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1.
The Early Cretaceous coal deposits of the Khasyn coalfield are intruded by Palaeogene diabase dikes. The coal has vitrinite reflectance values of 2.0–2.5% Ro, and characteristics of normal anthracite at some distance from the dikes, but at direct contact with the dike two morphological coal varieties occur: coal inclusions in the diabase dike and dispersed carbonaceous matter within the dike rock. Both types of coaly matter have properties typical of anthracites: strong anisotropy, altered internal structure and high vitrinite reflectance values ranging from 3.8 to 5.5% Ro. The X-ray diffraction measurements of the interplanar spacing d(002) and the crystallite sizes Lc and La show rather similar values for coal inclusions in the dike and dispersed carbonaceous matter. The additional reflection at 3.37 Å, corresponding to semi-graphite admixture, occurs in the coal and carbonaceous matter inside the dike and is absent in the natural coal outside the dike.  相似文献   

2.
Epidote phenocrysts in dacitic dikes,Boulder County,Colorado   总被引：1，自引：0，他引：1  
Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H2O and O2 activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.  相似文献   

3.
The Zaldívar porphyry copper deposit, Northern Chile, consists of two major intrusions, the 290 Ma Zaldívar, and the more recent Miocene (38.7 Ma) Llamo porphyry. Five types of inclusions have been identified in quartz phenocrysts from Llamo porphyry, including melt inclusions (M), and four types of fluid inclusions, called MS (multi solids), B (brines), G (vapor-rich) and W (aqueous), respectively.Melt remnants, well preserved as M-inclusions, homogenize around 1000 °C. They show a rhyolitic composition, comparable to the most evolved acidic rhyolitic end member found elsewhere in the regional magmatism and to worldwide volcanic rhyolitic glass. High silica content in some inclusions can, however, be due to partial remelting of the quartz host during the heating run. Copper content in the same inclusions ranges between 0.03 and 0.57 wt.%, with an average concentration of 0.10 wt.%, suggesting a major magmatic source for the copper (orthomagmatic model).MS inclusions, which contain a number of solids at room temperature, mostly H2O-bearing phases (system NaCl–KCl–((Fe, Mg, Cu)Cl)–H2O, average salinity 70 wt.% NaCl equiv.), homogenize at magmatic temperatures (around 1000 °C). They represent the first fluids to have exsolved from the magma at depth, at a pressure of about 2 kbar. Their high homogenization temperature, comparable to values measured for melt inclusions (1000 to 1050 °C), may indicate trapping of MS and M inclusions in host phenocrysts from an immiscible mixture of silicate melt and highly saline fluids expelled from the magma during the early stage of quartz crystallization.The data indicate a magmatic origin for copper, as well as extremely high melt temperatures. These features are interpreted by magmatic differentiation of mantle-derived primitive melts, corresponding to major changes in the tectonic regime of the Andean margin, which occurred in Miocene times.  相似文献   

4.
At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) 2 – , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO2. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.  相似文献   

5.
SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures.The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids (δ18Ofluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes.Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks (δ34S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation.The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.  相似文献   

6.
Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10−3 (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR0–30, pAAR30–60 and pAAR60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of Kmax inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (Kmax<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.  相似文献   

7.
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   

8.
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9.
In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H2O–CO2–CH4–NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ18O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δD compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δD and the calculated δ18O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ34S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.  相似文献   

10.
Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu–Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2–10 wt% NaCl eq.; ~0.1–0.3 g cm–3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic–hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1 ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H2S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS– ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic–hydrothermal systems, including porphyry-Cu–Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore styles.Editorial handling: B. Lehmann  相似文献   

11.
Proterozoic calcsilicate rocks in contact with the different types of granite from the granitic belt of northern Guinea show particular mineral assemblages, recording different steps of the tectono-metamorphic and magmatic evolution of the area. Petrological study provides evidence of a clockwise metamorphic P–T path with a metamorphic peak at temperature around 800 °C and pressure of 4–6 kb, corresponding to the emplacement of both generations of massive granite between 2115 and 2075 Ma. Retrograde metamorphism is characterized by decompression to 2–3 kb, associated with the emplacement of late small granite stocks and followed by cooling until 450–600 °C. Hydrothermal alteration involved by late fluid circulation is only weakly developed and limited to calcsilicate/granite contact (specially small stocks) and shear zones. Early fluids were essentially metamorphic and magmatic fluids, largely buffered by calcsilicate mineral assemblages, whereas surface-derived fluids were introduced at the end of the tectonic evolution via shear-zones.  相似文献   

12.
Gold mineralisation at the Wallaby gold deposit is hosted by a 1,200 m thick mafic conglomerate. The conglomerate is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic-monzonite, monzonite, syenite, syenite porphyry to late-stage carbonatite. In the mine area, a pipe-shaped zone of actinolite-magnetite-epidote-calcite (AMEC) alteration overprints the conglomerate. Gold mineralisation, associated with dolomite-albite-quartz-pyrite alteration, is hosted in a series of sub-horizontal, structurally controlled zones that are largely confined within the magnetite-rich pipe. The deposit has a current ore reserve of 2.0 Moz Au, and a total resource of 7.1 Moz Au.TIMS U–Pb analysis of magmatic titanite and SHRIMP U–Pb analysis of gold-related phosphate minerals are used to constrain the timing of magmatism and gold mineralisation at Wallaby. Monzogranite and carbonatite dykes of the Wallaby syenite intruded at 2,664±3 Ma, at least 5 m.y. and probably 14 m.y. before gold mineralisation at 2,650±6 Ma. The significant hiatus between proximal magmatism and gold mineralisation suggests that gold-bearing fluids were not derived from magmas associated with the Wallaby syenite, particularly since intrusive events are unlikely to drive hydrothermal systems for more than 1 m.y.Analysis of the C and O isotopic compositions of carbonates from regional pre-syenite alteration and AMEC alteration at the Wallaby gold deposit suggests that AMEC alteration formed via interaction between magmatic fluids and the pre-syenite wallrock carbonate. The C and O isotopic composition of gold-bearing fluids, as inferred from ore-carbonate, are isotopically distinct from proximal magmatic fluids, as inferred from magmatic carbonate in carbonatite dykes.Thus, detailed isotopic and geochronological studies negate any direct genetic link between proximal magmatic activity related to the Wallaby syenite and gold mineralisation at Wallaby. The gold endowment of the Wallaby gold deposit, combined with the relatively low solubility of gold as thiosulfide complexes in low-salinity ore fluids at temperatures of about 300°C, implicates the influx of very large volumes of auriferous hydrothermal fluids. No large-scale shear-zones nor faults through which such large fluid-volumes could pass have been identified within the immediate ore environment, so fluid influx most probably occurred largely in a unit-confined, brittle-ductile fracture system. This was the ~500-m diameter AMEC alteration pipe, which was a brittle, iron-enriched zone in an otherwise massive conglomerate. During compressional deformation, the competency contrast between unaltered and AMEC-altered conglomerate created a zone of increased fracture permeability, and geochemically favourable conditions (high Fe/Fe+Mg ratio), for gold mineralisation from a distal source.Editorial Handling: C. Brauhart  相似文献   

13.
Changes in high-volatile bituminous coal (Pennsylvanian) near contacts with two volcanic intrusions in Illinois were investigated with respect to optical properties, coal chemistry, and coal pore structure. Vitrinite reflectance (Ro) increases from 0.62% to 5.03% within a distance of 5.5 m from the larger dike, and from 0.63% to 3.71% within 3.3 m from the small dike. Elemental chemistry of the coal shows distinct reductions in hydrogen and nitrogen content close to the intrusions. No trend was observed for total sulfur content, but decreases in sulfate content towards the dikes indicate thermochemical sulfate reduction (TSR). Contact-metamorphism has a dramatic effect on coal porosity, and microporosity in particular. Around the large dike, the micropore volume, after a slight initial increase, progressively decreases from 0.0417 cm3/g in coal situated 4.7 m from the intrusive contact to 0.0126 cm3/g at the contact. Strongly decreasing mesopore and micropore volumes in the altered zone, together with frequent cleat and fracture filling by calcite, indicate deteriorating conditions for both coalbed gas sorption and gas transmissibility.  相似文献   

14.
The Lackawanna synclinorium of Pennsylvania contains abundant coal that was altered to anthracite rank during the Late Paleozoic Alleghanian orogeny. Why did the coal in the synclinorium undergo anthracitization? Two alternatives have been suggested. (1) The region was buried deeply during and/or after the Alleghanian orogeny and thus became hot in response to Earth's geothermal gradient; and (2) hot fluids migrating from the orogen toward the foreland carried heat into the coal and caused anthracitization. If the second hypothesis is correct, the region should contain evidence that hot fluids passed through the coal. Field observations, illite-crystallinity studies and fluid-inclusion analyses indicate that the coal-bearing Pottsville and Llewellyn Formations, and an underlying detachment fault, called the Pottchunk fault (at or slightly below the base of the Pottsville Formation), acted as a regional aquifer for the migration of hot fluids during the Alleghanian orogeny. The presence of quartz veins and the hydrothermal minerals tosudite and pyrophyllite in strata above the Pottchunk fault, and the apparent absence of an illite-crystallinity burial gradient in the underlying strata, suggest that fluids migrated through large pores in coarse-grained sandstones, through abundant fractures that developed in response to Alleghanian deformation, and along the Pottchunk fault. The quartz veins, tosudite and pyrophyllite mineralization do not occur below the Pottchunk fault (except at one locality), suggesting that fluid flow was greater above the fault, perhaps because fracturing accompanying deformation increased permeability of the fault's hanging wall. Fluid inclusions in the Pottsville and Llewellyn Formations indicate fluids achieved a minimum temperature of 270 °C at a depth of 3.1–8.5 km. Heat-flow calculations constrained by fluid-inclusion data show the heat carried by the migrating fluids through the coal-bearing strata was sufficient to elevate the rock temperature to anthracite-grade conditions (250 °C), assuming that the fluid event lasted about 1 year. Thus, deep burial (6–9 km) of the coal-bearing strata in the Lackawanna synclinorium was not required for anthracitization. Anthracitization was likely the result of stratigraphically controlled hot fluid migration through the coal-bearing strata at shallow depths (≤5 km).  相似文献   

15.
In the Goiás Alkaline Province, in central Brazil, some mafic–ultramafic alkaline bodies crop out, such as Morro do Engenho Complex (ME), whereas other intrusive bodies are completely covered by Quaternary sediments, such as A2 body, and can only be defined by their magnetic and gravity responses. The bodies give rise to strong gravity and magnetic anomalies. We inverted the gravity and magnetic data to estimate 3D density and magnetic intensity contrasts. The gravity inversion reveals that ME and A2 intrusions, totally emplaced in the upper crust, have maximum depths of about 10 and 6 km, respectively. The estimated density contrast distribution indicates a NE–SW preferential direction for ME intrusive body. Since the remnant magnetization is very important in the area, we inverted the magnetic data, testing two hypotheses: only induced field and induced plus remnant magnetizations. The results of the inversion using only induced field could not explain the observed magnetic field, which was about 1000 nT larger than the calculated one. Induced plus remnant field inferred from wandering polar path fitted the observations. 3D magnetic inversions when remnant magnetization was included recover deeper and thinner bodies compared with gravity inversion. The inverted bodies have spherical shapes which are more consistent with the model of magmatic chambers instead of plug intrusions.  相似文献   

16.
Unlike long-term heating in subsiding sedimentary basins, the near-instantaneous thermal maturation of sedimentary organic matter near magmatic intrusions is comparable to artificial thermal maturation in the laboratory in terms of short duration and limited extent. This study investigates chemical and H, C, N, O isotopic changes in high volatile bituminous coal near two Illinois dike contacts and compares observed patterns and trends with data from other published studies and from artificial maturation experiments. Our study pioneers in quantifying isotopically exchangeable hydrogen and measuring the D/H (i.e., 2H/1H) ratio of isotopically non-exchangeable organic hydrogen in kerogen near magmatic contacts. Thermal stress in coal caused a reduction of isotopically exchangeable hydrogen in kerogen from 5% to 6% in unaltered coal to 2-3% at contacts, mostly due to elimination of functional groups (e.g., OH, COOH, NH2). In contrast to all previously published data on D/H in thermally matured organic matter, the more mature kerogen near the two dike contacts is D-depleted, which is attributed to (i) thermal elimination of D-enriched functional groups, and (ii) thermal drying of hydrologically isolated coal prior to the onset of cracking reactions, thereby precluding D-transfer from relatively D-enriched water into kerogen. Maxima in organic nitrogen concentration and in the atomic N/C ratio of kerogen at a distance of ∼2.5 to ∼3.5 m from the thicker dike indicate that reactive N-compounds had been pyrolytically liberated at high temperature closer to the contact, migrated through the coal seam, and recombined with coal kerogen in a zone of lower temperature. The same principle extends to organic carbon, because a strong δ13Ckerogen vs. δ15Nkerogen correlation across 5.5 m of coal adjacent to the thicker dike indicates that coal was functioning as a flow-through reactor along a dynamic thermal gradient facilitating back-reactions between mobile pyrolysis products from the hot zone as they encounter less hot kerogen. Vein and cell filling carbonate is most abundant in highest rank coals where carbonate δ13CVPDB and δ18OVSMOW values are consistent with thermal generation of 13C-depleted and 18O-enriched CO2 from decarboxylation and pyrolysis of organic matter. Lower background concentrations of 13C-enriched carbonate in thermally unaffected coal may be linked to 13C-enrichment in residual CO2 in the process of CO2 reduction via microbial methanogenesis.Our compilation and comparison of available organic H, C, N isotopic findings on magmatic intrusions result in re-assessments of majors factors influencing isotopic shifts in kerogen during magmatic heating. (i) Thermally induced shifts in organic δD values of kerogen are primarily driven by the availability of water or steam. Hydrologic isolation (e.g., near Illinois dikes) results in organic D-depletion in kerogen, whereas more common hydrologic connectivity results in organic D-enrichment. (ii) Shifts in kerogen (or coal) δ13C and δ15N values are typically small and may follow sinusoidal patterns over short distances from magmatic contacts. Laterally limited sampling strategies may thus result in misleading and non-representative data. (iii) Fluid transport of chemically active, mobile carbon and nitrogen species and recombination reactions with kerogen result in isotopic changes in kerogen that are unrelated to the original, autochthonous part of kerogen.  相似文献   

17.
新疆东准噶尔松喀尔苏铜金矿区斑岩型矿床成因研究   总被引：1，自引：0，他引：1  
松喀尔苏铜金矿区位于卡拉麦里石炭纪陆相火山岩带。文章通过矿床地质、围岩蚀变、含矿斑岩、流体包裹体和同位素研究,探讨了矿床成因类型。研究表明,松喀尔苏矿床具斑岩型矿床的特征,铜金矿化体产于岩体接触带,围岩蚀变具有分带性,从岩体向围岩依次发育绢英岩化带、高岭石化带和青磐岩化带,绢英岩化带与成矿相关。含矿斑岩复式岩体系同期陆相火山活动产物,成矿作用在时间、空间和成因上与复式岩体中晚期花岗斑岩有关。花岗斑岩具有富水、富挥发性组分和岩浆爆破作用的氧化性岩浆特点,具有后碰撞花岗岩类的地球化学亲缘性,其岩浆起源于后碰撞挤压-伸展转换期的壳-幔岩浆过渡带。幔源岩浆注入、软流圈地幔底侵作用和壳-幔岩浆混合作用是形成含矿斑岩岩浆的主导因素。流体包裹体包括液相包裹体、气相包裹体和含子晶多相包裹体,激光拉曼探针分析表明,气相成分以CO2和CH4为主。成矿流体具有从高温、高盐度岩浆体系向低温、低盐度与大气降水混合的演化过程,流体沸腾或不混溶作用及温度、盐度降低是导致流体中成矿物质沉淀的主要因素。氢、氧同位素组成表明成矿流体以岩浆水为主,在成矿晚期混有大气降水。硫同位素具幔源硫的特征。铅同位素组成显示成矿作用起源于下地壳-上地幔过渡带的岩浆作用。综上所述,该矿床属于与陆相火山-侵入岩有关的斑岩型铜金矿床。  相似文献   

18.
Jun Matsushima  Yasukuni Okubo   《Tectonophysics》2003,371(1-4):141-152
We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.  相似文献   

19.
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   

20.
The Jurassic Shah-Kuh granite pluton was emplaced in the northeastern part of the Lut Block (Eastern Iran) while this block was a part of the active margin under which the Tethys ocean, that separated Arabia from Central Iran, was subducting. Since this time, the Lut Block has rotated and migrated northward up to its present position. This structural study of the Shah-Kuh aims at strengthening the evidence of its original geodynamical location. Thanks to a systematic collection of oriented samples in the field, well-defined fabric and microstructural patterns were deduced from magnetic fabric measurements and optical microscopy observations. The overall magnetic fabric of the pluton yields vertical, north–south striking foliations and shallow lineations plunging to the north. The corresponding microstructures attest to their formation in the magmatic stage, i.e. during emplacement. Subsequent deformation, characterized by (sub)mylonitic microstructures, has modified the original fabric into a northwest and locally a west–northwest trend inside a two kilometre-wide corridor. This corridor reflects the trace of a sinistral shear zone that offsets the previous north–south magmatic pattern of the pluton. The latter magmatic pattern is proposed to result from the shear component of strain that was parallel to the ancient active margin during pluton emplacement, as a result of strain partitioning, a situation frequently documented at convergent margins. Sinistral shear along an  E–W directed south-facing active margin, assuming a northeastward slip vector for the Tethys ocean during the Jurassic, and accepting a  150° counterclockwise rotation of the Lut Block after emplacement of the Shah-Kuh pluton, best explains both the magmatic and the solid-state lineation patterns.  相似文献   

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