首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 62 毫秒
1.
Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature (P–T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P–T conditions of the dioritic rocks are 680–725 °C and 4.9–6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0–6.4 wt.% NaCleq) H2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P–T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of 17–22 km, with a minimum average exhumation rate of 12–15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.  相似文献   

2.
The release of irradiation-produced noble gas isotopes (38ArCl, 80KrBr, 128XeI and 39ArK) during in vacuo crushing scapolite has been investigated and is compared to quartz. Three thousand crushing strokes released 98% of fluid inclusion-hosted noble gas from quartz. In comparison, 3000 crushing strokes released only 4% of the lattice-hosted 38ArCl from a scapolite gem. In vacuo crushing released lattice Ar preferentially relative to lattice Kr or Xe and prolonged crushing released 88% of the lattice-hosted noble gas in 96,000 crushing strokes. We suggest fast diffusion pathways generated by crushing are an important noble gas release mechanism and we demonstrate two applications of prolonged in vacuo crushing on irradiated scapolite.Firstly, scapolite molar Br/Cl and I/Cl values are shown to vary over a similar range as crustal fluids. The Cl-rich scapolite gem from Hunza, Pakistan has Br/Cl of 0.5–0.6 × 10−3 and I/Cl values of 0.3–2 × 10−6, that are similar to fluids that have dissolved evaporites. In contrast, three out of four skarn-related scapolites from the Canadian Grenville Province have molar Br/Cl values of 1.5–2.4 × 10−3, and I/Cl values of 11–24 × 10−6, that are broadly consistent with skarn formation by magmatic fluids. The fourth Grenvillian scapolite, with only 0.02 wt% Cl, has an exceptionally elevated molar Br/Cl value of up to 54 × 10−3 and I/Cl of 284 × 10−6. It is unclear if these values reflect the composition of fluids formed during metamorphism or preferential incorporation of Br and I in Cl-poor meionitic scapolite.Secondly, the Grenvillian scapolites give plateau ages of between 830 Ma and 400 Ma. The oldest ages post-date regional skarn formation by 200 Myr, but are similar to feldspar cooling ages in the Province. The age variation in these samples is attributed to a combination of factors including variable thermal history and the presence of mineral sub-grains in some of the samples. These sub-grains control the release of 39ArK, 38ArCl and 40Ar* during in vacuo crushing as well as the samples 40Ar* retentivity in nature. Scapolite is suggested as a possible analogue for K-feldspar in thermochronologic studies.  相似文献   

3.
This study provides evidence for the existence of halite and sylvite solid inclusions in igneous quartz and feldspars, the first to be reported in intrusive rocks, and to partially constrain the physicochemical environment that lets halides crystallize under magmatic conditions.Halite and sylvite solid inclusions were found included in quartz and feldspars from a micrographic–granophyric assemblage in a miarolitic aplite and, rarer, in alkali-feldspar from a miarolitic monzogranite. Monzogranite and aplite represent I-type, K-enriched postcollisional rocks of the Late Cambrian–Early Ordovician Sierra Norte–Ambargasta batholith in the Eastern Sierras Pampeanas. Both granitoids fall among the most evolved felsic rocks of the batholith, with aplite approaching haplogranitic compositions. Halite is far more common than sylvite and the presence and distribution of one or both halides are erratic within the felsic intrusive bodies. Halides occur as small skeletal grains, commonly in cross-shaped aggregates of less than 50 μm. No K or Na was found at the detection limits of EDS in either halite or sylvite respectively. Textural relationships suggest that the alkali-chlorides separated from the melt near the minima along the quartz–feldspar cotectics of PH2O > 160 < 200 MPa in a silica-, and potassium-rich magmatic system at approximately 750–700 °C, prior to the H2O-vapor saturated miarole-forming stage.Computed ratios for the magmatic volatile phase (MVP) coexisting with melt at the early stage of aplite crystallization are: NaCl/HCl = 0.11–0.97 and KCl/HCl = 0.24–1.62, being the highest range of values (0.79–0.97 and 1.45–1.62, respectively) found in those alkali-chloride-bearing samples. Maximum HCl/ΣCl(MVP) (0.28 to 0.31) indicates higher total Cl concentration in the MVP of alkali-chloride-bearing aplites, which is much higher in the halite-free aplite samples (HCl/ΣCl(MVP) = 0.59 to 0.74). One miarolitic monzogranite sample, where halite solid inclusions are present, also yielded the highest ratios for NaCl/HCl(MVP) (0.91) and KCl/HCl(MVP) (1.46), and the HCl/ΣCl(MVP) is 0.30. A high HCl concentration in the fluid phase is suggested by the log f(HF)/f(H2O) = − 4.75 to − 4.95, log f(HCl)/f(H2O) = − 3.73 to − 3.86, and log f(HF)/f(HCl) = − 0.88 to − 1.22, computed at 750 °C after biotite composition. The Cl concentrations at 800 °C, computed with a Dv/lCl = 0.84 + 26.6P (P at 200 MPa), yielded values within the range of  70 to 700 ppm Cl in the melt and  4000 to 40 000 ppm Cl in the coexisting MVP. The preferential partitioning of Cl in the vapor phase is controlled by the Dv/lCl; however, the low concentration of Cl in the melt suggests that high concentrations of Cl are not necessary to saturate the melt in NaCl or KCl.Cl-saturation of the melt and coexisting MVP might have been produced by a drop in Cl solubility due to the near-haplogranitic composition of the granitoids after extreme fractionation, probably enhanced by fluctuating reductions of the emplacement pressure in the brittle monzogranite host. Liquid immiscibility, based in the differential viscosity and density among alkali-chloride saturated hydrosaline melt, aluminosilicate felsic melt, and H2O-rich volatiles is likely to have crystallized halite and sylvite from exsolved hydrosaline melt. High degrees of undercooling might have been important at the time of alkali-chloride exsolution. The effectiveness of alkali-chloride separation from the melt at magmatic temperatures is in line with the interpretation of “halite subtraction” as a necessary process to understand the origin of the “halite trend” in highly saline fluid inclusions from porphyry copper and other hydrothermal mineralizations, despite the absence of the latter in the Cerro Baritina aplites, where this process preceded the exsolution of halite-undersaturated fluids.Pervasive alteration of the monzogranite country rock as alkali-metasomatic mineral assemblages, the mineral chemistry of some species, and the association of weak molybdenite mineralization are compatible with the activity of alkaline hypersaline fluids, most likely exsolved during the earliest stages of aplite consolidation.  相似文献   

4.
The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu–Au–(Fe) skarn deposits and Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu–Pb–Zn–Au–(S) vein-type and a few Cu–Fe–(Au) skarn-type occurrences.Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl−(S)–Na+–K+ chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components.Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid–melt partitioning. The ore fluids of magmatic origin formed large Cu–Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits.The sulphur isotopic composition of the skarn- and vein-type deposits varies from − 11.3‰ to + 19.2‰ and from + 4.2‰ to + 10.0‰, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water–rock interaction. Complex water–rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids.Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 × 10− 1 to 9.8 × 10− 7 m/s and 3.6 × 10− 7 to 4.6 × 10− 7 m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl2−, CuCl32− and CuClOH, whereas Au was transported as Au2(HS)2S2−, Au(HS)2−, AuHS and AuH3SiO4 complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO2, which resulted from fluid boiling and mixing.Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 °C my− 1, which permitted sufficient heat supply by magma to the ore-forming system.  相似文献   

5.
Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation (kMAX–kINT) parallel to dike walls and a lineation (kMAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility (kMAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow (kMAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for 240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α95 = 3.9; mean TUB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α95 = 8.7; mean TUB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-component unblocking temperatures (TUB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as 274 /+ 33 (88 Ma).  相似文献   

6.
The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China. Gold occurs mainly in pyrite- and polymetallic sulfide–quartz vein/veinlet stockworks. Fluid inclusions in the deposit are divided into three main types, namely CO2–H2O, H2O–CO2 ± CH4 and aqueous ones. Microthermometric data show that the pre-gold fluids were CO2-dominant (XCO2 up to 0.53), and the total homogenization temperatures fall in the range of 298377 °C. These fluids, modified by fluid/wallrock reactions, gradually evolved into fluids with less CO2 (XCO2 = 0.010.19) in the main ore-forming stage, and the total homogenization temperatures range from 170 to 324 °C. Hydrogen and oxygen stable isotope data suggest that ore-forming fluids were mixture of magmatic and meteoritic origin. Co-occurrence of gold and sulfides implies that gold was most likely transported in the form of gold–sulfide complexes. The wide distribution of CO2 inclusions means that the pH variation during gold transportation was controlled by CO2 buffering.  相似文献   

7.
A deep epithermal vein system hosted in Late Proterozoic to Cambrian granodiorite has been identified in the Sierra Norte de Córdoba, the easternmost range of the Sierras Pampeanas Orientales of Argentina. The vein swarm extends over an area of 3 km2 parallel to a mylonitic belt and formed in fractured granodiorite. Thicknesses of veins are less than 0.5 m and their visible strike length is less than 100 m. Veins are either barren or weakly mineralized in base-metal sulfides. Most veins have mineral associations dominated by calcite and quartz with lesser amounts of chlorite, sericite, pyrite, and minor illite. In other less exposed albite-rich, adularia-bearing veins, chalcopyrite, bornite, galena, sphalerite, chalcocite and covellite may occur. The widespread occurrence of bladed calcite without any petrographic or microthermometric evidence of boiling implies that this particular habit of calcite may also develop under sub-near boiling fluid conditions. Thermometric calculations based on fluid inclusion data, chlorite composition and oxygen isotopes in the quartz–calcite pair, constrain the formation of the system between 300 and 350 °C, at pressures between 42 and 64 MPa (1.5–2.3 km). Stable isotope data suggest that W/R interaction might have been the most probable mechanism of alteration, involving the participation of meteoric fluids; nevertheless, the metallic signature of some weakly mineralized veins as well as intermediate fluid inclusion salinities favor a magmatic input and a mixed origin for the fluids. Textures and mineral associations, as well as the absence of evidence of boiling in fluid inclusions, all suggest that the silica–carbonate vein system formed deeper than typically shallow Au and Ag-bearing boiling solutions. A 485 (±25) Ma lamprophyre dike crosscuts some of these veins locally producing metasomatic reactions and skarn formation, which constrains the age of the hydrothermal system to the Cambrian-Early Ordovician time span.  相似文献   

8.
Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO2 inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ13C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO2 formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ13C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO2 fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.  相似文献   

9.
The Sar-Cheshmeh porphyry Cu–Mo deposit is located in Southwestern Iran (65 km southwest of Kerman City) and is associated with a composite Miocene stock, ranging in composition from diorite through granodiorite to quartz-monzonite. Field observations and petrographic studies demonstrate that the emplacement of the Sar-Cheshmeh stock took place in several pulses, each with associated hydrothermal activity. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper was concentrated later. Four main vein Groups have been identified: (I) quartz+molybdenite+anhydrite±K-feldspar with minor pyrite, chalcopyrite and bornite; (II) quartz+chalcopyrite+pyrite±molybdenite±calcite; (III) quartz+pyrite+calcite±chalcopyrite±anhydrite (gypsum)±molybdenite; (IV) quartz±calcite±gypsum±pyrite±dolomite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred in the peripheral parts of the stock, contemporaneously with potassic alteration, and phyllic alteration occurred later, overprinting earlier alteration. The early hydrothermal fluids are represented by high temperature (350–520 °C), high salinity (up to 61 wt% NaCl equivalent) liquid-rich fluid inclusions, and high temperature (340–570 °C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent an orthomagmatic fluid, which cooled episodically; the brines are interpreted to have caused potassic alteration and deposition of Group I and II quartz veins containing molybdenite and chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower temperature (220–310 °C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system and mixing with magmatic fluid produced albitization at depth and shallow phyllic alteration. This influx also caused the dissolution of early-formed copper sulphides and the remobilization of Cu into the sericitic zone, the main zone of the copper deposition in Sar-Cheshmeh, where it was redeposited in response to a decrease in temperature.  相似文献   

10.
Glide systems of hematite single crystals in deformation experiments   总被引：1，自引：0，他引：1  
The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}<m>-slip in the temperature range 25 °C to 400 °C, at 400 MPa confining pressure, and a strain rate of 10− 5 s− 1 by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 °C and 400 °C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133–150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 °C at strain rates of 10− 5 s− 1 and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}<m>-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)<a>-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 °C to  5 MPa at 800 °C and for {a}<m>-slip from > 560 MPa at 25 °C to  40 MPa at 700 °C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 °C  60 MPa. The data indicate that the CRSSs above 200 °C seem to be between the values for r-twinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600 °C could be evaluated to  60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.  相似文献   

11.
P. Barbey  D. Gasquet  C. Pin  A.L. Bourgeix   《Lithos》2008,104(1-4):147-163
This study deals with the origin of igneous layering in plutons, and, especially, the extent layering is related to mafic–silicic magma interactions. The Budduso pluton (Sardinia) shows three main scales of organization.(i) Large scale lithological variations correspond to three main magmatic units, with differentiation increasing from the Outer (hornblende-bearing biotite granodiorite/monzogranite) to the Middle (biotite monzogranite) and the Inner (leucomonzogranite) units. The striking homogeneity of 87Sr/86Sr initial ratios (0.7090 ± 4) and εNd(t) values (− 5.6 ± 0.1) strongly suggests that magma isotopic equilibration was achieved prior to emplacement, whereas mixing/mingling structures observed within the pluton reflect second-stage processes involving broadly cogenetic components.(ii) Metre to decametre-scale igneous layering may be isomodal or modally-graded, locally with cross-layering. Biotite and plagioclase compositions are similar in both biotite-rich and quartzofeldspathic layers, as are the trace-element patterns which differ only by relative abundances. This precludes an origin by fractional crystallization. A penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation.(iii) Composite layering and schlieren are commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones or disrupted synplutonic dykes. In that case, there is a progressive shift in biotite XFe values from the core of enclave ( 0.65) to the host monzogranite ( 0.72): schlieren in the monzogranite show biotite XFe values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower XFe values ( 0.69) towards those of enclave rims.These features can be ascribed to three main processes: (i) assembly of differentiated (± mixed/mingled) magmatic pulses; (ii) local hydrodynamic sorting related to density currents in a mush, and segregation of residual melt; (iii) mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics. None of these processes affect the whole pluton but they are limited to specific magmatic units. Therefore, pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes.  相似文献   

12.
The bulk properties and bitumen molecular compositions of a rank-series of 38 humic coals from the New Zealand Coal Band (Cretaceous–Cenozoic) have been analysed to investigate early maturation processes affecting coaly organic matter through diagenesis to moderate catagenesis (Rank(Sr) 0.0–11.8, Ro 0.23–0.81%). The samples comprise a relatively restricted range of vitrinite rich coal types formed largely from higher land plant material under relatively oxic conditions, but with a significant contribution from microbial biomass. With increasing rank, total organic carbon contents show a general increase, whereas moisture and asphaltene contents decrease. Bitumen yields also decrease through the stages of diagenesis and early catagenesis (Rank(Sr) < 9, Ro < 0.55%), indicating partial loss of initial bitumen during early maturation. Thermal generation of hydrocarbons begins slowly at Rank(Sr)  5–6 (Ro  0.40%) as indicated by the constant occurrence and gradual increase of isoprenoids (e.g., pristane and phytane) and hopanoids in their more mature αβ configuration. This early phase of catagenesis, not previously recognised in New Zealand coals, is followed at Rank(Sr)  9 (Ro  0.55%) by the main catagenesis phase characterised by a more rapid increase in the generation of hydrocarbons, including total n-alkanes, isoprenoids and αβ-hopanes. Changes in the maturity of New Zealand coals can be traced by the Carbon Preference Index and several hopane maturity parameters, including 22S/(22S + 22R), αβ/(αβ + βα) and ββ/(αβ + βα + ββ).  相似文献   

13.
Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO2-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO2, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO2 contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO2-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (δ18Oquartz from 9.9‰ to 10.9‰, δDH2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (δ18Oqtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (δ18Omica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins (δ18Oqtz-H2O=−6.4‰) and late quartz (δ18Omica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.  相似文献   

14.
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   

15.
《International Geology Review》2012,54(14):1593-1621
Hasbey Pb-Zn-F mineralization in the Bitlis Massif, south of Lake Van, lies within the Neotethyan suture of the Alpine orogenic belt. Mineralization occurs in two different lithologies and locations: Type-I is present in dolostone fractures and faults as veins and veinlets, whereas Type-II occupies a fault zone between black marbles and calc-schists. Sphalerite and argentiferous galena are the main ore minerals in both types. The dominant gangue minerals are quartz and dolomite in Type-I ore and calcite, quartz, and green-white fluorite in Type-II. Analysed fluid inclusion data from sphalerite, fluorite, and quartz indicate that high-temperature (>500°C) mineralization was initiated from low-salinity fluids (4.3 wt.% NaCl equiv.). As temperatures dropped from 400°C to 160°C, the salinity of solutions increased and appreciable CO2 was contributed to the fluid system. In the absence of immiscibility, assemblages of fluid inclusions containing CO2 indicate that the solutions were homogeneous during entrapment and that mineralization took place under pressure conditions between 5 and 2 kb.

Analysed δ34S CDT (‰) values (?1.5 and??3.8, n?=?15) of sphalerite and galena indicate that the source of the sulphur is consistent with a magmatic origin for Hasbey Pb-Zn-F mineralization. The stable isotopic compositions and fluid inclusions in fluorite are also suggestive of an origin related to high-temperature, high-salinity magmatic fluids. In the region, volcanic rocks are abundant, and they document the magmatic events associated with the closure of the neo-Tethys.

The timing of mineralization is restricted to post-early Oligocene, inasmuch as mineralization occurs in faults that cut post-Eocene–Oligocene thrust faults and because of the relationship between mineralization and wall-rock deformation.  相似文献   

16.
The Late Middle Permian ( 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma.The oxide-bearing intrusions have higher Al2O3, FeO (as total iron) and total alkalis (Na2O + K2O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of Nd(t) values. All the intrusions, however, have Nd(t) values ranging from − 3.9 to + 4.6, and initial 87Sr/86Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe2O3 (2.5 to 13 wt.%) with initial 87Sr/86Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.  相似文献   

17.
Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)   总被引：1，自引：0，他引：1  
Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO2 (pCO2) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO2 emission to the atmosphere (9.1 ± 14.2 mmol m−2 d−1). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ13C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ13C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.  相似文献   

18.
The carbonate-hosted Kabwe Pb–Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE–SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu.Galena, sphalerite and pyrite from the Pb–Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean δ34SCDT permil (‰) values of − 17.75 ± 0.28 (1σ), − 16.54 ± 0.0.27 and − 15.82 ± 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of − 11.70 ± 0.5‰ δ34S for sphalerite and of − 11.91 ± 0.71‰ δ34S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur.Carbon and oxygen isotope ratios of the host dolomite have mean δ13CPDB and δ18OSMOW values of 2.89‰ and 27.68‰, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO2 content introduced during silicification of the host dolomite. The depletion in 18O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and 18O-rich hydrothermal solution.Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 °C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt.% NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt.% NaCl.Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.  相似文献   

19.
Deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation (CPO) development. Cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol.% of added water and 20 mg of Mn2O3 powder in a Griggs solid medium deformation apparatus in different crystallographic orientations with compression direction: (i) parallel to <c>, (ii) at 45° to <c> and 45° to <a> and (iii) parallel to <a>. The experiments were performed at a temperature of 800 °C, a confining pressure of 1.2 GPa, a strain rate of  10− 6 s− 1, to bulk finite strains of  14–36%. The deformed samples were analysed in detail using optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Two different microstructural domains were distinguished in the deformed samples: (i) domains with undulatory extinction and deformation lamellae, and (ii) domains with new recrystallised grains. Within the domains of undulatory extinction, crystal-plastic deformation caused gradual rotations of the crystal lattice up to  30° away from the host orientation. New recrystallised grains show a strong CPO with c-axis maxima at  45° to the compression direction. This is the case in all experiments, irrespective of the initial crystallographic orientation. The results show that c-axes are not continuously rotated towards the new maxima. The new grains thus developed through a mechanism different from subgrain rotation recrystallisation. New grains have a subeuhedral shape and numerous microcavities, voids, fluid channels and fluid inclusions at their grain boundaries. No host control is recorded in misorientation axes across their large angle grain boundaries. New grains might have been created by nucleation from solution in the μm-scale voids and microfractures. The CPO most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions.  相似文献   

20.
Cleats and fractures in southwestern Indiana coal seams are often filled with authigenic kaolinite and/or calcite. Carbon- and oxygen-stable isotope ratios of kaolinite, calcite, and coalbed CO2 were evaluated in combination with measured values and published estimates of δ18O of coalbed paleowaters that had been present at the time of mineralization. δ18Omineral and δ18Owater values jointly constrain the paleotemperature of mineralization. The isotopic evidence and the thermal and tectonic history of this part of the Illinois Basin led to the conclusion that maximum burial and heat-sterilization of coal seams approximately 272 Ma ago was followed by advective heat redistribution and concurrent precipitation of kaolinite in cleats at a burial depth of < 1600 m at  78 ± 5 °C. Post-Paleozoic uplift, the development of a second generation of cleats, and subsequent precipitation of calcite occurred at shallower burial depth between  500 to  1300 m at a lower temperature of 43 ± 6 °C. The available paleowater in coalbeds was likely ocean water and/or tropical meteoric water with a δ18Owater  − 1.25‰ versus VSMOW. Inoculation of coalbeds with methanogenic CO2-reducing microbes occurred at an even later time, because modern microbially influenced 13C-enriched coalbed CO2 (i.e., the isotopically fractionated residue of microbial CO2 reduction) is out of isotopic equilibrium with 13C-depleted calcite in cleats.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号