On the Migration of Gold in Natural Waters

L. L. Shabynin

(Presented by Prof. P. A. Udodov)

Up to the present time, the migration of gold in natural waters has been poorly studied. In this connection, the application of the hydrogeochemical method to the search for gold-ore mineralization is difficult. The accumulated experimental and literature material \[1, 2, 6, 9\] does not give a clear answer to questions concerning the conditions and forms of migration of gold in the natural waters of gold-ore deposits.

During 1961–1964 the author carried out field work at gold-sulfide deposits in regions with different geological and climatic settings. A brief characterization of the regions and the results of the work in each of them are given below.

1. Eastern Transbaikalia. Field investigations were carried out within the area of the Ara-Ilya ore field, in the region of a fault zone which, in structural-metallogenic terms, belongs to the Shilka–Olekma gold-molybdenum belt \[7\]. The area of the ore field studied, situated within this fault zone, is composed of widely developed Paleozoic biotite, biotite–hornblende granites and a complex of Mesozoic small intrusions of dioritic composition. In addition to the small intrusions, Mesozoic magmatism was manifested in the form of outpourings of acidic effusives of the felsite and felsite-porphyry type and their tuffs, having the form of a cover, with a thickness from 80 to 200 m, and overlapping the fault zone in separate sectors.

The fault zone described has a northeastern strike and is confined to granites dislocated to varying degrees. It favored the manifestation of hydrothermal processes expressed in chloritization, sericitization, beresitization, tourmalinization, and the development of quartz–carbonate mineralization. Gold mineralization is confined to the following structural elements: 1) to areas of junction of two systems of fracture zones of northeastern and northwestern strike; 2) to places where ore-bearing veinlets are bounded by tectonic clay of the crumpling zone; 3) along dikes of quartz porphyries; 4) to a system of feathering fractures of submeridional direction.

The chemical composition of the groundwater of the region is very complex. Here bicarbonate, chloride, and sulfate waters occur, with all possible transitions from one type to another. Waters directly associated with the fault zone are distinguished by particular diversity. The latter, along with a complex chemical composition, are characterized by a complex assemblage of microcomponents. Spectral analysis of aqueous con-

concentrates collected by the TPI method [10] showed the presence in these waters of 18 elements: arsenic, silver, antimony, zinc, lead, copper, bismuth, nickel, cobalt, molybdenum, beryllium, cadmium, barium, titanium, manganese, chromium, vanadium, and gold [7]. Such an extensive suite of elements is due to the presence of varied sulfide mineralization: gold–arsenic in the northeastern part of the ore field and gold–bismuth in its southwestern part.

Gold in the waters was detected in samples from adit drip waters and in samples from springs that drain the groundwaters both of Quaternary deposits and of the fault zone in the area overlain by a felsite unit.

2. Kuznetsk Alatau. Field investigations were carried out in the area of the Saralinsk ore field, composed of a thick sequence of effusive-sedimentary rocks and small bodies of intrusive rocks, which are cut by abundant tectonic disturbances that unite into several systems. The age of the sequence has not been definitively established: some consider it Precambrian, others Cambrian. The geological structure of the ore field is very complex. A number of formations are distinguished here, represented by shales of various composition, sandstones, marbled limestones, and basic, intermediate, and acid effusives. Effusive rocks that have undergone greenschist alteration make up about 50% of the area of the ore field. Intrusive rocks—diorites, gabbro-diorites, and plagiogranites—are not widespread. However, vein rocks, participating in the structure of the ore field, occupy about 30% of its area. They are represented by dikes of acid composition (albitites, granite-porphyries, pegmatites, and aplites) and of basic composition (diabases, diabase and labrador porphyrites).

The ores of the deposits of the Saralinsk field belong to the gold-sulfide type and consist of quartz veins with poor and moderate sulfide contents. The paragenetic association of the principal minerals is as follows: quartz, carbonates, native gold, pyrite, gray ores, chalcopyrite, sphalerite, galena, arsenopyrite, antimonite. Gold in most cases is established only by chemical analysis and occurs in a dispersed state in sulfides. Its distribution is uneven.

The groundwaters of the ore field (excluding mine waters), in their chemical composition, are predominantly bicarbonate and bicarbonate-chloride, with mineralization of about 100 mg/l. The pH values range from 5.8 to 8.0. Gold was encountered at four points at contents of 0.5–1.5 μg/l. All these points are confined to natural groundwater outlets. The most characteristic elements accompanying gold in the waters are silver, arsenic, copper, lead, zinc, and nickel.

3. Gorny Altai. Middle-mountain region. The investigated area is located at the junction of the Lebed geosynclinal trough with the Biysk horst massif, within a single branching zone of deep faults. The central part of the district is occupied by a diorite massif elongated in the latitudinal direction; in the west it is truncated by the Turochak granite pluton, and to the east it plunges beneath a sequence of effusive-sedimentary formations of Sinian–Cambrian age. The composition of the massif is complex, varying from pyroxenites to syenite-diorites, with a predominance of diorites and gabbro-diorites. The effusive-sedimentary sequence is subdivided into several formations and is represented by basic effusives (pyroxene porphyrites, diabases, amygdaloids, tuffs and tuff breccias), sandstones, shales, bituminous and marbled—

limestones and marbles. Directly within the area of the gold-ore occurrence near the Turochak granite massif, owing to contact metamorphism, skarns of varied composition, hornfelses, and metamorphosed shales have developed, and strong silicification is observed. Karstified coarse-grained marbles are also widely developed here. The mineralization, associated with wollastonite and garnet skarns, belongs to the gold–copper–polymetallic type. The most widespread minerals with which gold is associated are bornite and chalcopyrite; more rarely, chalcocite, covellite, and pyrite. The principal association is gold—copper—silver.

The groundwaters of this area are uniformly hydrocarbonate and hydrocarbonate-chloride, with mineralization of about 50–60 mg/l. The pH of the waters is predominantly slightly alkaline and neutral. The suite of elements in the waters corresponding to areas with gold-sulfide mineralization is lead, copper, zinc, and silver. Gold was not detected at any of the sampling points.

4. Rudny Altai. Work was carried out at the Zyryanovsk deposit, enclosed in strata of effusive-sedimentary rocks of Middle Devonian and Lower Cambrian age, strongly metamorphosed and dislocated. The main tectonic structure of the area is the Revnushensky anticlinorium of NNW strike. The deposit itself is situated on a second-order structure—the Zyryanovsk anticline. The ore of the Zyryanovsk deposit essentially consists of enclosing rocks located in the inner part of the anticline and metamorphosed to one degree or another. The ores are polymetallic, massive and disseminated, with the latter predominating.

Gold is distributed rather uniformly both in massive and in disseminated ores. It is present mainly in the native state, in close association with the principal sulfide minerals: galena, chalcopyrite, sphalerite, and gray ores. It should be especially noted that in the enclosing rocks and ores, irrespective of the degree of mineralization of the latter, the iron content is 4–5%, of which ferrous iron accounts for about 50% of the total content.

Among the groundwaters in the area, two aquifer horizons have been distinguished: the horizon of Paleozoic deposits and the horizon of Cenozoic deposits. The first is predominantly weakly confined. In chemical composition its waters are hydrocarbonate, with mineralization of about 400 mg/l and a slightly alkaline reaction (pH = 7.0–7.5). The loose sequence of Cenozoic deposits is water-bearing everywhere. The chemical composition of these waters is also hydrocarbonate, with mineralization of about 100 mg/l and pH = 7.2–8.2. The suite of metals in the waters corresponding to the character of the mineralization is as follows: zinc, cadmium, copper, lead, nickel, cobalt, silver. In mine waters the content of individual elements, for example zinc, reaches several milligrams per liter (more than 2.5). Gold, however, was not detected in the waters.

The investigations carried out showed that, irrespective of the type of mineralization of gold-sulfide deposits, the possibility of migration of gold in natural waters over more or less considerable distances is determined by the degree of carbonatization of the enclosing rocks. Thus, in the first two cases (Eastern Transbaikalia and the Kuznetsk Altai), we have three types of mineralization: gold-arsenic, gold-bismuth, and gold-polymetallic. The enclosing rocks, with respect to water, are weakly active and neutral [11]. As a result, these waters possess

predominantly acidic and neutral pH. Under these conditions, as indicated above, gold was found in waters in isolated, rather rare points.

The essentially carbonate setting of the deposit in the Gorny Altai, with a gold–copper–pyrite type of mineralization, led to a low content of metals in the waters, in particular gold, which was not detected at the sensitivity of spectral analysis for it of 0.001%.

The considerable carbonatization of the host rocks of the Zyryanovsk deposit, with gold–polymetallic ores, and the relatively high content of divalent iron (about 2%) in the rocks and up to 4–5 mg/l in the waters, apparently also caused the absence of gold in the waters, although the content of other metals reaches significant values.

From the works of a number of authors [2, 1, 9, et al.] one may judge the migration of gold in natural waters as a function of the degree of carbonatization of the rocks. L. V. Razin and I. S. Rozhkov note, for example, that “...analyses of waters filtering through gold-bearing deposits of the Kuranakh type made it possible to establish that gold is often present in these waters, and the more so the more acidic the natural-water environment is. In fissure-karst waters of karstified limestones located below the sole of the gold-bearing karst bands, gold was not established.” They also note, for mine waters, a direct dependence of the gold content on the amount of divalent manganese ions and ferrous iron, and a limited migration capacity of gold. “Outside the ore-bearing areas, gold was not established in the objects studied.”

The results of field studies and literature data led to the idea of testing experimentally, under laboratory conditions, the influence of sulfides and carbonates on the migration of gold. For this purpose, a series of experiments was set up using the method for determining gold proposed by K. S. Pakhomova and T. A. Vysotskaya [8]. Quartz, pyrite, porphyrite with a content of 10% divalent iron, and limestone were taken. Weighed portions of these rocks, with a volume of 100 cm³ and particle diameter 0.1–0.25 mm, were loaded into tubes to which a slight inclination was given in the working position (Fig. 1). The quartz sample was first treated with aqua regia and then washed to the pH of the distillate. Pyrite of sedimentary origin was similarly treated with hydrochloric-acid solution to remove carbonates. The rocks were first analyzed for their gold content. The analysis showed only in one of three determinations in pyrite 0.0000029% gold. After preliminary treatment of the rocks, a standard solution of metallic gold in aqua regia was prepared, with a titer of 5.929 μg/l and pH = 4.

Fig. 1. Diagram of the experimental apparatus.

Five experiments were set up with the following rocks: quartz (first tube), quartz + 15% pyrite (second tube), quartz + 40% pyrite (third tube), porphyrite, and limestone. The prepared gold solution was passed through these rocks for 6 days, with samples of 250 ml taken after one, two, and three days and analyzed for gold content (Table 1).

In samples of the solution that, under the same conditions, had passed through porphyrite and limestone, gold was not detected by analysis.

Table 1

Then, at intervals, samples of the rocks were taken from the tubes (Fig. 1) and analyzed for gold (Table 2).

Table 2

As the analyses showed, the main mass of gold was extracted from the solution in the first moments of contact between the solution and the rocks. It is possible that the precipitation of gold from solution is due to the combined action of physical (adsorption) and chemical phenomena (precipitation of gold from solution according to the reaction:

\[ \mathrm{AuCl_3 + Fe_2(SO_4)_3 \to Au + Fe_2(SO_4)_3 + FeCl_3}). \]

The content of the sulfate ion, reaching 75 mg/l, indicates the possibility of such a reaction taking place. In the latter case, the gold should have been deposited on pyrite. To confirm this supposition, a sample was taken from interval B of the third tube and, under a binocular microscope, was separated into pyrite and quartz. The analysis showed that the gold content in the pyrite (0,5688%) was twice the gold content in the quartz (0,2857%).

To determine the form of gold migration under the experimental conditions during filtration of the solution through rocks with different contents of sulfi-

electrodialysis of daily water samples from tubes one and three was carried out. The dialysate showed that the anolyte contains two to three times more gold than the catholyte. From this it may be assumed that gold migration occurs in the form of a negative complex ion, possibly the ion \([AuCl_4]^{-1}\), which agrees with the conclusions of V. M. Kreiter, S. A. Brusilovsky, I. K. Bazhenov, and others [3, 5, 6] concerning the forms of gold migration in natural waters.

Next, to determine the stability of the adsorbed gold, bidistillate was passed through these same rocks. Samples of equal volume were likewise taken after one, two, and three days (Table 3).

Table 3

After the completion of this series, rock samples were again taken from each tube at intervals and analyzed for gold content (Table 4).

Table 4

From the data in Tables [1, 2, 3, 4], graphs were constructed illustrating the behavior of gold in aqueous solutions and rocks under the experimental conditions as a function of time (Fig. 2).

During the first period of filtration of the standard solution through the rocks, the gold content in the waters decreases, and then increases slightly, which is possibly connected with a limitation of the adsorption capacity of the rocks. When bidistillate is passed through, the opposite picture is observed, which may be explained by the irreversibility of the reaction of gold precipitation from solution. Only in the case of a large amount of sulfi-

there occurs, evidently, a slight reverse dissolution and removal of gold (Fig. 2, c). The behavior of gold precipitated from solution is also quite interesting. If, when the standard solution was passed through, the gold was deposited in the first moments of its contact with the rocks, then when the rocks were washed with bidistillate the main mass of gold shifted to the end of the tube (Fig. 2). It may be assumed that gold, separating from solution, forms colloidal particles which

Fig. 2. A — determination of gold in the filtrate when passing the standard solution and bidistillate;
B — determination of gold in the rocks through which the solutions were passed.

are transported, even by a weak current of water, in the direction of its movement and, as the filtration rate decreases (approximately less than 3 mm/hour), remain in place.

Conclusions

1. Field investigations showed the applicability of the hydrogeochemical method in prospecting for gold-ore deposits by means of gold pathfinder elements in natural waters, using the procedure of coprecipitation with TPI.

2. The laboratory-experimental work carried out confirmed the conclusions of the field investigations and made it possible partially to elucidate the possible form of migration compounds of gold in natural waters with different pH values.

3. The role of pyrite and of associated divalent iron in rocks in the migration of gold under conditions imitating the oxidation zone of sulfide deposits has been partially elucidated.

References

1. S. P. Albul, A. D. Miller. On the possibility of prospecting for gold-ore deposits by hydrogeochemical dispersion halos. Tr. VITR, 1959.

2. M. N. Al'bov. On the forms of migration of gold in the oxidation zone of ore deposits. Izv. AN SSSR, Geol. Ser. 4, 1952.

3. I. K. Bazhenov. Complex compounds and their role in geochemical processes. Materials on Geology and Mineral Exploration of Western Siberia, Tomsk, 1964.

4. N. I. Brodskaya, I. P. Vychuzhanina, Z. V. Komarov, M. S. Leshchinskaya. Concentration of trace amounts of elements from natural waters on a mixed sorbent with subsequent spectral determination. VITR, issue 55, 1962.

5. S. A. Brusilovskii. On the migration forms of elements in natural waters. Hydrogeochemical Materials, vol. 35, AN SSSR, 1963.

6. V. M. Kreiter. The behavior of gold in the oxidation zone. Gosgeoltekhizdat, 1958.

7. V. M. Matusevich, G. V. Shubin, L. L. Shabynin. Results of hydrogeochemical investigations within the fault zone of one of the regions of Eastern Transbaikalia. Materials on Geology and Mineral Exploration of Western Siberia, Tomsk, 1964.

8. K. S. Pakhomova, T. A. Vysotskaya. Determination of gold in comprehensive geochemical investigations. Tr. YaF SO AN SSSR, Geol. Ser., issue 16, 1963.

9. L. V. Razin, I. S. Rozhkov. On the geochemistry of gold in the weathering crust and biospheric soil of permafrost terrain on the Aldan Shield. Tr. YaF SO AN SSSR, Geol. Ser., issue 16, 1963.

10. P. A. Udodov, I. P. Olufrienok, Yu. S. Parilov. Experience in hydrogeochemical investigations in Siberia. Izd. “Vysshaya shkola,” Moscow, 1962.

11. P. A. Udodov, I. N. Onufrienok, P. V. Kristalev. Chemical activity of rocks with respect to hydrogen ions. Experience in developing hydrogeochemical methods for prospecting ore deposits. Gosgeoltekhizdat, 1959.