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1.
Regional, national and global scale geochemical mapping projects have been carried out in China since the late 1970s, due to the development of cost‐effective, low detection limit analytical methods. These projects have provided a huge mass of high‐quality, informative and comparable data for mineral resource exploration and are now making contributions to environmental assessment. In this paper, four national‐scale geochemical mapping projects are described. (1) The Regional Geochemistry‐National Reconnaissance Project (RGNR project), which is China's largest national geochemical mapping project, has covered 6 million km2 of upland regions since 1978. Generally, stream sediment samples were collected at a density of 1/km2 and four samples were composited into one sample and analysed for thirty‐nine elements. (2) The deep‐penetrating geochemical mapping project (DEEPMAP Project) has been conducted since 1994 in covered terrains, including sedimentary basins, at a density of 1 sample per 100 km2 with thirty to seventy elements determined per sample. In the past 10 years, an area of approximately 800 000 km2 has been covered and this project has played an important role in finding sandstone‐type uranium deposits in basins. (3) The seventy‐six geochemical element mapping project (76 GEM project) has been carried out since 1999 and involved the collection of stream sediment samples from the RGNR project targets which were analysed for seventy‐six elements. Samples from each 1:50 000 map sheet were composited into one analytical sample (approximately one composite sample per 400 km2). Approximately 1 million km2 have been surveyed to date. (4) The multi‐purpose eco‐geochemical mapping project has been conducted since 1999 in Quaternary plain areas for environmental and agricultural applications. Surface soils (depths from 0–20 cm) were collected at a density of one sample per km2, and four samples were composited into one for analysis. Deep soils (from a depth of 150 to 200 cm) were collected at a density of one sample per 4 km2 and four samples were composited into one analytical sample. All the composite samples were analysed for fifty‐four elements.  相似文献   

2.
Guidelines for a low-density geochemical survey were described in 1990 by the Western European Geological Surveys. A low-density geochemical survey of Hungary was carried out in 1991–1995. The results are useful for future surveys and for the IGCP 360 project ‘Global Geochemical Baseline’. In regions with well-developed drainage systems in Hungary, 196 catchment basins of approx. 400 km2 were delineated and flood-plain deposits sampled at their outlets. The samples were taken from 0 to 10 cm and from 50 to 60 cm depths. Samples were analysed by ICP-AES and AAS techniques in two laboratories. A Geochemical Atlas of Hungary is in preparation that will show the distribution of 25 elements in the two sampled layers. Maps for the lower layer represent regional geochemical baseline values and a geochemical subdivision of the country (maps showing the distribution of element associations) was made on the basis of factor variables. Maps constructed from the data of the upper sampling level show us the present state of contamination of the surface. The results of this survey have contributed to the establishment of guidance values for soils prepared by the Hungarian Ministry of the Environment in 1995. Safe levels were established for As, Cd, Cr, Cu, Hg, Pb and Zn and regional environmental loads plotted. Differences between the median values of the two levels are generally small. However, the concentrations of certain elements like P, Pb and S are significantly greater in the upper layer reflecting contamination from agriculture. In certain regions, the rate of sedimentation was fairly fast such that the environmental effects of ore mining in Transylvania and southern Slovakia as well as those of heavy industry in northern Hungary can be observed in samples from the lower level. The main factor controlling the geochemical pattern in Hungary is the predominance of young (Pleistocene or Miocene) clastic sediments at the surface. Approx. 90% of the surface is covered by these young sediments. This kind of survey has the disadvantage of not providing enough contrast to differentiate geologically dissimilar areas but it has the advantage to provide regional surface background geochemical data and it helps to outline areas of possible surface contamination. Based on the results of this survey we conclude that it would be much better to sample smaller, but geologically homogeneous areas in mountainous terrain to obtain data characteristic of the geochemical background of lithologic units. This approach would mean a sampling density of a few tens of km2/sample for hilly areas, and a few hundred km2/sample for lowland areas.  相似文献   

3.
文章以浙江省平湖市为典型三角洲平原区,开展了多种尺度、不同采样方法的对比试验。结果表明,田块内土壤元素分布较为均匀,小范围内元素的空间分异性较小,而田块之间土壤元素含量差异性明显增加;Hg,Cd等典型污染元素空间分异性较强,而人为污染扰动较弱的元素的空间分布较为均一;不同采样方法取得的区域性资料,其统计值接近,空间分布模式也相似,但有些元素仍有较明显差异。因此,中大比例尺地球化学调查时,应根据调查研究的目标任务,充分考虑地块分布、土壤类型和土地利用方式,选择合理的采样点。土壤样品可采取多坑点采集、多子样组合的方式,以保证样品的代表性,同时减少分析测试工作量。  相似文献   

4.
A geochemical survey of Belgium and Luxembourg was carried out as part of an international research project entitled ‘Regional geochemical mapping of Western Europe towards the year 2000'. The aim of this research was to map regional background geochemical patterns based on pristine or at least pre-industrial overbank samples and to deduce regional information on the degree of environmental pollution of floodplain and present-day river sediments. Over the entire study area (about 33,000 km2), 66 overbank sites have been sampled. Catchment areas range between 60 and 600 km2. At each site an overbank profile has been dug out in the immediate vicinity of the river and described in detail. A first composite sample was taken 5–25 cm below the surface. This sample is supposed to represent deposition over the last centuries. Human interferences in this interval are often inferable based on changed sedimentary characteristics and the presence of anthropogenic particles such as charcoal, slags and brick fragments. A second composite sample was taken at depth, usually >1.5 m below the surface over an interval of about 20 cm. In most sites, the profile characteristics allowed to assume pre-industrial or even pristine conditions for this lower overbank sample. In some profiles this was confirmed by 14C-dating and/or by the absence of anthropogenic particles. Finally, a present-day stream sediment was sampled on the site to infer the actual pollution status. After drying at 80°C, disaggregation and sieving, the <125 μm fractions of the three sediment samples were analysed by XRF for major elements and several trace elements. Lower overbank samples generally show a direct link with the geological substrate and allow to assess natural background concentrations. Results from the mapping exercise as well as from the statistical analyses display a clear contrast between the northern part of Belgium where Cenozoic unconsolidated sandy and silty formations dominate which are especially vulnerable for erosion, and the southern part of Belgium and Luxembourg where Paleozoic and Mesozoic sandstones, carbonates, marls and shales are the prevalent lithologies. Here the shales are the most intensively eroded lithologies. This is especially reflected in the element patterns of Al2O3, MgO, K2O, Ga, Ni, Rb, Sc and V which negatively correlate with SiO2. Despite the human related pollution, the geological contrast between north and south Belgium is still recognisable in the geochemical pattern of the upper overbank and present-day stream sediment samples for the above-mentioned elements. Furthermore there is a clear increase in heavy metal contents (Zn, Cd, Pb, Cu), As and in certain locations in Ba from the lower to the upper overbank sediment, as well as to the present-day stream sediment. The relative increase in element content allows to assess the degree of pollution and helps to define those drainage areas where more detailed research is needed.  相似文献   

5.
Research undertaken by IGCP 259 (International Geochemical Mapping) indicates that wide-spaced sampling is a fundamental concept of international geochemical mapping as it appears to provide the only practical way to obtain a relative rapid (10–20 years) overview of global geochemistry. The main aim of this study is to test the suitability of floodplain sediment as a global sampling medium.Thirteen floodplain sediment samples and 13 stream sediment samples were taken at the exit of 13 super large drainage basins (SCB). The areal extents of these basins are in the order of 1000–10,000 km2. Within each SCB, 3–11 stream sediment samples each representing a component catchment basin (CCB) of 100–1000 km2 were also taken. Fourty nine elements were analyzed. The results were compared with the average values derived from calculating thousands of stream sediment data available from China's National Geochemical Mapping (RGNG) program. Strong similiarities were demonstrated in distribution and trends among the three levels of data. The set of floodplain sediment data shows great coincidence with RGNR stream sediment data.  相似文献   

6.
The Geochemical Atlas of Slovak Republic (49,036 km2) at a scale of 1 : 1,000,000 was compiled during 1991–1995 together with maps of associated geochemical and ecological features at a scale of 1 :200,000. Investigations were aimed at the evaluation of concentrations and distributions of Al, As, Ba, Be, Ca, Ce, Co, Cd, Cs, Cr, Cu, Fe, Ga, Hg, K, La, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, Sb, Se, Sn, Sr, Th, U, V, W, Y, Zn, Zr in groundwater (16,359 samples), stream sediments (24,422 samples), rocks (3839 samples), soils (9892 samples from 4946 profiles; A and C horizons of each profile were sampled) and forest biomass (the foliage of the forest tree species from 3063 plots was sampled). In groundwater field measurements of temperature, pH, conductivity, dissolved O2, acidity, alkalinity were done. The evaluation was oriented to the natural radioactivity of the Slovak territory as follows: rocks (K, U, Th, Utot and dose rate, 15,573 reference spectrometric points), radon risk and water (Unat, 226Ra, 222Rn; 5271 samples). The main objective of the Geochemical Atlas was to construct single-component maps showing concentrations of chemical elements, compounds and measured parameters in the researched media of Slovakia at a scale of 1 : 1,000.000 and to create interactive databases of chemical composition and/or measured parameters of groundwater, biomass, rocks, soils, stream sediments and natural radioactivity, for the entire territory of the Slovak Republic. The single-component maps are not constructed in the part ‘Rocks’, where all the main rocks types of Slovakia are presented in the ‘Map of lithogeochemical rock types of Slovakia at a scale of 1: 500,000’. The edition of six atlases has been planned. In 1997 the first three atlases will appear (Groundwater, Biomass, Natural Radioactivity). Publishing of last three atlases (Rocks, Soils, Stream Sediments) is planned for 1998. Since the analytical works on stream sediments were finished during 1997, it was not possible to present here the results of that part of the Geochemical Atlas.  相似文献   

7.
Many large ore deposits have been discovered in eastern China along the Circum-Pacific Rim. However, alluvium, which covers most of the terrain, prevents a complete assessment of the mineralization potential by geological and conventional geochemical approaches. Two deep-penetrating geochemical methods—collection of nanoscale metals in earthgas (NAMEG) and selective leaching of mobile forms of metals in overburden (MOMEO) have been used to investigate the possibility to delineate geochemical blocks generated from large ore deposits buried by alluvial terrains. A wide-spaced sampling was carried out in an area of approximately 250 000 km2 at density of one sample per 400–800 km2. The soil samples were subjected to MOMEO leaching and were analyzed for Au, Ag, Cu, Pb, Zn, Fe, and Mn by atomic absorption spectrometry (AAS). The earthgas samples were analyzed for 17 elements by INNA. The results show that water-extractable metals by MOMEO processing can give prominent expression to concealed deposits and can delineate large-scale geochemical anomalies in the alluvial terrains. The distribution of gold in earthgas is situated along the largest deep fault systems and the concentration centers distribute in the secondary faults on the both sides of the deep fault systems.  相似文献   

8.
The Isola del Toro is a small island (∼1 km2) about 5 nautical miles SW of Sardinia entirely made up of mildly alkaline sodic evolved rocks (benmoreites and trachytes) related to each other by fractionation processes. 40Ar/39Ar dating of an anorthoclase mineral separate from a trachyte gave an age of 11.83 ± 0.10 Ma. This age estimate falls within an interval previously considered a‐magmatic in Sardinia. The results of this study shift the beginning of the Middle Miocene–Quaternary anorogenic volcanism of Sardinia back to the Late Serravalian. The transition between orogenic and anorogenic activity in Sardinia is markedly different from that recorded in the rest of the circum‐Mediterranean area, where this transition is much more gradual, both in time and in terms of magma composition. On the other hand, the transition between orogenic and anorogenic magmatism in Sardinia is rather sharp and lacks hybrid compositions. The Isola del Toro volcanic rocks represent the first anorogenic products produced during the opening of the western Mediterranean Sea.  相似文献   

9.
《Applied Geochemistry》1995,10(5):517-529
A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km2 is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks.Forty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L−1 Au). Gold concentrations in solution up to 3 ng L−1 were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution.The best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.  相似文献   

10.
Multi-element lithogeochemical surveys of aureoles surrounding tin-tungsten-bearing quartz vein systems have been utilized infrequently in exploring for these deposits. At Panasqueira, Beira Beixa, Portugal, a Wrich quartz vein mineralization occurs in the Beira schists surrounding a greisenized cupola and offers a suitable test case for such lithogeochemical studies. Therefore, shale samples were collected at a density of about 6 samples per km2. Pronounced anomalies of tin and tungsten have been detected in these samples, together with those of other elements coeval with the mineralizing processes.The lithogeochemical anomalies at the surface coincide with the extent of the main ore fields. Statistical procedures have been applied to reveal interelement relationships, and contour maps were prepared for single elements and factor scores.Trace element analysis in combination with multivariate statistical data is an effective and rapid aid in locating such mineralizations, apparently also when hidden under a schist cover.  相似文献   

11.
The Council for Geoscience (CGS, South Africa) has a statutory mandate to carry out regional geochemical mapping in South Africa that needs to be rapidly and accurately analysed. Both simultaneous X-ray fluorescence spectrometry (S-XRF) and a newly developed method using inductively coupled plasma-mass spectrometry (ICP-MS) were employed. Various trace elements that could not previously be analysed by S-XRF can now be analysed by ICP-MS for the regional geochemical mapping programme, e.g., Cd, Mo, Te and Li. Using both techniques, the CGS aims to report element distributions for some fifty elements. To ensure that element concentration levels correlate over map boundaries, quality control measures in the sampling, sample preparation and analyses were of critical importance. This paper aims to discuss the sample preparation and quality control measures as applied to the ∼5500 samples of the Giyani and Tzaneen 1:100000 scale map sheets sampled at a density of one soil sample per km2. ICP-MS batch- and instrumental drift-correction procedures will be discussed. As a final step, geochemical data were overlain over simplified geological maps using geographical information system software. These maps complement existing geological information of South Africa, help in the identification of exploration targets, test exploration models and initiate further geological research.  相似文献   

12.
Over 3 million samples were collected from southern China at a density of 1–2 stream sediment samples per square kilometre as part of the Regional Geochemistry National Reconnaissance (RGNR) programme of China initiated in the late 1970s. Approximately 5244 composite samples of stream sediments from twelve provinces of southern China were prepared from the original RGNR samples collected from a territory with an area of 2300000 km2 at a density of one composite sample per 1:50000 map sheet (about 400 km2). Seventy‐six elements were determined by the geoanalysis laboratory of Henan Province (platinum‐group elements) and by the Institute of Geophysical and Geochemical Exploration (IGGE) laboratory for the remainder. Internal quality and external quality control methods were applied to ensure that the analytical data were comparable. Statistics were used to derive the mean and background values and indicate the average concentrations of the seventy‐six elements. Comparisons were made against the mean values obtained from stream sediments, floodplain sediments, overall sediments and overall soils of the entire territory of China. The concentrations of Hg, Cd, rare earth elements and 24 other elements were higher than their background for the whole of China. In contrast, the concentration of Na2O, CaO, Ba and Sr were lower in the stream sediments in southern China than their whole China background.  相似文献   

13.
A collection of 235 samples were taken from 115 sites (representing a density of 1 sampling site ca. 130 km2) on rural soils derived from the major rock types in the southern Iberian Massif. The geochemical baselines of selected trace elements (As, Co, Cr, Cu, Ni, Pb and Zn) were determined on the < 2 mm soil fraction. The sampling sites were not directly influenced by external pollution. Soil geochemical baseline and threshold values were calculated for each element in two geologically different zones: the Ossa-Morena Zone (OMZ) and the South-Portuguese Zone (SPZ).  相似文献   

14.
As the most important source rocks in the eastern Junggar Basin, the middle Permian Pingdiquan (P2p) source rocks have attracted increasing attention after the discovery of tight oil in the Shazhang uplift. The P2p source rocks are widely distributed (up to 7546 km2) and have an elevated thickness in the eastern Junggar Basin. To explore the P2p tight oil resource in the eastern Junggar Basin, 113 core samples from 34 exploration wells were analysed geochemically and re-examined for their organic matter abundance, type and thermal maturity, hydrocarbon potential and sedimentary environment. Geochemical analysis results indicate that the P2p source rocks are fair to good source rocks dominated by Type II kerogen, presently in a low mature–mature stage, and biomarkers and trace elements indicate deposition in a terrestrial to coastal environment under oxic to dysoxic, and fresh to brackish conditions, with possible intermittent seawater influence, implying proximity to the open sea. Based on hydrocarbon expulsion modelling, hydrocarbon expulsion began at 0.87% Ro, and the peak expulsion occurred at 1.1% Ro. Hydrocarbon generation intensities in the Shazhang uplift and the Wucaiwan sag are relatively large, with values centred at 4–6.5 million t/km2 and 4–6 million t/km2, respectively, with total hydrocarbon generation and expulsion from the P2p source rocks approximately 4.56 × 109 t and 1.44 × 109 t, respectively, indicating significant tight oil exploration potential in the eastern Junggar Basin. The Shazhang uplift and the Wucaiwan sag are two hydrocarbon expulsion centres in the study area with the largest hydrocarbon expulsion intensity centred around the Shazhang uplift, exceeding 3 × 106 t/km2. We suggest that the area with high hydrocarbon expulsion intensities is a favourable target for tight oil accumulation and exploration.  相似文献   

15.
Interpolated geochemical maps contain problems of sample catchments and vacant sample areas. However, the resemblance of these maps with regional geological maps implies that there are strong correlations between them. A conceptual model of the geological units from multivariate chemical elements is constructed by decision tree technique. The reproducibility of the geological map with the conceptual model is about 80% in the Chungbuk province map of Korea. Statistical uncertainties indicate that rock units could be predicted reasonably well by decision tree if the geological map polygon is big enough. Misinterpreted points are due to interpolation problems between samples from adjacent rock units and from different sample catchments. In the Chungbuk example, the latter is less significant. The size of the map polygon is one of the most critical factors, and a polygon size greater than 50 km2 is reliable for such analyses for the Chungbuk example. Combined lithologies in a single mapping unit are not critically affected by decision tree prediction.  相似文献   

16.
Antimony in the environment: Lessons from geochemical mapping   总被引：2，自引：0，他引：2  
The distribution of Sb in a variety of sample materials, including soils, plants and surface water, was studied at different scales, from continental to local, combining published data sets with the aim of delineating the impact and relative importance of geogenic vs. anthropogenic Sb sources. Geochemical mapping demonstrates that variation is high at all scales – from the detailed scale with sample densities of many sites per km2 to the continental-scale with densities of 1 site per 5000 km2. Different processes govern the Sb distribution at different scales. A high sample density of several samples per km2 is needed to reliably detect mineralisation or contamination in soil samples. Median concentrations are so low for Sb in most sample materials (below 1 mg/kg in rocks and soils, below 0.1 mg/kg in plants, below 0.1 μg/L in surface water) that contamination is easier to detect than for many other elements. Distribution patterns on the sub-continental to continental-scale are, however, still dominated by natural variation. Given that the geochemical background is characterised by a high variation at all scales, it appears impossible to establish a reliable single value for “good soil quality” or a “natural background concentration” for Sb for any sizeable area, e.g., for Europe. For such a differentiation, geochemical maps at a variety of scales are needed.  相似文献   

17.
The results of a first systematic study of spatial distribution of different elements in surface soil over of the Kavadarci region, Republic of Macedonia, known for its nickel industrial activity are reported. The investigated region (360 km2) is covered by a sampling grid of 2 × 2 km2; whereas the sampling grid of 1 × 1 km2 was applied in the urban zone and around the ferronickel smelter plant (117 km2). In total 344 soil samples from 172 locations were collected. At each sampling point soil samples were collected at two depths, topsoil (0–5 cm) and bottom soil (20–30 cm). Inductively coupled plasma-mass spectrometry (ICP-MS) was applied for the determination of 36 elements (Ag, Al, As, Au, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, Hg, K, La, Mn, Na, Mg, Mo, Ni, P, Pb, S, Sb, Sc, Se, Sr, Th, Tl, Ti, U, V, W and Zn). Data analysis and construction of maps were performed using the Paradox (ver. 9), Statistica (ver. 6.1), AutoDesk Map (ver. 2008) and Surfer (ver. 8.09) software. Four geogenic and three anthropogenic geochemical associations were established. Within the research, natural and anthropogenic enrichment with heavy metals was determined. Principally, the natural enrichment is related especially to Ni. Pollution by As, Cd, Co, Cr, Cu, Hg, Mo, Pb and Zn is basically insignificant.  相似文献   

18.
This paper reviews some aspects of the use of “loam” (soil) concentrates in geochemical surveys in arid, deeply weathered environments.An orientation survey at a small Ni-Cu-Co prospect in Western Australia has shown that discrimination between mineralized and unmineralized samples could be achieved using Ni, Cu, Co, Cr, Zn, As, Sn, Sc, Ti, Yb and Y in the coarse fraction of heavy concentrates. However, at the same prospect the best contrast for Ni, Cu and Co in surface samples was provided by analysis of the same fraction following a cold ammonium citrate/hydroxylamine hydrochloride digestion.At a nearby, larger prospect, some 54 km2 in area, concentrates were separated, by jigging, from bulk soil samples, themselves composites of representative subsamples. Sampling at a density of 4 samples per km2 revealed 1–2 km2 size anomalies of Cu, Ni, Co, Cr, As and Au which could be related, variously, to known Ni-Cu and Au mineralization.In Botswana, analysis of concentrates, separated by tabling from samples collected at a density of 1 sample per 7.5 km2 over an area of 5400 km2, identified distinctive geochemical districts. Enhanced values of Au and of Cu-Ni in the concentrates were relatable to known mineralization and the results suggested that there were also Sn-W-Mo-Bi (granitoid) and Au-Pb-Zn-Bi-Sn (volcanogenic) associations which could lead to new prospecting targets. Anomalies of certain elements (for example, Cu in an ultramafic environment) may be more readily detected in surface material by “enrichment indexing” the concentrate data.  相似文献   

19.
Fractal geometry is receiving increased attention as a model for many natural phenomena. In this paper, we address two issues in dealing with unorganized 3-D measured data using fractal geometry: extraction of fractal characters and fractal surface (geochemical landscapes) reconstruction. We start from a set of randomly measured 3-D data on a plane. After classification and concentration of the input data, we present methods for estimating the fractal dimension and texture deviation as fractal characters, and then an improved subdivision scheme is developed to reconstruct fractal surface on the basis of the extracted fractal characters. We demonstrate both characterization and reconstruction with irregularly measured geochemical data from 1767 stream sediment samples in the middle district (450km2 in area) of Zhejiang, China.  相似文献   

20.
Jun Zhou    Ping Han    Lei Liu    Qin-hu Tian  De-cheng Li 《Resource Geology》2007,57(1):76-89
The Bumo gold deposit is a low‐sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias). Tectonically, the SW‐NE‐oriented Gezhen Ductile‐Shearing Zone was largely attenuated in Bumo. It is hosted by gneisses and schists of Proterozoic age (locally called the Baoban Group), but the radiometric age of ores mainly is Jurassic–Cretaceous, being coeval to the regional metallogenesis along the Eurasian margin. The affinity with the Phanerozoic orogenic gold deposits is indicated by low salinity (2.4 wt% NaCl–10.3 wt% NaCl), intermediate temperature (150–300°C) and appropriate δ18O value (approximately 7–11‰). The gold metal tonnage, proved previously, within an area smaller than 1.0 km2 in the gold belt II amounted to 11.59 t, with the average gold grades for the major Au quartz veins (7 and 8) in this belt of 52.7 and 71.1 g/t, respectively. This shows significant enrichment of gold and an unusual “nugget effect”. A ring structure, mainly represented by ridges, valleys, dikes, big bull quartz veins and lithological borders or pertinent faults, marks the gold field, and its origin was linked to a possible magmatic diapir. It was viewed as macroscopic evidence that the dip angles of strata tend to decline gradually from deeper ones in the center area to nearly horizontal in the outskirts, and certain intrusion‐affiliated elements such as Bi, As and W are considerably abundant in soil samples. Two areas, namely the area A (3.06 km2) in the outskirts of the ring structure or in northeastern Bumo and the area B (1.32 km2) in the field center or to the west of the gold belts I and II, were selected for the geochemical survey in a 50 × 50‐m sampling grid. The analytical results of the samples from horizon C and in a ?80 mesh size showed that the average contents of Bi, As and W amounted to 0.55, 19.3 and 3.67 ppm in the area A (1225 samples) and to 0.79, 68.3 and 4.14 ppm in the area B (529 samples), respectively. This geochemical exploration led to the discovery of several Au quartz veins in both areas A and B, which also manifested an Au nugget effect (the average Au grades between 50 and 100 g/t) in spite of their limited scales (from a couple of centimeters to 50 cm wide and from several meters to 15 m long). A mylonite zone in area A was also found by this study. An intimate spatial correlation between a lithological border (gneiss and schist or pegmatite) or the mylonite zone and Au anomalies indicates an important prospect for further gold exploration.  相似文献   

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