calculations for proximal alteration zones in mafic wall rocks relative to least altered examples at Beebyn display enrichment in LOI, F, MgO, Ni, Fe₂O_3total, C, Zn, Cr and P₂O₅ and depletions of CaO, S, K₂O, Rb, Ba, Sr and Na₂O. The Y/Ho and Sm/Yb ratios of mineralised BIF at Windarling and Koolyanobbing reflect distinct carbonate generations derived from substantial fluid–rock reactions between hydrothermal fluids and igneous country rocks, and a chemical carbonate-inheritance preserved in supergene goethite.Hypogene and supergene fluids are paramount for the formation of high-grade BIF-hosted iron ore because of the enormous amount of: (1) warm (100–200 °C) silica-undersaturated alkaline fluids necessary to dissolve quartz in BIF, (2) oxidized fluids that cause the oxidation of magnetite to hematite, (3) weakly acid (with moderate CO₂ content) to alkaline fluids that are necessary to form widespread metasomatic carbonate, (4) carbonate-undersaturated fluids that dissolve the diagenetic and metasomatic carbonates, and (5) oxidized fluids to form hematite species in the hypogene- and supergene-enriched zone and hydroxides in the supergene zone.Four discrete end-member models for Archean and Proterozoic hypogene and supergene-only BIF hosted iron ore are proposed: (1) granite–greenstone belt hosted, strike-slip fault zone controlled Carajás-type model, sourced by early magmatic (± metamorphic) fluids and ancient “warm” meteoric water; (2) sedimentary basin, normal fault zone controlled Hamersley-type model, sourced by early basinal (± evaporitic) brines and ancient “warm” meteoric water. A variation of the latter is the metamorphosed basin model, where BIF (ore) is significantly metamorphosed and deformed during distinct orogenic events (e.g., deposits in the Quadrilátero Ferrífero and Simandou Range). It is during the orogenic event that the upgrade of BIF to medium- and high-grade hypogene iron took place; (3) sedimentary basin hosted, early graben structure controlled Urucum-type model, where glaciomarine BIF and subsequent diagenesis to very low-grade metamorphism is responsible for variable gangue leaching and hematite mineralisation. All of these hypogene iron ore models do not preclude a stage of supergene modification, including iron hydroxide mineralisation, phosphorous, and additional gangue leaching during substantial weathering in ancient or Recent times; and (4) supergene enriched BIF Capanema-type model, which comprises goethitic iron ore deposits with no evidence for deep hypogene roots. A variation of this model is ancient supergene iron ores of the Sishen-type, where blocks of BIF slumped into underlying karstic carbonate units and subsequently experienced Fe upgrade during deep lateritic weathering.     

基于航空高光谱遥感的沉积变质型铁矿找矿预测—以北祁连镜铁山地区为例

沉积变质型铁矿是北祁连山西段重要的矿床类型,一直以来都是该地区矿产勘查的重要对象,区内产有知名的镜铁山铁矿床。由于北祁连山地区山势陡峻、交通不便,地质工作程度相对较低,开展传统的地质调查工作难度较大。航空高光谱遥感具有高空间分辨率和高光谱分辨率特点,其在矿物信息识别上较多光谱遥感有了质的提升,同时在该地区开展航空高光谱遥感调查是发挥遥感技术的高效性和先行性优势所在。本文基于调查区内沉积变质型铁矿床地质特征,利用航空高光谱遥感数据提取的赤铁矿、菱铁矿信息开展针对沉积变质型铁矿找矿预测。结果表明,提取的赤铁矿信息或直接指示铁（化）体产出位置,或指示含赤铁矿地层产出信息,这为直接或间接寻找铁矿床提供了重要信息;菱铁矿信息同样可作为寻找铁矿床的重要依据,分布范围较大的菱铁矿信息可直接指示富菱铁矿的铁（化）体产出位置。     

吉林省小栗子铁矿地质特征及成因探讨

小栗子铁矿是矿体赋存一定层位;矿体与围岩整合接触;围岩蚀变微弱或无蚀变;组成矿石矿物的成分简单;铁矿石是在胶体溶液中化学沉积形成;铁矿石是由菱铁矿、赤铁矿受后期变质、迭加改造作用的中元古时期形成的沉积变质矿床,即＂大栗子式铁矿＂。     

Gold Mineralization in Banded Iron Formation in the Amalia Greenstone Belt,South Africa: A Mineralogical and Sulfur Isotope Study

The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO₂ in a H₂S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe²⁺ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe³⁺ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO₂ conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO₂ during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ³⁴S_ΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.     

The beach placer iron deposit of Portman Bay,Murcia, SE Spain: the result of 33 years of tailings disposal (1957–1990) to the Mediterranean seaside

Between 1957 and 1990, the Peñarroya Mining and Metallurgical Company (SMMPE) disposed about 60 million tonnes of tailings materials directly to the Mediterranean Sea. A substantial part of it (12.5 Mt) was dragged back by the sea currents progressively infilling the Portman Bay (Murcia, SE Spain), thus making the shoreline advance between 500 and 600 m seaward. The Roberto froth flotation plant processed mineral from manto-type deposits belonging to the Sierra de Cartagena-La Unión lead-zinc district. One of the mineral assemblages present in these deposits comprises greenalite, magnetite, sulfides, carbonates, and silica. Despite that magnetite recovery was undertaken by SMMPE between 1959 and 1967, we estimate that magnetite contained in the tailings hosts a substantial resource that could be as large as 2.3 Mt of iron ore. The ore contains magnetite ± hematite ± siderite. Tidal waves and sea currents led to gravimetric classification of the tailings material, with concentration of the dense iron oxides in the sandy fractions, eventually forming a coastal placer iron deposit. A major problem for magnetic separation is the intimate intergrowth between magnetite, hematite, and siderite. Besides, the sands contain large concentrations of Pb (0.27 %), Zn (0.72 %), and As (559 ppm).     

甘肃北山地区红山铁矿床成因及成矿机制

甘肃北山地区红山铁矿床为赋存于元古界青白口系顶部的沉积变质型铁矿床,其铁矿石主要类型为磁铁矿,少量为赤铁矿、镜铁矿。矿石储量可达亿吨,经野外调研和室内综合研究,本矿床形成于裂陷海盆中,与多处含铁的热卤水喷流柱上涌有关,属于"Sedex"型铁矿类型。     

Primary Mineral Information and Depositional Models of Relevant Mineral Facies of the Early Precambrian BIF—A Preliminary Review

The primary mineral compositions of BIF are regarded as ferric oxyhydroxide or iron silicate nanoparticles (mainly greenalite and stilpnomelane ) whichcan transform into minerals like hematite, magnetite and siderite. On the basis of predominant iron minerals, three distinctive sedimentary facies are recognized in BIF: oxide facies, silicate facies and carbonate facies. Marked by the Great Oxidation Event (GOE, 2.4~2.2 Ga), sedimentary facies can be divided into two models: “anoxic and reducing” model and “stratified ocean” model. The ancient ocean was anoxic and reducing before GOE, and under this circumstance, BIF was distributed from the distal to proximal zones transforming from hematite facies through magnetite facies to carbonate facies, such as West Rand Group BIF (2.96~2.78 Ga) and Kuruman BIF (~2.46 Ga) in south Africa. However, the ancient ocean was a stratified ocean during and after GOE, which means that shallow seawater was oxidizing while deeper seawater was reducing, leading to an opposite sedimentary facies distribution compared to the former one: BIF was distributed from the distal to proximal zones transforming from carbonate facies through magnetite facies to hematite facies, such as Yuanjiacun BIF in China (~2.3 Ga) and Sokoman iron formation in Canada (~1.88 Ga). Overall, BIF is an unrepeatable formation in geological history, which can only form in specific sedimentary environment. The key point to speculate the paleo-ocean environment, namely the problems to be solved at the moment, is to identify and derive the primary mineral compositions, to make sure the genetic mechanism of sedimentary facies especially silicate facies, to restrict the sedimentary conditions and to study microbial activities contacting with BIF.     

Carbonatitic Volcanic Genesis of Hetaoqing Fe-Cu Deposit in Central Yunnan, China

Abstract. The Kunyang rift on western margin of Yangtze Platform is a continental rift, and also a rare Precambrian Fe-Cu mineralization zone in China. The Wuding-Lufeng mineralization area in the middle section of the rift is an important part of the zone, and an elliptic-shaped volcanic collapsed basin, controlled by a ring fracture system with carbonatitic volcanic rocks mainly occurring along the northwestern edge of the basin. The Hetaoqing Fe-Cu ore deposit at the western side of the basin is hosted in carbonatitic volcanic rocks and pyroclastic sedimentary rocks. The original ore bodies occur as layers, bands and lenses conformable to the host carbonatitic rocks. The ores usually appear as massive, impregnated and granular in carbonatitic rocks, and as brecciform and sandy in pyroclastic sedimentary rocks. Ore-forming minerals are magnetite, hematite, chalcopy-rite, bornite, pyrite, carrollite, molybdenite, cobaltite and skinnerite, and secondary minerals limonite, chalcocite, azurite, malachite and tenorite. Gangue minerals are calcite, dolomite, ankerite, common hornblende, arfvedsonite, augite, aegirine-augite, albite, phlogopite, biotite, chlorite and apatite. Evidences of mineral chemistry, trace elements and isotopic ratios of ores, as well as geological features, suggest that the original ores are igneous in origin. Chemical features of magnetites in the deposit belong to carbonatite type, and are similar to those from the Bayan Obo carbonatites. The ores are rich in iron, titanium, rare earth elements, niobium, tantalum, gold, silver, phosphor and sulfur. These features indicate that the Fe-Cu deposit associated with volcanic activity in the Wuding-Lufeng basin is alkali-carbonatite volcanic type.     

沉积矿床成因亚类的划分及沉积菱铁矿的成因

沉积矿床的形成过程,可明确地划分为同生作用、成岩作用与后生作用等三个矿化阶段。现在所见到的沉积矿床,主要是最后矿化阶段改造的结果。根据沉积矿床所经历矿化阶段的不同和表现强度的不同,作者将沉积矿床划分为四种成因亚类:(1)沉积同生矿床;(2)沉积成岩矿床;(3)沉积成岩-后生矿床;(4)沉积后生矿床。产于沉积岩和沉积矿床中的青灰色微细均粒菱铁矿系成岩作用形成,米黄色或淡黄色的粗至巨粒的成分较纯的菱铁矿系后生作用形成。它们都不是同生作用形成的。