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1.
ABSTRACT

Stable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.  相似文献   

2.
A geochemical investigation was carried out mainly in a 14-km thick sedimentary sequence of late Precambrian age. The project included analysis of rock samples and drainage samples for Cu, Co, and Zn. The drainage samples were heavy-mineral concentrates and stream sediment samples.The results for the drainage sediments distinguish three well defined cupriferous horizons in the sedimentary column. Lithogeochemical studies of quartzites, pelites, and carbonates confirm that the anomalous values obtained in the drainage sediments arise from several Cu-mineralized horizons.The study has shown that geochemical prospecting appears to be well suited to Cu prospecting in Central East Greenland under arctic climate conditions even with a low sampling density of one sample per 5–10 km2 for the drainage samples, and one sample per 10–100 m of the sedimentary stratigraphic column for the rock samples.  相似文献   

3.
The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.  相似文献   

4.
An anomalous area (100 km2) revealed by a previous regional overbank sediment sampling survey by the Geological Survey of Norway was followed-up for Ti, Nb, Y, Zr, La and Ce by drainage and lithogeochemical sampling and analyses. The drainage samples were anomalous in the same elements as in the overbank samples and contained similar concentrations. The lithogeochemical results did not indicate mineralization and showed only a small enhancement of Ti, Zr, La and Ce in mylonitic rocks.Mineralogical studies of the drainage samples revealed, apart from proportions of quartz and feldspars, that stream sediments and heavy mineral concentrates are similar in mineralogy, with epidote predomination and several vol% of fine-grained titanite and zircon. Drainage samples contained amounts of Nb, Y, and Zr up to ten times larger than average contents of rock samples. Enhanced light REE (La and Ce) and Nb reflect host minerals, titanite, and enhanced Y possibly zircon and/or allanite.Anomalies in both media do not relate to mineralization. The overbank samples and the drainage samples provide comparable geochemical data.  相似文献   

5.
Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (<0.18 mm fraction) with sample densities of 1 site per 106 km2. Concentrations of 51 elements in 173 stream sediments were determined. Areas with especially high Cr, Ni, Mg and Co concentrations run intermittently and linearly through Shikoku, and coincide with exposures of the Mikabu greenstone complex composed of oceanic basaltic and gabbroic rocks with minor ultramafic bodies. Areas with higher concentrations of Fe, V, Sc, Ti, Mn, Cr, Ni, Mg, Co and Eu are distributed widely along the Shikoku Island axis; they mainly overlap with the zones of the high-pressure type metamorphic rocks (Sanbagawa Belt) and of the Jurassic accretionary complexes with sedimentary rocks (Chichibu Belt): in contrast, areas with lower concentrations spread over the northern coastal zone of Cretaceous granitoids (Ryoke Belt) and in the southern coastal zone of Cretaceous-Paleogene accretionary complexes (Shimanto Belt). Distribution patterns of these elements reflect that various amounts of mafic and ultramafic materials, possibly of oceanic origin, are associated with the rocks in the Sanbagawa and Chichibu Belts, although such components occur sparsely in the Shimanto and Ryoke Belts. Calcium, Sr, Na, Nb, Sm and Gd contents are lower along the southern coastal zone. High values of Th, U, La, Ce and Ba are associated with granitic rocks. The positive anomalies of Cu and Cd coincide with the distribution of stratabound Cu deposits in the Sanbagawa Belt. Positive anomalies of Sb in the northern area are associated with Sb deposits near the large Median Tectonic Line fault zone. A high-Hg zone is present in the southern fore-arc area. The median concentrations for stream sediments in Shikoku are higher in Hg, Cu, Ni, Cr, Rb, Li, Cs, K, slightly higher in Nb, Ta, La, Ce, Sn and lower in Ca than those for average Japanese stream sediments. The median concentrations for the four geologic zones show systematic wide variations of mafic associated elements, and narrow variations of lithophile elements generally more abundant in felsic rocks. The chemical compositions of the stream sediments in Shikoku largely reflect the concentrations in the pre-Neogene accretionary complexes and in the high-P metamorphic rocks mainly formed from clastic materials derived from continental margins, and ratios of mafic and ultramafic materials within surface rocks; they are partly influenced by sulfide mineralization and fluid migration.  相似文献   

6.
As part of a regional prospecting programme, 600 panned stream sediment concentrates from 1000 km2 have been examined for scheelite (CaWO4) using ultraviolet light. The mineralogical compositions of the heavy mineral concentrates were determined and related to bedrock geology. The results of the investigation are presented on maps showing the distribution of scheelite, magnetite, and ilmenite. Follow-up work resulted in the discovery of nine new tungsten occurrences. Magnetite dominates in areas of acid rocks, and ilmenite in areas of basic rocks. The areal distribution of hornblende and hypersthene reflects the metamorphic grades.  相似文献   

7.
Regional exploration for pegmatite hosted Sn-mineralization in Sweden by LKAB Prospektering has, since the early 1980s, generally been based on the use of heavy-mineral concentrates from till collected by roadside sampling. The average sampling density has been 0.5–1.0 sample/km2, but with an uneven distribution of sample points within areas investigated.Planning of the regional sampling grid in this study has been based on a statistical analysis to answer the following question: What sample density and grid-spacing should be used to hit an anomalous target with the highest possible probability.The statistical analysis indicates that using a rectangular grid, 750 × 1,500 m (0.88 sample/km2), the probability of intersecting an anomalous target is 100%. Heavy-mineral till concentrate sampling using this grid design in northern central Sweden in 1981 revealed one single, extremely high sample with 4,622 ppm Sn indicating mineralization in the area north of the village of Järkvissle.In 1982–1983, while boulder tracing up-ice from the anomalous regional sample point, cassiterite and spodumene-bearing boulders and outcrops were found. Two parallel mineralized pegmatites within an area of approximately 5,000 m2 with grades of 0.04–0.07% Sn and 0.40–0.92% Li2O have recently been located NW of Järkvissle.Semi-regional (250 × 500 m) and local (20 × 20–160 m) sampling of till gave some anomalous values of Sn and Li in concentrates, which to some extent could be correlated with mineralized boulders and outcrops, and also indicated new targets for drilling.The results from a simplified sample preparation technique for heavy-mineratl till concentrates show that a new device, named the Goldhound, can be used in future geochemical prospecting for Sn and Li in Sweden.  相似文献   

8.
Mineralogical and geochemical data from shallow overburden surveys are examined to ascertain parameters which govern the distribution of gold in overburden in a 2400-km2 area of southwestern Gaspésie, Quebec, Canada.The area is a deeply dissected plateau underlain by faulted and gently folded Siluro-Devonian strata. Complex geomorphic and glacial histories are reflected in the non-glacial character of the landscape, the preservation of very old erosional landforms and extensive variation in the composition and distribution of overburden.Total sample analysis and heavy-mineral studies show that the composition of overburden changes across the area in approximate correspondence with changes in underlying bedrock. Three broad zones related to bedrock and overburden types are delimited. Gold analyses of <250 μm overburden are insensitive to regional variations, with only 15 samples out of 300 registering above the detection limit of 2 ppb. Better contrasts of gold concentrations are obtained from chemical analyses of nonmagnetic heavy-mineral concentrates (NM HMC). Although sample density is low, NM HMC data show anomalies which can be related to particular bedrock and structural settings. Particulate gold was not observed in any of the NM HMC. Gold is associated with secondary iron-oxide phases replacing primary sulphide minerals. High concentrations of gold in NM HMC of overburden collected north of the mouth of the Assemetquagan River support the hypothesis of a local source to the north or northwest for the alluvial gold in the lower 2 km segment of the river.Dilution by far-travelled, shield-derived heavy minerals is by far the most important cause of regional mineral variation. Conversion of gold concentrations in NM HMC to concentrations in total size fraction eliminates some of the erratic behavior of NM HMC data caused by variations in heavy-mineral abundance and corrects for the dilution effect where the proportion of heavy minerals in the far-travelled component of overburden is much greater than in the local component. These calculations suggest a contribution of gold to the background in the fine sand fraction of overburden of 0.07 ppb. Where the diluting component is local, conversion of NM HMC data to total size fraction may or may not correct for differences in heavy-mineral contributions of underlying bedrock, depending on local conditions. Where dilution by far-travelled components is excessive, NM HMC analyses are inadequate to reflect conditions in underlying bedrock.  相似文献   

9.
Blueschist-facies rocks of the central Seward Peninsula cropout over 8000 km2. Protoliths were Lower Paleozoic-Precambrian(?) shallow-water miogeoclinal sediments that were metamorphosed during the Middle Jurassic. Thermobarometric estimates yield ‘peak’ metamorphic conditions of 10–12 kbar at 460 ± 30°C. Crystallization of blueschist-facies minerals was synkinematic with development of a transposition foliation. This foliation is parallel to lithologic contacts and is axial planar to recumbent mesoscopic isoclinal folds. These folds are refolded by larger scale recumbent tight to isoclinal folds. Both fold sets have hinges parallel to a well-developed N—S stretching lineation. Sheath folds are also present. The long axes of the sheath folds also parallel the stretching lineation. This deformation was non-coaxial as indicated by microstructures and quartz c-axis fabrics. Folds nucleated, then rotated into parallelism with the stretching direction. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the Brooks Range continental margin and a N-facing island arc (Yukon-Koyukuk). Convergence of these two plates is believed to have been nearly N—S (in present co-ordinates).  相似文献   

10.
Trace levels of chalcophile elements that form volatile sulfide minerals are determined in stream sediments and in the nonmagnetic fraction of a heavy-mineral concentrate of stream sediments by a carrier distillation emission spectrographic method. Photographically recorded spectra of samples are visually compared with those of synthetic standards for the two sample types. Rock and soil samples may also be analyzed by comparison with the stream-sediment standards. A gallium oxide spectrochemical carrier/buffer enhances the early emission of the volatile elements.Detection limits in parts per million attained are: Sb 5, As 20, Bi 0.1, Cd 1, Cu 1, Pb 2, Ag 0.1, Zn 2, and Sn 0.1. A comparison with other methods of analysis, total-burn emission and atomic absorption spectroscopy, shows good correlation for standard reference for materials and samples from a variety of geologic terranes.  相似文献   

11.
A reconnaissance exploration survey over 14 000 km2 of Precambrian terrain in South Greenland using stream-sediment and stream-water samples delineated a central uranium district of 2000 km2 with enhanced uranium levels and smaller anomalous zones in the south of the field area.The area is underlain by Archean and Proterozoic gneisses, granites and metasediments all of which have been intruded by late Proterozoic alkaline intrusions (Gardar Province). The terrain is mountainous and the streams are either steep torrents or impeded drainages typical of glaciated terrains with boggy organic rich sediments.The central uranium district was defined by a high uranium background in both stream sediments (5–20 ppm) and stream waters (0.5–1.0 ppb) and a markedly higher frequency of very anomalous values in the order of 50–100's ppm in the stream sediments and 1–10 ppb in the stream waters. An areal correlation of uranium, in this district, with high pH and conductivity in the stream water in addition to a higher organic content noted in the stream sediment raised the question of a possible enhancement of uranium values due to secondary environmental effects. On the other hand, an areal correlation of uranium with niobium and other trace elements characteristically associated with alkaline rocks, and the geographic proximity of this uraniferous district to the alkaline intrusions suggested a genetic relationship between uranium mineralization and the alkaline igneous activity.Limited follow-up work located 8 pitchblende occurrences in this extensive district. The pitchblende is in veins which contain quartz, calcite, iron oxide, fluorite and minor sulphides. The isotopic (U-Pb) age of the pitchblende, which ranges from 1180-1090 Ma, corresponds to the late stages of Gardar alkaline igneous activity. It is concluded, therefore, that the reconnaissance geochemistry reflects a district-wide hydrothermal event related to the late volatile differentiates derived from the highly fractionated alkaline magma. A combination of primary and secondary features have complemented each other in enhancing the geochemical reconnaissance data and emphasized its importance but has not materially altered the interpretation.The south of the field area also has a relatively high uranium background in both the sample media with some discrete anomalous zones, usually with a slightly lower order of magnitude than the central area, but still with a distinct contrast of 5–10 times. Fine-grained uraninite has been found in the area occurring as disseminated grains in pegmatitic elements as in the central district. Isotopic ratios (U-Pb) suggest an age of 1728 ± 30 Ma which probably reflects the long cooling of the granite.It is concluded that the geochemical reconnaissance data delineated two uranium metallogenic districts characterized by distinctly different types of uranium mineralization. It is suggested that South Greenland may be part of a much wider uranium geochemical province which includes parts of Labrador. To the present plate-tectonic models, which suggest such a connection (Le Pichon et al., 1977), must be added the comparable reconnaissance geochemical results (G.S.C. Open Files nos. 748 and 749), and the similar 1730 Ma age of the Kitts uranium mineral occurrence in Labrador (Gandhi, S.S , 1978) to that of the uraninite found in the south of the field area in Greenland.  相似文献   

12.
Overbank and medium-order stream sediment samples were collected in Belgium and Luxembourg from 66 sampling locations (area of about 33,000 km2) and analysed for major and trace elements among which Zn, Pb, Cu and As. At each sampling location large bulk samples were taken, namely in the lower (normally at ≥1.5 m depth, over an interval of about 20–40 cm) and upper (normally upper 5–25 cm) parts of the overbank profiles and from the stream sediments. Furthermore, at a number of these sites, a detailed geochemical analysis of vertical overbank sediment profiles (sampling intervals of 10–20 cm) was subsequently carried out to unravel element variations through time and to help in the overall evaluation. For most sampled sections evidences such as 14C-dating and the absence of anthropogenic particles point towards a pre-industrial and often pristine origin of the lower overbank sediment samples. From the latter bulk samples, mean background concentrations were deduced. They reveal the existence of significant differences between the northern and southern part of Belgium (incl. Luxembourg) which relate to the difference in geological substrate. In the north dominantly non-lithified Quaternary and Tertiary sands, marls and clays occur while in the south Palaeozoic sandstones, shales and carbonate rocks outcrop. Consequently separate mean background values were calculated for the two areas. In the southern study area, some anomalous metal concentrations have been recorded in pre-industrial sediments. They are derived from mineralised Palaeozoic rocks, a feature which could be of interest for base metal exploration. In the upper overbank and stream sediments, in general, higher heavy metal and As contents were recorded with highest values in areas with metal mining, metal melting and cokes treatment industries. By comparing the trace element concentrations of the upper overbank or stream sediment samples with the concentrations detected in the lower overbank samples at each of the sampling locations, and by evaluating the vertical distribution patterns where available, the degree of pollution of the alluvial plain and the present-day stream sediments can be assessed. From this exercise, it is clear that highest pollution occurs in the northern part of Belgium, which relates to its high population density and industrial development.  相似文献   

13.
A systematic approach for identifying mineral exploration target areas from reconnaissance stream sediment data without sufficient a-priori control information has been demonstrated in a portion of western Albay Province in the southern Bicol Peninsula of the Philippines. The approach involved devising a rapid method of catchment basin mapping using a geographic information system (GIS) so that the areal influence of the catchment basins may be incorporated in the geochemical data analysis. Areal proportions of mapped rock units occurring in the sample catchment basins and observed Mn and Fe contents in stream sediments are used as independent variables in multiple regression analysis to predict element contents in stream sediments related to lithologic and chemical controls. The predicted element contents are filtered-out from the original data to leave residuals in which the effects of other factors (e.g., mineralization) may be seen. A simple correction for the effects of downstream dilution is applied; this allows for the different sizes of the sample catchment basins so that positive geochemical residuals are enhanced. The inter-relationship of the different positive residuals in ‘highly enriched’ samples are investigated through principal components analysis to determine and quantify an ‘anomalous geochemical signature’. Lastly, the ‘anomalous geochemical signature’ is integrated with ‘proximity’ to faults/fractures to determine favourable target areas. For the test region, the lithologic controls explain between 80% and 100% of the variability in most of the elements studied. Chemical controls account for generally less than 5% of the variability in the data. Most of the dilution-corrected residuals reveal high relative enrichment in certain areas underlain by andesite and/or diorite. An anomalous Cu-Mg-Fe-Zn geochemical signature is disclosed by the principal components analysis of the dilution-corrected residuals in ‘highly enriched’ samples. Most sample catchment basins defined by this ‘anomalous geochemical signature’ pertain to areas underlain by andesitic rocks. Integration of the ‘anomalous geochemical signature’ and ‘proximity’ to faults/fractures reveals that some of these anomalous sample catchment basins are favourable target areas. These areas are interpreted to contain andesite-hosted stockwork or stringer zones that once formed part of a complete stratigraphic sequence of a volcanogenic massive sulphide occurrence. The results demonstrate the usefulness and ability of the procedures followed to extract significant anomalies from the reconnaissance geochemical data without the benefit of sufficient a-priori control data to aid in anomaly recognition. Similar procedures could also be applied elsewhere.  相似文献   

14.
The areal distribution of stream sediment and source-rock heavy mineralogy was studied to determine the mineralogical relations between source and derived sediment in the Piney Creek drainage basin of Wyoming. Heavy-mineral distribution in the streams was characterized using factor analysis. The results of analyses show that for an actively degrading stream system, the heavy-mineral composition of stream sediment is controlled to a considerable extent by the composition of local sediment sources and by continuous addition of detritus from bank cutting. Only the most common minerals from upstream sources persist as statistically significant variables downstream. In some cases the heavy-mineral mineral composition of stream sediment can be controlled by comparatively few, or volumetrically rare source rocks, even though more abundant source rocks supply a wide range of heavy minerals. Specific drainage configurations such as lakes and proximity of streams to major sediment sources significantly modify sediment mineralogy immediately downstream.  相似文献   

15.
The nature of gold dispersion in soils and stream sediments associated with a copper-gold-mineralized system in northeastern Thailand has been investigated as a basis for identifying appropriate geochemical exploration techniques for the search for comparable deposits in similar environments.Soils were collected with varying relationships to mineralization as a basis for determining sample representativity, size distribution of gold, variation with soil horizon and possible pathfinder elements. Similarly, stream sediments were collected to estimate sample representativity, size distribution of gold, variation of gold with depth in the stream sediment profile and to compare the relative recoveries of gold in field-panned and laboratory-prepared heavy-mineral concentrates. Samples were analyzed for Au and potential indicator elements by a variety of methods but mostly by instrumental neutron activation analysis.Results indicate the consistent distribution of fine-grained gold in soils which allows Au analysis of relatively small samples from B-horizon soils to be used effectively and reliably to identify the surficial patterns of gold mineralization in the study area. Anomalous patterns of other indicator elements, Co, As, Cu, Sb, W, Pb, Zn, Ag, Fe and Mn, may contribute additional information regarding type of mineralization. This finding indicates the effectiveness of soil surveys in gold exploration, particularly in areas of deep weathering where fresh bedrock exposures are infrequent.Unlike soils, size distributions of gold in stream sediments, as a result of the local flow regime, vary both between sampling sites and at depths within a sampling site. Exploration requires Au analysis of the fine fraction (minus 63 μm) of active stream sediments to reduce the problem of sampling representativity. The presence of coarse-grained gold in the stream channel has drawn attention to the possible benefit of using the conventional field-panning method as a semiquantitative technique for providing immediate results. However, highly erratic distribution of pannable gold on a very local scale together with variable proportions of the total gold recovered in field-panned or heavy-mineral concentrates highlights a potentially serious drawback of the method. Combination of analysis of the minus 63 μm fraction and field panning appears warranted to cover the possible existence of gold of a wide size range in stream sediments.The overall results indicate the utility of geochemical exploration techniques in the search for gold mineralization. However, particular care is necessary in the design and implementation of geochemical techniques to ensure maximum reliability of exploration.  相似文献   

16.
《Applied Geochemistry》2006,21(3):492-514
Geochemical mapping of northern Honshu in the Northeast Japan Arc was carried out using stream sediments at a sampling density of one sample per 130 km2. More than 50 elements were determined in 395 stream sediment samples (<0.18 mm fraction). In geochemical maps, areas with especially low concentrations of large ion lithophile elements (LILE), Be and Li widely overlap with the distribution of Quaternary volcanic rocks along the volcanic front. The areas rich in mafic elements are associated with mafic rocks in many cases. On a regional scale, Ni, Cr and Cu contents are higher in the eastern Paleozoic–Mesozoic basement zone, Pb and Tl tend to be higher on the western zones, and Zn and Cd are high in the western back-arc zone. The areas especially rich in Cu, Zn, Cd, Pb, Bi and Tl coincide with the distribution of large mineral deposits. High concentrations related to Kuroko, hydrothermal-vein, and skarn-type deposits are recognized. High values of As and Sb are related to active geothermal areas near volcanoes and ore deposits. Chemical variations of K, Ce, Th and Sn in the stream sediments are concordant with chemical variations in major rocks. The median and mean concentrations for the stream sediments in northern Honshu, showing arc signatures, are lower in Rb, Cs, Th, Li, K, Be, Ta, LREE, Ni, Hg and Sn, and higher in Sc, Ca and Cd relative to the whole area of Japan, largely because of the contribution of Cenozoic island-arc volcanic rocks that are generally poor in incompatible elements. The averaged chemical compositions of the stream sediments for the geologic zones show systematic variations of many elements. The contrasting variations of LREE and Th contents, which are lower in the zones of Cenozoic rocks relative to the zones of pre-Neogene basements, reflect the regional variations in the main rocks, and also reflect the change of geological settings of the studied area from the continental margin to an island arc during the Cenozoic.  相似文献   

17.
An elucidation of the background levels of heavy metals, including certain toxic elements, is very essential to accomplish an important environmental assessment. A regional geochemical mapping in Hokkaido, Japan was undertaken by the Geological Survey of Japan, AIST as part of a nationwide geochemical mapping for this purpose. There were 692 stream sediments collected from the active channel (1 sample) / (100 km2) in Hokkaido and the fine fraction sieved through a 180 μm screen was analyzed using the AAS, ICP-AES, and ICP-MS techniques. The regional geochemical maps for 51 elements were created as a 2000 m mesh map using the geographic information system software. Spatial distribution patterns of elemental concentrations in stream sediments, particularly Neogene–Quaternary volcanic and pyroclastic rocks, are primarily determined by surface geology. The correspondence of elemental concentrations in stream sediments to parent lithology is clearly indicated by ANOVA and a multiple comparison. Sediment samples supplied from mafic volcanic and felsic–mafic pyroclastic rocks are significantly rich in MgO, Al2O3, P2O5, CaO, Sc, TiO2, V, MnO, Total (T)-Fe2O3, Co, Zn, Sr, and heavy rare earth elements (REEs) (Y and Eu–Lu), but significantly lacking in alkali elements, Be, Nb, light REEs (La–Nd), Ta, Tl, Th, and U. Accretionary complexes with sedimentary rocks derived from sediments are in stark contrast to volcanic and pyroclastic rocks. Accretionary complexes with mafic–ultramafic rock have significantly elevated Nb, Ta, and Th abundances in sediments besides MgO, Cr, Ni, Co, and Cu. This inexplicable result is caused by the mixed distributions of granite and ultramafic–mafic rocks.The watersheds with mineral deposits relate to the high concentrations of certain elements such as Zn, As, and Hg. The geochemically anomalous pattern, which is a map of the regional anomalies, and a scatter diagram were applied to examine the contribution of mineral deposits to MnO, T-Fe2O3, Cr, Cu, Zn, As, Cd, Sb, Hg, Pb, and Bi concentrations. Consequently, they were grouped into four types: 1) Mineral deposits with no outliers resulting from mineralization (MnO, T-Fe2O3, and Cr), 2) sediments supplied from watersheds without metal deposits conceal high metal inputs from known mineral deposits (Cu), 3) deposits from a geochemically anomalous area that closely relates to the presence of mineral deposits (As, Sb, and Hg), and 4) deposits from the widely altered zone associated with the Kuroko as well as hydrothermal deposits corresponding to geochemically anomalous patterns (Zn, Cd, and Pb). This study provides an important regional geochemical database for a young island-arc setting and interpretational problems, such as complicated geology and active erosion, that are unique to Japan.  相似文献   

18.
A reconnaissance geochemical survey of stream drainages within 21,000 km2 of southeastern Arizona and southwestern New Mexico shows broad zones of low-level to moderate contrast anomalies, many associated with mid-Tertiary eruptive centers and Tertiary fault zones. Of these eruptive centers, few are known to contain metallic deposits, and most of those known are minor. This, however, may be more a function of shallow erosion level than an indication of the absence of mineralization, since hydrothermal alteration and Fe-Mn-oxide staining are widespread, and geochemical anomalies are pervasive over a larger part of the region than outcrop observations would predict. Accordingly, interpretations of the geochemical data use considerations of relative erosion levels, and inferred element zonalities, to focus on possible undiscovered deposits in the subsurface of base-, precious-, and rare-metal deposits of plutonic-volcanic association. In order to enhance the identification of specific deep targets, we use the empirically determined ratio: This ratio is based on reported metal contents of nonmagnetic heavy-mineral samples from the drainage sediment, determined by emission spectrographic analysis. Before the ratio was computed for each sample site, the data were normalized to a previously estimated regional threshold value. A regional isopleth map was then prepared, using a cell-averaging computer routine, with contours drawn at the 25th, 50th, 75th, 80th, 90th, 95th and 99th percentiles of the computed data.  相似文献   

19.
锂因其在新能源领域的应用广泛性和不可替代性而备受关注。以往锂矿勘查主要集中于盐湖型和硬岩型锂矿,针对沉积型锂矿开展工作较少。本文聚焦于中国西南滇黔桂地区,以岩石和水系沉积物中锂的时空分布特征为基础,分析区域内沉积型锂矿成矿潜力。结果显示,中国西南滇黔桂地区水系沉积物锂含量平均值为46. 7×10-6,显著高于全国背景值32×10-6。以57. 0×10-6为异常下限,共圈定9个锂地球化学省,与已知的沉积型锂矿床分布具有良好的对应性。同时,锂异常浓集区与某些特定时代和岩性的地层分布密切相关。区内水系沉积物中锂元素含量受到岩石锂含量制约,以碳酸盐岩为主的构造单元水系沉积物中的锂显著次生富集,以陆源碎屑岩为主的构造单元内水系沉积物中锂富集程度较弱。综合分析认为,西南滇黔桂地区锂异常带的形成受区域岩石类型、次生富集作用和气候条件等多重因素制约。该研究对了解西南滇黔桂地区锂元素次生聚集就位机制和指导沉积型锂资源的找矿勘查具有重要的参考意义。  相似文献   

20.
Deposits of very large rock avalanches were identified at the southern foot of the Rocky Range of the Northern Caucasus. Cliffs facing the Ardon River are 1–1.5 km high and composed of Cretaceous and upper Jurassic, hard, crystaline limestone, underlain by softer, middle Jurassic shale, siltstone and sandstone flysh. The largest rock avalanche, at Karivhoh, is ~2×109 m3 in volume, travelled more than 7 km, and covered about 18 km2 with deposits up to 200–300 m thick. All rock-avalanche bodies are composed of intensively crushed debris overlain by a blocky carapace. Numerous subsequent landslides develop within these deposits, and pose a threat to villages built on them.  相似文献   

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