Which water formed Australian sediment-hosted precious and potch opal?

The means and timing of the formation of Australian sediment-hosted precious opal remain a subject of continuing debate. In this study, the question of which water formed the opal is addressed by examination of rare earth element data for opals and host rocks. The available data, mainly for Lightning Ridge, NSW, suggest a positive Eu anomaly, relative to the neighbouring Sm and Dy, occurs in opals whereas no such anomaly was found for the weathered Cretaceous sediments hosting the opal. Such anomalies may be inherited from the source rock with a similar positive Eu anomaly or generated in situ by severe reduction. There is no indication of major reduction processes during the opal formation that could have led to such a Eu anomaly so this is likely inherited from a source rock. As the opal host rocks did not show this anomaly, the source rocks must be external to the opal fields. Calcite cements within rocks hosting the aquifers of the Eromanga and Surat basins of the Great Artesian Basin have been reported to have a positive Eu anomaly, which strongly suggests that opal was formed by upwelling Great Artesian Basin artesian waters. This work has also highlighted variations in trace-element concentrations in opals, which indicate significant variation in the source water composition during opal formation or different water sources were involved. Either of these is indicative of the source for the opal with its trace elements derived from external sources. These conclusions have significant implications to considerations of how opal formed, and hence, for the exploration for other deposits and to the chemistry and timing that led to the formation of opal.     

Exceptional silica speleothems in a volcanic cave: A unique example of silicification and sub‐aquatic opaline stromatolite formation (Terceira,Azores)

Silica stromatolites occur in a number of modern hydrothermal environments, but their formation in caves is very rare. The silica stromatolitic speleothems of the Branca Opala cave (Terceira Island, Azores), however, provide an excellent opportunity for their study. These formations may be analogous to ancient silica stromatolites seen around the world. Petrographic, mineralogical and geochemical analyses were undertaken on the silica speleothems of the above cave, and on the silica‐tufa deposits outside it, with the aim of understanding their genesis. The possible hydrothermal origin of their silica is discussed. X‐ray diffraction analyses showed opal‐A to be the sole silica phase. Negligible ordering of this opal‐A showed ageing to be insignificant, as expected for recent silica deposits. Most of the silica speleothems examined were definable as sub‐aquatic opaline stromatolites that are not currently growing. Optical microscopy clearly revealed a lower microlaminated, an intermediate and an upper microlaminated zone within the stromatolites. Stromatolite types (I, II and III) were classified with respect to their internal structure and distribution throughout the cave. Scanning electron microscopy showed silicified bacterial filaments within the stromatolites, the silicified plant remains and the silica‐tufa deposits. Bacteria therefore played a major role in the precipitation of the opal‐A. Plasma emission/mass spectrometry showed major, minor and rare earth elements to be present in only small quantities. The rare earth elements were mainly hosted within volcanic grains. Rapid silica precipitation from highly super‐saturated water would explain the intense silicification of the plant remains found inside and outside the cave. The opaline stromatolites, the silica‐tufa deposits and the above‐mentioned intense general silicification suggest a local hydrothermal source for the silica. Indeed, these deposits strongly resemble plant‐rich silica sinter associated with low‐temperature hot spring deposits that include bacterial filaments. However, no geochemical signals that might indicate a hydrothermal origin could be found.     

Structure of a silica diagenetic transformation zone: the Gjallar Ridge,offshore Norway

Three‐dimensional seismic data from the Gjallar Ridge were used together with X‐ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high‐amplitude reflections; (ii) a strong reflection event at the top of this interval which cross‐cuts stratigraphy, interpreted as the transformation boundary between opal‐A‐rich and opal‐CT‐rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal‐A and opal‐CT; and (iv) a second, deeper, cross‐cutting reflection event within the interval, interpreted as the transformation boundary between opal‐CT‐rich and quartz‐rich sediment. The base of the interval containing the anomalously high‐amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica‐rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0·8 to 2·5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal‐A to opal‐CT transformation boundary and probably results from more complete transformation of opal‐A to opal‐CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross‐cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.     

Miocene silcretes in argillaceous playa deposits, Madrid Basin, Spain: petrological and geochemical features

Four regressive sequences are present in the opaline rocks and related deposits of the Miocene Intermediate Unit of the Madrid Basin. The sequences consist of silty mudstones and argillaceous opals, separated by transitional facies. The silty mudstone consists mainly of dioctahedral smectites, whereas the argillaceous opal is principally opal-CT and variable amounts of sepiolite. In the transitional facies, lamina of dioctahedral smectite co-exist with neoformed opal-CT and sepiolite. Petrological and geochemical features (major, trace and REE elements) indicate that the opaline levels and the transitional facies are related and are a consequence of silcrete formation in an argillaceous playa deposit. The isocon method was used to calculate changes in element concentration associated with silcrete formation. The geochemical data suggest silicification in an arid environment. The silcrete profile occurs four times, possibly as a result of highstand–lowstand fluctuations of the lake level caused by climatic changes. Structures and cements in the silcretes indicate that, although silicification may have commenced at the top of a groundwater table, it continued in the unsaturated zone above the water table.     

Seasonal variations of microbial sulfate and iron reduction in alkaline Pb–Zn mine tailings (Ontario, Canada)

Several studies have shown that SO₄-reducing bacteria (SRB) are active in acidic sulfide-rich mine tailings and sediments impacted by mining activities. SRB activity in acidic tailings has been shown to vary with seasons as a result of fluctuating in situ physico-chemical conditions. Iron-reducing bacteria (FeRB) also play an important role in Fe cycling in sediments impacted by mining activities, but their activity in mine tailings is poorly understood, despite the fact that geochemical evidence indicates that they might be active. The present study was undertaken to assess the seasonal changes in SRB and FeRB abundance and activity in alkaline Pb–Zn mine tailings (Calumet tailings) located near Ottawa, ON, Canada. Results showed that FeRB and SRB populations were present throughout the year at two different sampling sites at the Calumet tailings, but SO₄ reduction rates (SRR) were lower in the spring than in the summer, indicating that SRB activity was affected by organic C availability and/or temperature. Surface agricultural runoff at one site provided ample nutrients and organic C to the tailings, but SRB activity remained lower than the site not impacted by nutrient runoff, suggesting that the type of organic C was different between the two sites and that less labile organic substrates were available to SRB in the organic-rich site. High SRB activity in the site containing low organic C inhibited the abundance of FeRB, and possibly their activity, as a result of abiotic reduction of Fe(III)-rich minerals by biogenic sulfides, which lowered the pool of final electron acceptors. The abiotic reduction pathway was consistent with the porewater data which showed that sulfide was consumed and SO₄ produced, along with Fe(II). These results show a strong interdependence between SRB and FeRB activity, as observed in other environments, such as saltmarsh sediments. Low temperature did not appear to hinder FeRB abundance in alkaline tailings. Finally, despite evidence that SRB populations were active at both sites, the |S isotopic composition of the AVS and CRS fractions were not representative of biogenic sulfides, indicating that the overall S-isotope signature of mine tailings is more representative of abiotic sulfides originating from the ore body.     

The role of water in opal

The occurrence of water in natural opaline silicas has been studied by differential thermal analysis, thermogravimetric analysis, infra‐red analysis and nuclear magnetic resonance. The results show that in the “crystalline” opals studied, some 90 per cent or more of the total water is physically adsorbed whereas in “amorphous” opals, at least 20 per cent but perhaps much more of the total water is held as hydroxyl groups chemically bonded to the silica surface. The rate of water loss on heating is also different, being chiefly controlled by the pore structure in “crystalline” opals but to a significant extent by the surface structure in “amorphous”.     

Stepwise fluorination—a useful approach for the isotopic analysis of hydrous minerals

Analytical uncertainties in oxygen isotopic studies of hydrous silica have been investigated using a partial fluorination procedure in which fractional oxygen yields are achieved by reducing the amount of fluorine. Stepwise reaction of opaline silica results in a set of sequential oxygen fractions which show a wide range of δ¹⁸O values due to variable amounts of water, organic matter, and other impurities, δ-values for successive fractions in non-biogenic opal systematically increase as water is reacted away and then remain constant to within ±0.2%. as the remaining silica reacts, δ-values in biogenic silica increase similarly but then decrease when low ¹⁸O oxide(?) impurities begin to react.The troublesome water component in opal is readily removed by Stepwise fluorination. This technique allows more precise oxygen isotope analysis of non-biogenic opal-A, and may improve the analytical precision for biogenic silica and any silicate mineral containing a significant water component.     

Mineralization of a Proterozoic Sulfide Black Smoker Chimney and Thermophilous Microorganisms in Eastern Hebei, China

A sulfide black smoker chimney exists in the Gaobanhe seabed exhalation massive sulfide deposit in the Xingiong-Kuancheng secondary fault basin of the Proterozoic Yaniiao rift trough in Hebei Province, taking the shape of mounds, individually about 2-3 cm high. Abundant fossils of thermophilous bacteria and algae in perfect preservation are found in the ore surrounding the black smoker chimney. Scanning electron microscopy （SEM） and molecular biomarker studies on the microorganismai ore fabric show that the microorganism in the sulfide ore is in fact a sedimentary product of probiotic bacteria and algae. In the special food chain based on black smoker chimney at ancient seabed- thermophilous bacteria, the thermophilous bacteria and algae reproduce in large quantity. Intermittently erupting of fluid from the chimney creats conditions for formation of sulfide deposit. In the process of exhalation action of hot fluid, thermophilous bacteria and algae grow and reproduce around the sulfide black smoker chimney, absorbing mineralizing substances brought by the fluid. Massive sulfide deposits are formed in this process of absorption of seabed black smoker chimney exhalation-mineralizing fluid pulsation-thermophilous microorganism.     

Silica diagenesis of Neogene diatomaceous and volcaniclastic sediments in northern Japan*

Marine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal-CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal-CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K. In subsurface sections, values of d(101) spacing of opal-CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal-CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal-CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal-CT; one is progressively ordered opal-CT and the other is additionally cemented opal-CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.     

埃塞俄比亚欧泊充填处理及鉴定特征研究

埃塞俄比亚WOLLO地区产出的欧泊品种丰富,但火欧泊和水欧泊不稳定,易因外界温度或水分变化形成较多裂纹,严重影响其稳定性及加工性能。因此,本实验对埃塞俄比亚WOLLO地区产出的多裂纹火欧泊、水欧泊样品进行了充填处理,并对充填前、后的样品进行了显微放大观察和红外光谱、相对密度、紫外荧光的测试及分析,总结出了有机充填欧泊的一些鉴定特征。测试及分析结果显示,在2800-3100cm叫范围内红外吸收峰为有机充填欧泊的主要鉴定特征,表面及内部显微充填特征、偏小的相对密度、裂隙处的蠕虫状及空洞缺陷处的斑块状强蓝白色荧光可作为辅助鉴定特征。从样品的充填及测试分析结果可见,埃塞俄比亚WOLLO地区产出欧泊的有机充填方案可行,比较明显的开放裂隙易被充填。     

洛川黄土剖面中蛋白石的发现及其意义

洛川黄土剖面位于陕西省中部,洛川县城南5km 的坡头村附近。该剖面厚度大、地层发育较完整,系第四纪(Q₃—Q₁)风成堆积黄土中的典型剖面。前人从多方面进行过广泛深入研究。黄土的主要矿物成分为石英、长石和/或云母、方解石。在物质成分研究中提到过50多种矿物,但未涉及蛋白石。在黄土的风化成土作用研究中提到“植物蛋白岩偶见”(刘东生等,1985),但未作蛋白石矿物学工作。笔者这次发现的蛋白石,作了较多的矿物学研究。     

Sequence of mineral formation in clastic ores of the Saf’yanovka volcanic-hosted copper massive sulfide deposit,the Central Urals

The bedded clastic ore widespread on the slopes and flanks of the deeply eroded sulfide mound at the Saf’yanovka volcanic-hosted copper massive sulfide deposit consists of products of destruction of the Paleozoic black smoker along with diverse newly formed sulfides. The size of ore clasts gradually decreases with distance from the massive ore mound, from more than tens of centimeters to a few millimeters. The clastic sediments are characterized by good preservation of sulfide material composed of hydrothermal sedimentary colloform pyrite, chalcopyrite with lamellae of relict isocubanite, and concentrically zoned sphalerite. Numerous pyrite framboids, nodules, and euhedral crystals; chalcopyrite segregations; and twinned sphalerite are typical of sulfide-bearing black shale. Enargite, tennantite, and galena were formed after pyrite, filling interstices between nodules or partially replacing and corroding the previously formed minerals. The interrelations between minerals show that the fine-clastic sulfide-bearing black shale underwent diagenesis in the presence of organic matter.     

贵州遵义黑色岩系多金属层中铂族元素的赋存状态

本文运用逐级化学提取和重液分离,结合电感耦合等离子体质谱分析,研究了遵义地区黑色岩系多金属层中铂族元素的赋存状态及其沉淀富集规律。实验结果表明,铂族元素主要赋存于硫化物类矿物中,其中Pt和Pd的赋存形式有所不同,有一部分Pt还存在于粘土矿物中,Pd则除部分在粘土矿物中外,还可能有独立矿物存在。结合前人的研究,可以认为成矿溶液中铂族元素可呈不同形式的络合物迁移;粘土矿物、有机质及金属硫化物在PGE富集过程中起着地球化学障的作用。     

The geochemistry of gem opals as evidence of their origin

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si⁴⁺ by Al³⁺ and Fe³⁺. The induced charge imbalance is compensated chiefly by Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺, K⁺, and Na⁺. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.     

Aluminosilicate cementation of saprolites,grits and silcretes in Western Australia

Amorphous to poorly crystalline aluminosilicates have been found as cementing agents within saprolites, hardpans and silcretes, particularly in granitic terrains in the Yilgarn Block, Western Australia. The cements range mineralogically from siliceous allophane to a kaolinite‐opal‐CT assemblage probably derived from the allophane. The allophane is non‐crystalline, with no distinctive X‐ray or electron diffraction patterns, and has close optical similarities to opaline silica. It is characterised by Al/Si ratios of 0.35–1.0, with small amounts of adsorbed iron and titanium. It occurs as colloform void and vein fillings, and permeating the kaolinitic matrix of saprolites. The kaolinite and opal‐CT have a similar occurrence, with the kaolinite oriented parallel to colloform and related banding. The aluminosilicate cements are found in saprolite, usually in the upper zones, and in overlying grits and silcretes. At Gabbin, in the central Yilgarn, the grits are up to 10 m thick and have aluminosilicate as the only cement. Although very hard when in situ, the grits tend to disaggregate on exposure and rarely, if ever, crop out. In profiles exposed in breakaways of the Barr‐Smith Range in the northern Yilgarn, the saprolite is overlain by silcretes with a quartz‐anatase‐zircon assemblage as cementing matrix, in turn overlain by alumino‐silicate‐cemented grits. The contact is gradational, and most silcretes and grits contain both types of cement, with the aluminosilicate increasing in abundance upwards. Kaolinitic spherules in sand plains are possibly derived from similar aluminosilicates.     

Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge

The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems.     

Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°-10°N

To characterize the hydrothermal processes of East Pacific rise at 9o-10oN, sulfide mineral compositions, textural, and geochemical features of chimney ores were studied using ore microscope, scanning electron microscope, X-ray diffraction analysis, and electron microprobe techniques. Results show that there are three mineral assemblages for the hydrothermal chimney ores, namely: (i) anhydrite marcasite pyrite, (ii) pyrite sphalerite chalcopyrite, and (iii) chalcopyrite bornite digenite covellite. Mineral assemblages, zonational features, and geochemical characteristics of the ore minerals indicate that ore fluid temperature changed from low to high then to low with a maximum temperature up to 400 ℃. The chimney is a typical black smoker. The initial structure of the chimney was formed by the precipitation of anhydrites, and later the sulfides began to precipitate in the inner wall.     

Occurrences of iron sulfides in Ohio coals

The iron sulfides in seven different Ohio coals have been studied by polished-section ore microscopy, scanning electron microscopy augmented with EDS analyses, and secondary ion mass spectrometry. The iron sulfides in the coals exhibit a large array of textures and interrelationships which reflect site-specific environmental changes that occurred during the deposition of the sulfides. Sulfide deposition occurred principally during the depositional and diagenetic phases of the formation of the precursor peats. Pyrite and marcasite are present in most of the samples examined. Pyrite occurs as isolated and clustered euhedra, isolated and clustered framboids and spheres, cell-fillings, cleat- and fracture-fillings, replacements of plant debris, and as a porous or spongy-textured variety deposited within and around sulfide masses and grains. Marcasite occurs as polycrystalline spheres, polycrystalline rims and bands within and around clusters of pyrite spheres and framboids, as cell-fillings, and as replacements of plant debris. A typical sequence of iron sulfide deposition in texturally complex sulfide grains and masses is: (1) pyrite framboids or spheres; (2) deposition of marcasite around the relict framboids and clusters of framboids; and (3) spongy pyrite deposited as an outer fringe around sulfide masses and as infillings within the masses. The sulfides exhibit a persistent, although not universal, association with clays, and it is likely that much of the iron now present as sulfides was delivered to the depositional environment adsorbed on clay minerals. The iron sulfides tend to be localized in zones parallel to the banding in the coals. Such localization is most pronounced with respect to specific varieties of iron sulfides such as marcasite spheres, pyrite framboids, zones of pyrite euhedra, and occurrences of texturally complex grains and masses. Such zones are believed to represent depositional environments favorable for the precipitation of specific types of iron sulfides. These stratigraphic microenvironments changed during the times of deposition and diagenesis of the precursor peats and resulted in sequential deposition of the different forms of iron sulfides particularly evident in texturally complex sulfide grains. Chemically significant variables most likely were pH and availability of certain trace elements. The factors favoring precipitation of marcasite rather than pyrite are not clearly understood.The textures of the iron sulfides will prove to be important in understanding the reactivity of pyrite and marcasite in causing acid mine drainage and possibly spontaneous combustion of coal and mine waste, and will be important in the continuing development of effective methods of coal cleaning.     

河北兴隆地区硫化物黑色页岩地球化学及海底喷气成因研究

河北兴隆－宽城一带块状硫化物矿床 板发育厚度不等的纹层状硫化物黑色页岩。此类黑色页岩Ｚｎ的质量分数平均为４．０４％,Ｓ的质量分数平均为１１．７３％,其中闪锌矿,黄铁矿均呈纹层状赋存在黑色页岩内。     

Geochemistry and morphology of metalliferous sediments and oxyhydroxides from the Endeavour segment, Juan de Fuca Ridge

We present first data on the geochemistry, mineralogy and morphology of near-vent sediments (35 and 200 m from active vent) and ridge flank sediments (approximately 3 km from the vent field) as well as oxyhydroxide deposits from the Endeavour segment, Juan de Fuca Ridge. The purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains. Hydrothermal components in sediments are Fe-Si ± S-rich and Mn-Fe-Si-rich phases, sulfides and barite, which were exclusively derived from plume fallout. Sulfides are only a minor constituent of near-vent sediments (2-4 wt%) and were not detected in ridge flank sediments. The study suggests that the distribution of hydrothermal phases and associated elements in near-vent and ridge flank sediments is affected mainly by processes of agglomeration, dissolution, absorption and settling that take place within a plume and to a lesser extent post-depositional processes. Rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the Cu concentrations in sediments with increasing distance from the vents. Besides sulfides, important carriers of Pb, Cu, Zn and Co in near-vent sediments are Fe-Mn oxyhydroxides that occur together with silica as aggregates of gel-like material and flaky particles and as coatings on filaments. Away from the vents, trace metals are mostly in Fe-Mn oxyhydroxides and authigenic Fe-rich montmorillonite. Oxyhydroxides at the Main Endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary Fe-oxyhydroxide + silica from low-temperature fluids. At the Mothra field, seafloor deposits and chimney crusts composed of Fe-oxyhydroxide ± Mn + silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater. Oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities.