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1.
Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.  相似文献   

2.
Heavy mineral separates of peat from a mineralotrophic bog contain sulphide minerals with distinctive textures. Pyrite framboids, consisting of spherical aggregates of subhedral pyrite crystals, are surrounded by a thin rim of chalcopyrite or a layer of massive marcasite. Clusters of framboids are cemented by covellite which also occurs as small idiomorphic grains, with rectangular or hexagonal outlines, surrounded by chalcopyrite. The sulphides appear to have resulted from discharge of groundwaters, enriched in copper from weathering of primary sulphides in bedrock and in iron by reduction of the till underlying the peat, into the hydrogen sulphide charged bog.  相似文献   

3.
Metal-cystine complexes of iron, lead, zinc, copper and nickel under mild artificial diagenesis give rise to crystalline metal sulphides and insoluble organic matter as well as gaseous and “oily” organic products. Under confined reducing conditions at 200°C for 100 h a virtual 100% conversion of metal complex to metal sulphide occurs, while < 10% of the associated organic material may remain as kerogen. Such a mechanism could account for the formation of metal sulphides and in particular pyrite from protein- or amino acid-rich material in carbonaceous sediments during diagenesis.  相似文献   

4.
Origin of the Kupferschiefer polymetallic mineralization in Poland   总被引：2，自引：0，他引：2  
The Kupferschiefer ore series, between the Lower Permian (Rotliegendes) terrestrial redbeds/volcanics and the Upper Permian (Zechstein) marine sequence, is developed as dark-grey organic matter-rich and metal sulphide-containing deposits (reduced zone) and as red-stained organic matter-depleted and iron oxide-bearing sediments (oxidized zone?=?Rote Fäule). The transition zone from oxidized to reduced rocks occurs both vertically and horizontally. This zone is characterized by sparsely disseminated remnant copper sulphides within hematite-bearing sediments, replacements of copper sulphides by iron oxides and covellite, and oxide pseudomorphs after framboidal pyrite. These textural features and copper sulphide replacements after pyrite in reduced sediments imply that the main oxide/sulphide mineralization postdated formation of an early-diagenetic pyrite. Hematite-dominated sediments locally contain enrichments of gold and PGE. The Kupferschiefer mineralization resulted from upward and laterally flowing fluids which oxidized originally pyritiferous organic matter-rich sediments to form hematitic Rote Fäule areas, and which emplaced base and noble metals into reduced sediments. It is argued that long-lived and large-scale lateral fluid flow caused the cross-cutting relationships, expansion of the hematitic alteration front, redistribution of noble metals at the outer parts of oxidized areas, and the location of copper orebodies directly above and around oxidized and gold-bearing areas. The Rote Fäule may be a guide to favourable areas for both the Cu-Ag and new Au-Pt-Pd Kupferschiefer-type deposits.  相似文献   

5.
Geochemically anomalous, pyritic sediments occur directly above a Mid Silurian unconformity in the Quidong area of southeastern New South Wales. The composition of these sediments reflects derivation from a mixture of: (a) feldspar- and mica-depleted detritus reworked from underlying quartz-rich flysch; (b) Mg-rich clay or chlorite precipitated from hydrothermal exhalations; and (c) pyrite formed by reaction of iron in clays or oxides with reduced sulphur derived largely from sea-water sulphate and possibly a magmatic source. Three types of base metal sulphide mineralisation occur at Quidong including: (a) weak syngenetic concentrations in the pyritic sediments; (b) stratabound and fault-controlled bodies of massive sulphides hosted by the pyritic sediments and containing higher grade Pb, Zn and Cu; and (c) small vein and cavity fillings of galena, barite and other minor sulphides in overlying limestones. All types of mineralisation are related to hydrothermal activity which occurred during and after deposition of the pyritic facies. The geochemistry of the immediately underlying basement rocks and Pb isotope data indicate that the source of the metal-bearing fluids was deeper in the crust and probably related to widespread partial melting and magmatic processes. The sulphidic sediments and stratabound sulphide deposits represent syngenetic-epigenetic, sediment hosted mineralisation developed in a shallow marine environment, distal from major volcanic centers. This style of mineralisation has not previously been described from the region. It has some similarities to the Irish-Alpine type spectrum of deposits best known in Europe.  相似文献   

6.
New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5  m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.  相似文献   

7.
Various Fe–S minerals of the mackinawite–greigite–pyrite association, ubiquitous in biogenic remains from Jurassic mudstones, have been described in detail in an SEM–EDS study. Two diagenetic stages of Fe sulphide formation and preservation in the Jurassic organic skeletons are identified. In the first stage, pyrite formed as euhedra and framboids shortly after deposition, mainly in the interiors of the skeletons which still contained labile organic matter. The second stage of iron sulphide formation was related to the later stages of diagenesis, when the influence of the surrounding sediment was more dominant, although some organic matter was still present in the biogenic skeletons. A Fe-rich carbonate–aluminosilicate cement was then introduced between the earliest iron sulphides and later subsequently sulphidized, to form a metastable iron monosulphide of mackinawite composition and then greigite.  相似文献   

8.
Sulfide minerals in coal bed III at the Chinook Mine, Indiana, are pyrite, marcasite, and rarely sphalerite. Pyrite occurs as framboids concentrated mainly in exinite, as bands or lenses in vitrinite and clay partings, as cell fillings in fusinite, and in cleats. Marcasite normally occurs in association with clusters of pyrite framboids within micro-organic remains. Sphalerite occurs exclusively in fusinite associated with cleat pyrite. The iron sulfides, which are of authigenic origin, were formed during the biochemical stage of coalification during the accumulation and compaction of peat. The factor that limited their formation in such an environment was the availability and reactivity of iron. Chemical heterogeneity in the peat swamps where the sulfides formed existed even on a microscopic scale. The iron sulfides were commonly precipitated in localized micro-environments that were favorable for their formation. The metamorphic stage of coalification did not affect the iron sulfides significantly, although it may have been responsible for the recrystallization of pyrite framboids and minor deformation of pyrite in fusinite and its local mobilisation.  相似文献   

9.
The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to 210Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS2 aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.  相似文献   

10.
The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide 34S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar 34S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734  相似文献   

11.
黄铁矿是富有机质沉积的特征矿物。根据TOC/S、TOC/DOP、S/Fe关系以及S TOC Fe多重线性回归分析结果对三水盆地古近系〖HT5”,6”〗土〖KG-*3〗布〖HT5”SS〗心组红岗段黑色页岩中沉积黄铁矿的形成及其控制因素进行了分析。土布心组红岗段黑色页岩的黄铁矿有成岩黄铁矿和同生黄铁矿两种成因组分。红岗段下部（亚段A）有机碳含量普遍较低,底部水体以弱氧化条件为主,硫酸盐还原作用发生于沉积物/水界面以下,黄铁矿为成岩成因,其形成主要受有机质的限制。红岗段中上部（亚段B和C）的沉积条件变化频繁,其有机碳含量变化幅度大。富有机质（TOC>4%）岩层形成于缺氧的底部水体条件下。水体中可含H2S,碎屑铁矿物在埋藏之前即与之在水体中反应形成同生黄铁矿。这一过程不受有机质的限制,而是受活性铁与H2S接触时间的限制。同时,由于大量淡水输入导致硫酸盐浓度的降低,从而对硫化物形成有一定的限制作用。对于低有机质（TOC<4%）样品,黄铁矿由同生和成岩组分组成。其中以成岩黄铁矿为主,其形成过程主要受有机质限制,而同生黄铁矿受铁矿物与H2S接触时间的限制。  相似文献   

12.
The Pimpama River floodplain has developed over the last several thousand years as a result of sea-level fluctuations that shaped the lower catchment and enabled the formation of sedimentary pyrite. The subsequent production of sulfuric acid due to the oxidation of this pyrite enhances the breakdown of metal-bearing sediments and can lead to leaching of major and trace metals into the waters of the region. The seasonal pattern of rainfall and current land-use activities are important aspects that intensify the natural production of acid and influence the release and distribution of metals. To identify the source and migration of metals in the Pimpama catchment and to understand the impact of pyrite oxidation on the distribution of metals in sediments and waters, several components of the drainage system were analyzed: bedrock, sediments from river bed and bank, and water. The elements analyzed in this study (V, Cr, Co, Ni, Cu, Zn and Pb) are all present in the bedrock material which explains their occurrence in the unconsolidated sediments of the floodplain. These metals concentrate in the upper section of the sedimentary sequence and their presence is related to clay minerals such as smectite, organic matter and iron phases. However, Zn, Mo and Co occur in higher amounts than the local background and within standard shale. This comparison suggests that the diagenetic processes alone cannot explain the higher concentrations and it is concluded that these metals also have an anthropogenic source. The formation of sulfuric acid creates conditions for higher mobility of some metals, such as Cr, Co, Ni, Cu, Zn, but does not affect less mobile ones such as Mo and Pb. Over the longterm, the production of acid influences the breakdown of mineral phases and enhances the process of weathering. Over the short term, every rain event leaches acid from sediments and mobilizes metals resulting in a substantial reduction in the quality of river water. Received: 2 October 1998 · Accepted: 16 February 1999  相似文献   

13.
A distinct vertical zonation very similar to that described for the Kuroko deposits of Japan, is displayed by both mineralogy and textures of sulphides from the Lahanos and Kzlkaya massive sulphide deposits of northeastern Turkey. A deeper erosional level is exposed at the Kzlkaya deposit, so that only remnants of the massive sulphide ore zone are present. The zonation is from an upper zone of massive Cu and Zn sulphides (black and yellow ore) with fine-grained, colloform, banded, framboidal, and spherulitic textures, downwards through an intermediate zone of low Cu-Zn massive pyrite with transitional textures, to a lower zone of stockwork and impregnated pyrite displaying euhedral, zoned textures. The fine-grained and colloform pyrite of the upper zones is progressively overgrown by, and recrystallized to, the massive and euhedral pyrite of lower zones. The original textures of these deposits are best preserved by pyrite. The previous interpretation of these textures, of sulphide deposition from colloidal solutions ponded by an impermeable pyroclastic horizon, is reexamined in the light of present observations. Although ultra-fine-grained sulphides, framboids, and radially-cracked spherules could have formed by replacement of pre-existing minerals by a colloidal solution, the colloform and banded textures are indicative of growth in open spaces. It thus seems likely that the fine-grained colloform sulphides, including chalcopyrite, sphalerite, and tennantite as well as pyrite, were initially deposited on or near the surface of the sea-floor. Additional evidence for this interpretation is seen in the progressive recrystallization of the sulphide textures to massive, much coarser, pyrite in the lower zones. This recrystallization may in part be due to diagenetic and hydrothermal processes operating after formation of the original layered sulphides. These conclusions are in agreement with those reached for the similar, but larger Madenköy deposit 100 km to the east.  相似文献   

14.
Three major types of dolomite occur in the Trenton Formation (Mid-Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture-related’ dolomite which occurs in association with both large- and small-scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture. The regional dolomite is fine-grained, has <0.34 mol% FeCO3, and mean δ18O of ?6·8‰OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO3 and has a mean δ18O of ?7·7‰. Fracture-related dolomites are coarse-grained, low in iron, and have the most depleted δ18O ratios (x?=–9·0%PDB). Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture-related dolomites post-date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture-related dolomites using oxygen isotope data. The distribution of the cap dolomite was controlled by the availability of Fe2? which was in turn controlled by the availability of S2?. In the centre of the basin Trenton-Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H2S from seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe2?. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H2S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe2- was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole-rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron sulphide is minor and primarily in the form of iron monosulphide.  相似文献   

15.
Quantitative mineral data from the lead-zinc bearing sediments at Mount Isa were studied using linear correlation analysis and R-mode cluster analysis. Pyrrhotite was found to be preferentially associated with galena and sphalerite. It is postulated that during sedimentation, formation of lead and zinc sulphides depleted an already limited sulphur supply to the point where the field of FeS stability was entered. The primary iron monosulphide formed was, or has since become, pyrrhotite. This hypothesis is in contrast to the widely held opinion that pyrrhotite in stratiform ores formed by metamorphic decomposition of pyrite. Empirical support for the sedimentary formation of pyrrhotite is provided by textural and qualitative mineralogical data from Mount Isa and other stratiform lead-zinc deposits.  相似文献   

16.
Framboidal pyrite has been prepared by precipitation of oxyhydroxides of iron, peptized by humic acids using a solution of Na-sulphate or hydrogen sulphide, spherical grains of elemental sulphur being present. By peptization of iron oxyhydroxides using humic acids, a stable negatively charged colloidal system arises. At room temperature and a pressure of 1 atm., and at pH widely ranging between 6.5–7.5, sulphidation of this colloidal system gives rise to a monodispersive sulphidic sol whose stability depends on the concentration of iron in the solution, the amount of humic acids and the presence of electrolytes. In the presence of spherical grains of sulphur, arising by rapid oxidation (acidification) of Na2S solution or a saturated solution of hydrogen sulphide, particles of the sulphidic sol precipitate on their surface and the grains become gradually replaced by sulphides of iron. At the same time, framboids of an average diameter of 14 μm are generated.  相似文献   

17.
Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe7S8, and its absolute magnetic susceptibility is 2.10− 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism.The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, (206Pb/204Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member (206Pb/204Pb  18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits.The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO2 conditions and neither a high temperature nor a low aH2S (below 10− 3) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds.Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.  相似文献   

18.
The Marl Slate, the English equivalent of the Kupferschiefer, has been studied with particular reference to the relationships between dolomitization and the origin of the metal sulphides. Dolomite occurs as: 1) discontinuous lenses of ferroan dolomicrite, 2) micronodules of finely crystalline dolospar in association with length-slow chalcedony and 3) discrete laminae of ferroan or non-ferroan dolospar. The ferroan dolomicrite has excess CaCO3, and is more abundant in the lower, sapropelic facies of the Marl Slate. It is considered to have formed by the penecontemporaneous alteration of calcium carbonate under hypersaline conditions. Small micronodules (typically about 0.3 mm in diameter) are also more abundant in the sapropelic Marl Slate. These frequently contain cores of length-slow chalcedony (quartzine) fibres and sometimes quartz megacrysts. Textural observations clearly indicate that this silica is of authigenic origin and the dolomite/chalcedony micronodules are interpreted as diagenetic replacements of a calcium sulphate mineral such as anhydrite. The discrete laminae of finely crystalline dolospar are often inter-laminated with calcite in the upper part of the Marl Slate. This dolomite is also calcium rich and represents a replacement, possibly of anhydrite, during a later phase of diagenesis. Metal sulphides occur in two distinct forms: as disseminated framboidal pyrite and as discrete lenses of pyrite, chalcopyrite, galena, sphalerite and rarer sulphides. The framboidal pyrite originated during early diagenesis by reaction of sulphide, produced by reduction of sulphate by organic material and micro-organisms, with iron also released in the reducing environment. The sulphide lenses are often in intimate association with dolospar, length-slow chalcedony and authigenic quartz megacrysts. This indicates that the lenses were produced during diagenesis by the reduction and replacement of calcium sulphate (anhydrite). Various sources, such as co-precipitation with dolomite precursors and the underlying Yellow Sands, may have supplied metals which were mobilized and transported by connate brines as diagenesis progressed.  相似文献   

19.
Massive and stockwork Fe-Cu-Zn (Cyprus type) sulphide deposits in the upper parts of ophiolite complexes represent hydrothermal mineralization at ancient accretionary plate boundaries. These deposits are probable metallogenic analogues of the polymetallic sulphide deposits recently discovered along modern oceanic spreading centres. Genetic models for these deposits suggest that mineralization results from large-scale circulation of sea-water through basaltic basement along the tectonically active axis of spreading, a zone of high heat flow. The high geothermal gradient above 1 to 2 km deep magma chambers emplaced below the ridge axis drives the convective circulation cell. Cold oxidizing sea-water penetrating the crust on the ridge flanks becomes heated and evolves into a highly reduced somewhat acidic hydrothermal solvent during interaction with basaltic wall-rock. Depending on the temperature and water/rock ratio, this fluid is capable of leaching and transporting iron, manganese, and base metals; dissolved sea-water sulphate is reduced to sulphide. At the ridge axis, the buoyant hydrothermal fluid rises through permeable wall-rocks, and fluid flow may be focussed along deep-seated fractures related to extensional tectonic processes. Metal sulphides are precipitated along channelways as the ascending fluid undergoes adiabatic expansion and then further cooling during mixing with ambient sub-sea-floor water. Vigorous fluid flow results in venting of reduced fluid at the sea-floor/sea-water interface and deposition of massive sulphide. A comparison of sulphide mineralization and wall-rock alteration in ancient and modern spreading centre environments supports this genetic concept.Massive sulphide deposits in ophiolites generally occur in clusters of closely spaced (< 1–5 km) deposits. Individual deposits are a composite of syngenetic massive sulphide and underlying epigenetic stockwork-vein mineralization. The massive sulphide occurs as concordant tabular, lenticular, or saucer-shaped bodies in pillow lavas and pillow-lava breccia; massive lava flows, hyalcoclastite, tuff, and bedded radolarian chert are less commonly associated rock types. These massive sulphide zones are as much as 700 m long, 200 m wide, and 50 m thick. The pipe-, funnel-, or keel-shaped stockwork zone may extend to a dehpth of 1 km in the sheeted-dike complex. Several deposits in Cyprus are confined to grabens or the hanging wall of premineralization normal faults.Polymetallic massive sulphide deposits and active hydrothermal vents at medium- to fast-rate spreading centres (the East Pacific Rise at lat. 21°N, the Galapagos Spreading Centre at long. 86°W, the Juan de Fuca Ridge at lat. 45°N., and the Southern Trough of Guaymas Basin, Gulf of California) have interdeposit spacings on a scale of tens or hundreds of metres, and are spatially associated with structural ridges or grabens within the narrow (< 5 km) axial valleys of the rift zones. Although the most common substrate for massive sulphide accumulations is stacked sequences of pillow basalt and sheet flows, the sea-floor underlying numerous deposits in Guaymas Basin consists of diatomaceous ooze and terrigenous clastic sediment that is intruded by diabase sills. Mound-like massive sulphide deposits, as much as 30 m wide and 5m high, occur over actively discharging vents on the East Pacific Rise, and many of these deposits serve as the base for narrow chimneys and spires of equal or greater height. Sulphides on the Juan de Fuca Ridge appear to form more widespread blanket deposits in the shallow axial-valley depression. The largest deposit found to date, along the axial ridge of the Galapagos Spreading Centre, has a tabular form and a length of 1000 m, a width of 200 m, and a height of 30 m.The sulphide assemblage in both massive and vein mineralization in Cyprus type deposits is characteristically simple: abundant pyrite or, less commonly, pyrrhotite accompanied by minor marcasite, chalcopyrite, and sphalerite. With few exceptions, the composition of massive sulphide ranges from 0.3 to 5 wt. % Cu, from 0.1 to 3 wt. % Zn, from 0.5 to 30 ppm Au, and from 1 to 50 ppm Ag. The only common gangue minerals — quartz, chlorite, calcite, and gypsum generally make up less than 10 percent of the massive zone.Sulphide assemblages in massive sulphide samples recovered from the Juan de Fuca Ridge (abundant sphalerite, wurtzite, and pyrite; minor marcasite, chalcopyrite, and galena), East Pacific Rise (abundant sphalerite, pyrite, and chalcopyrite; minor wurtzite, marcasite, and pyrrhotite), and Guaymas Basin (abundant pyrrhotite and sphalerite; minor chalcopyrite) contrast with ophiolitic deposits. Bulk analyses of two zinc-rich sulphide samples from the Juan de Fuca Ridge yield the following average values: Zn, 56.6 wt. %; Cu, 0.2 wt. %; Pb, 0.15 wt. %; Fe, 4.9 wt. %; Ag, 260 ppm; and Cd, 775 ppm. Other minerals precipitated with sulphides at hydrothermal-vent sites include anhydrite, barite, gypsum, Mg-hydroxysulphate-hydrate, talc, sulphur, and amorphous silica.Massive sulphide lenses in some Cyprus-type deposits are underlain by a silica-rich zone consisting of massive quartz, opaline silica, red jasper, or chert mixed with disseminated and veinlet Fe-Cu-Zn sulphides. Some deposits are overlain by ochre, a gossanous Mn-poor Fe-rich bedded deposit composed of goethite, maghemite, quartz, and finely disseminated sulphide. In the Solomon Islands, ochre is overlain by siliceous sinter containing anhydrite, barite, and sulphide; the sinter contains anomalous Ag, Au, Cu, Zn, and Hg, and grades upward into Fe-rich chert and manganiferous wad. Amorphous Fe-Mn deposits (umber) and Mn-bearing chert enriched in Ba, Co, Cu, Ni, Cr, Pb, and Zn are common features near the top of ophiolite sequences. Although their genetic relation to sulphide mineralization is uncertian, they probably formed during off-axis hydrothermal discharge.At modern, medium-rate spreading centres, thin blankets of unconsolidated hydrothermal sediment have been observed near hydrothermal sulphide deposits. Basalt fragments recovered with massive sulphide from the Juan de Fuca Ridge have surfaces coated with smectite, magnetite, hematite, opaline silica, and Fe---Mn-oxyhydroxides. Sediment mounds composed largely of nontronitic clay and hydrated Fe and Mn oxides, and more distal metalliferous (Fe, Mn, Cu, Ni, Pb, Zn) sediment on the flanks of ceanridges, are also products of off-axis hydrothermal processes.Pillow lavas, diabase dikes, and gabbro in ophiolite sequences, and deeper, layer 2 basalt and diabase recovered from oceanic ridges, are altered to greenschist-facies assemblages (albite + chlorite + actinolite ± sphene ± quartz ± pyrite) during high-temperature sub-sea-floor hydro-thermal metamorphism near the axis of spreading. Chemical changes in the wall-rock during this large-scale sea-water/rock interactive episode depend on the water/rock ratio and temperature but generally include gains in Mg, Na and H2O and losses of Ca. Subsequent low temperature sea-water/rock interaction away from the axis of spreading results in fracture-controlled zeolitefacies alteration, characterized by smectite, caledonite, zeolite, calcite, prehnite, hematite, marcasite, and pyrite. This retrograde alteration involves increases in total Fe, K, and H2O and decreases in Mg and Si in the wallrock; Ca may be lost or gained.Wall-rock alteration in Cyprus type stockwork zones is more striking, in that the basalt and diabase between veins of Fe---Cu-Zn sulphides, quartz, and chlorite have undergone partial to complete conversion to fine-grained aggregates of quartz + chlorite + illite + pyrite; kaolinite and palygorskite may be present in minor amounts. Calcium and Na are strongly depleted; K, Al, Ti, Mn, and Ni are leached to a lesser extent; and Fe, S, Cu, Zn, and Co are strongly enriched in the wall-rock underlying massive sulphide. Mafic rocks at depth in the volcanic pile may be enriched in K, Rb, and Li, and depleted in Cu, Co, and Zn. Lavas lateral to and overlying massive sulphide mineralization may have low concentrations of Cu and high concentrations of Zn and Co relative to background levels.Mutual consideration of hydrothermal sulphide deposits and associated wall-rock alteration in ophiolites and at modern oceanic spreading centres can provide useful criteria for the development of regional exploration models for ophiolitic terrains.  相似文献   

20.
Sulphur isotopic compositions of copper and iron sulphides (dispersed and vein mineralization) from the Polish part of the Kupferschiefer were determined and compared with data from the literature. Most of the δ34S values of sulphides range from about -40 to -25%, indicating sulphide precipitation during bacterial sulphate reduction in an open system which gradually chanes into a closed system. Sulphides from veins are usually enriched in 34S compared to finely dispersed mineralization and were probably formed in a more closed system. Copper sulphides are generally a few permil heavier than pyrite. Coupled with detailed microscopical observations the isotope data suggest that the mineralization is either syngenetic or early diagenetic.  相似文献   

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