Geochemical Mapping——Evolution of Its Aims,Ideas and Technology

The development of geochemical mapping progressed from local geochemical prospecting through regional geochemical exploration and regional geochemical mapping to national and global geochemical mapping. This paper discusses the evolution of aims, ideas and methodology of geochemical mapping in Western countries, Russia and China. The sophistication of geochemical mapping methodology will make great contributions to solving resources and environmental problems in the 21^st century.

Outlines of New Global Geochemical Mapping Program

Since 1988,great efforts and enthusiasm had been paid by applied geochemists in the implementation of global geochemical mapping through the International Geological Correlation Program＇s Projects 259 and 360,and the Task Group on‘Global Geochemical Baselines＇ established by the International Union of Geological Sciences（IUGS）,in collaboration with the International Association of Geochemistry（IAGC）.But how to use extremely low-density sampling to obtain a global picture of the distribution of most elements in the periodic table in a reasonably short time is still a great challenge faced by the applied geochemistry community.It will depend on the continuous development of new mapping concept,and the advisable and courageous innovation of methodology for searching other suitable sample media and sampling layout.Based on the encouragement results obtained from the representativeness study of delta sediments conducted at the mouth of Yangtze River,and at the mouths of its four major tributary,it is expected to broadly apply the geochemical fractal self-similarity nature to main rivers and their estuaries with catchments up to hundreds of thousands or over a million square kilometers in the world.With this new mapping concept,a new outlines of a Global Geochemical Mapping Program was advanced and the establishment of an International Research Center of Global Geochemical Mapping was also suggested to facilitate the programs implementations.

Geochemical Mapping:With Special Emphasis on Analytical Requirements

More than 40 national and regional geochemical mapping projects in the world carried out from 1973 to 1988 do not conform to common standards. In particular they have many analytical deficiencies. In the period 1988 to 1992, the International Geochemical Mapping project （Project 259 of UNESCO＇s IGCP Program） prepared recommendations designed to standardize geochemical mapping methods. The analytical requirements are an essential component of the overall recommendations. They included the following： 71 elements should be analyzed in future mapping projects; the detection limits of trace and ultratrace elements must be lower than the corresponding crustal abundances; and the Chinese GSD and Canadian STSD standard sample series should be used for the correlation of global data. A proposal was also made to collect 5000 composite samples, at very low sampling densities to cover the whole Earth＇s land surface. In 1997 an IUGS Working Group on Global Geochemical Baselines was formed to continue the work which began with IGCP 259. From 1997 up to now, new progress has been made especially in China and FOREGS countries under the aegis of this working group, including the study of suitable sampling media, development of a multi-element analytical system, new proficiency test for selection of competent laboratories and role of wide-spaced mapping in mineral exploration. One of the major problems awaiting solution has been the inability of many laboratories to meet the IGCP recommendations to generate high quality geochemical maps. Fortunately several laboratories in China and Europe have demonstrated an ability to meet the requirements and they will be well placed to render technical assistance to other countries.

Gold enrichment characteristics and exploration prospects in Zambia: Based on 1∶1000000 geochemical mapping

The 1∶1000000 geochemical mapping of Zambia provides catchment sediment geochemical data for 58elements including Au from 746 sediment samples at 736 sampling sites,corresponding to a sampling density of about one site per 1000 km2.Under strict quality control using field duplicates,certified reference materials,and analytical replicate samples,the Au was determined by Inductively Coupled Plasma Mass Spectrometry(ICP-MS).The detection limit of Au was 0.20×10^(-9).The 95%range(2.5%–97.5%)of Au concentrations was from 0.24×10^(-9) to 1.36×10^(-9),and the median value was 0.40×10^(-9).The most noticeable Au distribution patterns shown on the map are mainly located between Lusaka and Ndola(Lufilian Arc Belt).In addition,several high Au value areas occurred in Mansa,Muyombe,Chipata,and Livingstone.The spatial distribution patterns of Au in tectonic units,drainage basins,and geomorphological landscapes could be related to the Lufilian Arc Belt and Bangweulu Block.The Au concentrations show metallogenic belts between Muyombe and Mbala areas,between Mansa and Ndola areas,and between Lusaka and Kasempa areas.

Characteristics of B,Zn and Se in stream sediments of Tanzania and their agricultural applications:Based on 1∶1000000 geochemical survey

Tanzania is located in eastern Africa with a predominantly agricultural ecomomy,the potential for developing and utilizing cultivated land are promising,but scientific guidance is required.B,Zn and Se are essential micronutrients for plants and human body with crucial biological functions,in particular,Se is significant for human health and considered as“the king of anti-cancer”.As these elements required by human or plants are mainly absorbed from soil directly or indirectly,therefore,it is important to understand the contents and distributions of them in the soil of cultivated land for guiding agricultural production.In this work,low-density geochemical survey at the scale of 1∶1000000 was carried out in Tanzania,and the results show that the concentrations of B,Zn and Se in stream sediments are low and their distributions are heterogeneous.According to the distributions of geological units,the existing cultivated land resources can be divided into five regions in Tanzania.Compared with the national background values,the concentrations of B,Zn and Se are insufficient overall but enriched locally in these regions.In general,element concentrations in stream sediments and soil have a positive correlation because of their similar sources,which is essential in agriculture application.Based on the information provided by low-density geochemical data and maps,the Se-sufficient and Se-rich regions were delineated in Tanzania,where can be used to develop Se-rich industries.Finally,this paper believes that geochemical survey is a powerful tool for cultivated land evaluation,agriculture management and land development.

History and evaluation of national-scale geochemical data sets for the United States

Six national-scale,or near national-scale,geochemical data sets for soils or stream sediments exist for the United States.The earliest of these,here termed the ＇Shacklette＇ data set,was generated by a U.S. Geological Survey（USGS） project conducted from 1961 to 1975.This project used soil collected from a depth of about 20 cm as the sampling medium at 1323 sites throughout the conterminous U.S.The National Uranium Resource Evaluation Hydrogeochemical and Stream Sediment Reconnaissance（NUREHSSR） Program of the U.S.Department of Energy was conducted from 1975 to 1984 and collected either stream sediments,lake sediments,or soils at more than 378,000 sites in both the conterminous U.S.and Alaska.The sampled area represented about 65%of the nation.The Natural Resources Conservation Service（NRCS）,from 1978 to 1982,collected samples from multiple soil horizons at sites within the major crop-growing regions of the conterminous U.S.This data set contains analyses of more than 3000 samples.The National Geochemical Survey,a USGS project conducted from 1997 to 2009,used a subset of the NURE-HSSR archival samples as its starting point and then collected primarily stream sediments, with occasional soils,in the parts of the U.S.not covered by the NURE-HSSR Program.This data set contains chemical analyses for more than 70,000 samples.The USGS,in collaboration with the Mexican Geological Survey and the Geological Survey of Canada,initiated soil sampling for the North American Soil Geochemical Landscapes Project in 2007.Sampling of three horizons or depths at more than 4800 sites in the U.S.was completed in 2010,and chemical analyses are currently ongoing.The NRCS initiated a project in the 1990s to analyze the various soil horizons from selected pedons throughout the U.S.This data set currently contains data from more than 1400 sites.This paper（1） discusses each data set in terms of its purpose,sample collection protocols,and analytical methods;and（2） evaluates each data set in terms of its appropriateness as a national-scale geochemical database and its usefulness for nationalscale geochemical mapping.

The IUGS/IAGC Task Group on Global Geochemical Baselines

The Task Group on Global Geochemical Baselines,operating under the auspices of both the International Union of Geological Sciences(IUGS) and the International Association of Geochemistry(IAGC),has the long-term goal of establishing a global geochemical database to document the concentration and distribution of chemical elements in the Earth's surface or near-surface environment.The database and accompanying element distribution maps represent a geochemical baseline against which future human-induced or natural changes to the chemistry of the land surface may be recognized and quantified.In order to accomplish this long-term goal,the activities of the Task Group include:(1) developing partnerships with countries conducting broad-scale geochemical mapping studies;(2) providing consultation and training in the form of workshops and short courses;(3) organizing periodic international symposia to foster communication among the geochemical mapping community;(4) developing criteria for certifying those projects whose data are acceptable in a global geochemical database;(5) acting as a repository for data collected by those projects meeting the criteria for standardization;(6) preparing complete metadata for the certified projects;and(7) preparing,ultimately,a global geochemical database.This paper summarizes the history and accomplishments of the Task Group since its first predecessor project was established in 1988.

Geochemical Soil Survey for Base and Precious Metals in Dagbala-Atte District, Southwestern Nigeria

Geochemical survey of residual soils was undertaken in Dagbala-Atte District of Igarra Schist Belt, southwestern Nigeria to explore for base and precious metals (Cu, Pb, Zn, Au and Ag). It involved sampling residual soil from 49 sites in a grid pattern;analyzing the soil samples for the base and precious metals and other commonly associated elements including As, Cd, Fe, Hg and Sb;subjecting the geochemical data generated to statistical analysis;and preparing the geochemical distribution map of the district for each element.? On the basis of the multivariate statistical analysis (correlation matrix and factor analysis) of the soil geochemical data, three mineralization types were inferred to be present in the district. The suspected types of mineralization consist of: 1) Cu-Pb-Zn-Ag mineralization with associated As, Hg, Sb and Fe;2) Au-Cu mineralization with associated As;and 3) minor Hg-bearing Au-Pb mineralization. With the aid of the elemental geochemical distribution maps, the suspected mineralization was linked to rock types from which the sampled residual soils were apparently derived. Hence, the suspected Cu-Pb-Zn-Ag mineralization is linked to the quartz-biotite schist at the southwestern part of the district while the suspected Au-Cu mineralization is located on the silicified sheared rock at the northern and southern parts and the minor Hg-bearing Au-Pb mineralization within the granitic gneiss at the northeastern and eastern areas of the district. Based on the foregoing findings, it is recommended that lithogeochemical survey for the base and precious metals should be done in the area.

Siltstone Geochemical Compositions: Applications for Event Size and Correlation

Thinly laminated siltstone and sandy siltstone are major components of the Upper Permian Brushy Canyon Formation, west Texas and south New Mexico. These rocks have been variously interpreted as the deposits of low-density turbidity currents or as windblown sediment deposited over water. Nevertheless, all models agreed that this lithology was deposited without subsequent reworking by bottom currents or burrowing organisms. These siltstones, thus, are ideal test units for quantitatively estimating hydraulic properties of the flows that formed them. In particular, the Zr/Ti ratio was tested as a geochemical proxy for flow size and transport distance. In situ geochemical abundance and grain size of particles with contrasting susceptibility to erosion—Zr- and Ti-rich particles—were mapped and measured by X-ray fluorescence analytical microscopy, μXRF. Lamination thickness was measured from Fe fluorescence intensity, which increased sharply at the top of each layer. Within the same sample, zircon grains were systematically finer than rutilated quartz grains. Zr/Ti fluorescence ratio positively correlated with lamination thickness, not particle sizes. In other words, Zr/Ti fluorescence ratio fluctuations resulted from variations in mineral abundance. Therefore, variations of Zr/Ti fluorescence ratio in these siltstones are likely caused by fluctuations in the intensity of erosional events rather than transport distance. High Zr/Ti ratios and thick laminations reflect periods of enhanced erosion. The average wind velocity during typical events was estimated to be at least 150 km?hr?1, or the equivalent of a Category 1 hurricane. The method used here could be applied to both outcrop and subsurface strata correlation.

基于MapGIS-Surfer联合的地球化学制图

首先对比Surfer和MapGIS软件的优缺点,主要在于:Surfer具有强大的数据处理功能,图形处理较差;MapGIS具有完美图形处理功能而数据处理功能较差。其次指出部份学者在应用这二款软件中的不足之处,并提出以MapInfo文件格式为数据交换基础的,基于MapGIS-Surfer联合图方法。最后给出了基于MapGIS和Surfer联合制作地球化学图及规范化操作,旨在提供一种将这二款软件优势充分发挥出来,且具有快速、高效、高质量、低成本的地球化学图件制作之途径。

在MAPGIS下快速实现柱状剖面图的绘制

将油气化探数据用FORTRAN短程序生成MAPGIS点、线、面明码文件后,再利用MAPGIS图形编辑软件对图形进行编辑加工,达到快速制作油气化探柱状剖面图的目的。使用该方法能充分利用MAPGIS系统资源,降低编制绘图程序的难度。

MapGIS在矿产资源评价中的应用

针对矿产资源评价中存在地质学数据获取方法先进,而处理这些数据的方法较落后的问题,提出了一种基于MapGIS K9的矿产资源评价方法。该方法首先在MapGIS K9资源中心建立老矿山周围和深部数据的数据库,然后采用MapGIS K9软件处理这些数据,生成坐标转化点位图及地球化学图,从而完成矿产资源评价过程。处理结果表明,该方法不仅缩短了矿产资源数据处理的周期。

MAPGIS地理信息系统在化探工作中的应用——以洛古河普查区面积性土壤化探测量为例

在洛古河普查区面积性土壤化探测量中,将GPS(全球卫星定位系统)与MAPGIS(地理信息系统)结合在一起,对化探数据进行处理及成图.该方法技术的应用,具有成图快速准确、精度高、可操作性强的特点.绘制的异常图与地形地质图套合后,有利于对异常的解释和推断,指导探矿工程的布置.

应用MAPGIS处理地质、物化探图件

目前MAPGIS地理信息系统软件已在地质行业中广泛应用。本文叙述的是作者在使用该系统进行物化探图件编制过程中的一点体会和经验。

应用MAPGIS地形库管理整合1∶200000区域化探数据

利用1∶200 000地球化学数据,从整个成矿地球化学环境的角度出发,最大限度地开发、挖掘三十九种化学元素中每个元素以及元素组合所蕴含的矿化信息,为成矿远景区预测提供更加翔实的地球化学依据,是地球化学工作者的研究任务之一。MAPGIS功能强大,其地图库管理模块为多幅1∶200 000地球化学异常图的拼接提供了技术支持。这里论述了1∶200 000地球化学数据的特征和MAPGIS的地图库管理的功能,并记述了在实际工作中应用MAPGIS的地图库管理对多幅1∶200 000地球化学异常图的拼接,实现了对1∶200 000地球化学数据的另一种应用方式。

SURFER、MAPGIS在地球化学图制图中的对比研究

Surfer和Mapgis都是绘制地球化学图的常用软件,但各有优劣。研究结果表明:Surfer易于操作,而且省时,绘制的地球化学图与研究区域的大体状况相吻合;Mapgis操作的步骤较多,但是绘制出来的地球化学图能较准确、详细的反映研究区域的实际概况。

基于MapGIS的极值无偏地球化学等值线图生成方法

在MapGIS中自动生成地球化学等值线时,用三角剖分方法能最大程度地保留原始信息,但其图面可视化效果差,后期修改整饰的工作量巨大。用Gird网格化数据生成等值线,又会出现元素浓集中心和极值点位置存在一定程度的偏差和位移的情况,对后期的地球化学特征分析和异常查证工作带来了不便。经过研究发现,把离散采样点分析数据中的极值点提取出来,按照一定的阀值给极值点建立缓冲区,通过点对区空间分析,把规则网格数据中位置距离极值点较近,可能影响到三角剖分网优化效果的畸形网格节点剔除;再把离散的极值点数据和剔除了畸形网格节点的规则网格数据合并,进行三角剖分等值线追踪,很好地弥补了三角剖分生成等值线可视化效果差和网格化数据生成等值线出现浓集中心偏移的不足。该方法无需借助其他软件工具,在MapGIS中即可实现极值无偏移的地球化学等值线生成,既兼顾了制图精度,又满足了可视化效果,有很强的实用性。

MAPGIS化探图数据网格化方法选择探讨

根据新疆维吾尔自治区多目标区域地球化学调查(1:25万项目GZTR20070117)中昌吉部分区域的地球化学土壤表层样Cu化学分析数据,利用MAPGIS软件中的GRD模型生成化探异常图,为找矿、环境地球化学质量分析提供依据。

基于MapGIS三角剖分数据扩边方法的探讨

由于1∶5万水系沉积物测量样品布设原则和采样条件的限制,我们无法把样品设计并采集到图幅的边沿上,导致后期数据处理成图时,用三角剖分勾绘出的平面等值线图周边呈现不规则锯齿状,且与图幅边沿有一定的距离,既不完整也不美观。利用MapGIS的离散数据网格化功能,先对离散分布的1∶5万水系沉积物测量原始分析数据进行网格化插值处理,用插值后的网格数据填充到图幅边沿的空缺部位,再进行三角剖分绘制平面等值线图,就可以科学合理的解决三角剖分绘制等值线图的扩边问题。