Issues in the Methodology of Exploration of Placer Gold Deposits

(Using Martaiga as an Example)

E. D. Shpaikher (ZSGU)

At the present time, considerable experience has been accumulated in the exploration and exploitation of the placers of the Mariinsk taiga; however, to this day this experience has not been generalized and is not being used in the exploration of new placers. Numerous cases are known of significant discrepancies between exploration data and the subsequent exploitation of gold-bearing placers. In this respect, the Mariinsk district is no exception.

In terms of the parameters and distribution of gold, most of the placers of Martaiga (those explored and those being worked by dredges) belong to the second group (IIa), namely: “medium-sized placers, whose balanced reserves of rock mass amount to 3–12 million m³” (A. P. Bozhinskii, 1960). Such objects can be worked by dredges with bucket capacities of 150–210 l. Larger placers have not been found in the district.

In terms of the degree of uniformity of the content of the useful component, they belong to groups 2 and 3, i.e., they are characterized by an uneven distribution of gold and are often affected by mining operations (Kundat, Shaltyr-Kozhukh, Kiya-Shaltyr, and others).

According to the instructions for applying the classification of reserves to placer gold deposits (1962), the following is required:

a) an exploration grid for placers of group 2:

category B — 150–200 × 10–20 m;

category C₁ — 300–400 × 20–40 m;

b) for deposits of group 3:

category C₁ — 100–200 × 10–20 m.

As for the grid used in prospecting exploration, it is adopted empirically, depending on the parameters of the valley, and ranges within 800–1200 × 20–40 m, approximately corresponding to the recommendations of A. P. Bozhinskii (1960).

Exploration of the Martaiga placers was carried out by drilling holes with 4.5″ Empire drill sets, followed by verification of the drilling data by control shafts. Placers of small objects for hyd—

...by scattered small-scale manual mining, and also individual sections of dredging blocks (Kundusuyul) were explored by prospecting shafts.

An analysis of the exploration work leads to the conclusion that there were significant shortcomings in the existing methodology for placer exploration. The placers were explored unsystematically; the sequence of work was not observed, i.e., exploration was carried out bypassing the prospecting stage; drilling was not accompanied by the necessary amounts of control shaft sinking, which led to low reliability of the data obtained. At the dredging block on the Kundusuyul River, 95 shafts were sunk to verify the drilling data, of which 82 were within the commercial contour; 54 shafts checked barren drill holes. Although even after excluding the barren holes and the paired shafts the percentage of verification (about 7%) is sufficient, the inclusion of barren workings led to a significant overstatement of the correction coefficient (Tables 1 and 2).

Some of the placers were explored with an excessive degree of detail. Thus, the placer of the Tulu-yul River was explored by 197 lines; the distance between the lines averaged 110 m, and in individual sections it was reduced to 50–80 m.

There were cases of obvious over-exploration of blocks for dredging operations; for example, the block in the lower part of the Kundat River was explored on a 200 × 10 m grid, i.e., it was assigned to the third group of deposits, although it had not previously been mined, is located on a river where dredging is being carried out, and is a continuation of the section explored upstream and should be assigned to the second group. It should be added that the assignment of placers to a particular group according to the persistence and distribution of metal is carried out arbitrarily, without determining the coefficient of variation of grades or of vertical reserves.

As noted above, exploration of most of the gold-bearing placers of Martaiga was carried out with Empire sets 4.5″ in diameter, with an internal diameter of the drill bit of about 100 mm. The methodology of placer exploration using holes 100 mm in diameter leads to a significant underestimation of the gold grade and, consequently, to their non-objective evaluation (Kallistov, 1957).

For the dredging blocks of Martaiga, the correction coefficient for grade proves to be 1.36, and for individual placers (Kiya-Shaltyr, Kundusuyul) it is even higher.

Table 1

Significant discrepancies in the data from drilling and control shaft sinking are also explained by the incorrect selection of workings to be checked. In Table 2 the author has determined correction coefficients for two variants:

1. All paired workings located within the reserve contour are included in the calculation.
2. The coefficient is derived only from shafts that checked boreholes containing gold.

Under variant II, which in our opinion is the most acceptable, the average correction coefficient is 1.03, i.e., considerably lower than that determined from Table 1.

Table 2

To date there are no clear recommendations for selecting workings (drill holes) subject to verification. In some cases it is proposed to place shafts uniformly over the area of the placer, checking boreholes with low, medium, and rich metal contents, although in itself the concept of “low, medium, etc. contents” is entirely abstract.

In Salair it is customary to classify workings by content classes of 1–50 mg/m³; 51–100 mg/m³, etc. However, the practice of mining some placers in this district does not confirm the reserve calculations performed using correction coefficients by classes.

It is more correct to drive verification lines of shafts, since they provide—albeit on a sparse grid—complete geological sections, while at the same time checking workings with different metal contents. Based on the data from driving such “reference lines,” studies are made of the granulometric composition of loose deposits, the sieve characteristics and morphology of the placer metal, and laboratory-technological investigations. Still greater discrepancies occur between exploration data and subsequent mining. In the course of exploration, the gold present in placers is recovered less completely than during mining (Yu. N. Trushkov, 1955). This situation is confirmed by the practice of applying washing coefficients, which, as a rule, are greater than unity and most often fluctuate within the range 1.4–1.8, in individual cases reaching even higher values.

An analysis of the factual material made it possible to identify regularities in the change of the washoff coefficient as a function of the class of gold grades. The washoff coefficient changes even within a single placer deposit at different grades. For example, on the Kundat River, at gold grades up to 100 mg/m³ this coefficient is 3.6, while at grades of 100–200 mg/m³ it varies within the range 1.73–2.29. On the whole, for the placers of the Mariinsk taiga, a regularity is observed whereby the washoff coefficient increases from higher grade classes to lower ones (Table 3).

Table 3

And finally, the discrepancies between exploration and exploitation data are also clearly illustrated by the example of the Kundat River placer, worked by a 150-liter dredge (Table 4).

Table 4

Substantial discrepancies between exploration and exploitation lead to a mismatch between the contours of calculated and worked reserves.

It is evident from Table 4 that the washoff coefficient for the Kundat River placer was obtained both for the rock mass and for the metal. The correction to the volume of the rock mass is explained by the fact that the area actually worked is significantly larger than that contoured in the reserve calculation according to exploration data. The narrowing of the contours for calculating balance reserves and the resulting need to introduce a correction to the volume of the rock mass occur because of the understatement of grade during sampling, by which

is also responsible for the decrease in the metal reserve within the calculation contour. As a result, all three corrections depend on errors in determining gold in the process of exploration and sampling.

The reasons for the low accuracy of exploration are:

a) insufficiency of the sample volume when exploration is carried out by small-diameter boreholes;

b) imperfection of drilling technology, leading to distortion of sample volumes and incomplete recovery of gold during drilling;

c) imperfection of the methods used for washing and recovering gold during sampling.

Assuming that the predominant size of the gold particles constituting the commercial value in the Martaiga placers lies in the class \(-0.84 + 0.59\) mm, we determine that one gold particle is contained in 14,000 m³; i.e., in a sample with an interval of 1 m (8000 cm³), the probability of encountering gold particles of size \(-0.84 + 0.59\) mm barely exceeds 50%. At the same time, it should be taken into account that even the presence in 8000 cm³ of one gold particle of such size does not represent commercial value. From the calculation given, the low reliability of exploration of placers by small-diameter boreholes is clearly evident. Furthermore, percussion drilling technology does not ensure observance of the necessary conditions for accurate sampling or completeness of rock recovery. Incomplete recovery of loose material and gold leads to distortion of the metal content in the placer toward understatement. Understatement of grades is also connected with the imperfection of methods for processing exploration samples. Additional recovery of gold (after blowing off and picking under a binocular microscope) amounts to 8.0–45.6% (V. V. Syrovatskii, E. D. Shpaikher, 1968). Large losses are observed in placers with a significant admixture of fine material, and with fine and thin gold (the Bolshaya Elan and Zolotoi Kitat placers).

Along with methodological issues, questions of work technology and technique are also of great importance for the reliability of placer exploration. Taking this circumstance into account, in recent years the Martaiga expedition has introduced new drilling and sampling installations, and the methodology of analytical investigations has been changed as well. Instead of manual Empire sets, UBR-1 rigs began to be used, the introduction of which revealed a number of advantages (E. D. Shpaikher, A. T. Doryatin, 1968). In washing bulk samples from pits at the placer of the Zolotoi Kitat River, the POU-4M installation was used; the gold content under mechanized processing of samples proved to be, on average, 21% higher than when washing on a butara. For extracting gold from concentrates, laboratory amalgamation by the Sherwood method is used; this method, approximating the existing technology of operation of mining units, makes it possible to obtain more accurate data on the metal content in the placer.

Summarizing the above, the following conclusions may be drawn:

1. It is necessary to observe the staged character (sequence) of conducting prospecting and exploration work for placer gold.

2. Assignment of placers to a group under the classification should be carried out on a substantiated basis, with determination of the coefficient of variation of grades or of vertical reserves, which will make it possible to choose the exploration grid more accurately and to conduct exploration without unnecessary expenditures.

3. Introduction of progressive equipment and technology for exploration of placers, namely:

a) UBR-1 rigs, as well as large-diameter rigs;

b) field concentrating installations for washing samples from pits and boreholes;

c) the use of amalgamation in extracting gold from concentrates, especially in placers with a predominance of fine metal size classes;

1. The materials on washing coefficients for the Martaiga placers confirm the view that exploration data are of low reliability, which depends on a number of causes, including the grain size of the gold. Therefore, in prospecting and exploration work this factor must be taken into account, and placers with gold contents close to industrial grades should not be given a negative assessment.

REFERENCES

1. Bozhinskii A. P. Methods of exploration and reserve estimation for placer mineral deposits. “Nedra,” 1965.

2. Kallistov P. L. Improvement of methods for exploration and sampling of gold-bearing placers. Collection of materials on the geology of nonferrous, rare, and precious metals, issue 1, 1957.

3. Syrovatskii V. V., Shpaikher E. D. Losses of gold in concentrates and their influence on the evaluation of placers. Kolyma, No. 8, 1968.

4. Trushkov Yu. N. Accuracy of exploration data and reserve estimation in placers. ONTI Dalstroy, Magadan, 1955.

5. Shpaikher E. D., Dvorin A. T. Mechanization of exploration of placer gold deposits. Kemerovo Book Publishing House, 1968.