Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing,Southwestern China: Their mobilization,redistribution, and fractionation

In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO₂) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)_N and (La/Sm)_N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.     

Study of the distribution of Trace elements in soils in and around Mysore city,Karnataka

Soil samples collected from various places in and around Mysore were analyzed for the total trace elements such as Fe, Mn, Cu, Zn, Pb, and Cd. The results of the analysis indicate that the concentration of lead and cadmium in soils is below 2.5 g ml^–1 and 0.2 g ml^–1, respectively, which are the minimum detection levels, whereas the concentration of iron, manganese, copper, and zinc in most of the samples is within the global average ranges of 3%, 500–1000 g g^–1, 15–40 g g^–1, and 50–100 g g^–1, respectively. The investigated area has the presence of gneisses and schists, in which partly there are igneous intrusions and pegmatitic intrusions. There are amphibolite enclaves in gneisses that account for the higher concentration of trace elements. The lower concentration may be attributed to the presence of silicic type of rock.     

Platinum group elements and gold in ferromanganese crusts from Afanasiy-Nikitin seamount,equatorial Indian Ocean: Sources and fractionation

The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5–2) and Pt/Ru ratios (7–28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values (<2) similar to CI-chondrite (∼1). The chondrite-normalized PGE patterns are similar to those of igneous rocks, except that Pd is relatively depleted. The water depth of Fe-Mn crust formation appears to have a first-order control on both major element and PGE enrichments. These relationships are defined statistically by significant (r > 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r < 0.5). The strong enrichment of Pt (up to 1 ppm) relative to the other PGE and its positive correlations with Ce and Co demonstrate a common link for the high concentrations of all three elements, which likely involves an oxidation reaction on the Mn-oxide and Fe-oxyhydroxide surfaces. The documented fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.     

Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex.The serpentinites are characterized by high SiO₂ and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb)_N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern.All element contents increase highly from the parent rock to the coarse saprolite.In the weathering profiles, Fe₂O₃ contents decrease from the bottom to the top contrarily to Al₂O₃, SiO₂ and TiO₂. The contents of alkali and alkaline oxides are under detection limit.Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules.REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce^*]_NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce^*]_NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb)_N ratios values are lower in the Nkamouna profile than in Mang site.PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies.The mass balance evaluation, using thorium as immobile element, reveals that:

– major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite;

– all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite;

– rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles;

– platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil.

Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.     

四川天宝山铅锌矿床硫化物微量元素组成:LA-ICPMS研究

"川滇黔接壤铅锌矿集区"是我国西南大面积低温成矿域的重要组成部分,对于其中铅锌矿床是否属于MVT型矿床存在较大争议。本文以该矿集区中代表性矿床——四川天宝山矿床为例,通过LA-ICPMS原位元素分析,结合元素Mapping,以认识该矿床中闪锌矿和方铅矿微量元素组成特征及其赋存状态。研究表明,矿床中闪锌矿以富集Cd、Ge贫Fe、Mn、In、Sn、Co为特征,这些元素均以类质同象形式赋存于闪锌矿中,但含量变化范围较大,这可能与其成矿流体属于低温混合流体有关,这类盆地卤水流体在长期和长距离运移过程中,流经不同基底地层,活化出其中不同微量元素,因此成分变化较大,但以低温元素为主。此外,矿床中闪锌矿Ge和Cu呈现较好正相关关系,暗示其与Zn置换方式为:nC u~(2+)+Ge~(2+)(n+1)Zn~(2+),这可能是该矿床富集Ge的重要原因之一;矿床中方铅矿以富集Ag、Sb贫Bi为特征,含微量Cd和Tl,类质同象是这些元素主要赋存形式,其置换方式为(Ag)~(1+)+(Sb)~(3+)2Pb~(2+);矿床中Ge主要赋存于闪锌矿中,而方铅矿中不含Ge。总体上,本矿床硫化物微量元素组成与MVT型矿床基本一致,明显有别于喷流沉积型矿床、岩浆热液型矿床和远源夕卡岩型矿床,其成矿温度属于低温范围,成矿流体运移方向可能为深部→浅部。结合其矿床地质地球化学特征,本文认为天宝山铅锌矿床属于MVT型矿床,但其中闪锌矿中富集Cu,而方铅矿中富集Ag,可能暗示其形成具有一定特殊性。     

云南大坪金矿白钨矿微量元素、稀土元素和Sr-Nd同位素组成特征及其意义

利用ICP-MS和同位素质谱分析了大坪金矿含金石英脉中白钨矿的微量元素、稀土元素和Sm-Nd、Rb-Sr同位素组成,结果显示大坪白钨矿中富Sr、Ba,而亏损Mo、Bi、Sn、Nb、Ta等,指示原始成矿流体与岩浆的结晶分异作用无关,并非前人普遍认为的岩浆水和大气降水的混合流体;样品的REE球粒陨石标准化配分曲线为高度一致的右倾和MREE富集型,Eu出现正异常,表明白钨矿与流体之间REE元素发生了强烈分异,白钨矿中REE的配分行为主要表现为REE^3＋与Na^＋成化合价补偿形式替代Ca^2＋选择性进入白钨矿晶格中,成矿流体是相对封闭的高温、富Na^＋的还原性热液体系;Sr-Nd同位素组成显示本区原始成矿流体主要来自下地壳,但不排除有幔源物质加入.原始成矿流体的形成与区域性剪切带的活动有关,韧性剪切作用导致下地壳富CO2流体上升,并与闪长岩发生强烈的水岩反应,而剪切带中脆性断裂的形成是成矿流体迁移、集中、沸腾和矿质沉淀的触发因素.     

云南永平卓潘新生代碱性杂岩体的元素地球化学和Nd-Sr-Pb同位素特征及地质意义

云南永平卓潘碱性杂岩体是金沙江-哀牢山富碱斑岩带中的重要岩体,位于兰坪走滑拉分盆地西部.该岩体的主要岩石类型为辉石正长岩、正长岩和霞石正长岩等.岩石地球化学显示高碱、高K2O/Na2O比值、低TiO2、高Al2O3的超钾质钾玄岩系列岩石.微量元素表现为富集K、Rb、Sr、Ba等大离子亲石元素（LILE）,亏损Ta、Nb、Ti、P、Zr等高场强元素（HFSE）,表现为明显的“Ta-Nb-Ti”负异常,同时富集过渡性元素Cr、Ni、Co等.稀土元素为稀土总量高,富集LREE、（La/Yb）N比值高、Eu异常不明显为特征.（^87 Sr/^86 Sr）i比值为0.707611～0.709167、εNd（t）值为-4.89～-6.57,206Pb/204Pb变化于18.6886～18.7022、207Pb/204Pb变化于15.6169～15.63493、208Pb/204Pb变化于38.7972～38.8927,显示岩浆源区具有富集地幔（EM Ⅱ）与下地壳共同作用的壳幔过渡带特征,构造环境判别为大陆弧（CAP）环境.岩体是在碰撞造山过程中大陆板片俯冲,使曾经遭受古洋壳板片流体强烈交代的壳幔过渡带在上升的软流圈熔体的注入而发生部分熔融,在走滑拉分盆地的核心部位沿构造断裂上升侵位而形成.     

扬子板块周缘MVT型铅锌矿床闪锌矿微量元素组成特征与指示意义:LA-ICPMS研究

扬子板块周缘铅锌多金属成矿带内分布着数以百计的沉积岩容矿型铅锌矿床,它们不仅是我国主要的铅锌矿产地,同时也是重要的稀散元素(Ge、Ga等)生产基地。本次研究采用LA-ICPMS技术分别测定了扬子板块西南缘的会泽铅锌矿床、金沙厂铅锌矿床、大梁子铅锌矿床,扬子板块北缘的马元铅锌矿床以及扬子板块东南缘的凤凰茶田锌(铅)汞矿床中闪锌矿的微量元素组成,以揭示闪锌矿中微量元素(稀散元素)的富集规律和赋存状态,并为矿床成因类型的厘定及稀散元素矿产资源综合利用提供更多依据。LA-ICPMS微量元素测定结果显示闪锌矿中不同微量元素(稀散元素)分布不均匀,但这些矿床中闪锌矿总体以富集稀散元素Ge、Ga、Cd,贫In、Se、Tl、Te为特征,其Fe、Mn含量要明显低于与岩浆热液有关的高温闪锌矿,指示了扬子板块周缘铅锌矿床可能形成于中-低温成矿流体,而与岩浆热液无直接的成因联系,此外这些矿床中闪锌矿富Ge贫In的特征与其他的密西西比河谷型铅锌矿床(MVT)一致。同时,本次研究综合分析了闪锌矿中不同微量元素(稀散元素)之间的相关关系,并与闪锌矿微量元素LA-ICPMS时间分辨率特征相结合,研究表明:这些铅锌矿床中稀散元素Ge可能主要通过3Zn2+?Ge4++2(Cu+,Ag+)和2Zn2+?Ge4++□(晶体空位)的替代方式进入闪锌矿,Ga在闪锌矿中富集机理主要为2Zn2+?(Cu,Ag)++(Ga,As,Sb)3+。此外,为进一步揭示不同成因类型铅锌矿床中稀散元素的富集规律,本文还系统对比了全球范围内不同类型铅锌矿床闪锌矿的稀散元素(均为LA-ICPMS数据)组成特征,并初步探讨了造成不同成因闪锌矿中稀散元素(Ge、Ga和In)差异性富集的主要控制因素,研究表明:(1) Ge在中低温盆地卤水成矿系统(MVT和SEDEX矿床)和岩浆-火山热液成矿系统(浅成脉状铅锌矿床和VMS矿床)形成的闪锌矿中均可能富集成矿,但中低温浅成脉状矿床中Ge的富集程度要明显高于高温脉状矿床,指示了成矿温度是控制闪锌矿中Ge富集的一个重要因素。(2)铅锌矿床闪锌矿中In主要为岩浆来源,In倾向于在成矿温度较高的岩浆及火山热液成因铅锌矿床中富集成矿,而壳源的MVT和SEDEX型铅锌矿床中闪锌矿均贫In。可见除形成温度外,成矿物质来源是决定闪锌矿是否富In的关键因素。(3)除矽卡岩型铅锌矿床外,其他不同成因类型、不同形成温度的铅锌矿床中闪锌矿均可能富Ga。矽卡岩型铅锌矿床闪锌矿具有明显的贫Ga、Ge的特征,这可能是由于矽卡岩化过程中稀散元素Ga、Ge大量进入早期矽卡岩矿物,进而导致了成矿流体以及随后形成的闪锌矿中Ga、Ge的贫化。综上所述,闪锌矿中稀散元素富集与否和富集程度受成矿物质来源、成矿流体性质以及流体演化过程等多因素的综合控制。(4)扬子板块周缘铅锌矿床闪锌矿的微量元素(稀散元素)组成特征指示了它们形成于中低温成矿环境,稀散元素的富集规律与其它MVT型铅锌矿床类似。     

Origine du matériel altéré et hétérogénéité du matériel originel de la météorite Foum Tatahouine (Tunisie) : éléments traces et isotopes du SrOrigin of altered material and heterogeneity of original material in the Foum Tatahouine meteorite (Tunisia): Trace elements and Sr isotopes

Foum Tatahouine is one of the rare meteorites that allow studying weathering effects since their fall on Earth. The comparison between clasts collected the very day of the fall (1931) and in 1994 showed some chemical modifications. We report data obtained on samples collected in 2000. The purpose of this study is: (1) to define (REE, Sr isotopes) the terrestrial weathering origin that is to be found in soil carbonates, (2) to show (Sr, Rb, REE) that the weathering effects are small on centimetre-sized samples, and (3) to emphasise (HREE) Foum Tatahouine samples heterogeneity. To cite this article: Y. Bentahila et al., C. R. Geoscience 334 (2002) 267–272.