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1.
The Kristineberg massive sulfide deposit is hosted by metamorphosed volcanic and subvolcanic rocks of the Palaeoproterozoic Skellefte Group. The deposit consists of: (1) two main massive sulfide horizons, the A-ores and B-ores, which dip steeply southwards and are separated by 100–150 m; and (2) the Einarsson Zone, a complex interval of Cu–Au-rich ‘stockwork‘ sulfides and small massive sulfide lenses in altered and deformed rocks near the 1,000 m level. The Einarsson Zone occurs some 20–100 m south of the B-ores. There are no definite younging indicators in the mine sequence. In many areas of the mine, the original host rocks are impossible to identify petrographically due to the abundance of secondary minerals such as quartz, chlorite, muscovite, cordierite, andalusite, phlogopite, pyrite and talc, combined with variably schistose fabrics. Application of immobile-element methods to 600 recent whole-rock chemical analyses has, however, allowed the original rock types to be identified and correlated. Rhyolite X lies immediately north of the A-ore, while andesitic to dacitic to rhyodacitic rocks make up the 100–150 m interval between the A-ore and B-ore, and massive rhyolite A lies immediately south of the B-ore. The felsic rocks are mostly of calc-alkaline affinity, excluding rhyolite X, which is transitional. The mine porphyry, which lies north of the A-ore and forms the marginal phase of the synvolcanic Viterliden Intrusive Complex, is compositionally similar to dacite and rhyodacite. Mass changes calculated for all rock types indicate that most of the volcanic rocks in the mine area are strongly depleted in Na and Ca, and have gained variable amounts of Mg and Fe, whereas Si changes range from negative to positive. Gains in Fe and changes in Si are largest within 5–10 m of the massive sulfide lenses. Cordierite-bearing schists of andesitic to felsic compositions that lie between massive sulfide lenses A and B are not as altered. The Einarsson Zone commonly shows large gains in Fe and Mg, while Si shows large gains to large losses. Immobile-element ratios indicate that very different secondary assemblages in the mine, e.g. andalusite–quartz–muscovite and cordierite–chlorite–talc, can be produced from the same precursor volcanic unit, e.g., rhyolite. Conversely, the same secondary mineral assemblage can be produced from different rocks, e.g. weakly altered andesite and strongly altered rhyolite. The common presence of cordierite + andalusite in the mine area, without anthophyllite, is unusual in the alteration systems of volcanic-hosted massive sulfide deposits, and is proposed to have formed by the metamorphic reaction of the synvolcanic alteration minerals kaolinite and chlorite to produce cordierite. Where kaolinite was in excess of chlorite, andalusite was also formed. We propose that highly acidic alteration fluids locally produced high-Al minerals such as kaolinite that either overprinted, or occurred in place of, a more typical sericite–chlorite–quartz alteration assemblage that otherwise formed near the massive sulfide lenses. Application of lithogeochemical methods to the altered, deformed and metamorphosed Kristineberg rocks has identified specific volcanic contacts with massive sulfide potential, and quantified the effects of synvolcanic hydrothermal alteration. Such an approach can increase the effectiveness of mineral exploration in metamorphosed terrains.  相似文献   

2.
The Paleoproterozoic Ruttan Cu–Zn volcanogenic massive-sulfide (VMS) deposit is a large, relatively low grade, bimodal-siliciclastic type deposit in the Rusty Lake volcanic belt of northern Manitoba. The deposit contained over 82.8 million tonnes of massive sulfide, of which 55.7 million tonnes were mined from 1973 to 2002. The deposit consists of a series of moderately to steeply dipping, south-facing lenses that extend along strike at the surface for 1.1 km and to a depth of 1.0 km. These lenses occur within a steeply dipping, bimodal volcanic, volcaniclastic and siliciclastic sequence. In the immediate mine area, transitional calc-alkalic to high-silica (tholeiitic), felsic, and intermediate volcanic/volcaniclastic rocks of the Mine Sequence are host to, and intercalated with, the massive-sulfide lenses. Transitional tholeiitic to calc-alkalic basalt and andesite are present in the footwall sequence, approximately 500 m down-section from the ore horizon. The overlying rocks are predominantly fine-grained volcaniclastics and siliciclastics, but include polyfragmental agglomerate that contains mafic bombs and scoriaceous felsic fragments. Syn-depositional felsic and mafic dikes, sills, and apophyses are ubiquitous throughout the Mine Sequence, including the ore lenses, indicating continued, near-vent magmatism, and volcanism during ore formation. Fabrics in altered hostrocks have consistent, down-plunge stretching lineations to the SSE that suggest the deposit has been elongated by a factor of ~1.2–1.5; otherwise, the deposit is remarkably undeformed. Syn- and post-depositional faults in the mine area have relatively minor displacements up to tens of meters. Proximal (within 200 m) footwall rocks exhibit moderate to strong chloritization, characterized by the upper greenschist to lower amphibolite facies assemblages that include cordierite–almandine–andalusite–sillimanite–biotite ± staurolite ± anthophyllite ± talc, and local silicification. The proximal hanging wall rocks are characterized by sericite ± gahnite alteration, which is restricted to within approximately 75 m of the uppermost lenses. Additional gangue minerals are anhydrite and carbonate minerals (siderite, dolomite, ankerite, and calcite), as well as chlorite, sericite, biotite, talc, and quartz. Carbonate (excluding siderite), potassium feldspar, silicification and epidotization are common distal alteration zones in the footwall to the Mine Sequence several kilometers to the northeast. There are three principal groups of massive sulfide lenses; the East lenses, the West lenses, and the Western Anomaly lenses to the far west. In general, Cu is relatively enriched at the stratigraphic base and in the center of the deposit, whereas Zn is enriched upsection and at the outer margins. Some of the Zn-rich ore exhibits primary mineralogical layering. Parts of the West and Western Anomaly lenses show two layers with Cu-rich bases and Zn-rich tops. The massive sulfide is typically 10–40-m thick; one area along the margin of the main lenses is over 130-m thick and may represent deposition adjacent to a syn-depositional fault. The main sulfide phases are pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena, with tetrahedrite as the most abundant trace phase. Gahnite is ubiquitous in the chlorite-rich assemblages adjacent to the ore lenses. The average base, precious and trace metal contents estimated from Cu and Zn concentrates, and from millhead grades and recoveries. Metals easily transported as chloride and bisulfide complexes in hydrothermal fluids including: Pb, Ag, In, Cu, Cd, Au, and Zn are enriched by 1.5–2.5 orders of magnitude in comparison to the bulk continental crust. Other elements such as Sn, Mo, and As are at near-crustal concentrations, whereas Mn, Ga, and Co are significantly depleted in comparison to the crust. Calculated metal concentrations in the average hydrothermal fluid based on the average metal contents are comparable to, or higher than those measured at sediment covered ridge hydrothermal systems, which precipitate much of their metal budget in the subsurface. Average rare earth element contents for the sulfide are light rare earth element enriched (LaN/YbN=22) and range from 0.45 to 0.02x chondritic values, with a moderate negative Eu anomaly (Eu*=0.51). Metal and trace element contents in the Ruttan exhalite horizon, and in proximal (within 1–2 km) exhalites along strike from the 0.6 million tonne Dar-2 Cu–Zn deposit 12 km south of Ruttan, have positive Eu anomalies, whereas negative Eu anomalies are present at distance. The positive Eu anomalies reflect high temperature paleoseafloor hydrothermal venting and precipitation of Eu2+-enriched clays and possibly carbonates, and indicate proximity to base-metal deposits. Silver and lead are also enriched in the exhalites near the deposits, whereas Mn is enriched at ~1–3 km along strike, but not consistently. Editorial handling: B. Gemmel An erratum to this article is available at .  相似文献   

3.
The Boliden deposit (8.3 Mt at 15.9 g/t Au) is interpreted to have been formed between ca. 1894 and 1891 Ma, based on two new U–Pb ID-TIMS ages: a maximum age of 1893.9?+?2.0/?1.9 Ma obtained from an altered quartz and feldspar porphyritic rhyolite in the deposit footwall in the volcanic Skellefte group and a minimum age of 1890.8?±?1 Ma obtained from a felsic mass-flow deposit in the lowermost part of the volcano-sedimentary Vargfors group, which forms the stratigraphic hanging wall to the deposit. These ages are in agreement with the alteration and mineralization being formed at or near the sea floor in the volcanogenic massive sulfide environment. These two ages and the geologic relationships imply that: (1) volcanism and hydrothermal activity in the Skellefte group were initiated earlier than 1.89 Ga which was previously considered to be the onset of volcanism in the Skellefte group; (2) the volcano-sedimentary succession of the Vargfors group is perhaps as old as 1892 Ma in the eastern part of the Skellefte district; and (3) an early (synvolcanic) deformation event in the Skellefte group is evidenced by the unconformity between the ≤1893.9?+?2.0/?1.9 Ma Skellefte group upper volcanic rocks and the ≤1890.8?±?1 Ma Vargfors sedimentary and volcanic rocks in the Boliden domain. Differential block tilting, uplift, and subsidence controlled by synvolcanic faults in an extensional environment is likely, perhaps explaining some hybrid VMS-epithermal characteristics shown by the VMS deposits of the district.  相似文献   

4.
Mineralium Deposita - The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The...  相似文献   

5.
The Aznalcóllar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65?ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30?Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcóllar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcóllar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (VA1) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the VA1 cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (VA1) to hydroclastic subvolcanic conditions for the VA2. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F1), and development of folds that evolved to thrusts by short flank lamination. These thrusts caused tectonic repetition of massive and stockwork orebodies. In Aznalcóllar, some of the stockwork mineralization overthrusts massive sulphides. These structures are cut by large brittle overthrusts and by late wrench faults. The original geometric features of massive sulphide deposits correspond to large blankets with very variable thicknesses (10 to 100?m), systematically associated with stockworks. Footwall rock alteration exhibits a zonation, with an inner chloritic zone and a peripheral sericitic zone. Silicification, sulphidization and carbonatization processes also occur. Hydrothermal alteration is considered a multi-stage process, geochemically characterized by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilized in the inner chloritic zones. Three ore types occur, both in stockworks and massive sulphides, named pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Zoning of sulphide masses roughly sketches a typical VHMS pattern, but many alternating polymetallic and barren pyritic zones are probably related to tectonics. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300?°C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350?°C). Fluid inclusion data suggest that hydrothermal fluids changed continuously in temperature and salinity, both in time and space. Highest Th and salinities correspond to inner stockworks zones and later fluids. Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200?°C), intermediate (200–300?°C) and high temperatures (300–400?°C). 34S values in massive sulphides are lower than in stockwork mineralization suggesting a moderate bacterial activity, favoured by the euxinoid environment prevailing during black shale deposition. The intimate relation between massive sulphides and black shales points to an origin of massive sulphides by precipitation and replacement within black shale sediments. These would have acted both as physical and chemical barriers during sulphide deposition. Hydrothermal activity started during black shale deposition, triggered by a rise in thermal gradient due to the ascent of basic magmas. We suggest a three-stage genetic model: (1) low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales; locally, channelized high-T fluid flow occurs; (2) hydrothermal cyclic activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and (3) a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralization, mainly in the stockwork zone.  相似文献   

6.
Wilga is one of several Au-bearing volcanogenic massive sulphide deposits localized within Silurian felsic volcanics of the Lachlan Fold Belt of NE Victoria. Exploration has delineated a resource of 3.7 Mt averaging 3.4% Cu, 5.5% Zn, 0.4% Pb, 31 g/t Ag and 0.5 g/t Au within a lensoid-shaped sulphide body which strikes NE, dips NW and occurs at a depth of 50–160 m below surface. This body has maximum dimensions of 470 m strike, 350 m down-dip and 37 m true width delineated by 59 surface diamond drill holes and an exploration adit with 40 underground diamond drill holes. There are two main types of mineralization: Stratiform massive sulphides , (mainly pyrite, low-Fe sphalerite, variable chalcopyrite and minor galena with very minor silica/dolomite gangue) and Stratiform chloritic sulphides (mainly chalcopyrite, with subordinate pyrite and sphalerite, in a gangue of chlorite and minor dolomite). The deposit is layered with at least 6 distinct stratigraphic horizons/cycles of mineralization each characterized by a Cu-rich base and Pb-(Zn)-Au-As-rich top. Facies variations are locally rapid. Economically the deposit consists of 3 sub-lenses of roughly equal tonnage: a Cu-rich central lens of Chloritic Sulphides (av. 5.9% Cu, 3.6% Zn, 0.3% Pb, 32 g/t Ag and 0.2 g/t Au) between upper and lower lenses of Zn-rich Massive Sulphides (av. 1.8% Cu, 6.7% Zn, 0.5% Pb, 31 g/t Ag and 0.7 g/t Au). Gold mineralization, locally up to 7 g/t over 6 m, shows a strong spatial association with Pb, (Zn) and As in pyritic massive sulphides.  相似文献   

7.
The Um Samiuki Zn–Cu–Pb–Ag mineralisation, south Eastern Desert, Egypt is hosted by felsic volcanic rocks which form part of the 712-Ma-old, east-west-trending Shadli Volcanic Belt. Two major occurrences of massive sulphides are present at the top of rhyolitic breccia in the Western and Eastern mine areas. In each occurrence, a bornite-bearing zone is overlain by a pyrite-chalcopyrite-bearing zone and underlain by a disseminated, Cu-depleted zone. In the massive sulphide ore, sphalerite, chalcopyrite, pyrite, galena, bornite and tetrahedrite–tennantite are major minerals, whereas arsenopyrite, pyrrhotite, molybdenite and magnetite are accessory phases. Covellite and digenite are common secondary minerals. Bornite, tetrahedrite–tennantite and covellite contain high amounts of silver (averages of 1.97, 1.39 and 1.82 wt% respectively). Based on mineralogical balance calculations, bornite and covellite accommodate 80% of silver in the Um Samiuki deposit. Ag was incorporated in the crystal structure of the early-crystallised copper sulphides and sulphosalts and silver minerals. The temperature, sequential precipitation of the fluids and the structure of the crystallising phases control the distribution of silver. Post-depositional deformation and metamorphic processes caused liberation, remobilisation and redeposition of silver within the massive sulphides.Editorial handling: D. Lentz  相似文献   

8.
Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional DI deformation event (corresponding to the local DP1 to DP3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F3 folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.  相似文献   

9.
The Mobrun Zn-Cu-Ag-Au deposit in the Noranda mining camp is hosted by Archean mafic and felsic submarine volcanic rocks. The deposit comprises three massive sulfide complexes: the Main and Satellite Lenses near surface, and the 1100 orebody at depth. The rocks have been subjected to lower greenschist-facies metamorphism and related deformation, which resulted in changes in ore textures, development of shear zones and veins systems, remobilization of gold, and formation of a new mineral (electrum) within the orebodies. Both mechanical and chemical processes operated to produce secondary textures and structures resulting from brittle deformation, ductile deformation, and annealing. The specific deformation mechanisms include brittle failure and cataclastic flow, dislocation glide, dislocation creep and solution-precipitation creep. The Main and Satellite Lenses are characterized by excellent preservation of primary sulfides deposited from and reworked by synvolcanic hydrothermal fluids. These orebodies were affected to a limited degree by mechanical processes of deformation. In contrast, the 1100 orebody is characterized by a higher degree of development of textures and structures related to metamorphism and deformation, especially those formed by chemical processes. The differences may be due to the greater depth of the 1100 orebody relative to the other lenses, as regional metamorphic isograds are subhorizontal, and more extensive interaction between metamorphic fluids and the 1100 Lens.  相似文献   

10.
A multi-element geochemical study of the wall rocks of intermediate to felsic volcanic-hosted massive sulfide deposits was carried out to identify pathfinder elements which significantly enlarge the size of exploration targets. Drill core samples from the Crandon massive sulfide deposit in Wisconsin, and outcrop samples from the United Verde and Iron King deposits in Arizona, and from the Captains Flat, Mt. Costigan, and Wiseman Creek deposits in New South Wales, Australia were analyzed. Because anomalously high fluorine values have been described in several volcanic-hosted ore systems, fluorine was included in the study.All of the above deposits have patterns of fluorine enrichment around ore. Drill core samples from two noneconomic prospects within ten miles of the Crandon deposit contain background to only weakly anomalous fluorine values.At the large Crandon deposit (> 50 million tons of zinc, copper ore), fluorine enrichment extends approximately 320 m into the footwall rocks and at least 220 m into the hanging wall rocks. At the large United Verde deposit (> 50 million tons of copper, zinc ore), fluorine enrichment is recognizable in the footwall rocks at least 650 m from the ore. At the smaller Iron King deposit (five million tons production of zinc, lead, copper ore), fluorine enrichment extends for a distance of approximately 60 m into the footwall rocks. At the small deposits in New South Wales (< five million tons production of zinc, lead, copper ore), fluorine enrichment is easily recognizable, but with the samples collected, the limits of the anomalous patterns cannot be defined.Fluorine occurs in some hydrothermal systems unassociated with mineralization and is therefore not a specific signature of ore-forming processes. From the work completed, many massive sulfide deposits in volcanic rocks occur in hydrothermal systems which contain fluorine. On the basis of the data presented, if anomalously high fluorine values do exist in an exploration search area, the chances of finding a massive sulfide ore deposit are improved.Genetic models for volcanic-hosted massive sulfide ore deposits have concentrated on rock textures, alteration mineralogy, and geochemistry of the ore metals. From the data presented, fluorine should be considered as a component of massive sulfide systems in intermediate to felsic volcanic rocks, and should be considered as a possible complexing agent for the ore metals.  相似文献   

11.
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.  相似文献   

12.
Ore lead isotope ratios have been analysed in galenas and sulphosalts from nineteen massive sulphide deposits hosted by Svecofennian (1.9 Ga) supracrustals in the Skellefte ore district, northern Sweden. The ore lead data can be grouped on the basis of their geographical distribution. Most probably, this feature is reflecting a number of lead isotopic provinces which correspond to crustal blocks. The obtained ore lead data define linear trends in conventional Pb-Pb plots. The ore lead signatures are due to synvolcanic mixing processes as lead was leached from mafic and acid volcanic sources. The relationships inferred for initial ratios in source rocks at 1.89 Ga require a pre-Svecofennian crustal history. It is suggested that magmatic processes at c. 2.0 Ga involved recycling of Archean sedimentary material into the mantle and the formation of a crystalline, felsic basement. The metasomatized mantle and the basement melted at the time of Svecofennian magmatism (1.89 Ga) which created mafic and acid magma sources. Subsequently, basalts and rhyolites were extruded onto the sea floor. Ore was formed as hot solutions penetrated isotopically different levels of the volcanic pile.  相似文献   

13.
Cu-sulphide ores at Carolusberg and East Okiep have Cu/Ni ratios of up to 80, an order of magnitude higher than most magmatic sulphide ores elsewhere. In contrast, Se/S ratios (500–1700 × 10−6) and PGE tenors (up to 5 ppm) of the sulphides are in the range of more typical magmatic sulphide ores. The observed metal patterns may be explained by a process of monosulphide solid solution (mss) fractionation of a magmatic sulphide melt at depth, but this model is currently considered unlikely, due to the paucity of refractory ores in the district. Assimilation of Cu-rich country rocks during ascent of the Koperberg magmas proved difficult to test with the available data, but this provides no explanation for the common high-grade metamorphic setting of similar ores elsewhere. A restitic origin of the pyroxenites appears to explain many of the observed ore features and is presently favoured here. Desulphidization of a primary magmatic sulphide ore could not have yielded the observed metal patterns and is therefore considered to be of relatively minor importance in ore genesis. Received: 12 April 1999 / Accepted: 27 November 1999  相似文献   

14.
The Heath Steele massive sulphide deposit in northern New Brunswick lies conformably within a sedimentary-volcanic sequence of probable Ordovician age which has been metamorphosed to the greenschist stage. The dominant sulphide mineral is pyrite, and the main economic minerals are sphalerite, galena, and chalcopyrite; the general grade of the ore is 5% Zn, 2% Pb, and 1% Cu.The distribution of Pb and Zn in acid volcanic rocks stratigraphically above the massive sulphides is compared with the distribution in similar rocks stratigraphically below the sulphides. Whereas there are discernable differences in the populations, there is also considerable overlap between them. To enable individual samples to be classified, linear discriminant functions were calculated for the two groups; Pb and Zn were found to be the most useful variables to separate the two populations. The functions were then tested on hanging wall and footwall samples not used in computing the functions. A halo region, extending about 1,200 ft above the sulphides and 4,000 ft along the same stratigraphic horizon as the sulphides was outlined by samples classified as “hanging wall”. Beyond the halo zone there is no significant difference in the distribution of Pb and Zn between the hanging wall and footwall acid volcanic rocks.The results demonstrate that rocks at Heath Steele, which show no evidence of mineralogical alteration attributable to mineralization, have a trace element halo of considerable extent spatially associated with the sulphides. If similar halos can be shown to be a general feature of massive sulphide deposits, the technique described should have wide application for exploration for deeply buried deposits of this type.  相似文献   

15.
The Canatuan and Malusok massive sulfide deposits are located near Siocon, Zamboanga del Norte, in southwestern Mindanao, Philippines. The Canatuan–Malusok area is underlain by the Jurassic–Cretaceous Tungauan schists, which form much of the Zamboanga Peninsula. The volcanic strata at Canatuan and Malusok can be traced for >7 km along strike and is host to at least three discrete massive sulfide bodies: Canatuan, Malusok and SE Malusok. Basal basaltic andesite volcanic rocks are generally chemically uniform and show only moderate alteration. The massive sulfide deposits occur in overlying rhyolitic to rhyodacitic volcanic rocks that are altered to a schistose assemblage of quartz, sericite, chlorite and pyrite. The alteration is texturally destructive but graded clastic beds are locally observed. Despite tropical saprolitic weathering, four lithogeochemical subunits of the felsic package are identified. Stratigraphic interleaving, however, has made correlation of these units over any significant distance difficult. The sulfide lenses are overlain by a few metres of felsic schists which locally contain manganese-bearing silicates and oxides that serve as a stratigraphic marker. Hangingwall andesitic volcaniclastic rocks are discontinuously preserved, although where present, they consist of regularly bedded mafic volcanic sandstones. The lateral continuity of a manganese-bearing marker and flanking felsic volcaniclastic intervals indicate that locally the volcanic strata form a homoclinal sequence. The Canatuan Au–Ag–Cu–Zn deposit consists of a gossan overlying a massive sulfide lens. The sulfides and gossan are flat lying and hosted within felsic volcanic rocks. The gossan is gold–silver-rich, and was formed by a combination of oxidation and volume collapse of the original sulfide lens. The sulfide minerals present below the current water table, are auriferous massive pyrite with base metal sulfides, with some supergene chalcocite. The transition from gossan to sulfides is very sharp, occurring at the water table. Massive sulfide deposits at Malusok are hosted in the same felsic sequence as Canatuan and they have similar base and precious metal contents. Only limited gossan has been found at Malusok. The bimodal nature of the volcanic rocks at Canatuan, together with their low HFSE contents, near-flat REE patterns and tholeiitic affinities, suggest that they formed in an intra-oceanic arc setting above a depleted mantle source. Mafic and felsic volcanic rocks of similar composition have been recovered from the Tonga-Kermadec and Izu-Bonin-Marianas island-arc systems in the western Pacific. Mafic rocks at Canatuan show no evidence for LILE enrichment that characterizes melts derived from metasomatized mantle under more mature arcs, suggesting that they are the product of a nascent, rather than a mature arc. There is no evidence from the REE, or other incompatible trace elements, that continental crust or evolved arc crust was involved in the generation of the Canatuan-Malusok volcanic rocks. Although it has been proposed that the Zamboanga metamorphic complex comprises microcontinental fragments of Eurasian affinity, our data do not support an evolved crustal setting for the Canatuan-Malusok volcanic rocks, which we suggest were derived from an intra-oceanic arc and subsequently accreted to the eastern Mindanao terrane.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00126-003-0350-7Editorial handling: R.R. Large  相似文献   

16.
The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like (chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved. The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite (MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith. Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides, which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO. The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel.  相似文献   

17.
Diopside-rich, skarn-hosted, copper–gold ore derived primarily from carbonaceous metapelites at Mount Elliott forms a distinctive member of the spectrum of Cu–Au–(Fe oxide) deposit styles in the Cloncurry district of the Paleoproterozoic to Mesoproterozoic Mount Isa Block. The mine sequence is a package of carbonaceous metapelites and metagreywackes containing amphibolites derived from tholeiitic basic rocks. A 40Ar–39Ar age spectrum with an extensive plateau-like segment at 1,510 ± 3 Ma from an actinolite associated with sulfides is taken to represent the age of mineralization and is identical within error to the ages of most of the nearby batholithic granitoids. The mine sequence is locally intruded by 1- to 10-m-thick late- to post-tectonic trachyandesite dykes, which were emplaced during the hydrothermal activity that created the orebodies and have affinities with the regional high potassium “Eureka” supersuite granitoids. Stable isotope data are consistent with dominantly magmatic fluids during mineralization and the regionally distinctive skarn (Ca–Mg) and Cu–Au–Ni–Co–Te–Se (low Pb–Zn–Ag–Sb) chalcophile element associations may reflect a primitive magmatic fluid source and/or leaching of these elements from country rocks. Mount Elliott is an unusual skarn deposit characterized by pronounced early albitization (K–Fe–Mg depletion) of the host rocks succeeded by predominantly open-space deposition of sodic diopside ± actinolite ± scapolite ± andradite ± magnetite ± sulfides ± apatite ± allanite ± tourmaline ± calcite. The Ca–Fe–Mg(–Na)-rich (manganese-poor) chemistry was imposed from the fluid phase in the absence of carbonate-rich protoliths. Immobile trace element (Ti, Zr, Nb) geochemistry shows that Mount Elliott skarns formed in both metasedimentary and mafic metavolcanic host rocks, but the former are the main hosts of ore in upper and lower ore zones that represent most of the resource. Banded skarns derived from a distinct calc-silicate/marble package at the nearby SWAN prospect have higher Nb/TiO2 and Zr/TiO2 ratios than the Mount Elliott metasediment-derived skarns, consistent with different provenance of the detrital components in the two sequences. Medium- to coarse-grained massive skarn and skarn breccia in the Mount Elliott lower ore zone formed in pelites and the trachyandesite dykes are the only intrusive rocks that could be genetically related to the mineralization in the immediate vicinity of the orebodies. Received: 1 September 1999 / Accepted: 28 September 2000  相似文献   

18.
The Konkola deposit is a high grade stratiform Cu–Co ore deposit in the Central African Copperbelt in Zambia. Economic mineralisation is confined to the Ore Shale formation, part of the Neoproterozoic metasedimentary rocks of the Katanga Supergroup. Petrographic study reveals that the copper–cobalt ore minerals are disseminated within the host rock, sometimes concentrated along bedding planes, often associated with dolomitic bands or clustered in cemented lenses and in layer-parallel and irregular veins. The hypogene sulphide mineralogy consists predominantly of chalcopyrite, bornite and chalcocite. Based upon relationships with metamorphic biotite, vein sulphides and most of the sulphides in cemented lenses were precipitated during or after biotite zone greenschist facies metamorphism. New δ34S values of sulphides from the Konkola deposit are presented. The sulphur isotope values range from −8.7‰ to +1.4‰ V-CDT for chalcopyrite from all mineralising phases and from −4.4‰ to +2.0‰ V-CDT for secondary chalcocite. Similarities in δ34S for sulphides from different vein generations, earlier sulphides and secondary chalcocite can be explained by (re)mobilisation of S from earlier formed sulphide phases, an interpretation strongly supported by the petrographic evidence. Deep supergene enrichment and leaching occurs up to a km in depth, predominantly in the form of secondary chalcocite, goethite and malachite and is often associated with zones of high permeability. Detailed distribution maps of total copper and total cobalt contents of the Ore Shale formation show a close relationship between structural features and higher copper and lower cobalt contents, relative to other areas of the mine. Structural features include the Kirilabombwe anticline and fault zones along the axial plane and two fault zones in the southern limb of the anticline. Cobalt and copper behave differently in relation to these structural features. These structures are interpreted to have played a significant role in (re)mobilisation and concentration of the metals, in agreement with observations made elsewhere in the Zambian Copperbelt.  相似文献   

19.
安徽铜陵冬瓜山铜、金矿床两阶段成矿模式   总被引：21，自引：0，他引：21  
冬瓜山铜金矿床包括层状硫化物矿体、矽卡岩型和斑岩型矿体。层状硫化物矿体具层状形态和层控特征，矿石具块状、层纹状和揉皱状构造。燕山期岩浆岩及其岩浆流体对层状矿体进行了叠加和改造，改变了其结构构造、矿物组合和矿石成分，并在其上叠加蚀变和矿化。层状矿体中的铜是由含铜流体交代块状硫化物矿石形成的。冬瓜山铜金矿床经历了两次成矿作用：第一成矿阶段．在石炭纪中期，海底喷流作用形成了块状硫化物矿床，矿石成分以硫、铁矿为主；第二成矿阶段。燕山期岩浆侵人，一方面岩浆热液与围岩相互作用发生矽卡岩化、硅化、钾长石化、石英绢云母化和青磐岩化，形成矽卡岩型和斑岩型矿体，另一方面岩浆流体对块状硫化物矿体进行叠加改造，致使块状硫化物矿体富集铜等成矿物质。  相似文献   

20.
The lithostratigraphic sequence in the Rosário–Neves Corvo antiform comprises the Phyllite–Quartzite Group, whose top is of Famennian age, the Volcanic Sedimentary Complex, of Strunian to upper Visean age, and the Mértola Formation (the lower unit of the Baixo Alentejo Flysch Group) of upper Visean age. The volcanic sedimentary complex comprises a lower sequence of Strunian (Late Famennian) age and an upper sequence of lower to upper Visean age. Detailed mapping of the antiform towards NW of the Neves Corvo mine, supported by palynological dating, identified two new lithostratigraphic units: the Barrancão member (upper Famennian) ascribed to the Phyllite–Quartzite Group and made up of laminated dark shales with siliceous lenses and nodules, and the Ribeira de Cobres Formation of the Volcanic Sedimentary Complex, containing shales, siltstones and fine volcaniclastic rocks. Based on zircon U–Pb isotope dating, five discrete felsic magmatic events were identified at approximately 354, 359, 365, 373 and 384 Ma. This suggests that the volcanic activity in the area has extended for about 30 Ma, in a context of high regional heat flow as indicated by the geochemical signatures of the felsic volcanic rocks. The characteristics of magmatism and the depositional environment indicated by the sedimentary record should therefore have been highly favourable for massive sulphide formation. However, evidence of massive sulphide mineralization in the study area is still to be found. Moreover, reconstruction of the volcanic facies architecture demonstrated that the volcanic units in the Rosário area are strongly dominated by coherent facies typical of the inner part of thick lavas/domes. In fact, most of their external part, the more favourable location for possible massive sulphide mineralization, is missing. Palynological dating indicates a significant hiatus, recognised between the lower and upper sequences of the volcanic sedimentary complex, which implies erosion of the top of the volcanic centre, where VHMS deposits could possibly have formed. However, lateral areas of this volcanic centre, eventually preserved at depth, have good potential to host massive sulphide mineralization.  相似文献   

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