河北迁安杏山铁矿床地球化学特征及其对成矿物质来源的指示

不同学者曾对迁安地区铁矿床的前寒武地质、岩石学和地球化学等方面进行了深入的研究,但是其成矿物质来源至今没有进行深入探讨。危机矿山勘察在迁安杏山矿床中发现了富大铁矿体,但其成因不明。本文通过对迁安富矿和普通矿石的主量、微量元素和稀土元素研究,结果表明它们的化学成分主要由Fe₂O₃(T)、SiO₂组成,并且Al₂O₃和TiO₂具有较低的含量,指示其形成时几乎没有碎屑物质的加入。而经PAAS标准化后,稀土元素的配分模式表现轻稀土亏损、重稀土富集的特征,无论是富矿还是普通矿石,都具有Eu正异常、其Co/Zn和Ni/Zn比值与热液类似的特征,表明形成时有高温热液加入;其Y/Ho > 44、Y的正异常表明其有海水的成因;La/La*表明其没有陆源碎屑加入;La_N/YV_N < 1,表明既有海水特征,又有热液特征,所有这些数据都显示了迁安铁矿矿石的物质来源为海水和热液,与其他地方BIF铁矿物质来源一致。由于富矿和普通矿石的物质来源一致、主微量及稀土元素含量和分布类似、铁矿物主要为磁铁矿、原始沉积条带明显,推断富矿可能是火山一沉积建造原始沉积时由于局部富铁环境而形成的。      

Partial least squares-discriminant analysis of trace element compositions of magnetite from various VMS deposit subtypes: Application to mineral exploration

The petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide (VMS) deposits in Canada, and the Lasail VMS deposit in Oman, as well as from two VMS-associated banded iron formations (BIF), Austin Brook (New Brunswick, Canada) and Izok Lake (Nunavut, Canada), were investigated using optical microscopy, electron probe micro-analyzer (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The method of robust estimation for compositional data (robCompositions) was applied to investigate geochemical censored data. Among thirty-seven elements analyzed by EPMA and/or LA-ICP-MS in magnetite from the studied deposits/bedrock lithologies, only the results for Si, Ca, Zr, Al, Mg, Ti, Zn, Co and Ni contain < 40% censored values, and thus could be imputed using robCompositions. Imputed censored data were transformed using centered log-ratios to overcome the closure effect on compositional data. Transformed data were classified by partial least squares-discriminant analysis (PLS-DA) to identify different compositional characteristics of magnetite from VMS deposits and BIFs. The integration of petrography and mineral chemistry identifies three types of magnetite in VMS settings: magmatic, hydrothermal, and metamorphic. Magmatic magnetite in VMS deposit host bedrocks is characterized by ilmenite exsolution and may be overprinted by metamorphism. Some VMS deposits contain hydrothermal magnetite, which is intergrown with sulfides, and shows a metamorphic overprint as it is partly replaced by common metamorphic minerals including chlorite, sericite, anthophyllite, and/or actinolite, whereas the majority of the deposits are characterized by metamorphic magnetite formed by replacing pre-existing sulfides and/or silicates, and is intergrown with metamorphic minerals. Among VMS deposits of the Noranda mining district, the West Ansil deposit is characterized by hydrothermal-metamorphic magnetite zoned by inclusion-free cores and Si- and Mg-rich rims. Magnetite from the studied VMS-associated BIFs is also metamorphic in origin. Aluminum, Ti and Zn contents of magnetite can separate BIF from the other mineralized and un-mineralized bedrock lithologies in the studied VMS settings.PLS-DA shows that variable compositions of magnetite slightly discriminate different studied deposits/bedrock lithologies. The geochemical observations suggest that the variation in magnetite chemistry from different VMS settings might be sourced from differences in: 1) the composition and temperature of parental magmas or hydrothermal fluids, 2) the composition of host bedrocks, 3) the composition of co-forming minerals, and 4) oxygen fugacity. PLS-DA distinguishes magnetite compositions from the studied VMS deposits and BIFs from that of the other ore deposit types including Ni–Cu, porphyry Cu-Mo-Au, iron oxide-copper- gold, iron oxide-apatite, and the Bayan Obo REE-Fe-Nb deposit. Magnetite from the VMS settings on average contains lower concentrations of Si, Zr, Al, Mg, Ti, Zn, Co and Ni relative to that from the other mineral deposit types. PLS-DA of magnetite data from VMS deposits and BIFs of the Bathurst mining camp as well as PLS-DA of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for VMS deposits using indicator minerals in Quaternary lithified sedimentary rocks.     

Comparative geochemistry of shell phosphorites and dictyonema shales of the Baltic

Major and trace element composition of the Ordovician Obolus phosphorites and associated Dictyonema shales were determined by ICP-MS and chemical and microchemical elemental analyses. Relative to the phosphorites, the Dictyonema shales are substantially enriched in a variety of trace elements, except for As, Be, Co, Y, REE, Sr, and Pb. The Obolus phosphorites show enrichment of As, Bi, Hg, Mo, La, Y, Pb, and Sr and depletion of Ag, Ba, Be, Cd, Cr, Cu, Hf, Ni, Sc, Sn, U, V, Zn, and Zr relative to the world average phosphorite composition. The average trace element composition of the Dictyonema shales is close to the mean shale composition, except for higher contents of Mo, Hg, Pb, Se, Ta, Te, Th, V, and U and lower contents of Ba, Bi, Cd, Co, Re, Sr, and Zn. The results suggest that the change from phosphate sedimentation in aerated environments to anoxic carbonaceous sedimentation was accompanied by changes in the composition and concentration of trace elements in the sediment. Both facies show similar trends of trace element distribution indicative of the stability of the composition of seawater and terrigenous sediment input.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Cu-(Au,Fe) deposits in the Khetri copper belt in Rajasthan Province,NW India

Magnetite is common in many ore deposits and their host rocks, and is useful for petrogenetic studies. In the Khetri copper belt in Rajasthan Province, NW India, there are several Cu-(Au, Fe) deposits associated with extensive Cu ± Fe ± Au ± Ag ± Co ± REE ± U mineralization hosted in phyllites, schists and quartzites of the Paleoproterozoic Delhi Supergroup. Ore bodies of these deposits comprise dominantly disseminated and vein-type Cu-sulfide ores composed of chalcopyrite, pyrite, and pyrrhotite intergrown with minor magnetite. There are also Fe-oxide ores with minor or no Cu-sulfides, which are locally overprinted by the mineral assemblage of the Cu-sulfide ores. In addition to the Fe-oxide and Cu-sulfide ores, the protolith of the Delhi Supergroup includes banded iron formations (BIFs) with original magnetite preserved (i.e. magnetite-quartzites) and their sheared counterparts. In the sheared magnetite-quartzites, their magnetite and quartz are mobilized and redistributed to magnetite and quartz bands. Trace elemental compositions of magnetite from these types of ores/rocks were obtained by LA-ICP-MS. The dataset indicates that different types of magnetite have distinct concentrations of Ti, Al, Mg, Mn, V, Cr, Co, Ni, Zn, Cu, P, Ge and Ga, which are correlated to their forming environments. Magnetite grains in magnetite-quartzites have relatively high Al (800–8000 ppm), Ti (150–900 ppm) and V (300–600 ppm) contents compared to those of BIFs in other regions such as the Yilgarn Craton, Western Australia and Labrador, Canada. The high Al, Ti and V contents can be explained by precipitation of the magnetite from relatively reduced, Al–Ti-rich water possibly involving hotter, seafloor hydrothermal fluids derived from submarine mafic volcanic rocks. Magnetite in sheared magnetite-quartzites is generally irregular and re-crystallized, and has Ni, Mn, Al, Cu and P contents lower than the magnetite from the unsheared counterparts, suggesting that the shearing-related mobilization is able to extract these elements from original magnetite. However, elevated contents of Ti, V, Co, Cr, Ge and Mg of the magnetite in the sheared magnetite-quartzites can be ascribed to involvement of external hydrothermal fluids during the shearing, consistent with occurrence of some hydrothermal minerals in the samples.Compositions of magnetite from the Fe-oxide and Cu-sulfide ores are interpreted to be controlled mainly by fluid compositions and/or oxygen fugacity (fO₂). Other potential controlling factors such as temperature, fluid–rock interaction and co-precipitating minerals have very limited impacts. Magnetite in the Cu-sulfide ores has higher V but lower Ni contents than that of the Fe-oxide ores, likely indicating its precipitation from relatively reduced, evolved fluids. However, it is also indicated that the two types of magnetite do not show large distinctions in terms of concentrations of most elements, suggesting that they may have precipitated from a common, evolving fluid. We propose a combination of Ge versus Ti/Al and Cr versus Co/Ni co-variation plots to discriminate different types of magnetite from the Khetri copper belt. Our work agrees well with previous studies that compositions of magnetite can be potentially useful for provenance studies, but also highlights that discrimination schemes would be more meaningful for deposits in a certain region if fluid/water chemistry and specific formation conditions reflected in compositions of magnetite are clearly understood.     

Origin of native sulfur ball from the Kueishantao hydrothermal field offshore northeast Taiwan: Evidence from trace and rare earth element composition

We first report the trace and rare earth element compositions of native sulfur ball with sulfur contents varying from 97.08 wt.% to 99.85 wt.% from the Kueishantao hydrothermal field, off NE Taiwan. We then discuss the sources of trace and rare earth elements incorporated into the native sulfur ball during formation. Comparison of our results with native sulfur from crater lakes and other volcanic areas shows the sulfur content of native sulfur ball from the Kueishantao hydrothermal field is very high, and that the rare earth element (REE) and trace element constituents of the native sulfur balls are very low (∑REE < 35 ppb). In the native sulfur ball, V, Cr, Co, Ni, Nb, Rb, Cs, Ba, Pb, Th, U, Al, Ti and REE are mostly derived from andesite; Mg, K and Mn are mostly derived from seawater; and Fe, Cu, Zn and Ni are partly derived from magma. Based on the sulfur contents, trace and rare earth element compositions, and local environment, we suggest that the growth of the native sulfur ball is significantly slower than that of native sulfur chimneys, which results in the relatively higher contents of trace and rare earth element contents in the native sulfur ball than in the native sulfur chimneys from the Kueishantao hydrothermal field. Finally, we suggest a “glue pudding” growth model for understanding the origin of the native sulfur ball in the Kueishantao hydrothermal field, whereby the native sulfur ball forms from a mixture of oxygenated seawater and acidic, low-temperature hydrothermal fluid with H₂S and SO₂ gases, and is subsequently shaped by tidal and/or bottom currents.     

Trace Element Geochemistry of Magnetite from the Fe(-Cu) Deposits in the Hami Region, Eastern Tianshan Orogenic Belt, NW China

Laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS) was used to determine the trace element concentrations of magnetite from the Heifengshan, Shuangfengshan, and Shaquanzi Fe(–Cu) deposits in the Eastern Tianshan Orogenic Belt. The magnetite from these deposits typically contains detectable Mg, Al, Ti, V, Cr, Mn, Co, Ni, Zn and Ga. The trace element contents in magnetite generally vary less than one order of magnitude. The subtle variations of trace element concentrations within a magnetite grain and between the magnetite grains in the same sample probably indicate local inhomogeneity of ore–forming fluids. The variations of Co in magnetite between samples are probably due to the mineral proportion of magnetite and pyrite. Factor analysis has discriminated three types of magnetite: Ni–Mn–V–Ti(Factor 1), Mg–Al–Zn(Factor 2), and Ga– Co(Factor 3) magnetite. Magnetite from the Heifengshan and Shuangfengshan Fe deposits has similar normalized trace element spider patterns and cannot be discriminated according to these factors. However, magnetite from the Shaquanzi Fe–Cu deposit has affinity to Factor 2 with lower Mg and Al but higher Zn concentrations, indicating that the ore–forming fluids responsible for the Fe–Cu deposit are different from those for Fe deposits. Chemical composition of magnetite indicates that magnetite from these Fe(–Cu) deposits was formed by hydrothermal processes rather than magmatic differentiation. The formation of these Fe(–Cu) deposits may be related to felsic magmatism.     

Metasedimentary rocks of the paleoarchean Dniester-Bug Group,Ukrainian Shield: Composition,age, and sources

Paleoarchean granulite-facies metasedimentary rocks (quartzites, garnet quartzites, garnet-pyroxene gneisses, pyroxene-magnetite and magnetite quartzites) attributed to the Dniester-Bug Group of the Ukrainian Shield were studied. On the basis of geochemical data, including REE, the primary composition of these rocks was reconstructed as association of Fe-rich sandstones and sublitharenites, Fe-shales, and BIFs. This sedimentary association is similar to the rocks of other ancient greenstone belts and ascribed to the Algama-type iron formation. The sum of Al₂O₃, CaO, Na₂O, and TiO₂, high Zr contents (>100 ppm in quartzites), and the presence of detrital zircon grains of different ages are consistent with the terrigenous nature of sedimentary rocks. The Sm/Nd, Ti/Zr, Sc/Zr, and Ni/Zr ratios indicate the predominance of granitoid rocks in the source areas. The elevated Cr contents suggest that, in addition to granitoids, the source area contained ultramafic rocks. Geochemical characteristics, such as Fe/Mn ratio, low REE contents, and variations of REE versus the sum of Ni, Co, and Cu testify that sedimentation occurred under shallow-water conditions on the continent or its slope, similarly as the formation of ancient (3.5–3.2 Ga) basalt-komatiitic series intercalated with sedimentary rocks in the Pilbara Craton. The age of supracrustal rocks of the Dniester-Bug Group was constrained within the time interval of 3.4–3.2 Ga on the basis of U-Pb zircon dating and determination of Nd isotope composition. The DM model age of quartzites varies from 3.37 to 3.5 Ga. Sedimentary rocks together with volcanic rocks represent the oldest supracrustal association of the East European Platform.     

Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR‐ICP‐MS: A Combined Approach Using a Direct Digestion/Dilution Method and Preconcentration by Coprecipitation

A procedure is described for the determination of thirty‐seven minor and trace elements (LILE, REE, HFSE, U, Th, Pb, transition elements and Ga) in ultramafic rocks. After Tm addition and acid sample digestion, compositions were determined both following a direct digestion/dilution method (without element separation) and after a preconcentration procedure using a double coprecipitation process. Four ultramafic reference materials were investigated to test and validate our procedure (UB‐N, MGL‐GAS [GeoPT12], JP‐1 and DTS‐2B). Results obtained following the preconcentration procedure are in good agreement with previously published work on REE, HFSE, U, Th, Pb and some of the transition elements (Sc, Ti, V). This procedure has two major advantages: (a) it avoids any matrix effect resulting from the high Mg content of peridotite, and (b) it allows the preconcentration of a larger trace element set than with previous methods. Other elements (LILE, other transition elements Cr, Mn, Co, Ni, Cu, Zn, as well as Ga) were not fully coprecipitated with the preconcentration method and could only be accurately determined through the direct digestion/dilution method.     

Iron isotope, major and trace element characterization of early Archean supracrustal rocks from SW Greenland: Protolith identification and metamorphic overprint

The iron isotope, trace and major element compositions of Eoarchean supracrustal rocks from southern West Greenland (Isua Supracrustal Belt, the islands of Akilia and Innersuartuut) were analyzed in order to identify protoliths and characterize the imprints of metamorphism and metasomatism. Banded iron formations (BIFs) from the Isua Supracrustal Belt (ISB) have trace element characteristics that are consistent with seawater derivation, including high Y/Ho ratios, positive Eu/Eu^∗ anomalies, positive La/La^∗ anomalies, and concave upward REE patterns. These rocks also have heavy Fe isotopic compositions relative to surrounding igneous rocks (∼+0.4‰/amu). The most likely interpretation is that this signature was inherited from partial oxidation in a marine setting of Fe emanating from a source similar to modern mid-ocean ridge hydrothermal vents (∼−0.15‰/amu).Banded quartz-rich rocks from the island of Akilia with high Fe/Ti ratios share many similarities with bona fide BIFs from Isua (heavy Fe isotopic compositions up to +0.4‰/amu, elevated Y/Ho ratios compared to igneous rocks, sometimes positive Eu/Eu^∗ anomalies) suggesting a chemical sedimentary origin.Iron-poor metacarbonates from the southwestern part of the ISB have light Fe isotopic compositions (∼−0.4‰/amu). This is consistent with derivation of these rocks by fluid flow through surrounding ultramafic rocks and deposition as metasomatic carbonates. Iron-rich metacarbonates from the northwest and northeast parts of the ISB have Fe isotopic compositions (from +0.1 to +0.4‰/amu) and trace element patterns (high Y/Ho ratios, positive Eu/Eu^∗ and La/La^∗ anomalies, and concave upward REE) similar to associated BIFs. The most likely interpretation is that these iron-rich metacarbonates were derived from mobilization of Fe in BIFs by metasomatic fluids.     

Neoproterozoic banded iron formations

Two epochs of the formation of ferruginous quartzites—Archean-Paleoproterozoic (3.2–1.8 Ga) and Neoproterozoic (0.85–0.7 Ga)—are distinguished in the Precambrian. They are incommensurable in scale: the Paleoproterozoic Kursk Group of the Kursk Magnetic Anomaly (KMA) extends over 1500 km, whereas the extension of Neoproterozoic banded iron formations (BIF) beds does not exceed a few tens of kilometers. Their thickness is up to 200 m and not more than 10 m, respectively. The oldest BIFs are located in old platforms, whereas Neoproterozoic BIFs are mainly confined to Phanerozoic orogenic (mobile) zones. Neoproterozoic BIFs universally associate with glacial deposits and their beds include glacial dropstones. In places, they underlie tillites of the Laplandian (Marinoan) glaciation (635 Ma), but they are more often sandwiched between glaciogenic sequences of the Laplandian and preceding Sturtian or Rapitan glaciation (730–750 Ma). Neoproterozoic BIFs are rather diverse in terms of lithology due to variation in the grade of metamorphism from place to place from low grades of the greenschist facies up to the granulite facies. Correspondingly, the ore component is mainly represented by hematite or magnetite. The REE distribution and (Co + Ni + Cu) index suggest an influence of hydrothermal sources of Fe, although it was subordinate to the continental washout. Iron was accumulated in seawater during glaciations, whereas iron mineralization took place at the earliest stages of postglacial transgressions.     

Geochemistry and Si–O–Fe isotope constraints on the origin of banded iron formations of the Yuanjiacun Formation,Lvliang Group,Shanxi, China

Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al₂O₃, TiO₂, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain ³⁰Si depleted quartz, with high δ¹⁸O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce^4 +; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (⁵⁶Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe^2 + to Fe^3 + in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.     

湘西北下寒武统黑色岩系钒镍钼矿微量元素地球化学

湘西北下寒武统黑色岩系是中国南方下寒武统黑色岩系的重要组成部分,富含钒镍钼等多金属元素。文中对湘西北下寒武统黑色岩系钒镍钼矿进行了微量元素研究,研究区内Ni、Cd、Mo、Sb、V、Zn、W、Ba等元素特别富集,高含量的Sb和Ba表明其为热水沉积;Mo含量极高说明其为缺氧的还原环境;高的V/（V+Ni）、V/Cr、Ni/Co以及δU值表明其沉积环境为缺氧环境;高的 U/Th表明本区有热水沉积作用;稀土元素配分模式、Ce和Eu异常及La/Yb Ce/La和La/Yb ΣREE图解投点表明其沉积环境为还原环境,并有热水沉积作用。可见黑色岩系形成于缺氧环境,热液活动为其提供了丰富的热液来源。     

Trace element distribution,Co:Ni ratios and genesis of the big cadia iron-copper deposit,new south wales,australia

Analyses of pyrite, chalcopyrite and magnetite from the volcanic-hosted Big Cadia stratabound iron-copper deposit in Central Western New South Wales show considerable variation in the minor elements Mn, Ba, Ag, Pb, Zn, Cd, Se, Co and Ni. The preferential concentration of Co and Cd in pyrite, Zn and Ag in chalcopyrite and Mn in magnetite can be attributed to variations in activities of the ions in the hydrothermal fluid at the time of crystallisation of the mineral phases, or in cases such as the concentration of Co in pyrite, dependent on compatible electronic spin states between Co²⁺ and Fe²⁺. Trace element concentrations, especially Co and Ni contents and Co:Ni ratios in pyrite (average Co:Ni ratio=17.1) support a volcanic exhalative origin of mineralisation at Big Cadia. Differences in trace element composition such as higher Ni contents in pyrite in contrast with other volcanic-hosted ores may reflect the more basic character of volcanic rocks underlying the Big Cadia deposit.     

古元古代大氧化事件(GOE)前后海洋环境的变化: 来自华北条带状铁建造(BIF)岩相学和地球化学的证据*

条带状铁建造(BIF)是形成于前寒武纪海洋中的化学沉积岩,记录了古海洋氧化还原状态的重要信息。华北克拉通广泛分布的新太古代和古元古代BIF,是了解古元古代大氧化事件(GOE)前后古海洋氧化还原环境变化的理想对象。初步研究表明,华北克拉通新太古代BIF主要为磁铁矿型氧化物相和硅酸盐相,极少数出现碳酸盐相;古元古代BIF包括赤铁矿型和磁铁矿型氧化物相、硅酸盐相和碳酸盐相,其中赤铁矿相是古元古代BIF独有的。以上矿物学特征表明,新太古代和古元古代水体的氧化还原条件是不同的。华北克拉通新太古代BIF的稀土元素组成缺乏强烈的负Ce异常,反映同期海水氧含量非常低,为缺氧状态; 但少量BIF也包含有负Ce异常,同时具有较大变化范围的Th/U值,指示新太古代海洋的局部水体氧含量相对较高,呈弱氧化状态。与新太古代BIF相比,古元古代BIF的Ce异常变化较大,包括无异常、正异常和负异常,尤其是赤铁矿相BIF具明显的负Ce异常,表明古元古代水体的氧含量和氧化还原结构已发生了明显变化; 结合华北克拉通BIF的Ni/Co、V/(V+Ni)和Th/U等比值特征,认为古元古代海洋呈次氧化—氧化环境。新太古代BIF 强烈富集重铁同位素,S同位素非质量分馏效应较为明显;而古元古代BIF相对富集轻铁同位素,S同位素非质量分馏效应不明显。综上,新太古代海洋环境整体缺氧,但局部可能存在氧气“绿洲”,暗示光合产氧作用在太古代晚期已经存在;大氧化事件期间及之后的古海洋总体具上部氧化、下部还原的分层特征。     

Changes of oceanic environment before and after the Paleoproterozoic Great Oxidation Event(GOE): Evidence from petrography and geochemistry of banded iron formation(BIF)from the North China Craton

Banded iron formation(BIF)belongs to sedimentary rocks formed in Precambrian marine,which can directly reflect the redox state of the ancient oceans. Mineral composition and geochemistry of BIF can reveal the relative changes of oxygen contents of ancient atmosphere-ocean. The Neoarchean and Paleoproterozoic BIFs widely distributed in the North China Craton(NCC),are the ideal research objects for understanding the changes of the ancient ocean redox environment before and after the Paleoproterozoic Great Oxidation Event(GOE). Our previous studies indicated that the sedimentary facies of the Neoarchean BIF in the NCC are mainly magnetite-type oxide and silicate,with minor carbonate. The sedimentary facies of the Paleoproterozoic BIF are hematite- and magnetite-type oxide,silicate and carbonate,of which the hematite-oxide facies is unique to the Paleoproterozoic BIF. The above mineralogical features suggest that the redox conditions of the Neoarchean and Paleoproterozoic seawater are different. The rare earth element composition of the Neoarchean BIF in the NCC lacks a strong negative Ce anomaly,reflecting that the oxygen content of contemporary seawater is very low and the marine is anoxic. However,a small amount of BIFs in the NCC also present the negative Ce anomalies and a wide range of Th/U ratios,indicating that the local water of the Neoarchean ocean had relatively high oxygen content and was at a weak oxidation state. Compared with the Neoarchean BIFs,the Paleoproterozoic BIFs present a wide range of Ce anomalies(i.e.,no Ce anomalies,positive Ce anomalies and negative Ce anomalies). The hematite-bearing BIF has an obvious negative Ce anomalies,implying that the oxygen content and redox state of Paleoproterozoic seawater changed significantly. Combined with the ratios of Ni/Co,V/(V+Ni)and Th/U of the BIFs in the NCC,the Paleoproterozoic oceans exhibited a suboxidation to oxidation environment. Besides,Neoarchean BIF is strongly enriched in heavy iron isotopes and the non-mass fractionation of S isotope is obvious,whereas the Paleoproterozoic BIF is relatively enriched in light iron isotopes and the non-mass fractionation of S isotope is not obvious. It is summarized that the Neoarchean marine is anoxic,but the oxygen‘oasis' may exist locally,implying that photosynthetic oxygen production already existed in the Late Neoarchean. The ancient ocean presented a layered characteristics during and after the GOE,indicating that the shallow water was generally oxidized and the deep water was reduced.     

Hydrothermal alteration in andesitic volcanoes: Trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)

The mineralogy and the trace element compositions of hydrothermally-altered volcanic materials collected from ash fall deposits and in four debris-avalanche deposits (DADs) at La Soufrière volcano in Guadeloupe have been determined. Phreatic explosions of the 1976 eruption and flank collapse events have sampled various parts of the active and ancient hydrothermal systems of the volcano. Hydrothermal mineral assemblages (smectite + silica polymorphs ± pyrite/jarosite ± gypsum) are typical of rock alteration by low-temperature acid-sulphate fluids. High-temperature mineral assemblages are rare, indicating that phreatic explosions and flank collapse events have sampled mainly the upper parts of the volcanic edifice.Andesitic eruptive products affected by shallow hydrothermal alteration are complex assemblages of volcanic materials (glass, phenocrysts and xenocrysts with complex magmatic histories) of different ages and compositions. The use of incompatible element ratios and REE compositions normalised to an unaltered reference material overcomes the interpretation difficulties related to mass balance effects of alteration processes and the petrologic heterogeneity of the initial material.REE and other incompatible elements (Th, U, Hf, Zr) are mainly concentrated in the glassy matrix of unaltered andesitic rocks. Secondary S-bearing mineral phases (e.g., gypsum, jarosite) that have precipitated from acid-sulphate fluids do not contain substantial incompatible elements (REE, U, Th, Hf, Zr). Compositional variations of incompatible elements in hydrothermally-altered andesitic materials reflect mainly volcanic glass–smectite transformation, which is characterised by (i) strong depletion of alkalis and alkaline earths (Ba, Sr) and first transition series elements (Zn, Cu, Cr, Co, Ni), (ii) immobility of highly incompatible elements (Th, Zr, Hf, LREE) and (iii) strong depletion of MREE and HREE. The sigmoid shape of normalised REE pattern is characteristic of glass–smectite transformation by low-temperature acid-sulphate fluids. This transformation also produces significant variations in U/Th values, which offer the opportunity to date the cessation of hydrothermal alteration and to reconstruct the evolution in space and time of hydrothermal activity in a volcanic edifice.     

黔东注溪黑色岩系地球化学特征及矿化富集规律

目前急需分析黔东注溪钒矿形成的环境、成矿的物质来源以及矿化富集规律,指导实际地质勘查工作.系统研究了注溪矿区内中洞、老屋基和坪哨3个典型岩性剖面中黑色岩含矿岩系及矿层的全岩主微量元素组成.结果表明,含矿岩系具有较高的SiO₂、MnO、Ce/Ce*和Eu/Eu*值,而矿层则含相对较高的Al₂O₃、Fe₂O₃、TiO₂、CaO、Na₂O、K₂O、P₂O₅、V₂O₅、REE、As、Cu、Pb、Zn、Mo、Ni、Ti、Cr、Rb、Sr、Th、U和V.含矿岩系与矿层的主微量地球化学特征显示注溪钒矿床的成矿物源具有一定程度陆源物质的输入,且在成矿阶段受到了热水作用及生物作用的影响.另外,由南至北各剖面的热水成矿作用逐渐减弱;含矿岩系及矿层沉积环境均属缺氧环境,其中坪哨剖面的矿层沉积环境的缺氧程度要高于其他剖面.因此,注溪钒矿床钒富集成矿主要受古环境的还原条件和热液活动的影响,其中还原环境对钒元素的富集成矿起主要作用.据此推测坪哨剖面矿层形成时的海水深度应最深,北矿段中洞剖面的最浅;喷流热水带来的V等大量多金属元素在喷口及其附近大规模成矿.      

长江河流沉积物磁铁矿化学组成及其物源示踪

运用电子探针分析了长江干流和主要支流河漫滩沉积物中磁铁矿的元素组成.磁铁矿中的FeO平均含量稍高于其标准组成,而Fe2O3平均含量则明显低于标准组成;Ti、Al、Cr、V、Mn、Mg、Co和Zn等元素在磁铁矿中含量变化大,不同支流的磁铁矿的元素组成不同,同一取样点不同样品磁铁矿的元素组成变化也较大.金沙江、湘江、汉江及长江干流磁铁矿与钛磁铁矿、钛尖晶石、钒钛磁铁矿和铬铁矿等出溶交生,TiO2、Cr2O3和V2O3等元素含量高且变化大.金沙江磁铁矿富Mg、Al和Cr;大渡河、雅砻江和岷江磁铁矿中微量元素含量大多低于0.5%;涪江、汉江磁铁矿富Ti和V,而湘江磁铁矿富Ti和Al;总体上,长江干流上游磁铁矿富Ti,而下游磁铁矿中Ti、Al、Cr、V、Mg和Mn含量低于0.15%.干流磁铁矿的元素组成变化反映主要支流源岩组成及对干流影响程度的差异.     

鹤庆锰矿小天井矿区微量元素特征与沉积成矿环境研究

小天井矿区是鹤庆锰矿储量最大开采最早的矿区,通过对该矿区含锰岩系的60个样品进行微量及稀土元素测试,运用相关分析、因子分析等多元统计方法分析和探讨微量及稀土元素的地球化学特征与锰矿的沉积环境关系,确定Mo、U、V、Mn、Zn为该矿区重要成矿元素组合,这对本区地球化学找矿中判别地层与构造的含矿性、推断隐伏矿体寻找接替资源具有重要意义。对Co/Ni、Sr/Ba、Th/U的分析,认为锰矿层物源主要是陆源,推测沉积环境为陆缘近海环境,并对该区锰的沉积成矿环境进行了进一步探讨。     

In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit,North China

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit.