Regional geochemical secondary negative anomalies and their significance

Research involving secondary negative anomalies and their application in regional stream sediment surveys has been scarce. Conventionally, negative anomalous threshold values have been calculated in the same way as positive anomalous threshold values. But the conventional methods all have drawbacks which hinder their application. In this paper, we have chosen to delineate negative (and/or positive) anomalies using contrast values in order to overcome the drawbacks.Regional stream sediment surveys at a scale of 1 : 200 000 have been carried out in western Jungger, Shanxi, Kunlun, northwestern Jiangxi, and other areas in China. The geochemical data were processed using System RESMA.On the basis of the distribution of negative and positive single-element anomalies, three possible arrangements may occur: (1) negative anomalies accompanied by positive anomalies; (2) only positive anomalies occurring with no negative anomaly nearby; (3) only negative anomalies, with no positive anomaly nearby. These situations reflect different geological settings and different mineral forming processes. Basically, two different distribution patterns of regional negative anomalies in relation to the backgrounds — low background (LB) and high background (HB) —may be observed in different geological environments: (1) regional negative anomalies are distributed only around the positive anomalies in the LB area; (2) regional negative anomalies can exist on the periphery of positive anomalies in both LB and HB areas.Two kinds of patterns for regional multi-element negative and positive anomalies reflecting different geological processes have been noted: (1) coincident positive anomalies for one group of elements and negative anomalies for another group of associated elements can be used to uniquely define ore deposition; (2) regional positive multi-element anomalies of some elements (including ore and associated elements) occurring over a deposit are accompanied by negative anomalies on the periphery of the deposits. Two regional models of negative and positive anomalies are established for Au and Cu deposits.Integration of multi-element positive and negative composited geochemical anomalies are much more useful than positive anomalies or positive composited anomalies to delineate regional structures.     

多重地球化学背景下地球化学弱异常增强识别与信息提取

为对钦州湾-杭州湾成矿带(南段)庞西垌地区地球化学数据进行异常识别研究与信息提取,利用含量-面积法(C-A)得出庞西垌地区成矿主元素的异常下限,得到各元素异常分布图,并与已知矿(床)点进行叠加分析,发现已知矿(床)点与C-A法分析得到的异常区基本吻合,可根据该异常区预测未知矿床,从而为该研究区矿产资源潜力评价提供依据。为进一步从研究区复杂的地球化学背景中分离出与成矿有关的地球化学异常,采用分形滤波技术(S-A)提取致矿异常。研究表明,S-A法可在C-A法揭示的区域异常的基础上更深层次地提取出与矿化有关的局部异常用以反映研究区的多重地球化学背景,S-A法可有效地使弱异常增强进而提取出致矿异常,为庞西垌地区探寻隐伏矿体提供依据。     

The effect of scale on the interpretation of geochemical anomalies

The purpose of geochemical surveys changes with scale. Regional surveys identify areas where mineral deposits are most likely to occur, whereas intermediate surveys identify and prioritize specific targets. At detailed scales specific deposit models may be applied and deposits delineated.The interpretation of regional geochemical surveys must take into account scale-dependent difference in the nature and objectives of this type of survey. Overinterpretation of regional data should be resisted, as should recommendations to restrict intermediate or detailed follow-up surveys to the search for specific deposit types or to a too limited suite of elements. Regional surveys identify metallogenic provinces within which a variety of deposit types and metals are most likely to be found. At intermediate scale, these regional provinces often dissipate into discrete clusters of anomalous areas. At detailed scale, individual anomalous areas reflect local conditions of mineralization and may seem unrelated to each other. Four examples from arid environments illustrate the dramatic change in patterns of anomalies between regional and more detailed surveys.On the Arabian Shield, a broad regional anomaly reflects the distribution of highly differentiated anorogenic granites. A particularly prominent part of the regional anomaly includes, in addition to the usual elements related to the granites, the assemblage of Mo, W and Sn. Initial interpretation suggested potential for granite-related, stockwork Mo deposits. Detailed work identified three separate sources for the anomaly: a metal-rich granite, a silicified and stockwork-veined area with scheelite and molybdenite, and scheelite/powellite concentrations in skarn deposits adjacent to a ring-dike complex.Regional geochemical, geophysical and remote-sensing data in the Sonoran Desert, Mexico, define a series of linear features interpreted to reflect fundamental, northeast-trending fractures in the crust that served as the prime conduits for mineralizing fluids. At a larger scale, the linear, northeast-trending anomalies can be shown to result from a series of discrete mineralized systems with different ages and mineral assemblages. The linear pattern of anomalies disintegrates.A regional geochemical survey in the Sonoran Desert in southwestern Arizona displays a cluster of samples anomalous in Pb, Mo, Bi and W. In detail, the original regional anomaly separates into four discrete anomalous areas, each with its own distinctive suite of elements, geographic distribution and age of mineralization.A prominent regional gold anomaly in the Gobi Desert, Xinjiang, Peoples Republic of China, extends southeastward for 30 km from known lode gold deposits. Because the anomaly cuts both lithologic units and the structural grain, and because it parallels the prevailing direction of high-velocity winds, it was originally attributed to eolian dispersion. In detail, the regional anomaly consists of several east-west-trending anomalies, parallel to local lithology and structure that most likely reflect independent sources of lode gold. The regional anomaly results from smoothing of an en-echelon set of local anomalies.These examples emphasize that interpretation of regional anomalies must be tempered to consider regional-sized geologic features. Attempts to overinterpret anomalies by assigning deposit-scale attributes to regional anomalies can lead to confusion and incorrect interpretations. Potential targets that can be readily resolved only at intermediate or detailed scales of study may be overlooked.     

R-mode factor analysis applied to uranium exploration in the Montrose quadrangle, Colorado

R-mode factor analysis is used to describe the relationships among 18 remotely sensed and geochemical data sets (variables) for the Montrose 1° × 2° quadrangle, Colorado, a region that covers 19 000 km². The data sets contain reconnaissance-scale information and include Landsat imagery, airborne geophysical information (eU, eTh, K40, aeromagnetics), elevation, and hydrogeochemical and stream sediment analyses. The elements U, K, Dy, Hf, V, Th, Ca and Ba in sediments and U in waters were selected.The results of the factor analysis for the entire quadrangle are compared to the results for a 50 km × 50 km test area containing several known uranium occurrences. Four factors account for 70.0% of the total variance in the data. These are interpreted as a felsic factor, Landsat factor, economic or mineralization factor (in terms of uranium mineralization and potential mineralized areas), and a volcanic factor.Graphical representations (maps) of the raw data, factor approximations, residuals for each data set, and the four-factor model greatly aid interpretation of the analytic results. We find that data integration techniques and R-mode factor analysis can be used with some success in uranium resource appraisal.     

Geochemical evidence for a brooks range mineral belt, Alaska

Geochemical studies in the central Brooks Range, Alaska, delineate a regional, structurally controlled mineral belt in east—west-trending metamorphic rocks and adjacent metasedimentary rocks. The mineral belt extends eastward from the Ambler River quadrangle to the Chandalar and Philip Smith quadrangles, Alaska, from 147° to 156°W. longitude, a distance of more than 375 km, and spans a width from 67° to 69°N. latitude, a distance of more than 222 km. Within this belt are several occurrences of copper and molybdenum mineralization associated with meta-igneous, metasedimentary, and metavolcanic rocks; the geochemical study delineates target areas for additional occurrences.A total of 4677 stream-sediment and 2286 panned-concentrate samples were collected in the central Brooks Range, Alaska, from 1975 to 1979. The −80 mesh (< 177 μm) stream sediment and the heavy (specific gravity > 2.86) nonmagnetic fraction of the panned concentrates from stream sediment were analyzed by semiquantitative spectrographic methods.Two geochemical suites were recognized in this investigation; a base-metal suite of copper-lead-zinc and a molybdenum suite of molybdenum-tin-tungsten. These suites suggest several types of mineralization within the metamorphic belt. Anomalies in molybdenum with associated Cu and W suggest a potential porphyry molybdenum system associated with meta-igneous rocks. This regional study indicates that areas of metaigneous rocks in the central metamorphic belt are target areas for potential mineralized porphyry systems and that areas of metavolcanic rocks are target areas for potential massive sulfide mineralization.     

Using robust staged R-mode factor analysis and logistic function to identify probable Cu-mineralization zones in Khusf 1:100,000 sheets,east of Iran

Factor analysis method is a multivariate analysis technique that is widely used for the interpretation of stream sediment geochemical data. The purpose of factor analysis is describing the changes in a set of multi-element geochemical data by reducing the dimension of the data and variables to a number of factors that can present the hidden association between elements. Differences in mobility, physical, and chemical properties of the elements and the nature of the factor analysis method in which the matrix of all data is used cause paragenes elements not to be found on the output of factor analysis. In this research, to improve the output of factor analysis for deriving the best reagent multi-element mineralization, robust staged factor analysis method was used according to the close nature of geochemical data in order to identify the Cu-mineralization potential in Khusf 1:100,000 sheets located at the east of Iran. The robust staged factor analysis enhances the recognition of anomalous geochemical signatures and increases geochemical anomaly intensity and the percentage of the total explained variability of data. As indicated by the results of the study, few anomalous zones have been found in the study area. The observation of chalcopyrite and malachite mineralization in andesite and dacite–andesite rocks in a region during the field study confirms the effectiveness of the robust SFA technique. Such studies can be used by mine engineers and geologists for designing an optimum grid exploration on the next exploration steps.     

Unmasking multivariate anomalous observations in exploration geochemical data from sheeted-vein tin mineralization near Emmaville, N.S.W., Australia

Previous interpretations of surface-rock geochemical data from the sheeted-vein tin mineralization in the Emmaville district have been carried out using classical statistics. These investigations revealed low-contrast geochemical patterns of 3 to 5 ppm Sn, supported by 80 to 160 ppm F, block-average contours defining four of the six known mineral occurrences. Principal component scores for the association dominated by F-Li-Rb have defined the same four mineral occurrences. For the prospecting of similar deposits it is highly desirable to improve the data processing techniques to achieve more acceptable geochemical contrasts between anomalous and background levels. Minimum volume ellipsoid (MVE) estimation, a high-breakdown method (capable of accommodating up to 50% outliers) recently developed in robust statistics is applied to a subset of the data from the northeastern part of the Emmaville district. The anomalies related to mineralization in this part of the district are not as well developed compared to those in the west. The data set used in this study consists of 133 observations with 6 elements, namely Cu, Li, Rb, F, As and Sn.The detection of multivariate outliers (anomalous observations) by Mahalanobis distance calculation was carried out on the surface rock geochemical data. The robust Mahalanobis distances computed from MVE estimates of location and scatter shows little variation over background areas but are sharply enhanced over mineralization. In contrast, the usual Mahalanobis distances either fail to indicate the presence of mineralization altogether, or, at best, respond with feebly enhanced values that do not satisfactorily indicate the presence of mineralization.Graphical display of results from classical RQ-PCA performs poorly, revealing only 6 weakly anomalous observations related to mineral occurrences. Several additional observations from these occurrences have also gone undetected. On the other hand, results from MVE-robust RQ-mode principal component analysis show that the background observations cluster tightly within the 95% tolerance ellipse while the anomalous observations (related to mineral occurrences) are greatly enhanced and the variables that characterize them are clearly indicated. Results are consistent with those of robust Mahalanobis distance procedure; both techniques indicate essentially the same observations as being anomalous.     

青海省纳日宗地区水系沉积物地球化学特征及找矿远景

纳日宗地区位于南祁连岩浆弧和宗务隆山—夏河—甘加裂谷的接触部位,该区及其周边已发现金属矿床(点)10余处,具有良好的金、铅、铜矿找矿前景。以该区1∶5万水系沉积物测量的16种元素含量数据为基础,统计分析了水系沉积物地球化学参数、元素相关性、成矿有利度系数等指标,总结了元素组合、富集贫化规律及其与地质背景、地质构造与成矿作用的关系。区内主要富集元素为Sn、Pb、Bi、As、Sb,成矿有利度系数高的元素有As、Pb、W、Au、Bi、Sb。结果显示,水系沉积物元素异常沿NW—NWW向断裂构造分布,初步认为构造发育地段具有较好的Au、Pb、W找矿潜力。依据单元素异常特征和成矿地质条件,圈定综合异常74处,划分出3个找矿远景区,指出了各找矿远景区的找矿方向。     

长江中下游地区不同尺度铜地球化学异常的意义与大型矿床预测

以长江中下游1∶20万铜区域地球化学数据为基础,研究了铜元素地球化学异常特征,认为不同尺度的地球化学异常图具有不同的研究意义:(1)1∶20万地球化学异常可以圈定矿床异常,用于大型矿床预测。研究区内13个大型矿床中有12个落在具有三层套合结构的地球化学异常中,已知矿床储量与异常面金属量、异常面积之间的相关系数分别为0.94、0.95,显示区域地球化学异常规模与储量之间的较好相关性。(2)1∶50万地球化学异常可以圈定矿区异常,用于在成矿带中预测有利成矿区。(3)1∶100万地球化学异常可以圈定大型矿集区或成矿带,用于矿集区预测。如果把研究区内面积大于1 000km2且含有3个以上已知矿床的异常作为矿集区的话,则长江中下游存在3个大型矿集区:马鞍山—南京矿集区、九江—瑞昌—大冶矿集区和德兴—黄山—安庆—铜陵矿集区(实际上包含德兴和铜陵2个矿集区)。大型矿床多产于多层套合的地球化学异常中,大型矿集区所形成的异常具有至少3层套合结构,浓集中心与大型矿床存在对应关系,这些规律的发现为在不同成矿域预测新的大型矿集区提供了重要地球化学标志。     

基于盲源分离理论的地球化学矿致异常识别研究

尝试将盲源分离技术用于地球化学领域中,应用盲源分离理论中的FastICA算法对西藏洞嘎普铜矿勘查区1∶10 000土壤地球化学测量数据进行了矿致异常识别研究。首先根据盲源分离算法建立反演化探数据元素组合模型,以此确定地球化学成矿元素组合;然后,利用分形方法确定地球化学单元素及元素组合的异常下限,圈定异常浓集中心,进而确定异常分带性;最后,将元素分带特性研究与研究区地质特征相结合,对比单元素异常图及组合异常图,对研究区的地球化学元素作出异常分类和异常评价解释,划分的4个异常区域在后期的工程验证中取得了很好的效果。     

Separation of a geochemical anomaly from background by fractal and U-statistic methods,a case study: Khooni district,Central Iran

Separation of geochemical anomalies from background are one of the important steps in mineral exploration. The Khooni mineral district (Central Iran) has complex geochemical surface expression due to a complex geological background. This region was chosen as a study area for recognition of the spatial distribution of geochemical elements and separating anomalies from background using stream sediment geochemical data. In the past decades, geochemical anomalies have been identified by means of various methods. Some of these separation methods include: statistical analysis methods, spatial statistical methods and fractal and multi-fractal methods. In this article, two efficient methods, i.e. U-statistics and the fractal concentration-area for separation and detection of anomalous areas of the background were used. The U spatial statistic method is a weighted mean, which considers sampling point positions and their spatial relation in the estimation of anomaly location. Also, fractal and multi-fractal models have also been applied to separate anomalies from background values. In this paper, the concentration–area model (C–A) was suggested to separate the anomaly of background. For this purpose, about 256 stream sediment samples were collected and analyzed. Then anomaly maps of elements were generated based on U spatial statistics and the C-A fractal methods for Au, As and Sb elements. According to obtained results, the U-statistics method performed better than C-A method. Because the comparisons of the known deposits and occurrences against the anomalous area created using thresholds from U-statistics and C-A method show that the spatial U-statistics method hits all of 3 known deposits and occurrences, the C-A fractal method hits 1 and fails 2. In addition, the results showed that these methods with regard to spatial distribution and variability within neighboring samples, in addition to concentration value frequency distributions and correlation coefficients, have more accurate results than the traditional approaches.     

Regional mineral resources assessment based on rasterized geochemical data: A case study of porphyry copper deposits in Manzhouli,China

This study used regional geochemical survey data (1:200,000 scale) from the Manzouli area of China to assess mineral resources. Geochemical survey data was rasterized and a geochemical atlas was generated, with the image pixel size determined according to geochemical exploration sample point spacing. The Wunugetushan, Babayi, and Badaguan porphyry copper deposits were selected as model areas for the assessment of copper mineral resources. Three parameters were considered for the calculation of the mineral resources. An ore-bearing hydrothermal alteration coefficient was determined based on geological characteristics and geochemical characteristics of the model area, in order to determine alteration intensity; a denudation coefficient was calculated to determine denudation extent; and a mineralization intensity coefficient was calculated to determine the intensity of mineralization within each pixel. Resource estimation was conducted through regression analysis of model deposit resources and coefficients. The results can be used to determine prospecting target areas based on frequency classification and can be used to estimate the number of ore deposits. Results show that resource estimation using rasterized geochemical data provides high prediction precision and accurate positioning.     

Evaluation of lake sediment surveys using the relationship of metal tenor to organic content

In geochemical surveys, including those of lake sediments, it is convenient to classify logarithmic cumulative frequency distributions of elements as anomalous, background, or intermediate. The third, and commonest, group presents the dilemma - Is a given distribution in that group tending towards an anomalous or a background distribution? Data from a series of surveys suggest that the relationship of loss on ignition to metal tenor can be used as an indicator of an element's “potential” or “availability” in the region of the survey. In turn, increased availability implies an anomalous source, such as an ore deposit. Areas with the best potential are considered to be those where logarithmic cumulative distributions of particular elements have the same or greater positive slopes than the associated loss on ignition distribution. The method does not define areas within the survey that can be considered more anomalous, only whether or not the given survey area exhibits a greater than expected concentration of a given element.     

Identification of Geochemical Anomaly by Multifractal Analysis

The separation of anomalies from geochemical background is an important part of data analysis because lack of such identifications might have profound influence on or even distort the final analysis results. In this article, 1 672 geochemical analytical data of 11 elements, including Cu, Mo, Ag, Sn, and others, from a region within Tibet, South China, are used as one example. Together with the traditional anomaly recognition method of using the iterative mean ±2σ, local multifractality theory has been utilized to delineate the ranges of geochemical anomalies of the elements. To different degrees, on the basis of original data mapping, C-A fractal analysis and singularity exponents, Sn differs from the other 10 elements. Moreover, geochemical mapping results based on values of the multifractal asymmetry index for all elements delineate the highly anomalous area. Similar to other 10 elements, the anomalous areas of Sn delineated by the asymmetry index distribute along the main structure orientations. According to the asymmetry indexes, the 11 elements could be classified into 3 groups: (1) Ag and Au, (2) As-Sb-Cu-Pb-Zn-Mo, and (3) Sn-Bi-W.This paragenetic association of elements can be used to interpret possible origins of mineralization, which is in agreement with petrological analysis and field survey results.     

Delineation of Integrated Anomaly with Generative Adversarial Networks and Deep Neural Networks in the Zhaojikou Pb-Zn Ore District,Southeast China

Geochemical maps are of great value in mineral exploration. Integrated geochemical anomaly maps provide comprehensive information about mapping assemblages of element concentrations to possible types of mineralization/ore, but vary depending on expert’s knowledge and experience. This paper aims to test the capability of deep neural networks to delineate integrated anomaly based on a case study of the Zhaojikou Pb-Zn deposit, Southeast China. Three hundred fifty two samples were collected, and ea...     

东昆仑拉浪麦地区地球化学勘查异常下限确定方法对比研究

东昆仑地区是我国重要成矿带,地球化学勘查作为该区基础性的勘查手段在区域找矿、矿区及其外围找矿发挥了重要作用。地球化学勘查工作中,异常下限的确定是圈定地球化学异常的基础和关键。拉浪麦钨多金属矿区地球化学勘查工作中利用传统统计法、EDA法、含量—面积分形法分别对土壤地球化学分析元素计算确定异常下限,并结合成矿地质条件对三种方法计算出的异常下限进行综合对比研究,发现EDA法得出钨等元素异常不漏掉隐伏矿床形成的矿致异常,从而达到快速精准高效的找矿效果,具有很好代表性。因此,在东昆仑地区寻找钨多金属矿地球化学勘查中,采用EDA法确定异常下限是可行的。     

A catchment basin approach to the analysis of reconnaissance geochemical-geological data from Albay Province,Philippines

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.     

Statistical map analysis of regional stream-sediment data from Australia

In a regional stream-sediment survey of the Seigal and Hedleys Creek areas, covering approximately 6000 km², in all 2508 stream-sediment samples were collected with an overall sample density of 1 per 2–3 km². The minus 180 μm fraction of the samples was analyzed for up to 25 elements including As, Ba, Be, Bi, Ce, Co, Cr, Cu, F, Fe, Li, Mn, Mo, Nb, Ni, Pb, Rb, S, Sn, Th, Ti, U, W, Y and Zn.Selected elements were processed by a statistical map-analysis technique to: (a) display the broad-scale regional distribution patterns of the elements using a cell-average gap-fill mapping technique followed by low-pass filtering (LPF); and (b) delineate anomalous areas using the picture frame filter (PFF) and the Kolmogorov-Smirnov filter (KSF).The regional distribution patterns of elements clearly relate to geologic belts, plutons and stratigraphic units. Anomaly filtering clearly identifies the various known mineral occurrences including U, U-Cu-Sn, and Pb-Zn. The technique also delineated several anomalies, unrelated to known mineral occurrences, having similar anomalous element combinations and occurring in similar geological settings as those related to known mineral occurrences. Some of these filter-enhanced anomalies are too subtle to be immediately apparent in the unprocessed data.     

Multidimensional statistical technique for detection of low contrast geochemical anomalies

In this article we propose an advanced technique for detecting low contrast geochemical anomalies using a set of features. There are three principal elements in this technique: (1) a statistical measure of the contrast of the anomaly, denoted as τ; (2) selection of a background population; and (3) reduction of the dimensionality of the feature space. In the frame of the model, which describes the statistical distribution of geochemical background as a multidimensional normal distribution of logarithms of concentrations, the index, τ, is a powerful test statistic for the hypothesis of abnormality of an observation. Maps of τ anomalies can be rigorously interpreted on the basis of statistical inferences. Under all equal conditions this technique allows the detection of geochemical anomalies with at least the same contrast (if the chemical elements in a background population are correlated, then even the better) as using selective extractions of metals from soil or other techniques for data processing. The advantages of the proposed technique are demonstrated both theoretically and on examples of rare-metal and copper–nickel mineral deposits.     

Uranium districts defined by reconnaissance geochemistry in South Greenland

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.