The geochemistry of arsenic and its use as an indicator element in geochemical prospecting

The geochemistry of arsenic is reviewed, and the use of the element as an indicator in geochemical prospecting for various types of mineral deposits is outlined.Arsenic is a widespread constituent of many types of mineral deposits, particularly those containing sulphides and sulpho-salts. In these and other deposits arsenic commonly accompanies Cu, Ag, Au, Zn, Cd, Hg, U, Sn, Pb, P, Sb, Bi, S, Se, Te, Mo, W, Fe, Ni, Co, and Pt metals. Under most conditions arsenic is a suitable indicator of deposits of these elements, being particularly useful in geochemical surveys utilizing primary halos in rocks, and secondary halos and trains in soils and glacial materials, stream and lake sediments, natural waters, and vegetation. Some of the natural arsenic compounds (e.g., arsine, dimethylarsine) are volatile, but methods utilizing gaseous arsenic halos for geochemical prospecting have not yet been developed.     

Landscape geochemistry as the theoretical basis of geochemical prospecting methods

This article is a review of A. I. Perel'man's book "Landscape Geochemistry" (1961). The first part of the book deals with the theoretical aspects and basic theory of geochemical landscapes. This is followed by a short section on the main aspects of the geography of geochemical landscapes. Geochemical landscapes are classified on the basis of the motion of matter in the third part of the book, which is followed by a discussion of the geochemical features of landscapes of the geologic past. The last part of the book deals with the geochemistry of individual elements in landscapes, and contains sections on the practical applications of landscape geochemistry. The reviewer notes that geologists concerned with geochemical prospecting methods should make this well-written book part of their basic library. — R. D. DeRudder.     

“三江”地区北段区域地球化学元素组合异常提取及其找矿意义

由于地球化学元素携带了某种矿化信息,因此元素地球化学异常往往是指示矿床存在的一种直接标志。随着勘查地球化学的深入发展,有关矿致地球化学异常圈定方法已成为人们关注的焦点。利用"三江"北段1∶20万地球化学数据,运用R型因子分析,查明该区不同矿化类型的指示元素组合,以因子得分为综合指标,采用泛克里格剩余值圈定地球化学异常。结果得到2种元素组合:Ag-Zn-Pb-Sb-Bi组合(与热液型铅锌银矿床有关)和Mo-Cu组合(与斑岩型钼矿床有关)。圈定的组合异常与已知的矿床点在空间上具有很好的对应关系,说明利用R型因子分析与泛克里格法相结合圈定元素组合异常的方法能够较好地确认地球化学元素组合异常,并提示异常与矿化类型之间密切相关的规律性,为"三江"北段地区找矿提供信息。     

“三江”地区北段区域地球化学元素组合异常提取及其找矿意义

由于地球化学元素携带了某种矿化信息,因此元素地球化学异常往往是指示矿床存在的一种直接标志。随着勘查地球化学的深入发展,有关矿致地球化学异常圈定方法已成为人们关注的焦点。利用“三江”北段1∶20万地球化学数据, 运用R型因子分析,查明该区不同矿化类型的指示元素组合,以因子得分为综合指标,采用泛克里格剩余值圈定地球化学异常。结果得到2种元素组合：Ag-Zn-Pb-Sb-Bi组合(与热液型铅锌银矿床有关)和Mo-Cu组合(与斑岩型钼矿床有关)。圈定的组合异常与已知的矿床点在空间上具有很好的对应关系,说明利用R型因子分析与泛克里格法相结合圈定元素组合异常的方法能够较好地确认地球化学元素组合异常,并提示异常与矿化类型之间密切相关的规律性,为“三江”北段地区找矿提供信息。     

The use of tourmaline in geochemical prospecting for gold and copper mineralization

Chemical composition, unit cell parameters, and trace elements of tourmalines from Mesozoic gold-quartz-sulphide and gold-bearing copper-porphyry ore-magmatic systems of the Trans-Baikal area and Mongolia show that they belong to the specific schorl-dravite highly ferruginous oxytourmaline series. They are low in alumina (Al₂O₃ = 16–33%) and have MgO contents (up to 10%) and Fe₂O₃ (1%). There is a direct correlation of unit cell parameters (a,c,V) with total iron, which permits composition estimates from X-ray diffraction analyses. As a rule, these tourmalines contain high concentrations of Au, Pb and Cu, which are mainly hosted by inclusions of native gold and ore minerals. The highest As abundances are contained in the tourmalines of the copper-porphyry field.Two trends of isomorphic replacement are related to increasing Fe content of oxyferruginous tourmalines:(1) “Acid leaching” trend (less ferruginous part of the series) Mg + Fe²⁺ + 4Al + 40 4Fe³⁺ + 2 + 4(OH,F); and (2) “conjugate deposition” trend Mg + 1.5Fe²⁺ + 1.5Al + 4(OH,F) 4Fe³⁺ + 4O.These features distinguish tourmalines from gold-bearing systems from schorl-dravites of tin and rare-metal deposits. They may be used in metallogenic analyses, interpretation of the origin of primary and secondary anomalies, and assessment of the type and zonation of ore fields.     

The use of major elemental ratios in detailed geochemical prospecting utilizing primary halos

The behaviour of the ratios K₂O/Na₂O, SiO₂/CO₂, and $\text{SiO}{}_2\text{H}{}_2\text{O}\text{+ CO}{}_2\text{+ S}$ in the alteration envelopes of a variety of epigenetic deposits is documented. It is concluded that the ratio K₂O/Na₂O is the most suitable for estimating proximity to ore during exploration drilling programs. The other two ratios are useful in indicating proximity to ore only under certain geological conditions. Before the use of ratios is contemplated in detailed exploration programs within a given mineral belt orientation studies should be carried out to determine the trend of the ratios with proximity to mineralization.     

Ion-microprobe zircon geochemistry as an indicator of mineral genesis during geochronological studies

This paper considers the distribution of trace elements (including rare earth elements) in zircons dated by the ion-microprobe U-Th-Pb isotope method and its genetic implications. Two problems were addressed on the basis of the investigation of trace element compositions of zircons: (1) genesis of zircons from subalkaline magmatic rocks, sysenites, and sanukitoids and their comparison with tonalites as exemplified by the rocks of the Karelian region, and (2) determination of trace element signatures of zircons from the oldest granulite-facies rocks of the Ukrainian shield. It was shown that the REE distribution patterns of the tonalites, which crystallized in equilibrium with melt, are strictly governed by crystal-chemical laws. The REE distribution patterns show a positive slope with an increase from La to Lu, a positive Ce anomaly, and a negative Eu anomaly. Similar patterns were observed in zircons from the syenites. The trace element contents of zircons are related to those of melts through partition coefficients. Zircons from the sanukitoids show a considerable LREE enrichment, which is inconsistent with the calculated zircon/melt partition coefficients and presumably related to the inherently imperfect zircon structure. Such a structure was formed during zircon crystallization from melt at high temperatures and the anomalous fluid regime that is characteristic, in particular, of sanukitoid melts. The REE distribution patterns of zircons that crystallized under granulite-facies conditions are sharply different from typical distributions in HREE depletion, which was caused by the competitive growth of garnet during zircon crystallization.     

The use of hydroxyacids as geochemical indicators

The distributions of hydroxyacids in a variety of Recent and ancient sediments have been determined by gas chromatography-mass spectrometry. A lacustrine sediment (Rostherne Mere, U.K.) and two marine situations (Cariaco Trench, Black Sea) were analyzed as examples of Recent depositional environments. β-hydroxyacids occurred in all of the Recent sediments, consistent with their presumed microbial origin, whereas di- and trihydroxyacids were present in the sediments with a significant higher plant input (Black Sea and Rostherne Mere sediments). The two ancient (Eocene) sediments examined, viz. the Greene River (U.S.) and Messel (Germany) oil-shales, contained only trace amounts of hydroxyacids. These results provide evidence that hydroxyacids are in general not well preserved over geological time.     

甘肃地球化学景观特征及区域地球化学方法技术评价

在总结甘肃省地质矿产局20年来区域地球化学工作方法的基础上,系统地分析了甘肃省的地质、地形、地貌及气候特征,将甘肃省划分为6个不同的景观地球化学分区。通过统计过去区域化探的工作方法,分析现有数据,总结近年来的找矿效果,认为在甘肃省的区域化探工作中,除甘肃东南部工作方法正确,找矿效果突出外,其他各景观区1:20万水系沉积物测量的工作方法技术不同程度地存在问题,有待进一步研究。     

碎屑金红石地球化学特征对库车坳陷中生代沉积物源的启示

本文主要对库车坳陷的库车河流域和巴音布鲁克盆地中生代砂岩样品进行了碎屑金红石微量元素电子探针分析。结果显示,巴音布鲁克地区侏罗纪砂岩中的碎屑金红石来自变质泥质岩和来自基性变质岩的比例大致相当,而库车坳陷中生代砂岩中碎屑金红石则主要来自变质泥质岩。两地金红石的变质级别以角闪岩相/榴辉岩相为主,并有一定比例麻粒岩相变质的金红石。库车坳陷中生代砂岩中麻粒岩相金红石的含量,从三叠纪至白垩纪呈现出低—高—低的特点,说明侏罗纪时期物源方向有所变化,可能有东南部物源的参与。白垩纪时期,库车河地区砂岩中麻粒岩相变质金红石含量减少,可能与南天山再度抬升隆起成为优势物源区有关。     

Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system

A mantle value of 17.5 for Nb/Ta appears well established; less well established are crustal values of 11–12, although it appears that Nb/Ta for crustal-derived melts is less than mantle Nb/Ta, demonstrating fractionation of these two elements during crustal evolution, and suggesting that Nb/Ta variation may be indicative of a particular chemical process within the crust-mantle system.

Experimental studies on silicate and carbonatitic liquids at high pressure indicate that, although silicate minerals such as garnet, amphibole and clinopyroxene do fractionate Nb and Ta, the partition coefficients (D's) for both elements are very low. Thus involvement of these minerals may explain relatively small changes in Nb/Ta, but appears inadequate to explain the crust-mantle variation. However, high-quality data for Nb, Ta may be used to provide information on mantle melting or metasomatic processes (e.g., amphibole in the source region decreases Nb/Ta in derived melts, while carbonatitic metasomatism will increase Nb/Ta in affected mantle). Titanate minerals have high D's for Nb and Ta, and do fractionate these elements (e.g., D_Nb/D_Ta rutile/liquid of 0.5–0.8), and their involvement in crystal fractionation would increase Nb/Ta in derivative liquids. In contrast, D_Nb/D_Ta for rutile/fluid is 1.25, so that rocks affected by fluid equilibrated with residual rutile will show a decrease in Nb/Ta

Some Archaean gneisses appear to have high Nb/Ta, and may be a complementary component to that part of the crust which has a relatively low Nb/Ta, such as crustal-derived magmas (e.g., A- ad I-type granites and silicic volcanics). Within the crustal system pegmatites are known to have extremely high and variable Nb, Ta contents, often with low Nb/Ta. A fluid is generally considered to be involved in the generation of these rocks. Thus it is possible that fluid/melt partitioning may be the key to fractionating Nb and Ta, with preference for Ta in the fluid, and enrichment of Ta relative to Nb into the mid-upper crustal system, as the crust evolved, through upward movement of fluid.     

The fractal properties of geochemical landscapes as an indicator of weathering and transport processes within the Eastern Alps

Variations in surface morphology and lithology provide an opportunity to study lithologic and morphologic influences on the spatial pattern of stream-sediment geochemistry within two contrasting environments of the Eastern Alps (Hohe Tauern Range and Gurktaler Alpen Range). The fractal dimension, a measure of surface roughness over a variety of scales, is used to model the dissipation of erosive products due to climatic controlled denudation and fluvial mass transport. Based on a spatial correlation analysis, specific elemental concentrations are used as indicators for a dominant lithotype. Fractal geometry of these elements has been estimated by sequential Gaussian simulation of the area/perimeter relationship (Dal) and by the estimation of multifractal spectra. It is shown that within a 510–780 km² survey area the spatial variations of Al, Ga, Ni and Ca can be approximated by single fractals but for those of Ag and Sn multifractal models must be used. Fractal properties derived from simulated surfaces are explainable by the process controlling the spatial structure of the data. Climatic and tectonic parameters apparently influences Dal at large scales. At smaller scales rock-type variation exert an additional influence on Dal.     

Garnet in silicic liquids and its possible use as a P-T indicator

Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K _D values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher value than coexisting liquid, but below 950 ° C the garnet value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids.Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains 10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically <4 % MnO) to an Mn-richer rim (typically >4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma.     

Basalt geochemistry as a diagnostic indicator of tectonic setting

Basalt geochemistry can be used as a diagnostic indicator for determining the tectonic setting of origin, because specific plate tectonic settings often impart distinctive geochemical characteristics. For example: (1) mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) have clearly distinguishable trace element and Sr-Nd isotope geochemical characteristics; (2) arc related basalts, including IOAB (intra-oceanic arc basalts), IAB (island arc basalts) and CAB (continental arc basalts), exhibit following distinguishing features: all are characterized by low Nb/La ratios (<0.85) and negative Nb, Ta and Ti anomalies; most exhibit low Nb concentrations (<8 ppm), high positive ɛ_Nd values and low enrichment of incompatible elements except the continental arc shoshonitic basalts that possess high concentrations of incompatible trace elements and lower to negative ɛ_Nd values; (3) although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated continental intraplate basalts as arc related, there are still some essential differences between continental intraplate basalts and arc related ones; such as: uncontaminated continental intraplate basalts have high Nb concentrations, Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛ_Nd values that distinguish them from the arc related ones; whereas, the contaminated continental intraplate basalts are characterized by pronounced negative Nb, Ta and Ti anomalies, but their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones; (4) an important difference between back-arc basin basalts (BABB) and the MORB is that the former exhibit both MORB-like and arc-like geochemical characteristics; (5) most oceanic plateau basalts (OPB) show diagnostic geochemical characteristics of enriched MORB (E-MORB) to transitional MORB (T-MORB); only the Kerguelen Plateau is an exception; the early (pre 90 Ma) volcanism of the Kerguelen Plateau is associated with the Early Cretaceous break-up of Gondwana and displays features of continental flood basaltic volcanism; with time, the tectonic setting of the Kerguelen plume-derived volcanism changed from a rifted continental margin setting (133–118 Ma) through a young, widening ocean (118–40 Ma), finally to an oceanic intraplate setting (~40 Ma to the present).Tectonic discrimination diagrams should not be used in isolation, but can still be useful as part of holistic geochemical characterization. For example: (1) MORB and OIB are distinguishable from each other in the 3Tb-Th-2Ta diagram; (2) the arc related basalts, including IOAB, IAB and CAB, constantly plot in the arc-related basalts fields in the Th/Yb-Ta/Yb diagram; (3) the 3Tb-Th-2Ta diagram can be utilized to fully illustrate both MORB-like and arc-like characteristics of BABB; (4) some discriminant diagrams (such as Zr/Y-Zr, Th/Yb-Ta/Yb, 3Tb-Th-2Ta and Hf/3-Th-Nb/16 diagrams) can be used to distinguish continental intra plate basalts from arc related ones; (5) although there are not any discrimination diagrams published that delineate an OPB field, some trace element diagrams can still reveal diagnostic characteristics of the OPB.     

壤中游离气方法及其在油气化探中的应用

油气藏中的轻烃气体能够以微弱但可检出的量近似垂直地渗漏到地表土壤中,其中一部分烃类以游离态赋存在土壤或岩石颗粒空隙中.壤中游离气中的烃类能反映深部油气藏烃类的现代补偿性活跃微渗漏,被认为是油气地球化学勘探的最可靠方法之一.壤中游离气采集是获得第四系沉积层中微渗漏烃类地球化学异常的关键技术.自行研制了便携式游离气采集新装置,具有操作简便、携带方便、密封性好等特点.通过长期实践,总结了土壤游离气保真采集的方法.游离气方法在济阳坳陷惠民凹陷南坡临南—钱官屯地区、塔里木盆地阿克亚苏地区X井区的油气化探应用,获得的游离烃异常较好地反映了油气垂向微渗漏近地表信息,以及下伏油气的分布特征,油气微渗漏异常区与背景区分离明显,表明壤中游离气方法应用效果显著,在油气勘探中值得广泛推广.     

地球化学综合异常的圈定及找矿效果

能否正确有效地圈定地球化学综合异常是地球化学数据处理的重要内容,对找矿效果起决定作用.传统方法多在单元素异常的基础上圈定综合异常,而忽略了异常可能由地层本身高背景富集引起.笔者引用马氏距离识别地球化学离群点的方法圈定综合异常.由于马氏距离是基于多元正态分布理论,是多变量统计方法的直接推广和应用,其全面考虑了元素均值、方差及元素间协方差3个参数,是直接针对样本的计算过程,同时也是一元方法的直接推广,所以其具有独特的异常识别功能.经各种不同比例尺和不同采样介质化探数据处理应用显示,利用马氏距离方法求得的地球化学综合异常具边界唯一、界线清楚,减少了人为圈定异常的影响;异常强度突出,指标单一,可作为评价异常的一个重要参数使用;发现矿体(矿化)的概率程度高等优势.该方法可在实际中推广应用.     

云南省三江地区区域地球化学特征及铜多金属矿找矿意义

根据云南省境内三江地区1∶20万地球化学调查资料,结合区域地质特征及成矿规律,对该区地球化学特征与矿产资源找矿潜力进行研讨,重点对与铜及铜多金属矿床有关的地球化学特征进行分析。本区主要成矿元素有Cu、Zn、Pb、Cd、Ag、As、Sb、Hg、Be、W、Sn、Bi等,其中铜矿相关元素Cu、Pb、Zn、Cd、Ag、As、Sb、Hg、Be等与岩浆活动和沉积热液改造有关。以本区三类典型铜矿床模型为依据,利用GIS技术进行预测,提出铜多金属成矿预测区14个,其中A类5个,B类6个及C类3个,为进一步矿产勘查工作提供参考。     

Barium orthoclase from southern Yakutia and importance of barium as an indicator element

Barium anomalies in orthoclase formed during post-magmatic phlogopitization of microline pegmatites and certain other rocks are sufficiently high and consistent to justify the use of such barium as an indicator in prospecting for industrial mica. — IGR Staff.     

皖南黄山区坞石垄岩体地质地球化学特征及找矿意义

坞石垄岩体位于江南造山带与下扬子台坳接合部,沿江南深断裂侵入,岩性由花岗闪长岩和二长花岗岩构成的杂岩体,系燕山晚期同源岩浆不同演化阶段的产物。岩石类型属同熔型花岗岩,具有高硅、富铝、偏碱岩石化学特征,为过铝质钙碱性花岗岩类。与周边同类型太平岩体、洪田都岩体、旌德岩体等相比,Mo、W元素丰度值高。中型规模的萌坑钼矿产于坞石垄岩体中,具有一定的找矿前景。