Potential Field,Geoelectrical and Reflection Seismic Investigations for Massive Sulphide Exploration in the Skellefte Mining District,Northern Sweden

Multi-scale geophysical studies were conducted in the central Skellefte district (CSD) in order to delineate the geometry of the upper crust (down to maximum ～ 4.5 km depth) for prospecting volcanic massive sulphide (VMS) mineralization. These geophysical investigations include potential field, resistivity/induced polarization (IP), reflection seismic and magnetotelluric (MT) data which were collected between 2009 and 2010. The interpretations were divided in two scales: (i) shallow (～ 1.5 km) and (ii) deep (～4.5 km). Physical properties of the rocks, including density, magnetic susceptibility, resistivity and chargeability, were also used to improve interpretations. The study result delineates the geometry of the upper crust in the CSD and new models were suggested based on new and joint geophysical interpretation which can benefit VMS prospecting in the area. The result also indicates that a strongly conductive zone detected by resistivity/IP data may have been missed using other geophysical data.     

DETERMINATION OF ELECTRICAL PROPERTIES OF THE GROUND AT SHALLOW DEPTH WITH AN ELECTROSTATIC QUADRUPOLE: FIELD TRIALS ON ARCHAEOLOGICAL SITES1

Knowledge of both the electrical resistivity ρ and the dielectric permittivity ? of the ground is important for the determination of characteristics such as granularity, porosity, moisture and salt content. Whereas the measurement of ρ is very common and can be achieved using either the d.c. resistivity method or a wide variety of EM devices, ? remains practically unknown in the low-frequency domain between the IP domain and the high-frequency domain. Following the principle of the quadrupole technique used in d.c. prospecting and of the quadrupolar probe for measuring the complex permittivity of the ionosphere, we propose a new approach which does not require any galvanic contact between the poles and the ground. The transfer impedance can be evaluated using the quasi-static approximation for low frequency or the full EM theory for higher frequencies. The conditions under which both calculations apply are discussed. At low frequencies and for low resistivity ground, the electrostatic quadrupole measures ρ exactly as with the d.c. technique; for higher resistivities or frequencies the simultaneous measurement of both properties becomes possible. Examples in archaeological prospecting are presented and checked against independent d.c. resistivity measurements or excavation analyses.     

INDUCTION ANOMALY DUE TO AN ELONGATED COVERED ORE-ZONE EXCITED BY A LONG CURRENT CARRYING CABLE*

A detailed analysis of the electromagnetic anomaly due to an elongated mineralized zone of low resistivity in presence of a fixed-transmitter (a long cable carrying an, alternating current Ie^Iωt) is presented where (i) the host rock is not highly resistive and might contain some disseminated mineralization, (ii) the ore-body has an inhomogeneous conductivity, (iii) there is a contrast in magnetic properties of the ore-body and the host rocks, and (iv) source is close to the conducting system i.e., anomalous zone is lying shallow. The numerical computation of the generalized analytical expressions have been made. The selective screening behaviour of the cover is delineated and suitable frequency ranges for maximum detectability of a covered conductor have been obtained. A paradoxical decrease of the in-phase component of the response function with the increase of the core conductivity has been found for large conductivities of the cover. For moderate values of response parameter the inhomogeneity in the conductivity is found to significantly affect the induced field. The present analysis of the various features of the secondary field will aid to the existing interpretation sensitivity of the induction prospecting data for porphyry conducting ore deposits with zonal wall-rock alteration and sulphide distributions.     

COMPATIBILITY OF EM AND IP SURVEYS *

Methods and typical case histories of EM and IP surveys are compared in order to check their compatibility for the investigation of large areas for base metal sulphides. It is demonstrated that the fast, low cost EM may miss sulphide concentrations which cause no reduction of apparent resistivity, or which act like horizontal slabs, where EM anomalies occur marginally. In such cases only IP can recognize the whole extent of the mineralization. The misleading role of graphite exists for both methods. The conclusion is that EM remains the only economical method to survey whole ore districts, but that the shortcomings should always be taken into account in the interpretation. They can be overcome by employing geological, geochemical or other geophysical indicators in selecting promising targets for follow-up IP. Even if less than 20% of the area is covered by the expensive IP, there is a fair chance that all sulphide bearing rocks will be found. Therefore, the question is not whether to apply EM or IP methods, but how to combine them best.     

3D electrical structure of porphyry copper deposit: A case study of Shaxi copper deposit

Located in Lu-Zong ore concentration area, middle-lower Yangtze metallogenic belt, ShaXi porphyry copper deposit is a typical hydrothermal deposit. To investigate the distribution of deep ore bodies and spatial characteristics of host structures, an AMT survey was conducted in mining area. Eighteen pseudo-2D resistivity sections were constructed through careful processing and inversion. These sections clearly show resistivity difference between the Silurian sandstones formation and quartz diorite porphyry and this porphyry copper formation was controlled by the highly resistive anticlines. Using 3D block Kriging interpolation method and 3D visualization techniques, we constructed a detailed 3D resistivity model of quartz diorite porphyry which shows the shape and spatial distribution of deep ore bodies. This case study can serve as a good example for future ore prospecting in and around this mining area.     

The modified pseudo-depth section as a tool in resistivity and IP prospecting — A case history

Following the depths of investigation of different electrode arrays, given byRoy andApparao (1971) andRoy (1972), a modified pseudo-depth section was suggested for any array byApparao andSarma (1981) as a tool in resistivity and IP prospecting. The tool was used for the interpretation of IP and resistivity anomalies obtained in a virgin area—Jonnagiri village, Pattikonda Taluq, Kurnool district (A.P.) India. Two of the four bore holes recommended encountered sulphide mineralization, while the other two met with white altered ashy material at the position of the maximum anomaly contour in the depth sections. The drilling results prove the utility of the modified pseudosections in the interpretation of the field data.     

Deep massive sulphide exploration using 2D and 3D geoelectrical and induced polarization data in Skellefte mining district,northern Sweden

Geoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high‐chargeability low‐resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high‐resolution image of the top ～450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep‐resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ～1‐km depth.     

激发极化法的“饱和效应”问题

西格尔（H.O.Seigel）从理论计算得到导电颗粒含量为25％的极化球体有最大异常,因此认为:激发极化法只宜于寻找中等浸染程度的矿体,对于含量很高的矿体或良导致密型矿体,激发极化法是不行的。据称这种“饱和效应”的理论得到野外结果的证实。 本文根据我国激发极化法在良导致密型矿体上大都能获得明显视极化率异常的事实,说明西格尔的“饱和效应”理论与事实不符。本文认为良导致密型极化体不存在“饱和效应”问题,激发极化法对它们的勘探也是有效的。可能由于西格尔把低含量理论公式,不计条件地推导到高含量、甚至良导矿体,因此他的结论是错误的。本文最后讨论了电阻率因素对浸染型球体视极化率强度的影响。     

水下激发极化测深异常特征研究

利用有限单元法对水下激发极化测深进行数值模拟以研究其异常特征和适用条件.水下的电阻率测深和激电测深均可对水底地质体进行近距离探测,提高探测分辨率.而水底地形对视电阻率测深的影响很大.水底极化体有限元正演模拟的结果表明,水底地形对激发极化测深结果没有影响.采用水下激发极化法进行地质体探测是可行、有效的.通过计算不同水体和水底岩石电阻率对视极化率幅值的影响发现,水体电阻率是影响水下极化率测深的主要因素.由于海水的电阻率过低,观测视极化率异常微弱而不适合开展水下激电工作.     

Integrated geological and geophysical exploration for concealed ores beneath cover in the Chaihulanzi goldfield, northern China

An integrated geological and geophysical investigation was carried out in the near‐mine areas of the Chaihulanzi gold‐field, Inner Mongolia, northern China, to determine the south‐eastern strike extension of the principal mineralized shear system, in the search for new resources. In this case study, surface geophysical surveys played an important role in defining the mineralized structures beneath cover. A re‐investigation of the mine geology by observing the underground exposures of mineralization and its hosting structures, coupled with re‐evaluation and re‐examination of previous exploration data, revealed that the mineralization style at the Chaihulanzi gold‐field is not the skarn‐type as previously assumed, but is structurally controlled by an oblique‐sinistral shear structure. Moreover, a south‐eastward shallow‐plunging mineralized corridor within the principal mineralized shear system was also identified, implying that the ore‐forming fluid‐flow might move through the mineralizing system, from the south‐east at depth to the north‐west. These new geological findings imply that the mineralized shear system should extend much farther south‐eastwards along strike beneath cover, and thus a conceptual target area was proposed. A detailed geophysical survey program, involving the application of VLF‐EM, Stratagem EH4, CSAMT and gradient IP measurements, was carried out to test the validity of the conceptual target. Orientation surveys on the geologically known Line‐18 traverse revealed that VLF‐EM and Stratagem EH4 were the most effective methods of detecting the unseen mineralized system, whereas CSAMT could only provide low‐resolution data, and IP proved to be unsuitable in this environment due to the ubiquitous presence of graphite‐bearing schists in and around the mineralized system. A follow‐up systematic VLF‐EM survey highlighted the principal mineralized shear system as a linear conductive belt, tracing the principal mineralized system for a further 750 m of strike length beneath cover from the previous exploration limit (Line 18). Stratagem EH4 soundings over six parallel traverses perpendicular to the mineralized trend revealed that the principal mineralized shear structure extends for more than 500 m in the dip direction, a strong indication of the presence of potential deep mineralization under the surveyed area. Detailed modelling of the Stratagem EH4 sounding images provided well‐defined targets for test drilling. Subsequent test drilling on some of these targets returned encouraging results as several core‐intercepts of economic gold mineralization were encountered. This led to a more extensive drilling and underground prospecting program, focusing on deep mineralization south‐eastwards along the mineralized trend.     

地气测量寻找深部隐伏金矿及其机理研究

根据９个地区４种不同的金和金多金属矿区地气研究、室内上升气流和纯扩散模拟模型实验以及地气采样片在高分辨显微技术（ＡＦＭ,ＴＥＭ,ＳＥＭ）下观察的结果,讨论了地气测量的找矿机理,认为地壳内上升气流携带深部矿化元素的纳米级微粒垂直向上迁移至地表,形成地气异常．文中对地气法的方法技术做了概述和评论,并列举了应用实例,地气法兼有物、化探的优点,在寻找深部隐伏矿化方面具有广阔的应用前景。     

ACHIEVEMENTS IN COPPER SULPHIDE EXPLORATION IN ALBANIA WITH IP AND EM METHODS1

The copper sulphide exploration programme in Albania involves a number of geophysical methods. The most important ones are the Induced Polarization (IP) and the Turam methods. This paper reports some recent achievements in increasing the depth of investigation and in discriminating sulphide ore textures by the IP, spectral IP and Turam methods.     

广东始兴南山矿区钨锡多金属矿床特征及资源潜力

广东始兴南山钨锡多金属矿床位于南岭成矿带东段,是中国地质调查局南京地质调查中心在新一轮钨多金属矿找矿工作实施中根据＂模型找矿＂找到的具有大型远景的钨锡钼铋多金属矿床。最新研究和工程钻探成果显示矿区分南北两个矿带,已圈定规模较大的钨锡多金属矿脉（体）10多条（个）,矿化类型有矽卡岩型、蚀变花岗岩型（云英岩型）、斑岩型和石英脉型。各种类型的矿体在空间上既有独立产出,又有共存一体、相伴而生,构成＂多位一体＂的复合矿床。其中以矽卡岩型钨锡铋多金属矿体、斑岩型钨钼多金属矿体和蚀变花岗岩型钨钼多金属矿体规模最大。本文通过该区地质、地球化学及主要钨锡钼铋多金属矿体特征的分析研究,指出了南山矿区的资源潜力,提出南山地区钨锡钼铋多金属矿的找矿方向。     

Geophysical Investigation of Fresh‐Saline Water Interface: A Case Study from South Punjab,Pakistan

The importance of the study of fresh‐saline water incursion cannot be over‐emphasized. Borehole techniques have been widely used, but they are quite expensive, intrusive, and time consuming. The electrical resistivity method has proved very successful in groundwater assessment. This advanced technique uses the calculation of Dar‐Zarrouk (D‐Z) parameters, namely longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity has been employed by using 50 vertical electrical sounding points to assess the groundwater and delineate the fresh‐saline water interface over 1045 km² area of Khanewal in Southern Punjab of Pakistan. The x‐y plots and maps of D‐Z parameters were produced to establish a decipherable vision for the occurrence and distribution of different water‐bearing formations of fresh‐saline water aquifers through a complicated situation of intermixing of different resistivity ranges for fresh‐saline water bodies. This technique is useful to reduce the ambiguity produced by the process of equivalence and suppression which cause intermixing in differentiating fresh, brackish, and saline aquifers during interpretation. The fresh‐saline water interface is correlated very well with the previous studies of water quality analysis carried out in Khanewal area. The results suggest that the D‐Z parameters are useful for demarcating different aquifer zones. The behavior and pattern of D‐Z parameters with respect to occurrence and distribution of different water‐bearing formations were effectively identified and delineated in the study area.     

基于二次耦合势的广域电磁法有限体积三维正演计算

与可控源音频大地电磁（CSAMT）相比,广域电磁法通过采用全区视电阻率定义,突破了卡尼亚视电阻率所需的远区条件限制,极大拓展了可控源电磁观测区域和探测深度.考虑到电偶源激发场的三维特征以及地下复杂三维结构,为提高广域电磁数据解释精度,本文实现了基于二次耦合势的广域电磁法三维正演计算.该算法利用Helmholtz定理将麦克斯韦方程转化为库伦规范下的磁矢势和电标势耦合方程,有效改善了离散所得大型线性方程组的谱性质,并通过强加散度条件来消除电场伪解的影响.此外,采用散射场方法,其中一次场使用准解析法求解,二次场使用有限体积法求解,克服了局部激发场源奇异性问题.通过与一维层状模型下电偶源产生的电磁场准解析解对比,验证了本文算法的正确性.在此基础上,利用本文的正演算法对比分析了广域电磁法与CSAMT对典型三维目标体的探测能力,结果表明在相同的观测条件下,广域电磁法能够更准确地反映地下目标体信息,拥有更优的分辨能力.     

洛宁七里坪多金属矿直流激电测深方法的应用效果研究

直流激电测深方法的应用,使得物探技术变得更加简捷、高效和经济。本文介绍了直流激电测深技术在洛宁七里坪多金属矿勘探中初步研究与应用结果,讨论了该技术的原理和操作方法,详细阐述了该技术应用于矿产资源勘察的技术原理与科学依据、操作装置与数据采集、观测仪器与系统控制、数据处理与层析算法、极化方式与参数计算、技术特点与注意事项等。     

Frequency distributions of major elements (Pb,Zn, Cu) in the main polymetallic deposits of the Baia Mare district (Romania)

The frequency distributions of Pb, Zn, Cu in the hydrothermal sulphide deposits of the Baia Mare district are established. It is shown that the distribution of Pb is similar to that of Zn, unlike that of Cu. This law appears to be valid whatever the composition and grade of the ore, the type of the host rock, and the type of hydrothermal alterations. This law can provide arguments to establish the genesis of mineralizations of unknown origin.     

福建皇山铅锌多金属矿地质特征及成矿标志

文章介绍了福建皇山铅锌矿矿区地层、构造和岩浆岩,通过对矿体地质特征的描述、岩浆岩化学成分组成特征及成矿条件分析,提出矿区找矿标志。     

安徽省霍山县东溪-南关岭金矿地质特征与成矿条件

东溪-南关岭金矿为北淮阳东段发现的第一个原生金矿,产于磨子潭-晓天中生代火山盆地边缘,容矿围岩为安山质-英安质火山岩,矿化和蚀变受桐柏-桐城深大断裂带的次级构造带控制。该金矿发育陡倾斜的脉状矿体和缓倾斜的似层状矿体,成因类型为冰长石-绢云母型浅成低温热液金矿。火山岩盆地之下的变质基底可能起到了初始矿源层作用,桐柏-桐城断裂带及其次级构造破碎带为重要的导岩导矿构造,早白垩世强烈的岩浆活动提供了热源、动力源和部分矿源。参照国内外类似金矿的特征,认为本区成矿地质条件良好,有可能隐伏着与石英正长岩体有关的细脉浸染型矿化,晓天火山盆地具有良好的找矿前景。     

西藏班戈日阿铜多金属矿床地质特征及找矿前景

西藏班戈日阿铜多金属矿床位于班戈-腾冲多金属成矿带,是近年来新发现的矽卡岩型铜多金属矿床,已探明资源量达中型规模。最新研究和找矿成果显示矿带呈北西向展布,受燕山晚期白垩纪花岗岩类岩体侵入作用控制,矿体分布于岩体与下白垩统灰岩地层接触带及附近部位。在空间上,矿床主要分为三个矿段：主矿体分布在矿区中西部,目前在西矿段已圈定5个（条）铜矿体,矿体呈似层状、透镜状,其中最大的Cu13号矿体长221 m,平均厚9 m,沿倾向已控制延深100 m以上,铜品位：0.45%~2.01%,平均0.9%。主要金属矿物为黄铜矿、磁铁矿。中矿段已经圈定3条矿体,成矿元素除铜外,铅、锌、银含量也较高,为铜多金属矿体。矿体呈似层状、透镜状。其中以Cu-23号矿体最大,长达300 m,矿体最宽处达110 m。矿体平均品位Cu1.48%,Pb 1.25%,Zn 2.52%,A g 32.52×1-0 6。本文在矿区地质特征研究的基础上,通过对1：5万水系沉积物测量成果分析,对比中、西矿段成矿条件,对矿区东段进行了预测评价,指出该矿区具有较好的铜多金属找矿前景。同时建议对已圈定的1：5万水系沉积物异常开展1：1万土壤地球化学测量工作,进而圈定该区铜多金属矿带或矿（化）体,为矿区深入勘查工作提供依据。