Tellurium-bearing minerals in zoned sulfide chimneys from Cu-Zn massive sulfide deposits of the Urals, Russia

Tellurium-bearing minerals are generally rare in chimney material from mafic and bimodal felsic volcanic hosted massive sulfide (VMS) deposits, but are abundant in chimneys of the Urals VMS deposits located within Silurian and Devonian bimodal mafic sequences. High physicochemical gradients during chimney growth result in a wide range of telluride and sulfoarsenide assemblages including a variety of Cu-Ag-Te-S and Ag-Pb-Bi-Te solid solution series and tellurium sulfosalts. A change in chimney types from Fe-Cu to Cu-Zn-Fe to Zn-Cu is accompanied by gradual replacement of abundant Fe-, Co, Bi-, and Pb- tellurides by Hg, Ag, Au-Ag telluride and galena-fahlore with native gold assemblages. Decreasing amounts of pyrite, both colloform and pseudomorphic after pyrrhotite, isocubanite ISS and chalcopyrite in the chimneys is coupled with increasing amounts of sphalerite, quatz, barite or talc contents. This trend represents a transition from low- to high sulphidation conditions, and it is observed across a range of the Urals deposits from bimodal mafic- to bimodal felsic-hosted types: Yaman-Kasy → Molodezhnoye → Uzelga → Valentorskoye → Oktyabrskoye → Alexandrinskoye → Tash-Tau → Jusa.     

Identification of diagnostic geochemical alteration in the wallrocks of archean volcanic-exhalative massive sulfide deposits

The present investigation is concerned with the identification of diagnostic lithogeochemical alteration signatures around volcanic-exhalative massive sulfide deposits in the Superior Province, with the overall objective of deriving lithogeochemical criteria, applicable in the search for new deposits of this type.Previous work on these deposits has indicated that, in general, the footwall alteration halo is marked by iron and magnesium enrichment, and calcium and sodium depletion. These features are often only detectable if the over-riding effects of igneous differentiation are compensated. It is apparent that the relative contribution of individual elements to the geochemical alteration varies from deposit to deposit, preventing the recognition of any universal geochemical alteration criterion.The role of discriminant analysis has been examined to establish the possibility of the technique indicating a more reliable expression of geochemical alteration. Discriminant analysis establishes the optimum weighted combination of variables to distinguish two or more populations from each other, in this case mineralized from barren environments. The application of the procedure to the data relating to the composition of wall rock associated with eight volcanic-exhalative massive sulfide deposits has drawn attention to the existence of two distinct types of alteration. The Joutel and Poirier deposits are characterized by Fe₂O₃, MgO, Zn and Ag enrichment, and CaO and Na₂O depletion; this alteration style has been termed the “Joutel” type. At the South Bay, Sturgeon Lake and Mobrun deposits, Na₂O is also strongly depleted, but Fe₂O₃ and MgO are usually depleted and K₂O is strongly enriched; this alteration style has been termed the “South Bay” type. Both of these alteration styles are displayed at the Mattabi and East Waite deposits. “Joutel” type alteration appears to be in close spatial association with discharge vents, while “South Bay” alteration is more laterally widespread and is representative of at least one deposit thought to be formed distally from its associated discharge vent.These geochemical signatures are more strongly expressed in pyroclastic rocks, and andesites, relative to massive rocks, and rhyolites.The geochemical alteration patterns delineated in this way constitute significantly larger exploration targets than the readily observable mineralogical alteration haloes.Results of the current investigations indicate that the mineralizing processes associated with Archean volcanic-exhalative massive sulfide deposits have given rise to more than oversimplification to aim exploration at the detection of a single type of response. The application of discriminant analysis provides a potential means of identifying and comparing as many responses as are present at the deposits studied. In this respect it is superior to any univariate statistical method, and has considerable application in exploration.     

Mercury dispersion halos as ore guides for massive sulfide deposits,West Shasta district,California

In the West Shasta district, California, flat-lying deposits of massive pyrite containing chalcopyrite, sphalerite, galena, and tetrahedrite occur within a section of unmineralized rhyolites and tuffs. Mercury was determined in residual soils derived from apparently unmineralized rocks at stratigraphic levels from 50 to 200 feet over known ore at the Early Bird, Keystone, and Mammoth mines. In each case, pronounced mercury anomalies were found. The mercury content of anomalous soils ranges up to 340 ppb (parts per billion, 10^–9 g/g) over a background of 20 to 60 ppb.

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Zusammenfassung Geochemische Untersuchungen im West-Shasta-Distrikt (California/USA) zeigten, daß über den bekannten, aber verborgenen Erzkörpern der Early Bird, der Keystone-und der Mammoth-Mine Quecksilber-Anomalien in Residualböden auftreten. Die flach liegenden, linsenförmigen Erzkörper bestehen aus massivem Pyrit, kleineren Mengen Kupferkies und untergeordnet Bleiglanz, Zinkblende, Fahlerz, Magnetkies und Magnetit. Sie werden von unmineralisierten Rhyolith- und Tuffhorizonten überlagert, die im Bereich der untersuchten Vorkommen etwa 20 bis 70 m mächtig sind. Anomale Bodenproben enthalten bis zu 340 ppb Quecksilber. Die Untergrund-Werte liegen in einem Bereich zwischen 20 und 60 ppb Hg.

     

Geochemical behavior during mineralization and alteration events in the Baiyinchang volcanic-hosted massive sulfide deposits,Gansu Province,China

The Zheyaoshan deposit is the largest within the Baiyinchang (BYC) volcanic-hosted massive sulfide (VHMS) district, located in the northern Qilian orogenic belt of North China. The deposit is hosted by quartz keratophyre tuffs, with wall-rock alteration mainly comprising chlorite, sericite, quartz, pyrite and epidote. Mineral assemblages within the altered host rocks can be divided into a sericite-quartz-dominant assemblage (sericite-silicified zone), a chlorite-dominant assemblage (chlorite-dominant zone) and a pyrite-dominant assemblage (mineralized zone) based on geochemical analysis and alteration characteristics. We have conducted detailed processing and critical analysis of the geochemical data of both the altered and least-altered host rocks in order to investigate the problem of closure in the geochemical dataset to eliminate the influence that each component has on the other in terms of mass change, and have applied the standardized method of the mass change calculation to analyze this data. The results show that: (1) the sericite-silicified zone formed along fissures due to the ingress of hydrothermal fluids, with MnO₂, Na₂O and CaO being mobilized into the hydrothermal fluids leached and MgO, Fe₂O₃, SiO₂, K₂O, BaO deposited. Additionally, Ag, Cu and chalcophile elements (Ag, As and Bi) were enriched while Pb, Zn and large ion lithophile elements (LILEs) (Cs, Sr, Eu, Be) were mobilized into hydrothermal fluids; (2) the physiochemical conditions and pH levels of the hydrothermal fluids changed during sericitization, with MgO, Fe₂O₃, BaO being further enriched and MnO, Na₂O, CaO further depleted, leading to formation of chlorite and the initial precipitation of metallogenic the (Cu, Zn, Pb) and chalcophile elements (Ag, As, Bi); (3) the negative Eu anomaly was mainly due to its strong activity when Eu is mobilized into the hydrothermal fluids during since plagioclase break-down during the sericite-silicification process; (4) AI and CCPI values gradually increase towards the orebody. The chlorite-dominant assemblage and sericite-quartz-dominant assemblage on the periphery of the chlorite-dominant zone can all be used as vectors towards the volcanic massive sulfide orebody and for regional-scale mineral exploration. Either leached elements or enriched elements can be considered as significant indicator elements and as prospective indicators for geochemical exploration within the BYC district. The Eu anomaly may be especially useful as an indicator for distinguishing the least-altered rocks which has great significance for exploration on the regional scale.     

Subsea-floor replacement in volcanic-hosted massive sulfide deposits

Recent research on volcanic-hosted massive sulfide (VMS) deposits indicates that syngenetic subsea-floor replacement ores form an important component of many deposits. In the context of VMS deposits, subsea-floor replacement can be defined as the syn-volcanic formation of sulfide minerals within pre-existing volcanic or sedimentary deposits by infiltration and precipitation in open spaces (fractures, inter- and intra-granular porosity) as well as replacement of solid materials.There are five criteria for distinguishing subsea-floor replacement in massive sulfide deposits: (1) mineralized intervals are enclosed within rapidly emplaced volcanic or sedimentary facies (lavas, intrusions, subaqueous mass-flow deposits, pyroclastic fallout); (2) relics of the host facies occur within the mineral deposit; (3) replacement fronts occur between the mineral deposit and the host lithofacies; (4) the mineral deposit is discordant to bedding; and (5) strong hydrothermal alteration continues into the hanging wall without an abrupt break in intensity. Criteria 1–3 are diagnostic of replacement, whereas criteria 4 and 5 may suggest replacement but are not alone diagnostic. Because clastic sulfide ores contain accessory rock fragments collected by the parent sediment gravity flow(s) during transport, criteria 2 can only be applied to massive, semi-massive, disseminated or vein style deposits, and not clastic ores.The spectrum of VMS deposit types includes deposits that have accumulated largely subsea-floor, and others in which sedimentation and volcanism were synchronous with hydrothermal activity, and precipitation of sulfides occurred at and below the sea floor over the life of the hydrothermal system. Deposits that formed largely subsea-floor are mainly hosted by syn-eruptive or post-eruptive volcaniclastic facies (gravity flow deposits, water-settled fall, autoclastic breccia). However, some subsea-floor replacement VMS deposits are hosted by lavas and syn-volcanic intrusions (sills, domes, cryptodomes). Burial of sea-floor massive sulfide by lavas or sediment gravity flow deposits can interrupt sea-floor mineralization and promote subsea-floor replacement and zone-refining.The distance below the sea floor at which infiltration and replacement took place is rarely well constrained, with published estimates ranging from less than 1 to more than 500 m, but mainly in the range 10–200 m. The upper few tens to hundreds of metres in the volcano-sedimentary pile are the favoured position for replacement, as clastic facies are wet, porous and poorly consolidated in this zone, and at greater depths become progressively more compacted, dewatered, altered, and less amenable to large scale infiltration and replacement by hydrothermal fluids. Furthermore, sustained mixing between the upwelling hydrothermal fluid and cold seawater is regarded as a major cause of sulfide precipitation in VMS systems, and this mixing process generally becomes less effective with increasing depth in the volcanic pile.The relative importance of subsea-floor replacement in VMS systems is related principally to four factors: the permeability and porosity patterns of host lithofacies, sedimentation rate, the relative ease of replacement of host lithofacies (especially glassy materials) and early formed alteration minerals during hydrothermal attack, and physiochemical characteristics of the hydrothermal fluid.     

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.     

Volcanogenic massive sulfide deposits of ophiolite associations

Volcanogenic massive sulfide deposits in ophiolite complexes are usually attributed to the Cyprus type. They associate with basaltic volcanics that are formed in mid-ocean or back-arc spreading centers and much less frequently in intra-plate settings. The deposits are characterized by copper or copper-zinc ores that are enriched in Ni, Co, and in places Mn and As, but are very poor in Pb and demonstrate a low to moderate content of Ag and Au. Typically, the deposits are low to very low in ore and metal reserves. Cyprus-type deposits were irregularly distributed during geological history. The most ancient of them were formed in the Neoproterozoic, while the bulk of the deposits are Ordovician or Cretaceous in age. Their possible Paleoproterozoic analogues can be found in the Svecofennian belt (Outokumpu ore district), while modern ones are confined to the Explorer and Endeavour Ridges and southern segment of the Juan de Fuca Ridge.     

北祁连块状硫化物矿床中矿石和主要矿石矿物地球化学特征

对北祁连山白银矿田和郭密寺矿田中主要矿床的矿石和矿石矿物组分特征研究表明，由于各矿田的成矿条件和地球化学背景存在差异，造成不同矿床的矿石和矿石矿物元素组合各具特色。但作为同一类型矿床，它们之间又有很多共性，特别是同一矿田内各矿床的地球化学特征具有相似性和过渡性，反映了成矿条件变化的趋势。     

Precious metals in massive base metal sulfide deposits

Gold and silver are ubiquitous, sometimes minor but economically important metals in massive base metal sulfide ores. Their content, proportions and distribution in the ores depend on complex, interrelated factors of their source, mobilization, transport and deposition.Different types of these deposits are formed by similar seafloor hydrothermal systems operating, however, in widely differing tectono-stratigraphic environments which span a spectrum from ensimatic-oceanic, through continent-margin to ensialic-continental ones. Like those of the base metals, the proportions and distribution of the precious metals in the ores vary regionally with these changing depositional environments. This suggests that precious metal content of the sub-seafloor rocks in which the generative fluids circulate is one factor that governs the amounts and distribution in the ores. The lithology of these source-rocks is also important. Pillowed, tholeiitic basalts have high permeability, golddepleted crystalline pillow interiors and relatively gold-rich palagonitic rims, and are consequently particularly favorable sources.Mobilization of gold from the sub-seafloor rocks may require basalt-water, and/or carbonaceous sediment-water reactions to produce strongly reduced bisulfide, carbonyl or cyanide complexes that promote gold transport. Chloride complexing and transport are less important for gold but more so for silver and the base metals.Seafloor hydrothermal discharge at shallow depth is commonly accompanied by boiling, steamblast explosions in the vent and resulting deep penetration and mixing of cool, oxygenated seawater with rising hot, reduced metalliferous fluid. This results in deposition of both chloride- and isulfide-complexed gold at depth and centrally in the footwall stockwork or in copper ore in the base of the massive body. Chloride-complexed silver, stable to lower temperatures, is carried farther and deposited with higher-level and more distal, massive zinc-lead ores. Boiling in deep water, however, although possible, is rare. This fact minimizes deep fluid mixing and allows transport of lower temperaturestable, bisulfide-complexed gold to the seafloor and outward from the vent. Gold too, is then deposited with the shallower, distal, massive zinc-lead-silver ore. Late-stage changes in fluid Eh, salinity and activity of sulfur during evolution of the generative hydrothermal system, and by discharge through previously deposited, early stage sulfides around the vent also cause diagenetic remobilization of gold, moving it to shallower, more distal locations in the system. In combination, these relationships explain the three associations of gold in primary, in-situ massive sulfide deposits; in central, deep footwall stockwork mineralization with or without copper, in central copper ore in the base of the massive body and in shallow, peripheral pyritic zinclead-silver ore.Primary, in-situ ore near the vent is sometimes reworked by seafloor density flows which transport clasts of the primary sulfides down-slope, mix them with rock and sedimentary detritus and redeposit them to form secondary, transported ore. Gold, like iron and the base metals, is diluted during this clastic transport. But silver and barite may be enriched indicating transport in the density flows not only as clasts of primary ore but partly also m solution in the hydrothermal fluids that, in this case, must have lubricated the density flows.

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Zusammenfassung Gold- und Silbervorkommen in massiven Metallsulfid-Lagerstätten sind stets ökonomisch wichtige Metalle, auch wenn sie nur in geringen Konzentrationen vorliegen. Der Gehalt an diesen Metallen und ihre Verteilung innerhalb der Lagerstätte hängt von komplexen, sich gegenseitig beeinflussenden Faktoren wie Metallquelle, Art der Mobilisation, Transport und Fällung ab.Unterschiedliche Lagerstättentypen werden von ähnlichen hydrothermalen Systemen auf den Ozeanböden gebildet. Die tektonostratigraphischen Environments unterscheiden sich dabei allerdings beträchtlich; sie befinden sich in ensimatisch-ozeanischen, kontinentalrandlichen und ensialischkontinentalen Bereichen. Innerhalb dieser regional wechselnden Ablagerungsbedingungen variiert Konzentration und Verteilung der Edelmetalle in den Lagerstätten wie bei den einfachen Metallen. Dies bedeutet, daß der Gehalt an Edelmetallen der Gesteine, die den Meeresboden unterlagern und durch die die metallhaltigen Lösungen zirkulieren, ein Faktor ist, der Menge und Verteilung der Metalle in der Lagerstätte steuert. Ebenso ist die Lithologie dieser Gesteine von Bedeutung. Als besonders gut geeignete Quellen gelten kissenartige tholeitische Basalte mit hoher Permeabilität, goldarmen Kisseninneren und relativ goldreichem palagonitischem Rand.Um das Gold aus diesen Gesteinen mobilisieren zu können, bedarf es einer Reaktion zwischen Basalt und Wasser und/oder eines karbonatischen Sediments mit Wasser, um stark reduziertes Bisulfid, Carbonyl-oder Cyanidkomplexe zu bilden, die den Goldtransport ermöglichen. Chlorid-Komplexbildung und -Transport sind zwar wichtig für Silber und einfache Metalle, für Gold spielen sie nur eine untergeordnete Rolle.Der Austritt hydrothermaler Lösungen an Ozeanböden in geringer Tiefe wird in der Regel von Sieden und explosionsartigem Dampfaustritt begleitet und führt deshalb zu einem tiefen Eindringen und Durchmischen von kaltem, sauerstoffreichen Meereswasser mit den aufsteigenden heißen, reduzierten metallischen Lösungen. Daher kommt es zur Fällung von sowohl an Chloridkomplexe als auch an Bisulfidkomplexe gebundenem Gold. Diese Ausfällung findet in größerer Tiefe statt und zwar hauptsächlich im liegenden Stockwerk oder mit Kupfer zusammen an der Basis der massiven Lagerstätte. An Chloridkomplexe gebundenes Silber ist auch bei niedrigeren Temperaturen stabil, wird also weiter transportiert und in einem höheren Niveau in distal gelegenen Blei-Zink-Lagerstätten gefällt. In größeren Wassertiefen kommt es seltener zu dem beobachteten Sieden der austretenden Lösungen. Diese Tatsache reduziert das Durchmischen der Lösungen in größeren Tiefen und ermöglicht den Transport von Gold, das an Bisulfidkomplexe gebunden ist. In diesem Fall ist die Verbindung auch bei niedrigeren Temperaturen noch stabil also transportfähig und kann bis zum Meeresboden oder außerhalb des Schlotes in Lösung bleiben. Dabei kann das Gold zusammen mit Blei, Zink und Silber in mehr distalen Lagerstätten angereichert werden. Späte Änderungen in Eh, Salinität und Schwefelaktivität der Lösungen während der Entwicklung des hydrothermalen Systems, sowie der Austritt durch früher abgelagerte den Schlot umgebende Sulfide, können eine diagenetische Gold-Remobilisation auslösen. Auch dabei kann das Metall zu in geringer Tiefe liegenden, distalen Ablagerungsorten transportiert werden. Berücksichtigt man alle Faktoren, so erklären diese Verhältnisse die drei möglichen Goldvorkommen in primären, in-situ vorliegenden Sulfid-Lagerstätten: Mit Kupfer vergesellschaftet, allerdings nicht unbedingt, zentral im liegenden Stockwerk; an der Basis der Kupferlagerstätte und in geringer Tiefe in Verbindung mit peripheren Blei-Zink-Silber-Vorkommen.Primäre, in-situ neben Schloten vorkommende Lagerstätten werden in einigen Fällen von meeresbodennahen Masseströmen aufgearbeitet. Diese transportieren Sulfidkomponenten, die während des Transports mit Sediment und Gesteinsbruchstücken vermischt und schließlich als sekundäre sedimentäre Lagerstätte abgelagert werden. Durch diesen Transport und die Mischung der Klastika wird die Goldkonzentration in der späteren Lagerstätte stark reduziert. Silber und Barit können dagegen in Ausnahmefällen während des Transports angereichert werden, da diese Komponenten nicht nur als Sulfidbruchstücke transportiert werden, sondern auch in Lösung in den hydrothermalen Lösungen vorhanden sein können. Diese Lösungen dienen in solchen Fällen den Masseströmen als Gleithorizont.

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Résumé Dans les gisements de sulfures métalliques massifs, l'or et l'argent sont des métaux ubiquistes, parfois mineurs, mais toujours d'importance économique. Leur teneur et leur distribution dans les corps minéralisés dépendent de facteurs complexes, en relation les uns avec les autres, tels que: leur source, leur mobilité, leurs modalités de transport et de dépôt.A partir des mêmes systèmes hydrothermaux en action sur le fond de la mer, divers types de gisements peuvent être engendrés, selon leur environnement tectono-stratigraphique: océanique ensimatique, de marge continentale ou continental ensialique. Les teneurs et la répartition des métaux précieux, comme celle des autres métaux varient régionalement selon ces divers milieux. Ceci suggère que le contenu en métaux précieux dans les roches sous-jacentes au fond marin à travers lesquelles circulent les solutions minéralisantes est un facteur qui détermine leurs teneurs et leurs répartitions dans les minerais. La lithologie de ces roches-sources est également importante. Une source particulièrement significative est représentée par les coussins des basaltes tholéiitiques, très perméables, avec leur coeur pauvre en or et leur couronne palagonitique relativement riche.Le lessivage de l'or dans les roches situées sous le fond marin peut impliquer des réactions eau-basalte et/ou eausédiments carbonatés, réactions susceptibles d'engendrer les bisulfures très réduits et les complexes carbonés ou cyanurés qui permettent le transport de l'or. Le transport par complexes chlorurés joue un rôle subordoné dans le cas de l'or, mais important dans le cas de l'argent et des autres métaux.L'arrivée de solutions hydrothermales sur les fonds marins peu profonds est d'ordinaire accompagnée d'ébullitons et d'émissions explosives de vapeur, ce qui provoque la pénétration profonde d'eau de mer froide et oxygénée et son mélange avec les fluides métallifères chauds et réducteurs ascendants. Il en résulte le dépôt de complexes aurifères bisulfurés et chlorurés. Cette précipitation s'opère en profondeur, particulièrement dans les roches sous-jacentes ou dans le minerai de cuivre, à la base des corps minéralisés massifs. L'argent des complexes chlorurés, stables à plus basse température, est transporté plus loin et se dépose, en situation plus distale, dans les minerals massifs de Pb-Zn. Dans les mers profondes, l'ébullition, sans être impossible, est néanmoins un phénomène rare; cette circonstance minimise le mélange des fluides en profondeur et permet le transport de l'or jusqu'à la surface du fond et même loin des évents sous la forme de complexes bisulfurés stables à basse température. L'or est alors déposé en situation distale peu profonde avec les minerals massifs de Zn-Pb-Ag. Des modifications tardives d'Eh, de salinité et d'activité du soufre dans les solutions au cours de l'évolution du système hydrothermal, de même que le lessivage des sulfures déjà accumulés autour des évents entraînent une remobilisation diagénétique de l'or vers des situations distales d'eau peu profonde. La combinaison de ces divers facteurs permet d'expliquer les trois occurrences de l'or dans les dépôts in situ de sulfures massifs primaires: dans les parties centrales des masses sous-jacentes en association ou non avec le Cu, à la base des corps minéralisés en Cu, et à faible profondeur, en liaison avec les gisements périphériques de Pb-Zn-Ag.Les gisements primaires, formés in situ près des évents sont parfois remaniés par des courants de densité, qui emportent des clastes de sulfures, les mélangent aux débris sédimentaires et les redéposent sous forme de minerais secondaires. De tels transports provoquent la dilution de l'or, en même temps que celle du fer et des autres métaux. Par contre, l'argent et la barite peuvent subir un enrichissement car leur transport dans les courants de densité ne s'effectue pas seulement sous forme de clastes, mais également en solution dans des fludies hydrothermaux, lesquels, dans ce cas, contribuent à lubrifier le courant de densité.

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An assessment of the utility of staurolite in U-Pb dating of metamorphism

Pb isotope data, major and trace element compositions, fission track and synchrotron X-ray fluorescence analyses are presented for staurolites from nine pelitic schists in the continental United States to evaluate their potential use in U-Pb geochronology. Seven U-Pb analyses from Lanzirotti and Hanson (1995) are reexamined with respect to this additional data which was not available at the time. These data are then compared to 21 new U-Pb analyses of staurolite of varying composition from a variety of localities. The primary goals of this study are to: (1) evaluate the variability in U and Pb abundance and U/Pb ratio in staurolites of varying composition; (2) constrain how much of the measured U and Pb is derived from radiogenic solid inclusions such as monazite and zircon; (3) constrain how much of the measured U and Pb is derived from staurolite itself and evaluate any possible correlation of U and Pb abundance and U/Pb ratio to major element composition; (4) place preliminary constraints on closure temperature to Pb diffusion in staurolite; (5) evaluate how meaningful U-Pb ages can better be calculated for the low U/Pb ratio minerals. In the staurolite fractions analyzed U abundances range from 0.2 to 24.9 ppm, Pb from 0.13 to 2.41 ppm, the ²³⁸U/²⁰⁴Pb ratios vary from 135 to 9447, and the ²⁰⁶Pb/²⁰⁴Pb ratios from 23 to 623. For many of the fractions analyzed precise U-Pb ages can be calculated (±10 Ma or better) that appear to be consistent with available age constraints on the time of peak metamorphism. Mass balance calculations, fission track analysis, and synchrotron X-ray fluorescence trace element mapping show that although radiogenic inclusions are almost always present in large staurolite porphyroblasts, it is difficult for inclusions to account for the measured Pb isotopic compositions. It is also demonstrated that the U-Pb ages calculated for staurolites from Connecticut are at least 20 Ma older than nearby Rb-Sr muscovite and ⁴⁰Ar-³⁹Ar hornblende ages. This is consistent with staurolite having a closure temperature to U and Pb diffusion significantly higher than 500 °C. Received: 14 July 1995 / Accepted: 16 May 1997     

Formation of volcanogenic massive sulfide deposits: The Kuroko perspective

The main objective of this paper is to identify the geochemical, hydrological, igneous and tectonic processes that led to the variations in the physical (size, geometry) and chemical (mineralogy, metal ratios and zoning) characteristics of volcanogenic massive sulfide deposits with respect to space (from a scale of mining district size area to a global scale) and time (from a < 10 000 year time scale to a geologic time scale).All volcanogenic massive sulfide deposits (VMSDs) appear to have formed in extensional tectonic settings, such as at mid ocean spreading centers, backarc spreading centers, and intracontinental rifts (and failed rifts). All VMSDs appear to have formed in submarine depressions by seawater that became ore-forming fluids through interactions with the heated upper crustal rocks. Submarine depressions, especially those created by submarine caldera formation and/or by large-scale tectonic activities (e.g., rifting), become most favorable sites for the formation of large VMSDs because of hydrological, physical and chemical reasons.The fundamental processes leading to the formation of VMSDs include the following six processes:

1. (1) Intrusion of a heat source (typically a 10³ km size pluton) into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton. The radius of a circulation cell is typically 5 km. The temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350°C, and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years. The majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity.

2. (2) Reactions between low temperature (T < 150°C) country rocks with downward percolating seawater cause to precipitate seawater SO²⁻₄ as disseminated gypsum and anhydrite in the country rocks.

3. (3) Reactions of the “modified” seawater with higher-temperature rocks at depths during the waxing stage cause the transformation of the “seawater” to metal- and H₂S-rich ore-forming fluids. The metals and sulfide sulfur are leached from the county rocks; the previously formed gypsum and anhydrite are reduced by Fe²⁺-bearing minerals and organic matter, providing additional H₂S. The mass of high temperature rocks that provide the metals and reduced sulfur is typically 10¹¹ tons ( 40 km³ in volume). The roles of magmatic fluids or gases are minor in most massive sulfide systems, except for SO₂ to produce acid-type alteration in some systems.

4. (4) Reactions between the ore-forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores.

5. (5) Mixing of the ore-forming fluids with local seawater within unconsolidated sediments and/or on the seafloor causes precipitation of “primitive ores” with the black ore mineralogy (sphalerite + galena + pyrite + barite + anhydrite).

6. (6) Reactions between the “primitive ores” with later and hotter hydrothermal fluids cause transformation of “primitive ores” to “matured ores” that are enriched in chalcopyrite and pyrite.

Variations in the mineralogical and elemental characteristics, the geometry, and the size of submarine hydrothermal deposits are controlled by the following four parameters:

1. (A) The chemical and physical characteristics of seawater (composition, temperature, density), which depend largely on the geographical settings (e.g., equatorial evaporating basins),

2. (B) The chemical and physical characteristics of the plumbing system (lithology, fractures),

3. (C) The thermal structure of the plumbing system, which is determined largely by the ambient geothermal gradient, and the size and temperature of the intrusive, and

4. (D) The physical characteristics of the seafloor (depth, basin topography).

For example, the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in Pb and Ba compared to those developed in felsic plumbing systems. The lower temperature systems are generally poorer in sulfides, but richer in iron oxides and sulfates. The higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores. Contrasts in the metal ratios between the Noranda-type Archean VMSDs and the younger VMSDs reflect the differences in the geothermal gradient of the plumbing systems. The submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins.The basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history, from at least 3.5 billion year ago to the present.     

冀东黄柏峪—羊崖山地区太古宙岩浆作用和变质作用——SHRIMP锆石U-Pb定年

报道了冀东黄柏峪—羊崖山地区太古宙变质辉长岩、黑云斜长片麻岩、片麻状二长花岗岩、片麻状富钾花岗岩等不同类型变质岩浆岩(8个样品)的SHRIMP锆石U-Pb定年结果。首次发现3.1Ga二长花岗岩,黑云斜长片麻岩形成时代约为3.0Ga。变质辉长岩的侵入时代很可能为新太古代晚期,存在少量3.2~3.6Ga捕获锆石。几乎所有岩石样品都记录了约2.5Ga的变质锆石年龄。结合前人资料认为,(1)冀东地区新太古代晚期构造热事件十分发育,被认为与地幔软流圈上涌导致的岩浆板底垫托有关;(2)冀东地区中太古代以前的陆壳物质广泛分布,黄柏峪—羊崖山地区很可能存在一古老陆核。     

The gold content of volcanogenic massive sulfide deposits

Volcanogenic massive sulfide deposits contain variable amounts of gold, both in terms of average grade and total gold content, with some VMS deposits hosting world-class gold mines with more than 100?t Au. Previous studies have identified gold-rich VMS as having an average gold grade, expressed in g/t, exceeding the total abundance of base metals, expressed in wt.%. However, statistically meaningful criteria for the identification of truly anomalous deposits have not been established. This paper presents a more extensive analysis of gold grades and tonnages of 513 VMS deposits worldwide, revealing a number of important features in the distribution of the data. A large proportion of deposits are characterized by a relatively low gold grade (<2?g/t), with a gradual decrease in frequency towards maximum gold grades, defining a log-normal distribution. In the analysis presented in this paper, the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of VMS deposits based on gold grade, especially when comparing deposits within individual belts and districts. The geometric mean gold grade of 513 VMS deposits worldwide is 0.76?g/t; the geometric standard deviation is +2.70?g/t Au. In this analysis, deposits with more than 3.46?g/t Au (geometric mean plus one geometric standard deviation) are considered auriferous. The geometric mean gold content is 4.7?t Au, with a geometric standard deviation of +26.3?t Au. Deposits containing 31?t Au or more (geometric mean plus one geometric standard deviation) are also considered to be anomalous in terms of gold content, irrespective of the gold grade. Deposits with more than 3.46?g/t Au and 31?t Au are considered gold-rich VMS. A large proportion of the total gold hosted in VMS worldwide is found in a relatively small number of such deposits. The identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in VMS. At the district scale, the gold-rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes. The gold-rich VMS are commonly associated with transitional to calc-alkaline intermediate to felsic volcanic rocks, which may reflect a particularly fertile geodynamic setting and/or timing (e.g., early arc rifting or rifting front). At the deposit scale, uncommon alteration assemblages (e.g., advanced argillic, aluminous, strongly siliceous, or potassium feldspar alteration) and trace element signatures may be recognized (e.g., Au?CAg?CAs?CSb ± Bi?CHg?CTe), suggesting a direct magmatic input in some systems.     

Mineralogy and geochemical behavior of trace elements of hydrothermal alteration types in the volcanogenic massive sulfide deposits,NE Turkey

Volcanogenic massive sulfide (VMS) deposits of the Eastern Pontides, Turkey, are hosted by the Maastrichtian–Eocene dacite and rhyodacite series, accompanied by lesser andesite and basalts, as well as their pyroclastic equivalents, with tholeiitic to calc-alkaline affinity. The ore mineral assemblages are chalcopyrite, sphalerite, galena, chalcocite, covellite, bornite, and tetrahedrite. Potassic-, phyllitic- (sericitic), argillic- (kaolinitic and smectitic), silicic-, propylitic- and hematitic-alteration is commonly associated with these deposits.HFSE, LILE, TRTE and REE contents show strong variability in different alteration types resulting from interaction with acid or alkaline fluids. Sample groups showed chondrite-normalized enrichment of LREE relative to HREE and sub-parallel trends, except for the hematitic- and phyllitic-alteration types. MREE are strongly depleted in the zones of most intense silicification and kaolinization. Most sample groups have strongly- to slightly-negative Eu anomalies, ranging from 0.35 to 0.88 (mean); hematitic- (1.45) and propylitic-altered rocks (1.11) have slightly- to moderately-positive anomalies. The negative Eu anomalies indicate the low temperatures of fluids (< 200 °C). In contrast, the positive Eu anomalies result from high-temperature hydrothermal conditions (> 200 °C). No Ce anomaly was observed, except for phyllitic alteration where a slight positive anomaly was noted. The chondrite-normalized trace and REE patterns of the altered rocks are similar to each other, suggesting that they were derived from a common felsic source. The alteration groups formed from acid, intermediate, and alkaline hydrothermal solutions. Some transition, base and precious metals and volatile elements were clearly enriched, especially in the hematitic-, silicic-, kaolinitic- and phyllitic-altered samples. The other elements exhibit different behaviors in different sample groups. REE behavior is relatively immobile in the silicic-, hematitic-, kaolinitic- and partially in moderately- and propylitic-altered rocks, based on mass-balance calculations. LILE and HFSE appear mobile in the altered sample groups, except in the propylitic-altered rocks. TRTE behave as relatively immobile in most of samples, except in some of the silicic- and phyllitic-altered rocks, and especially in the hematitic-altered samples. HFSE, most of the transition (W, Mo, Cu, and Sb) and some other trace elements (Pb, As, Hg, Bi, Se and Tl), are enriched in the hematitic-altered samples and in the some silicic-altered samples. The highest As, Bi, Mo, Se and Hg concentrations in the hematite-altered samples can be used to distinguish other alteration types and may be a useful indicator in a prospect-scale base metal exploration.     

甘肃白银矿田变酸性火山岩锆石LA-ICP-MS测年——白银式块状硫化物矿床形成时代新证据

结合阴极发光,对白银矿田与成矿密切相关的变酸性火山岩中的锆石进行了LA_ICP_MS(激光剥蚀等离子体质谱)U_Pb同位素测年。获得变石英角斑岩内单颗粒锆石的微区U_Pb年龄分别为(467.3±2.9)Ma、(414.2±2.7)Ma,糜棱岩化石英角斑岩内单颗粒锆石的微区U_Pb年龄分别为(467.1±2.2)Ma、(435.9±3.6)Ma和(412.6±2.1)Ma。认为白银矿田变酸性火山岩的形成时代为中奥陶世,由韧性剪切作用引起的糜棱岩化的时代为早志留世晚期,主变质期时代为早泥盆世早期。提出白银铜多金属矿床形成时代为中奥陶世。这一新认识,对进一步深入研究北祁连造山带的构造演化过程以及白银式块状硫化物矿床的形成环境、找矿方向具有重要意义。     

Hydrogeochemical exploration for volcanic-hosted massive sulfide deposits in semi-arid Australia

Research on hydrogeochemistry for mineral exploration for inland Australia includes development of weathering models and extensive mine-scale and regional groundwater data. Mineral saturation indices for groundwater, activity–activity plots and reaction modelling simulate weathering of volcanic-hosted massive sulfide (VHMS) deposits in deeply weathered environments. At 10 m or more below surface, dissolved O₂ is very low and other solutes such as sulfate, carbonate and nitrate are more likely oxidants. Modelling indicates that these processes differ from oxic weathering of highly eroded terrains, and provide the framework to develop robust hydrogeochemical exploration procedures in covered terrains. Sulfide weathering potentially occurs in two or more phases that effect surrounding groundwaters in differing manners. Deeper oxidative alteration of sulfides (e.g. bornite to chalcopyrite), occurring tens to hundreds of metres below surface, uses sulfate and carbonate as oxidants, causing neutral to alkaline conditions. In this zone, only pyritic massive sulfides potentially generate acidic conditions. Thus, deep sulfide-rich rocks are indicated by sulfate-depleted groundwater. Closer to the surface, sulfides are oxidised to soluble sulfates by dissolved nitrate, with much less acid production than if dissolved oxygen was the main oxidant. Thus, in shallow groundwater, sulfides are indicated by sulfate enrichment and nitrate depletion. Elements are released from sulfides and wall rocks by acid or alkaline conditions. The derived FeS (pH–Eh + Fe + Mn) and AcidS (Li + Mo + Ba + Al) indices distinguish sulfide systems through tens of metres of cover. VHMS systems are distinguished from other non-economic sulfide deposits where there is little transported cover, using various dissolved elements, including Zn, Pb and Cu. Elsewhere, ‘patchiness’ and limited aerial extent of metal signals are due to adsorption effects, that intensify with depth. Other elements such as Mn and Co have lesser diminution effects, but are less selective indicators for VHMS. There is exploration potential for elements such as Pt or Ag. These varying sulfide indicators have moderate utility, even for large-scale (～5 km spacing) sampling. Results indicate that hydrogeochemistry can add value to greenfields exploration for VHMS ore deposits in deeply weathered terrains. It is also moderately successful at indicating the presence of sulfide-rich systems (whether magmatic or hydrothermal) under >100 m cover, thus providing a rapid and cost-effective regional prospectivity tool for deeply buried terrains. Such mineral exploration tools will encourage exploration investment for more difficult regions of Australia and in other deeply weathered regions of the world.     

Vesuvianite-new tool for the U-Pb dating of skarn ore deposits

Summary Vesuvianites from the Early Proterozoic Björntjärn tungsten skarn deposit in northern Sweden were dated with the U-Pb and Pb-Pb methods. Low²⁰⁶Pb/²⁰⁴Pb values make the U-Pb dating of vesuvianite rather sensitive to the common lead correction. However, the combination of Pb-Pb and U-Pb data on the same material permits the deduction of precise ages on Proterozoic vesuvianites. Vesuvianite can be used to date the formation of skarn mineralizations and possibly also the metamorphism and metasomatism of argillaceous limestones.

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Vesuvianit-ein neues Werkzeug zur U-Pb Datierung von Skarnerzlagerstätten

Zusammenfassung Vesuvianit von der Wolfram-Skarnerzlagerstätte Björntjärn in Nordschweden wurde mit der U-Pb and Pb-Pb Methode datiert. Das tiefe²⁰⁶Pb/²⁰⁴Pb Verhältnis des Vesuvianits macht die U-Pb Altersbestimmung kräftig von der Korrektur des gewöhnlichen Bleis abhängig. Die Kombination von Pb-Pb und U-Pb Altersbestimmung am gleichen Probenmaterial erlaubt jedoch die präzise Datierung proterozoischer Vesuvianite. Vesuvianit kann zur direkten Altersbesimmung von Skarnlagerstätten und möglicherweise der Metamorphose und Metasomatose von mergeligen Kalksteinen verwendet werden.

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With 4 Figures     

PGE distribution in massive sulfide deposits of the Iberian Pyrite Belt

We present the first platinum group elements (PGE) data on seven massive sulfide deposits in the Iberian Pyrite Belt (IPB), one of the world largest massive sulfide provinces. Some of these deposits can contain significant PGE values. The highest PGE values were identified in the Cu-rich stockwork ores of the Aguas Teñidas Este (Σ PGE 350 ppb) and the Neves Corvo (Σ PGE 203 ppb) deposits. Chondrite normalized PGE patterns and Pd/Pt and Pd/Ir ratios in the IPB massive, and stockwork ores are consistent with the leaching of the PGE from the underlying rock sequence.