Ore-bearing Solutions Research Articles

Study on the evolution of ore-formation fluids for Au-Sb ore deposits and the mechanism of Au-Sb paragenesis and differentiation in the southwestern part of Guizhou Province, China

Ore deposits (occurrences) of Au, As, Sb, Hg, etc. distributed in Southwest Guizhou constitute the important portion of the low-temperature metallogenic domain covering a large area in Southwest China, with the Carlin-type Au and Sb deposits being the most typical ones. In this paper the Au and Sb ore deposits are taken as the objects of study. Through the petrographic analysis, microthermomitric measurement and Raman spectrophic analysis of fluid inclusions in gangue minerals and research on the S and C isotopic compositions in the gold ore deposits we can reveal the sources of ore-forming materials and ore-forming fluids and the rules of ore fluid evolution. Ore deposits of Au, Sb, etc. are regionally classified as the products of ore fluid evolution, and their ore-forming materials and ore fluids were probably derived mainly from the deep interior of the Earth. Fluid inclusion studies have shown that the temperatures of Au mineralization are within the range of 170–361°C,the salinities are 0.35 wt%–8 wt% NaCl eq.; the temperatures of Sb mineralization are 129.4–214°C and the salinities are 0.18 wt%–3.23 wt% NaCl eq.; the ore-forming fluid temperatures and salinities tend to decrease progressively. In the early stage (Au metallogenic stage) the ore-forming fluids contained large amounts of volatile components such as CO2, CH4, N2 and H2S, belonging to the H2O-CO2-NaCl fluid system; in the late stage (Sb metallogenic stage) the ore-forming fluids belong to the Sb-bearing H2O-NaCl system. The primitive ore-forming fluids may have experienced at least two processes of immiscibility: (1) when early ore-bearing hydrothermal solutions passed through rock strata of larger porosity or fault broken zones, CO2, CH4, N2 would escape from them, followed by the release of pressure, resulting in pressure release and boiling of primitive homogenous fluids, thereafter giving rise to their phase separation, thus leading to Au unloading and mineralization; and (2) in the late stage (Sb metallogenic stage ) a large volume of meteoric water was involved in the ore-forming fluids, leading to fluid boiling as a result of their encounter, followed by the drop of fluid temperature. As a result, the dissolubility of Sb decreased so greatly that Sb was enriched and precipitated as ores. Due to differences in physic-chemical conditions between Au and Sb precipitates, Au and Sb were respectively precipitated in different structural positions, thus creating such a phenomenon of Au/Sb paragenesis and differentiation in space.

Isotope and geochemical evidence for the influence of ore-bearing solutions on basic dykes of the Dal’negorskii ore region

Isotope and geochemical evidence for the influence of ore-bearing solutions on basic dykes of the Dal’negorskii ore region

Influence of fluid systems of the consolidated crust on degassing of the earth and oil formation

Fluid systems in the consolidated continental crust associated with large permeable zones have been distinguished on the basis of geophysical data. These zones serve as migration pathways for fluids uplifting from the mantle. The conditions of interaction of such fluids with large horizontal and subvertical fluid systems during degassing are considered. Horizontal systems, fixed at geophysical profiles as waveguides, contain significant amounts of high-temperature fluids, which interact there with enclosing rocks and fluids uplifting during degassing with the formation of oil- and ore-bearing solutions. The experiments on the interaction of water and solutions with rocks under increased temperatures carried out in the second half of 1960s gave an idea about the character of similar reactions. It has been revealed that significant amounts of organic matters pass into solutions, and bitumen is generated at 150–250°C. Similar reactions promote formation of new oil fields.

Geological-hydrogeological characteristics and paleohydrogeological reconstructions of the Balei Gold Field, Transbaikalia

The Balei epithermal vein gold-quartz deposit is located in the Lower Cretaceous sedimentary sequence. The detailed geological and hydrogeological study of the deposit allowed us to reconstruct the paleohydrogeological conditions of the ore formation. This was particularly promoted by the preservation of the initial geological and hydrogeological structure—a graben-like depression hosting a subsurface water basin. The ore field was formed due to the Early Cretaceous high-temperature hydrothermal activity. The ore-bearing solutions originated via the interaction of the deep fluids with rocks and local subsurface waters. The solutions boiled at the depth of ∼600 m and gold was deposited from the degassed liquid phase.

Specific genesis of gold and silver sulfides at the Yunoe deposit ( Magadan Region, Russia)

Hypogene uytenbogaardtite, acanthite, and native gold parageneses have been revealed at the epithermal Yunoe gold-silver deposit, Magadan Region, Russia. Thermodynamic calculations in the system Si–Al–Mg–Ca–Na–K–Fe–Pb–Zn–Cu–Ag–Au–S–C–Cl–H 2O were carried out at 25–400 °C and 1–1000 bars to elucidate the role of hydrothermal solutions in the formation of gold and silver sulfides. Several most probable scenarios for ore-forming processes in the deposit are considered: (1) interaction between cold and heated meteoric waters percolating along cracks from surface to depth and reacting with the host rock—rhyolite; (2) evolution of ascending postmagmatic fluid resulting in chloride–carbonic acid solution, which interacts with rhyolite at 100–400 °C; (3) stepwise cooling of hydrothermal ore-bearing solutions; (4) rapid cooling of ore-bearing hydrotherms on their mixing with cold surface waters. Rhyolite with Pb, Zn, Cu, Cl, S, Ag, and Au clarke contents was taken as an initial host rock. Calculations by model 3 showed the possible formation of uytenbogaardtite and petrovskaite at low-temperature stages. Gold and silver sulfides can be deposited during the mixing of ore-bearing acid chloride–carbonic acid hydrothermal solutions with surface alkaline waters.

The world-class Natalka gold deposit, northeast Russia: REE patterns, fluid inclusions, stable oxygen isotopes, and formation conditions of ore

REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H2O-CO2-NaCl-MgCl2 with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO2-CH4, and (3) H2O-NaCl-MgCl2 with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H2S and destabilization of the Au(HS)2− complex. The fluid with the third composition arose due to the boiling of the H2O-CO2-CH4-NaCl-MgCl2 liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ18O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by \( \delta ^{18} O_{H_2 O} \) = +6.3 to +8.8‰ at 350°C and +3.6 to +5.9‰ at 280°C, respectively. The narrow interval of oxygen isotopic compositions shows that this source was homogeneous. The data obtained allow us to suggest that the deposit formation was related to magmatic activity, including the direct supply of ore components from a magma chamber and mobilization of these components in the processes of dehydration and decarbonation during contact and regional metamorphism.

Mineralogical-geochemical features of sulfide ores from the Broken Spur hydrothermal vent field

A representative collection of hydrothermal sediments was sampled practically from all the hydrothermal mounds of the Broken Spur hydrothermal vent field from the Mir manned submersibles during three cruises of R/V Akademik Mstislav Keldysh. Mineral associations characteristic of different morphological types of sulfide ores from hydrothermal pipes, plates, and diffusers are assessed. Particular attention is paid to the distribution of minor elements and their distribution patterns determined by the mineralogical zonation. The measured isotopic value of the sulfur in the sulfide minerals appeared to vary from 0.4 to 5.2‰, which indicates their similarity with the ores from the Snake Pit vent field and is related to the dilution of hot ore-bearing solutions by seawater and reduction of the water sulfate ions to H2S with a heavy isotopic composition.

Some aspects of the interrelations between magmatism and ore formation: Evidence from oxygen isotope data on ore-magmatic systems, Armenia

The results of oxygen-isotope study of the Kokhb-Shnokh (J3) and Vokhci (N1) intrusive complexes and the related Tekhut and Kadzharan Cu-Mo porphyry deposits of different types, Armenia, were used to discuss the relevant problems of rock and ore genesis and their relationships. It was established that the assimilation of parental mantle basaltic melts by crustal, mainly feldspathic material is a decisive factor in the formation of multiple gabbro-granite complexes. Specifics in the composition and crystallization of the hybrid melt causes the enrichment of the residual melt and hydrothermal fluids in Fe and other elements, in particular, Cu and Mo. The ore-bearing solutions are generated over the solidification time of the melt. They are removed by the hydrothermal system of directionally moving solutions, a process that gives way to a convective-circulation hydrothermal system. A change in the spatial position of the hydrothermal solutions during the formation of the deposit supplies them to different sites, creating a seeming discontinuity in ore deposition, which is observed at many deposits. In fact, the supply of the ore-bearing solutions is continuous process. The ore stage is not a time interval with particular ore-bearing solutions but rather a certain physicochemical state of the solutions attained during their interaction with the environment. This process resulted in the simultaneous precipitation of different mineral assemblages and the asynchronous precipitation of the same assemblages. The formation of deposits of various metals related to a common intrusive complex, as changes in the composition of mineral assemblages, is caused by the influence of various geochemical barriers rather than by compositional variations in the initial hydrothermal solutions.

Hydrothermal sulfide deposits of the Lucky Strike vent field, Mid-Atlantic Ridge

Several hydrothermal sulfide structures were sampled using Mir manned submersibles in the relatively shallow Lucky Strike vent field, Mid-Atlantic Ridge; the bathymetric position of these structures varies by approximately 100 m. The investigation of the chemical and mineral compositions of hydrothermal ore occurrences led to the conclusion that the initial high-temperature ore-bearing solution ascending toward the surface became unstable and experienced phase separation beneath the ocean floor. The phase separation was responsible for the bathymetric control of hydrothermal ore formation in the ocean.

Deep lithospheric structure and hypogene metallogeny at convergent plate margins

The correlation between hypogene mineral deposits and seismically active fracture zones, cutting the continental lithosphere of convergent plate margins in South America, Indonesia and Mexico, was investigated. It was found that all large mineral deposits and most important mining districts in Andean South America between 18-34° S, Sumatra and Java, and central and southern Mexico are situated in the outcrops of deep rooted seismically active fracture zones induced and/or activated by the process of subduction of the Nazca, Indo-Australian and Cocos plates. We suppose that active fracture zones, penetrating the whole thickness of the continental lithosphere, effectuate a channeling of ore-bearing solutions liberated from the subducting oceanic lithosphere. The repeated seismic activity stimulates the long-term permanent re-opening of import paths for ore-bearing solutions that causes the accumulation of enormous amounts of metals in restricted domains of the earth crust. It appears that the fracturing of the continental lithosphere remains preserved in time and space, being re-activated under favourable conditions by a sequence of subduction cycles of different age.

Pattern and metallogenic consequences of deep-rooted seismically active fracture zones in Sumatra and Java

Seismically active fracture zones were delineated and correlated with hypogene mineral deposits in the region of Sumatra and Java. It appears that accumulations of metals are situated in the outcrops of these deep rooted fracture zones which may effectuate a channeling of ore-bearing solutions liberated by flow of volatiles from the subduction zone.

Geochemical and isotopic composition of organic matter in the Kupferschiefer of the Polish Zechstein basin: relation to maturity and base metal mineralization

vs the present depth of the Kupferschiefer, soluble organic matter (SOM) yields, and relative proportions of saturated and aromatic hydrocarbons of the SOM provide evidence for an oxidative alteration of organic matter in highly mineralized Kupferschiefer samples near the Rote Faule zones. This is confirmed by differences in the composition of the saturated and aromatic hydrocarbon fractions of the soluble organic matter: Saturated hydrocarbons from Rote Faule samples are dominated by short-chain n-alkanes and higher abundances of pristane and phytane relative to heptadecane (n-C17) and octadecane (n-C18), respectively, compared with samples more distant to the Rote Faule zone. Compositional changes of the aromatic hydrocarbon fractions with decreasing distance to that zone are characterized by the occurrence of polycyclic aromatic hydrocarbons and elevated ratios of phenanthrene to methylphenanthrenes that are attributed to demethylation reactions and resulted in a decrease of the methylphenanthrene index (MPI 1). Kupferschiefer samples from the barren zone of the Polish Basin do not show these alteration patterns. The observed variations in organic matter composition with burial depth are consistent with changes due to increasing thermal maturation. Maturity assessment is achieved from MPI 1 and the methyldibenzothiophene ratio (MDR). From the relationship between the maturity of organic matter in terms of vitrinite reflectance values and depth of the Kupferschiefer strata, a continuous increase in reflectance of vitrinite is obtained within the Polish Basin. The alteration pattern of organic matter related to base metal mineralization of the Kupferschiefer corresponds to changes in the isotopic composition of organic carbon and calcite. Kerogen within, or close to, Rote Faule zone is enriched in 13C caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter. The opposite tendency towards lower δ 13C and δ 18O values of calcite provides evidence for isotopic exchange between carbonate and the oxidizing, ore-bearing solutions and for organic matter remineralization. In contrast, organic matter and calcite from the Kupferschiefer do not show regular trends in δ 13C with increasing thermal maturation.

Seismically active fracture zones and distribution of large accumulations of metals in the central part of Andean South America

The analysis of the geometry of distribution of earthquake foci in the central part of Andean South America between 18° and 34°S made the delineation of several seismically active fracture zones in the continental wedge overlying the subducting Nazca plate possible. Correlation of their position with the distribution of hypogene accumulations of metals revealed that the majority of large mineral deposits and mining districts are situated in the outcrops of these fracture zones. We present geometrical documentation (map of epicentres, vertical and longitudinal cross sections) of the most important fracture zones and data on mineralogical composition, genetic type and available radiometric ages of mineral deposits. Sixteen mining districts in Chile, and 24 in Argentina, were attributed to the seismically active fracture zones. Major mining districts and individual large mineral deposits occur in six seismically active fracture zones roughly parallel to the axis of the Peru-Chile trench (Carachas-Portillo, Choquelimpie, Iquique, Domeyko, Rio Blanco-Los Bayos and Farellones F.Z.), in six fracture zones roughly perpendicular to the trench (El Salvador, Maricunga, Jaroma, Ujina, Tumbaya and Incahuasi-Leon Muerto F.Z.) and in two fracture zones oriented at an angle of about 45° in relation to the direction of the presently active Andean subduction (Aconcagua and Sierra del Volcan F.Z.). The occurrences of large mineral deposits of different ages show that these fracture zones were also active in the geological past and represent sites of permanent re-opening of paths allowing ore-bearing solutions and long-term accumulation of large amounts of metals to occur in relatively restricted domains of the Earth's crust. The mining districts with dated mineral deposits are arranged into four periods of hypogene mineralization: Upper Miocene-Pliocene, Upper Oligocene-Middle Miocene, Upper Eocene-Middle Oligocene, Lower Paleocene-Upper Eocene. These periods of metallogenic activity correlate well with four supposed Andean subduction cycles active in the Tertiary. The occurrence of mineral deposits of different ages in recently active fracture zones can be used as an important evidence in favour of long-term spatial permanence and activity of these zones and as a guide for the discovery of further mineral deposits hidden under young sedimentary and volcanic cover in the fracture zones.

Stable isotopic variations of clay minerals: A key to the understanding of Kupferschiefer-type mineralization, Germany

Isotopic compositions of silicate minerals and carbonates of selected Kupferschiefer samples from the Hessian Depression and from the Lower Rhine Basin have been studied in detail. In the area of the Richelsdorf ore district, stable isotope analyses of the Cu-mineralized sections in the Kupferschiefer show that the metal enrichment processes are associated with significant changes in the oxygen and hydrogen isotope composition of the clay minerals. The isotopic compositions of the < 2-μ-size fractions reflect a decrease of approximately 4%. in δ 18O (18.4 to 13.7%.) and an increase of nearly 50%. in δD (−74 to −25%.) with decreasing distance from the “Rote Fäule” zone. This isotopic zonation in the Kupferschiefer shale is explained by fluid-rock interaction with ascending, oxidizing, hypersaline solutions, which transported high amounts of base metals from the Lower Permian red beds into the Kupferschiefer horizon acting as a geochemical trap. The estimated isotopic composition of the involved water ( δ 18O between +3.0 and +6.5%.; δD between −10 and +3%.) supports the characterization of the ore-bearing solutions as basinal brines. Internal oxygen isotope fractionation of illite from samples next to “Rote Fäule” corresponds to a temperature of approximately 130°C, which is suggested as the temperature of metal precipitation. In the Lower Rhine Basin, the metal content does not exceed values commonly observed in black shales. Enhanced base metal (Pb, Zn) mineralization is observed only in the bottom section of the Kupferschiefer. The results of the oxygen and hydrogen isotope analyses of the selected core samples provide no evidence for correlation between isotopic composition of the silicate minerals and the extent of lead and zinc sulphide precipitation. Futhermore, they do not point towards isotopic exchange reactions between clay minerals within the Kupferschiefer shale and fluids out of isotopic equilibrium with the investigated mineral phases. However, it is shown that the lack of detectable interaction between the rock and external fluids, as reflected by these data, in general coincides with the proposed concept that the base metals Pb and Zn as well as Ba have been coprecipitated from Carboniferous formation waters.

Stratiform and vein U, Mo and Cu mineralization in the Novoveská Huta area, CSFR

U-Mo and Cu mineralization occurs in horizons as well as in veins in the Permian formations near Novoveska Huta. Ore mineralization is represented by uraninite, U-Ti oxides, coffinite, molybdenite, chalcopyrite, tennantite and pyrite. The isotopic composition of S and C displays a larger variability in the stratiform ores (δ34S from −32.7 to +2.7 and δ13C from −27.1 to — 0.5‰) These data suggest mixing of meteoric solutions with fluids of volcanic origin and a complex history. There is a narrower range of δ34S from −18.8 to −4.6 and δ13C from −6.3 to −2.5% in quartz-carbonate veins with Cu mineralization suggesting a deep source of ore-bearing solutions. The Permian volcanics were a significant source of ore elements.Their contents of U, Mo, Cu and Y are from two to eight times higher than in sedimentary rocks. Accumulations of ore elements in the horizons were formed by the reduction and adsorption processes 240 ± 30 Ma ago according to U-Pb isotopic dating. Due to Alpine tectonism, these low-grade ores (U 0.1 wt%) were formed 130 ± 20 Ma ago at temperatures ranging from 110 to 120° C, according to fluid inclusions. Younger veins with Cu mineralization were formed 115 ± 10 Ma according to the model age of Pb at temperatures ranging from 95 to 190°C.

The role of organic matter in the metallogeny of the Bohemian Massif

Formations rich in organic matter occurred throughout most of the geologic history of the Bohemian Massif. Accumulation of organic matter was connected with basic submarine volcanic activity, with transgressive sedimentary regimes, and with limnic and paralic environments.The largest carbon-rich marine sediment accumulation occurred in late Proterozoic and early Paleozoic time. In these periods, substantial concentrations of trace elements and sulfur accumulated in black claystones, where the metal associations were determined mostly by synchronous volcanicity. In spite of the substantial amount of accumulated trace elements, only subeconomic concentrations of pyrite-pyrrhotite and U-V-Sb-Au mineralizations were formed in association with Proterozoic stromatolites. Subsequently, Cadomian and Variscan metamorphic processes caused substantial remobilization of metals and sulfur in black claystones and produced disseminated gold and uranium deposits. During the emplacement of postkinematic Variscan granitoids, organic matter and some metals were leached from the surrounding low metamorphic, carbon-rich sediments by hydrothermal solutions of deep crustal origin. A typical feature of these deposits is the presence of isotopically light bitumens (delta 13 C PDB = -40.8 to -56.5ppm) and the biological tracers phytane and pristane, indicating a biogenic origin. As a reducing agent, organic carbon (graphite) controlled the localization of Variscan hydrothermal uranium mineralization which occurs in graphitized shear zones in the highly metamorphosed basement of the Bohemian Massif. The crystallinity of the graphitized organic matter decreases sharply in these zones, probably due to hydrothermal decomposition of the graphite lattice. The low crystallinity of graphite and anomalously light isotopic composition of the hydrothermal carbonates in this type of deposit (delta 13 C PDB = -6 to -ppm) indicate that graphitized organic matter was involved in the reduction of uranium-bearing hydrothermal solutions.During the formation of sandstone-type uranium deposits in the carbon-rich platform sediments of the Bohemian Massif, the organic matter of host rocks became oxidized by the action of oxygen dissolved in ore-bearing solutions. This process gave rise to organic polymers similar to humic acids, which reacted with UO (super +2) 2 to form insoluble uranium organic compounds.

Application of the zoning of ore-associated elements to exploration for volcanogenic massive sulphide deposits: A case study of the Pukkiharju Zn-Cu prospect, central Finland

Abstract The zoning of ore-associated elements Volcanogenic massive sulphide deposits are characterized by mineralogical alteration zones that are associated with distinct geochemical zones. The geochemical zones commonly form larger exploration targets than do the observable alteration zones. Use has been made of the depletion of Na and Ca and enrichment of Mg, Fe, and sometimes of K as a standard procedure in lithogeochemical exploration. The enrichment of ore-associated minor or trace elements that are often found in a restricted zone in the area of most intense alteration are also used in locating orebodies, even though their halos are sometimes less distinct and well defined than those of the major elements. The concept of zoning of ore-associated elements in hydrothermal deposits has been extensively applied in the Soviet Union to interpret the level of erosion and type of deposit and also to detect blind mineralization. The direction of movement of the ore-bearing solutions is expressed as axial zoning, and the standard sequence for elements considered there is (from top to bottom): Ba-Ag-Pb-Zn-Au-Cu-Ni-Co. In this study, zoning is compared to features of volcanogenic massive sulpide deposits.

Distribution and significance of Pb and Tl in the sulfides and host rocks from the hydrothermal mineralization of the Tolfa Mountains (Latium, Central Italy)

Tl and Pb contents of 29 samples of sulfide minerals (mainly pyrite) from the hydrothermal Pliocene vein mineralization of the Tolfa Mountains volcanic district have been measured by a polarographic technique. Abundances of Tl and Pb have also been measured in selected host rocks and travertines to establish the relationship between ores and possible source rocks. Hydrothermal pyrites of the various stages of the mineralization show similar ranges of Tl contents (220–7320 ppb, X ̄ = 2101 ppb ), while the Pb content is higher in late-stage pyrites (23–3650 ppm, X ̄ = 1791 ppm ) relative to early ones (8–246 ppm, X ̄ = 95 ppm ). This latter feature suggests a different Pb content of the ore-bearing solutions of the two stages of mineralization. The Pb/Tl ratio distinguishes the pyrites of the main stage of mineralization from the different mines and is roughly constant in most pyrites from the same locality. It is suggested that Pb and Tl contents in pyrites merease with evolution of the mineralizing process. Volcanic pyrites are enriched in Tl (9000–28000 ppb, X ̄ = 16,666 ppb ) relative to hydrothermal pyrites due to the greater availability of Tl in magmatic processes. The Pb content of volcanic pyrites is as low as that of early hydrothermal pyrites. Disseminated pyrites in the unaltered flysch rocks show higher Pb and Tl levels than those expected in sedimentary pyrites and are comparable with those of hydrothermal pyrites, thus suggesting an origin or interaction with hydrothermal solutions. Tl and Pb contents in the hydrothermally-altered flysch ( X ̄ = 962 ppb and 16 ppm, respectively) are much higher than in the unaltered flysch ( X ̄ = 5 ppb and 4 ppm, respectively), indicating that Pb and Tl were carried by the hydrothermal fluids. Thus the origin of mineralization appears to be strictly related to the hydrothermal process, the ore potential being limited to the alteration zone. The source of Pb and Tl was represented by sedimentary rocks and magmatic fluids involved in the formation of the buried skarns. Tl is retained in the gossans formed on the mineralized zones, whereas the element is absent in ironstones not related to ores. This suggests its potential use as a pathfinder during prospecting for sulfide ores. On the contrary, Pb is retained in both gossans and ironstones at roughly comparable levels, and is, therefore, of limited use for geochemical prospecting. Finally, Pb and Tl are detectable in the travertines from the eastern and southern parts of the Tolfa Mountains area, where the thermal waters leached Pb and Tl, circulating deeply through volcanic rocks and/or sulfide ores. Thus, both the Pb and Tl contents in travertines may be used in prospecting.

Geological and genetic aspects of the Hällefors sulfide ores, Bergslagen, Sweden

Abstract The Hallefors sulfide ores are located in the westernmost part of Bergslagen Ore Province, south central Sweden. The ore-bearing rock sequence consists of K-rich felsic metavolcanics. In the eastern part of the field, stratiform ore occurs in the upper part of a 30–40 m thick limestone. This ore consists of carbonate-bearing manganiferous skarn rock with conformable sulfide- and/or magnetite-rich beds. In the ore there are interlaycred beds of limestone, dolomite and tuffite. The sulfide skarn ironoxide ore is interpreted as a product of regional metamorphism of pre-existing strata. It comprises four chemical components, which are suggested to represent tuff materials (Si, Al, K), and chemically precipitated oxides (Fe, Mn), carbonates (Ca, Mg) and sulfides (S, Zn, Pb, As, Ag). The precipitates are considered to be products of submarine hydrothermal activity. The absence of wallrock alteration and channelways for ore-bearing solutions indicates that the ore was deposited on the sea floor distally...

Neptunic dikes and their relation to the hydrodynamic circulation of submarine hydrothermal systems

Neptunic dikes in Lower Jurassic sedimentary rocks of the Tethyan margins consist of internal sediment in cavities and submarine cement. The textures of the infilling sediment indicate downward and lateral movements of seawater beneath what was probably a deep-sea floor. This circulation probably was part of the hydrodynamic circulation that supplied hot, ore-bearing solutions to hydrothermal vents near a spreading-ridge system. This working hypothesis is being tested by stable-isotope studies now in progress.