Mineralogical and textural changes accompanying ageing of silica sinter

Twenty nine samples of silica sinter, ranging in age from modern to Miocene, record temporal changes in both mineralogy and texture. When first deposited, sinters consist largely of noncrystalline spheres (<1–8 μm diameter) of opal-A exhibiting varying degrees of close-packing. Particle densities range from 1.5 to 2.1 g cm⁻³, total water 4–10 wt%, and porosities 35–60%. Changes over ∼10,000 years following deposition are slight although the spheres may be invested by an additional film of secondary silica. For the next 10,000 to ∼50,000 years, the silica incrementally crystallises to become poorly crystalline opal-CT and/or opal-C; spherical particles of thin-bladed crystals (lepispheres) replace opal-A particles and coalesce in microbotryoidal aggregates (∼10–30 μm diameter). Amygdaloidal fibrous clusters occur with lepispheres. As silica lattice ordering becomes enhanced, total water content drops to <7 wt%, particle density increases to ∼2.3 g cm⁻³, and porosity reduces to <30%. The change from opal-A to opal-C takes place over a briefer periods (∼50 years) in silica sinters that contain other materials (e.g. calcite, sulfur, alunite, plant remains). Sinters older than ∼50,000 years have recrystallised to microcrystalline quartz. With the onset of quartz crystallisation at ∼20,000 years, total water is <0.2 wt%, particle density approximates quartz (2.65 g cm⁻³), and porosity is <4%. The progressive changes in silica species and texture yield ageing profiles for sinters that may serve as guides to the paleohydrology of geothermal systems and/or epithermal ore deposits in areas where surface thermal activity has declined or ceased. Received: 18 November 1998 / Accepted: 6 July 1999     

Thermoluminescence spectra of igneous quartz and hydrothermal vein quartz

Variations in thermoluminescence spectra are reported for four types of geological quartz examined with a new spectrometer featuring dual imaging photon detectors that separately and simultaneously detect (1) uv-blue (200–450 nm) and (2) blue to near infrared (400–800 nm) emission. Samples show striking differences which appear to be characteristic of their geological origin. Volcanic quartz phenocrysts from acid volcanics show red thermoluminescence (TL) emission bands centered at 620–630 nm that are 100 times more intense than similar bands in other quartz, while a violet emission at 420–435 nm was observed exclusively in igneous quartz (volcanic and granitic). A broad emission band centered at 560–580 nm was observed only in quartz formed hydrothermally. Massive quartz from Li-rich pegmatite bodies shows narrow, intense 470 nm emission bands at 230° C apparently related to Al and to Ge defects detected with electron paramagnetic resonance (EPR), and emission bands at 330 and 280 nm, possibly related to recombination at oxygen vacancies. The common 380 nm emission band of quartz was observed in both volcanic and granitic quartz, but was not detected in either the pegmatitic or the hydrothermal vein quartz. Observed spectral variation is identified as a potential source of error in luminescence dating.     

Mixed biogenic and hydrothermal quartz in Permian lacustrine shale of Santanghu Basin,NW China: implications for penecontemporaneous transformation of silica minerals

International Journal of Earth Sciences - The cycling of various isomorphs of authigenic silica minerals is a complex and long-term process. A special type of composite quartz (Qc) grains in...     

Strontium isotope systematics of hydrothermal vein minerals in deposits of West Germany

Vein minerals in West Germany hosted by crystalline basement rocks and Paleozoic and Mesozoic sedimentary rocks are characterized by⁸⁷Sr/⁸⁶Sr ratios which indicate continental derivation of Sr incorporated into the minerals. Neither mantle-derived Sr nor prominent contribution of this element from sea water (except for the strontianites of Münsterland) can be substantiated by the data presented.Minerals in veins hosted by sedimentary rocks have acquired their Sr most plausibly from buried sedimentary rock sequences.Mineralization of veins hosted by Moldanubian crystalline rocks in the Bayerischer Wald is confined to a Permian-Triassic time span. The⁸⁷Sr/⁸⁶Sr ratios of the minerals cover a relatively narrow range reflecting the time-dependent Sr isotopic composition of the host environment.Mineralization of veins in the Schwarzwald crosscutting crystalline basement rocks and partly extending into Mesozoic caprocks obviously occurred episodically or continuously from the Late Variscan event to Tertiary time. Mineral sequences of the Schwarzwald veins incorporated Sr with time-integrated isotopic composition most likely derived predominantly from gneisses.

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Zusammenfassung Gangminerale im kristallinen Grundgebirge und in paläozoischen und mesozoischen Sedimentgesteinen Westdeutschlands zeichnen sich durch⁸⁷Sr/⁸⁶Sr-Verhältnisse aus, die eine kontinentale Herkunft des Sr anzeigt. Weder Sr vom Erdmantel noch ein wesentlicher Beitrag dieses Elementes aus Meerwasser in den Mineralen (abgesehen von den Strontianiten des Münsterlandes) kann aus den vorliegenden Daten abgeleitet werden.Gangminerale in Sedimentgesteinen haben sehr wahrscheinlich Sr aus tieferliegenden Sedimentgesteinsabfolgen eingebaut.Die Mineralisation von Gängen in Gesteinen des kristallinen Grundgebirges im Moldanubikum des Bayerischen Waldes ist auf den Zeitraum Perm-Trias beschränkt. Die⁸⁷Sr/⁸⁶Sr-Verhältnisse umfassen eine relativ enge Spanne und spiegeln die zeitabhängige Isotopenzusammensetzung der umgebenden Gesteine und der des Untergrundes wider.Gänge im Schwarzwald durchschneiden kristallines Grundgebirge und setzen sich teilweise in mesozoisches Deckgebirge fort. Die Mineralisation erfolgte episodisch oder kontinuierlich von der variszischen Orogenèse bis ins Tertiär. Die Mineralabfolgen der Schwarzwaldgänge enthalten Sr mit einer zeitintegrierten Isotopenzusammensetzung, das sehr wahrscheinlich vorwiegend aus Gneisen stammt.

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Résumé Les minéraux des filons hydrothermaux contenus dans le socle cristallin et dans les sédiments paléozoïques et mésozoïques de l'Allemagne occidentale présentent des rapports initiaux⁸⁷Sr/⁸⁶Sr qui indiquent une origine continentale du Sr. Les résultats présentés dans cette note ne permettent d'accepter ni une origine mantellique ni un apport à partir de l'eau de mer (à l'exception des strontianites du Münsterland).Les minéraux des veines contenues dans les roches sédimentaires doivent très probablement leur Sr aux séries sédimentaires profondément enfouies.La minéralisation des veines contenues dans les roches cristallines moldanubiennes du Bayerische Wald date du Permo-Trias. Les rapports initiaux embrassent un éventail relativement étroit qui traduit la composition isotopique, fonction du temps, des roches encaissantes.Les filons de la Forêt Noire, qui traversent le socle cristallin et se prolongent partiellement dans la couverture mésozoïque, se sont foïmés manifestement, par épisodes ou de manière continue, depuis l'orogenèse varisque jusqu'au Tertiaire. Leurs séquences minérales renferment du Sr de composition isotopique temporellement intégrée, qui provient principalement des gneiss.

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Oxygen three-isotope fractionation lines in terrestrial silicate minerals: An inter-laboratory comparison of hydrothermal quartz and eclogitic garnet

Geochemical—and perhaps biochemical—processes may yield tell-tale proxies in rocks and minerals on the Earth or other planetary bodies, in the form of distinctive slopes of linear fractionation arrays on the oxygen three-isotope plot. It is generally recognized that kinetic and equilibrium fractionation processes are described by different mass fractionation laws. We show that coupled laser fluorination, dual-inlet IRMS procedures for oxygen three-isotope analysis of silicates, at high precision, gave reproducible accuracy for the slope value as measured independently by two different laboratories, using replicates of the same silicate samples. As far as we are aware, this is the first such inter-laboratory comparison. Hydrothermal quartz (together with one chalk flint sample) with a range in δ¹⁸O of 31‰ gave respective slope λ values of 0.5240 ± 0.0010 and 0.5242 ± 0.0010, using Prism III and MAT 253 mass spectrometers, respectively, at the Open University (OU). Errors were computed from weighted linear regression and are reported at the 95% confidence level. The comparable result obtained at the Geophysical Laboratory (GL), Carnegie Institution of Washington, was 0.5240 ± 0.0015. A MAT 252 mass spectrometer was used for the latter measurements and the oxygen extraction and purification procedures differed in detail from those used at the OU. In contrast, slopes measured for replicates of seven garnet samples, metamorphosed under high-temperature, high-pressure conditions, and spanning 20‰ in δ¹⁸O, gave 0.5262 ± 0.0008 at OU (Prism III analyses) and 0.5266 ± 0.0012 at GL.     

Mineralogical and geochemical characteristics of the hydrothermal illite from Hoam granite,South Korea: Implications for episodic fluid injections in the hydrothermal alteration system

Two stages of illite mineralization are recognized in the hydrothermal alteration zone of the Hoam granite. These illites are formed as a result of pervasive alteration by re-equilibration with high water/rock in a brittle environment below <2 km; the mineralization timing is middle Oligocene (26–27 Ma), coinciding with the timing of crustal deformation related to the opening of the East Sea (Sea of Japan). The mineralogical and geochemical characteristics of the clearly distinguished illites at each site indicate that they were mineralized from different fluid injections in distinct geological environments. Illites at the site-1 alteration zone are characterized by high-K content [K_0.84 per O₁₀(OH)₂], 2M₁ polytype of 99 %, hexagonal plate shape, and coexistence with pyrite. These observations indicate that the illites were formed in a slow cooling system (>250 °C), high fS₂, and a relatively acidic environment. The pseudomorphic replacement combined with matrix-filling texture indicates that the illites at the site-1 alteration zone recorded the changes in fluid conditions from low to high water-rock ratio. In contrast, the illites at the site-2 alteration zone show the coexistence of polytypes (2M₁, 1M, and 1M_d), high-K illites [(K_0.83 per O₁₀(OH)₂]/low-K illites [K_0.63 per O₁₀(OH)₂], platy/hairy shapes, and presence of magnetite. Furthermore, this alteration zone no longer exhibits primary textures because of pervasive alteration induced by the dissolution-precipitation process. These results indicate that they were formed in a rapid cooling system and were continuously under conditions of high water-rock ratio, as well as in a less acidic and fS₂ environment than that observed at the site-1 alteration zone. The behavior of trace elements for each illite primarily depends on the constituents of the hydrothermal fluid, which reflect different degrees of fluid evolution. The enrichment of high field strength elements (Nb and Ta), large ion lithophile elements (B, Be, and Cs), rare earth elements, and actinide elements (U and Th) in illite at the site-2 alteration zone shows that these elements formed by a more evolved fluid than that of the illite at the site-1 alteration zone. In addition, negative Ce anomalies at the site-2 alteration zone indicate that these crystallized in a reducing environment. Considering the mineralogical and geochemical properties of illites at the site-1 and site-2 alteration zones, the illite mineralization in the Hoam granite was likely generated by at least two episodes of hydrothermal illite mineralization, which originated from episodic injections of fluids, rather than continuously evolved fluids.     

维拉斯托稀有金属-锡多金属矿床铌铁矿族矿物特征及其对岩浆-热液演化的指示

大兴安岭南段维拉斯托大型稀有金属-锡多金属矿床的成矿岩体从深部的中粒花岗岩向上逐渐过渡为斑状细粒碱长花岗岩,记录了岩浆演化的不同阶段。为查明铌铁矿族矿物成分、结构变化及其与岩浆演化过程的耦合关系,文章对采自不同深度的3种类型花岗岩（中粒花岗岩、石英斑晶不具雪球结构的斑状细粒富云母碱长花岗岩和具雪球结构的斑状细粒碱长花岗岩）中的铌铁矿族矿物进行了详细的电子探针成分分析和元素面扫工作。维拉斯托矿床铌铁矿族矿物主要为铌铁矿,发育渐变环带结构,从核部到边部Ta含量增加,且斑状细粒富云母碱长花岗岩和斑状细粒碱长花岗岩铌铁矿族矿物边部的铌铁矿、铌锰矿相较于幔部和核部的铌铁矿,其Ta含量具有明显的组分间隔;铌铁矿族矿物Mn/(Fe+Mn)原子比具有多种演化趋势,与分离结晶的矿物相和流体交代作用有关。中粒花岗岩和斑状细粒富云母碱长花岗岩中的铌铁矿结晶于岩浆阶段,斑状细粒碱长花岗岩中的铌铁矿族矿物结晶于岩浆-热液过渡阶段。其中,斑状细粒富云母碱长花岗岩铌铁矿边部Ta含量增加与铁锂云母的分离结晶作用有关,斑状细粒碱长花岗斑岩铌铁矿族矿物边部Ta含量的突变与岩浆-热液过渡阶段的水硅酸盐液体中Ta溶解度的增加...     

Bedding-parallel shear,thrusting and quartz vein formation in witwatersrand quartzites

Evidence is presented which clearly demonstrates the presence of thrust faults in the gold-bearing sedimentary rocks of the Witwatersrand Supergroup. A package of these rocks subjected to shear strain allowed overlying beds on the northern margin of the Witwatersrand Basin to move outwards in a N-NW direction. Movement took place on ductile shear zones which were parallel to the bedding and often reveal excellent examples of ramp and duplex structures. It has also been established that syntectonic foliation planes were utilized as planes of shear in certain circumstances. Quantitative estimates of strain for the pile, which are of necessity minimum values, give γ ≈ 1. Fluids, from which vein quartz formed, played a critical role in the shear plane movement and may have relieved a portion of the overburden load during deformation.     

Regularities in the impurity element content of minerals during hydrothermal ore formation

Many goldfields host scores of discovered gold ore occurrences with unclear prospects because of their insufficient geologic exploration. Since exploration is expensive, the occurrences should be first appraised by low-cost methods. Studying the typomorphic features of minerals from the goldfields of the Upper Yana–Kolyma region, we have established a relationship between the contents of isomorphic impurity elements (IIE) and the degree of solution supersaturation (DSS). Preliminary studies show that, based on this relationship, criteria can be developed for revealing “concealed” mineralogical–geochemical zonation and predicting mineralization to depth. The determining factor for the behavior of IIE, depending on the DSS, is their distribution coefficient (K) with respect to the host element. If K > 1, the content of these elements decreases with increasing DSS during ore deposition; if K < 1, it increases. When a solution contains IIE with K > 1 and K < 1, they behave according to their distribution coefficients. In other words, when the DSS increases during ore deposition, IIE with K > 1 will be typical of early mineral segregations, and IIE with K < 1, of late ones. If the crystallization front migrates, this must give rise to a vertical zonation in the content of both groups of elements. The content of elements with K > 1 will decrease upsection, and that of elements with K < 1 will increase (“concealed” mineralogical–geochemical zonation). In connection with this subject, the paper reports data using native gold as an example. The behavior of Ag, Sb, Hg, Cu, Ni, and Co is discussed depending on the depth of deposit formation from the paleosurface (deep-seated → shallow); in the vertical section of individual deposits; depending on mineralization stages; in the cross-section of individual crystals with a zonal structure. The behavior of Ag, Sb, and Hg has been shown to be opposite to that of Cu, Ni, and Co in all these aspects. With respect to Au, the first group of elements has K < 1, and the second one, K > 1. Copper and antimony are of particular interest. In the vertical section of individual deposits, the Cu content of gold increases with depth and the Sb content decreases. The Cu/Sb ratio can be used to estimate the erosional truncation level of deposits and thus predict their extension to depth. Examples of well-explored deposits are given.     

Symplectite formation in the presence of a reactive fluid: insights from hydrothermal experiments

This study describes the microstructural and chemical development of symplectites, obtained in fluid‐mediated mineral replacement experiments. During the experiments polymineralic feldspar‐rich samples were exposed to aqueous Na–SiO₂ solution at 600 °C and 2 kbar confining pressures for durations of 12 h to 20 days. The resulting reaction rims display high mineralogical and structural complexity and contain two varieties of symplectites, represented by nanometre‐scale intergrowths of gehlenite–zeolite and grossular–zeolite grains. The experimental fluid was enriched in ¹⁸O isotope in order to trace oxygen redistribution during the reaction. The elevated ¹⁸O concentration in the reaction products and the heterogeneity in its distribution suggest that symplectite formation was controlled by dissolution–precipitation mechanisms rather than volume‐diffusion processes. Microstructural and chemical observations suggest that symplectite formation occurred in multiple stages in response to spatially heterogeneous and temporarily evolving fluid composition at the reaction interfaces. Hence, our results shed light on the fundamental processes involved in symplectite formation improving our ability to interpret symplectite microstructures.     

湘西大溶溪钨矿床矽卡岩矿物的矿物学、地球化学特征及其形成机制

大溶溪钨矿床为湘西地区一层控型白钨矿矿床,白钨矿主要分布于矽卡岩和石英网脉中。在详细野外地质调查和室内镜下观察的基础上,利用EPMA和高精度LA-ICP-MS测试技术,对该矿主要矽卡岩矿物的矿物学和地球化学特征进行了研究,并初步揭示了其形成机制。研究表明,大溶溪矿区含钨矽卡岩为还原型类矽卡岩,矽卡岩矿物主要为辉石和石榴子石;辉石为透辉石—钙铁辉石—锰钙辉石系列;石榴子石以钙铝榴石为主,但锰铝榴石+铁铝榴石含量也较高。该区辉石REE具有总量较低、轻重稀土元素分馏不明显、正Ce和正Eu异常的特征;而石榴子石REE具有较高的稀土总量,呈现明显的重稀土元素富集、轻稀土元素亏损、负Ce异常、强烈正Eu异常的特点。辉石Ce和Eu异常可能与其形成时的水/岩反应及氧化还原条件有关。REE进入石榴子石的方式以REE~(3+)等价置换Al~(3+)的形式为主,正Eu异常主要为Eu~(2+)等价置换钙铝榴石Ca~(2+)所致,而负Ce异常则反映出热液沉淀正Ce异常辉石之后热液体系中贫Ce元素。该区含钨矽卡岩主要为热液与含锰灰岩在平衡机制下发生扩散交代作用所形成;而含钨石英网脉内发育的辉石、石榴子石等钙硅酸盐矿物,则是热液沿裂隙、孔隙对富钙变质砂岩进行交代的产物。矽卡岩形成过程中,相对贫Mn的辉石和石榴子石主要受交代流体作用控制,形成于一种相对高温、中等水/岩比的条件下;而相对富Mn的辉石和石榴子石,则可能是形成于靠近围岩一侧的相对低温、低水/岩比的环境中,部分锰质来自被交代的围岩。     

Mineralogical and chemical compositions of glauconite and glauconitic rocks and assumption for the formation of the minerals in the Kyzyltokoy basin,Kyrgyzstan

<正>This study is focused on the mineralogical and chemical compositions,deposition environment and mechanism of formation of sediments of Kyzyltokoy basin.By an interpretation of formation,an environment of sedimentation of a basin was separated into three general conditions:a condition where glauconitization process interrupted,where process reached a completion and where occurred decay of glauconites,i.e.,the beginning and interruption in the middle of glauconitization,completion of the process     

Rare earth element distribution in some hydrothermal minerals: evidence for crystallographic control

Rare earth element (REE) abundances were measured by neutron activation analysis in anhydrite (CaSO₄), barite (BaSO₄), siderite (FeCO₃) and galena (PbS). A simple crystal-chemical model qualitatively describes the relative affinities for REE substitution in anhydrite, barite, and siderite. When normalized to ‘crustal’ abundances (as an approximation to the hydrothermal fluid REE pattern), log REE abundance is a surprisingly linear function of (ionic radius of major cation—ionic radius of REE)² for the three hydrothermal minerals, individually and collectively. An important exception, however, is Eu, which is anomalously enriched in barite and depleted in siderite relative to REE of neighboring atomic number and trivalent ionic radius. In principle, REE analyses of suitable pairs of co-existing hydrothermal minerals, combined with appropriate experimental data, could yield both the REE content and the temperature of the parental hydrothermal fluid.The REE have only very weak chalcophilic tendencies, and this is reflected by the very low abundances in galena—La, 0.6 ppb; Sm, 0.06 ppb; the remainder are below detection limits.     

Sapphirine + quartz assemblage in contrasting textural modes from the Eastern Ghats Belt, India: Implications for stability relations in UHT metamorphism and retrograde processes

Stability of the assemblage sapphirine + quartz in Mg–Al-rich granulites implies ultrahigh temperature (UHT) condition of metamorphism but their direct contact is rarely preserved in natural rocks. The present study shows contrasting textural relations between sapphirine and quartz in different parts of the same occurrence of a Mg–Al-rich granulite, Eastern Ghats Belt, India. Textural data suggest stabilization of the assemblage sapphirine + quartz with orthopyroxene and cordierite during the metamorphic peak. Thermometric estimates yield temperature exceeding 950 °C for the stability of this assemblage. Most of such sapphirine grains (Spr₁) are texturally separated from quartz and cordierite grains by double corona of sillimanite + orthopyroxene that results due to isobaric cooling during the post-peak stage. Sapphirine (Spr₂) also forms a symplectic intergrowth with quartz and orthopyroxene at the fringe of coarse orthopyroxene. This textural feature can be explained by the breakdown of (Fe, Mg)-Tschermak components of orthopyroxene during the same isobaric cooling episode from UHT peak condition. The preservation of grain contact of this intergrown sapphirine and quartz can be attributed to a problem in reaction kinetics. In the other mode, sapphirine (Spr₃) occurs with quartz with a thin skin of cordierite near a quartz vein. Such texture could result from isothermal decompression of the cooled crust. Alternatively and more possibly, cordierite could form from ingress of CO₂–H₂O rich fluid during terminal stage of cooling. Finally, sapphirine (Spr₄) and quartz show direct contact close to the quartz vein. Direct contact of such sapphirine and quartz represents textural disequilibrium as this particular quartz is introduced as a vein much later than the peak metamorphism but prior to the major foliation-forming deformation. Coarse sapphirine and vein quartz, therefore, accidentally came in contact with each other and persisted metastably. Therefore, though coexistence of sapphirine and quartz is considered to be a strong evidence for ultrahigh temperature condition, care should be taken to decipher their stable coexistence. Different types of textural relations involving this mineral pair could originate in a single rock, probably in different stages of its metamorphic history.     

The origin of clay minerals in active and relict hydrothermal deposits

Samples of Fe-oxide-rich hydrothermal sediments were collected from active and inactive portions of the TransAtlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Clays separated from TAG metalliferous sediments in this study all consist of Al-poor nontronite. Oxygen isotope thermometry of the clays yields formation temperatures of 54-67°C for samples from the inactive Alvin mound compared with 81-96°C for samples from the active TAG site. The latter are the highest recorded temperatures for authigenic hydrothermal clays. Sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater-dominated fluids, containing less than 15% hydrothermal end-member fluid. In contrast, nontronite from the inactive Alvin mound has ⁸⁷Sr/⁸⁶Sr values that closely resemble that of detrital North Atlantic clays, suggesting a dominantly continental source for the Sr. Rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid. Crystallographic fractionation of the trivalent REE is apparent in the heavy REE enrichments for all nontronite samples. The inferred formation-mechanism for nontronite-rich Fe-oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids. At the inactive Alvin site, in contrast, the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and Fe-oxyhydroxide. These two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit.