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1.
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2.
碳酸盐岩的反射光谱特征的研究及应用   总被引：2，自引：0，他引：2  
傅碧宏 《岩矿测试》1996,15(3):207-209
在室内对新疆塔里木盆地西北缘柯坪地区碳酸盐岩的反射光谱进行了测量。分析结果表明：碳酸盐岩的特征吸收谱带位于２３００～２３５０ｎｍ，随着碳酸盐岩中Ｍｇ２＋含量的增加，其特征吸收谱带的中心波长位置具有向短波长方向移动的特点；同时，随着碳酸盐岩中碳酸盐矿物含量的增加，其特征吸收谱带的深度（Ｄ）也有增大的趋势，并且岩石的平均反射率（Ｒ）也随着增高。根据这些特征可以判定碳酸盐岩的岩石类型及碳酸盐矿物的相对含量  相似文献   

3.
Continuous surface cores of cold-seep carbonates were recovered offshore Pacific Nicaragua and Costa Rica from 800 to 1,500-m water depths (Meteor 66/3) in order to decipher their evolution and methane enriched fluid emanation in contrasting geological settings. Cores from the mounds Iguana, Perezoso, Baula V and from the Jaco Scarp escarpment were used for a multi-method approach. For both settings aragonite was revealed as dominant authigenic carbonate phase in vein fillings and matrix cementation, followed by Mg-calcite as second most abundant. This common precipitation process of CaCO3 polymorphs could be ascribed as indirectly driven by chemical changes of the advecting pore water due to anaerobic oxidation of methane. A more direct influence of seep-related microbial activity on the authigenic mineral assemblage in both settings is probably reflected by the observed minor amounts of dolomite and a dolomite-like CaMg carbonate (MgCO3 ~ 42 %). δ13C data of Jaco Scarp samples are significantly lower (?43 to ?56 ‰ PDB) than for mound samples (?22 to ?36 ‰ PDB), indicating differences in fluid composition and origin. Noteworthy, δ18O values of Scarp samples correlate most closely with the ocean signature at their time of formation. Documenting the archive potential, a high resolution case study of a mound core implies at least 40 changes in fluid supply within a time interval of approximately 14 ky. As most striking difference, the age data indicate a late-stage downward-progressing cementation front for all three mound cap structures (approx. 2–5 cm/ky), but a significantly faster upward carbonate buildup in the bulging sediments on top of the scarp environment (approx. 120 cm/ky). The latter data set leads to the hypothesis of chemoherm carbonate emplacement in accord with reported sedimentation rates until decompression of the advective fluid system, probably caused by the Jaco Scarp landslide and dating this to approximately 13,000 years ago.  相似文献   

4.
The Emirate of Abu Dhabi is famed for its coastal carbonate, sabkhas and sand dunes; it is located in the NE part of the Arabian Plate, which formed during the Late Neoproterozoic (~820–750 Ma) by the accretion of island arcs and microcontinents to early Gondwana. Most of Arabia seems to have spent its existence within the Southern Hemisphere until it crossed the Equator during the Mesozoic; parts were involved in four glaciations, two in the Proterozoic (~750–630 Ma—Iceball or Slushball Earth?), and two more in the Palaeozoic (Late Ordovician and Permo-Carboniferous transition). In the early Palaeozoic the Arabian Plate was oriented about 90° counter clockwise relative to today’s poles. Gondwana later skirted the South Pole, migrating to the other side of the planet, eventually emerging the ‘right-way up’ with the Arabian Plate oriented to the poles more or less as seen today. Cold and temperate climate conditions ensured that for much of its early existence, Arabia was the site of mainly quartz-rich deposits. Later in the Neoproterozoic, however, extensive stromatolitic carbonate deposition took the lead, culminating around the Cambro-Precambrian boundary with deposition of the extensive Ara and Hormuz evaporites. Since south Arabia’s Permo-Carboniferous glaciation, the Arabian plate has been drifting northward, crossing temperate climatic zones conducive to fluvial and aeolian sandstone deposition and, from the later Permian, to tropical shallow-marine carbonates and evaporites In parallel with the above, the rifting of Gondwana opened an oceanic trough in the Late Permian off the NE flank of Arabia. Slope carbonates and deepwater Hawasina turbidites with a clear flow to the NE were deposited until they were obducted (together with associated ophiolites) in the Late Cretaceous on the edge of the Arabian plate in Oman and Iran. The deposition of widespread Early Silurian hydrocarbon source rocks in east-central Arabia was followed in the later Permian by extensive reservoir rocks with more during the mid-Late Mesozoic, giving rise to major oilfields both on- and off-shore, including Abu Dhabi. Arabia and Africa began to separate late in the Miocene with the opening of the Red Sea and Gulf of Aden. SSW–NNE compressive stresses caused uplift and volcanic activity in west Saudi Arabia and Yemen. Some products of erosion flowed eastward into Abu Dhabi. At the NE margin of Arabia, the Tethys Ocean narrowed, the NE flank of the newly forming Zagros Mountains of Iran is being subducted beneath southern Asia. To the SE, roughly coeval crustal compression adjacent to the Gulf of Oman led to uplift of the Oman Mountains and deposition of erosional products flanking the mountains mainly to the W and SW. The Oman Mountains are currently rising at about 2 mm/a, while northern Musandam is subsiding into the Strait of Hormuz at some 6 mm/a in association with subduction of the Arabian plate margin below the Eurasian plate. Alternations between polar glaciations and interglacials over the past few 100 ka resulted in considerable climatic changes over Arabia; slow glacial build-ups lasting some 80 to 120 ka led, somewhat erratically, to a fall in sea level of up to 130 m, to strong winds and the building of systems of extensive sand dunes such as the Rub’ al Khali. The joint Tigris–Euphrates river system flowed through a desert landscape, reaching the ocean only SE of the Strait of Hormuz. The peak of the last glaciation about 21 ka was followed by its rapid collapse and flooding of the Arabian Gulf to its present level between about 12 or 10 and 6 ka, a horizontal marine advance of some 200–300 m/a. Abu Dhabi is now the site of shallow-marine carbonates offshore and classical sabkhas and carbonate-rich sand dunes onshore.  相似文献   

5.
Detrital zircon U–Pb age distributions derived from samples representing ancient or relatively young large-scale continental drainage networks are commonly taken to reflect the geochronological evolution of the tapped continental area. Here, we present detrital zircon U–Pb ages and associated heavy mineral data from Pleistocene Rhine River Middle Terrace sands and equivalents between the Swiss–German border and Cologne in order to test the commonly assumed Alpine provenance of the material. Samples from eight localities were analyzed for their heavy mineral assemblages. Detrital zircon U–Pb ages were determined by laser ablation inductively coupled mass spectrometry on selected samples from five locations along the Rhine River. The zircon age populations of all samples show a similar distribution, their main peaks being between 300 and 500 Ma. Minor age populations are recognized at 570 and 1,070 Ma. The 300–400 Ma maximum reflects the Variscan basement drained by or recycled into the Rhine River and its tributaries. The 400–500 Ma peak with predominantly Early Silurian ages points to Baltica or to the mid-German crystalline rise as original sources. One distinct peak at c. 570 Ma probably represents input from Cadomian terranes. The Precambrian U–Pb ages are compatible with derivation from sources in Baltica and in northern Gondwana. The heavy mineral populations of Middle Terrace sands and equivalents are characterized to a variable extend by garnet, epidote, and green hornblende. This association is often referred to as the Alpine spectrum and is considered to be indicative of an Alpine provenance. However, hornblende, epidote, and garnet are dominant heavy minerals of collisional orogens in general and may also be derived from Variscan and Caledonian units or from intermittent storage units. A remarkable feature of the detrital zircon age distribution in the Rhine River sediments from the Swiss–German border to Cologne is the absence of ages younger than 200 Ma and in particular of any ages reflecting the Alpine orogeny between c. 100 and 35 Ma. Sediments from rivers draining the equally collisional Himalaya orogen contain detrital zircons as young as 20 Ma. Our results question the assumption that Pleistocene Rhine River sediments were directly derived from the Alps. The lag time between the formation and deposition age of the youngest zircon in the studied Pleistocene Rhine River deposits is 200 Ma. Together with the absence of Alpine zircon ages, this stresses that detrital zircon age data from ancient sedimentary units found in poorly understood tectonic or paleogeographic settings need to be interpreted with great care, one could miss an entire orogenic cycle.  相似文献   

6.
为增强植被覆盖区羟基和碳酸盐矿物的吸收特征, 提高矿物信息提取精度.通过模拟单像元内新鲜植物、干枯植物、羟基和碳酸盐矿物的混合光谱, 发现在一定波段范围内4种端元的特征波段处吸收深度呈显著线性关系, 并建立了羟基和碳酸盐矿物的植物校正吸收深度(vegetation corrected continuum depths, VCCD)模型.将模型应用于黑龙江呼玛的Hyperion影像, 提取了高岭石和方解石矿物信息.在去除河床、道路等干扰信息后, 经野外实地验证和室内岩石鉴定, 矿物信息提取结果较好.   相似文献   

7.
The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ18Ocarbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ13Ccarbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (δ18Oporefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ18Oporefluid ≈5 ‰) in Mound 11. A positive correlation between δ13Ccarbonate and δ18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ13Cporefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ13Cporefluid ≈?70 ‰). Furthermore, the δ18Oporefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ13CCH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).  相似文献   

8.
The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ18O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.  相似文献   

9.
Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ～460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.  相似文献   

10.
《International Geology Review》2012,54(11):1350-1362
ABSTRACT

Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ26Mg values and higher 87Sr/86Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined 87Sr/86Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ26Mg values of ?0.60 to ?0.30‰ and relatively low initial 87Sr/86Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.  相似文献   

11.
We have measured the infrared spectrum of aragonite-structured PbCO3-cerussite to 41 GPa at 300 K in the diamond anvil cell. We observed a phase transition from an orthorhombic to a trigonal structure beginning at ~15 GPa, manifested by a splitting of the ν2-out-of-plane bending vibration and a broadening and dramatic decrease in amplitude of the ν1-symmetric stretching vibration of the carbonate group. While the locations of the ν1-symmetric stretching and ν4-in-plane bending bands are similar between the low- and high-pressure phases, their mode shifts and peak shapes change markedly near the transition. In particular, the ν1 symmetric stretch has an essentially zero pressure shift in the high pressure phase, and its dramatically enhanced peak width indicates that it may be symmetry forbidden. The decreased mode shifts of the carbonate vibrations after the phase transition suggest that the carbonate group is less compressible in the new structure. The spectral changes observed are consistent with a small, trigonal unit cell, with space group ${P\bar{3}{1c}}$ and two formula units, instead of a previously proposed orthorhombic cell with sixteen formula units. This structure is identical to that of the high-pressure phase of BaCO3, and likely CaCO3 as well. Our results thus indicate that the post-aragonite, high-pressure phase of divalent-cation carbonates may be a comparatively high-symmetry trigonal structure.  相似文献   

12.
通过区内505和530等主要矿脉石英红外吸收光谱、热爆裂及热发咣特征的系统研究，总结了该区无矿、贫矿及矿体石英脉评价的石英矿物学标志，表现为：①无矿石英起爆温度为210－330℃，热爆裂曲线无明显峰型；石英热发光总强度大于20000cps，峰值强度大于500cps;②贫金石英脉其石英红外吸收相对光密度DCO2／DH2O值大于1．1；起爆温度为210－280℃，石英热爆裂曲线为明显单峰形态，主爆峰对应温度值为420－440℃；石英热发光总强度介于10000－20000cps之间，峰值强度介于150－300cps之间；③矿体石英脉其石英红外吸收相对光密度DCO2／DH2O值小于1．0；起爆温度100－160℃，热爆曲线为单峰型，主爆峰对应温度为140－180℃；石英热发光总强度值小于10000cps，峰值强度小于150cps。根据本区含金矿脉矿化分带模式及石英红外吸收光谱、热及热发光参数系统变化规律，建立了该区石英脉型金矿床找矿评价的石英矿物学标志模型。  相似文献   

13.
Comparison of results for the original burial rate of carbonate sediments over Phanerozoic time, as calculated using the GEOCARBSULFvolc model, with their rate of preservation to the present (survival rate) shows a considerable loss of mass, partly by subduction of oceanic crust, during the past 250 million years. Before that time, despite the evidence that preserved Paleozoic carbonates appear to have been deposited only in shallow water, we contend that there was also inorganic deposition of carbonates in the Paleozoic deep sea with subsequent loss by subduction. Inorganic carbonate deposition may have been abetted by the vastly different seawater and atmospheric composition for most of the Paleozoic than those of post-Cretaceous and end Paleozoic–early Mesozoic times. The hypothesis helps to explain the loss of mass greater than that predicted for shallow-water carbonates prior to 250 Ma.  相似文献   

14.
The sodalite sample used in this investigation did not exhibit the characteristic orange-yellow luminescence due to the $ {\text{S}}_{ 2}^{ - } $ center, because there was no trace of sulfur impurity. The heat-treated samples exhibited green and red luminescence with maximum intensity at 496 and 687 nm, respectively, under 264 nm excitation at room temperature. Their luminescence intensities were extensively dependent on the treatment temperature. The green luminescence efficiency of the sample heat-treated at 900 °C was 6.5 times higher than that of unheated natural sodalite. At 8.5 K, the green luminescence showed a vibronic structure. After heating at 1,300 °C, the crystal structure of sodalite was transformed to NaAlSiO4 (carnegieite), and the intense red luminescence was exhibited in the NaAlSiO4 sample. The peak wavelength of the red luminescence shifted from 687 nm at 300 K to 726 nm at 8.5 K. The luminescence lifetimes of the green and red luminescence at room temperature were 2.1 and 5.1 ms, respectively. It was proposed that the origin of the green luminescence is Mn2+ replacing Na+, and that of the red luminescence is Fe3+ replacing Al3+ in sodalite or NaAlSiO4 (carnegieite).  相似文献   

15.
 Siliceous dolomites and limestones contain abundant retrograde minerals produced by hydration-carbonation reactions as the aureole cooled. Marbles that contained periclase at the peak of metamorphism bear secondary brucite, dolomite, and serpentine; forsterite-dolomite marbles have retrograde tremolite and serpentine; wollastonite limestones contain secondary calcite and quartz; and wollastonite-free limestones have retrograde tremolite. Secondary tremolite never appears in marbles where brucite has replaced periclase or in wollastonite-bearing limestones. A model for infiltration of siliceous carbonates by CO2-H2O fluid that assumes (a) vertical upwardly-directed flow, (b) fluid flux proportional to cooling rate, and (c) flow and reaction under conditions of local equilibrium between peak temperatures and ≈400 °C, reproduces the modes of altered carbonate rocks, observed reaction textures, and the incompatibility between tremolite and brucite and between tremolite and wollastonite. Except for samples from a dolomite xenolith, retrograde time-integrated flux recorded by reaction progress is on the order of 1000 mol fluid/cm2 rock. Local focusing of flow near the contact is indicated by samples from the xenolith that record values an order of magnitude greater. Formation of periclase, forsterite, and wollastonite at the peak of metamorphism also required infiltration with prograde time-integrated flux approximately 100–1000 mol/cm2. The comparatively small values of prograde and retrograde time-integrated flux are consistent with lack of stable isotope alteration of the carbonates and with the success of conductive thermal models in reproducing peak metamorphic temperatures recorded by mineral equilibria. Although isobaric univariant assemblages are ubiquitous in the carbonates, most formed during retrograde metamorphism. Isobaric univariant assemblages observed in metacarbonates from contact aureoles may not record physical conditions at the peak of metamorphism as is commonly assumed. Received: 19 September 1995 / Accepted: 14 March 1996  相似文献   

16.
Quantifying the abundance and physicochemical properties of minerals using reflectance spectroradiometry in the visible, near infrared and shortwave infrared (400–2500 nm) regions is an important tool in mineral exploration. In this study, the reflectance spectra of drill cores from the world-class N4WS iron deposit located in the Carajás Mineral Province, Brazil, were obtained. These spectra were validated using X-ray fluorescence (XRF) geochemical analyses and thin sections. The reflectance spectra were collected using a FieldSpec 3 spectroradiometer (ASD, Boulder, Colorado, USA) in 10 drill cores. The mineralogy of the deposit is mainly hematite, with lesser amounts of magnetite, goethite, quartz, kaolinite, gibbsite, smectite, talc, carbonate and chlorite. The mineralogy of the iron deposit was extracted from the spectral data using the geometry (depth and wavelength) of absorption features across the reflectance spectrum removed from the continuum. The depth of the absorption features is proportional to the mineral abundance, and the wavelength is proportional to the mineral chemical composition. The diagnostic absorption features of each mineral were used to determine the mineral abundance and composition. The final products include the abundance of iron (hydro) oxide (11.6% root-mean-square error [RMSE] Fe2O3); abundance of aluminous clays (RMSE 6% Al2O3); abundance of talc (8% RMSE MgO); identification of clay type (kaolinite, montmorillonite or gibbsite); composition of carbonate (dolomite vs. calcite); and composition of chlorite (Mg vs. Fe). The mineral abundance and composition results provided an effective characterisation of the ore, protore and host rocks and showed variations within the ore body.  相似文献   

17.
Jharkhand is well known for the largest iron deposits in India. Noamundi is the main deposit, which is associated with shale, keolinite. The rock succession exposed in the southern Singbhum and Keonjhar and lying unconformably over the older metamorphics are known as Iron Ore Series (Ravindrakumar, 1986). This paper relates the assessment of grades of iron ore of the area. The study involves with pre-processing, FLAASH (Fast Line-of-Sight Atmospheric Analysis of Spectral Hypercubes) Atmospheric correction algorithms of EO-1 Hyperion data and evaluating the spectral characteristic by using ENVI 4.7 software package. The position of peak reflectance and absorption trough, continuums removal and absorption band depth are considered for assessment the spectral characters. The strong absorption observed is between the 860 to 900nm wavelength region and peak reflectance is observed in the 750 to 780nm wavelength region of the image spectra. The position of the NIR absorption trough shift to the direction of the longer wavelengths is due to decrease of iron content. Higher values of band depth indicate a better possibility of the iron occurrence. As a result, this spectral study enables the possibility to differentiate the grades of iron ores from hyperspectral satellite images.  相似文献   

18.
Twenty thousand metres of diamond drill core representing a 14 km cross-section from weakly to intensely altered Roxby Downs Granite through the Olympic Dam Breccia Complex, host to the Olympic Dam iron-oxide–copper–gold–uranium deposit in South Australia, was analysed using the HyLogger-3 spectral scanner. Thermal and shortwave infrared spectroscopy results from 30 drill holes provide insight into the spatial relationships between quartz, orthoclase–microcline, albite–oligoclase and progressively changing sericite and chlorite compositions. The relative proportions of quartz, feldspars and phyllosilicates were mapped with thermal infrared spectroscopy. Variations in the chemistry of sericite and chlorite were extracted by proxy from their shortwave infrared spectral response, together with their relative spatial distribution. HyLogger scanning has revealed four deposit-scale mineralogical trends, progressing from least-altered Roxby Downs Granite into mineralisation where most of the feldspar has been replaced by sericite + hematite + quartz: (1) a progressive Al–OH wavelength shift of 2205 nm to 2210 nm for sericite, followed by a spatially rapid reversal corresponding to lower phengite/muscovite abundance ratios; (2) progressive Mg/Fe–OH wavelength shift of 2248 nm to 2252 nm reflecting an increase in the Fe:Mg ratio of chlorite; (3) increasing ratio of microcline to orthoclase followed by a rapid decrease; and (4) slightly decreasing ratio of albite to oligoclase followed by plagioclase destruction prior to albite replacement by sericite. The HyLogger feldspar results support recent petrographic evidence for hydrothermal albite and K-feldspar at the Olympic Dam deposit, not previously reported. The spectral results from continuous HyLogger scans also show that the microscopic observations and proposed feldspar replacement reactions are not locally isolated phenomena, but are applicable at the deposit and regional-scale. A modified quartz–K-feldspar–plagioclase ternary diagram utilising mineralogy interpreted from HyLogger thermal infrared spectra (QAPTIR) diagram along with supporting data on the abundance ratios of orthoclase/microcline and albite/plagioclase, and the wavelength shifts in characteristic absorption features for sericite and chlorite, can be used as empirical vectors towards mineralisation within the Olympic Dam mineral system, with potential application to other IOCG ore-forming systems. Intrusion of Gairdner Dyke Swarm dolerite dykes into sericite ± hematite altered Roxby Downs Granite results in retrograde albite–chlorite–magnetite alteration envelopes (up to tens of metres thick) overprinting the original sericite ± hematite alteration zone and needs to be carefully evaluated to ensure that such areas are not falsely downgraded during exploration.  相似文献   

19.
This paper carried out mineral and geochemical studies on a profile through a diagenetic methane-derived authigenic carbonate sample that was collected from southwestern Dongsha area of the northern South China Sea. Five samples locating in the cross-sectional middle mainly consist of dolomite and quartz, and two samples close to the surface have a small amount of Mg-calcite. The δ13C values of the samples vary between ?30.59 and ?0.30 % VPDB, with δ18O values ranging from 3.07 to 3.59 % VPDB, δ44/40Ca values ranging from 1.35 to 1.47 % SRM915a, indicating a contribution of methane to the carbon pool where the precipitation of authigenic carbonates occurred. Based on the isotope values alone, it can not be distinguished if the carbon source is thermogenic gas or a mixture of biogenic methane and marine dissolved inorganic carbon. The δ18O values are in general consistent with dolomite precipitation from a fluid similar to present seawater. The observed small variation might be related to the oxygen isotope composition of seep fluid. The relative small range in calcium isotope values suggests that relatively constant growth conditions and precipitation from seawater. The central part of the carbonate nodule formed under the strong influence of methane seepage, and the external part is less influenced by methane, either due to reduced methane flux to the surface or caused by erosional exhumation of the carbonate nodule from greater depth to the sediment surface.  相似文献   

20.
Estimates of early atmosphere compositions from metamorphosed banded iron formations(BIFs)including the well-studied ≥3.7 BIFs of the Isua supracrustal belt(Greenland)are dependent on knowledge of primary versus secondary Fe-mineralogical assemblages.Using new observations from locally well preserved domains,we interpret that a previously assumed primary redox indicator mineral,magnetite,is secondary after sedimentary Fe-clays(probably greenalite)±carbonates.Within ~3.7 Ga Isua BIF,pre-tectonic nodules of quartz+Fe-rich amphibole±calcite reside in a finegrained(≤100 μm)quartz+magnetite matrix.We interpret the Isua nodule amphibole as the metamorphosed equivalent of primary Fe-rich clays,armoured from diagenetic oxidative reactions by early silica concretion.Additionally,in another low strain lacunae,~3.76 Ga BIF layering is not solid magnetite but instead fine-grained magnetite+quartz aggregates.These magnetite+quartz aggregates are interpreted as the metamorphosed equivalent of Fe-clay-rich layers that were oxidised during diagenesis,because they were not armoured by early silicification.In almost all Isua BIF exposures,this evidence has been destroyed by strong ductile deformation.The Fe-clays likely formed by abiotic reactions between aqueous Fe~(2+)and silica.These clays along with silica±carbonate were deposited below an oceanic Fe-chemocline as the sedimentary precursors of BIF.Breakdown of the clays on the sea floor may have been by anaerobic oxidation of Fe~(2+),a mechanism compatible with iron isotopic data previously published on these rocks.The new determinations of the primary redoxsensitive Fe-mineralogy of BIF significantly revise estimates of early Earth atmospheric oxygen and CO_2 content,with formation of protolith Fe-rich clays and carbonates compatible with an anoxic Eoarchean atmosphere with much higher CO_2 levels than previously estimated for Isua and in the present-day atmosphere.  相似文献   

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