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1.
Ore deposits are frequently associated with various types of rock alteration that may affect the physical properties of rocks, and result in a characteristic signature of the deposits. The present work focuses on the use of geophysical data in targeting areas hosting Fe-oxide Cu–Au (IOCG) deposits west of the Kiruna mining district, northern Sweden. It is noted that IOCG deposits in different areas worldwide (e.g., Australia, Canada) are preferentially located in proximity to faults associated with deformation zones of regional relevance, and that the deposits are hosted in areas affected by potassic to sodic alteration. This study shows that at a semi-regional scale IOCG deposits occur within areas of high positive gravity anomalies, high values of estimated potassium content and high K/Th ratios, which is an expression of potassic alteration. High magnetic anomalies are associated with magnetite occurrences, while low linear magnetic anomalies mark fault/shear zones, in proximity of which the majority of the IOCG deposits occur.  相似文献   

2.
The Mesoproterozoic deeply eroded Keurusselk? impact structure in central Finland is situated within the ??1860?C1890 Ma Central Finland Granitoid Complex. An estimate for the original size of the structure is 30 km, yielding a 5 km wide central uplift with insitu shatter cones and shock metamorphic features in quartz. Petrophysical and rock magnetic properties of the three shallow drill cores (V-001, V-002 and V-003) in the vicinity of the central uplift are determined in order to assess the dimensions of the central uplifts magnetic anomalies. The drill core lithologies consist of schists (metagraywackes), metavolcanic rocks, gneisses and breccia. Petrophysical properties of the drill core rocks show average densities (D) of 2644?C2752 kg/m3, susceptibilities (??) of 160?C761 × 10?6 SI and natural remanent magnetization (NRM) of 3?C306 mA/m and Koenigsberger Q ratios of 0.1?C10. Rock magnetic measurements with temperature dependence of susceptibility (??-T) curves and hysteresis indicated mostly paramagnetic behaviour. However, a fraction of fine-grained ferromagnetic minerals (pyrrhotite and magnetite) was detected from all lithologies. Breccia veins cutting the parautochthonous subcrater floor show lower values of petrophysical properties (D, ??, NRM, Q) and this could be related to the impact event. Amphiboles and micas in the breccia are strongly altered and replaced by secondary chlorite. Chloritization may indicate widespread impact-induced hydrothermal alteration of the target rocks or it may be related to regional tectonic shearing. However, planar deformation features in quartz, found from shatter cones in the central uplift area, are decorated with fluid inclusions indicating that alteration by post-impact processes was present.  相似文献   

3.
A magnetic method is proposed for reconstructing the paleoclimate existing at the formation time of rocks of the Paleolithic Kostenki-12 site section. Results obtained from an experimental study of scalar magnetic characteristics (SMCs) of the rocks (magnetic susceptibility, anhysteretic remanent magnetization, and saturation isothermal remanent magnetization) are presented. These results indicate complex along-section SMC behavior distinguished by alternating larger and smaller values of the parameters. The magnetic minerals of the section are mainly represented by pseudo-single-domain magnetite grains. The size of the magnetite grains in the pseudo-single-domain range varies along the section, increasing in its lower half. The SMCs values plotted on different scales for the upper and lower halves of the section clearly revealed 12 magnetic zones (MZs). Comparison of the MZs with palynological zones obtained in a parallel section showed, on the whole, a convergence of data: MZs with larger (smaller) SMC values correlate with thermomers (cryomers). Comparison of along-section SMC variations with the GRIP (Greenland Ice Core Project) curve of the time dependence of paleotemperatures confirmed or refined the ages of lithologic horizons. As a result, it is established that constraints on paleoclimatic changes in the rock formation time can be gained from SMC variations if the main carriers of magnetization in rocks are pseudo-single-domain magnetite grains even with magnetic grain size varying along the section.  相似文献   

4.
华北地块早古生代地层单元的岩石磁学特征研究   总被引：1，自引：1，他引：1       下载免费PDF全文
通过对采自华北地块西部鄂尔多斯盆地边缘早古生代地层单元中的４４块定向岩芯样品的岩石磁学实验（饱和等温剩磁及剩磁矫顽力的测量，三轴磁化的饱和等温剩磁的系统热退磁和低温实验）研究，揭示出华北地块早古生代地层单元的各类岩石中，主要载磁矿物的构成具有磁铁矿或赤铁矿与中等居里温度的磁黄铁矿、磁赤铁矿等共存的特征；个别地层单元的岩石中以极低居里点的针铁矿为主要载磁矿物．  相似文献   

5.
A thick sequence of alkaline intrusions and volcanic rocks underlies a Quaternary caldera on Lihir Island. The sequence is host to a still-active subaerial hydrothermal system and associated epithermal gold mineralization. Chondrite-normalized (La/Lu)cn and (La/Sm)cn ratios progressively increase up the alteration sequence from the potassic alteration zone, to the argillic zone to the advanced argillic zone. (Tb/Lu)cn ratios only significantly increase in argillic and advanced argillic assemblages. Surface oxide alteration lithologies and acid sulphate water precipitates possess distinctly lower (La/Lu)cn and (Tb/Lu)cn ratios than the underlying subsurface alteration units. The changes in the REE, LREE and HREE fractionation trends from subsurface to surface alteration zones reflect the transition from a magmatic-hydrothermal, neutral chloride fluid regime at depth to acid sulphate meteoric waters in the upper portion of the alteration profile.Boiling of the LREE-Eu-enriched magmatic fluids occurred at a depth of at least 750 m. It is proposed that pronounced differential flow rates of the vapour and liquid phases and solution chemistry changes approximately above 300–350 m caused the incorporation of LREE and Eu into anhydrite-calcite veins and the deposition of LREE and HREE into wallrocks of upper parts of the potassic alteration unit. Condensation of the vapour phases into meteoric waters gave rise to low-temperature acid fluids that deposited large amounts of LREE within argillic and advanced argillic alteration units. This was also accompanied by HREE mobility due to large fluid volumes, acid fluid conditions and abundant sulphate complexes within the solutions. The HREE were either lost from the hydrothermal system or deposited in oxide assemblages and acid sulphate water precipitates.  相似文献   

6.
Hydrothermal alteration zones have been investigated by X-ray diffraction, mineralogical–petrographical techniques, and geochemical analysis. Examination of cores and cuttings from two drill sites, obtained from a depth of about 814–1020 m, show that the hydrothermal minerals occuring in the rock include: K-feldspar, albite, chlorite, alunite, kaolinite, smectite, illite, and opaque minerals.In the studied area, silicified, smectite, illite, alunite, and opal zones have been recognized. These alteration mineral assemblages indicate that there are geothermal fluids, which have temperatures of 150–220°C in the reservoir.The distribution of the hydrothermal minerals shows changes in the chemical composition of the hydrothermal fluid, which are probably due not only to interaction with host rock, but also to dilution of the Na–K–Cl-rich hydrothermal fluid of the deep reservoir by cold sea water at shallow levels. Geochemical analyses of the solid and liquid phases indicate that the hydrothermal fluids of the Tuzla geothermal system are in equilibrium with alteration products.The tectonic structure of the studied area is caused by NW–SE and NE–SW directional forces. The volcanic rocks where hydrothermal zones are observed in the studied area are of Lower–Middle Miocene age comprise latite, andesite, dacite, rhyolite-type lavas, tuff, and ignimbrites.  相似文献   

7.
Dunite samples from a borehole drilled in the platiniferous concentrically-zoned Kondyor Massif are studied by electron spin resonance (ESR). The spectrum profiles, relative intensities I, and volume magnetic susceptibilities κ are analyzed. These values experience strong irregular variations, sometimes by an order of magnitude, in the upper and medium parts of the column, at depths from 100 to 400 m; and the variations decay at greater depths. The magnetic properties of the samples are determined by iron (II) ions in the olivine lattice and by iron (III) ions in the magnetite and pyrrhotite microphases and in the products of breakdown of the solid solution: chromiferous magnetite, chromoferrite, etc. The I and κ values are directly related: κmax = 27.8 × 10−3 SI units, κmin=2.63 × 10−3 SI units, and κmean = 12.7 × 10−3 SI units. The maximum κ values are found in the zones with elevated contents of magnetite and pyrrhotite particles, and the minimum ones, in zones with few medium and small clusters with Fe3+ ions. The uneven distributions of solid solutions and magnetic phases over depths are suggested to be related to the disturbances in the conditions of crystallization.  相似文献   

8.
The processes that result in arc magmas are critical to understanding element recycling in subduction zones, yet little is known about how these systems evolve with time. Nicaragua provides an opportunity to reconstruct the history of a volcanic arc since the Cretaceous. Here we present the stratigraphy of the Cretaceous–Eocene volcanic units in Nicaragua and their relationship to the different tectonic units where the arc developed. We discovered an evolution from an arc‐dominated by calc‐alkaline compositions in the Cretaceous–Eocene, to transitional compositions in the Oligocene–Miocene, to finally tholeiitic magmas common in the modern volcanic front. Our petrographic studies confirm that in the Cretaceous–Eocene the olivine + clinopyroxene cotectic was followed by clinopyroxene + plagioclase ± amphibole. Given the abundance of amphibole and the lack of this mineral in the modern volcanic front, the Cretaceous–Eocene Arc melts were likely more water‐rich than modern Nicaragua, suppressing the crystallization of plagioclase after olivine. We also found temporal changes in element ratios that are sensitive to variations in sediment input. The Cretaceous–Eocene Arc is characterized by a lower Ba/Th compared to the Oligocene–Miocene and modern volcanic front samples, suggesting that the sediment input was lower in Ba, possibly analogous to old deep siliceous sediment subducting in the western Pacific. Both U/Th and U/La are higher in the modern volcanics, reflecting higher U/Th in the subducting sediments following the 'Carbonate Crash'. Finally, we found that the orientation of the arc axis also changed, from northeast‐southwest in the Cretaceous–Eocene to northwest‐southeast after the Oligocene. This change probably records variations in the location of the subduction zone as this region shaped into its current geographic configuration.  相似文献   

9.
Multifractal theory was developed for handling scale invariant fields instead of geometry only[1―4]. From a multifractal point of view, some fractal models, ordinary physical processes and relevant probability distribution types can be considered as special cases of multifractal models which provides new insight into the interrelationships between systems and subjects. For example, the low order moment exponents τ (0), τ (1), τ (2) or τ ″(1) obtained by means of the moment method determi…  相似文献   

10.
The loci and abundance of U and Th were examined in tuffaceous rocks encompassing hydrothermal systems at the Long Valley caldera, California and the Valles caldera, New Mexico. Aspects of these systems may be analogous to conditions expected in a potential site for a high-level waste repository in welded tuff. Examination of radioelements in core from scientific drill holes at these sites was accomplished by gamma-ray spectrometry and fission-track radiography. In the lateral-flowing hydrothermal system at the Long Valley caldera, where temperatures range from 140 to 200 °C, U is concentrated to 20 ppm in Fe-rich zones of varved tuff and to 50 ppm with Fe-rich mineral phases in tuff fragments of a calcite-cemented breccia. U-series disequilibrium in some of these samples suggests mobilization/deposition of parent U and/or its daughters. In the vapor zone of the Valles caldera's hydrothermal system (temperature ˜ 100 °C), the concordance of high U, low Th/U and decreasing whole-rock O-isotope ratios suggests that U was concentrated in response to hydrothermal circulation when the system was formerly liquid-dominated. In the underlying present-day liquid-dominated zone (temperature to 210 °C), U, up to several tens of parts per million, occurs with pyrite and Fe-oxide minerals, and in concentrations to several percents with a Ti-Nb-Y-rare earth mineral. In the Valles system's outflow zone, U is also concentrated in Fe-rich zones as well as in carbonaceous-rich zones in the Paleozoic sedimentary rocks that underlie the Quaternary tuff. Th, associated with accessory minerals, predominates in breccia zones and in a mineralized fault zone near the base of the Paleozoic sedimentary sequence. Relatively high concentrations of U occur in springs representative of water recharging the Valles caldera's hydrothermal system. In contrast, considerably lower U concentrations occur in hot waters (> 220 °C) and in the system's outflow plume, suggesting that U is concentrating in the hotter part of the system. The Long Valley and Valles observations indicate that U and Ra are locally mobile under hydrothermal conditions, and that reducing conditions associated with Fe-rich minerals and carbonaceous material are important factors in the adsorption of U, and thus can retard its transport in water at elevated temperature.  相似文献   

11.
Rock magnetic investigations of Permo-Carboniferous carbonate sediments from two areas on Spitsbergen are described, conducted to identify the carriers of the NRM in these rocks. Since microscopic and magnetic separation techniques could not profitably be applied, the nature of magnetic minerals was investigated by thermal demagnetization of the NRM and decay of saturation isothermal remanence (Irs) during heating to 600°C, as well as by the distribution of the median destructive fields of the NRM and observation of magnetic susceptibility after subsequent heatings. The results show that the NRM of these limestones resides mainly in magnetite, but creation of magnetic pyrrhotite and of fresh magnetite is observed during heating to 600°C. Presence of sulphides indicates that magnetite is an oxidation product of pyrite or of non-magnetic pyrrhotite. Examination of rock magnetic properties of limestones leads to the conclusion that most of the magnetite in the rocks of the Bellsund area is of detrital origin, whereas the rocks at Festningen contain magnetite derived from pyrite probably during an early stage of the diagenetic process.  相似文献   

12.
A number of lode–gold occurrences are hosted by hydrothermally altered greenstones along the southern boundary of the Palaeoproterozoic Central Lapland Greenstone Belt. The hydrothermally altered and mineralised zones are related to a major thrust and shear zone system that extends much across northern Finland. Spatial correlation between mineralized zones, brittle structural features and chemical alteration was explored and identified from high-resolution aeromagnetic data, in combination with airborne electromagnetic and gamma-ray spectrometric data and coupled with petrophysical and palaeomagnetic studies. The most prominent magnetic, ductile signatures formed during the Svecofennian Orogeny (1900–1800 Ma), resulting in elastic, curved, continuous magnetic patterns. These elastic anomaly patterns were disturbed by tectonic stress from S–SW, resulting in parallel, regularly oriented fracture families and thrust faults normal to the main stress direction. According to aeromagnetic, palaeomagnetic and structural evidence, the thrust zone was active during the latest stage of the orogenic event, but was also reactivated at a later date. Airborne gamma-ray data reveals zones of potassic alteration in the ultramafic rock units in the vicinity of cross-sections of these two fault sets. Chemical and mineralogical changes during alteration and metamorphism strongly affected the mafic and ultramafic host rocks throughout the deformation zone. The strong potassium enrichment and coinciding destruction of magnetic minerals resulted in enhanced potassium concentration and reduction of magnetic anomaly amplitudes. Palaeomagnetic results indicate that the remanent magnetization for the altered ultramafic rocks along the thrust zone is of chemical origin (CRM) and was acquired at 1880–1840 Ma, which is presumed also to be the age of the chemical alteration related to gold mineralization.  相似文献   

13.
A characteristic magnetic transition at 30–34 K is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low-temperature techniques as applied to magnetite and hematite, extended down to liquid helium temperature. A review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals. Unblocking temperature up to 350°C and extreme resistance against AF may be encountered in fine grained pyrrhotite.  相似文献   

14.
对采自河南嵩县祁雨沟7号角砾岩筒的黄铁矿样品进行了详细的磁化率随温度变化特征研究，加热的最高温度为700℃，结果表明黄铁矿受热分解最终生成磁黄铁矿.黄铁矿通过两个可能的途径转变为磁黄铁矿：一个是黄铁矿颗粒首先经表面吸附氧的氧化转变为磁铁矿，随温度升高新生成的磁铁矿与黄铁矿晶格中挥发出的硫进一步反应转变为磁黄铁矿；另一可能途径是黄铁矿直接脱硫转变为磁黄铁矿，此反应为磁黄铁矿的主要来源.氩气环境下加热过程中，从约380℃开始即有磁铁矿生成，但直到约535℃才有磁黄铁矿的生成.在535℃～560℃的加热过程中生成的磁黄铁矿居里温度不稳定，直到加热至高于约560℃时才生成居里温度稳定的磁黄铁矿.氩气环境下，黄铁矿受热生成的磁性矿物的类型仅受最高温度的控制，与高温段的滞留时间无明显关系，而其生成量同时受最高温度和高温段滞留时间的控制.  相似文献   

15.
Bata.  AT 《地球物理学报》1997,40(2):239-246
根据航磁、重力和地震数据以及地质和辅助地球物理资料，对约旦东北部前寒武纪岩石的轮廓和变化及其上覆的沉积岩石的厚度进行了研究．识别出5个具有特定磁性特征的磁场区，每个磁场区都有其特征的样式和突变的边界，每个磁场区的物质组成存在明显差异，其构造边界均与断层相对应．计算表明基底表面有很大起伏，磁化的前寒武纪岩石深度变化范围为-5000m至-10000m，可以识别出由基底下陷相对应的5个盆地或拗陷带，同时可见3个起伏较大的构造隆起．探测结果表明，古生代建造中发育的主要断裂呈N-S与NNE向，而在新生代建造中发育的断裂则呈NE-SW，NW-SE和E-W向．研究区构造发展的第一构造阶段与E-W向张力有关，第二构造阶段的产物明显受到第三构造阶段发生的构造变形的改造，并与阿拉伯板块的逆时针旋转有关．磁场区之间的移位、错断、拖曳和并置被认为剪切断层所造成．剪切形式表明位移是左行的，即北侧的块体向北西方向移动．  相似文献   

16.
The results of palaeomagnetic, rock magnetic, and microscopic study of Early Paleozoic metabasites and granulites from the Orlica?nie?nik Dome (OSD, Sudetes) have been combined with geochronological data. In the eastern part of the OSD (?nie?nik Massif, SM) ferrimagnetic pyrrhotite is prevalent, accompanied by various amounts of Fe-oxides. In the western part of the OSD (Orlica-Bystrzyca Massif, OBM) Fe-oxides dominate. All magnetic minerals originated during hydrothermal and weathering processes. The palaeomagnetic study revealed the presence of three secondary components of natural remanence: Late Carboniferous, Late Permian, and Mesozoic. Two Paleozoic components are related to volcanic activity in the Sudetes. They are carried by pyrrhotite and Fe-oxides and were isolated only in SM rocks. The Mesozoic component was determined in both parts of the OSD and is carried by Fe-oxides. It covers a time span, from ～160 to ～40 Ma, corresponding to a long period of alteration.  相似文献   

17.
Paleomagnetic, rock magnetic, and sedimentary micro-textural data from an early Miocene mudstone sequence exposed in Okhta River, Sakhalin, Russia, indicate the presence of pyrrhotite and magnetite at different stratigraphic levels. Sites that contain only magnetite have a reversed polarity characteristic remanent magnetization (ChRM) with a low-coercivity overprint, which coincides with the present-day geomagnetic field direction. Pyrrhotite-bearing sites have stable normal polarity ChRMs that are significantly different from the present-day field direction. After correction for bedding tilt, the ChRM data fail a reversals test. However, the normal polarity pyrrhotite ChRM directions become antipodal to the tilt-corrected magnetite ChRM directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40% partial unfolding. These data suggest that the pyrrhotite magnetization was acquired during folding and after lock-in of the magnetite remanences. Electron microscope observations of polished sections indicate that fluid-associated halos surround iron sulphide nodules. Pyrrhotite is present in randomly oriented laths in and around the nodules, and the nodules do not appear to have been deformed by sediment compaction. This observation is consistent with a late diagenetic origin of pyrrhotite. Documentation of a late diagenetic magnetization in pyrrhotite-bearing sediments here, and in recent studies of greigite-bearing sediments, suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn-depositional magnetization.  相似文献   

18.
The ground follow-up of a magnetic and radiometric air survey had to cope with the usual dilemma to check a great number of anomalies within a short time. A limited aggregate of magnetic anomalies, expected to correspond to magnetite quartzites was statistically selected for ground identification by this method: the ratios length/width times amplitude were listed for all coherent contours and the calculation of the standard deviation per unit area resulted in different key-numbers for a new lithological “Salem Unit” and for the charnockitic or gneissic environment. The ground work thus directed and reduced by 85% yielded a substantial potential of iron ore. This was supported by by abundant determinations of the magnetic susceptibilities, confirming the sources of anomalies and revealing the amenabilities of iron ores for the magnetic separation process. The lines of truncation of anomalies were found to represent a system of local and regional faults and shear zones, which segregated the area into different tectonic blocks. These sutures have also provided the ways of intrusion for alkaline and basic magmae in the style of a “Rift” structure, housing several carbonatites and impregnations of metal sulphides. The airborne radiometrics obtained many uranium indications by gamma ray spectrometry. However, they led only to disseminated uranium-silicates, associated with syenites, granites and pegmatites, each emanating a characteristic photon energy spectrum. But in general the radiation of thorium prevails, marking northern Madras as a “Thorium Province”.  相似文献   

19.
The Pingchuan iron deposit, located in the Yanyuan region of Sichuan Province, SW China, has an ore reserve of 40 Mt with ~60 wt% Fe. Its genesis is still poorly understood. The Pingchuan iron deposit has a paragenetic sequence of an early Fe-oxide–Pyrite stage(Ⅰ) and a late Fe-oxide–pyrrhotite stage(Ⅱ). Stage Ⅰ magnetite grains are generally fragmented, euhedral–subhedral, largesized crystals accompanying with slightly postdated pyrite.Stage Ⅱ magnetite grains are mostly unfragmented, anhedral, relatively small-sized grains that co-exist with pyrrhotite. Combined with micro-textural features and previously-obtained geochronological data, we consider that these two stages of iron mineralization in the Pingchuan deposit correspond to the Permian ELIP magmatism and Cenozoic fault activity event. Both the Stage Ⅰ and Ⅱ magnetites are characterized with overall lower contents of trace elements(including Cr, Ti, V, and Ni) than the ELIP magmatic magnetite, which suggests a hydrothermal origin for them. ‘‘Skarn-like' enrichment in Sn, Mn, and Zn in the Stage Ⅰ magnetite grains indicate significant material contributions from carbonate wall-rocks due to water–rock interaction in ore-forming processes. Stage Ⅱ magnetite grains contain higher Mn concentrations than Stage Ⅰ magnetite grains, which possibly implies more contribution from carbonate rocks. In multiple-element diagrams, the Stage Ⅰ magnetite shows systematic similarities to Kiruna-type magnetite rather than those from other types of deposits. Combined with geological features and previous studies on oxygen isotopes, we conclude that hydrothermal fluids have played a key role in the generation of the Pingchuan low-Ti iron deposit.  相似文献   

20.
Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.  相似文献   

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