Barite Precipitation Research Articles

An experimental investigation of barite formation in seawater

We report results from time-series decay and sequential leaching experiments of laboratory cultured and coastal plankton to elucidate the mechanisms controlling barite formation in seawater. Batch-cultured diatoms ( Stephanopyxis palmerina) and coccolithophorids ( Emiliania huxleyi) were let to decay in the dark for 8–10 weeks, suspended in aerated seawater. The development of barite crystals was monitored by Scanning Electron Microscopy (SEM). A similar experiment was conducted with plankton collected during the spring-bloom in Vineyard Sound (MA). In addition to SEM, suspended particles were sequentially leached for Ba (distilled water rinse; 10% (v/v) HNO 3 rinse at room temperature; 30% (v/v) HCl at 80°C overnight; 50% (v/v) HNO 3 at 80°C overnight) immediately after collection, and after 10-week decay in seawater, in seawater poisoned with HgCl 2, and in seawater spiked with 135Ba. Both experiments showed an increase in the number of barite crystals during decay. The spring-bloom plankton had initially a large pool of labile Ba, soluble in distilled water and cold dilute HNO 3 that was lost from the plankton after 10-week decay in both axenic and nonaxenic conditions. In contrast, Ba in the decayed plankton samples was predominantly in forms extracted by hot HCl and hot HNO 3 acids, which were attributed to presence of barite Ba and refractory organic Ba respectively. The increase in barite crystal counts under a Scanning Electron Microscope (SEM), the increase in HCl extractable Ba relative to organic carbon, and the loss of a large fraction of Ba during plankton decay suggest that living plankton consists of a relatively large pool of labile Ba, which is rapidly released during plankton decomposition and acts as the main source of Ba for barite formation in supersaturated microenvironments. Since mass balance indicates that only a small proportion (2 to 4%) of the labile-Ba pool is converted to barite, the availability of microenvironments that could locally concentrate Ba released by plankton decay seems to be the main limiting factor in barite precipitation.

Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins

Stratiform (bedded) Paleozoic barite occurs as large conformable beds within organic- and chert-rich sediments; the beds lack major sulfide minerals and are the largest and most economically significant barite deposits in the geologic record. Existing models for the origin of bedded barite fail to explain all their characteristics: the deposits display properties consistent with an exhalative origin involving fluid ascent to the seafloor, but they lack appreciable polymetallic sulfide minerals and the corresponding strontium isotopic composition to support a hydrothermal vent source. A new mechanism of barite formation, along structurally controlled sites of cold fluid seepage in continental margins, involves barite remobilization in organic-rich, highly reducing sediments, transport of barium-rich fluids, and barite precipitation at cold methane seeps. The lithologic and depositional framework of Paleozoic and cold seep barite, as well as morphological, textural, and chemical characteristics of the deposits, and associations with chemosymbiotic fauna, all support a cold seep origin for stratiform Paleozoic barite. This understanding is highly relevant to paleoceanographic and paleotectonic studies, as well as to economic geology.

Real-time ocean chemistry for improved biogeochemical observation in dynamic coastal environments

We describe a new ocean observation system that integrates mass spectroscopy, hydrographic instrumentation, and satellite imagery (SeaWIFS). We used a quadrupole ICP mass spectrometer at sea to acquire continuous trace element data during separate surveys of the Baja California coastal margin, and the San Diego Bay and coastal environment. There is evidence for extreme Ba depletion in surface waters off the Baja coast, which is the result of biological productivity and marine barite precipitation. The synoptic data are used to elucidate the biochemical mechanism of barium removal; to constrain the spatial and temporal boundaries over which the phenomenon occurs; and to quantify surface flux to sediments. Further systems application led to mapping of the distribution of a suite of biologically active trace metals (Mn, Ni, Zn, Cu, and Cd) in San Diego Bay, and the Bay signature was tracked in tidal plumes into the coastal ocean. The continuous data provided chemical gradients within the Bay, with which we estimate the contaminant metal flux that is discharged from the Bay into the coastal ocean during tidal pumping.

Silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits: an example of meteoric‐dominated diagenesis from the Triassic of central Spain

Cauliflower‐shaped nodules are widespread in a single red mudstone bed in the Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly of quartz, dolomite and calcite, but other minerals, such as barite, kaolinite and iron oxyhydroxides, are also present. The nodules are spherical, ovoid or elongate in shape and range from 1 to 8 cm across. The surface of the nodules is irregular, and some show a pedogenic coating of microspar. The sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of Ca2+ and SO42– in a vadose environment, occasionally under the influence of pedogenic processes. Partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting, as indicated by the presence of organic filaments on the quartz crystal surfaces. In type A nodules, the dissolution of the innermost anhydrite was complete, and different types of quartz cement filled the porosity. Fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules. In type B nodules, the dissolution of the anhydrite was incomplete, inhibiting quartz cementation and enabling later dolomitization of the anhydrite. Dolomitization appears to have been driven by sulphate reduction, as indicated by the presence of bacterial bodies within the dolomite crystals. Dedolomitization and precipitation of barite, kaolinite and calcite spar cements occurred later under the influence of meteoric solutions. The nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin. Their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions.

Geochemistry of suspended particles in the Subantarctic and Polar Frontal zones south of Australia: Constraints on export and advection processes

Filtered particles from a Southern Ocean meridional transect (from 42°S to 54°S along 142°E) off Tasmania were studied between surface and 600‐m water depth (SAZ 1998 expedition, February‐April 1998). Particulate Ba, Sr, Ca, P, U, Th, Ti, Al, Mn, light rare earth elements, and Zr contents were analyzed by inductively coupled plasma‐mass spectrometry and inductively coupled plasma‐atomic emission spectroscopy. Mesopelagic Baxs (total Ba minus a lithogenic fraction estimated from Al) contents were higher within the Polar Front Zone (PFZ) in comparison with the northern areas, indicating that C export to the intermediate and deep ocean is more important in the PFZ. Average surface Uxs versus latitude covaries with mesopelagic Baxs, contrasting with organic matter proxies such as PP and chlorophyll a. This similar behavior suggests that U might be preferentially adsorbed on organic material, which subsequently becomes involved with barite precipitation. Srp and Cap contents decrease sharply with depth, following total particle content. For the northern area (42°–47°S), surface Ce anomaly reflects essentially a lithogenic signature (0.6<Ce/Ce*<1) ascribed to the supply from the Tasmanian continental shelf. Suspended matter from surface waters at stations south of 47°S shows an enhanced seawater signature (0.3<Ce/Ce*<0.6). In all stations, Ce anomalies increase with depth, toward values close to 1. This is likely due to the increase of lithogenic/authigenic ratio of particles with depth, rather than redox processes. Finally, an advective event is identified at 45°S below 200 m by a strong increase of lithogenic element contents, which affects Baxs.

Impact‐induced hydrothermal activity within the Haughton impact structure, arctic Canada: Generation of a transient, warm, wet oasis

Abstract— Field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot, impact‐generated rocks following formation of the 24 km diameter, 23 Ma Haughton impact structure. Hydrothermal alteration is recognized in two settings: within polymict impact breccias overlying the central portion of the structure, and within localized pipes in impact‐generated concentric fault systems. The intra‐breccia alteration comprises three varieties of cavity and fracture filling: (a) sulfide with carbonate, (b) sulfate, and (c) carbonate. These are accompanied by subordinate celestite, barite, fluorite, quartz and marcasite. Selenite is also developed, particularly in the lower levels of the impact breccia sheet. The fault‐related hydrothermal alteration occurs in 1–7 m diameter subvertical pipes that are exposed for lengths of up 20 m. The pipes are defined by a monomict quartz‐carbonate breccia showing pronounced Fe‐hydroxide alteration. Associated sulfides include marcasite, pyrite and chalcopyrite. We propose three distinct stages in the evolution of the hydrothermal system: (1) Early Stage (>200 °C), with the precipitation of quartz (vapor phase dominated); (2) Main Stage (200‐100 °C), with the development of a two‐phase (vapor plus liquid) zone, leading to calcite, celestite, barite, marcasite and fluorite precipitation; and (3) Late Stage (<100 °C), with selenite and fibroferrite development through liquid phase‐dominated precipitation. We estimate that it took several tens of thousands of years to cool below 50 °C following impact. During this time, Haughton supported a 14 km diameter crater lake and subsurface water system, providing a warmer, wetter niche relative to the surrounding terrain. The results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems.

Barium peaks at glacial terminations in sediments of the equatorial Atlantic Ocean—relicts of deglacial productivity pulses?

In sediment cores recovered from oligotrophic settings of the western and eastern north-equatorial Atlantic Ocean, Ceará Rise and Sierra Leone Rise, distinct peaks of excess barium were detected at glacial/interglacial transitions. These Ba maxima are unrelated to any other potential productivity proxy, e.g. organic carbon, carbonate or opal. Moreover, they coincide with minima in organic carbon contents. Despite the lack of correlation between excess barium and organic carbon, we ascribe these Ba spikes to pulses of productivity that occurred during glacial/interglacial transitions. The discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward-progressing oxidation fronts during deglacial nonsteady-state depositional conditions. The oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced NADW production during interglacial periods. The fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short-time productivity events. This assumption is supported by the depth arrangement of the solid phase peaks of Ba, Fe, and Mn along the 6/5 oxygen isotope stage boundary in sediments of the Ceará Rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern Mediterranean sediments. It remains unclear whether productivity was proportional to the magnitude of the Ba excess maxima. Possibly, higher dissolved Ba concentrations in intermediate and deep waters during glacial/interglacial transitions caused the precipitation of barite to increase out of proportion of the (postulated) deglacial productivity pulses.

Preliminary Fluid Inclusion Constraints on Fluid Evolution in the Bayan Obo Fe-REE-Nb Deposit, Inner Mongolia, China

The Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China, is the world’s largest REE resource and is also exploited as a major Fe oxide deposit. The deposit consists of replacement bodies hosted in dolomite marble and of magnetite, hematite, and REE minerals associated with apatite, aegirine, amphibole, fluorite, calcite, and barite. Fluid inclusions from monazite-(Ce), bastnasite-(Ce), apatite, aegirine, fluorite, and barite were studied from eight samples to provide new constraints on ore genesis. During early disseminated monazite mineralization the fluids are best represented by the system H 2 O-CO 2 -NaCl, with salinities ranging from 1 to 5 wt percent NaCl equiv and X (CO 2 ) values from 0.3 to 0.45, at temperatures of >280° to 330°C and pressures of >0.7 kbar. Fluids in apatite and bastnasite from the main-stage banded ores were initially aqueous, with salinities ranging from 6 to 10 wt percent NaCl equiv. They developed significant CO 2 contents (X (CO 2 ) = 0.1–0.3) due to carbonate dissolution at temperatures of >400° to ~300°C and pressures of >0.9 to 1.4 kbars. Evidence for phase separation from coexisting brine and CO 2 -rich fluid inclusions in vein fluorite indicates pressures of 0.8 to 1.0 kbar during vein formation. These pressures can be used to infer temperatures of 430° to 460°C for aegirine alteration and of 340° to 240°C for apatite, bastnasite, and fluorite vein fill. Fluids responsible for aegirine alteration were dominantly aqueous, with salinities from 5 to 15 wt percent NaCl equiv, whereas those responsible for apatite and bastnasite mineralization had salinities of 1 to 6 wt percent NaCl equiv and X (CO 2 ) values from 0.25 to 0.75. Inclusions in late-stage fluorite and barite have homogenization temperatures from 240° to 150° and 200° to 130°C, respectively. The general salinity trend in these inclusions suggests dilution from around 15 to 5 wt percent NaCl equiv down to very low salinities (<1 wt % NaCl equiv) during barite precipitation. Halite-bearing inclusions may indicate episodes of boiling or aqueous-carbonic fluid immiscibility. These fluids are similar to those in some carbonatitic, alkaline igneous, and other igneous-related REE- and Fe-rich magmatic hydrothermal systems. However, they are different from fluids in igneous-related Fe oxide-Cu-Au-U-REE deposits, which have previously been compared to those at Bayan Obo.

Mass wasting, ephemeral fluid flow, and barite deposition on the California continental margin

Research Article| April 01, 2000 Mass wasting, ephemeral fluid flow, and barite deposition on the California continental margin T. H. Naehr; T. H. Naehr 1Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA Search for other works by this author on: GSW Google Scholar D. S. Stakes; D. S. Stakes 1Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA Search for other works by this author on: GSW Google Scholar W. S. Moore W. S. Moore 2Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (4): 315–318. https://doi.org/10.1130/0091-7613(2000)28<315:MWEFFA>2.0.CO;2 Article history received: 17 Sep 1999 rev-recd: 24 Dec 1999 accepted: 11 Jan 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. H. Naehr, D. S. Stakes, W. S. Moore; Mass wasting, ephemeral fluid flow, and barite deposition on the California continental margin. Geology 2000;; 28 (4): 315–318. doi: https://doi.org/10.1130/0091-7613(2000)28<315:MWEFFA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An area of active fluid discharge characterized by authigenic barite formation and populated by vestimentiferan tubeworms has been discovered by remotely operated vehicle exploration in Monterey Canyon. Slumping and mass wasting, presumably triggered by seismic activity, have exposed barium-rich pore fluids to sulfate-rich bottom water, leading to barite precipitation on the sea floor. Age estimates based on 210Pb/226Ra dating of the barite samples and growth rates of vestimentiferan tubeworms indicate that the seep site might be as young as 100 yr. Systematic variations in sulfur and strontium isotope ratios, the lack of visible fluid flow, and the abundance of dead tubeworms indicate decreasing flow rates. This newly discovered site shows many similarities to a previously described barite deposit in the California Borderland, possibly misinterpreted as a hydrothermal vent site. The discovery of a cold seep associated with barite deposits on the seismically active, transpressional California margin raises the possibility that numerous unexplored areas along the continental margins might have similar low-temperature deposits. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A model for growth of steep‐sided vent structures on the Endeavour Segment of the Juan de Fuca Ridge: Results of a petrologic and geochemical study

Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep‐sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia‐ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia‐poor fluids present at most other unsedimented mid‐ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans‐Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.

Mechanisms of Surface Precipitation and Dissolution of Barite: A Morphology Approach

Barite (BaSO4) was synthesized at normal pressure and room temperature by mixing sodium sulfate and barium chloride solutions by gentle flow (0.5 ml/min.). The well-formed rectangular and rhombohedral crystals with smooth faces formed from solutions with low degrees of supersaturation. Most precipitates became single crystals. We employed the scanning electron microscope (SEM) to observe barite dissolution by chemical agent diethylenetrinitrilopentaacetic acid. Molecular modeling was used to assist the interpretation of the surface reaction. A kinetic model for the pit growth (initial dissolution) on the surface was evaluated. With SEM microanalysis of barite and kinetic model of pit growth, important surface phenomena and mechanisms of dissolution can be hypothesized. Surface phenomena and composition are very important in barite dissolution. Experimental results suggest that the initial dissolution starts on the two lowest energy surfaces: (001) and (210), and that the etch pits are elongated in one direction on the (210) surface and formed cavities on the (001) surface. The simulated surface structures by molecular modeling can aid the interpretation the pit formation on those selected surfaces.

Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120°E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (4) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.

Rapid barium removal in the Delaware estuary

Six profiles of dissolved barium covering the entire salinity range of the Delaware River and Bay estuary from March through September 1996 were collected and analyzed. The profiles are similar to one another in both shape and magnitude except for one attribute. A sudden (≤24 days), nearly complete (>90%) removal of dissolved Ba in midestuary occurs in mid-May followed by an 80% recovery in early June. This removal appears to be temporally and spatially coupled to the end of the spring bloom. Based on such episodic behavior, and on recent work with flocculation of diatom exudates, we conclude that the Ba depletion is caused by barite precipitation in the estuary during the late stages of the bloom. This would imply that lower estuary and inner coastal margin sediments associated with eutrophic estuaries receive a seasonal pulse of barite. The suddenness of this event also implies that sedimentary barite is strongly influenced by high productivity events. Comparison of the riverine Ba concentration with the effective riverine end member after desorptive barium release yields an estimated 30–40 nM Ba available from the suspended sediments as they enter the estuary. This estimate is supported by excess barium in unfiltered samples over filtered samples taken from the river and also by calculations done elsewhere.

Yttrium and rare earth elements in fluids from various deep-sea hydrothermal systems

Rare earth element (REE) and yttrium (Y) concentrations were measured in fluids collected from deep-sea hydrothermal systems including the Mid-Atlantic Ridge (MAR), i.e., Menez Gwen, Lucky Strike, TAG, and Snakepit; the East Pacific Rise (EPR), i.e., 13°N and 17–19°S; and the Lau (Vai Lili) and Manus (Vienna Woods, PacManus, Desmos) Back-Arc Basins (BAB) in the South-West Pacific. In most fluids, Y is trivalent and behaves like Ho. Chondrite normalized Y-REE (Y-REE N) concentrations of fluids from MAR, EPR, and two BAB sites, i.e., Vai Lili and Vienna Woods, showed common patterns with LREE enrichment and positive Eu anomalies. REE analysis of plagioclase collected at Lucky Strike strengthens the idea that fluid REE contents, are controlled by plagioclase phenocrysts. Other processes, however, such as REE complexation by ligands (Cl −, F − SO 4 2−), secondary phase precipitation, and phase separation modify REE distributions in deep-sea hydrothermal fluids. REE speciation calculations suggest that aqueous REE are mainly complexed by Cl − ions in hot acidic fluids from deep-sea hydrothermal systems. REE concentrations in the fluid phases are, therefore, influenced by temperature, pH, and duration of rock-fluid interaction. Unusual Y-REE N patterns found in the PacManus fluids are characterized by depleted LREE and a positive Eu anomaly. The Demos fluid sample shows a flat Y-REE N pattern, which increases regularly from LREE to HREE with no Eu anomaly. These Manus Basin fluids also have an unusual major element chemistry with relatively high Mg, SO 4, H 2S, and F contents, which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation. REE distribution in PacManus fluids may stem from a subseafloor barite precipitation and the REE in Demos fluids are likely influenced by the presence of sulfate ions.

The petrology and paragenesis of fracture mineralization in the Sellafield area, west Cumbria

SUMMARY The petrology and paragenesis of fracture mineralization hosted in the Ordovician, Dinantian (Carboniferous Limestone) and Permo-Triassic rocks of the Sellafield area were studied as part of the United Kingdom Nirex Limited programme of site investigations around Sellafield in west Cumbria for a deep repository for radioactive waste. This paper summarizes the petrological and mineralogical observations from 23 of the deep (up to 2 km) boreholes drilled by Nirex. A paragenetic sequence of nine Mineralization Episodes (ME1 to ME9) was recognized, and has been interpreted in the context of the geological and hydrogeological history of the Lake District massif and East Irish Sea Basin margin areas. ME1 to ME3 produced silicate and sulphide-dominated Palaeozoic epithermal to mesothermal mineralization associated with pre-Acadian hydrothermal circulation and late-Caledonian (mid-Devonian) intrusions. ME4 to ME7 represent carbonate-sulphate-fluorite-hematite-dominated mineralization associated with warm, complex, Na-Cl-Ca-SO 4 brines expelled from the East Irish Sea Basin area during progressive burial of the thick Carboniferous and Permo-Triassic sedimentary sequence. Sulphate-carrying brines expelled to the basin margin in the Sellafield area mixed locally with cooler, more dilute, sulphate-poor groundwaters carrying barium, resulting in the precipitation of barite. ME8 and ME9, by contrast, are closely related to the development of the present-day groundwater system. ME8 involved telodiagenetic, supergene alteration and weathering by near surface oxidizing groundwaters. Isotopic evidence indicates that ME8 was probably initiated following Late Tertiary uplift and may still be ongoing in some areas. ME9 may be coeval with ME8, but represents mineralization in the deeper, more-reducing groundwater environment. This mineralization episode is dominated by calcite (sometimes associated with pyrite, marcasite, barite, anhydrite or gypsum) precipitated from groundwaters of similar composition to those found in the Sellafield area at the present day. Observations suggest that ME9 mineralization occurred during the Quaternary and is ongoing in the present groundwaters.

Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼ 3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120 E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (5) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.

Element mobility during diagenesis: sulphate cementation of Rotliegend sandstones, Southern North Sea

Several wells in the Amethyst Gas field of the North Sea's Southern Basin are poor producers and have been since they were drilled. The lack of gas flow from these wells is due to pervasive cementation of the Rotliegend sandstone reservoir by either anhydrite and/or barite. Both minerals precipitated late in the diagenetic history of the sandstones. Such cements from up to 20% of the total rock. Isotopic and geochemical evidence indicate that the source of the elements for these sulphate cements was outside the Rotliegend sandstone. The sulphur and oxygen isotope data for the anhydrite and barite are unlike those which could have precipitated in Lower Permian times from an evaporating marine basin. Nor are they like any sulphur and oxygen isotope ratios that might be derived from a provenance area for the Rotliegend Sandstone. However the sulphur and oxygen data are identical to anhydrite in the overlying Upper Permian, Zechstein carbonate and evaporate sequences. The Rotliegend sandstones did not contain much barium at deposition. However, unlike the sulphate, the Zechstein sequences are a highly unlikely source for barium, given the highly insoluble nature of barite. A more probable source for barium is the underlying Carboniferous Coal Measures. Precipitation of barite and anhydrite occurred during basin inversion. Specifically the cements are most abundant in gas wells that lie close to inverted normal faults that cut both the Permian and Carboniferous intervals. Both the timing and distribution of these cements is taken to indicate that faulting allowed, or indeed promoted, mixing of sulphate-rich and barium-rich formation waters derived from the Zechstein and Carboniferous, respectively.

Formation on the sea floor of the Hellyer volcanogenic massive sulfide deposit

New fluid inclusion data from the stockwork veins beneath the Hellyer orebody, a relatively undeformed, Late Cambrian polymetallic volcanogenic massive sulfide deposit, allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide. The earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies. Subsequent vent fluids, buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater, mixed with the cooler brine. During and after filling the basin to the level of its lowest outlet, density inhomogeneities within the brine pool were reduced by double-diffusive convection across the brine-seawater interface and internal boundaries, and mixing by currents within the brine pool. Estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage (avg 200°C, 11 wt % salinity), assuming a steady or quasisteady state, had been established, and using volume fluxes similar to those observed in modern oceans, indicate temperatures ≪100°C. Sulfides (mostly pyrite, sphalerite, and galena) precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor, a pattern that would have continued whether or not a true steady state was reached. If the early fluids were buoyant in seawater they ponded after mixing, forming a hybrid brine pool that would have required longer periods to homogenize. At the peak temperature (≥300°C) and volume flux the salinity of the vent fluids fell to <7.5 wt percent, but the flow paths would have been maintained unless the brine pool had been drained. At this time that part of the massive sulfide body over the vent (the Cu core) grew by dilatational veining together with leaching, cavity infilling, replacement, and recrystallization. Chalcopyrite was deposited both in the Cu core and probably at the same time in a layer over the flanking massive sulfide. In the final stage as temperature fell and salinity rose (avg 250°C, 11 wt % salinity), the fluid oxidation potential increased so that barite precipitation dominated, resulting in a formation of a discontinuous barite-rich cap over the massive sulfide. The brine pool model explains several features of the Hellyer ore that differ from those of the ores of the Hokuroko basin, namely, the large metal content, high Zn/Cu ratio, low aspect ratio, absence of chimney fragments, presence of mineral banding, and high barite sulfur isotope values, features seen in many other massive sulfide orebodies.

Morphological and compositional evidence for biotic precipitation of marine barite

Barite formation in the surface oceans is generally assumed to be dominated by abiotic precipitation. Acceptance of this pathway is largely the result of the absence of a pelagic marine organism known to precipitate the ovoid to rounded-rectangular barite crystals typically observed in marine waters and sediments. Barite crystals observed in net-tow particles and on substrates retrieved from the seafloor (both in the central North Pacific) were examined by scanning electron microscopy and energy dispersive X-ray spectrometry. Three distinct crystalline forms of bar&e were observed: ovoid and hexagonal crystals several microns in diameter, and aggregates of submicron-sized crystals. Ovoid and hexagonal-type crystals contained between 0 and 26 mole percent SrS04. The microcrystalline barite contained no detectable Sr (CO.05 percent). Hexagonal-type crystals were precipitated by an unusual benthic foraminifera. Comparison of the morphology and composition of the barite crystals observed in this study to crystals precipitated by a variety of biotic and abiotic processes suggests a biotic origin for the ovoid barite crystals, the most common form of barite observed in this region.

Volcanic‐hosted sulfide‐barite deposit from the Kita‐Bayonnaise submarine caldera, Izu‐Ogasawara arc, northwestern pacific

Volcanic‐hosted sulfide and barite mineralization with high Au content (19.3 ppm) was discovered by submersible study (Shinkai 2000) on the eastern floor of the Kita‐Bayonnaise submarine caldera on the Shichito‐Iwojima Ridge, Izu‐Ogasawara Arc, northwestern Pacific. The hydrothermally mineralized rhyolitic tuff is partly coated by Mn oxyhydrox‐ide, and associated with hydrothermal Mn crusts and debris of mussel shells in the vicinity of the hydrothermally altered tuff‐breccias. The minearlized rhyolitic tuff occurs as a fragment of a large block (about 2 ×1 ×1 m) on talus, while the crusts occur on the caldera floor as a few thin patches several meters across or smaller. Ore‐mineral composition of the mineralized rhyolitic tuff consists mainly of sulfides and barite. Randomly interstratified seriate and montmorillonite is the only hydrothermal clay mineral from glass shards in the mineralized rhyolitic tuff. The mineralized tuff is significantly enriched in Zn, Fe, Cu, Ba, Au, Ag, and As, and has high Au /Ag ratio (0.036 to 0.071) compared with other marine volcanogenic sulfides. Au is probably present as submicrometer‐sized particles within sulfide minerals or is contained within the crystal lattice of the sulfide minerals. The δ34S values of sulfides of subsamples from the mineralized rhyolitic tuff indicate a fairly narrow range between +3.4 and +4.8% but are characterized by lighter isotopes in a drusy cavity compared with vein and disseminated sulfides. The sulfide sulfur is mainly from volcanic rocks of the caldera basement. A single barite sulfur isotopic value (+20.7%o) and salinity of fluid inclusions in barite are close to modern seawater values, suggesting a major contribution of seawater sulfate to the barite precipitation.