Chemical and structural relations of epitaxial xenotime and zircon substratum in sedimentary and hydrothermal environments: a TEM study

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤10 μm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-μm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone (≤90 nm—diagenetic sample, 200–300 nm—hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution–re-precipitation processes at the zircon rim along with precipitation from the respective fluid.     

Microtextures of authigenic Or-rich feldspar in the Upper Jurassic Humber Group, UK North Sea

ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and ⁴⁰Ar/³⁹Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.     

Tourmaline in a low grade clastic metasedimentary rock: an example of the petrogenetic potential of tourmaline

Detrital tourmaline grains and their associated tourmaline overgrowths provide a means to unravel the provenance and petrogenetic history of low grade clastic metasedimentary rocks. Evidence derives from tourmaline grains found in a lithic wacke metamorphosed to chlorite zone conditions. The detrital tourmaline cores are diagnostic indicators of the source rocks of the sediment whereas the overgrowths record both diagenetic and metamorphic reactions in the rock. Tourmaline grains consist of a detrital core surrounded by asymmetric overgrowths comprised of inner and outer rims. Abrupt chemical discontinuities between each of these zones implies that volume diffusion within tourmaline was minor under the conditions of formation. Compositions of the detrital cores vary widely, yet can be correlated with source rock types that are consistent with lithic fragments recognizable in the metawacke. At either the analogous or antilogous pole, inner rim compositions proximal to the detrital cores converge, despite the substrate tourmaline composition, indicating an approach to chemical equilibrium. However, significant dufferences in Al and X-site vacancies at the expense of Mg, Na and Ti between the analogous and antilogous poles of the inner rims demonstrate the presence of significant amounts of compositional polarity. Outer rim compositions at either pole also converge but compositional polarity between the analogous and antilogous poles persists. The presence of the inner and outer rims separated by a compositional discontinuity suggests punctuated evolution of the overgrowth. This implies that boron was sporadically available during diagenesis and metamorphism. Based on boron contents of minerals, this may correspond to a mechanism such as boron release due to polytypic change of illite or consumption of illite and/or muscovite. As such, tourmaline growth stages may serve as a monitor of chemical reactions in low grade metamorphic rocks.     

安塞油田长2油层成岩作用及其对储层物性的影响

安塞地区长2油层储层以中细粒长石砂岩为主，主要成岩矿物为绿泥石环边、方解石胶结物、石英和长石加大生长、伊利石、伊一蒙混层、钠长石和高岭石等。绿泥石环边的发育对原生粒间孔的保存起到有利的作用。石英加大级别达Ⅲ级，加大边含有丰富的有机包体，均一温度范围48．1～76．5℃，烃类进入储层的时间为早白垩世中期至晚白垩世。储层发育多种孔隙类型，面孔率平均为11．60％．主要类型有粒间孔、骨架颗粒溶孔和微裂缝等。残余粒间孔和骨架颗粒溶孔是本区长2储层的主要孔隙类型。根据成岩作用的矿物岩石学标志、有机质成熟度及古温度，将长2油层砂岩的成岩作用划分为3个成岩阶段：早成岩阶段、晚成岩阶段和表生成岩阶段。储层性质明显地受到沉积微相和成岩作用的影响，沉积物粒度较粗、厚度较大的分流河道储集物性明显优于各种粒度较细、厚度较薄的分流间席状砂体。     

成岩作用对河流-三角洲相砂岩储层物性演化的影响--以延长油区上三叠统延长组长2砂岩为例

延长油区上三叠统长 2地层河流相-三角洲相砂岩储层的物性明显受埋藏-成岩作用事件的影响。埋藏压实作用是导致砂岩孔隙丧失的主要原因,造成的平均孔隙度丧失为 17.8%。其中黑云母的早期成岩蚀变是造成原生孔隙丧失的一个重要原因。胶结作用造成的平均孔隙度丧失为 7.1%。其中碳酸盐胶结物和次生石英加大是造成砂岩物性降低的主要胶结物。碎屑颗粒周围绿泥石薄膜的存在阻止了一部分石英次生加大及碳酸盐胶结物的沉淀,使一部分原生粒间孔隙得以保存。晚期成岩阶段有机质分解形成的酸性流体及表生成岩作用阶段的大气降水是形成次生孔隙的主要原因,从而使长 2砂岩的物性得到改善     

Determination of Hydrocarbon Charging History by Diagenetic Sequence and Fluid Inclusions: A Case Study of the Kongquehe Area in the Tarim Basin

Petrographic analysis of hydrocarbon inclusions in reservoirs is the basis and prerequisite for study of hydrocarbon charge history using fluid inclusion analysis.Samples from Silurian reservoirs in the Kongquehe area were studied with microscopy,cathode luminescence and scanning electron microscopy,and the paragenetic sequence of diagenetic events was established.Aqueous and oil inclusions were found in four different occurrences,i.e.,① in healed cracks in detrital quartz grains,② in quartz overgrowths that were formed relatively early in diagenesis,③ in healed cracks crosscutting quartz overgrowths and detrital quartz,and ④ in paragenetically late calcite cements.Solid bitumens were found in intergranular pores and in late fractures,whereas gas inclusions occur in healed cracks crosscutting quartz overgrowths and detrital quartz.The homogenization temperatures of aqueous(Th_(aq)) and oil incluisons(Th_0) within individual fluid inclusion assemblages are very consistent,suggesting that the microthermometric data are reliable.The Th_(aq) values are generally larger than Th_0,indicating the oil charging events took place at significant depths.The results suggest that there were at least two episodes of hydrocarbon charging in the Kongquehe area:the early hydrocarbon charging occurred in late Caledonian,dominated by oil,and the late hydrocarbon charging occurred in the Yanshan-Himalayan,first by oil and then gases.In addition,two episodes of hydrocarbon reservoir adjustment and destruction occurred in the Hercynian and Himalayan,respectively,forming solid bitumen.     

Strain determination using cathodoluminescence of calcite overgrowths

Cathodoluminescence is used in a new application to study extensional strains recorded by calcite overgrowths on detrital calcite grains. The geometry of the overgrowths yields an extension of +0.39 that is parallel to a pressure solution cleavage. A volume change of −0.21 was calculated from this geometry and an independently determined strain ratio.     

Diagenesis of Tertiary playa sandstones of Northern Chile; implications for Andean uplift and metallogeny

The Oligo-Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub-environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa-marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro-magnesian minerals and calcic feldspar resulted in the release of ions into intra-stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals. These include early hematite coatings on framework grains, pore-lining zeolite and pore-filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K-feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit. A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K-feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle to upper Miocene.     

Authigenic iron and titanium oxides in Triassic red beds: (St. Bees Sandstone), Cumbria,Northern England

Authigenic iron and titanium oxides occur in three main textural varieties in the St. Bees Sandstone, a Triassic red bed succession in Cumbria. These are: (a) overgrowths of haematite, titaniferous haematite, anatase, and rutile which generally occur on detrital host grains of similar compositions. The overgrowths may occur as syntaxial rim cements or rhombohedral and prismatic projections and are always in optical continuity, (b) discrete crystals of anatase (including octahedrite) and haematite 10-40 μm in size occur in pore spaces, and (c) fine grained ( < 1 μm) pigmentary haematite occurs as grain coatings around detrital grains and in the interstitial matrix. The euhedral nature and pore-filling habit clearly indicate the authigenic origin of these iron- and titanium oxides. All three textural varieties are believed to have been precipitated from ground waters containing dissolved ferrous and titanium ions. These ions were derived from the intrastratal solution of detrital iron and titanium bearing grains including ferromagnesian silicates and iron-titanium oxides. The precipitation of iron oxides from migrating ground waters in the manner described here goes some way to explaining how thick, uniformly red successions can be formed.     

Authigenic iron and titanium oxides in triassic red beds: (St. Bees Sandstone), Cumbria,Northern England

Authigenic iron and titanium oxides occur in three main textural varieties in the St. Bees Sandstone, a Triassic red bed succession in Cumbria. These are: (a) overgrowths of haematite, titaniferous haematite, anatase, and rutile which generally occur on detrital host grains of similar compositions. The overgrowths may occur as syntaxial rim cements or rhombohedral and prismatic projections and are always in optical continuity, (b) discrete crystals of anatase (including octahedrite) and haematite 10-40 μm in size occur in pore spaces, and (c) fine grained ( < 1 μm) pigmentary haematite occurs as grain coatings around detrital grains and in the interstitial matrix. The euhedral nature and pore-filling habit clearly indicate the authigenic origin of these iron- and titanium oxides. All three textural varieties are believed to have been precipitated from ground waters containing dissolved ferrous and titanium ions. These ions were derived from the intrastratal solution of detrital iron and titanium bearing grains including ferromagnesian silicates and iron-titanium oxides. The precipitation of iron oxides from migrating ground waters in the manner described here goes some way to explaining how thick, uniformly red successions can be formed.     

富县探区上三叠统延长组成岩作用及孔隙结构特征

根据薄片观察、扫描电镜及X—衍射资料的详细分析,富县探区延长组成岩作用已经普遍进入晚成岩A期,主要成岩作用有压实—压溶、胶结充填和溶解作用。压实—压溶及胶结充填作用是导致砂岩孔隙丧失的主要原因。其中碳酸盐、粘土矿物胶结物和石英、长石加大是造成砂岩物性降低的主要原因,碎屑颗粒周围形成的绿泥石衬边阻止了部分石英、长石次生加大及碳酸盐胶结物的沉淀,使部分原生粒间孔隙得以保存。晚期成岩阶段有机质分解形成的酸性流体及表生成岩作用阶段的大气降水使得碎屑部分碎屑长石和早期的浊沸石胶结物溶解是形成次生孔隙的主要原因。浊沸石胶结物主要发育在长3油层组,其原因是该层位长石碎屑含量高,并在成岩过程中蚀变形成浊沸石。高岭石在上部长2、长3油层组含量高,可能是印支期大气淡水淋滤对长石溶蚀并发生了高岭石化。孔隙类型以残余粒间孔和长石溶孔为主,划分出小孔细喉、小孔微细喉、小孔微喉、微细孔微细喉和微细孔微喉型五类孔喉结构,其中最后一种为无效储层。     

The effect of diagenesis on reservoir quality of Mamu Sandstone,Anambra basin,Nigeria

The petrography and powder diffraction analyses were integrated to study the effect of diagenesis on the reservoir quality of the Mamu Sandstone in areas around Enugu, southeastern Nigeria. The sands are moderately to well sorted, very fine to fine grained feldspathic arenite deposited in diverse environments of upper to lower shoreface and sub-environments of delta plain. Pressure solutions, authigenic clays and quartz and feldspar overgrowths are the main diagenetic features of the sandstones in the area. These diagenetic features result in mechanical and chemical compaction, alteration of framework grains and precipitation of dissolved grains. In varying degrees, coating of clasts and filling of pore-spaces by clay and heavy minerals reduced the pore throat diameter, which caused mild reduction of the porosity and permeability of sands. Despite these observed diagenetic effects, the Mamu sands still have the potential to serve as good hydrocarbons reservoir especially for gas.     

SHRIMP U–Pb ages of diagenetic and hydrothermal xenotime from the Archaean Witwatersrand Supergroup of South Africa

SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime ²⁰⁷Pb/²⁰⁶Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.     

Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?

Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ³⁴S values (? 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (? 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ³⁴S values (? 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ³⁴S values (? 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.     

西藏北部西雅尔岗地区白垩-第三系红层的成岩作用及成岩环境

西雅尔岗地区位于羌塘高原腹地，地理范围为东经88°00′至89°00′，北纬32°40′至33°40′.大地构造位置从属于羌塘地块的玛依岗日一阿木冈基底隆断带.在晚白坚一第三纪时期，受青藏高原整沐抬升的影响，沉积了一套巨厚的红色陆屑建造，其基本特征为沉积厚度大、粗粒沉积所占比例高、沉积类型复杂、横向及纵向上相变快、岩石的颜色多为红色及紫色.沉积相类型主要为冲积扇、扇三角洲及湖泊.化石类型卖要为植物〔柏型枝)，孢粉和鸟足印化石.反映了在干操气候条件下，快速堆积的沉积特征. 根据对该区岩石学的研究，发现岩石的矿物组合和结构类型皆因成岩作用的影响而发生了很大的变化.矿物组合的变化是由于沉积后附加的陆源机械渗人粘土，在地表条件下不稳定矿物及岩屑的溶解作用以及在成岩过程中自生矿物的形成作用所造成的.导致结构类型改变的原因是(1)机械渗人的碎屑粘土及自生的粘土矿物形成孔隙间的粘土质“成岩杂基”.而这些孔隙在最初沉积时则没有杂基存在;(2)松散沉积物在成岩过程中，结构颖粒的溶解作用;(3)由于压实及压溶作用而形成粉砂级及其它细小的颗粒;(4)交代作用及重结晶作用.这些成岩过程明显地反映了整个区域岩石的演化历史. 显微镜、扫描电镜的观察以及红外光谱的侧定结果表明该区红层的形成是由于在成岩过程中赤铁矿的沉淀而造成的.在扫描电镜下可清楚地识别出赤铁矿的自生晶形.由此可以断定赤铁矿在其形成过程中显然是起了染色剂的作用.它的形成主要受孔隙水的氧化还原条件所控制，似乎于气候的关系不大，尽管它是产于干旱条件的沉积物中. 基于对该区岩石的成岩作用及成岩历史演化序列的研究，建立了该区沉积后的成岩环境模式，亦即(l)早期的表生成岩浅埋藏环境，该环境明显受到沉积环境、沉积作用控制以及气候因素的影响;(2)晚期的成岩深埋藏环境，该环境则主要受地温梯度、压力、温度和孔隙水的地球化学条件的控制.     

Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at ~250°C

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 μm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of ∼250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.     

Mechanism of formation of Haddat Ash Sham ironstones (Oligo-Miocene), Makkah Al Mokaramah District, West Central Arabian Shield, Saudi Arabia

The present study aims to shed light on the mechanism of formation of the Oligo-Miocene oolitic ironstones of Haddat Ash Sham area, Saudi Arabia. These ironstones are enclosed within the middle part of the Oligo-Miocene siliciclastic succession of the western part of the Arabian Shield, western Saudi Arabia. The ironstone beds were formed during marine incursion and creation of short-lived starved time periods of high organic matter activities, ferrous iron, and low clastic input. The depositional and diagenetic processes involved in the formation of Haddat Ash Sham ironstones are summarized here as follows: (1) the deposition of detrital components (i.e., amorphous iron-bearing clays admixed with silt and sand-sized quartz grains) and their distribution by the waves and current actions in areas of different water depths (bars and inter-bar areas); (2) the deposition of the iron-bearing clays in slightly reducing transgressive conditions (dysaerobic zone) led to the authigenesis of green marine chamositic clays of variable mineralogical and chemical compositions according to the predominated depositional environments; and (3) in the upper parts of the depositional cycles, the iron-bearing clays become admixed with detrital quartz grains which resulted in the formation of silty and sandy ironstones of low iron content. The diagenetic processes led to the oxidation of the green chamositic clays and formation of amorphous Fe-oxyhydroxides, ferrihydrites, goethite, and hematite. These iron mineral phases are related to each other and show progressive steps of transformation during the diagenetic processes. The iron ooids represent in situ formed irregular domains formed during the diagenetic crystallization and dehydration of the amorphous iron oxyhdroxides resulted from the diagenetic oxidation of green chamositic clays. This is supported by the absence of detrital cores of the iron ooids, the gradational contact between the iron ooids and the enclosing matrix and also by the presence of many ooids of unclear and ill-defined internal structure.     

Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks

The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism. Such structures are most common in immature sandstones and siltstones. Mineral reactions, involving modification of detrital grains and diagenetic minerals, and forming a logical extension to diagenetic processes, are an integral part of the deformation mechanism, and the cleavage stripes represent accumulations not just of inert particles, but mostly of newly crystallized micas, the products of these reactions. The mechanism of deformation by pressure solution is now seen to involve mmetamorphic reactions, change in volume of solid phases during reaction, removal of some silica from the rock, rearrangement of reaction products to produce fabrics, solution of some detrital grains in cleavage stripes.Formal reactions have been written to describe the alteration of detrital felspar and epidote to white mica, the modification of greywacke matrix to white mica, and the transformation of diagenetic chlorite to white mica, all of which are observed to occur during formation of the pressure solution cleavages. These reactions emphasize the importance of metamorphic processes during pressure solution deformation, suggest that pressure solution may involve removal of silica released as a product of the reactions, indicate that the pH of the aqueous phase may be buffered to a level that silica solubility is increased, involve a volume reduction that contributes to the overall shortening during deformation, and also involve dehydration, the large scale circulation of released water possibly being important to the removal and redistribution of silica during pressure solution.     

The role of grain dissolution and diagenetic mineral precipitation in the cycling of metals and phosphorus: A study of a contaminated urban freshwater sediment

This paper describes the detrital mineralogy, early diagenetic reactions and authigenic mineral precipitates for freshwater contaminated sediments deposited in an urban water body (the Salford Quays of the Manchester Ship Canal, Greater Manchester, UK). These sediments contain a mix of natural and anthropogenic detrital grains. Detrital grains are dominated by quartz and clay grains, whilst anthropogenic grains are dominated by metal-rich glass grains, concentrated at a depth of 12–17 cm in the sediment as a result of historical inputs. Sediment porewaters contain significant concentrations of Fe, Mn, Zn and phosphate. Bacterial Fe(III) and Mn(IV) reduction are hypothesised to supply Fe²⁺ and Mn²⁺ to porewaters, with phosphate released from Fe oxide reduction or organic matter oxidation. Petrographic observations indicate that the metal-rich glass grains are undergoing chemical dissolution during early diagenesis, supplying Fe and Zn to porewaters.     

Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.