Geophysical exploration theory and technical method
In the past 20 years, new theories and concepts of metal mineral exploration have been constantly emerging, and new technologies and methods have been continuously applied, which has effectively promoted the development of mineral exploration and injected vitality into metal mineral exploration. Among them, surface geophysical methods and borehole geophysical methods with deep detection depth, such as surface time domain electromagnetic method (TEM), controlled source audio-frequency magnetotelluric method (CSAMT), high-precision gravity and magnetic method, metal mine seismic method and three-dimensional seismic tomography technology, are widely used in mineral exploration (Cas et al.,1995; Salisbury et al.,1996; Lv Qingtian et al., 200 1, 2004), which brings a new opportunity for the prospecting and discovery in the ore concentration area-the prediction and search of concealed ore.
2.4. 1 Development Status of Modern Geophysical Exploration Technology
In the past, some prospectors thought that geophysical prospecting method was a "black box" with high multi-resolution and low reliability. Now, with the continuous progress of technology and a large number of practical applications, every prospector finally realizes that geophysical exploration technology is a very effective prospecting method. The research and application of geophysical high-tech has become an important part of mineral exploration in many western countries, especially in developed countries such as Canada, Australia and the United States.
The progress of geophysical exploration technology is mainly reflected in two aspects: first, new inventions; The second is to improve and upgrade the existing technology, so as to continuously improve the accuracy and precision of measurement. New, more powerful and more complex aerogeophysical methods (such as Falcon, MegaTEM, SPECTREM, TEMPEST, HOISTEM, NEWTEM, Scorpion, etc.). ) has become an important force in mineral exploration, thus greatly improving the efficiency of regional mapping and target delineation (TheNorthern Miner, 2007; Zhang Changda, 2006).
Airborne geophysical prospecting has developed rapidly in recent years. TEMPEST, the world's most advanced aviation mineral exploration system developed by the Mineral Exploration Technology Department of Australian Cooperative Research Center, uses a high-sensitivity magnetic probe to measure the weak secondary magnetic field generated by geological bodies, and the detection depth can reach 300m m m. Australia's "GlassEarth" project includes airborne gravity gradient measurement, airborne magnetic tensor gradient measurement, advanced electromagnetic method, mineral chemical mapping, new drilling technology and three-dimensional earthquake, among which airborne magnetic tensor measurement technology and airborne gravity gradient measurement technology are the key research and development contents. The most advanced superconducting airborne gravity gradient measurement system has been successfully developed by ARKEX Company in Britain, and the measurement accuracy has been improved by 10 times. The airborne gravity gradient tensor measurement system (Falcon) of BHPBilli-ton Australia won the CSIRO scientific research achievement award CSIRO 2000. It was born out of American military technology and is an export control product of the United States. The United States once prevented the company from using Falcon system to conduct exploration flights in China (Zhang Changda, 2005). GEDEXd-Agg, a high-resolution airborne gravity gradiometer developed by Gedex Company of Canada, was awarded the Mining Research Award by London Mining Magazine in June 2006. It is said that the instrument can detect solid minerals, oil and natural gas with a depth of 12km, and its accuracy and speed greatly improve the exploration efficiency and reduce the exploration cost.
Great progress has also been made in surface geophysical exploration. Phoenix Company of Canada introduced V5-2000 and V8 array magnetotelluric systems while perfecting the V-5 magnetotelluric system. Canada's EM-57 and EM-67 series have become the representatives of time-domain electromagnetic instruments. The Zongge Engineering and Research Organization of the United States has successively introduced the multi-functional electromagnetic systems of GDP- 16 and GDP-32, as well as the multi-functional magnetotelluric system that can carry out long-term natural field magnetotelluric survey. While improving MT- 1 magnetotelluric system, American EMI Company introduced EH-4 electromagnetic system, which has become one of the important means of mineral exploration, and also introduced MT-24 array magnetotelluric system. Nabighian et al. (2005) think that no geophysical method has a very wide application range like magnetic method, from planetary scale to several square meters, which can not only cost less but also improve rich information. The combination of electromagnetic method and gravity magnetic method has become an important development direction and exploration means. Electromagnetic method systems mostly work in frequency domain and time domain, and can collect data by multiple methods, such as induced polarization method, transient electromagnetic method and controlled source audio magnetotelluric method. Electromagnetic and gravity magnetic geophysical techniques are developing towards digitalization, intelligence, multifunction and integration.
In the process of geophysical exploration technology development, China's pace is relatively slow, and it is still mainly in the stage of technology introduction, which is not suitable for the rapid development of mineral exploration in China. At present, the country proposes to speed up the independent development of scientific experimental instruments and equipment, and the independent research and development of geophysical exploration techniques and methods should also become an important part of this strategic goal. China Geological Survey is organizing the development of a number of geophysical instruments for deep prospecting.
2.4.2 Basic principles of geophysical exploration methods for gold deposits and selection of working methods.
Although Au itself has outstanding physical properties (high density and good conductivity), due to its low abundance in the earth's crust, even for gold deposits with important economic value, the content of Au will not change the physical properties of Au-bearing geological bodies. Trace element gold is difficult to be directly detected by geophysical methods.
The basic idea of geophysical exploration of gold deposits is to observe the corresponding geophysical field response (anomaly) and solve the structure (especially deep structure), rock mass, strata, ore source bed and sulfide mineralization zone related to mineralization by studying the associated relationship between gold deposits (bodies) and geophysical abnormal responses of some special surrounding rocks, prospecting indicator layers, ore-controlling structures (especially fault shear zones) and sulfide (pyrite). Zhao, 2001; Li Daxin, 2003).
The difference of physical parameters between geological bodies related to gold deposits and surrounding rocks is the basis of geophysical exploration. The physical properties of the observed underground medium include density, magnetism, electricity, elasticity, radioactivity and temperature. The corresponding geophysical exploration methods include electrical and electromagnetic exploration techniques, magnetic exploration techniques, nuclear exploration (radioactive survey) exploration techniques, gravity exploration techniques and shallow seismic exploration techniques. Compared with gold deposits, different geological bodies choose different prospecting techniques.
(1) tracing gold-bearing fault zone
Trace the ore-bearing fracture zone (low stop zone) by electromagnetic fields such as resistivity method and very low frequency method; Tracing the fracture zone in magnetic rock by magnetic method (low magnetic tape under high magnetic background); Delineation of fault zone (high radioactive zone) by radioactive method. These methods can sometimes be used to understand its occurrence.
(2) Looking for gold-bearing quartz veins
Mainly use electrical method (high resistance, high polarization), radioactive method and magnetic method, and can understand its occurrence.
(3) Searching for the gold-bearing sulfide enrichment zone.
Electrical prospecting (low resistivity and high polarization) can determine the location, range, general occurrence and buried depth of gold-bearing bodies.
(4) looking for associated gold deposits
Geophysical prospecting method directly indicates base metal deposits, which are associated with gold for prospecting. Such as magnetic method, electrical method and gravity method, to find and delineate the scope, buried depth and general occurrence of associated gold deposits, so as to provide basis for engineering verification.
(5) Identify various small structures and ore-forming geological bodies related to mineralization.
Spatial distribution of various ore-hosting structures and ore-forming geological bodies (such as ore-bearing porphyries). ) is covered by Quaternary or bedrock, and it is found out by means of magnetic method, electrical method and earthquake.
(6) Large and medium scale mapping.
Methods include magnetic method, electrical method and radioactive method. The purpose of mapping is to determine the rock-lithologic factors related to gold deposits. Carbonaceous and graphitized strata, volcanic sedimentary complexes and other signs showing gold mineralization, such as near-ore wall rock alteration zone, silicified zone, sericite zone, talc magnesite schist zone and pyrite fine schist zone.
2.4.3 Problems needing attention in geophysical exploration of gold deposits
Because of the particularity of gold deposit itself, it is complicated to use geophysical technology to find ore, which leads to the uncertainty of obtaining geophysical information and geological interpretation. For example, the diversity of genetic types, mineral assemblages and tectonic environments of gold deposits determines the polymorphism of geophysical and geometric properties of gold deposits; Although some types of gold deposits are obviously different from surrounding rocks in ore physical properties, it is often difficult to observe identifiable anomalies because of the small size of ore bodies and weak geophysical field information. The background of geological interference caused by complex geological environment often masks useful information. All these will lead to difficulties or mistakes in exception interpretation. Generally speaking, the interpretation of geophysical anomalies in gold deposits is more difficult and complicated than other minerals and fields.
Due to the complexity of geophysical anomalies and anomaly interpretation of gold deposits, this indirectness broadens the application field of geophysical gold prospecting, but at the same time makes the multiplicity of geophysical anomalies more prominent. Therefore, the role of geophysical prospecting for gold should not be exaggerated.
When deploying geophysical exploration of gold deposits, we should aim at the macroscopic target areas of gold deposits found in different exploration stages and the geological problems to be solved. Actively explore comprehensive geophysical prospecting information. Because there are many types of gold deposits and the geological background is often complicated, the geophysical field is very complicated. In addition, the geophysical information of gold deposits is weak, and it is difficult to explain anomalies. It is often difficult to find Au by a single geophysical method.
In the whole process of gold prospecting, comprehensive methods and comprehensive explanations should always be emphasized. When arranging geophysical exploration of gold deposits in the exploration area, we should choose the most effective geophysical exploration method or the best technical combination of various exploration methods according to the specific exploration problems or geological and geophysical conditions, and give full play to the advantages of fine analysis of various geophysical parameters and comprehensive interpretation of geological data. Practice shows that scientific synthesis of information of various physical fields and physical parameters can achieve obvious results in determining the occurrence space of gold deposits, delineating gold mineralization zones and gold bodies, and determining the content of metal ores related to gold. In fact, the potential information content of a parameter can only be brought into play by comprehensive application with other parameters. And the comprehensive information provided by several methods is not the algebraic sum of information provided by a single method. In the process of prospecting, we should try our best to find out the macroscopic physical characteristics of the mineralized zone or the mineralized enrichment part, and then choose the most representative and targeted method to obtain comprehensive geophysical information related to mineral resources with a certain observation grid and accuracy, and endow all kinds of objects with clear geological significance through geological interpretation and intuitive illustration.
Different from other base metal deposits, it is more difficult to find gold deposits by geophysical exploration. However, the geophysical technology of gold mines has played a prominent role in the resource replacement of crisis mines. The Institute of Gold Geology of the Armed Police Force used the high-density resistivity method and EH4 to carry out prospecting work in several gold mines, and found gold bodies below 300m outside the Yuerya gold mine in Hebei Province, and newly discovered large concealed porphyry gold bodies in the Bilihe gold mine in Suyouqi, Inner Mongolia. It is proved from different aspects that geophysical prospecting technology plays an important role in the breakthrough of deep resource exploration in crisis mines.