MINERAL RESOURCE DEPOSITS

Field of the Invention

The invention relates to the field of applied geochemistry and could be used in searching mineral resource deposits, in predictive-geochemical mapping of enclosed and semienclosed territories on the basis of data from geochemical mapping territories being investigated and following analyzing samples of soil.

Background of the Invention

Known are methods for searching mineral resource deposits [1-3]. All those methods based on statistical data processing of elemental analysis of great set of samples selected along the pre-assigned net are widely used for establish- ing the presence, nature, and parameters of secondary lithochemical halos and ore body dispersion trains, deposits or geochemical specialized terrains owing to a detected change in content of elements being determined in the samples. An element migration occurring in the process of deposit destruction in the hyper- genesis zone could be carried out in different forms: the elements can migrate in a solid, liquid, and gaseous state. As a result, dispersion halos are formed around deposits, the content of one or another element of interest within those halos being highest near the deposit and gradually decreased apart from it.

Known are methods for searching deposits on water element dispersion halos based on analyzing samples of natural water. The chemical composition of water depends to a great extent on a composition of those rock formation and ore mass through which the water circulates. Theoretically, water halos can have a great expansion, however, in practice, they are hardly succeed to be met at a great distance from deposits, since water will alter continuously its composition in the course of moving across different formations [4-6]. Elements being car- ried out from mineral resource deposits can be occluded by finely dispersed mineral and organic products built up the Earth crust. In such a way, sorption halos are originated in every place where underground water comes to the Earth surface and meets colloidal sediments presented in a solution.

Known are methods for searching mineral resource deposits by the so called mechanical (lithochemical) halos and dispersion trains that are not disrupted or small disrupted from the mother body. Widely known fragmentary- fluvial and bouldery-glacial methods are relevant here. The tillema lithochemical shooting is based on that minerals embosomed in other pieces can be carried by river water and glaciers to long distances [7-10]. However, the known method for searching is not applicable to deep-seated ore bodies, as well as to deposits that do not have conditions for mechanically transporting minerals of the elements being sought, or if those elements do not furnished their own mineral forms at all, i.e., if they exist in a diffused state.

The essence of the known methods [11-15] consists in that samples of soil, surface soil-forming loose sediments or bottom sediments of constant flows are yielded from the depth of 10-20 cm in accordance with the given net. The samples are dried and bolted through a sieve of size 0.5-1 mm. The extracted fraction of solid particles are abraded mechanically to the size of analytical powder (0.074mm) and analyzed by emission spectral analysis method or X-ray fluorescence method for determining an element content of mineralization indicators. Secondary lithochemical halos and dispersion trains are discovered by anomaly lithochemical halos for forecasting the presence of ore bodies and deposits. A disadvantage of those methods is in a low efficiency while applying to enclosed and semienclosed territories, where the primary rocks are overlapped with a cover of loose sediments. Under such conditions, the mechanical halos and dispersion trains can be absent in the loose sediment layer, and the sorption- salt halos and dispersion trains in the solid fraction of particles of size 0.5-1 mm can be manifested slightly and do not create geochemical anomalies, which will not allow to reveal ore zones, bodies or deposits. The analysis methods being in use are under-sensitive for detecting low content of rare elements.

In geochemical searching methods, various kinds of leaching are used quite often for selectively eluting elements [16-19].

For the samples enriched with organic matter, the leaching by sodium py- rophosephate is applied, the majority of manganese and iron oxide is dissolved when using hydroxylamine (hot and cold leaching), ethane diacid dissolves all oxide formations and partially - weak silicates, the mixture of potassium iodide and ascorbic acid dissolves oxide formations of iron, manganese and aluminium. The water extraction (hot and cold) is applied either for preliminary rinsing a sample, or for detecting a water-soluble salt, the extraction by hydrochloric acid dissolves acid soluble components (in the case of small sample weight, the concentration of the most elements appears to be lower than the detection thresh- old).

The method for selective extracting different salts allows not always to obtain the general pattern of dispersion of wide spectrum of chemical elements within one kind of analysis. When using aggressive reagents (acids), the selectivity of the element extraction decreases, and low-amplitude anomalies can be masked due to high background content of metals in leaching solutions and high detection threshold. No one from such teachings is selective with regard to rare and dispersed elements, and therefore does not lead to increasing the analysis sensitivity and, as a result, to detecting contrast anomalies of rare elements.

Known is the geochemical method for searching mineral resource deposits [20], which is the closest to the present invention by the technical solution and technical result, the method being based on analyzing the superfine fraction (Method for Analyzing the Superfine Fraction - MASF), chosen as the nearest analog. The method includes steps of collecting soil samples (250-300 g), ex- tracting the superfine fraction (< 10 μιη) from the samples of soil and loose sediments by the method of powdering at the mounting developed and designed in the "BCErEH" [All-Russian Research and Development Geological Institute]. It is assumed that ore and indicator elements being in a specific mobile form are reaffixed at sample particles. Further, a sample weight of 100 mg was processed with "aqua regia", and originated salt were dissolved in nitric acid. The resulted solution was analyzed for 25 chemical elements [21] by the method of inductively coupled plasma mass-spectrometry (ICP MS).

Disadvantages of the known method are the uncontrollability of the size of particles being extracted, the necessity for decomposing solid samples and solubilizing them using strong acids, a long time for obtaining the analytical data, abusive work of the personal. Such approach does not allow to obtain contrast anomalies and, as a result, the accuracy of the method for revealing the anomaly zones is low.

Thus, the low accuracy in revealing the anomaly zones is caused by:

- uncontrollability of the size of particles being extracted;

- use of strong acids for decomposing, which leads to an inaccuracy of analysis due to an inaccuracy in functioning an apparatus that must not, in accordance with the instruction, be used in the solution concentration, e.g. HC1, exceeding 12%;

- long time for obtaining the analytical data in view of the necessity for decomposing the samples;

- abusive work of the personal;

- high cost of the complete analysis cycle including cost for recycling the strong acids.

The known method for extracting the superfine fraction does not assume to fix a particle size. The particle size of the superfine fraction can vary substantially depending on natural content of sample. Herewith, the sample solubility degree being a function of the particle size will introduce some uncertainty into the data interpretation concerning the abnormal values. Besides that, an incomplete extraction of the salt forms of rare elements can take place, since, in contradistinction from ions, there are no reliable data on that those elements are absorbed by superfine fraction particles bigger than those elements themselves.

A series of preliminary steps for extracting the superfine fraction and a process for solubilizing solid particles require altogether a valuable time and people as a resource.

In solubilizing the superfine fraction, when preparing samples for analysis by the method of ICP MS, strong acids are used (e.g., "aqua regia"), from whence not only the absorbed salt forms of the elements are solubilized, but the rock matrix (carrier) is partially dissolved, and the contribution of the latter into the total element concentration can be predominant.

The method of inductively coupled plasma mass-spectrometry being used after the sample dissolving has limitation on chlorinity in solutions, the chlorine prevailing inevitably in a solution due to using the hydrochloric acid. Corrections should be introduced in the case of high chlorinity in solutions, which affects on the analysis accuracy.

When dealing with the strong acids, the personal must fulfil extraordinary safety measures. The utilization of acids being used also requires time and money expenses.

As a result, the method for geochemical searching the rare and dispersed chemical elements based on the method for analyzing the superfine fraction (MASF) and known from the nearest analog does not allow to obtain crisp and objective data on the rare element distribution in the territory being investigated.

The indicated circumstances do not permit to allocate precisely the deposit place, which requires additional costs for refining data in order for revealing anomaly zones in the prospecting territory. The technical result of the claimed invention consists in improving accuracy and reliability in determining the content of rare and dispersed chemical elements in the investigation territory, enhancing the contrast ratio of anomaly zones, and subsequently enhancing accuracy of the anomaly zone localization, i.e., high degree of anomaly localization. Besides that, the technical result of the claimed invention consists in saving the analysis execution time (analysis speed) and analysis cost (cheapness of the method), as well as personal operation security.

The checkability of particle size is achieved considering the use of Stokes law when selecting conditions for water extraction of those particles. Time expenditures for analyzing one sample are decreased substantially, which affects on the analysis cost.

The absence of strong acids in the sample preparation scheme allows to realize maximally the possibilities of the method of ICP MS. The personal does not handle with strong acids, which decreases the analysis health hardness. There is no need in utilizing the acids after usage thereof.

In the claimed method, the indicated technical result is achieved by means of considering and using the current level of scientific achievements in the field of researching deportment forms of chemical elements and modern possibilities of analytical equipment. The enhanced study of chemical element behavior in a dispersion state is the essential component in developing the geochemical methods for searching mineral resources, especially in the case of searching deposits of rare and dispersed chemical elements. Investigations in this field presume dealing with extreme low concentrations, which is limited by possibilities of analytical equipment.

The establishment of innovative methods of geochemical search dates from the end of 1990s, and the possibility of such investigations is conditioned by appearing the modern analytical methods and devices [22-25]. The transfer of the geochemical investigations to the modern level was promoted by development of the theory of mobile and secondary fixed forms of elements. The current searching methods are based on fixation of chemical element mobile forms and on their possibility for migrating through rock pores and micro fractures to far enough distances from deep ore body to the Earth surface.

The accounting of microelement forms in soils and rock formations has significant importance for both geochemists and analytical chemists. A considerable part of chemical elements forms a part of minerals as amorphous admixtures, replacing macro components in crystal lattice. Some of them are accumulated in gas-liquid occlusions, and a part is in a colloid-dispersed form in pore space of rocks.

Traditional analytic methods, such as the X-ray spectroscopy, have high sensitivity threshold for rare and dispersed elements (especially for gold), and for this reason, it is impossible to reveal anomalies of those elements in soils.

At present, in order for obtaining objective data with a required accuracy and repeatability, quantitative method are used, which methods ensure for reaching low detection levels including, primarily, atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and inductively coupled plasma-mass spectrometry (ICP-MS). However, their extension in routine geochemical searching methods is limited by a high cost of analysis. Besides that, the use of various chemicals for testing and solubilizing samples leads often to errors associated with incontrollable harmful influence of solution components and solvents, which content is in large excess over rare and dispersed elements. This influence stipulates an increase in the method detection limits, so the content of targeted elements appears to be lower than achieved thresholds. As the problem solution, a method had been developed, a multi-element analysis of superfine soil fraction using the laser ablation and inductively coupled plasma mass spectrometer (LA-ICP MS).

Summary on the Invention

The essence of the claimed method consists in that the positive result is achieved by congesting the sample density (not less than 1000 samples per 1 km ² ), which is especially important in searching small and middle gold ore objects. Picking samples weighing 50-60 g is carried out from the top layer of the illuvial horizon B l ; from each picked sample, a suspension is prepared on the base of water in the ratio 1 : 10; after that, from each suspension, a superfine fraction weighing 2-3 g and 2-35 μπι in size is extracted during 1 minute, dried at a room temperature for not less than 24 hours, and applied each dried superfine fraction on a glass palette made in the form of plane table with dimensions of 12x 10x0.3 cm and square marking 5x5 mm in number not less than 200 squares; the obtained dry superfine sample fractions are placed into those squares and analyzed for rare and dispersed elements by the method of laser ablation (LA-ICO MS) with the burning area 5x5 mm.

Based on the results of the analysis, the anomaly zones are segregated, and the conclusion is made on the presence of rare and dispersed element deposit at a depth.

The claimed method differs from the nearest analog in that:

a) a size of the sampling density. At present, a discovery of deposits small in resources is the most probable, and the effectiveness of the prospecting works for gold and other rare elements depends substantially on the number of tested and analyzed geochemical samples. For this purpose, the sampling must be carried out according to detail network exceeding the network size indicated in the instruction [9]. The number of the tested samples for one object is made in an amount of 1000 samples per 1 km ;

b) a sampling of small (50-60 g) samples instead of 200-300 g is carried out;

c) a sampling of the horizon B as the most informative in terms of chemical element accumulation is carried out;

d) an individual sample preparation can be done in the field environment, resulting in that the sample weight is reduced to 2-3 g, which is important in transportation from the remote area;

e) the sample analysis is carried out at an instrumental complex LA-ICP MS. The ablation mode: scanning speed 900 μηι/sec; burning depth 5 μιη; energy 65%; frequency 10 Hz; beam size 610 μιη; burning time 18 seconds for gold and 32-60 seconds for other elements.

Quality inspection is carried out using standards and also includes an inspection in an external laboratory. Distribution maps of chemical element content are drawn, and zones of anomaly indicator element content are revealed on the ground of the obtained analytical data. According to the revealed zones of anomaly chemical element contents, the presence of the targeted zones of ore mineralization, ore bodies and deposits are estimated.

The claimed method is developed theoretically and approved in the field laboratory of the Mineral Exploration Network Ltd (Finland) in the v. Ollola (Finland) and on the base of laboratories of the Sankt-Petersburg State University (SPbSU), and then in the field conditions in various landscape-climatic zones in the territory of Finland and Spain. The analysis inspection was carried out in the certified laboratories of Finland, Spain and Russia.

Samples of the soil horizon from the top portion of the illuvial horizon (B l) are collected on the network 50x5 m. From the sample of soil, the superfine fraction is extracted and analyzed by the laser ablation method (LA-ICP MS) for a wide range of chemical elements (Au, Pt, Pd, Re, Ag, Mo, W, Sn, Co, Ni, Ti, Zr, Nb, Ta, Sb, Rb, Se, Y, REE, Cr, Mn. Cu, Zn, As, Hg, Pb, Cd, Sr, Ba).

The data on content of gold as one of key strategic elements of any state is of the most interest. The gold contents of 0.2 g/t at an average were documented in the fine fraction of soil (FFS) comparing the background contents of 0.004 g/t. the revealed anomalies of the rare elements are authenticated with the geophysical data.

Detailed Description of the Invention

The geochemical method for searching mineral resource deposits according to the thin fraction in soil has been practiced, for example, within the region Logrosan (Spain) at the area of 67 km . A group of anomalies is presented by two parallel schistosity zones having a total expanse more than 10 km with the bulge thickness up to 200 m. In order for locating the anomaly source, 4 profiles of prospecting boreholes are drilled. Those boreholes enter the zones of quarts- sericite metasomatites in sedimentary-metamorphic rocks with the gold content of 0.1 -0.4 g/t.

The results of multiple approvals with the expected result can be found in the form of specific examples.

Example 1 demonstrates the informativity of the FFS analysis comparing the analysis of the soil as a whole.

Each sample of the soil horizon Bl with the total mass of 150 g were divided into two unequal parts (50 g and 100 g). The extraction and FFS analysis were performed similarly to the above. For comparison, the second part of the soil sample were analyzed by the atomic absorption method by the standard technique with the total acid coverage. The data of the analysis are shown in the table 1. Table 1. Content of Au, Pt, Pd in the samples of the soil horizon Bl and the fine fractions thereof, ppm.

The represented data indicate the substantial accumulation of noble metals in the fine fraction of soil in comparison with the soil as a whole. Due to the accumulation of the rare and dispersed elements in the FFS, their content happens to be by several orders higher than in the original sample of soil, i.e., in the samples having the content of individual element less than the detection threshold, in the FFS that content is defined as a real value. The FFS analysis allows to reveal real anomalies rather than obtain the values below the detection threshold, which does not allow for carry out the geochemical mapping and draw distribution maps of chemical element contents.

Example 2. Selection of the soil horizon for geochemical testing. An important moment of the search is in establishing the soil horizon that is the most informative when carrying out the testing. It is important to establish, in which horizon takes place the accumulation of chemical elements indicative for the search, and to perform hereafter the prospecting in that horizon. For this end, soil profile cuts are made, and their testing along the cut is performed. The investigation results are shown in tables 2 and 3.

Table 2. Gold content in various horizons of the soil profile cuts, ppm.

A is the humus horizon, Bl is the top portion of the illuvial horizon, B2 is the bottom portion of the illuvial horizon.

Table 3. Chemical element content in various horizons of the soil profile cut, ppm.

A is the humus horizon, B 1 is the top portion of the illuvial horizon, B2 bottom portion of the illuvial horizon. Thus, it had been established that the most informative horizon is the horizon B 1 , where gold and companion elements are accumulated.

Example 3 demonstrates a selection of the FFS deposition time during the extraction. The selection of the optimal time period allows for optimizing time expenses of the process.

Five grams of the standard sample having a known gold content and particle size of 0.074 μιη were mixed with 50 g of quarts silt sandstone that is similar in its content to the soil horizon B l content. Test gauges with gold content of 0.005, 0.012, and 0.04 ppm were prepared. The elutriation was performed in a glassware according to the standard technique from a suspension based on water in the ratio 1 : 10, which maintains the Stokes law. The result are represented in the table 4.

Table 4. Gold content in the fine fracture depending on the deposition time, ppm.

The results of investigations in this example show that 1 minute is enough for the deposition. When using the less time interval, the reliable result cannot be obtained, and the more time interval extends the time of experiment, but does not affect on the analysis result.

Example 4 allows to estimate possibilities of using water having various degrees of cleaning when extracting the fine fraction of soil (FFS).

As was mentioned above, in carrying out the geochemical searches, the time and material costs have a great significance. The preference is given to low-cost (in time and means) methods. Since the extraction of the FFS is performed in water medium, it is important to estimate, water of what quality can be used in extracting the fraction. The results are shown in the table 5.

Table 5. Experiment for extracting FFS by water of various degrees of cleaning.

In the experiment, potable, distillated and bidistillated deionized water were used. The table shows the results of the experiment for analyzing the test gauges having gold content of 0.005, 0.012, and 0.040 ppm. It follows from the experiment, that the quality of water cleaning does not affect on the analysis result in the case of gold, and so it is possible to use potable water when carrying out the geochemical searches. The result shows the absence of significant contents of gold in potable water.

Example 5 demonstrates results of experiment for selecting, by the method LA-ICP MS, the analysis area that is burned by the analyzing probe when sampling (table 6).

Table 6. Selection of the analysis area when using the method LA-ICP MS.

Analysis area Gold content in test gauges, ppm

(analyzing probe) Test gauge 0.005 Test gauge 0.012 Test gauge 0.040

Point, 1 μπι 0.003 0.008 0.02

Area of l x l mm 0.030 0.010 0.032

Area of 2x2 mm 0.004 0.012 0.033

Area of 5x5 mm 0.005 0.012 0.040 Thus, the experiment proved that it is possible to use the burning area of 5 x5 mm for obtaining correct results.

Example 6 demonstrates the reproducibility of gold analysis results in FFD by the method of laser ablation and results of inspection performed by the method of atomic absorption in an external laboratory (table 7). The external inspection is the necessary condition for checking the adequacy of the analysis results when developing new techniques.

Table 7. Results of gold analysis in FFS by the claimed method and by the method of atomic absorption in the external laboratory, ppm.

The represented data shows a good result reproducibility of analyzing FFS by the claimed method and method of atomic absorption made in the external laboratory. Thus, summarizing the advantages of the proposed method, it is necessary to note: high sensitivity of the analysis; high confidence, reproducibility and comparability of the analysis results; high operational efficiency (up to 400 samples per day); low operational cost (about 4 euro/sample). The extraction of FFS does not require special conditions for sample preparation and can be performed in the field. The proposed method is ecologically harmless one, since no chemicals are used during the sample preparation and analysis.

The claimed method for searching deposits on the base of analyzing the fine fraction of soil, besides increasing the authenticity and reliability of the analysis results, allows to obtain error-free information at extreme low levels of content, which leads to increasing the probability of revealing and reliability of estimating geochemical systems while reducing in expenses for performing analytical works.

References cited

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Appendix

to the patent application for an invention on "Geochemical method for searching mineral resource deposits"

(Explanations to pp.1-2 of the text)

Thus, according to analysis of the background of the invention, geochemical methods for searching mineral deposits are mainly based, up to date, on the "Instruction on geochemical methods for searching ore deposits" (Instruction, 1983), approved by the Ministry of Geology of the USSR. That Instruction being based on theoretical concepts of founders of exploration geochemistry [7, 10] and their followers had generalized the development experience accumulated by the early 1980s in theory and practice of geochemical method application for searching ore deposits. The Instruction regulates the techniques of organization of geochemical deposit searches. Compliance of the geochemical search methods to requirements of the Instruction guaranteed for many years the minimum required quality of the geochemical area prospecting. Meanwhile, forty years or so passed from the moment of putting the Instruction into execution. In this period, theory and practice of application geochemistry in Russia and abroad gained further development. In many instances, the new priority requires to recede from provisions of the Instruction.

(Explanations to pp.4-5 of the text)

In accordance with the standard geochemical classification, the group of rare and dispersed chemical elements includes elements having a content in lith- osphere within 1 to 0.0001 g/t (table 8).

Table 8. Abundance of some chemical elements in Earth crust

Group Degree of extension Chemical element

1 Middle-abundant Ti, P, Ba, Sr, Zr, Mn 2 Low-abundant Ni, Co, Cr, V, Li, Pb, Cu, Zn, Th, Nb, Y,

Ga, Ce, La, Rb, B, Sc

3 Rare Be, Sn, W, Mo, Ta, Hf, Ge, U, As

4 Very rare Sb, Cd, In, Hg, Se, Ag, Bi

5 Super rare Pd, Au, Pt, Re, Ir, Os, Rh

Herewith, it is well known that the low the average grade of chemical element is, the more its share in a disperse form. Meanwhile, just within this group, chemical elements being "strategical" elements of any state are present, i.e., gold, platinoids and uranium.

It is known that a soil sample consists of flakes and particles of different size. When fractionating samples, it turns out that a share of fractions having various size is non uniform for different samples. Generally accepted grade scales are used in fractioning samples. Thus, for example, the granulometric spectrum of sedimentaries is evaluated on the following scale, shown in the table 9.

Nable 9. Classification of particles in size

The bonding clay of samples is often enriched in chemical elements, and just this fraction is, as a rule, used in carrying out the geochemical methods of prospecting. Geochemical halos revealed on basis of the bonding clay of rock and soil samples have significantly higher sharpness and, consequently, higher informativity.

In some cases, it is necessary to divide the bonding clay of sample into finer grain-size classes. Such division is performed, for example , using the ap- paratus "Analizetta" in water; the method is based on the Stokes law - dividing particles in size. Fractions from 1 μηι (1 10^ ηι) ιο 0.005 mm are revealed.