Homogeneity of water content in obsidian from the coso volcanic field: Implications for obsidian hydration dating

Users of the obsidian hydration dating method have routinely assumed that artifacts which originate from the same geological flow will be of the same chemical composition and thus hydrate at the same rate under equivalent conditions of temperature and relative humidity. Recent laboratory experimentation into the hydration process has shown that the intrinsic water content of the glass is the dominant factor in establishing the rate of hydration. Water content determinations on a large suite of samples from numerous prehistoric quarries within the Coso volcanic field, California, indicated that water content values, and thus hydration rate, varied significantly on a within flow basis. It is recommended that water determinations be made on individual artifacts prior to obsidian hydration dating. © 1993 John Wiley & Sons, Inc.     

Low-temperature isotopic exchange in obsidian: Implications for diffusive mechanisms

While a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the glass transition, the nature of this interaction at lower temperatures is much more poorly understood. This paper presents the results of a series of isotopic exchange experiments aimed at further elucidating this process and determining the extent to which a point-by-point analysis of the D/H or ¹⁸O/¹⁸O isotopic composition across the hydrated rim on a geological or archaeological obsidian sample can be used as a paleoclimatic monitor. Experiments were performed by first hydrating the glass for 5 days in water of one isotopic composition, followed by 5 days in water of a second composition. Because waters of near end-member compositions were used (nearly pure ¹H₂¹⁶O, ¹H₂¹⁸O, and D₂¹⁶O), the relative migration of each species could be ascertained easily by depth-profiling using secondary ion mass spectrometry (SIMS). Results suggest that, during hydration, both the isotopic composition of the waters of hydration, as well as that of intrinsic water remaining from the initial formation of the glass vary dramatically, and a point-by-point analysis leading to paleoclimatic reconstruction is not feasible.     

Sulfur-poor intense acid hydrothermal alteration: A distinctive hydrothermal environment

A fundamentally distinct, sulfide-poor variant of intense acid (advanced argillic) alteration occurs at the highest structural levels in iron oxide-rich hydrothermal systems. Understanding the mineralogy, and geochemical conditions of formation in these sulfide-poor mineral assemblages have both genetic and environmental implications. New field observations and compilation of global occurrences of low-sulfur advanced argillic alteration demonstrates that in common with the sulfide-rich variants of advanced argillic alteration, sulfide-poor examples exhibit nearly complete removal of alkalis, leaving a residuum of aluminum-silicate + quartz. In contrast, the sulfur-poor variants lack the abundant pyrite ± other sulfides, hypogene alunite, Al-leached rocks (residual “vuggy” quartz) as well as the Au-Cu-Ag ± As-rich mineralization of some sulfur-rich occurrences. Associated mineralization is dominated by magnetite and/or hematite with accessory elements such as Cu, Au, REE, and P. These observations presented here indicate there must be distinct geologic processes that result in the formation of low-sulfur advanced argillic styles of alteration.Hydrolysis of magmatic SO₂ to sulfuric acid is the most commonly recognized mechanism for generating hypogene advanced argillic alteration, but is not requisite for its formation. Low sulfur iron-oxide copper-gold systems are known to contain abundant acid-styles of alteration (e.g. sericitic, chloritic), which locally reaches advanced argillic assemblages. A compilation of mapping in four districts in northern Chile and reconnaissance observations elsewhere show systematic zoning from near surface low-sulfide advanced argillic alteration through chlorite-sericite-albite and locally potassic alteration. The latter is commonly associated with specular hematite-chalcopyrite mineralization. Present at deeper structural levels are higher-temperature styles of sodic-calcic (oligoclase/scapolite – actinolite) alteration associated with magnetite ± chalcopyrite mineralization. These patterns are in contrast to the more sulfur-rich examples which generally zone to higher pyrite and locally alunite-bearing alteration.Fluid inclusion evidence from the systems in northern Chile shows that many fluids contain 25 to >50 wt% NaCl_eq with appreciable Ca, Fe, and K contents with trapping temperatures >300 °C. These geological and geochemical observations are consistent with the origin of the low-sulfur advanced argillic assemblages from HCl generated by precipitation of iron oxides from iron chloride complexes from a high-salinity fluid by reactions such as 3FeCl₂ + 4H₂O = Fe₃O₄ + 6HCl + H₂. Such HCl-rich (and relatively HSO₄⁼-poor) fluids can then account for the intense acid, Al-silicate-rich styles of alteration observed at high levels in some iron-oxide-coppe-gold (IOCG) systems. The geochemical differences between the presence of sulfide-rich and sulfur-poor examples of advanced argillic alteration are important to distinguishing between system types and the acid-producing capacity of the system, including in the modern weathering environment. They have fundamental implications for effective mineral exploration in low-sulfur systems and provide yet another vector of exposed alteration in the enigmatic IOCG clan of mineral deposits. Furthermore, understanding the geochemistry and mineralogy of this distinct geologic environment has applications to understanding the acid generating capacity and deleterious heavy metals associated with advanced argillic alteration.     

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.     

Calculated and observed long term performance of Leneghans embankment

This paper presents the finite element (FE) analysis of the consolidation of the foundation of an embankment constructed over soft clay deposit which shows significant time dependent behaviour and was improved with prefabricated vertical drains. To assess the capability of a simple elastic viscoplastic (EVP) model to predict the long term performance of a geotechnical structure constructed on soft soils, a well documented (Leneghans) embankment was analyzed to predict its long term behaviour characteristics. Two fully coupled two dimensional (2D) plane strain FE analyses have been carried out. In one of these, the foundation of the embankment was modelled with a relatively simpler time dependent EVP model and in the other one, for comparison purposes, the foundation soil was modelled with elasto-plastic Modified Cam-clay (MCC) model. Details of the analyses and the results are discussed in comparison with the field performance. Predictions from the creep (EVP) model were found to be better than those from Elasto-plastic (MCC) analysis. However, the creep analysis requires an additional parameter and additional computational time and resources.     

The Coso volcanic field reexamined: Implications for obsidian sourcing and hydration dating research

Obsidian from the Coso locality, Inyo County, California, has long been regarded by archaeologists as a single “source.” However, studies by the U.S. Geological Survey have provided evidence of geochemical variability among flows within the volcanic field. to determine whether these geochemical distinctions could be applied productively to archaeological research, nondestructive x-ray fluorescence analyses were conducted on samples recently collected from 15 obsidian-bearing loci. the results of this research show that geochemically distinct varieties of artifact-quality obsidian can be recognized within the Coso volcanic field, and subsequent nondestructive x-ray fluorescence analyses of artifacts from two nearby archaeological sites document that different geochemical varieties of obsidian were used prehistorically to manufacture tools. Implications of these results for studies of prehistoric exchange and obsidian hydration dating are discussed.     

Radiometric survey for exploration of hydrothermal alteration in a volcanic area

A radiometric survey on a sector of the Lipari volcanic island particularly affected by argillification due to hydrothermal processes was carried out. Preliminarily, on the basis of a wide set of field measurements over unaltered outcrops U and K distribution maps were obtained. The concentration of these radioelements increases with the degree of magma differentiation; the U and K content shows maxima (13.4 ppm and 3.7% on average, respectively) in the felsic materials, belonging to the youngest volcanic cycles, and minima (2.7 ppm and 2.0%) in the mafic rocks of the oldest cycles. The Th/U ratio insignificantly varies with magma differentiation and it equals 3.2. Laboratory measurements on volcanic products from altered zones were then used to identify radiometric anomalies related to hydrothermal processes. On the average, argillized rocks have radioactivity approximately five times lower than that measured in the unaltered volcanics. The Th/U ratio against U is rather uniform, whereas Th/K ratio is of the order of 3.6 in less altered rocks K > 0.2%) and it ranges from 7 to 11 in kaolin deposits having the largest depletion in potassium.     

Structural evolutions of an obsidian and its fused glass by shock-wave compression

Shock-recovery experiments for obsidian and its fused glass have been carried out with pressure up to 35 GPa. Structural evolution accompanying the shock compression was investigated using X-ray diffraction technique, Raman and infrared spectroscopy. The densities of obsidian and its fused glass increased with applied shock pressure up to 25 GPa. Densification reached a maximum of 4.7 and 3.6% for obsidian and its fused glass, respectively. The densification mechanism is attributed to reduction of the T–O–T angle, and changes in ring statistics in the structure. Density reduction observed at greater than 25 GPa of applied shock pressure is due to partial annealing of the high-density glass structures brought by high post-shock residual temperature. The density of fused glass is almost equal to its original value at 35 GPa while the shocked obsidian has a slightly lower value than its original value. Amorphization of crystallites present in the obsidian due to shock compression is probably the cause of the density decrease. The structural evolution observed in shock-compressed obsidian and its fused glass can be explained by densification resulting from average T–O–T angle reduction and increase of small rings, and subsequent structural relaxation by high post-shock temperature at applied shock compression above 25 GPa.     

Trace element mobility during hydrothermal alteration of oceanic basalts

Trace element analyses have been carried out on hydrothermally altered pillow basalts of greenschist facies dredged from the median valley of the Mid-Atlantic Ridge. Sr is leached from the rock, and its behavior is apparently controlled by the same reactions as Ca. Cu is also leached from the basalt, but often shows local precipitation in veins as sulfides. Fe, B, Li, Ba, Mn, Ni and Co show sufficient variations in concentration and location within the altered basalts to indicate that some mobilisation occurs, but there may be subsequent uptake or precipitation into the secondary mineral assemblages. V, Y, Zr and Cr do not appear to be affected by hydrothermal alteration.The production of a metal-enriched solution by hydrothermal alteration and subsequent precipitation of metal salts to form metalliferous sediments is indicated, as is precipitation of metal sulfides in the basaltic basement.     

The upper Gila river gravels as an archaeological obsidian source region: Implications for models of exchange and interaction

Recent geoarchaeological research in the Upper Gila River region of southeastern Arizona indicates the presence of two geochemically distinct archaeological obsidians occurring as small secondarily deposited nodules within the Quaternary gravels and alluvium. Approximately 32% of the nodules recovered in the sample are derived from the known and reported Mule Creek source upstream in western New Mexico. The remaining 68% of the nodules recovered are derived from another source somewhere in the area, probably in the Tertiary silicic volcanics along the San Francisco drainage or the Upper Gila proper. Additionally, analyses of the Mule Creek primary source data indicates significant geochemical variability not previously indicated by energy dispersive x-ray fluorescence (EDXRF) studies. The presence of Mule Creek glass 50 or more kilometers from the primary source bears significantly on distance to source based studies where secondary deposition is not accounted for.     

Sulfur in peridotites and gabbros at Lost City (30°N, MAR): Implications for hydrothermal alteration and microbial activity during serpentinization

Variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the Lost City Hydrothermal Field at the southern face of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were examined to better understand serpentinization and alteration processes and to study fluid fluxes, redox conditions, and the influence of microbial activity in this active, peridotite-hosted hydrothermal system. The serpentinized peridotites are characterized by low total sulfur contents and high bulk δ³⁴S values close to seawater composition. Low concentrations of ³⁴S-enriched sulfide phases and the predominance of sulfate with seawater-like δ³⁴S values indicate oxidation, loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites. The predominance of pyrite in both serpentinites and gabbros indicates relatively high fO₂ conditions during progressive serpentinization and alteration, which likely result from high fluid fluxes during hydrothermal circulation and evolution of the Lost City system from temperatures of ∼250 to 150 °C. Sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at Lost City show δ³⁴S values that reflect the influence of microbial activity. Our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long-lived, moderate temperature peridotite-hosted hydrothermal system and high seawater fluxes on the global sulfur cycle.     

Importance of volcanic glass alteration to sediment stabilization: offshore Japan

A minor amount (ca 1 wt%) of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the Nankai Trough subduction zone throughout the Shikoku Basin. The distribution and nature of the cement were examined via secondary and backscattered electron imaging. The amorphous silica occurs as altered material in contact with volcanic glass, coating grains (including grain contacts) and filling pores. Based on chemical and petrographic evidence, the cement is probably sourced from volcanic glass; this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica. Amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay‐dominated sediment throughout the Shikoku Basin. Measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the Shikoku Basin (at sites separated by >500 km). The cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration (i.e. Central America, Cascadia and the Gulf of Alaska).     

Effects of hydrothermal alteration on the compositions of chromian spinels

Chromite spinels in hydrothermally altered rocks from fracture-zone ultramafic rocks and from both ultramafic cumulate pods and sheeted dikes in the Josephine ophiolite, California, display a wide variety of compositions. Alteration of the spinel may not be visible in thin section. The primary composition changes accompanying hydrothermal alteration are increase in Cr/(Cr+Al) and/or Fe²⁺/(Fe²⁺+Mg). In general, altered spinel grains associated with hornblende and chlorite show an increase in Cr/(Cr+Al) from core to rim. Altered spinel grains associated with serpentine show an increase in Fe²⁺/(Fe²⁺+Mg) from core to rim but may not show an increase in Cr/(Cr+Al). The compositional zoning in some altered spinel grains appears to result both from reaction of clinopyroxene plus spinel to form hornblende, and from reaction of hornblende to form chlorite. These observations suggest that subsolidus hydrothermal metamorphic effects need to be considered when interpreting spinel compositions and the compositions should not be interpreted solely in terms of igneous processes. Further, the presence of highly altered spinels may be indicative of hydrothermal alteration in rocks where other evidence of such alteration is absent.     

Influence of glass composition and alteration solution on leached silicate glass structure: A solid-state NMR investigation

A multinuclear solid-state NMR investigation of the structure of the amorphous alteration products (so called gels) that form during the aqueous alteration of silicate glasses is reported. The studied glass compositions are of increasing complexity, with addition of aluminum, calcium, and zirconium to a sodium borosilicate glass. Two series of gels were obtained, in acidic and in basic solutions, and were analyzed using ¹H, ²⁹Si, and ²⁷Al MAS NMR spectroscopy. Advanced NMR techniques have been employed such as ¹H-²⁹Si and ¹H-²⁷Al cross-polarization (CP) MAS NMR, ¹H double quantum (DQ) MAS NMR and ²⁷Al multiple quantum (MQ) MAS NMR. Under acidic conditions, ²⁹Si CP MAS NMR data show that the repolymerized silicate networks have similar configuration. Zirconium as a second nearest neighbor increases the ²⁹Si isotropic chemical shift. The gel porosity is influenced by the pristine glass composition, modifying the silicon-proton interactions. From ¹H DQ and ¹H-²⁹Si CP MAS NMR experiments, it was possible to discriminate between silanol groups (isolated or not) and physisorbed molecular water near Si (Q²), Si (Q³), and Si (Q⁴) sites, as well as to gain insight into the hydrogen-bonding interaction and the mobility of the proton species. These experiments were also carried out on heated samples (180 °C) to evidence hydrogen bonds between hydroxyl groups on molecular water. Alteration in basic media resulted in a gel structure that is more dependent on the initial glass composition. ²⁷Al MQMAS NMR data revealed an exchange of charge compensating cations of the [AlO₄]⁻ groups during glass alteration. ¹H-²⁷Al CP MAS NMR data provide information about the proximities of these two nuclei and two aluminum environments have been distinguished. The availability of these new structural data should provide a better understanding of the impact of glass composition on the gel structure depending on the nature of the alteration solution.     

Incipient hydrothermal alteration of basalts and the origin of martian soil

The martian soil is a fine-grained regolith that is chemically basaltic in character with evidence for both gains and losses of volatile and mobile elements compared to martian basalt compositions. These chemical fractionations provide clues to geochemical processes on the surface of Mars. Geochemical processes contributing to the soil proposed in the past include the chemical and mechanical breakdown of rocks under surface conditions, the addition of volcanic aerosols containing S and Cl compounds, and the alteration of basaltic glass to palagonite. Our studies of terrestrial analogs suggest that hydrothermal alteration processes involving impact craters and volcanism could also contribute to the major element trends observed in martian soil. Data from Viking, Pathfinder, and the current MER missions consistently show that relative to basaltic martian meteorite compositions, the major element compositions of the soils are (1) depleted in the fluid-mobile element calcium, (2) generally similar or somewhat enriched in iron oxide and magnesium but MgO depleted compared to Gusev rocks, (3) locally variable in potassium, (4) possibly poorer in aluminum, and (6) very enriched in chlorine and sulfur. The major element trends, aside from the Cl and S enrichment, could be explained by the formation or addition of palagonite according to McSween and Keil (2000), but the missing CaO remains a problem. The chlorine and sulfur are probably derived from other processes such as volcanic aerosols and hydrothermal fluids. McSween and Keil (2000) also argued that hydrothermal alteration of basalts produce alteration trends that are inconsistent with the Mars soil, but this study concludes otherwise. We have used quantitative mass balance mixing models to investigate possible models involving mixtures of basaltic compositions with different types of alteration materials, including palagonite. We show that the Mars soil composition can be matched with a combination of unweathered basaltic martian meteorites with basaltic FeO-rich, CaO-poor alteration products. Palagonite is a possible, but not a necessary component of successful model mixtures. The hydrothermal alteration materials that form successful model mixtures are formed in low temperature, low water/rock ratio environments, and they can reproduce the required geochemical trends because they are poorer in CaO but not in FeO compared to their respective protoliths. These results argue that material altered by hydrothermal processes could be a plausible component of the soil, and that removal of CaO from the soil into some undiscovered reservoir after its formation is not required. The current soil on Mars, therefore, did not have to undergo an episode of in situ aqueous alteration but could represent a sink for materials that experienced aqueous processes in a different setting before erosion to form the soil. The soil can also represent a sink for mobile elements (e.g., S, Cl, and Br) derived from other sources such as volcanic aerosols and hydrothermal fluids.     

天然含放矿物的水热蚀变研究进展

矿物固化体作为第二代高放废物固化体,具有良好的热稳定性、化学稳定性以及辐照稳定性。然而,目前用于化学稳定性研究的矿物固化体,很少含有放射性核素,或者没有积累足够的辐照损伤缺陷,所以很难反应固化体在真实地质处置库中的行为,并且在实验室研究中,也难以长期(1×10~4～10×10~4年)模拟HLW处置库中的物理化学条件。而自然界中的一些含放射性核素的矿物,经过百万年甚至上亿年的地质作用后,仍然保持着稳定的成分和形态。这些天然矿物因自辐照作用而在结构上与含放矿物固化体具有相似性,本文比较了天然矿物与HLW固化体水热蚀变的异同,并通过大量文献调研总结了天然矿物水热蚀变的现状,以为矿物固化体长期化学稳定性的研究提供科学依据和评估数据。     

Spectra and coordination changes of transition metals in hydrothermal solutions: Implications for ore genesis

Optical absorption spectra of Fe(II), Co(II), Ni(II) and Cu(II) have been measured in aqueous solutions of up to 5 m NaCl at temperatures from 25°C to 300°C and at water-saturated vapor pressures. Ni and Co complexes exhibited a change from octahedral to tetrahedral coordination, this occurring at both higher temperatures and Cl^? concentrations. Similar transitions to lower coordination number are predicted for Cu and Fe but were not directly observed because of interference with water overtone bands. The coordination changes in response to ligand type and concentration, pressure and temperature. Formation of lower coordination complexes is attributed to the decreased dielectric constant of the solvent, the predominance of electrostatic forces and a decrease in the octahedral site preference energy at elevated temperatures. Our data suggest that lower coordination complexes with lower or neutral formal charge, will result in minerals having a higher solubility. The molecular properties and changes in coordination of these complexes are important in determining the transport and deposition of hydrothermal minerals.     

High-temperature hydrothermal alteration of the Boehls Butte anorthosite: origin of a bimodal plagioclase assemblage

The Boehls Butte anorthosite consists predominantly of an unusual bimodal assemblage of andesine and bytownite–anorthite. Oxygen isotope compositions of the anorthosite were profoundly altered by high temperature, retrograde interaction with meteoric-hydrothermal fluids that varied in composition from isotopically evolved to nearly pristine meteoric water. Oxygen isotope ratios of bulk plagioclase separates are in the range +7.0 to −6.2‰ V-SMOW, however, secondary ion mass spectrometry indicates spot-sized isotope values as low as −16‰. Typical inter- and intra-plagioclase grain variability is 3–6‰, and extreme heterogeneity of up to 20‰ is noted in a few samples. High-temperature hydrothermal alteration of intermediate plagioclase is proposed to explain the origin of bytownite–anorthite in the anorthosite and creation of its unusual bimodal plagioclase assemblage. The anorthite-forming reaction created retrograde reaction-enhanced permeability which, together with rapid decompression, extension, and unroofing of the anorthosite complex, helped to accommodated influx of significant volumes of meteoric-hydrothermal fluids into the anorthosite.     

REE redistribution during hydrothermal alteration of ores of the Kalahari Manganese Deposit

The Kalahari Manganese Deposit (KMD) is the largest land-based manganese deposit, hosting approximately 80% of the world's known, mineable manganese resources. The deposit, located near Kuruman in the Northern Cape Province of South Africa, is one of five erosional relics of the Paleoproterozoic (ca. 2.2 Ga) Hotazel Formation, with sedimentary manganese ores occurring as up to 50 m thick beds interbedded with banded iron-formation (BIF) and hematite lutite.The study focuses on the manganese ores of the Nchwaning–Gloria mining area of the northern KMD. In this area, pronounced mineralogical and major element alteration was imparted on the sedimentary manganese ores by a structurally-controlled hydrothermal fluid flow event. Most notable effects of hydrothermal alteration are the decomposition and leaching of Ca- and Mg-carbonate, and marked residual enrichment of manganese. On the basis of mineral assemblage, grade, texture and geochemical characteristics, three ore types were distinguished in the studied sample set, classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. Advanced altered ores may be further classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. The rare earth element (REE) geochemistry of these fundamental ore types was studied in detail, to document REE mobility during hydrothermal alteration.Total REE concentrations in LA ores were found to be very low (14–22 ppm) and remarkably uniform, within the range typically observed for BIF. Hydrothermal alteration results in residual enrichment and a much larger scatter in REE contents. A small Ce anomaly observed in the protolith remains similar in magnitude when observed in PAAS-normalised REE plots. The data define, however, a power trend in the (Ce/Ce*) vs (Pr/Pr*) diagram. Such behaviour is interpreted in terms of a conservative system that was predominantly protolith-buffered. Local remobilisation of REE during hydrothermal alteration is attributed to the dissolution of diagenetic apatite and redistribution of hydrothermal trace minerals, including neoformed apatite, monazite and cerianite.     

A study of rock alteration process based on kinetics of hydrothermal experiment

Experimental investigation was made on hydrothermal alteration of sericite in acid solution. It is found that the sericite is changed to pyrophyllite through kaolinite at 270° C and 260° C, and the alteration is considered to be a consecutive reaction involving two steps, from sericite to kaolinite and from kaolinite to pyrophyllite. On the other hand, pyrophyllite is not formed at 190° C and the sericite is altered to kaolinite. The rate of reaction in each step of this hydrothermal alteration is dependent on reaction temperature and chemical composition of the solution. From the results of the quantitative analyses of the experimental products, kinetic constants and related parameters were obtained.By using these numerical values, an alteration process can be followed in a given model. Computed results for some cases of an alteration of sericite in acid solution are presented. Geological phenomena such as rock alteration or weathering are thought to progress in a disequilibrium state and in an open system. An attempt described in this paper is a physicochemical approach to treat an alteration process accompanied with material transfer occurring in an open system from a kientic viewpoint.