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1.
A.M.R. Neiva   《Ore Geology Reviews》2008,33(3-4):221-238
Cassiterite and wolframite compositions from Sn > W- and W > Sn-bearing quartz veins in Northern and Central Portugal are compared to provide evidence on fluid compositions. In Sn > W-bearing quartz veins, euhedral cassiterite shows sequences of alternating parallel darker and lighter zones. The darker zones are pleochroic, oscillatory zoned, exhibit exsolutions of columbite and ixiolite and are richer in Nb, Ta and Fe than the lighter zones which consist of nearly pure SnO2. Cassiterite from W > Sn-bearing quartz veins is usually zoned, with homogeneous and slightly pleochroic darker zones, which are chemically similar to lighter zones. Both zones have inclusions of rutile and rare ilmenite. The darker zones of cassiterite from the former veins are richer in Nb, Ta and Fe contents and poorer in Ti than the darker and lighter zones of cassiterite from the latter veins. This is attributed to differences in the composition of magmatic hydrothermal fluids.Wolframite compositions from Sn > W- and W > Sn-bearing quartz veins do not show any significant distinction, because they precipitate from relatively similar magmatic hydrothermal fluids. In some deposits, most wolframite crystals are homogeneous, but others are heterogeneous. Inner patches, rich in a hübnerite component, rarely occur in crystals from the Filharoso and Panasqueira deposits. Zoned crystals, showing an increase in Fe and a decrease in Mn from core to rim, were found in the Vale das Gatas deposit. Complex oscillatory zoned crystals occur. In the Carris deposit, later wolframite contains inclusions of scheelite, partially replaces it and is richer in Fe and poorer in Mn than earlier wolframite. Wolframite from Sn > W-bearing quartz veins in the Argozelo deposit and W > Sn-bearing quartz veins from Vale das Gatas and Panasqueira deposits has significant Nb content. This does not depend on the Fe and Mn content of the wolframite, but W content is negatively correlated with Nb content. Only very rare single crystals of wolframite show an increase in W and a decrease in Nb from core to rim. Sn > W-bearing quartz veins contain wolframite poorer in Nb than the darker zones of cassiterite, which exsolved columbite and ixiolite. In W > Sn-bearing quartz veins from Panasqueira and Vale das Gatas, the wolframite has a higher Nb content than the cassiterite, which contains rutile inclusions enriched in Nb, because cassiterite and wolframite are derived from two distinct magmatic hydrothermal fluids of different age. The fluid responsible for wolframite precipitation will have a similar composition to that resulting from the evolution of the fluid responsible for cassiterite precipitation in the Sn > W-bearing quartz veins.  相似文献   

2.
The quartz veins containing scheelite from Fonte Santa mine cut the Lower Ordovician quartzites. A muscovite–biotite granite (G1) and a muscovite granite (G2), both S-type, crop out close to the Fonte Santa mine and are related to the Moncorvo–Bemposta shear zone. The most altered samples of G2 show intense muscovitization and microclinization and contain chlorite, columbite–tantalite, wolframite, W-ixiolite and Fe-oxides. The tin-bearing granites contain 18 ppm (G1) and 73 ppm (G2) Sn. The most altered samples of G2 correspond to a tungsten granite. The quartz veins contain muscovite, chlorite, tourmaline, scheelite, pyrrhotite, pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, iron oxides, Fe sulfates, phosphates of Pb, Fe and Al. The Fonte Santa mine area was exploited for W between 1942 and 1982. At the end of November 2006, a flood event damaged the tailings dam of Fonte Santa mine, releasing contaminated material and increasing contaminant levels in water within the area of influence of the mine. The waters related to the Fonte Santa mine are poorly mineralized, with electrical conductivity of < 965 µS/cm, and of a mixed type or HCO3− and SO42− types. Most pH values (5.0−8.5) indicate that there is no significant acidic drainage in the region, as found in other areas. More acidic values (pH = 3.4) were found in the mine's lagoon. Waters associated with mineralized veins and old mine activities have Fe and Mn concentrations that forbid their use for human consumption and agriculture. Natural Na, Mg and K water contents are associated with the alteration of albite, chlorite and muscovite of country rock, while Ca with the W-bearing quartz veins. Weathering agents are carbonic and sulphuric acids and the latter has a strong influence in areas draining fine-grained mine tailings.  相似文献   

3.
At Segura, granitic pegmatite veins with cassiterite and lepidolite, hydrothermal Sn–W quartz veins and Ba–Pb–Zn quartz veins intruded the Cambrian schist–metagraywacke complex and Hercynian granites. Cassiterite from Sn–W quartz veins is richer in Ti and poorer in Nb and Nb+Ta than cassiterite from granitic pegmatite. Wolframite from Sn–W quartz veins is enriched in ferberite component. The Sn–W quartz veins contain pyrrhotite, arsenopyrite, sphalerite, chalcopyrite, stannite, matildite and schapbachite and the Ba–Pb–Zn quartz veins have cobaltite, pyrite, sphalerite, chalcopyrite, galena and barite, which were analyzed by electron microprobe. The presently abandoned mining area was exploited for Sn, W, Ba and Pb until 1953. Stream sediments and soils have higher concentrations of metals than parent granites and schists. Sn, W, B, As and Cu anomalies found in stream sediments and soils are associated with Sn–W quartz veins, while Ba, Pb and Zn anomalies in stream sediments and soils are related to Ba–Pb–Zn quartz veins. Sn, W, B, As, Cu, Ba, Pb and Zn anomalies in stream sediments and soils are also related to the respective old mining activities, which increased the mobility of trace metals from mineralized veins to soils, stream sediments and waters. Stream sediments and soils are sinks of trace elements, which depend on their contents in mineralized veins and weathering processes, but Sn, W and B depend mainly on a mechanic process. Soils must not be used for agriculture and human residence due to their Sn, B, As and Ba contents. Waters associated with mineralized veins were analyzed by flame atomic absorption spectroscopy (FAAS) and ICP-AES have high As, Fe and Mn and should not be used for human consumption and agriculture activities. The highest As values in waters were all related to Sn–W quartz veins and the highest Fe and Mn values were associated with the Ba–Pb–Zn quartz veins. No significant acid drainage was found associated with the old mine workings.  相似文献   

4.
Abstract: El Mueilha area consists of post-collision granitic rocks intruding Pan-African metasediments, metavolcanics and granodiorites. Tin mineralization in Gabal El Mueilha is either of vein type or disseminated in the greisenized and albitized parts of the granitic rocks. Cassiterite and wolframite-bearing quartz veins also characterize a small intrusion of muscovite granite at El Mueilha tin mine area. Detailed geochemical prospecting for the rare metals Sn, Nb, Be, Li, U, Th and some other trace elements was carried out at Gabal El Mueilha area using stream sediments survey. Sixty-seven stream sediment samples were collected from the main drainage patterns of the study area. Statistical parameters were calculated for the analyzed elements. The sought elements Sn, Nb, Be and Li have relatively high background values in the studied sediments. This may reflect the role of the pathfinder elements (Nb, Be and Li) during secondary dispersion survey for Sn mineralization.
Geochemical maps were constructed to delineate anomalous areas with abnormally high rare metal contents. The anomalous Sn, Nb and Be areas are mainly encountered in the main stream draining the mineralized zones of El Mueilha tin mine and near the SW albitized parts of the post-collision granite. Correlation coefficient matrices show significant positive relation between Sn and the rare metals group (Nb, Rb and Li) at 99 % significant level. R-mode factor analysis for the concerned elements yields five factor–model.  相似文献   

5.
The purpose of this work is to characterize the hydrochemical behavior of acid mine drainages (AMD) and superficial waters from the Adoria mine area (Northern Portugal). Samples of superficial and mine drainage water were collected for one year, bi-monthly, with pH, temperature, Eh, conductivity and HCO3 determined in situ with chemical analyses of SO4, Ca, K, Mg, Na, Cl, Ag, As, Bi, Co, Cu, Fe, Mn, Ni, Pb, Zn and Cd. In the mine, there are acidic waters, with low pH and significant concentrations of SO4, and metals (Fe, Mn, Zn, Cu, Pb, Cd and Ni), while in the superficial natural stream waters outside the mine, the pH is close to neutral, with low conductivity and lower metal concentrations. The stream waters inside the mine influence are intermediate in composition between AMD and natural stream waters outside the mine influence. Principal Component Analysis (PCA) shows a clear separation between AMD galleries and AMD tailings, with tailings having a greater level of contamination.  相似文献   

6.
 Acid mine drainage (AMD) with a minimum pH of 0.52 was recorded at Iron Duke mine near Mazowe, Zimbabwe during an investigation of the environmental geochemistry of mine waters in the Greenstone Belts of Zimbabwe. Hydrochemical data for waters emanating from the Iron Duke waste-rock pile indicate their super-saturation with respect to Fe and SO4 2–. Extremely high dissolved concentrations of Al, Zn, Cu, Co, Ni, V, Cr, Cd and As also prevail. Substantial losses of metals from solution occur within 400 m of the AMD source through the precipitation of crystalline sulphates, principally melanterite. Further downstream, hydrous oxide precipitation forms the dominant mechanism of metal attenuation in waters characteristically under-saturated with respect to Fe sulphates. Speciation and saturation index data generated using the equilibrium model WATEQ4F, suggest that such codes have broad utility for generic prediction of the mineralogical contraints on metal mobility in acute AMD systems. Major discrepancies between modelled and empirical hydrochemistries are, however, evident for super-saturated waters in which the kinetics of Fe precipitation are slow, and in which total ionic strengths markedly exceed their theoretical maximum. Received: 28 August 1998 · Accepted: 7 December 1998  相似文献   

7.
A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na2O content (1.12–2.66 wt%), TiO2 content (0.63–1.68 wt% TiO2) and Fe content, expressed as Fe2O3 (up to 1.37 wt%). Major element compositions of samples were dominated by SiO2 (87.894–90.997 wt%), Al2O3 (6.759–7.741 wt%), Fe2O3 (1.136–1.283 wt%), and K2O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.  相似文献   

8.
The Canan area (Honduras) is characterized by a gold-bearing ore deposit that is associated with quartz-veined shear zones. Gold mineralization occurs in low-to medium-grade metamorphic host-rocks (graphitic and sericitic schists). Hydrothermal fluids, which are associated with the emplacement of Cretaceous-Tertiary granodioritic intrusions, are responsible for the formation of quartz veins and the hydrothermal alteration of wall-rocks. Three main altered zones have been detected in the wall-rocks as far as 150 cm from the quartz veins. The distal zone (up to 50-cm thick) contains quartz, chlorite and illite. The intermediate zone is the thickest (up to 80 cm) and is marked by quartz, muscovite, sulphides, kaolinite and native elements such as Au and Ag. The proximal zone, which is close to the quartz veins, is rather thin (up to 25 cm) and contains clay minerals, Al-oxides-hydroxides and sulphides. The transition from the distal to the proximal zone is accompanied by the enrichment of SiO2 and the depletion of all other major elements, except for Fe2O3(tot). Precious metals occur in the highest concentrations in the intermediate zone (Au up to 7.6 ppm and Ag up to 11 ppm). We suggest that gold was transported as a reduced sulphur complex and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with Fe2+ from the alteration of the mafic minerals of the host-rock. Fluid–wall-rock interactions seem to be the main cause of gold mineralization. Genetic relationships with a strike-slip fault system, hydrothermal alteration zones within the metamorphic wall-rocks, and an entire set of geochemical anomalies are consistent with orogenic-type gold deposits of the epizonal class.  相似文献   

9.
The Pemali tin deposit is located in a Triassic granite pluton the magmatic evolution of which is characterized by a decrease of compatible Ca, Mg, Ti, P and Zr in the sequence: medium- to coarse-grained biotite granite, megacrystic medium-grained biotite granite, two-mica granite/muscovite granite. The tin mineralization is confined to the two-mica granite and consists of disseminated cassiterite as well as greisen-bordered veins. The highly evolved muscovite granite is tin-barren and is distinguished from the two-mica granite by its low mica content and low loss-on-ignition values. The fluid inclusions in quartz and fluorite of the two-mica granite and of the greisen homogenize in the 115–410 °C temperature range; the salinities are in the range of 0.4–23 equiv wt% NaCl and the CO2 concentrations are < 2 mole%.  相似文献   

10.
Prosperous granite (Rb-Sr 2520±25 Ma) occurs as several plutons (1–380 km2 outcrop area) in a thick succession of metamorphosed greywacke-mudstone of the Yellowknife Supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz (32), plagioclase (31), K-feldspar (24), muscovite (9), biotite (3), and apatite (<1). Average trace-element concentrations (in ppm) are Li (140), Be (4), B (28), Zn (47), Rb (250), Sr (76), Zr (75) and Ba (360). The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica (Li in biotite/Li in muscovite=4.7), and Be and B in muscovite and plagioclase. Countless pegmatite dikes occur in the Sparrow pluton and in schist-hornfels to the east; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dikes vary greatly in size (1 km to a few cm in length), in mineral content (quartz, albite, K-feldspar, muscovite, tourmaline, beryl, spodumene), in major element composition (especially the NaK ratio), and in trace-element content (Li 18–5000 ppm, Be 5–260 ppm, B 20–150 ppm). Compared with Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr, and Ba. Dikes rich in tourmaline, beryl, and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to host rock; the highest concentrations of Fe and Zn occur in crystals from pegmetite and the highest concentrations of Mg and V occur in crystals from tourmalinized schist, while those from granite and quartz veins occupy on intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. Estimates of temperature (500°–600° C) and pressure (2–4 kb) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be, and B, were possibly derived from Yellowknife graywacke and mudstone.  相似文献   

11.
The acid mine drainage (AMD) discharged from the Hejiacun uranium mine in central Hunan (China) was sampled and analyzed using ICP-MS techniques. The analyzing results show that the AMD is characterized by the major ions FeTotal, Mn, Al and Si, and is concentrated with heavy metals and metalloids including Cd, Co, Ni, Zn, U, Cu, Pb, Tl, V, Cr, Se, As and Sb. During the AMD flowing downstream, the dissolved heavy metals were removed from the AMD waters through adsorption onto and co-precipitation with metal-oxhydroxides coated on the streambed. Among these metals, Cd, Co, Ni, Zn, U, Cu, Pb and Tl are negatively correlated to pH values, and positively correlated to major ions Fe, Al, Si, Mn, Mg, Ca and K. The metals/metalloids V, Cr, Se, As and Sb are conservative in the AMD solution, and negatively-correlated to major ions Na, Ca and Mg. Due to the above different behaviors of these chemical elements, the pH-negatively related metals (PM) and the conservative metals (CM) are identified; the PM metals include Cd, Co, Ni, Zn, U, Cu, Pb and Tl, and the CM metals V, Cr, Se, As and Sb. Based on understanding the geochemistry of PM and CM metals in the AMD waters, a new equation: EXT = (Acidity + PM)/pH + CM × pH, is proposed to estimate and evaluate extent of heavy-metal pollution (EXT) of AMD. The evaluation results show that the AMD and surface waters of the mine area have high EXT values, and they could be the potential source of heavy-metal contamination of the surrounding environment. Therefore, it is suggested that both the AMD and surface waters should be treated before they are drained out of the mine district, for which the traditional dilution and neutralization methods can be applied to remove the PM metals from the AMD waters, and new techniques through reducing the pH value of the downstream AMD waters should be developed for removal of the CM metals.  相似文献   

12.
Sn–W deposit of the Mueilha mine is one of many other Sn–W deposits in the Eastern desert of Egypt that associated with albite granite. Two forms of Sn–W mineralizations are known at the Mueilha Sn-mine area, namely fissure filling quartz veins and greisen. Cassiterite and/or wolframite, sheelite, and beryl are the main ore minerals in the greisen and quartz veins. Subordinate chalcopyrite and supergene malachite and limonite are also observed in the mineralized veins. To constrain the P–T conditions of the Sn–W mineralizations, fluid inclusions trapped in quartz and cassiterite, have been investigated. The following primary fluid inclusion types are observed: CO2-rich, two-phase (L?+?V) aqueous, and immiscible three-phase (H2O–CO2) inclusions. Low temperature and low salinity secondary inclusions were also detected in the studied samples. Microthermometric results revealed that Sn–W deposition seem to have taken place due to immiscibility at temperature between 260°C and 340°C, and estimated pressure between 1.2 to 2.2 kb. Microthermometric results of fluid inclusions in fluorite from fluorite veins illustrated that fluorite seems to be deposited due to mixing of two fluids at minimum temperature 140°C and 180°C, and estimated minimum pressure at 800 bars.  相似文献   

13.
Numerous pegmatite dikes occur in the Sparrow pluton (muscovite-biotite granite) and in the adjacent cordierite-zone schist-hornfels of the Yellowknife Supergroup. Where pegmatite dikes cut granite, the adjacent granite is enriched in muscovite and apatite, and depleted in K-feldspar. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that H, P, and locally B, Ti, Fe, and Ca were added, and K, Sr, Ba, and locally Na were removed (hydrogen metasomatism). In one alteration zone (8 cm wide) the calculated change (in terms of mols/gram of unaltered granite) is, 600 K-feldspar+24 biotite+190 plagioclase +[770 H+36 P+3 Ti+13 Fe+13 Ca] 400 muscovite+1100 quartz +11 apatite+[240 Na+260 K]. Where pegmatite dikes cut schist-hornfels (biotite-plagioclase-quartz), the adjacent rock is, in places, enriched in tourmaline, apatite, and quartz, and depleted in biotite and plagioclase. These alteration zones are variable in width; most are less than 20 cm wide. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that B, P, Zn, and locally Ca, Fe, and Al were added, and that Na, K, Fe, Rb, Sr, Ba, and locally Mg and Si were removed (boron metasomatism). In one zone, 2 cm wide, the calculated reaction (in units of mols/gram of unaltered schist) is, 730 biotite+1530 plagioclase +[1080 B+600 H+430 P+360 Ca] 480 tourmaline+480 quartz+115 apatite +[3630 Si+870 Na+590 K+110 Fe]. Changes in the volume fraction of muscovite, K-feldspar, tourmaline, and biotite, relative to distance from pegmatite, are progressive, and in most alteration zones may be expressed by use of an error-function equation. Some tourmaline zones are more complex. Zone formation is considered in terms of a steady-state reaction model in which grainboundary diffusion is the transport mechanism.  相似文献   

14.
Combined research in geochemistry and mineral chemistry of the hydrothermally altered W-Sn specialized granite of Regoufe and its derivatives in Portugal was undertaken to gain insight in the mineralogical changes associated with hydrothermal processes within a single granite cupola. Over 1000 unpolished rock sections were analyzed by automated X-ray fluorescence spectrometry (XRF). On the basis of the XRF data, a small number of these same sections was selected for investigation by electron probe microanalysis. The study focuses on fourteen elements of interest that are measurable with the chosen techniques. Major pervasive alteration within the Regoufe granite is virtually contemporaneous with mineralization in the form of Sn- or W-bearing quartz veins. Two phases of hydrothermal activity are discerned, characterized by different element associations. Fluids of the first phase were especially rich in Sn, Cs and F, whereas the second phase was marked by a W-Ta-Nb-Rb association and presumably carried less F. Phosphorus probably was an important fluid component in both phases. The fluids are inferred to have fractionated from a related granitic magma at depth.Tin, W, Nb and Ta are mainly found as substitutions or inclusions in biotite in the least altered part of the Regoufe granite. Tantalo-niobian rutile is an important control for the distribution of Nb. Tin occurs in rutile and rarely as cassiterite. Muscovitization caused leaching of Ti and Zr from the granite. Tin, supplied or mobilized by the hydrothermal fluids, behaves differently from W, Nb and Ta. In the most altered rocks, muscovite hosts significant amounts of Sn. Whereas Sn is still related to Cs and Ti, elements that probably represent altered biotite, W, Ta and Nb are related to newly formed Rb-rich muscovite. Columbitetantalite was detected embedded in late muscovite. In contrast to Sn, the fluid-supplied W was predominantly deposited as wolframite in quartz veins and the altered granite is not enriched in W compared to the relatively unaltered rocks. Strontium is preferentially hosted by K-feldspar in the least altered granite; Ca in this rock is still partly contained in albite. With increasing albitization and muscovitization, Sr and Ca were released and are partly bound in newly formed apatite. The primary magmatic apatites have near-ideal formula compositions, but mobilization of P during hydrothermal activity resulted in the formation of Mn-rich apatite in all parts of the granite, and Sr- and probably Li-rich varieties in the most altered rocks. Eosphorite, and scorodite as the oxidation product of arsenopyrite, were also formed as a result of P mobilization. In the least altered rocks, Cs is mainly contained in biotite. In the most altered granite and aplitic rocks, enrichment of Cs and Rb is evenly distributed over K-feldspar and micas. The processes that lead to increased Rb are partly independent of Cs enrichment, and apparently related to the W-Nb-Ta mineralization event, separate from the preceding Sn mineralization.  相似文献   

15.
Tungsten ore at Carrock Fell Mine comprises wolframite and scheelite in polyminerallic quartz veins which traverse the Grainsgill Granite cupola and surrounding country rocks. In the veins, a wolframite-scheelite-apatite assemblage pre-dates a scheelite-arsenopyrite-pyrite (plus other sulphides) assemblages. Temperatures of mineralisation declined from a peak near 350°C to 170°C, and the hydrothermal fluid contained about 6 weight% NaCl and 3 wt% NaHCO3. Contemporaneous greisenisation involved loss of Na, Cr, Ca and Ba from granite, but Si and K were retained while B, Be and Al increased slightly. Sn also increased but is always a trace constituent, and F appears to have decreased. Zones of intense alteration contain high concentrations of quartzhosted fluid inclusions resulting from penetration of the granite by fluid chemically similar to that in the vein quartz. The W-rich, Sn-poor nature of the mineralisation may relate to the weakly saline, F-deficient but CO2-rich fluid chemistry. The alteration and mineralisation processes took place during late cooling of the Lower-Devonian Skiddaw Granite. Cross-cutting quartz-ankerite veins and argillitic zones which may be considerably younger than those producing the tungsten ore, have a distinct mineral suite lacking W and As and including major Pb and Zn. Temperatures at this late stage were below 150°C, and the fluid is estimated to have contained approximately 12 wt% NaCl and 15 wt% CaCl2.  相似文献   

16.
The Sn-rich Qiguling topaz rhyolite dike intrudes the Qitianling biotite granite of the Nanling Range in southern China; the granite hosts the large Furong Sn deposit. The rhyolite dike is typically peraluminous, volatile-enriched, and highly evolved. Whole-rock F and Sn concentrations attain 1.9 wt.% and 2700 ppm, respectively. The rhyolite consists of a fine-grained matrix formed by quartz, feldspar, mica and topaz, enclosing phenocrysts of quartz, feldspar and mica; it is locally crosscut by quartz veinlets. Lithium-bearing micas in both phenocrysts and the groundmass can be classified as primary zinnwaldite, “Mus-Ann” (intermediate member between annite and muscovite), and secondary Fe-rich muscovite. Topaz is present in the groundmass only; common fluorite occurs in the groundmass and also in a specific cassiterite, rutile and fluorite (Sn–Ti–F) assemblage. Cassiterite and rutile are the only Sn and Ti minerals; both cassiterite and Nb-rich rutile are commonly included in the phenocrysts. The Sn–Ti–F assemblage is pervasive, and contains spongy cassiterite in some cases; cassiterite also occurs in quartz veinlets which cut the groundmass. Electron microprobe and LA-ICP-MS compositions were used to study the magmatic and hydrothermal processes and the role of F in Sn mineralization. The presence of zinnwaldite and “Mus-Ann”, which are respectively representative of early and late mica crystallization during magma differentiation, also suggests a significant decrease in f(HF)/f(H2O) of the system. Cassiterite included in the zinnwaldite phenocrysts is suggested to have crystallized from the primary magma at high temperature. Within the Sn–Ti–F aggregates, rutile crystallized as the earliest mineral, followed by fluorite and cassiterite. Spongy cassiterite containing inclusions of the groundmass minerals indicate a low viscosity of the late fluid. The cassiterite in the quartz veinlets crystallized from low-temperature hydrothermal fluids, which possibly mixed with meteoric water. In general, cassiterite precipitated during both magmatic and hydrothermal stages, and over a range of temperatures. The original fluorine and tin enrichments, f(HF)/f(H2O) change in the residual magma, formation of Ca,Sn,F-rich immiscible fluid, decrease of the f(HF) during groundmass crystallization, and mixing of magma-derived fluids with low-saline meteoric water during the late hydrothermal stage, are all factors independently or together responsible for the Sn mineralization in the Qiguling rhyolite.  相似文献   

17.
The Slyudyangorsk muscovite deposit in the southern Urals was explored and mined in 1926–1957. By the mid-1950s, 104 veins of quartz–feldspar pegmatites including 21 muscovite-bearing veins have been found. Pegmatites with giant black Y-bearing epidote crystals are crosscut by veins with giant muscovite crystals, which, in turn, are intersected by veins of two-mica–quartz–two-feldspar pegmatites with rare-metal and REE mineralization. Microprobe data on compositions of complex Ti–Ta–Nb oxides [fergusonite-(Y), samarskite-(Y), euxenite-(Y), polycrase-(Y), columbite-(Fe), pyrochlore supergroup] are characterized, as well as of uraninite, ilmenorutile, scheelite, Y-bearing epidote, certain sulfides and rock-forming minerals from the Slyudyanogorsk deposit. The morphology and interrelation of minerals indicate that they are the result of crystal growth in cavities rather than of metasomatic replacement of gneisses, as has been suggested earlier. Thus, it is more promising for rare-metal and REE minerals in the Slyudorudnik area to be found in igneous rocks (granitic muscovite–quartz–feldspar pegmatites with the Nb–Ta–Ti–Y–U–W–Mo mineralization) than in metasomatic rocks.  相似文献   

18.
《Applied Geochemistry》2001,16(11-12):1387-1396
The purposes of this study are to (i) determine the geochemical characteristics of Imgok creek impacted by acid mine drainage (AMD) generated from abandoned coal mines, (ii) to assess the pollution of heavy metals in the stream sediments and soils, and (iii) to identify the chemical form of Fe precipitates collected in the study area where there are 4 abandoned coal mines, which belong to the Grangreung coal field at the eastern part of Korea. AMD generated from mine adits and coal refuse piles shows low pH, and high concentrations of Fe, Al and SO4, especially in the Youngdong coal mine. In Imgok creek, pH values increased, and total dissolved solids (TDS) values decreased with distance. The concentrations of toxic heavy metals and major cations except Fe decreased by dilution, but the concentration of Fe decreased rapidly due to the formation of precipitates. The quality of groundwater samples did not exceed the Korean drinking-water standard. In the stream sediments, the concentrations of Fe are relatively high in the Youngdong tributary and Imgok creek, but the concentrations of heavy metals are similar to those of unpolluted sediments. Pollution indices of agricultural soils range from 0.28 to 0.47. Yellowish red Fe precipitates collected in the study area turned out to be amorphous or poorly crystallized minerals (determined by X-ray diffraction patterns and Feox/Fetot ratios) and to contain chemically bonded SO4 and OH [determined by infra-red (IR) spectral analysis]. With these, the mol ratios of Fe/S ranging from 4.6 to 6.1 determined by electron probe micro-analysis (EPMA) in precipitates strongly support the existence of schwertmannite.  相似文献   

19.
In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0 ± 0.5 Ma and 312.9 ± 2.0 Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite.  相似文献   

20.
Acid mine drainage (AMD) is one of the severe environmental problems that coal mines are facing. Generation of AMD in the northeastern part of India due to the coal mining activities has long been reported. However detailed geochemical characterization of AMD and its impact on water quality of various creeks, river and groundwater in the area has never been reported. Coal and coal measure rocks in the study area show finely disseminated pyrite crystals. Secondary solid phases, resulted due to oxidation of pyrite, occur on the surface of coal, and are mainly consisting of hydrated sulphate complexes of Fe and Mg (copiapite group of minerals). The direct mine discharges are highly acidic (up to pH 2.3) to alkaline (up to pH 7.6) in nature with high concentration of SO42−. Acidic discharges are highly enriched with Fe, Al, Mn, Ni, Pb and Cd, while Cr, Cu, Zn and Co are below their maximum permissible limit in most mine discharges. Creeks that carrying the direct mine discharges are highly contaminated; whereas major rivers are not much impacted by AMD. Ground water close to the collieries and AMD affected creeks are highly contaminated by Mn, Fe and Pb. Through geochemical modeling, it is inferred that jarosite is stable at pH less than 2.5, schwertmannite at pH less than 4.5, ferrihydrite above 5.8 and goethite is stable over wide range of pH, from highly acidic to alkaline condition.  相似文献   

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