Late Cenomanian environmental conditions at the submerged Tatric Ridge,Central Western Carpathians during the period preceding Oceanic Anoxic Event 2 – A palaeontological and isotopic approach

Micropalaeontological and isotopic studies of the upper Cenomanian turbiditic/hemipelagic sediments from the High-Tatric unit (Central Western Carpathians; Polish part of the Tatra Mountains) has been undertaken to characterize the sedimentary conditions in the Tatric basin, a part of the Western Tethys, related to the interval preceding the late Cenomanian oceanic anoxic event (OAE2). The deposition of these sediments, including organic-rich layers (TOC up to 0.7%), corresponds to the Rotalipora cushmani foraminiferal Zone. Microfacial, foraminiferal and palynological analyses show that the sea floor was located at upper bathyal depths and the water column was poorly oxygenated. The intrabasinal carbonate material indicates moderate primary productivity with rare periods of upwellings. The scarcity of marine fossils in redeposited material and features of carbonate lithoclasts suggest very low productivity in the nearshore surface water, most probably due to a low-density hyposaline cap as surface runoff from the southern margin of the basin. The carbon isotopic study documents the negative values of δ¹³C_carb in the whole section as an effect of transfer of isotopically light carbon sourced from various sources. Such negative values of δ¹³C_carb are characteristic of the upper Cenomanian sediments, deposited in relatively shallow water basins, characterized by input of terrestrial organic matter and/or carbonate particles known from the Western Interior sections, the Atlantic coastal plain, the northwestern African margin, the eastern margin of the Apulian Platform and shelf sediments in the NW Europe and Tethyan Himalayas. Most probably, all of these events could be related to the global sea level fluctuations that occurred ca. 95.5–94.5 Ma comparing with the Haq (2014) eustatic curve.     

湘西合仁坪金矿床硫、铅同位素地球化学

湘西柳林汊一带广泛分布钠长石_石英脉型金矿,合仁坪金矿床是其典型代表。文章对合仁坪金矿床的硫、铅同位素进行了研究,并与区域石英脉型金矿床进行对比,探讨了该矿的成矿物质来源,并初步确定了其矿床成因。研究表明,合仁坪金矿床硫化物的δ34S值范围较窄(-4.8‰~4.4‰),平均为-0.6‰,该矿床的硫为深源硫,由深部变质流体带入;铅的同位素组成较均一,并表现出明显的造山带铅的特点。结合区域成矿作用,进一步研究揭示,合仁坪金矿床为一典型的造山型金矿,其成矿可能与湘西雪峰山地区加里东期的造山作用有关。     

Iron isotopic systematics of UHP eclogites respond to oxidizing fluid during exhumation

The iron stable isotope compositions (δ⁵⁶Fe) and iron valence states of ultrahigh‐pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe³⁺/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid‐ocean ridge basalts (MORB)‐like (~0.10‰) δ⁵⁶Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle‐derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ⁵⁶Fe variation ranging from ?0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ⁵⁶Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe³⁺/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe³⁺/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe³⁺/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ⁵⁶Fe values and Fe³⁺/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe‐isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ²⁶Mg_{omphacite‐garnet} values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe³⁺/ΣFe ratios of omphacite from retrograde eclogites and variant Δ⁵⁶Fe_{omphacite‐garnet} values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ⁵⁶Fe values of omphacite during retrograde metamorphism.     

大兴安岭北段争光金矿床成因探讨:来自流体包裹体及稳定同位素的制约

争光浅成低温热液型金矿床位于大兴安岭成矿带北段,是多宝山矿集区内的一个重要矿床。文章通过流体包裹体和C_H_O_He_Ar同位素的系统研究,对该矿床成矿流体和矿床成因进行了深入探讨。矿床成矿作用可划分为4个主要阶段:石英_黄铁矿阶段(成矿前阶段)、石英_多金属硫化物阶段(主成矿阶段)、方解石_(石英)_多金属硫化物阶段(主成矿阶段)和方解石阶段(成矿后阶段)。流体包裹体研究表明,争光金矿床主要发育富液相流体包裹体。石英_黄铁矿阶段、方解石_(石英)_多金属硫化物阶段和方解石阶段流体包裹体的均一温度分别介于116~243℃(集中于150~170℃)、129~294℃(集中于140~160℃)和130~155℃(集中于130~150℃);w(NaCleq)分别介于0.9%~10.1%、1.2%~13.8%和2.7%~8.7%。成矿流体具有低温、低盐度、相对还原的特征,属H_2O_Na Cl体系。石英_黄铁矿阶段成矿流体的δD和δ18O分别为-127‰~-110‰和-5.9‰~0.6‰,蚀变围岩的δD值和δ18O值分别为-118‰~-108‰和6.3‰~7.9‰。方解石_(石英)_多金属硫化物阶段和方解石阶段方解石的δ~(13)C分别为-5.3‰~-2.0‰和-2.9‰~-2.2‰,δ18O分别为7.7‰~9.3‰和9.9‰~13.5‰。黄铁矿流体包裹体的~3He/~4He、~(40)Ar/~(36)Ar和~(40)Ar*/4He比值分别为1.75~3.06 Ra、683~1295和0.30~0.63。综合流体包裹体特征和稳定同位素组成,认为成矿早阶段成矿流体为大气降水与围岩发生水_岩反应后的演化水。随着成矿作用的进行,成矿流体变为大气降水与岩浆水的混合水,但仍以大气降水为主导。成矿流体与贫H_2S的流体混合和硫化物沉淀的共同作用可能是该矿床金沉淀的主要机制。     

莱州湾沿岸寿光、莱州和龙口地下水的稳定同位素与地球化学

依据地下水咸化机理:蒸发、溶解和混合,海水入侵是地下淡水与较之更咸的海水的混合,海水和淡水的混合包括水分子的混合和溶解盐分的混合。只有溶剂水分子和溶质溶解盐分都发生了相同比例的混合,才能确定二种水体发生了混合。地下淡水变咸并不能都归因于海水入侵,也有可能源于蒸发或溶解。鉴定是否存在海水入侵可运用惰性示踪剂和反应示踪剂来共同识别。惰性示踪剂包括δD、δ~(18)O、Cl~-和Br~-,反应示踪剂包括主要化学成分和微量元素。本研究对莱州湾沿岸寿光、莱州和龙口的地下水进行了化学成分形成过程的对比研究。莱州采集样品6件,龙口10件,寿光9件。莱州和龙口的碎屑沉积物以硅酸盐矿物为主,而寿光的沉积物来源于南部鲁中南山地的碳酸盐岩。根据莱州采样点地理条件和样品的稳定同位素δD-δ~(18)O分布域确定了当地降雨入渗的多年平均值,进而明确龙口样品也源于就地降雨入渗补给,根据高程效应和当地水文情况确定寿光样品补给来自南部山区的降水,经河道排入洼地后入渗地下。Cl~--δ~(18)O关系表明盐分都来源于淡水溶解的蒸发盐,而非与海水混合。Br~-/Cl~-比值证实了蒸发盐溶解,还存在人为污染物和海侵沉积物中有机质分解的影响。从三线图可看出龙口样品经历了离子交换反应。主要成分和微量元素(HCO_3~-,H_2SiO_3,F~-,Li~+,Sr~(2+)和Ba~(2+))的含量与样品所处含水层的矿物组成有关,反映出水岩作用。所有样品都没有淡水和海水混合的现象。惰性示踪剂分析提供了区域地下水补给的框架,明确了大气水源因溶解蒸发盐而咸化,排除了淡水和海水混合的可能性。用反应示踪剂中主要化学成分和微量元素来分析水中化学成分的形成过程,在和海洋盐分输入对比基础上,二者偏差值主要源自水-岩相互作用结果,包括溶解、沉淀、离子交换等,以及局部样品的污染物输入。所以沿海地带研究地下水咸化,先要解决溶剂的混合问题,在此基础上辅以水文地球化学分析,利用水化学的多种示踪剂,主要解决溶质混合以及盐分来源问题。     

Origin and evolution of geothermal fluids from Las Tres Vírgenes and Cerro Prieto fields,Mexico – Co-genetic volcanic activity and paleoclimatic constraints

Major and trace elements, noble gases, and stable (δD, δ¹⁸O) and cosmogenic (³H, ¹⁴C) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Vírgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured ³He/⁴He (R) ratios normalized to the atmospheric value (R_a = 1.386 × 10⁻⁶) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U–Th/⁴He ages for CP fluids (0.7–7 Ma) suggest the presence of a sustained ⁴He flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5–3 Ma). Radiogenic in situ ⁴He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 ± 0.11 pmC – 35.61 ± 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene – Early Holocene meteoric water (58–75%) with a fossil seawater component (25–42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. δD values for the LTV meteoric end-member, which are 20‰–44‰ depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9–13.6 °C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonic–volcanic activity in the Las Tres Vírgenes region. Fast hydrothermal alteration processes caused a secondary, positive δ¹⁸O-shift from 4‰ to 6‰ for LTV and from 2‰ to 4‰ for CP geothermal fluids since the Late Glacial infiltration.     

Application of Zn isotopes in environmental impact assessment of Zn–Pb metallurgical industries: A mini review

Zn and Pb smelters are the major contributors to Zn and Pb emissions among all anthropogenic sources, thus, it is essential to understand Zn isotopic variations within the context of metallurgical industries, as well as its fractionation in different environments impacted by smelting activities. This mini review outlines the current state of knowledge on Zn isotopic fractionation during the high-temperature roasting process in Zn and Pb refineries; δ⁶⁶Zn values variations in air emissions, slags and effluents from the smelters in comparison to the geogenic Zn isotopic signature of ores formation and weathering. In order to assess the environmental impact of these smelters, the available and measured δ⁶⁶Zn values are compiled for smelter impacted natural water bodies (groundwater, stream and river water), sediments (lake and reservoir) and soils (peat bog soil, inland soil). Finally, the discussion is extended to the fractionation induced during numerous physicochemical reactions and transformations, i.e. adsorption, precipitation as well as both inorganic and organic surface complexation.     

Water-rock-tailings interactions and sources of sulfur and metals in the subtropical mining region of Taxco,Guerrero (southern Mexico): A multi-isotopic approach

Multi-isotope (H, O, S, Sr, Pb) systems coupled with conventional (major and trace element) hydrogeochemical analysis were applied to determine the origin of water, to model water-rock-tailings interactions and for source apportionment of sulfur and associated toxic metals in the mining region of Taxco, Guerrero in southern Mexico. Oxygen and H isotopes indicate that meteoric water in the zone is rainwater undergoing varying degrees of isotopic fractionation by atmospheric evaporation whereas Sr isotopes trace the interaction of pristine water from volcanics of the regional recharge zone and subsequently flowing through sandstone and shale to spring points. Leachates form from two distinctive sources (spring water and surface water) having differential interactions with bedrocks prior to entering the tailings. Compared to pristine water, leachates are enriched in sulfate, metals (e.g. Fe, Mn, Pb and Zn) and metalloids (e.g. As). The sulfur isotopic composition of ore-sulfides, leachates, secondary precipitates, regional surface water and hypogenic sulfates is described in terms of a two-component mixing model with shale of Mexcala and limestone of Morelos formations representing the light and heavy end-members, respectively, whereas Sr isotopic composition is bracketed combining three lithogenic (Mexcala/Morelos, Tilzapotla and Taxco Schist) sources. Finally, leachates have a mixture of lead from ore-sulfides and Taxco Schist Formation (Family I) or from ore-sulfides alone (Family II). The application of multiple environmental isotopic techniques is an outstanding tool for elucidating complex interactions of water with bedrocks and tailings and for determining the source of sulfur and toxic metal from mining and other metal polluted environments.     

Magma associations in Ediacaran granitoids of the Cachoeirinha‒Salgueiro and Alto Pajeú terranes,northeastern Brazil: Forty years of studies

Granitic magmatism in the Cachoeirinha‒Salgueiro and Alto Pajeú terranes in the Transversal Zone Domain of the Borborema Province, northeastern Brazil, occurred in three main time intervals: 650–620 Ma, 590–560 Ma and 545–520 Ma. The oldest one is characterized by intrusions of magmatic-epidote (mEp) bearing calc-alkalic (some with trondhjemitic affinities) and high-K calc-alkalic plutons, synkinematic to the main regional foliation, under contractional tectonic regime, and exhibits T_DM < 2.0 Ga and ƐNd (0.6 Ga) from −1 to −4, and δ¹⁸O (zircon) values from 7.1 to 10‰_VSMOW. O- and Nd-isotope data for the 650‒620 Ma group of plutons is compatible with partial fusion of subducted oceanic basaltic crust (mEp-bearing calc-alkalic tonalites/granodiorites, equivalent to adakites). Voluminous intrusions in the 590–560 Ma interval are represented by abundant mEp-free high-K calc-alkalic, peralkalic, ultrapotassic, mEp-bearing high-K calc-alkalic, and less abundant shoshonitic magmas. Nd-model ages for this group of plutons vary from 1.5 to 2.5 Ga and ƐNd (0.6 Ga) ranges from −8 to −20; δ¹⁸O (zircon) varies from 6.4 to 7.9‰_VSMOW. Values of δ¹⁸O (zircon) for the 590‒560 Ma old group of plutons coupled with Nd isotope data are compatible with remelting of crustal (negative ƐNd, 1.6 to 2.0 Ga old) source rocks. O- and Nd-isotope data for this group of plutons are compatible with underplating of basaltic magma in the base of the lower crust for the high-K calc-alkalic granitoids, coeval to transcurrent movements along sigmoidal shear zones. Intrusion of one shoshonitic (Serrote do Arapuá), one calc-alkalic (Riacho do Icó) plutons besides the peralkalic Manaíra-Princeza Isabel dike set have witnessed this transition from contractional to transcurrent movements along shear zones, around 610‒600 Ma.     

湖南湘潭锰矿的地球化学特征及成矿机制

湘潭锰矿床的锰矿层赋存于新元古代南华系(成冰系)大塘坡组底部含锰黑色页岩中,含锰矿物主要为菱锰矿。湘潭锰矿的Fe/Mn值低,Th/U、V/(V+Ni)和V/Cr值等地球化学指标显示其发育在氧化-次氧化的沉积环境中,暗示菱锰矿并不是由Mn~(2+)和CO_3~(2-)直接沉淀形成的。湘潭锰矿稀土元素含量高,稀土元素配分模式存在轻微的中稀土元素富集,具有明显的Ce正异常,这些特征指示湘潭锰矿含锰矿物是以锰氧化物或氢氧化物的形式沉淀的。同时,锰矿的碳同位素富集碳的轻同位素,说明有机物参与了菱锰矿的形成过程。综合分析表明,湘潭锰矿成矿过程可以分为沉淀和转化两个阶段:在氧化性的水体中,Mn以氧化物或氢氧化物的形式沉淀;在缺氧且富含有机物质的成岩环境中,Mn氧化物或氢氧化物被有机物还原而转化生成菱锰矿。这与华南地区其他几个典型的大塘坡式锰矿的成矿机制一致。