On the Hydrogeochemical Method for Prospecting Non-Ferrous Metals and Some Rare Elements

P. A. Udodov and I. P. Onufrienok

(Presented by Professor, Doctor F. N. Shakhov)

Many regions have until recently remained poorly studied with respect to the presence of mineral resources in them, because geological conditions hinder the effective application of existing prospecting methods. Such regions include: 1) mountain massifs with widespread development of kurums, 2) regions with a thick cover of loose formations, 3) blind deposits of various mineral resources.

For the study of all these regions, new effective methods are required. One such method is the hydrogeochemical method being developed by the authors. This method is applicable not only to the previously mentioned difficult-to-develop regions, but, as thematic field studies have shown, it also produces a large practical effect under the most diverse geological conditions.

As a result of three years of work, both in the laboratory and in the field, the principal features of the methodology of field hydrogeochemical investigations have taken shape; at present they may be considered tested.

At first we developed this method of studying natural waters while taking into account the following features of their composition. 1. The composition of an aqueous solution depends on pH, and pH in turn depends on the composition of the solution. Consequently, pH is one of the most essential characteristics of natural waters. 2. The composition of the aqueous solution and its pH must change depending on the rocks that the water encounters along its path. 3. If aqueous solutions come into contact with ore minerals, they must assimilate certain quantities of metal ions. 4. The content of metal ions must depend on the rate of movement of the aqueous solution through the ore zone, i.e., on the duration of contact of the solvent with the substance being dissolved, on the area of contact between the substance being dissolved and the solvent, on the presence of a chemical reaction accelerating the transition into a dissolved state, on the pH value, and on the temperature of the water. 5. The content of ions of non-ferrous and rare metals in natural waters is small, on the order of 0.001 mg/l, and in rare cases greater; therefore ordinary chemical analysis of waters will not be able to record such small quantities, and even if it can, then in order to obtain the corresponding results it will be necessary to concentr-

concentrate a water sample by evaporating 10–20 liters down to a volume of 10–20 ml; the latter makes the method lengthy, cumbersome, and unprofitable.

These preliminary general considerations indicated to us that, in order to achieve the stated goal, it was necessary to work in several directions. First, to overcome the main obstacle, that is, to find a method for the rapid concentration of metals in natural waters, to find sensitive reactions that would make it possible at least qualitatively to record the presence of these ions under field conditions. Second, to study the paths of mineralization of natural waters, beginning with atmospheric water, fresh precipitation, and extending to ocean waters and deep waters, and also to trace those changes that occur in aqueous solutions depending on the position of these solutions in the system of the water cycle; to study the influence of pH on the content of ions of various metals in natural waters. Third, to develop organizational norms for equipping and conducting the hydrogeochemical method of prospecting for mineral resources, to develop criteria for prospecting deposits by this method, and also to develop ways of presenting the data obtained by this method against the background of the general geological setting.

In this article, primary attention is given to the method of concentrating metal ions from natural waters.

In the second half of 1951 we began developing a method that would make it possible rapidly to concentrate metal ions from an aqueous solution. Since sulfide compounds of metals have the lowest solubility, it was decided to precipitate metal ions in the form of sulfides with the aid of sodium sulfide. Experiments on the precipitation of small quantities of copper, lead, zinc, and other metals from an aqueous solution by sodium sulfide did not lead to the desired results, because at low metal contents in water the sulfide precipitates did not form for a long time; the sulfides were obtained as colloids, which easily passed through filters. At the beginning of 1952 it proved possible to accelerate the precipitation of sulfides by coagulating them with alum-potassium alum. Coagulation with alum-potassium alum made it possible, within a two-hour period, to concentrate metals from a 10-liter sample into a concentrate weighing about one gram. This first success gave us much: the possibility of determining metals increased almost 10,000-fold, and the time required to obtain the concentrate already made it possible to think about the profitability of quantitative determinations. Having obtained such concentrates, one can detect metals in them spectroscopically down to 0.001 mg/l (calculated for the water).

The microchemical method of analyzing such concentrates detects metals down to 0.01 mg/l; ordinary chemical analysis, down to 0.1 mg/l. Finally, the polarographic method of quantitative analysis makes it possible to determine fairly accurately the content of metals in concentrates within the range from 0.001 mg/l and higher (calculated for the water). Thus, the method of rapidly obtaining concentrates of metals from natural waters brought with it the full possibility of using analyses of these waters for prospecting mineral resources, which was first applied by us in field prospecting work in 1952, 1953, and 1954.

The experience of our three years of work, and the favorable reviews of this method received from production organizations, where it is already being introduced into the practice of geological-prospecting investigations, obliges us to publish the fundamentals of this method.

Fundamentals of the Hydrogeochemical Method

The ultimate goal of the hydrogeochemical method of prospecting for mineral resources is to delineate, on the ground, areas with an elevated content of metal ions in waters, and to compare the results of the investi-

…of natural waters with the geological structure, and a forecast of the metalliferousness of the area under study in order to substantiate further geological-prospecting and exploration work.

At present, on theoretical grounds, the following metals may be expected to occur in the concentrate: Ag, As, Au, Ba, Bi, \((\mathrm{Ce}^{4+})\), Cd, Co, Cu, Fe, Ga, Ge, Hf, Hg, In, Ir, Nb, Ni, Os, P, Pb, Pd, Pt, Pa, Ra, Rh, Ru, Sb, Sc, Se, Si, Sn, Ta, Te, Th, Ti, \((\mathrm{U}^{4+})\), W, Zn, Zr, Cr.

It should be noted, however, that so far field hydrogeochemical investigations have verified only the presence in concentrates of the following elements: Ag, As, Ba, Bi, Cd, Co, Fe, Hg, Ni, P, Pb, Sb, Sn, Te, Ti, \((\mathrm{U}^{4+})\), Zn, Mn, Th, B, Cr, Be, Mo.

The following cannot be completely separated: Be, Lu, Mo, Tu, Yb, V.

The following cannot be separated: the alkali metals, Mg, Ca, Sr, Sm, Eu, Er, Y, \((\mathrm{Ce}^{3+})\), \((\mathrm{UO}_2^{2+})\), Mn, Tl.

The hydrogeochemical prospecting method within the system of geological exploration service may be planned independently, separately, but it is better if it is used in combination with other prospecting or exploration methods; the latter greatly reduces the cost of prospecting work. It is most convenient to plan work by this method together with hydrogeological mapping, because the hydrogeochemical method of investigation must be based on a comprehensive geological study of the area.

Comprehensive field geological investigations of the area must include the study of lithology, stratigraphy, and tectonics, the carrying out of heavy-mineral and metallometric surveys, as well as the hydrogeochemical investigation of all natural waters.

In most hydrogeochemical surveys at scales of 1 : 100000 and 1 : 50000, the objects of study are chiefly the various natural outlets of groundwater and surface water bodies. In more detailed investigations, various exploratory workings are made, from which water samples and concentrates are taken.

A hydrogeochemical survey includes the following types of work: 1) geological examination of the prospecting area, 2) hydrogeochemical sampling of water objects with analysis of water samples and concentrates, 3) field and office processing of the material.

Geological examination of the prospecting area

Before beginning hydrogeochemical sampling, it is necessary to become thoroughly familiar with the geology of the area. The geological examination of the area is carried out on a geological base at the scale of the hydrogeochemical survey. In doing so, special attention must be given to the contact zones of intrusions with sedimentary and metamorphic rocks, to zones of dislocations, and to areas where vein rocks are developed. The geological examination of the area makes it possible to lay out the plan of field hydrogeochemical investigations more rationally.

Hydrogeochemical sampling

All water objects—outlets of soil and groundwater and the waters of surface streams—are sampled first of all for pH. For a survey at a scale of 1 : 100000, within one square kilometer the determination of pH in water objects is carried out, on average, at 5 points.

The pH is determined approximately with a universal indicator in a porcelain spot plate. To do this, the porcelain spot plate is rinsed several times with the water being sampled; 3–5 ml of water are taken into it and 3 drops of universal indicator are added; the mixture is stirred with a glass rod and the color is compared with a standard color scale.

After an approximate determination of pH, it is determined more accurately with the aid of indicators of the nitrophenol series. Such indicators, together with prepared standard scales, are contained in the Michaelis apparatus (manufactured by Agropibor). A detailed instruction manual is usually supplied with such an apparatus.

The taking of concentrates from various watercourses is carried out with the aim of fully characterizing the basin under study. Each collected concentrate must be characterized with respect to the pH of the water, the temperature of the water and air, the discharge of the spring or watercourse, and also the chemical and physical properties of the water.

When carrying out hydrogeochemical investigations at a scale of 1 : 100000, water concentrates are taken at the rate of 50 concentrates per 100 sq. km of the surveyed area.

The surveying rate and the composition of the hydrogeochemical party are provisionally, until standards are developed, adopted according to the SUSN as the same as for hydrogeological parties of the corresponding scale.

During 1954 and 1955 the method for obtaining water concentrates was gradually refined by us and is now used in the following form. To determine heavy metals in natural waters, a sample of one liter is taken in a bottle with a ground-glass or rubber stopper, and 2–3 drops of methyl-orange indicator are added to it. After this, the sample in the bottle is mixed, and six-normal hydrochloric acid is added dropwise until an acid reaction is obtained, which is determined by the appearance of a pale-pink color. Next, two milliliters of sodium sulfide (concentration 0.5N) are poured in, the bottle is closed with a stopper, and the contents are thoroughly mixed and allowed to stand for 5 minutes. Then one milliliter of a 2.5N solution of \(\mathrm{Al_2(SO_4)_3 \cdot 18H_2O}\), prepared in a 0.5N sulfuric-acid solution, is introduced, and the concentrate is mixed. Then a 1N solution of soda \((\mathrm{Na_2CO_3})\) is added to the bottle as calculated.

The calculation is made by the formula:

\[ x=\frac{(6a+3b)\,80}{100}, \]

where \(x\) is the number of ml of 1N soda solution,
\(a\) is the number of ml of 6N hydrochloric acid used to neutralize 1 liter of water with methyl-orange indicator, \(b\) is usually equal to one and corresponds to the number of ml of 2.5N solution of \(\mathrm{Al_2(SO_4)_3 \cdot 18H_2O}\) in 0.5N sulfuric acid added to the sample to obtain the concentrate.

The bottle is then closed with a stopper and thoroughly shaken. After 20–25 minutes the metal sulfides that have formed are coprecipitated together with aluminum hydroxide; the supernatant liquid above the precipitate is siphoned off, and the precipitate is transferred onto a filter, which is then placed in a test tube. At the end of the traverse the test tubes with concentrates are sent to a chemical laboratory for drying, weighing, grinding, and mixing. After these operations the concentrates are sent to a spectral laboratory.

When taking the concentrate, it is necessary to bear the following in mind:

1. Avoid taking a concentrate if the water of the spring is turbid. If the turbidity of the water is caused by a temporary phenomenon, for example by weather conditions, it is better to wait until the water in this spring becomes clear. The experience of field investigations has shown the necessity of filtering the water when taking all water concentrates.

1. All persons engaged in collecting the concentrate must, before beginning work, thoroughly wash their hands with water from the given source (slightly downstream from the sampling point), rinse all vessels that will be used for sampling, and only after this proceed to collect the concentrate.

Work and Tasks of the Field Chemical Laboratory

The tasks of a field chemical laboratory serving a hydrogeochemical prospecting party include: 1) further treatment of concentrates, 2) abbreviated chemical analysis of water samples, 3) sending concentrates for spectral analysis.

Work with the Concentrate

The concentrate delivered from the sampling point in a 100 ml vial is allowed to settle, which requires no more than 30 minutes. Then the settled water is siphoned into a clean beaker and this water is saved for rinsing the bottle when transferring the residues of the concentrate onto the filter. After the liquid has been siphoned off, the concentrate precipitate together with the remaining liquid is transferred onto a filter 7–9 cm in diameter in a white holder. The precipitate on the filter is washed from the walls of the filter to the bottom of the filter with small portions of the siphoned water. Then the concentrate precipitate together with the filter is removed from the test tube, placed on a watch glass, and put into a drying oven for one hour at an oven temperature of 105–110°. After one hour of drying, the precipitate with the glass and filter is weighed on an analytical balance to an accuracy of 0.1 mg; the concentrate is then again placed for drying for 30 minutes and weighed again. If the difference of the present weighing does not exceed 0.1 mg in comparison with the preceding one, the drying is considered complete. After drying, the concentrate precipitate is completely removed from the filter and placed in a small porcelain, or preferably agate, mortar, and the filter with the glass is again weighed on an analytical balance.

The weight of the concentrate is obtained from the difference between the weighings before and after pouring it into the mortar. In the mortar the concentrate is, if possible, finely ground with a pestle. The ground concentrate powder is kept in packets of thick clean paper. These packets are stored in such a place as to exclude the possibility of their contamination by foreign substances containing the metals being determined. When working with concentrates in the laboratory, the following data must be recorded for each concentrate: a) the color of the concentrate precipitate on the filter during filtration; b) the weight of the concentrate. In addition, it is necessary that all samples delivered to the laboratory, as well as those stored in it—water concentrates, etc.—be well labeled.

The results of analyses of concentrates are recalculated to content in mg/l. The calculations are carried out according to the following formula:

\[ \text{mg/l Me} = \% \text{Me}G \cdot 10, \]

where: \(\% \text{Me}\) is the percentage of the metal being determined in the concentrate,
\(G\) is the weight of the dry concentrate obtained from a volume of 1 l of water, in g,
\(\text{mg/l Me}\) is the content of metal in the sample of source water, expressed in mg/l\(^1\).

\(^1\) When checking the reagents shows the presence in them of any metals, the corresponding correction must be introduced into the final results of metal determinations.

Field Office Processing of Materials

Field office processing of materials consists, above all, in compiling a geological map of the study area. Particular attention is given to identifying the mineral resources of the area; for this purpose, data from heavy-mineral and metallometric surveys are used.

The results of the heavy-mineral survey are finally processed in the field, for which the party must necessarily have a mineralogical laboratory.

In addition, during the field season, water concentrates selected in the course of the investigations in the appropriate manner are systematically processed, and various ore elements in them are determined by spectral analysis.

At the same time, abbreviated chemical analyses of waters are carried out in the field laboratory according to the instructions of A. A. Reznikov (1950). All the obtained analytical results are checked and correlated with the geology of the area in the field.

Thus, by the end of the field work, a complete hydrogeochemical characterization of the natural waters of the study area will have been compiled.

All analytical data—polarographic, spectral, mineralogical, and chemical—must be systematically plotted on the corresponding maps.

The results of the field office processing of materials are expressed in the following maps: 1) a map of natural waters by pH and \(SO_4''\), 2) a hydrogeochemical map for one of the elements of the area, 3) a general hydrogeochemical map, 4) a map of the chemistry of natural waters, 5) a map of the water abundance of rocks, 6) a map of observation points, 7) graphs of the distribution of metals in natural waters.

Without dwelling in detail on the methodology for compiling the individual maps, we shall for the time being give only some of the most essential remarks.

The map of natural waters by pH and \(SO_4''\) is compiled on the geological base of the study area. The various values of pH and \(SO_4''\) should in most cases correspond to the geological-lithological composition of the rocks. The hydrogeochemical map is likewise compiled on the geological base of the area. This map shows quantitatively the presence and distribution, in the waters of the area, of the given metal for which the investigations are being carried out.

The general hydrogeochemical map is a resultant map. On it are delineated all promising areas for the planning of further prospecting and exploration work for various metals and certain rare elements. This map is compiled on the basis of data from geological study of the area, heavy-mineral and metallometric surveys, and analyses of water concentrates, expressed in milligrams per liter.

Usually, zones of ore occurrences, according to the results of polarographic and spectral analyses of concentrates from waters, are characterized by an elevated content of ions of heavy metals.

Indirect indicators of these zones also sometimes include acidic waters and the presence in them of divalent iron and the sulfate ion. Complete hydrogeochemical sampling of natural waters is carried out when conducting a geological traverse.

If any discrepancies are observed between the results of analyses of water concentrates and the geology of the area, control traverses are carried out and additional concentrates are collected.

On the basis of all the data obtained, a general hydrogeochemical map of the district is compiled.

The map of the chemistry of natural waters should reflect the geological history of the district’s development. The maps we have compiled for certain ore districts show the close connection between water chemistry and the geological structure and tectonics of the district.

A map of the water abundance of rocks during geological prospecting work is compiled, for districts that have been poorly studied geologically, from springs and the modulus of underground runoff. In doing so, it is desirable to take into account the coefficient of the basin’s characteristics.

The results of hydrogeochemical investigations must be illustrated by graphs showing changes in the metal content of waters from the source to the mouth of springs. These graphs clearly indicate the sectors with the greatest mineralization of waters. In addition, they also approximately establish the dimensions of dispersion flows.

Thus, as a result of field and office processing, all the necessary data are obtained, which subsequently should serve as the basis for carrying out geological prospecting and exploration work.

LITERATURE

Alekin O. A. — General Hydrochemistry. Gidrometizdat, 1948.

Brodsky A. A. — Hydrochemical Method of Prospecting for Copper. VSEGINGEO. Gosgeolizdat, 1956.

Vernadsky V. I. — History of the Minerals of the Earth’s Crust, vol. 2, History of Natural Waters, part 1, issue III, ONTI, Khimteoret., L., 1936.

Kamensky G. N., Klimentov P. P., Ovchinnikov A. M. — Hydrogeology of Mineral Deposits. Gosgeolizdat, 1953.

Malyuga D. P. — On the content of Cu, Ni, Co and other elements of the iron family in natural waters. Reports of the Academy of Sciences of the USSR, vol. 48, No. 2, 1943.

Ovchinnikov A. M. — Basic principles for the application of hydrogeological criteria to the search for mineral deposits. Problems of Theoretical and Applied Geology. Collection 2, Publishing House of the Moscow Geological Prospecting Institute, 1947.

Posokhov E. V. and Kalinin S. K. — Spectroscopic characterization of thermal and cold springs and mountain rivers of Eastern Kazakhstan. Izvestiya of the Academy of Sciences of the USSR, geological series, No. 6, 1948.

Reznikov A. A. and Mulikovskaya E. P. — Analysis of natural waters. Collection “Analysis of Mineral Raw Materials,” Leningrad, 1936.

Sergeev E. A. — Investigation of waters in the search for polymetallic deposits. Razvedka nedr, No. 2, 1943.

Smirnov S. S. — Oxidation Zone of Sulfide Deposits. Publishing House of the Academy of Sciences of the USSR, 1951.

Satkov A. A. — Geochemistry. Geolizdat, Moscow, 1951.

Safronov N. I. — On halos of dispersion of mineral deposits and their use in prospecting and exploration. Problems of Soviet Geology, No. 4, 1936.

Fersman A. E. — Selected Works, vol. II, Publishing House of the Academy of Sciences of the USSR, Moscow, 1953.

Khitarev N. N. and Mulikovskaya E. P. — On the geology of mine waters of sulfide deposits. Problems of Soviet Geology, No. 8, 1938.

Corrections and Noted Misprints

In S. A. Stroitelev’s article “Investigation of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.