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1.
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   

2.
The Sullivan Pb-Zn-Ag massive sulphide deposit in southeastern British Columbia occurs within middle Proterozoic argillite, siltstone and quartz wacke of the Purcell Group. Rock samples were collected from the hangingwall and footwall of the eastern section of the mine and from outcrop up to 50 km from the Sullivan deposit. The samples were analyzed for Cu, Pb, Zn, S, Mn, Ba, Fe, K, Ca, Na and specific conductance. A stepwise discriminant analysis applied to the analytical data determined the group of variables that differentiate between hangingwall, footwall and outcrop or “Background” samples. Hangingwall and footwall rock samples were most effectively discriminated from “background” rock samples on the basis of specific conductance with Cu, Pb, S, Na, and Ba selected as less efficient discriminators. The variables that discriminate hangingwall from footwall rock samples are Cu, Zn and S. The selection of the discriminating variables in each case can be explained in terms of the chemical changes that occur as a result of host rock alteration and sulphide deposition during the mineralizing event at the Sullivan deposit.Stepwise discriminant analysis was used to reduce a number of potential pathfinder variables to an optimum group of pathfinder variables. These optimum pathfinders represent the variables that most effectively differentiate the host rocks of the Sullivan deposit from rocks outside of the mineralized zone that apparently do not contain massive sulphide mineralization.  相似文献   

3.
The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints.  相似文献   

4.
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

5.
The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO3. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ18O, medium- to high-salinity, H2O-type fluid is the most important component during siderite deposition, whereas H2O–CO2-type fluid inclusion containing dense liquid CO2 and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (Th=90 to 180 °C) H2O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of 87Sr/86Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite.  相似文献   

6.
Electron paramagnetic resonance (EPR) spectroscopy of hot HNO3 insoluble residues of rock powders is used as a new exploration technique for the volcanic-hosted massive sulphide (VHMS) deposit in the Rosebery mine area. The EPR signal intensities measured in 326.5±5 mT sweeps are strong in the altered rocks, and show a negative correlation with Ca, Na and Sr, and a positive correlation with K/Na, Rb/Sr and (K × Rb)/(Ca × Na × Sr). The EPR intensities measured in 326.5±100 mT sweeps show high values in the footwall pyroclastics, host rocks and hanging wall pyroclastics near and around the Rosebery deposit, and correlate positively with K, Fe, Mn, Ba, F, Rb, Zn, Pb and Zr. The Rosebery deposit and associated footwall alteration zone are located at the intersection of two elongated paramagnetic halos. The first is characterized by strong intensities of [AlO4]° signals measured at magnetic flux density sweeps over 326.5±5 mT, trends NE–SW, and passes discordantly from the west to the east the White Spur Formation, altered footwall (footwall alteration zone), host rock of the Rosebery deposit, hanging wall and Mount Black Volcanics. The second, largely stratabound, halo is defined by strong intensities of Mn2+ sextets observed at magnetic flux density sweeps over 326.5±100 mT, runs N–S following the stratigraphic trend, and outlines the mineralized host rock and footwall alteration zone. It also extends toward the south into the unaltered footwall and hanging wall rocks. The first type of halo is considered to be related to wall rock alteration due to the VHMS mineralization processes as well to later Devonian metamorphism, and the second is thought to be related to massive sulphide mineralization alone.  相似文献   

7.
《International Geology Review》2012,54(10):1239-1262
The Chahgaz Zn–Pb–Cu volcanogenic massive sulphide (VMS) deposit occurs within a metamorphosed bimodal volcano–sedimentary sequence in the south Sanandaj–Sirjan Zone (SSZ) of southern Iran. This deposit is hosted by rhyodacitic volcaniclastics and is underlain and overlain by rhyodacitic flows, volcaniclastics, and pelites. Peperitic textures between rhyodacite flows and contact pelites indicate that emplacement of the rhyodacite occurred prior to the lithification of the pelites. The rhyodacitic flows are calc-alkaline, and show rare earth and trace elements features characteristic of arc magmatism. Zircons extracted from stratigraphic footwall and hanging-wall rhyodacitic flows of the Chahgaz deposit yield concordant U–Pb ages of 175.7 ± 1.7 and 172.9 ± 1.4 Ma, respectively, and a mean age of 174 ± 1.2 Ma. This time period is interpreted to represent the age of mineralization of the Chahgaz deposit. This Middle Jurassic age is suggested as a major time of VMS mineralization within pull-apart basins formed during Neo-Tethyan oblique subduction-related arc volcano-plutonism in the SSZ. Galena mineral separates from the layered massive sulphide have uniform lead isotope ratios of 206Pb/204Pb?=?18.604–18.617, 207Pb/204Pb?=?15.654–15.667, and 208Pb/204Pb?=?38.736–38.769; they show a model age of 200 Ma, consistent with the derivation of Pb from a Late Triassic, homogeneous upper crustal source.  相似文献   

8.
The lower valley of Changjiang, from Wuhan of the Hubei Province in the west to Zhenjiang of the Jiangsu Province in the east, contains more than 200 polymetallic (Cu–Fe–Au, Mo, Zn, Pb, Ag) deposits and is one of the most important metallogenic belts in China. This metallogenic belt, situated at the northern margin of the Yangzi craton and bordered by the Dabieshan ultrahigh pressure metamorphic belt to the north, consists mainly of Cambrian–Triassic marine clastic sedimentary rocks and carbonate and evaporite rocks, which overlay a Precambrian basement and are intruded by Yanshanian (205 to 64 Ma) granitoid intrusions and subvolcanic complexes. Repeated tectonism from Late Proterozoic to Triassic resulted in extensively developed networks of faults and folds involving the Cambrian–Triassic sedimentary strata and the Precambrian basement. The Yanshanian granitoid intrusions and subvolcanic complexes in the Lower Changjiang metallogenic belt are characterized by whole-rock δ18O of +8‰ to +10‰, initial 87Sr/86Sr of 0.704 to 0.708, and εNdt from −10 to −17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials. Also, the Yanshanian granitoids exhibit eastward younging and increase in alkalinity (i.e., from older calc–alkaline in the west to younger subalkaline–alkaline in the east), which are related to oblique collision between the Yangzi and Sino-Korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes. Most polymetallic deposits in the Lower Changjiang metallogenic belt are clustered in seven districts where the Yanshanian magmatism is particularly extensive: from west to east, Edong, Jiurui, Anqing–Guichi, Luzhong, Tongling, Ningwu and Ningzhen. Mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits: orebodies in Yanshanian granitoid intrusions, skarn orebodies at the contact zones between the Yanshanian intrusions and Late Paleozoic–Early Mesozoic sedimentary rocks, and stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata. The most important host sedimentary strata are the Middle Carboniferous Huanglong Formation, Lower Permian and Lower–Middle Triassic carbonate and evaporite rocks. The intrusion-hosted and skarn orebodies exhibit well-developed zonation in alteration assemblages, metal contents, and isotopic compositions within individual deposits, and apparently formed from hydrothermal activities related to the Yanshanian magmatism. The stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano-sedimentary exhalative processes in shallow marine environments. However, extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization. On the basis of geological relationships and isotope geochemical characteristics, we propose a carbonate-hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies. This model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the Yanshanian intrusions with carbonate and evaporite wall rocks. Mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion-hosted orebodies. The stratabound massive sulfide deposits of the Lower Changjiang metallogenic belt share many features with the well-studied, high-temperature, carbonate-hosted replacement deposits of northern Mexico and western United States, particularly with respect to association with small, shallow granitoid complexes, structural and stratigraphic controls on mineralization, alteration assemblages, geometry of orebodies, metal association, metal zonation and isotopic systematics.  相似文献   

9.
The Huize Zn–Pb–(Ag) district, in the Sichuan–Yunnan–Guizhou Zn–Pb–(Ag) metallogenic region, contains significant high-grade, Zn–Pb–(Ag) deposits. The total metal reserve of Zn and Pb exceeds 5 Mt. The district has the following geological characteristics: (1) high ore grade (Zn + Pb ≥ 25 wt.%); (2) enrichment in Ag and a range of other trace elements (Ge, In, Ga, Cd, and Tl), with galena, sphalerite, and pyrite being the major carriers of Ag, Ge, Cd and Tl; (3) ore distribution controlled by both structural and lithological features; (4) simple and limited wall-rock alteration; (5) mineral zonation within the orebodies; and (6) the presence of evaporite layers in the ore-hosting wall rocks of the Early Carboniferous Baizuo Formation and the underlying basement.Fluid-inclusion and isotope geochemical data indicate that the ore fluid has homogenisation temperatures of 165–220 °C, and salinities of 6.6–12 wt.% NaCl equiv., and that the ore-forming fluids and metals were predominantly derived from the Kunyang Group basement rocks and the evaporite-bearing rocks of the cover strata. Ores were deposited along favourable, specific ore-controlling structures. The new laboratory and field studies indicate that the Huize Zn–Pb–(Ag) district is not a carbonate-replacement deposit containing massive sulphides, but rather the deposits can be designated as deformed, carbonate-hosted, MVT-type deposits. Detailed study of the deposits has provided new clues to the localisation of concealed orebodies in the Huize Zn–Pb–(Ag) district and of the potential for similar carbonate-hosted sulphide deposits elsewhere in NE Yunnan Province, as well as the Sichuan–Yunnan–Guizhou Zn–Pb–(Ag) metallogenic region.  相似文献   

10.
Draa Sfar is a polymetallic (Zn–Pb–Cu) volcanogenic massive sulfide deposit with an actual resource of 13 Mt at 4.0% Zn and 1.3% Pb. It is part of the central Jbilets area known for its several Cu–Zn ore deposits. The ore is hosted in the upper Visean-Namurien sedimentary formation. Owing to the complexity of the geology of the ore deposits, numerical simulation approach was attempted to shed light into the temperature distribution, the circulation of the hydrothermal fluid and the genesis of massive sulfide ore bodies by evaluating the permeability, porosity, and thermal conductivity. On the basis of this simulation approach, the ore is predicted to be deposited at a temperature ranging between 230 and 290 °C. This temperature range is dependent on the pre-existing temperature of the discharge area where a metal-rich fluid precipitated the ore. The duration of the Draa Sfar ore body formation is predicted to be 15, 000 to 50, 000 years. Based on geological studies of Draa Sfar deposit together with the aforementioned results of the simulation approach, an ore genetic model for the massive sulfide ore bodies is proposed. In this model, the supply of ore-forming fluids is ensured by the combination of seawater and magmatic waters. Magma that generated rhyodacite dome acted as the heat source that remobilized the circulation of these ore-bearing fluids. The NW-SE trending faults acted as potential pathways for both the downward and upward migration of the ore-forming fluids. Due to their high permeability, the ignimbritic facies, host rocks of Draa Sfar ore bodies, have favored the circulation of the fluids. The mixing between the ore-forming fluids of magmatic origin and the descending seawaters and/or in situ pore waters led to the formation the ore bodies in 35,000 years. The position and size of the ore body, determined by the simulation approach, is consistent with the actual field geological data.  相似文献   

11.
Halogen elements play an important role in the metallogenesis of metallic ore deposits and are involved in the whole process of remobilization, transport and precipitation of metallic elements. However, with the exception of fluorine, which, as a component of fluorite and mica minerals, can be occasionally concentrated in ores, Cl, Br and I are hard to enrich in the ores. Investigations have found that the halogen elements tend to diffuse toward country rocks with the development of hydrothermal alteration in the process of their involvement in metallogenesis, especially during the post-ore stage when extensive halogen diffusion halos over orebodies would be formed. Such halogen element diffusion halos over the Chadong As–Ag–Au deposit extend as widely as 200 m. The largest diffusion extent is for I and the diffusion halos of Br are most noticeable 50–130 m away from the orebodies. In areas of ore exposure and the strongly altered zone, the Cl, Br and I contents are close to those of the strata with a V-shaped distribution pattern in the periphery of the mining district. Comparatively speaking, in going away from the altered zone, the major metallic elements Au and Ag in the deposit tend to decrease suddenly to their normal contents in the strata. This variation feature of halogen elements can be used as geochemical indicators for exploring concealed orebodies at depth. In the Chadong ore deposit, halogen element anomalies can be used to predict concealed orebodies at the depth range of 0–200 m.  相似文献   

12.
海相火山-沉积建造铁铜矿床类型及地质特征   总被引：4，自引：1，他引：4  
于浦生  邬介人 《地球学报》1996,17(Z1):50-56
铁-铜型矿床产出的时代从元古宙到新生代均有，与其有关的火山岩大多数为中基性与中酸性或偏碱性岩石。作者以镜的山桦树沟、陇山陈家庙和陕西铜厂不同时代的铁-铜矿床为例，概述了该类型矿床的地质特征、成矿环境并着重探了铁-铜矿床的成因机制，认为该类型矿床是与火山作用有关的喷气-沉积型矿床，同时指出柳沟峡地区及其以西（东缰地区）铁-铜型铜矿化带的发现，是进一步寻找铁-铜-金矿床的有利地段。  相似文献   

13.
The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded 13CVPDB values of –11 to +1 and 18OVSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. 34SVCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D1 deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz  相似文献   

14.
The metaturbidites of the Palaeoproterozoic Jormua–Outokumpu thrust belt in eastern Finland enclose m- to km-scale ultramafic massifs that are distributed over an area of more than 5000 km2. These bodies, which almost entirely consist of highly depleted mantle peridotites (now metaserpentinites and metaperidotites), are intimately associated with massive to semimassive, polymetallic Cu–Co–Zn–Ni–Ag–Au sulphide deposits that sustained mining in the region between 1913 and 1988. Currently, one deposit (Kylylahti) is proceeding into a definitive feasibility study emphasising the renewed economic interest for Outokumpu-type deposits.The origin of these Outokumpu-type Cu–Co–Zn–Ni–Ag–Au deposits is now re-interpreted to be polygenetic. First, their formation requires deposition of a Cu-rich proto-ore within peridotitic sea floor at  1950 Ma. Close modern analogues to the proto-ore setting include, for example, the Logatchev and Rainbow fields at the Mid-Atlantic Ridge, where venting of high-T–low-pH hydrothermal fluid resulted in accumulations of Cu–Zn–Co–Ag–Au sulphides on serpentinised ultramafic seafloor. Second, the Ni-rich composition of Outokumpu sulphide ores calls for a separate source for nickel: Some 40 Ma after the deposition of the Cu-rich proto-ore – concomitant with the obduction of the ultramafic massifs – disseminated Ni sulphides formed through chemical interaction between obducting peridotite massifs and adjacent black schists. This process was related to listwaenite–birbirite type carbonate–silica alteration at margins of the ultramafic massifs. Due to this alteration, silicate nickel was released from the primary Fe–Mg silicates and redeposited as Ni sulphides in the alteration fringes of the massifs.We propose that syntectonic mixing of these two “end-member” sulphides, i.e., the primary Cu-rich proto-ore and the secondary Ni-sulphide disseminations, resulted in the uncommon metal combination of the Outokumpu-type sulphides. Late tectonic solid-state re-mobilisation, related to the duplexing of the ore by isoclinal folding, upgraded the sulphides into economic deposits.  相似文献   

15.
The volcanic-hosted massive sulphide (VHMS) deposits in the Eastern Desert of Egypt (e.g., Um Saki deposit) are associated with Precambrian coarse acid pyroclastic rocks. The upper contacts of the massive sulphide body are sharp and well-defined; while the keel zone to the mineralization is always associated with pervasive alteration, characterized by the presence of septechlorite and talc, associated with variable amounts of carbonate and tremolite. On the other hand, the economic talc deposits in Egypt are hosted intensively altered volcanic rocks. Besides talc, chlorite, carbonates and tremolite that occur in variable amounts in these deposits, anomalously high concentrations of gold are also present.The present study showed that alterations in the talc deposits of Darhib, El Atshan, Abu Gurdi, Egat, Um Selimat and Nikhira are similar to those occurring in the keel zone underlying the VHMS of Um Samuki and that the chemical modifications due to alteration processes (additions of Mg, Fe, Mn and Ca coupled with depletions in silica, alkalies, alumina and titanium) are comparable, even the host rocks are different, thus reflecting a genetic relationship. It is suggested that, the examined localities of talc deposits are hosted in the intensively altered volcanics in the keel zones of volcanogenic massive sulphide deposits. Recently, detailed geophysical prospecting program, including electric (resistivity, self-potential and induced polarization), electromagnetic and magnetic methods, was carried out at Darhib, Abu Gurdi and Um Selimat talc deposits. The quantitative interpretation of these geophysical measurements revealed the presence of subsurface bodies of sulphides. The present distribution of talc and allied minerals in Darhib, El Atshan, Abu Gurdi, Egat, Um Selimat and Nikhira could be explained by a tectonic process in which the coarse acid pyroclastic rocks with massive sulphides have tilted in such way that the footwall rock alterations (talc and allied minerals) are exposed on the present-day surface at these localities. Structural studies are currently under way in an attempt to explain the deformation regime that led to the present situation of talc deposits.Two distinct spatial and mineralogical associations of gold mineralization could be identified in the volcanogenic massive sulphide deposits and their footwall alterations (the keel zone) in the Eastern Desert of Egypt. These are (1) gold–silver–zinc association, and (2) gold–copper association. In the former, gold grades are very low and silver is anomalous. This association occurs typically in the upper levels of the VHMS deposit where low-temperature sulphides are abundant. Gold was deposited because of the mixing between the ascending hot solutions and the sulphate-rich seawater. The upper levels of Um Samuki sulphide body represent this association. Gold–copper association, on the other hand, typically occurs in the footwall altered rocks (the keel zone) and the lowest parts of the massive sulphide body. Gold grades reach up to 5.54 ppm, but the average is 1 ppm. Silver is very low, usually in the range of 4–10 ppm. Lead usually, but not always, accompanies gold in this association. Deposition of gold probably took place due to decreasing of temperature and/or increasing pH of the ascending hot brines. The keel zones at Darhib, Abu Gurdi, El Atshan, Um Selimat, Nikhira and Egat talc mines better represent this association.  相似文献   

16.
CSA mine exploits a ‘Cobar-type’ Cu–Pb–Zn±Au±Ag deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (400 m) and narrow width (100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway.Fluid inclusions were examined from quartz±sulphide veins adjacent to now-extracted ore, from coexisting quartz–sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D2–D3 ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D2–D3 ore and vein emplacement. Post-decrepitation outflow (late D3 to early post-D3) is recorded by monophase CH4 inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH4 system; modification is recorded by H2O+CH4 and H2O+(trace CH4) secondary populations and by an H2O+(trace CH4) primary population. The contractional tectonics (D2–D3) of ore emplacement was superseded by relaxational tectonics (D4P) that facilitated meteoric water penetration and return flow.Under D2 prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Tt300–330 °C and Pt1.5–2 kbar≈Plith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (T350–380 °C) in mid- to late-D3, while in late-D3 to early post-D3, essentially monophase CH4 secondary inclusions were entrapped at Tt350 °C and Pt=1.5–2 kbar≈Plith. Subsequently, abundant CH4 and entrained meteoric water were entrapped as H2O+CH4 secondaries under slowly decreasing temperature (Tt330–350 °C) and constant pressure (Pt1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H2O+(trace CH4) populations record decreasing temperatures (Tt>300 to <350 down to 275–300 °C) at pressures of Phydrostatic<Pt (1 kbar) <Plith (1.5 kbar).The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit's evolution. They indirectly support the role of basin-derived CH4 fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from 395 to 385 Ma.  相似文献   

17.
The main Woodlawn ore lens is a polymetallic, massive sulphide deposit’ with pyrite the major constituent, variable sphalerite, galena and chalcopyrite, and minor arsenopyrite, tetrahedrite‐tennantite, pyrrhotite and electrum. The silicate gangue minerals are chlorite, quartz, talc and sericitic mica. Other mineralization in the vicinity consists of footwall copper ore in chlorite schist and several smaller massive sulphide lenses. The predominant country rocks are felsic volcanics and shales, with abundant quartz, chlorite and mica, and talc in mineralized zones.

An important textural feature of the massive ore is the fine compositional banding. Bands, which vary in thickness from a few tens of micrometres to several millimetres, are produced by variations in the sulphide content. Post‐depositional metomorphism and minor fracturing have only slightly modified this banding.

Apart from the major element constituents—Pb, Zn, Fe, Cu and S—the ore is characterized by significant (100–1000 ppm) values for Ag, As, Cd, Mn, Sb and Sn, and lower (1–100 ppm) values of Au, Bi, Co, Ga, Hg, Mo, Ni, Tl. In and Ge. Variations in the base‐metal sulphide content, the gangue mineralogy, and trace elements, are used to separate the orebody into hanging‐wall and footwall zones. The hanging‐wall zone shows a more variable trace element content, with higher Tl, Sn, Ni, Mn, Ge and Sb, but lower Ag, Cd, and Mo, than the footwall zone.

In general style of mineralization, mineralogy, and chemistry, the Woodlawn deposit resembles other volcanogenic massive sulphide deposits in eastern Australia, in New Brunswick in Canada, and the Kuroko deposits of Japan.  相似文献   

18.
Eight Zn–Pb–Cu massive sulphide deposits that appear to have formed on the sea floor (seven in Spain, one in Tasmania) are believed to have been precipitated in brine pools, based on the salinities and temperatures of fluid inclusions in underlying stockworks. Comparing the geological features of these deposits with those of the Zn–Pb–Cu massive sulphide ores of the Hokuroku Basin, Japan, which have formed as mounds from buoyant fluids of low salinity, shows that brine pool deposits have: (1) potentially very large size and tonnage, and high aspect ratio, (2) higher Zn/Cu and Fe/Cu values, (3) no evidence of chimneys, (4) relatively abundant framboidal pyrite and primary mineral banding, (5) reduced mineral assemblages (pyrite-arsenopyrite/pyrrhotite), and minor or rare barite in the massive sulphide, (6) associated stratiform and/or vein carbonates, (7) relatively unimportant zone refining, (8) lack of vertical variation in sphalerite and sulphur isotopic compositions, and (9) evidence of local bacterial sulphate reduction. Application of these criteria to the Rosebery deposit in Tasmania, for which there are no fluid inclusion data, leads to the conclusion that the southern section was deposited as separate lenses in a brine-filled basin or basins. Other potential candidates include Brunswick no. 12 and Heath Steele (Canada), Woodlawn and Captains Flat (New South Wales), Hercules and Que River (Tasmania), and Tharsis and the orebodies at Aljustrel (Spain and Portugal). Recently published fluid inclusion data for Gacun (China) and Mount Chalmers (Queensland) suggest that not all ores deposited from highly saline fluids have reduced mineral assemblages.  相似文献   

19.
The Xiangxi Au–Sb–W deposit, the largest of its type in northwestern Hunan, China, is a sulfide-dominated ore body hosted by low grade metamorphic red slates of the Neoproterozoic Madiyi Formation. Three stages of mineralization, quartz–scheelite, quartz–gold–pyrite, quartz–gold–stibnite, and one metal-barren stage of veining, quartz–calcite, are recognized. Arsenopyrite occurs only as a minor mineral phase in the second stage. Analyses for 21 trace elements show that the enrichment factors of As in the metal deposit (EC [=element concentration of sample/average content of an element in the upper crust]: 190; 43 samples) in ore veins and in the Guanzhuang and Yuershan reference sections (3.7 km and 2.7 km away from the Xiangxi mine, EC: 3.5; 96 samples) are much smaller than those of Sb (52855 [in ore veins], 117 [in the sections]), W (5665, 7.5) and Au (2727, 5.3). The background concentrations of Au and As in the two sections were 1.4 ppb and 1.4 ppm, respectively. Arsenic (with an anomaly coefficient [AC = number of anomalous samples/total number of samples] of 76%) forms a larger geochemical halo than W (AC: 8%) and Au (AC: 32%). Gold and As in the deposit were transported mainly as metal complexes such as Au(HS)−2, HnAs3S−(3−n)6 (n=1, 2 or 3) and HAsS02. Au(HS)−2 is rapidly precipitated by a geochemical oxidation barrier — the red slates of the Madiyi Formation. As–S complexes in the stratigraphic horizon can be transformed into As–O complexes (e.g., H3AsO03) under oxidizing conditions, and are continuously transported. Therefore, they can be widely distributed in the red slate units, thus forming extensive geochemical haloes, so that As can be used as an indicator element for Au exploration in the Xiangxi region.  相似文献   

20.
上世纪90年代中期通过成矿预测发现的水银洞金矿床,通过近10年的勘查,成为特大型层控金矿床。矿体呈层状、似层状产出于灰家堡背斜轴部附近300m范围内二叠统龙潭组的大孔隙度生物碎屑灰岩中,形态与背斜形态一致。矿体严格受控于碳酸盐岩,顶板和底板皆为粘土岩或粉砂质粘土岩,界线清楚。矿化表现为“硅化、白云石化、黄铁矿化”组合。硅化与白云石化是成矿的先决条件,碳酸盐岩最终能否成矿及品位的高低,取决于是否具有黄铁矿化热液蚀变及其强度。矿床具矿体多、厚度薄、品位富的特点。单矿体即达中型矿床规模。具中—低温超高压热液成矿的特点。金赋存于沿自形黄铁矿内核生长成的含砷黄铁矿环带中。  相似文献   

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