Diagenetic alunite in clastic sequences, Kuwait, Arabian Gulf

Diagenetic alunite occurs with calcrete, gypcrete and silcrete deposits in a Mio-Pleistocene clastic sequence at several locations in southern Kuwait, Arabian Gulf. Based on their physical properties and textural characteristics the alunite deposits were divided into (1) chalky quartzitic alunite, (2) chalky quartz-free alunite, (3) hard pink alunitic sandstone. The chalky alunite is composed mainly of hydronium-alunite (H₃O)Al₃(SO₄)₂(OH)₆, while the hard pink alunite is composed solely of well developed potassium alunite KAl₃(SO₄)₂(OH)₆. These minerals resulted from the action of sulphuric acid on clays and K-feldspars in the muddy sandstone and mudstone host sediments. The sulphuric acid is most probably produced by the oxidation of hydrogen sulphide that might have seeped in from the oil fields of this area. Four diagenetic stages are suggested for the genesis of the studied alunites: gypsification, sulphuratization, silicification and alunitization. In oil field areas, the occurrences of alunite would serve as an indicator for the presence at depth of hydrocarbons.     

Geochemical and mineralogical control of different genetic types of deep-sea nodules from the Pacific Ocean

Deep-sea nodules from the Northeast Pacific nodule belt and the Southeast Pacific (Sonne Basin), being formed in areas bordering the equatorial zone of high biological productivity, accumulate by two basically different growth processes: (A) early diagenetic growth by supply from pore water and (B) hydrogenetic growth by supply from near-bottom sea-water. These growth processes lead to different genetic types of nodules: early diagenetic type A, hydrogenetic type B, and mixed-type AB; a further type A_C, very rich in Mn, is being formed by increasing influence of early diagenesis. These types can clearly be distinguished by their shapes, surface textures, mineral constituents of oxide fraction, internal microstructures, and geochemistry. A genetical classification is being proposed on the basis of statistically computed interelement relationships. Todorokite, very poor in Fe, is the main Mn phase in the early diagenetic substance; -MnO₂ intimately intergrown with FeOOH · xH₂O is the main phase in the hydrogenetic substance. Consequently an important difference can be pointed out: the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of Me²⁺ (e. g. Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺), whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles. Mobilization of Mn²⁺ and fractionation from Fe is controlled by the amount of decomposing organic matter in the "peneliquid" layer of the sediments. The main factor controlling the intensity of early diagenesis is the biological productivity in surface waters. The crucial "point of reversal" at a Mn/Fe ratio of about 5, obtained by hyperbolical regression of the analyses of nodules from the Southeast Pacific, represents best concentrations in Ni and Cu. Mn/Fe quotients greater than 5 cause a decrease of Ni and Cu content. Nodules from the Northeast Pacific nodule belt generally contain higher concentrations in Cu than nodules from the Southeast Pacific. This can be explained by an additional supply of Cu transported below CCD by siliceous plankton.     

Nodular silcretes of the Cypress Hills Formation (upper Eocene to middle Miocene) of southern Saskatchewan, Canada

Siliceous nodules in the upper Eocene to middle Miocene Cypress Hills Formation in southern Saskatchewan formed on the distal portions of an extensive braidplain. The nodules are similar to silcretes described elsewhere in the literature but their morphology and occurrence suggests that they are of a rare variety. The silcretes are discrete equant to disc-shaped nodules, 1–3 cm thick, and up to 15 cm long. The silicretes form horizontal, discontinous layers parallel to bedding within an unweathered profile. Void spaces and fractures within the nodules are lined or filled with drusy quartz. The silcretes contain less than 0.07% TiO₂. The TiO₂/SiO²/Al²O³ and TiO²/SiO²/Fe²O³ ratios are similar to values obtained from nodules formed in an arid to semi-arid environment based on comparison with modern silcretes. This interpretation is confirmed by independent sedimentological and palaeontological climatic evidence for an arid to semi-arid climate throughout Oligocene time in the western Canadian plains.     

安徽庐枞盆地酸性蚀变岩帽地质地球化学特征研究

酸性蚀变岩帽是浅成低温热液系统演化的产物,形成于酸性高氧化性流体的化学条件下;在高硫化型浅成低温热液金矿床中广泛发育,是该类矿床的显著识别特征。通过对酸性蚀变岩帽的野外地质特征、矿物共生组合和地球化学特征研究,能较好阐明浅成低温成矿热液系统的特征、性质、发生和发展演化及成矿作用过程。庐枞矿集区是长江中下游成矿带重要的矿集区之一,盆地内广泛发育以明矾石为特征蚀变矿物的酸性蚀变岩帽,面积超过30km~2,指示盆地内高硫化浅成低温热液系统的存在。目前为止,前期工作主要针对明矾石矿床地质特征和明矾石资源储量进行,该酸性蚀变岩帽的地质地球化学特征研究尚未开展。本次工作通过对酸性蚀变岩帽系统的野外采样、全岩地球化学分析和短波红外光谱测试分析技术(PNIRS测试)分析,确定其主要赋存在砖桥组火山岩中,组成矿物为石英、明矾石、高岭石、地开石,此外有少量绢云母、伊利石、珍珠陶土、叶蜡石、褐铁矿,极少数的叶腊石和黄钾铁矾等,在钻孔深部存在浸染状和半自形粒状黄铁矿。由于受到地表风化剥蚀和不同热热中心的影响,水平方向从矾山明矾石矿床向外围发育石英+明矾石带、石英+高岭石/地开石+明矾石带、石英+高岭石/地开石带、硅化带以及最外围的泥质带即高岭石±绢云母±伊利石带。根据酸性蚀变岩帽的矿物组合和主量元素特征,可将其分为三类:硅质蚀变岩、明矾石蚀变岩和粘土蚀变岩。硅质蚀变岩中SiO_2含量发生明显的富集作用,其余主量元素(K_2O、Na_2O、Al_2O_3、Fe_2O_3、P_2O5)含量显著降低;明矾石蚀变岩和粘土蚀变岩具有相似的地球化学特征,SiO_2、Al_2O_3、Fe_2O_3、P_2O_5元素含量范围变大,K_2O和Na_2O含量降低,且Na_2O降低更加明显;而钛为不活泼元素,在岩石发生蚀变过程中TiO_2含量变化很小。矾山地区的酸性蚀变岩帽的产状、蚀变类型、地球化学特征受构造和地层的双重控制。     

邵东保和堂大冶灰岩沉积构造特征

保和堂大冶灰岩中发育丰富的沉积构造,对之研究有助于保和堂、乃至整个湘中的大冶灰岩形成环境的探讨。其第一段岩石发育了丘状波痕、丘状交错层理及粒序层理,是风暴作用的反映;泥裂、水雹痕属于暴露环境。说明大冶灰岩沉积的早期仍属潮上或潮间碳酸盐台地相。中期海水加深,发育了水平层理、变形层理及包卷层理等,近于台地边缘斜坡相。晚期的粒序层理、虫迹等构造,反映了海水再度变浅的趋势。     

Pressure-oxidation autoclave as an analogue for acid–sulphate alteration in epithermal systems

Gold extraction at the Macraes gold mine in New Zealand involves concentration of pyrite and arsenopyrite, oxidation of those sulphides, then cyanidation. The ore concentrate is predominantly Otago Schist host rock (andesitic composition) with up to 15% sulphides. The oxidation step is conducted on ore concentrate slurry in an autoclave at 225°C and 3,800 kPa oxygen gas pressure with continuous feed. The slurry takes ca. 1 h to pass through the autoclave, during which time the sulphides are almost completely oxidised. Sulphide oxidation causes strong acidification of the slurry, which is maintained at pH of 1–2 by addition of CaCO₃. Scales form on walls in the autoclave, with minerals reflecting progressive oxidation and alteration of the ore through the system. The schist in the ore feed has mineralogy similar to propylitically altered andesite: quartz, albite, muscovite, chlorite, and pyrite. Muscovite undergoes almost complete dissolution, with associated precipitation of quartz and alunite (KAl₃(SO₄)₂(OH)₆). Other principal minerals deposited and discharged include anhydrite (and/or gypsum), jarosite (KFe₃(SO₄)₂(OH)₆), hematite (and/or amorphous iron oxyhydroxide), and amorphous arsenates. Dissolved ferrous iron passes right through the autoclave, and variably hydrated Fe²⁺and Fe³⁺sulphate minerals, including rozenite and szomolnokite (both FeSO₄.hydrate) and ferricopiapite (Fe₅(SO₄)₆O(OH).hydrate), are formed along the way. The autoclave chemical system resembles acid–sulphate hydrothermal activity in geothermal systems and high-sulphidation epithermal mineral deposits formed in arc environments. These natural acid–sulphate systems are pervaded by volcanic vapours in the near-surface environment, where widespread dissolution of host rocks occurs and deposition of quartz, alunite, and anhydrite is common. Some of the volume loss associated with these natural systems may be due to dissolution of soluble sulphate minerals by later-stage groundwater incursion.     

Diagenetic mobilization of manganese in Peru Basin sediments

The Mn contents and sedimentation rates of two cores from different areas of the Peru Basin have been determined. The southern core is associated with Mn nodules of conventional slow accretion rates (~ mm/10⁶ yr) while the northern one accompanies nodules with very high growth rates (~10² mm/10⁶ yr). The depletion of Mn observed within the top 40 cm of the cores is interpreted as resulting from a diagenetic remobilization of Mn⁺² due to the oxidative characteristics of the sedimentary column. In both cores, the calculated flux of Mn provided by the sediment is higher than the total Mn recovered in the overlying nodules. This indicates that the Mn content of the Peru Basin nodules can be supplied by diagenetic processes from the sediment, not only for the slowly growing nodules but also for the extremely fast growing concretions found in the north of the basin.     

Silicification of sulphate evaporites and their carbonate replacements in Eocene marine sediments, Tunisia: two diagenetic trends

Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ¹³C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ¹⁸O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ¹⁸O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ¹⁸O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ¹⁸O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ²⁹Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.     

Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China

Molar-tooth (MT) structure is an enigmatic sedimentary structure consisting of variously-shaped cracks and voids filled with a characteristically uniform, equant calcite microspar. It is globally distributed but temporally restricted to rocks from Neoarchean to Neoproterozoic age. The origin of MT structures has been debated for more than a century and the topic continues to be highly contentious. Some features of MT structure occurring in micritic limestones of the Mesoproterozoic Gaoyuzhuang Formation (ca. 1500 Ma to ca. 1400 Ma), Jixian section, Tianjin City, North China show that: 1) there is a definite interface or lining, rich in organic material and pyrite, between the MT crack-filling calcite microspar and the micritic host rock, which is also rich in organic matter; 2) the micritic host rocks are notable for the absence of stromatolites and microbial laminites; 3) distinctive conglomeratic lag deposits made up of intraclasts of MT microspar result from storm reworking of the MT structures; 4) the MT structure is associated with possible algal megafossils such as Chuaria; 5) the MT microspar is made up of the larger calcite crystal and the MT crack is marked by the diversity of configurations; 6) both the TOC content and the carbon-isotopic value (δ13CPDB) among the host rock, the MT microspar and the possible algae fossil are obviously different. For the forming mechanism of the Gaoyuzhuang MT structure, these features can still indicate that: A) the MT microspar was formed by rapid precipitation and lithification; B) the MT microspar precipitated directly within the cracks; C) the decomposition of organic matter within the host micrite might be the chief mechanism producing gas bubbles; D) microscale gas-sediment interaction led to the generation of the MT cracks and the precipitation of microspar therein; E) the MT cracks might represent the track of migration and expansion of gas bubbles, and that the recrystallization of host micrites cannot be eliminated during forming process of the MT microspar; F) the MT structure is occurred in early diagenetic period; and G) the formation of MT microspars is a complex diagenetic process. Therefore, model of the microbially-induced gas-bubble expansion and migration is the best interpretation for the formation of the MT structure. Effectively, MT structures are a type of sedimentary structure that is formed in the early diagenetic period and is related to microbial activities and organic matter degradation.     

On the deformation sequence in the New Harbour Group of Holy Island,Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D₂ age. An earlier preferred orientation (S₁) is commonly preserved as crenulations within the Q-domain microlithons of the S₂ schistosity and is demonstrably non-parallel to bedding. F₃ folds are widely developed in S₂ and, to a lesser extent, in bedding. S₃ crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D₃ age since it clearly deforms S₂ schistosity and is consistent with the vergence of F₃ minor structures. All planar structures are deformed by folds of D₄ age. © 1997 John Wiley & Sons, Ltd.     

New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au–Ag and Cordilleran polymetallic ores

We present ⁴⁰Ar/³⁹Ar data acquired by infra-red (CO₂) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au–(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO₂-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9 Ma and lasted until ∼10.6 Ma. Four alunite samples associated with high-sulfidation epithermal Au–(Ag) ore gave ⁴⁰Ar/³⁹Ar plateau ages between ∼11.9 and ∼11.1 Ma (compared to the previously documented ∼11.6 to ∼11.3 Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au–(Ag) deposition period can be estimated to at least 0.4 My. Three new ⁴⁰Ar/³⁹Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06 Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au–(Ag) and the base-metal Cordilleran ores may suggest that SO₂-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO₂-bearing vapor degassing longer than 11.9 to 10.6 Ma may be recognized with further more detailed work. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molar tooth structures of the Neoarchean Monteville Formation, Transvaal Supergroup, South Africa. I: Constraints on microcrystalline CaCO3 precipitation

Molar tooth (MT) structures are enigmatic, contorted millimetre‐ to decimetre‐long veins and spheroids of microcrystalline calcite that formed during very early diagenesis in Precambrian sediments. MT structures in the ca 2·6 Ga Monteville Formation are 600–800 Myr older than previously reported occurrences and establish that conditions necessary for MT genesis were met locally throughout much of the Precambrian. In the Monteville Formation, MT structures were formed shallow subtidally, extending to depths near storm wave base, in shale host sediments intercalated with storm‐generated carbonate sand lenses. They are filled with microcrystalline calcite and rare pyrite. Microcrystalline calcite identical to that in MT structures fills other pore space, including porosity between grains in carbonate sand lenses, moldic porosity in sand grains, sheet cracks in columnar stromatolites, and shallow cracks on sandy bedding planes. Relationships in the Monteville Formation demonstrate that microcrystalline CaCO₃ precipitated in fluid‐filled cracks and pores; microcrystalline calcite characteristics, as well as the paucity of carbonate mud in host rocks, are inconsistent with injection of lime mud as the origin of MT structures. Locally, MT cracks were filled by detrital sediment before or during precipitation. Precipitation occurred in stages, and MT CaCO₃ evolved from granular cores to a rigid mass of cores with overgrowths – allowing both plastic and brittle deformation of MT structures, as well as reworking of eroded MT structures as rigid clasts and lime mud. Crystal size distributions and morphology suggest that cores precipitated through nucleation, Ostwald ripening and size‐dependent crystal growth, whereas overgrowths formed during size‐independent crystal growth.     

Processes controlling the heavy metal distribution in pacific ferromanganese nodules and crusts

The ferromanganese precipitates existing in deep-sea waters of the Pacific consist of two types of deposits: (1) nodules mainly are distributed in pelagic basins beneath the CCD (Calcite Compensation Depth) where the rate of sedimentation is low; (2) polymetallic encrustations are formed on exposed seamount rocks where currents prevent normal sediment accumulation. Nodules, being formed in areas bordering the equatorial zone of high biological productivity, grow by two different processes: (A) early diagenetic growth by supply of metals and metal compounds from pore water and (B) hydrogenetic growth by supply of colloidal particles from near-bottom seawater. These processes lead to different kinds of oxide and different metal contents. The diagenetic growth process takes place under oxidizing to suboxidizing conditions and is supplied by an ionic solution of Mn²⁺ and other divalent metal ions. The mobilization of Mn is caused by the decomposition of organic matter. The growth features of the early diagenetic nodules show alternating laminae of crystalline and amorphous material. These rhythmic sequences of different microlayers are explained by physico-chemical changes (variation of pH) in the microenvironment of the accreting nodule surface. The hydrogenetic crust growth on seamounts leads to ferromanganese precipitates which are in particular rich in Co. The Co concentration is inversely related to the water depth. Co is positively correlated to Mn which can be derived from the oxygen minimum zone. Contrary to the diagenetic nodule growth, the crust accretion is also a colloidal precipitation process. In the water column below the oxygen minimum zone, a mixture of particles of Mn-Fe-oxyhydroxide and silicate accrete together on the surface of substratum rocks. Surface chemical mechanisms control the enrichment of Ni, Co, Pb, and other metals from the seawater; for Pt, a coprecipitation with MnO₂ caused by a redox reaction is proposed. Distinct oceanographical and geological conditions enable or promote, respectively, the ferromanganese crust formation on seamounts.     

塔里木盆地库车坳陷白垩系辫状三角洲砂体成岩作用和储层特征

通过对库车坳陷白垩系辫状三角洲的研究,总结辫状三角洲的沉积特征是:1.由于辫状河流直入湖泊,沉积物比较粗,中砂岩和细砂岩占有相当的比例,可达 6 0 %～ 80 %,最高达 95 %。 2.成分和结构成熟度低-中等,一般为长石砂岩、岩屑长石砂岩和岩屑砂岩,石英含量 30 %～ 5 0 %,岩屑含量可高达 5 0 %以上,一般为 30 %左右,岩屑成分复杂。3.沉积构造以块状层理、大型槽状交错层理和斜层理为主,见平行层理、波状层理和极少量的水平层理。4.辫状河道的主要沉积期是洪水期,因而河道具快速迁移性,沉积作用具有阶段性,砂体之间多次重复叠加,在滨岸地区辫状砂体性质相似而垂向侧向连接,成为分布面积很大的砂体,砂层之间只有薄层的泥质夹层。对研究区露头和岩心的普通薄片、铸体薄片、阴极发光及扫描电镜等资料的分析研究,认为该区成岩作用包括压实、胶结、溶蚀、交代、压溶和重结晶等作用,并且以前三类为主。根据对自生矿物、粘土矿物、I/S间层中的S %、包裹体测温、成岩作用类型及特点等分析,判断研究区白垩系成岩阶段已达晚成岩A期。白垩系砂岩储层特殊的沉积-成岩水介质条件、构造-埋藏演化史、异常高压背景等决定了其特有的成岩序列演化的特点。白垩系巴什基奇克组上部和底部、巴西盖组砂     

安徽庐枞盆地矾山酸性蚀变岩帽形成时代及其地质意义

酸性蚀变岩帽是岩浆热液流体和围岩在近地表相互作用的产物,是斑岩-浅成低温热液成矿系统的重要指标。发育在长江中下游成矿带庐枞盆地内的矾山酸性蚀变岩帽产出面积较大( 20km~2)。前人对该酸性蚀变岩帽中的明矾石矿床的地质和地化特征进行了相关研究,但详细的年代学研究工作尚未开展。为精确厘定矾山酸性蚀变岩帽的形成时代,本文开展了明矾石~(40)Ar-~(39)Ar法和金红石原位U-Pb法定年。矾山酸性蚀变岩帽中明矾石共有三种类型:ⅠA型明矾石主要呈交代蚀变发生在热液蚀变早阶段,与石英、粒状黄铁矿或赤铁矿、少量金红石共生;ⅠB型明矾石形成于热液蚀变晚阶段,主要呈叶片状集合体充填在开放空间中,与石英、星点状赤铁矿、粒状金红石集合体共生,少量金红石和赤铁矿沿明矾石解理裂隙分布;Ⅱ型明矾石是表生明矾石,主要呈细粒集合体沿裂隙分布,与赤铁矿、高岭石、地开石共生。三类明矾石形成于不同环境下:ⅠA和ⅠB型明矾石形成于岩浆热液环境下,是大矾山明矾石矿区的主要产物;Ⅱ型细粒明矾石分布在矾山酸性蚀变岩帽的非明矾石矿区,是表生环境下的产物。ⅠA型明矾石的~(40)Ar-~(39)Ar定年的坪年龄为131±6Ma,代表了矾山酸性蚀变岩帽的形成时代。与Ⅱ型明矾石密切共生的金红石U-Pb定年结果为32. 7±4Ma,在该期间,整个盆地内无岩浆活动发生,该年龄反映了矾山酸性蚀变岩帽经历表生氧化作用的时间。明矾石和金红石定年结果分别对应岩浆热液和表生明矾石的形成时代。在利用明矾石进行找矿工作时需先明确明矾石成因,矾山酸性蚀变岩帽中深成明矾石是下一阶段的找矿研究的基础。     

Multiple origin and regional significance of bedding parallel veins in a fold and thrust belt: The example of a carbonate slice along the Appalachian structural front

The development and evolution of bedding parallel veins (BPV) are investigated in the Saint-Dominique carbonate slice (southern Québec Appalachian structural front), in order to emphasize the regional significance of BPV in a fold and thrust belt and their importance in establishing its structural and diagenetic evolution. Structural and microstructural analyses reveal that most BPV display a laminated structure locally crosscut by a massive one. Both structures show mutual crosscutting relationships with bedding parallel or bedding perpendicular stylolites and with bedding perpendicular veinlets, indicating that BPV cementation and deformation are continuous processes. Opening both sub-parallel and sub-perpendicular to the BPV walls are documented. Early BPV are planes of weakness that focus later reactivation, and evidence of successive and sometimes opposite senses of opening are locally preserved within a single BPV. The bedding parallel orientation of these veins proved particularly helpful in establishing the structural evolution of an area mostly characterized by front parallel structures with little crosscutting relationships otherwise. Petrographic and geochemical (δ¹⁸O _VPDB and δ¹³C) analyses of structural cements suggest a common, locally derived source for the fluids that percolated through BPV, non-bedding parallel veins and faults. The isotopic ratios also reveal a significant enrichment in ¹⁸O and ¹³C through time. This evolution is tentatively correlated with tectonic, syn-convergence exhumation of the Saint-Dominique slice during its imbrication along the Appalachian structural front, after its burial under Taconian thrust sheets.     

Composition and formation of fossil manganese nodules in Jurassic to Cretaceous radiolarites from the Nicoya Ophiolite Complex (NW Costa Rica)

Horizons of several types of Upper Jurassic to Lower Cretaceous manganese nodules occur locally in sequences of radiolarian cherts within the Nicoya Ophiolite Complex (NW Costa Rica). Field studies, X-ray diffraction analysis, petrographic, chemical and experimental studies give evidence of a sedimentary, early diagenetic origin of the nodules, in contrast to earlier suggestions. Smooth, discoidal, compact and very dense nodules with diameters of some mm to 9 cm dominate. They are characterized by braunite, hollandite, pyrolusite and quartz as well as 39–61% Mn, 0.9–1.6% Fe, 5–26% SiO₂, 1.3–1.9% A1₂O₃, 1.5–3.0% Ba, 460–5400 ppm Cu, 85–340 ppm Ni and 40–130 ppm Co, among others. It is suggested that the original mineralogy (todorokite?) was altered during thermometamorphic (braunite) and hydrothermal (hollandite, pyrolusite) events. Petrographic similarities between the fossil nodules and modern deep-sea nodules are striking. Using standard hydrothermal techniques in an experimental study it is shown that under special conditions, braunite can be produced from modern nodule material.     

Source and genesis of sulphate and phosphate–sulphate minerals in a quartz‐sandstone cave environment

Gypsum (CaSO₄·2H₂O), alunite (KAl₃(SO₄)₂(OH)₆), and rare phosphate–sulphate sanjuanite Al₂(PO₄)(SO₄)(OH) 9(H₂O) and rossiantonite (Al₃(PO₄)(SO₄) 2(OH)₂(H₂O)₁₄) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H₂S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.     

Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of the Vocontian Basin (SE France)

Several intervals rich in barite nodules occur in the mid-Cretaceous marls of the Vocontian Trough (SE France). These concretions, which grew on heterogeneities in the sediment, are arranged as horizons parallel to the bedding, and are often associated with carbonate and phosphate nodules. Detailed sedimentological and stratigraphical observations suggest that they represent the consequence of short-term pauses in sedimentation during early diagenesis, but the most important occurrences of barite are below prominent discontinuities. This may be explained by the frequency of pauses in sedimentation caused by a progressive starvation, or an increase in the energy of bottom-water currents. The high number of barite occurrences on the basin margins is directly linked to the recurrence of sedimentary breaks. The geochemical data show that the host sediment corresponds to a pelagic setting, far from hydrothermal influences, but generally in a restricted, dysoxic to suboxic, depositional environment. The sulphur isotopic composition of the barite sulphate (+16 to +76‰ rel. CDT) supports the hypothesis of an early diagenetic origin of the barite nodules. Their genesis may be explained by the fixation of a diagenetic front corresponding to the penetration of a sulphate reduction zone from the seawater–sediment interface down in the sediment which is in contact with barium-rich porewaters expelled upwards as a consequence of compaction. The barium of biogenic origin, initially stored in the underlying black shale, was mobilized under strongly reducing conditions. Such fixation is normally induced by a pause or a stop in sedimentation due to starvation or renewed bottom-water circulation. Together with other concretions, the barite nodules mark condensed stratigraphic intervals and often represent major breaks. As such they shed light on variations in sedimentary activity and their importance in the sedimentology and stratigraphy of pelagic marly sequences cannot be ignored.     

Prediction of Gibbs free energies of formation of minerals of the alunite supergroup

A method for the prediction of Gibbs free energies of formation for minerals belonging to the alunite family is proposed, based on an empirical parameter Δ_GO⁼ M^z+(c) characterizing the oxygen affinity of the cation M^z+. The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations, multiplied by the difference of oxygen affinity Δ_GO⁼ M^z+(c) between any two consecutive cations. The Δ_GO⁼ M^z+(c) value, using a weighing scheme involving the electronegativity of a cation in a specific site (12-fold coordination site, octahedral and tetrahedral) is assumed to be constant. It can be calculated by minimizing the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method gives values within 0.5% of the experimentally measured values. The relationships between Δ_GO⁼ M^z+(alunite) corresponding to the electronegativity of a cation in either dodecahedral sites, octahedral sites or tetrahedral sites and known as Δ_GO⁼ M^z+(aq) were determined, thereby allowing the prediction of the electronegativity of rare earth metal ions and trivalent ions in dodecahedral sites and highly charged ions in tetrahedral sites. This allows the prediction of Gibbs free energies of formation of any minerals of the alunite supergroup (bearing various ions located in the dodecahedral and tetrahedral sites). Examples are given for hydronium jarosite and hindsalite, and the results appear excellent when compared to experimental values.