First, geophysical methods.
Aerogeophysical exploration is an economical and rapid exploration method for concealed areas. Using aeromagnetism and aeroelectricity to quickly scan hidden areas, we can obtain direct and indirect information related to mineralization, find out the tectonic environment or stratigraphic units related to mineralization, and then carry out deep geological mapping.
Ground geophysical methods, such as gravity, magnetic survey, electromagnetic method and induced polarization method, are still important methods to find concealed ore. In recent years, some new geophysical exploration methods have been put forward in order to find deep buried hidden deposits.
1. Time domain electromagnetic method
The main development direction of aviation time-domain electromagnetic system is to increase the detection depth of good conductor targets by improving the transmission power and airborne data processing. In order to achieve this goal, on the one hand, the analog receiver in the traditional input system is replaced by the digital receiver; On the other hand, the coils are separated by a certain distance to reduce the power loss in the unit. Although the primary field is still a half sine wave pulse, the average transmission distance is twice as long as that of the traditional input system. Many hidden massive sulfide deposits have been discovered by time-domain electromagnetic methods on the ground and in wells abroad.
2. Controlled source audio magnetotelluric method
The power supply of controlled source audio magnetotelluric method is grounding wire, which is far from the receiver. By measuring the ratio of horizontal electric field to magnetic field at the receiving point (called ground impedance), we can get a quantity (Cagniar resistivity) which is not affected by the electromagnetic wave propagation path of the transmitting and receiving devices, but is related to the resistivity stratification below the measuring point. Effective detection depth is mainly a function of earth resistivity and working frequency. When the frequency range is 0. 125 ~ 4096 Hz and the transmission power is 30 kW, the detection depth can reach tens of meters to 2km. By gradually changing the frequency of transmitter and receiver in a certain range, sounding can be realized by sampling at different depths of the earth. CSAMT is extremely sensitive to geological noise, but compared with TEM, because its transmitter is far away from the receiver, the underground resistivity is mainly obtained from the electric field measurement results, so its lateral resolution is higher than that of TEM, but it is greatly influenced by good conductor overburden, so it is difficult to distinguish the response between overburden and ore body. Compared with the conventional electromagnetic method, this method has the characteristics of high working efficiency, large detection depth and high resolution.
3. Natural source audio magnetotelluric method (AMT)
AMT measures the ratio of the horizontal electrical component of natural field to the horizontal magnetic component perpendicular to it, which is related to the resistivity of uniform layered earth. Calibration of AMT means that the instrument can directly give the ratio at the selected frequency after calibration. The reproducibility of AMT is poor, and later CSAMT was proposed to improve it.
4. Small-scale charging method
The traditional feeding method is aimed at a single ore body, and the small-scale feeding method is aimed at ore fields. Charging points are not limited to bare good conductors, but are distributed as deep as possible in different parts of ore bodies, altered zones, intrusions or surrounding rocks. The horizontal detection range can reach several kilometers or even dozens of kilometers. Ore bodies have the lowest resistivity and are conductive cores. Alteration halo is generally better than unmodified surrounding rock. The ore halo can be a conductive window, and the alteration halo can be divided into an outer region and an inner region. Inner layer resistivity is the intermediate value between outer layer resistivity and ore body resistivity. Therefore, the electrical zoning of the ore field is very obvious both vertically and horizontally.
5. Seismic exploration technology for metal mines
Seismic method has been widely used in oil and gas exploration. Because the structure of metal mining area is much smaller than that of oil and gas area, and the measurement accuracy of earthquake is higher, effective measures must be taken to eliminate the influence of zone on measurement results. Therefore, different combinations of seismic source and geophone can be used to measure the surface velocity, the elevation of detection point and excitation point, so as to correct the observation signal. In addition, we can use signal enhanced seismograph and * * * deep point stacking technology, and use corresponding high-frequency detectors and Qualcomm detectors to filter the detection depth. In seismic exploration, inversion method and refraction method are mainly used. The former is used to study the structure of sedimentary rocks, extrusive rocks and metamorphic rocks, the shape and internal structure of intrusive rocks, and to track faults. Refractive wave method is used to draw bedrock and basement and study the weathering crust of basement rock.
Second, geochemical exploration methods
In geochemical exploration, the most outstanding progress is the appearance of geogas method, element molecular morphology method and ion halo method, which detect solid particles brought by geogas flow and create new methods and principles. Geochemists at home and abroad have done a lot of field experiments and indoor experiments on this method, which initially confirmed the existence of vertical migration mechanism of deep solid particles and the feasibility of the corresponding method. Xie et al. improved the geogas method to develop it into a strategic exploration method, and proved its effectiveness in pilot exploration at home and abroad. These achievements make an important supplement to the traditional mechanism of overhead halo formation, and it is considered that the material particles in ore bodies and primary halos can migrate anywhere near the surface with the airflow, so the anomalies found by this method can be used as direct information for prospecting. This greatly strengthens the theoretical basis for the application of geochemical methods in concealed areas. According to the western literature review, the overhead halo method has some shortcomings in determining the location of ore bodies, but it can determine the metallogenic prospect. This is enough for strategic exploration. In addition, the traditional exploration methods of concealed mineralization have also made some progress. Now, some main methods are selected for brief introduction.
1. GEOGAS method (GEOGAS), element molecular morphology method (MFE) and ion halo method.
In 1982, K. Christesen and others found that radon can migrate to the surface quickly when studying the radon migration model, and the migration time does not exceed the radon half-life of 3.8 days. It is difficult to explain this phenomenon with the conventional diffusion model, so they put forward a new hypothesis that radon atoms may be transported by a slow upward geological flow that can pass through rocks and reach the surface. The gas carrying radon atoms exists in the form of bubbles in groundwater, while radon atoms and radioactive precursors of radon are attached to the gas-water interface of bubbles.
If the above assumption of radon migration holds, why can't other metals migrate in this way? Inspired by this idea, K. Christesen and others first measured the snow samples above the known ore bodies in northern Sweden in 1984, and soon found that there were ore-forming element anomalies related to the deposit in the snow flakes. Because the snow in this area is only preserved for half a year, it shows that the deep elements related to the deposit can migrate to the surface in some way in half a year. However, snow flakes are easy to be polluted by the air during falling, so they use a special device to collect the metals carried by the ground flow, and as a result, good metal anomalies are obtained above the known ore bodies.
German company RULF GEO and Czech Institute of Geophysics have also conducted similar research. However, they believe that after the elements reach the surface in the form of molecules, they will soon be transformed into gases and enter the atmosphere. Therefore, they use the method of collecting the atmosphere directly to analyze the metal content, and at the same time observe the anomalies of the above metal elements, and call their method "molecular form of elements", which is referred to as MFE method for short.
Coincidently with geogas method, in recent years, Russian Grigorian has developed a new prospecting method in concealed areas-"ion halo method". The researchers of ion halo method believe that there are high concentrations of indicator elements above the concealed mineralization and near the atmospheric surface. Griya developed a special device to collect ions. According to197' s lecture in China Geological Library in March, a glass bottle with a diameter of about 10cm was filled with 150ml of 1N nitric acid, covered with a semi-permeable membrane and fastened with a rubber band. After 24 hours, the concentration of metal ions in HNO3 solution was analyzed for observation and comparison. This method is similar to both geo-electrochemical method and gas geochemical method, and uses the principle of chemical potential difference to capture active metal ions. Russian physicists believe that this is a new physical phenomenon, and this achievement is listed as one of Russian scientific discoveries.
2. Active metal ion method
In sequential extraction, no matter which scheme, the problem of water-soluble phase will be encountered first. Because the water-soluble phase method is simple to operate, active elements are often extracted, which is very attractive for finding concealed minerals. In Russia, the ion concentration of water-soluble phase (40g soil +40g water) in soil is effectively analyzed by ion-selective electrode method, such as,,,
Br-, K+, Na+, Ca2+, Eh, pH, etc. Good anomalies are obtained above the deposit covered by thick sediments.
A.W. Mann put forward the method of flowing metal ions. Their MMI uses one or several weak reagents to extract active metal ions, but the name of the extractant remains confidential. In essence, this method is to extract weakly bound ions by weak acid or enzyme cooking. Mainly analyze copper, lead, zinc, cadmium, nickel, gold, silver, palladium and so on. It is said that the geochemical anomalies obtained by this method have good reproducibility and can detect ore bodies 700 meters underground.
This method has entered the market on a large scale and has been registered as its registered trademark by Australia's Wamtch Company. In the literature about mineral exploration, this method is almost always listed as a new geochemical method to be popularized. For the formation mechanism of geochemical anomalies of active metal ions, A.W. Mann's statement is very vague. They think it is capillary action, plant root action, ion diffusion, electrochemical action and so on.
3. Enzymatic leaching method
Enzymatic leaching is a new method developed by J.R. Clark of the US Geological Survey in the mid-1980s. He believes that although amorphous manganese oxide in soil only accounts for a small part of the total manganese oxide, its irregular surface has a huge surface area and randomly distributed charges, which enables it to absorb anions and polar molecules with completely different chemical properties. Trace elements adsorbed by amorphous manganese oxide in sediments often reflect the geochemical characteristics of deep bedrock. J.R. Clark uses glucose oxidase to produce trace amounts of hydrogen peroxide and gluconic acid. Hydrogen peroxide can easily reduce dissolved amorphous manganese oxide, thus releasing oxides, while crystalline manganese oxide is only slightly eroded, and gluconic acid will release metal complexes, leaving them in the solution. Then, the concentration of metal ions in the solution was determined. It is said that this method is particularly effective in moraine-covered areas.
As for the geological driving force of the upward migration of elements related to mineralization, J.R. Clark thinks that the most important thing is the deep circulating groundwater, the upward migration of deep metal ions, which are absorbed by deep-rooted plants, and then enter the deciduous layer, where they are leached and enriched in amorphous manganese oxide in soil layer B. J.R. Clark also thinks that the formation of abnormal enzyme extraction of a group of elements such as Cl, Br, I and As, and the formation of Sb, Mo, W and As on the surface.
In terms of partial extraction technologies and methods, recently, in the implementation of the "EXTECH" plan, the Geological Survey of Canada conducted extensive research on the sequential extraction technology of selective phase extraction technology, and achieved important results.
4. Geoelectrochemical method
In Russian literature, the term "geo-electrochemical method" includes a large group of methods, such as element occurrence form method, thermomagnetic geochemical method, diffusion extraction method and partial metal extraction method. Among them, some metal extraction methods are the core of geo-electrochemical methods, which are usually called electro-extraction in China. Speciation of elements, thermomagnetic geochemistry and diffusion extraction are generally called "partial extraction techniques" in western literature and at home.
At present, the focus of the debate on the geo-electrochemical method is still whether the metal ions come directly from the ore body or from the active metal ions near the surface around the electrode. Although both of them reflect the information of deep mineralization, two different understandings have different effects on the prospect of geo-electrochemical methods. Recently, Russia's S.G. Aleksev (1996) expounded an important fact, which deserves attention. The elemental composition of geochemical halos is usually equivalent to the chemical composition of ore bodies. In view of this, he listed two ore bodies that are not far apart on the same section, of which Pb and Zn in ore body A are roughly the same, accounting for 5% ~ 7% respectively; In ore body B, the lead content reaches 10%, while the zinc content is only about 0.05%. There are lead and zinc anomalies above the lead-zinc ore body, but only lead anomalies above the lead-zinc ore body. We think it is difficult to explain what kind of understanding this phenomenon supports.
The US Geological Survey started this research from 1993. Through experiments, they found that the disadvantage of the Soviet Union's geo-electrochemical method is that the traditional electrochemical method only collects the substances in rocks and soil under natural humidity. The chemical extraction volume obtained by the electrode is less than 1 cubic inch. Therefore, they developed a new geo-electrochemical method (NEOCHIM), which can easily increase the sampling volume by 100000 times without moving the soil, thus greatly enhancing the sensitivity of the geo-electrochemical method. It is said that this new geo-electrochemical method can be used to detect the influence of soil moisture on geological variables, identify the change of soil moisture caused by distance pollution sources, and evaluate the effect of soil control.
5. Large sample heap leaching (BLEG) method
In the early 1980s, Australia introduced the commercial technology of bulk heap leaching of gold (BLEG), which was widely used in gold exploration under various geological background conditions. At first, it was mainly used in soil, but now it is mainly used in river sediments. This is a relatively cheap and highly sensitive area sampling technique. Since 1980s, major breakthroughs have been made in gold exploration in Australia and the Southwest Pacific (Indonesia, Papua New Guinea, etc.). ) In this way. However, so far, there are few documents describing this method in detail.
The principle of this method is to collect large samples of river sediments, leach them with weak sodium cyanide (NaCN) solution, and then determine them by atomic absorption spectrometry. The essence of this method is also partial extraction of gold, but the nature of the gold provided is not clear. However, it is generally believed that only free gold is extracted, and fine gold is easier to extract than crude gold. Experiments show that gold particles with a diameter of 45μm can be dissolved in the leaching process. The dissolution of gold is affected by the content of clay minerals and organic matter in the sample. It is usually necessary to collect 5 ~ 6 kg samples, but some recent data show that the sample collection amount can be relatively small. However, it is very important to keep the consistency of sample size in the measurement process. Usually, samples are screened into parts smaller than 2 mm, 4 mm or 6 mm. The sampling density is mainly determined according to the experience of various regions.
After the sample is collected, it is dried in the laboratory and then accurately weighed. Under the static condition or intermittent vibration condition, the sample was leached with sodium cyanide for more than 65438 02 or 24 hours. There are several ways to remove gold from mother liquor. The most commonly used methods are zinc powder precipitation and activated carbon adsorption. Another method is to determine the gold content in mother liquor by carbon column and atomic absorption spectrometry. The common method is to recover the gold extracted by zinc, activated carbon or organic reagents into the solution, and then directly determine it by spray atomic absorption spectrometry. The detection limit of most schemes is 0.005× 10-9. Because of the heavy weight of the samples to be collected and the heavy workload in the field, even western scholars call "BLEG" the "big" method, which means "heavy".
6. Geochemical gas measurement method
Gas has strong permeability, so geochemical gas measurement method is considered as one of the most promising methods. The research on geochemical gas measurement methods is very active, and its research objects mainly include mercury vapor, CO2, O2, hydrocarbon gas, sulfur, carbon and oxygen compounds (such as COS). With the deepening of oil and gas geochemical exploration, the application of hydrocarbon gas in metal deposits is increasing day by day. However, as a sign of mineralization, most gases are indirect prospecting signs, so they have great uncertainty and multiple solutions. The Journal of Exploration Geochemistry published the Special Collection of Gas Geochemistry in Soil and Rock in Volume 38 of 1990. In this special edition, S.E. Kaiser (1990) made an objective evaluation of the present situation of geochemical gas measurement methods. "The gas geochemical method in soil is far from a conventional tool for mineral exploration, and more research work is needed to explain the observed abnormal patterns. Generally speaking, the geochemical gas measurement method has not yet entered the practical stage in mineral exploration, but if we look back on the evolution of geochemical gas measurement technology, it is not difficult to find that the observation means and technical ideas have changed greatly.
In the initial stage of gas measurement, free gas is the main research object, but people soon find that this kind of free gas is affected by meteorology, landform, loose layer characteristics and microbial action, so it is difficult to compare the measurement results at different times. Subsequently, the study of gas adsorption by solid substances (soil and rock) appeared. For example, S.E. Cather (1984, 1990) applied desorption method and achieved good results. Although this method inhibits the influence of environmental conditions on gas measurement results to a certain extent, the adsorption capacity of soil for various gases is very different, especially for hydrocarbons, and some gases can compete with hydrocarbons for effective adsorption positions in the soil. In addition, after the sample is collected, the gas adsorbed in the wet sample will be destroyed by microorganisms; In the process of sample treatment, the adsorbed gas may be shielded by the thermoelectric gas of carbonate, which makes it difficult to quantitatively measure the adsorbed gas. Therefore, the researchers proposed to treat the sample with acid to extract acidolysis hydrocarbons, so that all carbonates were dissolved and all adsorbed gases were released.
From the evolution of geochemical gas measurement methods, it can be seen that studying various gases and their metal contents is an effective way to deepen geochemical gas measurement methods.
It should be noted that enzyme leaching method and active metal method have a broad market in western mining industry and have been widely used in mineral exploration in concealed areas, while other methods are far less valued than these two methods. However, in recent years, with Russia's reform and opening up, geo-electrochemical methods have been rapidly recognized in the West. The above methods are still used for local geochemical exploration, but not for regional exploration. Therefore, at present, we should strengthen the research on the basic theory and observation technology of geochemical exploration methods in concealed areas, improve the reliability and practicability of existing geochemical exploration methods, and make them develop into strategic prospecting methods in concealed areas.
Third, remote sensing methods.
The development of geological application methods of remote sensing technology provides a shortcut to comprehensively observe and analyze the point and surface laws in the prospecting evaluation of concealed deposits to a certain extent, and opens up a method for tracking the metallogenic process and quantitative research of indicators. In order to find and evaluate concealed deposits, the most important development of geological application methods of remote sensing technology at home and abroad so far is mainly manifested in the following four aspects.
1. Development and Utilization of High Spatial Resolution Spaceborne Data
High spatial resolution sensors such as satellite-borne imaging scanning and radar imaging system in visible-near infrared region are launched one after another, which constitutes a huge data source for multi-channel imaging and all-weather and multi-landscape coverage, and solves the work demand that the cost of evaluation area is increased or it is difficult to start, such as comparison and direct comparison between evaluation area and known typical area, which is often involved in prospecting evaluation. At present, almost all important metallogenic belts at home and abroad have available satellite-borne images. These images (data) have become an indispensable intuitive data to reflect geological characteristics in the study of regional metallogenic laws and specific prospecting work, and provide a basic data source for the comparison and enhancement of mineralization indicators of some minerals. In various stages of mineral exploration in western countries, different types of satellite-borne remote sensing imaging or non-imaging data are widely used to "steal" geological information of adjacent leased land to reduce manpower and investment.
2. Development and application of imaging spectrum system
The progress of sensing system lies in the determination of specific minerals or lithology by using the characteristic spectral bands of minerals, and accordingly, imaging spectral systems with dozens or even hundreds of working bands have been developed. This imaging spectral system is essentially a combination of indoor spectral analysis and aerial imaging. This is one of the most outstanding advances in the development of geological application methods of remote sensing technology in recent 20 years, which has promoted the development of a series of special or general data information processing software for practical problems in mineral exploration, and further penetrated today's computer technology into routine geological work. The formation, revision and verification of the usual field geological observation and metallogenic concept have become a process that can be demonstrated and operated indoors, and then verified on the spot after the target is determined. The progress of mineral exploration process after the target is determined makes the above-mentioned quantitative research technology of rocks and minerals more practical, which is helpful to better combine the analytical experiment of geological process with general digital processing and demonstration.
3. Study on the spectrum of ground objects
The physical basis of geological application method of remote sensing technology is the study of ground object spectrum, but the study of ground object spectrum aims to enhance the difference of spectral response of different ground objects, so as to distinguish and judge the results. Therefore, under the above working mode, it is of little benefit to test, store and manage the spectral data of massive ground objects. On the contrary, people pay more attention to the geological application value of spectral behavior of rocks and minerals developed on the surface.
Detection of volatile components —— Detecting the spectrum of minerals rich in ore-bearing components or compounds or complex inorganic salts can trace the distribution path, content distribution and composition change (composition and structure) of minerals containing volatile components, and help the target area to delineate or reveal the evolution relationship of metallogenic series (from the spatial position). These minerals include NH4-containing minerals, Cl-containing minerals, F-containing minerals and OH-containing minerals. For example, the ground and aerial exploration of ammonium-bearing mineral-ammonium feldspar proves that it can be used to indicate the path of ore-bearing fluid carrying hydrothermal solution and reveal the metallogenic series of precious metals and non-ferrous metals such as gold. In the detection of chlorite spectrum, the frequency shift of hydroxyl vibration band caused by the change of chlorite's Mg/(Mg+Fe) ratio is used as a sign to indicate the approach of massive sulfide ore bodies, and so on.
Spectral response detection of structural changes of endogenetic minerals on the surface —— In recent years, the research on the series structure of clay minerals has been widely carried out, with the aim of tracing its genesis by using the spectral response of clays with different mixed layered structures, and then revealing the metallogenic geological environment. It belongs to this kind of work to study the spectral behavior of different iron oxide aggregates (limonite or limonite) and use it to evaluate and distinguish endogenous and exogenous pig iron drop.
4. Quantitative simulation and interpretation of remote sensing data
In recent years, the technical composition and working strategy of mineral remote sensing exploration have obviously evolved, which can be summarized as follows:
1) Based on the geological model, a multi-step working procedure of air-ground-indoor is formed, and the technical idea is generally based on the conceptual exploration model composed of geological signs (features) of the target to be measured.
2) The interpretation and research of linear bodies gradually abandon the image-based interpretation method, and on the basis of the original geological structure pattern, use the advantages of imaging and data processing to extract the "unexpected" structural parts of oil and gas accumulation or ore body occurrence, and pay attention to the analysis of stress structure and paleostress field.
3) Up to now, the working band of the sensor has included the wavelength region where the characteristic bands of many hydrothermal alteration minerals and some silicate minerals are located. Almost all hydrothermal alteration zones can be enhanced and displayed by image processing, including many subtle alteration halos. These works are the comprehensive application of regional remote sensing and geochemical exploration. In particular, this macro-micro tracking method is realized, which combines regional geochemistry with metallogenic process indicators such as hydrothermal center revealed by characteristic spectral bands to evaluate the element anomalies of local ore fields, mining areas and even ore bodies, thus avoiding some subjective deviations caused by the "superposition" evaluation based on spatial positions.
4) At present, while studying the morphological expression of geological target images, we pay more attention to the design scheme based on the physical parameters of geological target imaging, and develop some software systems that reflect the mineral indicators and their two-dimensional or three-dimensional distribution patterns of specific geological processes. Improve the pertinence and practicability of geological research and avoid or reduce the limitations of general GIS.