An Eburnean base metal source for sediment-hosted zinc-lead deposits in Neoproterozoic units of Namibia: Lead isotopic and geochemical evidence

The lead isotopic composition of galena from the Neoproterozoic sediment-hosted Zn-Pb sulphide deposit at Rosh Pinah and the oxidised Zn deposit at Skorpion in the Pan-African Gariep Belt, southwestern Namibia, as well as that of galena from minor occurrences in the wider Rosh Pinah ore province was investigated and is compared with that of other major sediment-hosted base metal ore deposits hosted by Neoproterozoic strata in southwestern Africa and Brazil. The isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the Rosh Pinah deposit and its stratigraphic equivalents. The pre-orogenic Rosh Pinah deposit and the inferred progenitor for the secondary Skorpion deposits have very similar Pb isotopic compositions. In contrast, syn-orogenic deposits (e.g. Tsumeb-type) show a larger proportion of radiogenic Pb from the country rocks that were infiltrated by orogenic ore fluids.In all examples studied, the ore Pb has elevated ²⁰⁷Pb/²⁰⁴Pb ratios compared to Pb that evolved according to average crustal Pb growth models. The isotopic composition of the ore Pb is in agreement with that of the oldest crustal component known from the pre-Gariep basement, i.e. an Eburnean (c. 2.0 Ga) volcanic arc, best preserved in the Richtersveld Terrane, that represents the largest volume of post-Archaean juvenile crust in southern Africa. Erosion of that arc provided the main sediment source for the metasedimentary siliciclastic host to the Rosh Pinah deposit. Derivation of the Pb, and by analogy Zn, from this Eburnean volcanic arc is therefore inferred for the formation of Rosh Pinah-type syn-rift, early diagenetic replacement mineralisation.The calculated ₂- and ₂-values for the Rosh Pinah ore province (around 10.2 and 42, respectively) are higher than predicted by conventional Pb crustal growth models and are similar to those found in the wider region of southern Africa (Otavi Mountain Land) and eastern Brazil. The obtained data highlight not only that calculated Pb model ages may have no geochronological significance but might also point to a common crustal evolution of this part of SW-Gondwana.Editorial handling: M. Chiaradia     

Genetic Analogy Between Sediment—Hosted Gold Deposits and Sediment—Hosted Copper Deposits in Southwest China

The sediment-hosted copper deposits according to the authors‘ study were formed from connate formation water during the post-sedimentary or diagenetic stage while the sediment-hosted disseminated gold deposits are unanimously considered to be of post-sedimentary hydrothermal origin.Therefore,apart from their own individulities.These two types of deposits must share some characters in common.Comparisons are attempted,in this paper,between the sediment-hosted copper deposits in southwest China and the Triassic sediment-hosted disseminated gold deposits in the Yunnan-Guizhou-Guangxi Triangle in terms of geological and geochemical features.     

西南三江碰撞造山带沉积岩容矿Pb-Zn-Ag-Cu贱金属复合成矿与深部过程

西南三江特提斯造山带中新生代沉积盆地中(沱沱河、玉树、昌都和兰坪-思茅地区)发育包括金顶超大型铅锌矿床在内的一系列以沉积岩容矿的Pb-Zn-Ag-Cu贱金属矿床,构成长达千余千米的青藏高原东缘贱金属成矿带。作为大陆碰撞环境成矿谱系的重要矿床类型,加强这些矿床的理论研究对提高和完善大陆碰撞造山成矿理论和指导找矿勘查等具有重要意义。已有研究表明这些Pb-Zn-Ag-Cu矿床的分布受盆地形成后新生代大型逆冲推覆-走滑构造控制,其容矿岩石和成矿作用特征与SEDEX和MVT矿床存在明显的差异,矿床成矿流体表现出多来源混合的特征,成矿与深部过程密切相关。尽管取得重要进展,但由于缺乏高精度年代学数据制约,成矿动力学背景及其与碰撞造山的时空联系存在较大争议。一些矿床的研究显示复合成矿迹象,但是复合成矿过程与深部驱动等问题仍不清楚。近年来我们以兰坪和昌都盆地的Pb-Zn-Ag多金属矿床和Cu多金属矿床为重点研究对象,系统开展了成矿年代学、成矿流体源-运-储系统和复合成矿机制以及深部过程对成矿制约等方面研究。结果表明,兰坪盆地西缘Cu(Mo)多金属矿床主要形成于48～58Ma,兰坪和昌都盆地Pb-Zn-Ag多金属矿床主要形成于27～33Ma。成矿流体表现出明显的多来源混合的特征,主要存在三种类型:1)变质流体与盆地卤水或大气降水复合成矿,以金满-连城Cu矿床为代表; 2)盆地卤水与大气降水复合成矿,以金顶Pb-Zn矿床为代表; 3)盆地卤水和岩浆流体复合成矿,以拉诺玛Pb-Zn-Sb矿床为代表。兰坪盆地西缘Cu矿床主要形成于新生代印度-欧亚大陆主碰撞挤压阶段,与成矿密切相关的变质流体可能来源于陆-陆碰撞俯冲引起的高压变质。Pb-Zn矿床主要形成于印度-欧亚大陆晚碰撞构造转换环境,构造挤压和造山隆起驱动盆地流体迁移,同期的岩浆活动主要为成矿提供热驱动力或成矿物质。     

Manganese haloes surrounding metamorphic stratabound base metal deposits

Primary manganese haloes "survive" medium to high-grade regional metamorphism: stratabound base metal deposits in Namaqualand, N. W. Cape Province (Proterozoic) and in the Eastern Alps (Paleozoic and Mesozoic) reveal an increase in Mn as the ore horizon (and associated BIF) is approached from the hangingwall or the footwall. Electron microprobe analyses show Mn to be preferentially concentrated in garnets; compositions vary considerably and include pure spessartine (40 % MnO). Compositional inhomogeneities (zoning) appear to be confined to garnets in lower grade metamorphic terranes. Iron contents of associated sphalerite vary within meters of the stratigraphic sequence and even within one polished section; sphalerite geobarometry does not appear promising in these environments.     

A hypothesis for base and noble metal deposits

Chemical deposits in the Red Sea, Salton Sea, and Cheleken Peninsula have concentrations of Cu, Zn, Pb, Ag, and some Au. Brines emanating from these base metal systems have over 10% Cl and have chalcophile metals in excess of sulphide in solution. Siliceous sinters laden with Au, Ag, Hg, and Sb exist around vents and wells in California, western Nevada, and New Zealand. Hot CO₂-H₂O fluids that exhale from these noble metal systems have less than 2 % Cl and sulphide in excess of chalcophile metals. Some geothermal systems around the world have features intermediate between these two end-members. Cu-Fe and Ag-Au concentrations occur around the margin of an Archean felsic stock in the Wawa greenstone belt in Ontario, Canada. The interpretation is that an initial seawater dominated system concentrated exhalites rich in base metals. This culminated with regional metamorphism and the concentration of precious metal-bearing veins from low salinity fluids.

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Zusammenfassung Die chemischen Ablagerungen im Roten Meer, Salton Sea (Californien) und auf der Cheleken Halbinsel (UDSSR) haben Anreicherungen von Cu, Zn, Pb, Ag und etwas Au. Sole, mit der die Ag-Pb-Zn-Cu Lösungen kommen, hat über 10% Cl und einen Überschuß an chalcophilen Metallen. Kieselsinter, die reich an Au, Ag, Hg und Sb sind, erscheinen an den Austrittsstellen der heißen Lösungen in Californien, West-Nevada und Neu-Seeland. Heiße CO₂-H₂O-Lösungen, die mit diesem Sb-Hg-Ag-Au-System ausströmen, enthalten weniger als 2% Cl und anstelle von chalcophilen Metallen kommen Sulfide vor. In verschiedenen geothermalen Provinzen gibt es Mischsysteme zwischen diesen Endgliedern.Zn-Cu und Ag-Au-Konzentrationen treten am Rande des archaischen Komplexes im Wawa Grundschiefergürtel in Ontario, Canada auf. Dies wird interpretiert als ein zunächst vom Meerwasser dominiertes System, in dem Exhalationen reich an unedlen Metallen eindrangen. Während einer folgenden Regionalmetamorphose erfolgt die Konzentration von Edelmetallen aus Lösungen mit geringer Salinität.

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Résumé Les dépôts chimiques de la Mer Rouge, du lac de Salton (Californie) et de la presqu'île de Cheleken (URSS), présentent des enrichissements en Cu, Zn, Pb, Ag et, dans une certaine mesure, en Au. Les saumures qui émanent de ces systèmes à Ag-Pb-Zn-Cu renferment plus de 10% Cl et contiennent un excès de métaux chalcophiles par rapport à S. Des dépôts de précipitation siliceux, riches en Au, Ag, Hg, Sb se rencontrent autour d'évents et de sources chaudes en Californie, en W. Nevada et en Nouvelle Zélande. Les fluides chauds (H₂O - CO₂) qui émanent de ces systèmes à Sb - Hg - Ag - Au renferment moins de 2% Cl et les ions S^{– –} y sont en excès sur les métaux chalcophiles. Divers systèmes géothermaux à travers le monde présentent des caractères intermédiaires entre ces deux types.Dans le »Greenstone belt« de Wawa (Ontario, Canada), des concentrations en Zn-Cu et en Ag-Au se présentent autour de la bordure d'un massif felsitique archéen. Cette disposition est interprétée comme le résultat: d'abord de l'action d'un système à eau de mer dans lequel les métaux usuels se sont concentrés, ensuite, au cours d'une phase ultérieure de métamorphisme régional, de la concentration des métaux précieux à partir de solutions peu salées.

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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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Sulfur- and oxygen-isotopes in sediment-hosted stratiform barite deposits

Sulfur- and oxygen-isotope analyses have been obtained for sediment-hosted stratiform barite deposits in Alaska, Nevada, Mexico, and China to examine the environment of formation of this deposit type. The barite is contained in sedimentary sequences as old as Late Neoproterozoic and as young as Mississippian. If previously published data for other localities are considered, sulfur- and oxygen-isotope data are now available for deposits spanning a host-rock age range of Late Neoproterozoic to Triassic. On a δ³⁴S versus δ¹⁸O diagram, many deposits show linear or concave-upward trends that project down toward the isotopic composition of seawater sulfate. The trends suggest that barite formed from seawater sulfate that had been isotopically modified to varying degrees. The δ³⁴S versus δ¹⁸O patterns resemble patterns that have been observed in the modern oceans in pore water sulfate and water column sulfate in some anoxic basins. However, the closest isotopic analog is barite mineralization that occurs at fluid seeps on modern continental margins. Thus the data favor genetic models for the deposits in which barium was delivered by seafloor seeps over models in which barium was delivered by sedimentation of pelagic organisms. The isotopic variations within the deposits appear to reflect bacterial sulfate reduction operating at different rates and possibly with different electron donors, oxygen isotope exchange between reduction intermediates and H₂O, and sulfate availability. Because they are isotopically heterogeneous, sediment-hosted stratiform barite deposits are of limited value in reconstructing the isotopic composition of ancient seawater sulfate.     

The genesis of sediment-hosted,exhalative zinc + lead deposits

Large sediment-hosted lead+zinc deposits like Mount Isa, McArthur River, Navan, Rammelsberg and Sullivan form a distinctive group characterised by stratiform, syngenetic sulphide ores that formed in local basins on the sea floor as a result of protracted hydrothermal activity accompanying continental rifting. Generally there is a development of a sedimentary pre-ore phase mineralization often featuring manganese followed by zinc±lead, iron and chert. Lower main phase zinc+lead lenses are usually almost devoid of copper but Cu tenors increase toward the middle or top of the ore sequences. Hanging wall trace element haloes are common. These characteristics are accounted for by deriving the ore solutions from subsurface convective circulation of modified highly saline seawater. The circulation is initiated during rifting and driven by a high geothermal gradient. As a result of continued extensional strain and cooling of the rock column the brittle-to-ductile transition zone is depressed and the circulation penetrates to greater depth with time. Of the ore metals the downward-penetrating convection fluids first leach and transport zinc and lead, but with increasing temperature are later able to leach and transport some copper. Unless convective circulation ceases the metal sequence generally reverses as the cooling phase sets in. The minimum distance separating major coeval orebodies of this type is 18 km which is a function of the size of the convective systems.     

Lithostratigraphy, basin development, base metal deposits, and regional correlations of the Neoproterozoic Nguba and Kundelungu rock successions, central African Copperbelt

The Neoproterozoic Katangan Supergroup comprises a thick sedimentary rock succession subdivided into the Roan, Nguba, and Kundelungu Groups, from bottom to top. Deposition of both Nguba and Kundelungu Groups began with diamictites, the Mwale/Grand Conglomérat and Kyandamu/Petit Conglomérat Formations, respectively, correlated with the 750 Ma Sturtian and (supposedly) 620 Ma Marinoan/Varanger glacial events. The Kaponda, Kakontwe, Kipushi and Lusele Formations are interpreted as cap-carbonates overlying the diamictites. Petrographical features of the Nguba and Kundelungu siliciclastic rocks indicate a proximal facies in the northern areas and a basin open to the south. The carbonate deposits increase southward in the Nguba basin. In the southern region, the Kyandamu Formation contains clasts from the underlying rocks, indicating an exhumation and erosion of these rocks to the south of the basin. It is inferred that this formation deposited in a foreland basin, dating the inversion from extensional to compressional tectonics, and the northward thrusting. Sampwe and Biano sedimentary rocks were deposited in the northernmost foreland basin at the end of the thrusting. The Zn–Pb–Cu and Cu–Ag–Au epigenetic, hypogene deposits occurring in Nguba carbonates and Kundelungu clastic rocks probably originate from hydrothermal resetting and remobilization of pre-existing stratiform base metal mineralisations in the Roan Group.     

若干以沉积岩为容矿围岩的热液矿床硫同位素地球化学研究

文章对与沉积岩容矿有关的热液广西高龙微细浸染型金矿、新疆霍什布拉克铅锌矿、广西盘龙铅锌矿和下雷锰矿等矿床硫化物进行硫同位素测定,硫同位素组成分别为-15.3‰~+15.6‰,-18.8‰~+24.2‰,-21.0‰~+26.4‰,-32.6‰~+32.2‰。硫同位素可以达到平衡(高龙、霍什布拉克、下雷),也可以没达到平衡(盘龙)。δ34SΣS值在+13‰~+28‰之间,表明与沉积岩容矿热液矿床中硫来源于海水硫酸盐还原硫,但不排除有岩浆硫的加入。     

沉积岩型铅锌矿床的成矿系统研究

沉积岩型铅锌矿床是铅锌矿床中重要类型之一。在系统论述沉积岩型铅锌矿床成矿背景、地球化学和矿床类型的基础上,分别阐明了沉积喷流型(SEDEX)、密西西比型(MVT)和砂岩型(SST)3个成矿系统的成矿要素、作用过程、矿床和异常组合以及成矿模式。对3个铅锌成矿系统作了全面对比,获得一些认识:(1)SST矿床的矿化表现为对构造和高孔隙度有利岩性而不是地层层位的依赖性;(2)云南会泽碳酸盐型铅锌矿(MVT)成矿物质除热卤水沉淀来源外,还有火山岩系的提供,属后生混合流体成矿作用;(3)沿古陆边缘同生断裂构造发生的海底热水喷流沉积作用为SEDEX型铅锌成矿奠定了物质基础;裂谷伸展期常有“双峰式”火山活动,提供部分矿源、热源、水源;同生断层长期活动提供热水循环通道和矿质沉积场所;(4)风化剥蚀的变质基底为三者提供了成矿物质,成矿热卤水在盆地中经过了演化过程。     

Characteristics and origin of sediment-hosted disseminated gold deposits: a review

Sediment-hosted disseminated gold (SHDG) deposits comprise a major portion of the gold production and reserves in the US. Although presently known to be common only in western North America, SHDG deposits are a significant source of world gold production. These deposits are characterized by extremely fine-grained disseminated gold, hosted primarily by arsenian pyrite. Other metals show very little enrichment although in addition to As, anomalous concentrations of elements such as Sb, Hg, Tl and Ba are utilized as exploration tools. The host rocks are dominantly silty carbonates, but ore concentrations are also present in siliceous and silicified rocks as well as intrusive rocks. Alteration consists of decarbonatization, silicification (jasperoid formation) and argillization, which are arranged both spatially and temporally in that order. Argillic alteration is zoned from kaolinite-dominated cores to sericite-dominated margins. The deposits commonly exhibit significant structural (faults) and stratigraphic (composition/permeability) controls. Until the last few years, SHDG deposits were considered as near-surface, epithermal type deposits in origin. Because of their fine-grained nature and the lack of macroscopic features such as veins, it has proven quite difficult to extract geochemical data that are clearly related to their genesis. However, fluid inclusion data indicate pressures corresponding to depths of 2–4 km under lithostatic conditions. Temperatures are constrained by fluid inclusions and phase equilibria to near 225°C. Stable isotope data from alteration minerals and fluid inclusions indicate that the ore fluids were dominated by meteoric waters, some of which had clearly exchanged oxygen with wallrocks during their passage through the crust. Although the data vary, most ore fluids probably had δD values near −150‰ and δ¹⁸O values ranging from −10 to +5‰. Sulfur isotope values reported from SHDG deposits span a wide range, from −30 to +20‰ (sulfides) and 0 to >45‰ (sulfates). Ore-related sulfides (pyrite, realgar) fall at the upper end of the range reported for sulfides. The alteration and mineral assemblage indicate the ore fluids were probably near neutral and gold was likely carried as a bisulfide complex. The depositional mechanism(s) probably included mixing, cooling and oxidation. These mechanisms are consistent with the observed alteration features, i.e. quartz precipitation, calcite dissolution and sericite-kaolinite coexistence. It also explains the presence of both siliceous ores containing native Au and sulfide ores containing Au in pyrite. The extreme variations in sulfur isotopes as seen at Post and fluid inclusion data from Carlin may be indicative of some phase separation (‘boiling’), but such relations have not been documented in other deposits and the importance of phase separation to gold deposition appears minimal.     

Regional fluid flow and the genesis of Irish Carboniferous base metal deposits

The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal clastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.     

Juvenile contribution of the Neoproterozoic Rio Negro Magmatic Arc (Ribeira Belt,Brazil): Implications for Western Gondwana amalgamation

The ca. 790–600 Ma Rio Negro Complex (RNC) of the Ribeira belt (Brazil) consists of a plutonic portion of a magmatic arc built by the E-vergent subduction of the ESE border of the São Francisco paleoplate during the amalgamation of Western Gondwana.The plutonic series comprises low- to medium-K granitoids (ca. 790–620 Ma) and high-K granitoids and shoshonite rocks (ca. 610–605). The age span of 185 m.y. is suggestive of a long history of arc-related magmatism, continuously or not in time. The Nd isotopic signatures of the RNC consist of εNd(t) ratios from ? 3 to + 5 for the medium-K series shoshonite series and from ? 14 to ? 3 for the younger high-K group. This time-dependent trend of Nd isotopes is indicative of progressive maturity of the arc over time. The same evolution is indicated by Sr data, as the medium-K rocks have ⁸⁷Sr/⁸⁶Sr initial ratios < 0.705 while the high-K rocks yield values between 0.705 and 0.710. The predominance of intermediate rocks over mafic ones suggests an initial intra-oceanic to transitional stage, possibly developed in a distal portion of a passive margin, such as the Japanese arc, evolving to a more developed, differentiated felsic rock associations.The role of transform fault zones, such as the Luanda shear zone, is emphasized in order to explain the consumption of a wide oceanic plate in the inner portion of Western Gondwana.     

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif,Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.     

Genesis of sediment-hosted stratiform copper–cobalt deposits, central African Copperbelt

The Neoproterozoic central African Copperbelt is one of the greatest sediment-hosted stratiform Cu–Co provinces in the world, totalling 140 Mt copper and 6 Mt cobalt and including several world-class deposits (10 Mt copper). The origin of Cu–Co mineralisation in this province remains speculative, with the debate centred around syngenetic–diagenetic and hydrothermal-diagenetic hypotheses.The regional distribution of metals indicates that most of the cobalt-rich copper deposits are hosted in dolomites and dolomitic shales forming allochthonous units exposed in Congo and known as Congolese facies of the Katangan sedimentary succession (average Co:Cu = 1:13). The highest Co:Cu ratio (up to 3:1) occurs in ore deposits located along the southern structural block of the Lufilian Arc. The predominantly siliciclastic Zambian facies, exposed in Zambia and in SE Congo, forms para-autochthonous sedimentary units hosting ore deposits characterized by lower a Co:Cu ratio (average 1:57). Transitional lithofacies in Zambia (e.g. Baluba, Mindola) and in Congo (e.g. Lubembe) indicate a gradual transition in the Katangan basin during the deposition of laterally correlative clastic and carbonate sedimentary rocks exposed in Zambia and in Congo, and are marked by Co:Cu ratios in the range 1:15.The main Cu–Co orebodies occur at the base of the Mines/Musoshi Subgroup, which is characterized by evaporitic intertidal–supratidal sedimentary rocks. All additional lenticular orebodies known in the upper part of the Mines/Musoshi Subgroup are hosted in similar sedimentary rocks, suggesting highly favourable conditions for the ore genesis in particular sedimentary environments. Pre-lithification sedimentary structures affecting disseminated sulphides indicate that metals were deposited before compaction and consolidation of the host sediment.The ore parageneses indicate several generations of sulphides marking syngenetic, early diagenetic and late diagenetic processes. Sulphur isotopic data on sulphides suggest the derivation of sulphur essentially from the bacterial reduction of seawater sulphates. The mineralizing brines were generated from sea water in sabkhas or hypersaline lagoons during the deposition of the host rocks. Changes of Eh–pH and salinity probably were critical for concentrating copper–cobalt and nickel mineralisation. Compressional tectonic and related metamorphic processes and supergene enrichment have played variable roles in the remobilisation and upgrading of the primary mineralisation.There is no evidence to support models assuming that metals originated from: (1) Katangan igneous rocks and related hydrothermal processes or; (2) leaching of red beds underlying the orebodies. The metal sources are pre-Katangan continental rocks, especially the Palaeoproterozoic low-grade porphyry copper deposits known in the Bangweulu block and subsidiary Cu–Co–Ni deposits/occurrences in the Archaean rocks of the Zimbabwe craton. These two sources contain low grade ore deposits portraying the peculiar metal association (Cu, Co, Ni, U, Cr, Au, Ag, PGE) recorded in the Katangan sediment-hosted ore deposits. Metals were transported into the basin dissolved in water.The stratiform deposits of Congo and Zambia display features indicating that syngenetic and early diagenetic processes controlled the formation of the Neoproterozoic Copperbelt of central Africa.     

“三江”喜马拉雅期沉积岩容矿贱金属矿床基本特征与成因类型

"三江"的兰坪、昌都、玉树、沱沱河地区发育丰富的沉积岩容矿贱金属矿床,构成一条极具成矿潜力的巨型矿化带。综合分析表明,带内矿床形成于新生代印-亚大陆碰撞环境,发育在大陆碰撞造山带内部的褶皱-逆冲带内,与逆冲和走滑构造控制的新生代盆地相伴。不同矿区均发育逆冲断层,矿体主要赋存于逆冲断层上盘的碳酸盐岩或碎屑岩内,受与逆冲相关的盐底辟、逆冲断裂的次级断裂、热液溶洞、白云岩化、灰岩破碎、矿前溶洞垮塌、与褶皱有关的密集劈理或断裂等构造或岩相变化控制。金属呈现Pb-Zn、Pb-Zn(-Cu-Ag)、Cu-Co、Cu等组合,其中,Pb-Zn矿床主要发育闪锌矿、方铅矿、黄铁矿、±白铁矿、方解石、白云石,一些矿床富硫酸盐矿物和萤石,也常见沥青,矿化以交代和开放空间充填为主;Pb-Zn(-Cu-Ag)矿床主要发育闪锌矿、细/灰硫砷铅矿、方铅矿、黝铜矿、黄铁矿、方解石、白云石,矿化以脉体或热液溶洞内交代/开放空间充填式出现;Cu矿床由石英-方解石/白云石-Cu硫化物(黄铜矿、黝铜矿、斑铜矿、辉铜矿)脉构成。Pb-Zn和Pb-Zn(-Cu-Ag)矿床成矿流体以低温(<210℃)和高盐度主体[w(Na Cl)>10%]的盆地卤水为主,脉状Cu矿床成矿流体可能来自富CO2、具相对高温(180~230℃)和低盐度[w(NaCl)<11%]的变质流体。盆地卤水中硫酸盐遭受细菌还原±有机热还原或仅经历有机热还原,为Pb-Zn和Pb-Zn(-Cu-Ag)矿床提供了还原硫,下伏火山岩和沉积地层中的硫可能是脉状Cu矿床硫的来源,它们的成矿金属物质均来自于上地壳。该带矿床是一套与岩浆活动无关的后生矿床,以世界上已知的矿床类型来划分,金顶、赵发涌、东莫扎抓、莫海拉亨和茶曲帕查矿床等Pb-Zn矿床及白秧坪东矿带的Pb-Zn(-Cu-Ag)矿床可归为类MVT矿床,白秧坪西矿带脉状Pb-Zn(-Cu-Ag)矿床和脉状Cu矿床可归为多金属脉状矿床。其中,金顶等类MVT矿床以其形成于大陆碰撞带内部、受控于逆冲推覆构造等特点不能被已建立的MVT成矿模型所涵盖,成矿作用独特。     

Stable isotope study of base metal deposits from the Eastern High Atlas,Morocco

Tertiary epigenetic lead ± zinc and copper mineralizations occur in the Mesozoic carbonate cover and stibnite, barite and copper mineralization in the Precambrian to Palaezoic basement in the Eastern High Atlas, Morocco. The carbon isotope data from host carbonates range from +3 to -1‰ PDB. The data are typical for marine carbon. There is no difference between surrounding dolomites and younger vein dolomites. The oxygen isotopic composition of the samples is enriched in ¹⁶O; the range is from -3 to -11‰ PDB with respect to that of carbonates in equilibrium with marine water. It is postulated that the isotopic composition was changed during dolomitization being caused by transition of connate formation waters diluted to some degree by meteoritic water. The bacteriogenically reduced sulphur from syngenetic sulphides and Mesozoic marine sulphate mobilized during Tertiary orogenesis are thought to be the source of H₂S and SO₄. Cambro-Ordovicien sulphate is also supposed to be the source of sulphur in the Jebel Zelmou barite deposit in the basement. Isotopic composition of lead from galenas in the Tertiary deposits from the Mesozoic cover are strikingly homogenous. Model ages vary between 230–180 Ma. It is suggested that important mobilization processes are responsible for Tertiary metallogeny in Morocco.

     

Shale basins,sulfur-deficient ore brines and the formation of exhalative base metal deposits

The massive sulfide deposits of the Iberian Pyrite Belt are interbedded with felsic volcanic rocks and shale, and underlain by several thousand meters of siliciclastic sedimentary rocks known as the PQ Group. Isotope geochemistry and regional geology are both consistent with equilibration of the ore-forming fluids with the PQ Group, prior to ore deposition near the former seafloor. The average Cu:Zn:Pb ratio of the PQ Group rocks (ca. 26:55:19) is similar to the weighted average of all the massive sulfide orebodies combined (ca. 25:52:23).The genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model, proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore-forming fluid. Relatively metal-rich but organic-poor pyrite-bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur-deficient but relatively iron and base metal-rich brines. Thus, we propose that the giant deposits of the Iberian Pyrite Belt owe their existence not to exceptionally metal-enriched (e.g., magmatic) fluids, but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments, combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with H₂S-rich fluids at or near the seafloor.Essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks. Leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar, variably metal-rich but relatively sulfide-deficient fluids. Thus, the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic, Irish-type or sedex deposits, many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site.