Geology, geochemistry and sulfur isotope composition of the Late Proterozoic Jingtieshan (Superior-type) hematite-jasper-barite iron ore deposits associated with stratabound Cu mineralization in the Gansu Province, China

The Jingtieshan deposit occurs in a Precambrian tectonic-stratigraphic terrane within the Northern Qilian Caledonian Orogen, and is generally considered as a Superior-type iron formation. The deposit is characterized by Fe-Si-Ba and Cu mineralization and consists of two types of orebodies, an upper jasper-barite-iron deposit and a lower copper sulfide deposit. The iron orebodies occur as independent stratigraphic layers concordant within a thick argillaceous succession, and exhibit fine-grained textures and well-developed sedimentary layering. The ores are predominantly composed of specularite and jasper with lesser amounts of magnetite, hematite, siderite, and barite. The presence of barite, hematite and jasper as major components shows that the iron ores were precipitated in a relatively oxidized ocean floor environment. The Cu orebody directly underlies the iron ore and is hosted by chlorite-sericite-quartz phyllite. The Cu mineralization is composed of pyrite and chalcopyrite and is characterized by stockwork. The disseminated and stockwork Cu mineralization is metamorphosed and concordant with respect to foliation, indicating pre-fabric development, i.e. pre-metamorphism, and was probably originally formed by reduced fluids reacting at the base of and within the oxide iron formation. Geochemical data show that the jasper-barite-iron ores, which resemble Superior-type iron formations, have a high input of hydrothermal-hydrogeneous elements (SiO₂, av.=56%; Fe₂O_3t, av.=30%; Mn, av.=0.45%; BaO, av.=16.7%) with minimal terrigeneous input (<15% combined Al₂O₃, TiO₂, K₂O, MgO, etc.). The δ³⁴S of exhalative barite varies from 28 to 34‰, which is very heavy with respect to other Late Proterozoic sulfate-bearing deposits, except those of circa 600 Ma in which the sulfides range from 8 to 20‰. The sulfur isotope data indicate that the barite was formed by the mixing of a Ba-rich hydrothermal fluid with sulfate-rich ambient seawater and that the sulfides ores were most probably derived from the reduction of seawater sulfate during subsurface reaction with ferrous iron-bearing minerals. These data are consistent with the jasper-barite-iron deposit forming by hydrothermal exhalative and chemical sedimentary processes on the floor of an ocean basin, and with the Cu mineralization forming by hydrothermal filling and replacement in base of and within the iron formation. Received: 19 March 1997 / Accepted: 14 May 1998     

Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO₂ and SiO₂ from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness. Received: 28 May 1996 / Accepted: 22 January 1997     

辽宁弓长岭铁矿床二矿区类矽卡岩的岩石矿物学特征

辽宁弓长岭铁矿床二矿区是我国最重要的鞍山式沉积变质型富铁矿床.不同于鞍山-本溪地区其他贫铁矿床,弓长岭铁矿二矿区富铁矿体的附近分布有大量的类矽卡岩,这些类矽卡岩与富铁矿体具有密切的成因联系.本文在野外和岩相学研究的基础上,选择弓长岭二矿区类矽卡岩的岩相学、矿物学、矿物化学特征进行了研究.结果表明:类矽卡岩可分为石榴石岩、绿泥石岩、含石榴石绿泥石岩、含磁铁矿阳起石岩四种类型;类矽卡岩矿物中石榴石端员组分以铁铝榴石为主,角闪石属于钙角闪石系列中的透闪石,绿泥石属于蠕绿泥石.类矽卡岩和富铁矿是由热液交代改造磁铁贫矿形成的,二者是同一期热液活动的产物.     

Platinum-group elements in Cu-sulphide ores at Carolusberg and East Okiep, Namaqualand, South Africa

Cu-sulphide ores at Carolusberg and East Okiep have Cu/Ni ratios of up to 80, an order of magnitude higher than most magmatic sulphide ores elsewhere. In contrast, Se/S ratios (500–1700 × 10⁻⁶) and PGE tenors (up to 5 ppm) of the sulphides are in the range of more typical magmatic sulphide ores. The observed metal patterns may be explained by a process of monosulphide solid solution (mss) fractionation of a magmatic sulphide melt at depth, but this model is currently considered unlikely, due to the paucity of refractory ores in the district. Assimilation of Cu-rich country rocks during ascent of the Koperberg magmas proved difficult to test with the available data, but this provides no explanation for the common high-grade metamorphic setting of similar ores elsewhere. A restitic origin of the pyroxenites appears to explain many of the observed ore features and is presently favoured here. Desulphidization of a primary magmatic sulphide ore could not have yielded the observed metal patterns and is therefore considered to be of relatively minor importance in ore genesis. Received: 12 April 1999 / Accepted: 27 November 1999     

Geology and geochemistry of the Águas Claras and Pico Iron Mines, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Águas Claras and Pico Mines are two world-class iron-ore mines hosted within the Lower- Proterozoic banded iron-formations (locally known as itabirites) of the Minas Supergroup located in the Quadrilátero Ferrífero district, Minas Gerais, Brazil. The Águas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Cauê Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Águas Claras orebody, representing about 85% of the 284 Mt mined since 1973, with the remaining 15% comprising hard high-grade ore. Hematite is the main constituent of the iron ores. It occurs as martite, granular hematite and locally as specularite. Magnetite appears subordinately as relicts within martite and hematite crystals. Gangue minerals are very rare. These consist of dolomite, chlorite, talc, and apatite, and are especially common in contact with the protore. This virtual absence of gangue minerals is reflected in the chemistry of ores that are characterized by very high Fe contents (an average of 68.2% Fe).The Pico orebody is a continuous ~3,000-m-long body of a lenticular shape hosted within siliceous itabirite, which is the protore of the soft high- and low-grade ores at the Pico Mine. The soft high-grade ores, together with the low-grade ores, called iron-rich itabirite, are the main types of ore, and respectively represent approximately 51 and 29% of the reserves. The remaining 20% consists of hard high-grade ore. The iron oxide mineralogy is the same as that of the Águas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but are slightly more common in the iron-rich itabirite. Quartz is the dominant gangue mineral, and is found with minor quantities of chlorite. The chemistry of the high-grade ores is characterized by high Fe contents (an average of 67.0%) and low P, Al₂O₃, and SiO₂, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO₂.The genesis of the soft high-grade ores and iron-rich itabirites is related to supergene processes. Leaching of the gangue minerals by groundwater promoted the residual iron enrichment of the itabirites. This process was favored by the tropical climate and topographic situation. The original composition of the itabirites and the presence of structures controlling the circulation of the groundwater have influenced the degree of iron enrichment. The hard high-grade ores are of a hypogene origin. Their genesis is attributed to hydrothermal solutions that leached the gangue minerals and filled the spaces with hematite. This process remains a source of debate and is not yet fully understood.Editorial handling: S.G. Hagemann     

Fluid types and their genetic meaning for the BIF-hosted iron ores,Krivoy Rog,Ukraine

This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske–Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, microthermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52–56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60–70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed.The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO₂-rich, H₂O, H₂O–CO₂(± N₂–CH₄)–NaCl(± NaHCO₃) and H₂O–CO₂(± N₂–CH₄)–NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO₃) remains unresolved. The minimum P–T conditions for the formation of these veins, inferred from microthermometry are T_min = 219–246 °C and P_min = 130–158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske–Magnetitove deposit was metamorphosed at T = ~ 530 °C.The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1^st episode three types of fluids were introduced: CO₂–CH₄–N₂(± C), CO₂(± N₂–CH₄) and low salinity H₂O–N₂–CH₄–NaCl (6.38–7.1 mass% NaCl equiv.). The 2^nd episode included expulsion of the aqueous fluids H₂O–N₂–CH₄–NaCl(± CO₂, ± C) of moderate salinities (15.22–16.76 mass% NaCl equiv.), whereas the 3^rd event involved high salinity fluids H₂O–NaCl(± C) (20–35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH₄ and N₂. The high salinity fluids were most likely either magmatic–hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = ~ 430–500 °C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic–hydrothermal fluids or heated basinal brines.     

论中生代岩浆活动对海南石碌富铁矿床的改造作用

石铁矿位于海南省昌江县境内,是中国最大的富铁矿床.该矿区及其周边中生代侵入岩广泛发育,岩浆活动对矿床影响强烈,其结果主要产生两类改造型矿石,即石榴子石磁铁矿矿石和黄铁矿磁铁矿赤铁矿矿石.通过对改造型矿石的矿相学研究、矿石及矿物的硫同位素和微量元素分析,表明改造型矿石中的赤铁矿发生了磁铁矿化,其中的硫主要来源于岩浆,而铁...     

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.     

辽宁鞍本地区铁质活化再富集成因富铁矿的成矿时代——齐大山铁矿床辉钼矿Re-Os年龄证据

中国与早前寒武纪条带状铁建造有关的磁铁富矿集中分布在辽宁鞍本地区,主要由条带状铁建造经过后期热液改造而成,有去硅富铁和铁质活化再富集2种成因,前者以弓长岭铁矿床二矿区的富铁矿为代表,富铁矿的成矿时代为1.84 Ga左右;后者以齐大山铁矿床樱桃园矿区的富铁矿(樱桃园富铁矿)为代表,但是该富铁矿的成矿时代还不清楚。为了探讨铁质活化再富集型富铁矿的成矿时代,笔者对齐大山铁矿区的辉钼矿进行Re-Os同位素测年。该矿区的辉钼矿有3种产出方式:第一种产于花岗伟晶岩中,呈巨晶辉钼矿集合体;第二种为蚀变岩中石英透镜体边部薄膜状辉钼矿;第三种产于混合花岗岩中的石英脉中,呈浸染状产出。第一种辉钼矿的年龄(2503±33)Ma~(2538±36)Ma,代表了条带状铁建造铁质活化再富集形成富铁矿的主要时期,形成于2.5 Ga左右的华北克拉通发生岩浆、变质作用与克拉通化时期,钼来自地壳,佐证了新太古代末华北克拉通的第一次克拉通化主要是壳内物质的重组;第二种辉钼矿的年龄为(2088±28)Ma,其成矿物质来自地壳,佐证了华北克拉通2.3~1.95 Ga的裂谷-俯冲-增生-碰撞的陆内造山事件也主要是壳内物质的重组;第三种辉钼矿的年龄为(1834±28)Ma~(1853±29)Ma,与弓长岭二矿区"去硅富铁"型富铁矿的成矿时代一致,其成矿物质来自地壳,但混有地幔组分,佐证了1.85~1.65 Ga的华北克拉通基底抬升、镁铁质岩墙群侵入、裂陷槽和裂谷形成有地幔物质的参与。     

Mineralogy and trace-element geochemistry of the high-grade iron ores of the Águas Claras Mine and comparison with the Capão Xavier and Tamanduá iron ore deposits, Quadrilátero Ferrífero, Brazil

Several major iron deposits occur in the Quadrilátero Ferrífero (QF), southeastern region of Brazil, where metamorphosed and heterogeneously deformed banded iron formation (BIF) of the Cauê Formation, regionally called itabirite, was transformed into high- (Fe >64%) and low-grade (30%?2O₃, with a higher amount of detrimental impurities, especially MnO, in the soft ore. Both hard and soft ores are depleted in trace elements. The high-grade ores at the Águas Claras Mine have at least a dual origin, involving hypogene and supergene processes. The occurrence of the hard, massive high-grade ore within “fresh” dolomitic itabirite is evidence of its hypogene origin. Despite the contention about the origin of the dolomitic itabirite (if this rock is a carbonate-rich facies of the Cauê Formation or a hematite–carbonate precursor of the soft high-grade ore), mineralogical and geochemical features of the soft high-grade ore indicate that it was formed by leaching of dolomite from the dolomitic itabirite by meteoric water. The comparison of the Águas Claras, Capão Xavier and Tamanduá orebodies shows that the original composition of the itabiritic protore plays a major role in the genesis of high- and low-grade soft ores in the QF. Under the same weathering and structural conditions, the dolomitic itabirite is the more favorable to form high-grade deposits than siliceous itabirite. Field relations at the Águas Claras and Capão Xavier deposits suggest that it is not possible to form huge soft high-grade supergene deposits from siliceous itabirite, unless another control, such as impermeable barriers, had played an important role. The occurrence in the Tamanduá Mine of a large, soft, high-grade orebody formed from siliceous itabirite and closely associated with hypogene hard ore suggests that large, soft, high-grade orebodies of the Quadrilátero Ferrífero, which occur within siliceous itabirite, have a hypogene contribution in their formation.     

华北克拉通条带状铁建造中富铁矿成因类型的研究进展、远景和存在的科学问题

本文在查阅前人大量资料的基础上,对华北克拉通条带状铁建造中富铁矿的研究历史进行了回顾和总结,将研究历史分为1949年以前,1950~1965年期间,1978~1986年期间,1987~1994年期间和2009年以来5个阶段。重点介绍了鞍本地区、冀东-吕梁地区和河南舞阳地区富铁矿的基本地质特征以及典型富铁矿的研究概况,针对鞍本地区弓长岭二矿区磁铁富矿成因的复杂性,对不同成因观点以及目前已取得的共识进行了详细阐述。目前大多数学者不支持接触交代假说和菱铁矿经变质转化为富铁矿成矿假说,近半数学者支持变质热液成矿假说,半数学者支持混合岩化热液成矿假说。作者在综合分析前人大量资料后,认为变质热液成矿说依据不足,理由有四点:(1)磁铁富矿中往往见有磁铁贫矿的残体;(2)磁铁富矿与蚀变岩紧密伴生,蚀变矿物石榴子石、部分角闪石(透闪石)和部分绿泥石均属非变质热液成因;(3)研究区遭受区域高绿片岩相至低角闪岩相变质作用的时间为2500~2450Ma,而与蚀变矿物石榴石紧密伴生的热液锆石SHRIMP U-Pb定年结果为1840±7Ma,明显小于区域变质作用年龄,据此可将热液作用时间限定于古元古代晚期,相当于大陆地壳伸展阶段;(4)部分热液成因富铁矿利用Re-Os方法定年,除一种属原生沉积成矿外,年龄范围也在古元古代晚期,可作为参考。此种热液是否为混合岩化热液尚缺乏足够证据,故本文暂将其作为古元古代晚期热液。此外,本文对华北克拉通条带状铁建造中富铁矿成因类型及其远景进行了初步总结,认为古元古代晚期形成的磁铁富矿规模属大型矿床,有较好远景;原生较富贫铁矿因褶皱构造产生磁铁矿流变而形成的富铁矿(可能尚有热液叠加)规模较大,具有一定远景;其他类型均为小型规模,不具工业意义。最后,本文指出富铁矿成因研究中尚存在的主要问题,包括早元古代晚期热液的来源;热液的形成是一期还是多期;铁建造遭受区域变质达高绿片岩相时,贫铁矿的围岩变质演化机理等,尚需进一步探讨。     

宁乡式沉积铁矿床的时空分布和演化

宁张式铁矿是我国最重要的沉积型铁矿床,广泛分布于我国南部的鄂、湘、赣、川、滇、黔、桂诸省以及甘南地区。含矿建造主要赋存于中上泥盆统,而以上泥盆统为主,可划分出７个成矿区,其中最重要的鄂西－湘西北成矿区。在１个矿床中通常有１～４层铁矿,其中有１个是主矿层,矿石主要由鲕状赤铁矿组成,次有菱铁矿、鲕绿泥石和褐铁矿,含铁品位一般为３０％～４５％,含磷通过偏高,介于０．４％～１．１％,中泥盆世和晚泥盆世沉积铁矿在分布范围、矿床规模、赋太和围岩建造和矿石特征等方面有一定差异,文章对鄂西－湘西北成矿区的矿宋时空演化作了重点论述,对铁矿的分布与岩相古地理的关系及矿床生成条件进行了讨论,指出含矿建造大多产于海侵程序的沉积岩系中,在湿热环境下较封闭或半封闭的古海盆,古海湾或潮坪中的浅海－滨海相沉积组合是有利的成矿古地理条件,提出要     

西昆仑赞坎铁矿床地质特征、形成时代及高品位矿石的成因

新疆赞坎铁矿床位于西昆仑塔什库尔干地块西段,是近年新发现的一个大型沉积变质型磁铁矿床。赋矿岩系布伦阔勒群主要由黑云母石英片岩、斜长角闪片岩、变粒岩、硅质岩及磁铁石英岩等组成。目前探明工业矿体4条,单个矿体长度大于2.5km,矿体厚10~70m;局部见高品位铁矿段(mFe50%),长度达900m,厚度40m左右。矿石类型主要为2种,一种为原生的条纹-条带状磁铁矿(为主);另一种为热液改造形成的块状(高品位铁矿石)及浸染状磁铁矿。矿石稀土元素配分(PAAS)表明,原生条纹-条带状铁矿石Ce和Y元素异常不明显(~1.15、~0.94),Eu具正异常(~1.69),Y/Ho平均值为25,稀土配分模式与沉积变质型铁矿相似。而受改造的矿石中,浸染状矿石具有较高的稀土总量,明显富集轻稀土,La和Ce显示正异常(~1.46、~1.17),Y显示负异常(=0.66~0.72),Eu表现为强烈的正异常(~4.37),稀土配分模式明显不同于原生条纹-条带状铁矿石。矿体围岩斜长角闪片岩(变沉积岩)中的碎屑锆石U-Pb年龄为591±1Ma,结合前人对矿区内侵入体的年代学研究(霏细斑岩,533Ma),大致反映沉积铁矿的形成时代为新元古代至早寒武世。电子探针显示,条带状磁铁矿中的TiO_2、AL_2O_3、MgO、MnO含量较低,标型组分含量与沉积变质型磁铁矿颇为接近,在磁铁矿单矿物成因图解中,条带状磁铁矿整体显示磁铁矿为沉积变质型铁矿;浸染状矿石和块状矿石的组成与典型沉积变质型铁矿的偏离反映了后期岩浆-构造热事件对条带状铁矿石的改造;上述结果显示赞坎铁矿整体属于沉积变质型铁矿(BIF)。调查发现赞坎高品位铁矿体与早寒武世侵入的霏细斑岩联系密切,高品位矿石及其围岩发育一定程度的矽卡岩化,如阳起石化、碳酸盐化和黄铁矿化。本文推测高品位铁矿石的成因可能为霏细斑岩的岩浆热液溶解并运移早期沉积变质铁矿中的含铁物质,在构造发育处充填交代形成块状磁铁富矿石。在早寒武世侵入到矿区中部的霏细斑岩体中,同时发育有角砾状磁铁矿和脉状磁铁矿,因此,岩浆热液改造原生条带状铁矿石形成高品位铁矿石的时代应为早寒武世。     

Chemical composition of tourmaline from the Yunlong tin deposit, Yunnan, China: implications for ore genesis and mineral exploration

Summary ?The Yunlong tin deposit is located in the northern part of the Lancangjiang metamorphic zone of the Sanjiang Tethys orogen series in western Yunan province of China. It consists of vein-type cassiterite ores, which are mainly hosted in migmatites of Caledonian age. Abundant tourmaline is associated with the ores, quartz–tourmaline veins and barren migmatized gneiss and migmatites. A detailed electron microprobe study has been carried out to document the chemical compositions of tourmaline from this deposit. The results exhibit a systematic compositional change that might be used as tracer for ore genesis and in prospecting for tin mineralization. Tourmalines from the ore bodies are dravite with Fe/(Fe + Mg) ratios of 0.09 ∼ 0.31 and Ca/(Ca + Na) ratios of 0.03 ∼ 0.40. These tourmalines are also rich in chromium (up to 0.74 wt% Cr₂O₃) and tin (up to 0.42 wt% Sn). In contrast, tourmalines from the barren migmatites are mostly schorl with Fe/(Fe + Mg) ratios of 0.38 ∼ 0.94 and Ca/(Ca + Na) ratios of 0.00 ∼ 0.14. Tourmalines from quartz–tourmaline veins that occur between ore bodies and the migmatites show intermediate compositions, i.e., Fe/(Fe + Mg) = 0.09 ∼ 0.59, Ca/(Ca + Na) = 0.01 ∼ 0.22. It is suggested that the Mg-rich nature of the tourmaline can be used as an exploration tool in this region to target tin mineralization, because the tourmalines show increasing Mg contents and are more dravitic when approaching the ore bodies. It is likely that the formation of the Yunlong tin deposit was related to migmatitic-hydrothermal processes. The high Mg and Cr contents in tourmalines from the ore bodies were probably derived from the local meta-sedimentary and meta-volcanic rocks of the Precambian Chongshan Group rather than from the granites in the region. Received December 28, 2000; revised version accepted January 25, 2002     

Mode of occurrence of phosphorus in iron ores of eastern limb,Bonai Synclinorium,eastern India

The eastern limb of horse shoe shaped “Bonai Synclinorium” in India hosts Banded Iron Formations (BIFs), consisting of major high grade iron deposits. Phosphorus (P) gets adsorbed in the iron ore by way of ion exchange mechanism of clay minerals and hydrated secondary iron oxide minerals. Its concentration is lesser in hard ores and blue dust types of ores, while the highest in case of lateritic ores. P content reduces with increase in iron (Fe) content in individual ore types. Along the eastern limb, phosphorus content gradually reduces from north to south direction. Since phosphorus is mainly associated with secondary lateritization process, its concentration is very high in top weathered profile and along the weaker zones.     

The Hanshan gold deposit in the Caledonian North Qilian orogenic belt, NW China

The recently discovered Hanshan gold deposit in northern Gansu Province, northwestern China, is hosted by a WNW-striking shear zone in Ordovician andesite and basalt. Mineralization consists of surface to near-surface oxidized ore (the yellow sandy gossan type) and three types of primary ore, i.e. early-stage quartz-sericite-pyrite ores in stockworks, early-stage disseminated ore, and the most important late-stage quartz ± calcite-sulfide veins. The ore system is characterized by variable degrees of potassic and silicic alteration. Late-stage gold-related fluid inclusions have homogenization temperatures between 170 to 310 °C, with a peak around 260 °C and low salinities. The ore fluids had high contents of CO₂, CH₄, and N₂. Sulfur isotope measurements of −1.9 to +1.7 per mil for hydrothermal pyrites could be consistent with a hydrothermal fluid source from the mantle, but the oxygen and carbon isotope data from calcite and quartz suggest mixing between mantle and crustal fluid sources. K-Ar ages for hydrothermal sericite from ore zones are 213.9 ± 3.1 and 224.4 ± 3.2 Ma. Due to the arid Cenozoic climate, a yellow gold-bearing gossan developed, which consists of jarosite, gypsum, and relict quartz. It could be a widespread and useful prospecting guide for gold in northwestern China. Received: 1 February 1999 / Accepted: 1 August 1999     

Structurally controlled gold and sulfosalt mineralization: the Altenberg example, Salzburg Province, Austria

Summary ?In the south-eastern Altenbergkar–Silbereck area in the eastern Tauern window (Lungau, Salzburg) structurally controlled precious-metal (Au–Ag) mineralization is hosted in marbles of the Permo(?)-Mesozoic Silbereck Formation and in the underlying Variscan Central gneiss. During the Alpine otogeny both lithologies were affected by ductile deformation (shearing, D1; folding, D2/D3) and subsequent brittle deformation (tension gashes, D4; normal faulting, D5) related to the uplift and exhumation of the Tauern window. Mineralization is controlled by brittle D4 structures. NE–SW trending steeply dipping tension gashes of the “Tauerngoldgang” type occur within the Central gneiss. Three different marble-hosted ore types following fracture systems as well as foliation and bedding planes can be distinguished: 1) metasomatic replacement ores, 2) ores in tension gashes and 3) ores in talc-bearing structures, often containing high-grade gold and silver mineralization (native gold in association with Ag–Pb–Bi–sulfosalts). Four stages of mineralization can be distinguished which occur in all ore types: arsenopyrite–pyrite–pyrrhotite (first stage), Au–(Ag–Pb–Bi–sulfosalts) (second stage), base-metal sulfides and tetrahedrite–tennantite (third stage) and Ag-rich galena (fourth stage). Preliminary fluid inclusion data indicate temperatures of ore formation well above 300 °C (346 °C mean) for the second stage within the Central gneiss and temperatures between 310 and 230 °C for the second and third stages in the marble. Received October 12, 2001; revised version accepted September 5, 2002 Published online March 10, 2003     

Banded iron formation to high-grade iron ore: a critical review of supergene enrichment models

All the major worldwide direct-shipping iron ore deposits associated with banded iron formations (BIF) are characteristically deeply weathered. They extend to considerable depths below the water table and show well-preserved primary structures and textures, but characteristically most deposits contain no evidence of chert bands being present prior to weathering. Recent studies have found evidence of hydrothermal and/ or metamorphic influences in the development of certain ore deposits and new genesis models such as the supergene-modified hypogene model have been postulated for major high-grade iron ore deposits. Nevertheless, there are many high-grade deposits that show no evidence of hypogene alteration and for which a hypogene or metamorphic genesis is unreasonable that are automatically ascribed to supergene enrichment, commonly erroneously attributed to lateritic weathering in tropical environments. Laterite (sensu lato) is a soil formation in which primary textures are destroyed and is underlain by a pallid zone showing the preservation of chert and the depletion, not enrichment, of iron oxides and thus is totally incompatible with the formation of the high-grade ore deposits. Various theories and models that purported to explain the conditions under which such a uniquely BIF-related dissolution of quartz and residual accumulation of hematite could occur by supergene processes typically conflict with current understanding of groundwater hydrology, chemistry, weathering processes and soil formation.Supergene enrichment of ore is universal in the leaching of gangue minerals such as iron silicates, carbonates and apatite and supergene enrichment of BIF to low-grade ore is common in near surface environments above the water table such as ferrugenised BIF outcrops, detrital ore deposits, and some shallow ore deposits that have been subjected to prolonged exposure to fresh meteoric water. In all cases of supergene enrichment traces of the chert bands are visible and the dissolution or replacement processes for the removal of quartz are clear, in direct contrast to the most important deep saprolite ore deposits that show no trace of chert bands.The widespread acceptance of an inappropriate and untenable supergene enrichment model inhibits search for the true origin of the ore and our ability to predict and find concealed high-grade ore deposits.     

The composition of magmatic Ni–Cu–(PGE) sulfide deposits in the Tati and Selebi-Phikwe belts of eastern Botswana

We studied a number of magmatic Ni–Cu–(PGE) sulfide deposits in two distinct belts in eastern Botswana. The Tati belt contains several relatively small deposits (up to 4.5 Mt of ore at 2.05% Ni and 0.85% Cu) at Phoenix, Selkirk and Tekwane. The deposits are hosted by ca 2.7 Ga, low- to medium-grade metamorphosed gabbroic–troctolitic intrusions situated within or at the periphery of a greenstone belt. The deposits of the Selebi-Phikwe belt are larger in size (up to 31 Mt of ore grade). They are hosted by high-grade metamorphosed gabbronorites, pyroxenites and peridotites believed to be older than ca 2.0 Ga that intruded gneisses of the Central Zone of the Limpopo metamorphic belt. The composition of the sulfide mineralisation in the two belts shows systematic variation. Most of the mineralisation in the Tati belt contains 2–9% Ni and 0.05–4% Cu (Cu/Cu + Ni = 0.4–0.7), whereas most of the mineralisation in the Selebi-Phikwe belt contains 1–3% Ni and 0.1–4% Cu (Cu/Cu + Ni = 0.4–0.9). The Cu–Ni tenors of the ores in both belts are consistent with crystallization from a basaltic magma. The Tati ores contain mostly >3 ppm Pt + Pd (Pt/Pd 0.1–1), with Pd/Ir = 100–1,000, indicative of a differentiated basaltic magma that remained S-undersaturated before emplacement. Most of the Selebi-Phikwe ores have <0.5 ppm Pt + Pd (Pt/Pd < 0.1–1), with Pd/Ir = 10–500. This suggests a relatively less differentiated magma that reached S saturation before emplacement. The Tati rocks show flat mantle-normalised incompatible trace element patterns (average Th/Yb_N = 1.57), except for strong enrichments in large ion lithophile elements (Cs, Rb, Ba, U, K). Such patterns are characteristic of relatively uncontaminated oceanic arc magmas and suggest that the Tati intrusions were emplaced in a destructive plate margin setting. Most of the Selebi-Phikwe rocks (notably Dikoloti) have more fractionated trace element signatures (average Th/Yb_N = 4.22), possibly indicating digestion of upper crustal material during magma emplacement. However, as there are also samples that have oceanic arc-like signatures, an alternative possibility is that the composition of most Selebi-Phikwe rocks reflects tectonic mingling of the intrusive rocks with the country rocks. The implication is that orogenic belts may have a higher prospectivity for magmatic Ni–Cu ores than presently recognised. The trigger mechanism for sulfide saturation and segregation in all intrusions remains unclear. Whereas the host rocks to the intrusions appear to be relatively sulfur poor, addition of crustal S to the magmas is suggested by low Se/S ratios in some of the ores (notably at Selebi-Phikwe). External S sources may thus remain unidentified due to poor exposure and/or S mobility in response to metamorphism.     

Geological setting, mineralogy, geochemistry and genesis of the Middle Archaean Kalyadi copper deposit, western Dharwar craton, southern India

The Kalyadi polymetallic copper deposit occurs within the Middle Archaean (≥3.0 Ga), medium-grade Kalyadi schist belt which consists predominantly of ultramafic-mafic schists interbedded with chemogenic chert, detrital high Al-Mg schists and siliceous schists. This sedimentary exhalative type (SEDEX type) ore-body is the only copper deposit hosted in cherts in the western Dharwar craton. The Kalyadi supracrustal rocks are intruded by tonalite-trondhjemitic gneisses (ca. 3.0 Ga) and granite (ca. 2.6 Ga). The Kalyadi copper deposit is polygenetic in nature. The primary ores represented by disseminations of pyrite ± linneite and chalcopyrite ± magnetite essentially along the bedding lamination of the metachert are referred to as the metamorphosed chert-sulphide rhythmites of a primary stratiform type. The ore is of low-grade and records imprints of at least two events of deformation. Pyrite is characterised by high-Co values (262–4524 ppm) and high–Co/Ni ratios (3.0–19.7). Rare earth element patterns of the primary ores and the host metacherts are identical, characterised by La enrichment, absence of Eu anomalies and flat to depleted HREE patterns with δ ³⁴ S = −0.8‰. The secondary (remobilised) ores are structurally controlled occurring as veins and stringers discordant to the bedding lamination or schistosity. The constituent ores are chalcopyrite-pyrite-pyrrhotite with minor pentlandite. These sulphides with low-Co/Ni ratios (0.87–1.80), have either a strong positive or negative Eu anomaly and show slight HREE enrichment. The δ ³⁴ S value ranges from +2.64 to −4.29‰. It is interpreted that the primary stratiform ores and the cherts were derived from volcanogenic hydrothermal fluids as syngenetic/chemical deposits in a deep sea environment. The secondary epigenetic mineralisation is related to subsequent migmatisation, deformational events and granitic activity. Received: 8 September 1995 / Accepted: 18 November 1996