Metal-bearing Brines Research Articles

Methodology of experimental filtration studies (releases) from high-pressure inter-salt productive deposits of industrial lithium-bearing brines of the hydrogeological halogen-carbonate formation of the Siberian platform

It is known that all our knowledge about the industrial brines of the Siberian platform was obtained while drilling wells for oil and gas whereas targeted studies of brine-bearing horizons in deep wells for hydrocarbons are hardly conducted. One of the reasons is the high daily rate in the drilling cycle resulting in carrying out strictly limited additional works not related to drilling. Meanwhile, the geological results including the forecast, the estimation of the calculated parameters of the section productive intervals, the exploration and development methodology of deposits of industrial metal-bearing brines with abnormally high reservoir pressure directly depend on the data of the hydrodynamic studies. Today, large companies – subsurface users of the south of the Irkutsk region are tasked to determine the methodology of short-term pilot releases, which provides correct input data for reserve calculation of industrial brines of natural inter-salt carbonate fractured reservoirs of the hydrogeological halogen-carbonate formation. To determine a reliable permeability coefficient under the experimental filtration work, it is necessary to achieve a quasi-stationary filtration mode. The optimal mode is an intermittent one of brine release with a constant flow rate at the same time intervals with a continuous recording of pressure changes by the telemetry system. The results of pilot releases implemented at the Znamenskiy site of the Angara-Lena deposit of industrial lithium-bearing brines have been considered. The release methods enabling the performance of experimental filtration work in a short time, and applicable in well construction for hydrocarbon production have been proposed. Surface users can carry out the research using the proposed methodology in the case when the well bottom unexpectedly opens a brine formation with an abnormally high reservoir pressure of the fluid system. The implementation of experimental filtration work employing the described methods allows the subsoil users to substantiate the resource base of industrial brines and the elements in them including lithium, rubidium, cesium, bromine, iodine and other rare, trace elements and mineral salts.

Nanomaterial accumulation in boiling brines enhances epithermal bonanzas

Epithermal bonanza-type ores, characterized by weight-percent contents of e.g., gold and silver in a few mm to cm, are generated by mixtures of magmatic-derived hydrothermal brines and external fluids (e.g., meteoric) that transport a variety of metals to the site of deposition. However, the low solubilities of precious metals in hydrothermal fluids cannot justify the high concentrations necessary to produce such type of hyper-enriched metal ore. Here we show that boiling metal-bearing brines can produce, aggregate, and accumulate metal nanomaterials, ultimately leading to focused gold + silver ± copper over-enrichments. We found direct nano-scale evidence of nanoparticulate gold- and/or silver-bearing ores formed via nonclassical growth (i.e., nanomaterial attachment) during boiling in an intermediate-sulfidation epithermal bonanza. The documented processes may explain the generation of bonanzas in metal-rich brines from a range of mineral deposit types.

Trace metals in saline waters and brines from China: Implications for tectonic and climatic controls on basin-related mineralization

• Surficial metal-bearing brines can be regarded as the initial ore-forming fluids of basin-related mineral deposits . • Climate is the link between major tectonic events and global/large-scale basin-related mineralization . Research into mineralization using a deposit-centric approach has identified three principal factors (evolution of the hydrosphere–atmosphere, secular decrease in global heat production, and long-term global tectonic trends) that have influenced the global pattern of metallogenesis. However, these factors do not explain the peak occurrences of basin-related mineralization during particular periods of Earth’s evolution. Here we shift the research perspective from an ore-deposit-centric approach to one focusing on the formation processes of ore-forming fluids by using mass spectrometry to analyze the concentrations of metallic elements (U, Cr, Fe, Mn, Co, Ni, Cu, Mo, Cd, Sn, Pb, Zn, and V) of naturally occurring saline water bodies (brines) in currently developing basins in China. Our aim is to determine the presence of metal-bearing fluids and establish the formation processes of ore-forming fluids that can subsequently generate mineral deposits. The concentrations of trace metals in evaporated seawater and surficial saline waters (brines) in continental basins can reach up to mg/L level, with the increase in salinity resulting from the significant evaporation that occurs in semi-arid to arid environments. The high salinities increase the density of metal-bearing brines, which in turn causes these brines to circulate and mix with groundwater, resulting in enriched metal element concentrations in the groundwater. Groundwater containing dissolved evaporites also shows relatively high concentrations of metal elements (up to mg/L level) that compare with fresh groundwater. These metal-bearing brines can be regarded as the initial ore-forming fluids of basin-related mineral deposits, such as unconformity-type U deposits, red-bed Cu deposits, and some stratiform Pb–Zn deposits. The metal-bearing brines are the result of substantial evaporation under arid to semi-arid climates. Therefore, we conclude that climate is an important control on the formation of basin-related mineralization and that the tectonoclimatic setting plays an essential role in the formation of metal-bearing brines in closed inland basins. The major climatic events, such as the snowball and greenhouse event can affect the basin-related mineralization globally. The super mantle plume event could affect the global basin-related mineralization by changing the global climate significantly.

How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

Abstract Most of the world's Zn and Pb is extracted from sediment-hosted Zn-Pb deposits. The Zn-Pb deposits hosted in carbonate rocks are hypothesized to form by mixing of acidic metal-bearing brines with reduced sulfur-bearing fluids while dissolving sedimentary carbonate. To test the role of carbonate in this process, we conducted hydrothermal experiments simulating ore formation by reacting Zn ± Pb ± Ba–bearing brines with H2S and SO42– produced by native sulfur, with and without carbonate minerals (calcite or dolomite crystals), at 200 °C and water-saturated pressure. Sphalerite, galena, and barite (or anhydrite) crystals formed only when carbonate was present in the experiment, accompanied by carbonate dissolution. The textures of sphalerite clusters are similar to those observed in ancient and modern hydrothermal deposits. Thermodynamic modeling at 150 °C and 250 °C demonstrates that mixing of metal-rich brines and H2S causes most of the Zn in solution to precipitate as sphalerite only when carbonate dissolution occurs to buffer the pH, consistent with the experimental observations. The need for a pH buffer increases with increasing temperature, and different pH buffers may play a role for different deposit types. We propose that carbonate-buffered fluid mixing is a critical process for forming post-sedimentary Zn ± Pb ± Ba deposits in sedimentary carbonate rocks.

Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs.

He-Ar Isotopes and Trace Gas Compositions of Fluid Inclusions in Massive Sulphides from the Yushui Copper-Polymetallic Deposit, South China: Metallogenic Implications

The Yushui ore deposit, located in the middle section of the Yong’an-Meixian Hercynian depression, is a medium-sized Cu-polymetallic massive sulphide deposit in Eastern Guangdong Province, South China. This deposit is characterized by unusually high copper grade (up to 50–60 wt. % Cu). Other metallic elements, such as lead, zinc and silver, are also economically important in the Yushui ore bodies. The aim of this study was to apply N2–Ar–He systematics, together with organic gases (light-hydrocarbon tracers), to constrain the origin and evolution of ore-forming fluids. The helium-argon isotopes and trace gas compositions of fluid inclusions trapped within metal sulphide minerals were measured for a number of bonanza ores from the Yushui deposit. The noble gas concentrations in the studied samples vary over one to two orders of magnitude (4He: 2.27–160.00 × 10−5 cm3 STP g−1; 3He: 0.53–34.88 × 10−12 cm3 STP g−1; 40Ar: 6.28–37.82 × 10−7 cm3 STP g−1; 36Ar: 1.25–10.40 × 10−9 cm3 STP g−1). Our data show a narrow range of 3He/4He ratios from 0.006 to 0.056 Ra (~0.026 Ra on average, n = 8), which are considerably lower than the modern atmospheric end-member value; whereas the 40Ar/36Ar ratios (ranging from 333.76 to 501.68, with an average of 397.53) are significantly greater than that of air-saturated water. Most of the bornite samples have somewhat higher 3He/4He ratios of trapped fluids when compared to chalcopyrite. Overall, these He-Ar results are well within the range of crustal reservoir, thus implying a predominantly crustal source (originated from Caledonian basement) for ore-forming solutions, with little contribution from mantle-derived fluids. Analysis of the N2–Ar–He composition in Cu-rich sulphides indicates that the Yushui ore-forming fluids were probably derived from formation water (or basinal hot brines). Moreover, organic gas species identified in sulphide-hosted fluid inclusions are mainly composed of C1–C4 alkanes, while the concentrations of unsaturated olefins and aromatic hydrocarbons are very low. In particular, most chalcopyrite samples with relatively low 3He/4He ratios (0.006–0.016 Ra) and 40Ar*/4He values (0.0002–0.0012) are generally characterized by very high CO2/CH4 ratios (~60–102). All these suggest that main-stage Cu-Ag metallogenic processes might have not been affected by high-temperature magmatic activities or superimposed by strong metamorphic overprinting, although some chalcopyrite-rich ores appear to be influenced by later stage hydrothermal processes. In summary, neither magmatic input nor convecting seawater has played an important role in the formation of Yushui copper-polymetallic deposit. The massive sulphide ore bodies were products of water–rock interaction between metal-bearing basinal brines and the host sedimentary strata.

Polyphase vein mineralization in the Fennoscandian Shield at Åkerlandet, Järvsand, and Laisvall along the erosional front of the Caledonian orogen, Sweden

The Akerlandet, Jarvsand, and Laisvall deposits in Sweden are calcite-fluorite-sulfide vein deposits and occurrences located close to the current erosional front of the Caledonian orogen and hosted by crystalline basement rocks in the Fennoscandian Shield. At Laisvall, basement-hosted veinlets occur beneath Ediacaran to Cambrian sandstones that host a strata-bound Pb-Zn deposit. The mineralized fractures at Akerlandet and Jarvsand occur along fault systems oriented N–S to NNW–SSE. Veins or veinlets strike NNW–SSE and NW–SE at Akerlandet, NNE–SSW at Jarvsand, and NNW–SSE and NNE–SSW to NE–SW at Laisvall. At Akerlandet and Jarvsand, fractures acted as conduits for hydrothermal fluids of variable composition and formed during separate tectonic events. At Akerlandet, the fault zone with NNW–SSE strike shows kinematic indicators consistent with ~NE–SW bulk horizontal extension. At Jarvsand, the calcite-fluorite-galena veins formed along R-Riedel shears related to the host N–S to NNW–SSE fault system. The kinematic indicators are consistent with ~NW–SE bulk horizontal extension, similar to the extensional deformation during the later part of the Caledonian orogeny (Silurian to Devonian). At Akerlandet, adularia-quartz deposition was followed by sphalerite ± galena and finally by precipitation of fluorite and calcite. 40Ar-39Ar thermochronology of a single adularia sample did not yield a well-defined plateau age but the gas released at higher temperatures suggests an early Tonian (980 to 950 Ma) crystallization age, i.e., during the later part of the Sveconorwegian orogeny, although the data do not exclude other less likely interpretations. Previous fluid inclusion microthermometry and geochronological studies and new petrographic and geochemical results suggest that sphalerite ± galena mineralization formed from saline, relatively oxidizing, moderate-temperature, and slightly acidic hydrothermal fluids, either during the Ediacaran or the Middle Ordovician. Metals and H2S were derived from local basement rocks. Based on petrographic evidence, rare earth element composition, and S, C, and O isotope data, fluorite and calcite precipitated under near neutral and relatively reducing conditions. Occurrence of solid bitumen in veins at Akerlandet and C and O isotope data of calcite at Akerlandet and in the Laisvall basement veinlets suggest that the precipitation of calcite and fluorite was triggered by interaction of hot and evolved hydrothermal fluids (87Sr/86Sr = 0.718–0.732) with organic matter. Structural, petrographic, and geochemical data at Laisvall suggest that the basement structures hosting calcite-fluorite ± pyrite veinlets were utilized in the Middle Ordovician as the plumbing system for the oxidizing, slightly acidic, metal-bearing brines that caused the economic Pb-Zn mineralization in the overlying sandstones.

A Middle Ordovician Age for the Laisvall Sandstone-Hosted Pb-Zn Deposit, Sweden: A Response to Early Caledonian Orogenic Activity

Ten sphalerite separates isolated from mineralized samples in proximal and distal positions relative to the proposed main feeder fault systems at the Laisvall deposit were used to obtain an absolute age determination of this world-class Pb-Zn deposit hosted by autochthonous Ediacaran to Lower Cambrian sandstone and located currently along the erosional front of the Scandinavian Caledonides in northern Sweden. Residue and leachate fractions of each separate were obtained using the crush-leaching technique. All samples correspond to sphalerite formed using reduced sulfur derived from thermochemical sulfate reduction, three of them from disseminated ore in the Lower Sandstone, two from the disseminated ore in the Upper Sandstone, and five from steeply dipping galena-sphalerite-calcite veinlets interpreted in previous works as remobilization of disseminated ores. The isotope dilution-thermal ionization mass spectrometry (ID-TIMS) data yield an overall complex Rb-Sr isotope pattern with two distinct trends in the 87Sr/86Sr vs. 87Rb/86Sr isochron diagram. The three sphalerite residues of disseminated mineralization from the Lower Sandstone orebody show Rb-Sr isotope systematics indicative of undisturbed primary precipitates, and yield an isochron model age of 467 ± 5 Ma (mean square weighted deviation, MSWD, 1.4). Since the isochron is based on three points, the obtained age is to be considered as preliminary. Yet, the obtained age is fully consistent with geologic evidence reported by previous authors and pointing to Middle Ordovician timing of ore formation. The ID-TIMS data were complemented by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on the same sphalerite samples. The data support the hypothesis that the measured ID-TIMS Rb and Sr contents in these sphalerite residues are held in the sphalerite structure itself and are not related to micro-inclusions. The most viable hypothesis, in agreement with published work, is that during rapid growth, sphalerite may incorporate Rb and Sr ions from the hydrothermal fluids in its structure, most probably in octahedral voids. By contrast, the second trend in the 87Sr/86Sr vs. 87Rb/86Sr space defined by most other sphalerite residues and corresponding inclusion fluid leachates from the Upper Sandstone orebody and the veinlet samples is too steep to account for a realistic isochron age determination. This steep linear trend is interpreted to represent a postmineralization disturbance involving fluids rich in Sr. This disturbance of the Rb-Sr isotope system is consistent with the presence of the steeply dipping galena-sphalerite-calcite veinlets and the fact that the Upper Sandstone is, in places, tectonically disrupted because of its proximity to the basal Caledonian decollement. The attempt to date the Granberget deposit, located in tectonically disrupted allochthonous units inside the Caledonian orogen, failed because the Rb-Sr isotope systematics of the three analyzed sphalerite samples are also disturbed. The obtained Middle Ordovician (467 ± 5 Ma) mineralization age at Laisvall can be interpreted as a far-field foreland response to an early Caledonian arc-continent collision and the subsequent development of a foreland basin. Basinal brines formed in the foredeep of the orogen could be conveyed cratonward, interact with permeable Baltica crystalline basement rocks, and resurge as metal-bearing fluids in sandstone at Laisvall along reactivated Paleoproterozoic crystalline basement faults. Mixing of metal-bearing brines with hydrocarbon and H2S-rich fluids in Ediacaran to Lower Cambrian sandstone may explain the initial Sr isotope signature (87Sr/86Sr = 0.715900 ± 60) of the isochron intersect.

Microscale sulfur isotopic compositions of sulfide minerals from the Jinding Zn–Pb deposit, Yunnan Province, Southwest China

The Jinding Zn–Pb deposit, located in the Lanping basin in Northwest Yunnan Province, is the largest Zn–Pb deposit in China, and also probably the youngest sediment-hosted super giant Zn–Pb deposit in the world. Its genesis differs from the well-known major types of sediment-hosted Zn–Pb deposits. Based on mineral paragenesis and textures, there are two stages of mineralization: stage 1 that is typically characterized by fine-grained sulfide minerals (galena, sphalerite, pyrite and marcasite) disseminated in sandstones of the Lower Cretaceous Jingxing Formation (K1j), and massive sulfides in limestone breccias of the Paleocene Yunlong Formation (E1y); and stage 2 which mainly occurs as coarse-grained galena veins crosscutting stage 1 sulfides, and minor amounts of colloform sphalerite intergrown with galena. In situ sulfur isotopic analyses of galena, sphalerite and pyrite were determined by secondary ion mass spectrometry (SIMS), and showed highly variable δ34S values (−42.1‰–7.7‰) of different ore types. Stage 1 mineralization has δ34S values from −42.1‰ to −10.2‰ with the majority ranging from −26‰ to −14‰. Stage 2 mineralization has higher δ34S values (−8.3‰–7.7‰). Combined with the geological settings and mineral paragenesis, the sulfur isotopic data presented here suggest multiple sulfur sources (biogenic sulfur+evaporites) and formation mechanisms for reduced sulfur (H2S). H2S responsible for stage 1 sulfide precipitation was associated with bacterial sulfate reduction (BSR). However, H2S of stage 2 was likely derived from thermochemical sulfate reduction (TSR). The most reasonable scenario for the stage 1 mineralization is a metal-bearing brine mixing with an H2S-rich fluid, thereby causing rapid sulfide precipitation. Till the stage 2, the ore-forming fluid shifted to the meteoric water that infiltrated and reacted with evaporitic rocks, leached metals and transported them as sulfate- or sulfite-complexes to the Jinding dome where the oxidized sulfur was reduced by organic matters to H2S, leading to precipitation of metal sulfides. In contrast to other sulfide deposits in the Lanping basin, biogenic sulfur might have played a key role in the mineralization process, especially during the early stage of formation of the Jinding Zn–Pb deposit.

Metalliferous Manganese Oxide Mineralization Associated with the Boleo Cu-Co-Zn District, Mexico

Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boleo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanechite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %). Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO 2 -Fe 2 O 3 t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boleo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boleo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boleo Formation strata at elevations above the present-day water table.

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boleo district of the Miocene Santa Rosalia basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boleo Formation is characterized by modified marine carbon (δ13CPDB=−6.0 to +4.4‰) and oxygen (δ18OSMOW=+19.5 to +26.2‰) isotope composition due to the influx of 13C- and 18O-depleted fluvial water. Sulfate sulfur isotope composition (δ34SCDT=+17.21 to +22.3‰ and δ18OSMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ34S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ34S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ13C=−11.6 to −3.2‰ and δ18O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ13C=−7.9 to +4.3‰ and δ18O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.

Significance of strain localization and fracturing in relation to hydrothermal mineralization at Mount Isa, Australia

The rheology of layered meta-sedimentary rocks, and their orientation and position relative to major fault systems were the key controls on Proterozoic hydrothermal copper mineralization at Mount Isa, Australia. Compositional layering in the host rock partitioned mechanical behavior and strain, leading to selective permeability generation and the focusing of fluid flow. Shale layers preferentially failed by plastic shearing, whereas meta-siltstones remained elastic or failed in tension depending on magnitude of deformation and fluid pressure. Numerical simulations support the hypothesis that the orientation of layering and the proximity to major fault systems controlled fracturing and permeability increase in the Urquhart shale. The dilating shale provided a pathway for an upward-flowing, reduced basement fluid, from which quartz was precipitated during cooling. During a later event, the reactivation of steep structures provided access to surface derived oxidized metal-bearing brine, causing the precipitation of dolomite followed by chalcopyrite ore in the brecciated silicified shale.

The Paleoproterozoic carbonate-hosted Pering Zn–Pb deposit, South Africa. II: fluid inclusion, fluid chemistry and stable isotope constraints

The Pering deposit is the prime example of Zn–Pb mineralisation hosted by stromatolitic dolostones of the Neoarchean to Paleoproterozoic Transvaal Supergroup. The hydrothermal deposit centers on subvertical breccia pipes that crosscut stromatolitic dolostones of the Reivilo Formation, the lowermost portion of the Campbellrand Subgroup. Four distinct stages of hydrothermal mineralisation are recognised. Early pyritic rock matrix brecciation is followed by collomorphous sphalerite mineralisation with replacive character, which, in turn, is succeeded by coarse grained open-space-infill of sphalerite, galena, sparry dolomite, and quartz. Together, the latter two stages account for ore-grade Zn–Pb mineralisation. The fourth and final paragenetic stage is characterised by open-space-infill by coarse sparry calcite. The present study documents the results of a detailed geochemical study of the Pering deposit, including fluid inclusion microthermometry, fluid chemistry and stable isotope geochemistry of sulphides (δ34S) and carbonate gangue (δ13C and δ18O). Microthermometric fluid inclusion studies carried out on a series of coarsely grained crystalline quartz and sphalerite samples of the latter, open-space-infill stage of the main mineralisation event reveal the presence of three major fluid types: (1) a halite–saturated aqueous fluid H2O–NaCl–CaCl2 (>33 wt% NaCl equivalent) brine, (2) low-salinity meteoric fluid (<7 wt% NaCl) and (3) a carbonic CH4–CO2–HS− fluid that may be derived from organic material present within the host dolostone. Mixing of these fluids have given rise to variable mixtures (H2O–CaCl2–NaCl ±(CH4–CO2–HS−), 2 to 25 wt% NaCl+CaCl2). Heterogeneous trapping of the aqueous and carbonic fluids occurred under conditions of immiscibility. Fluid temperature and pressure conditions during mineralisation are determined to be 200–210°C and 1.1–1.4 kbar, corresponding to a depth of mineralisation of 4.1–5.2 km. Chemical analyses of the brine inclusions show them to be dominated by Na and Cl with lesser amounts of Ca, K and SO4. Fluid ratios of Cl/Br indicate that they originated as halite saturated seawater brines that mixed with lower salinity fluids. Analyses of individual brine inclusions document high concentrations of Zn and Pb (∼1,500 and ∼200 ppm respectively) and identify the brine as responsible for the introduction of base metals. Stable isotope data were acquired for host rock and hydrothermal carbonates (dolomite, calcite) and sulphides (pyrite, sphalerite, galena and chalcopyrite). The ore-forming sulphides show a trend to 34S enrichment from pyrite nodules in the pyritic rock matrix breccia (δ34S = −9.9 to +3.7‰) to paragenetically late chalcopyrite of the main mineralisation event (δ34S = +30.0‰). The observed trend is attributed to Rayleigh fractionation during the complete reduction of sulphate in a restricted reservoir by thermochemical sulphate reduction, and incremental precipitation of the generated sulphide. The initial sulphate reservoir is expected to have had an isotopic signature around 0‰, and may well represent magmatic sulphur, oxidised and leached by the metal-bearing brine. The δ18O values of successive generations of dolomite, from host dolostone to paragenetically late saddle dolomite follow a consistent trend that yields convincing evidence for extensive water rock interaction at variable fluid–rock ratios. Values of δ13C remain virtually unchanged and similar to the host dolostone, thus suggesting insignificant influx of CO2 during the early and main stages of mineralisation. On the other hand, δ13C and δ18O of post-ore calcite define two distinct clusters that may be attributed to changes in the relative abundance in CH4 and CO2 during waning stages of hydrothermal fluid flow.

Hypogene and supergene alteration of the Late Palaeozoic Ratburi Limestone during the Mesozoic and Cenozoic (Thailand, Surat Thani Province). Implications for the concentration of mineral commodities and hydrocarbons

An interdisciplinary study of the Upper Carboniferous to Middle Permian Ratburi Group, Peninsular Thailand, is presented. The investigation involved sedimentary petrography, inorganic geochemistry, Sr, C, O isotope analyses, micropalaeontology as well as radio-carbon age dating. Emphasis was placed on the post-depositional evolution of the Ratburi Limestone in the Surat Thani Province. The Holocene chemical residues and the various calcite and dolomite minerals which have formed since the Late Palaeozoic in the Ratburi Limestone are the product of a complex, multistage alteration which is called supergene and hypogene karstifications, respectively. Sedimentation took place in a shelf environment with some reefs evolving during the late Murgabian at the shelf margin. There was no pre-concentration of elements, except for Ca and F during sedimentation. Diagenetic neomorphism and cementation under marine and freshwater conditions caused the Ratburi Limestone to convert into a marble-like rock. Fabric-selective dolomitization is of local scale and has impacted only on part of the Ratburi Limestone during the Lower to Upper Permian. A significant enhancement of pore space and better conduits were generated during the Late Cretaceous epithermal alteration. The most favorable conditions for the accumulation of metals were provided during the high-temperature stage of epithermal alteration when a low-metal concentration with As, Zn, Sb, U, Co and Pb existed. Unlike the other elements, Sb was subject to a multiphase concentration, giving rise to a considerable Sb deposit in the region. The most recent stage of karstification produced numerous caves, dripstones, tufa terraces and encrustations around brine pools in the study area. This alteration originated from per descensum and per ascensum processes which may be traced back to 15,000 years before present. The alteration of the Ratburi Limestone may be subdivided into two parts. The prograde post-depositional alteration, beginning with diagenesis, reached its temperature climax during epithermal subsurface alteration I. The retrograde branch of alteration lasted until the most recent times. The initial stages deposition and diagenesis took place under more or less closed-system conditions relative to the succeeding stages of the prograde alteration which saw the strongest influx of metal-bearing brine during the epithermal stage I. The retrograde branch of alteration is “element-conservative”.

Processes of ore formation in the stratiform sediment-hosted Zn [sbnd]Pbdeposits of Northern Australia: testing the Century model

Numerical simulations of chemical processes of ore formation are used to test hypotheses concerning the genesis of northern Australian Proterozoic stratiform sediment-hosted Zn Pb deposits. Exhalation of metal-bearing brines into anoxic marine waters is shown to be effective for massive sulfide ore formation. The ‘Century’ model, late diagenetic massive sulfide deposition involving metal-laden brines interacting with hydrocarbon reservoirs, is also demonstrated to have potential as an ore-forming process. It requires that H 2S (g) is available in the gas reservoir and that acidification is prevented by dissolution of wall rock carbonate. Seawater mixing is capable of producing laminated sulfide ore. Brine-sour gas interaction will most likely produce discordant Zn Pb ore due to carbonate dissolution.

Relation Between Sedimentation and Diapirism at the Davis Hill Salt Dome, Liberty Co., Texas

ABSTRACT The Davis Hill dome is a relatively large salt stock located in the northeastern part of the Houston salt dome province. The minimum depth is about 800 ft to the cap rock and about 1,200 ft to salt. The source layer (Jurassic Louann Salt) lies at a depth of at least 30,000 ft. Seismic and well log interpretation suggests that diapir growth took place from Late Cretaceous to Plio-Pleistocene. Passive growth (downbuilding) of the dome ceased during the Early Oligocene (Vicksburg) and was followed by active growth (upbuilding) of the dome. Present-day topographic relief at Davis Hill is 175 ft above the surrounding coastal plain. A radial pattern of oxbow lakes above the dome suggests that the dome still is rising, but the topographic maximum does not correspond to the apex of the dome. Studies of cores indicate that the cap rock is about 440 ft thick and comprises, from bottom to top, anhydrite, gypsum and calcite zones. The anhydrite zone represents the less soluble components of the salt that accumulated as the halite was dissolved by pore fluids. The calcite zone formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone in these cores contains significant amounts of sulfide minerals and oil. The sulfiderich calcite zone is approximately 40 ft thick and is dominated by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool ground waters.

Mississippi valley-type mineralization in the Silurian paleoaquifer, central appalachians

This study used lead and sulfur isotope tracers to determine the nature and geometry of MVT-related brine flow in the Silurian paleoaquifer of the central Appalachians. Although the stratigraphy and lithology of this sedimentary package is similar to that of the important Lower Cambrian MVT-hosting paleoaquifer of the southern Appalachians, it lacks important mineralization. Lead isotope compositions from MVT deposits in the Lockport Dolomite, in the upper part of the Silurian paleoaquifer, form a cluster at about 206Pb 204Pb = 18.3 , 207Pb 204Pb = 15.6 , and 208Pb 204Pb = 38.2 , which is similar in form but slightly different in composition from those seen in deposits in underlying clastic rocks. Sulfur isotope compositions for Lockport MVT sulfides range from −19 to −6‰, and have lower δ 34S values than MVT sulfides hosted by underlying clastic rocks. These relations, in combination with NaClBr compositions of fluid inclusion leachates, suggest that MVT deposits in the lower part of the paleoaquifer formed when rising metal-bearing brines intersected reduced sulfur of probable thermogenic origin, whereas deposits in the upper part of the paleoaquifer (Lockport Dolomite) formed where metal-bearing brines intersected reduced sulfur of biogenic origin that was dissolved from sulfate evaporites in the overlying Salina Group and transported downward into the Lockport.

The possible role of organic matter in transport and accumulation of metals exemplified at the Permian Kupferschiefer formation

The Permian Kupferschiefer in the Central European Zechstein Basin has been deposited under anoxic sedimentary conditions. The shale contains on average about 5% of organic matter which is classified as kerogen Type II. In areas where an ore-grade mineralization of the Kupferschiefer is not observed (i.e. NW Germany), the organic matter follows solely a temperature-induced transformation process. Copper and silver enrichment in the Kupferschiefer is associated with the occurrence of Rote Fäule. At those places where Rote Fäule came in contact with or very close to the Kupferschiefer, secondary alteration reactions are detectable in the solvent-extracted organic matter as well as in the kerogen. This effect is pronounced in the Kupferschiefer of the Konrad mine investigated in the present study. The analysis of solvent extracts has been carried out using methods such as gas chromatography coupled to mass spectrometry (GC/MS). For the investigation of the kerogen pyrolysis experiments (“Rock-Eval”) have been undertaken. In a closely sampled Kupferschiefer profile from the Konrad mine (SW Poland), significant depletion of the organic matter and the hydrogen content of the organic matter is found in the bottom part of the section approaching the Rote Fäule horizon. The precipitation of copper from ascending oxidizing solutions occurred primarily in the inner part of the Kupferschiefer section, where sufficient hydrogen for abiotic thermochemical sulphide production was still available. Thus, the organic matter acted as a reducing barrier mediating the metal accumulation from upwards migrating metal-bearing brines (ascending brine theory). The determination of the maturity parameters indicates, that the organic matter was mature when oxidation occurred.

Epigenetic dolomitization and iron mineralization along faults and their possible relation to the paleohydrology of southern Israel

Iron mineralization occurs in carbonate rocks of the Cretaceous Judea Group in northern Sinai and the Negev. This mineralization, along with strong enrichment of trace metals, fills fissures and veins. The mineralization is not directly emplaced into the Judea Group limestones but rather within epigenetic dolomites or ankerites which postdate the country rocks. The epigenetic phenomena are narrowly restricted to regional tectonic lineaments. The association of mineralization phenomena with tectonic zones of high permeability suggests that fluid transfer between two distinctly different geochemical environments was the main mineral emplacing agent. Epigenetic dolomitization initially resulted from upflow of old Mg-rich brines along the fault face. During later stages, the residual CaCl 2 brines flowing through the underlying Paleozoic-Mesozoic clastic rocks accumulated Fe and trace metals by leaching of volcanic intrusions. The introduction, via faulting, of these pressurized, acidic, low Eh, metal-bearing brines into the overlying aerated and alkaline carbonate surroundings led to precipitation of Fe which in turn scavenged the trace metals. The Mg-rich brines developed during the Cambrian-Lower Cretaceous mostly continental time-interval. The episodic expulsion of pressurized dolomitizing and metal-bearing brines was related to various stages of tectonic and igneous activity occuring during the Early Miocene-Pleistocene.

Supercritical two-phase separation of hydrothermal fluids in the Troodos ophiolite

The hot (350 °C) black smoker springs of the ocean floor are the vents of hydrothermal systems in which cold seawater descends below the ocean floor, is heated, reacts with the host rock and is transformed into metal-bearing brine. The salinity of these brines varies between vents from ∼0.7 to 2.0 times seawater salinity1–4 this variation has been ascribed to crustal hydration5, crystallization of chloride-bearing minerals6 or two-phase separation of seawater into components of differing salinity4,7–12. Here we present fluid inclusion evidence for two-phase separation in the ancient Troodos sulphide-forming systems of Cyprus and discuss, with the NaCl-H2O system as a model, how this process could explain observed salinity variations in hydrothermal fluids.