Methodology for assessing forecast reserves, scale, and degree of ore-bearing potential of prospective areas
Abstract Generated abstract
The paper proposes a quantitative methodology for evaluating the gold ore-bearing potential of prospective areas during prognostic-metallogenic mapping. It defines an ore-bearing coefficient as the total gold in lode and placer deposits per unit ore-bearing area, compares node values with district averages to classify prospectivity, and applies the approach to regions including the Kuznetsk Alatau, Gornaya Shoria, and Salair. For estimating forecast gold reserves, the paper argues that coefficients must be corrected because much fine and very fine gold from primary sources is not retained in placers, using granulometric and extraction data to suggest increasing coefficients by roughly two to four times. The method is presented as a basis for ranking nodes by prospectivity and planning gold prospecting, exploration, and geological survey work.
Full Text
Methodology for Assessing Forecast Reserves, Scales, and Degree of Ore-Bearing Potential of Prospective Areas
V. V. Syrovatsky (ZSGU)
In the course of prognostic-metallogenic studies, essentially no attention has been paid to the question of the quantitative assessment of the ore-bearing potential of prospective areas. At the same time, practical work has shown that it is by no means always possible, on the basis of the metallogenic factors that control the distribution of gold mineralization, to identify—using the available geological data—in a sufficiently objective and reliable way, within the territory studied, areas, and within them specific sectors, according to the degree of their prospectivity with respect to ore gold.
The proposed methodology for the quantitative assessment of the ore-bearing potential of areas should be regarded as one of the possible variants of recommendations that may be applied in the process of compiling small- and large-scale prognostic-metallogenic maps.
Quantitative assessment of the ore-bearing potential of areas
By the present time, an enormous body of factual material has been accumulated on the metallogeny of lode and placer gold in Gornaya Shoriya, the Kuznetsk Alatau, and the Salair, which makes it possible, when assessing the intensity of gold mineralization and determining its scales, to pass from descriptive definitions (rich, abundant, intense, weak, dispersed, ore mineralization, etc.) to more precise quantitative estimates. We first applied a quantitative method for assessing gold-bearing potential when compiling a map of placer gold-bearing potential of the Salair Ridge (Syrovatsky, 1963). The standards for this assessment were the linear coefficient of gold-bearing potential (for placers) and the areal coefficient of ore-bearing potential (for primary mineralization).
The coefficient of ore-bearing potential (K) of nodes, ore zones, districts, and regions is understood as the total amount of gold in ore and placer deposits (production + metal reserves, including off-balance reserves) per unit of ore-bearing area. The coefficient is expressed in kilograms of metal per one square kilometer of area (kg/km²). In 1965, S. D. Sher, characterizing
the gold-bearing characteristics of large provinces of the world, introduces the concept of the density of gold production. By the coefficient of production density he understands the quantity of gold produced per unit area. It is not difficult to see that the ore-bearing coefficient proposed by us differs in principle from the concept of production density. It more fully reflects the degree of specific ore-bearing potential of prospective areas and, for this reason, corresponds to a greater extent to the aims and tasks of metallogenic investigations for gold.
The initial data for calculating ore-bearing coefficients are:
a) information on the production and reserves of gold in lode and placer deposits for individual ore nodes; b) the areas of ore-bearing tracts within gold-ore nodes.
By ore-bearing areas are meant the local areas of ore nodes that supplied ore material for the formation of gold-bearing placers. Ore-bearing areas are, as a rule, smaller than the areas of the gold-ore nodes proper (in the sense of E. T. Shatalov, 1963), since the latter were delineated not only according to the spatial position of presently known occurrences and deposits of lode and placer gold, but also according to geological-structural criteria. Adjacent areas where ore occurrences and gold deposits have not yet been identified, but where they are quite possible on the basis of the geological-structural setting, were also included in the nodes.
The quantity of metal reserves and production assigned to specific ore-bearing areas (nodes) is the basis for calculating the absolute value of the ore-bearing coefficients, which are then plotted on forecast-metallogenic maps.
In the practice of forecast-metallogenic investigations, the comparison of the ore-bearing potential of one node with another acquires the greatest importance. For this purpose the concept of an average district ore-bearing coefficient is introduced. It is derived arithmetically from the values of the ore-bearing coefficients of the ore nodes included in a given ore district.
The ore-bearing coefficients of nodes were compared with respect to the derived district coefficients, as shown by the example of the Kuznetsk Alatau (Fig. 1). Comparative analysis made it possible to divide the gold-ore nodes, according to the degree of ore-bearing potential and, consequently, prospectivity, into three categories:
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Nodes whose ore-bearing coefficients significantly exceed the average district ore-bearing coefficient are characterized by a high degree of ore-bearing potential;
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Nodes whose coefficients are close in value to the district coefficient are characterized by an average degree of ore-bearing potential;
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Nodes having a coefficient value below the average district value are characterized by a low degree of ore-bearing potential.
Thanks to the use in metallogenic investigations of a quantitative method for evaluating areas in combination with the identified regularities in the distribution of gold mineralization, an extremely high contrast is achieved in the subdivision of nodes—these basic units of metallogenic zoning—according to the degree of prospectivity and possible industrial value.
Analysis of the spatial position of ore nodes on the forecast-metallogenic map compiled for the Kuznetsk Alatau, Gornaya Shoria, and Salair (V. Syrovatskii, 1963, 1967) showed that nodes having an average and, especially, a high ore-bearing coefficient are distinguished by the most favorable geological-structural setting.
Fig. 1. Ore-bearing coefficients of the gold-ore nodes of the Kuznetsk Alatau
1 — graph of the ore-bearing coefficients of the nodes;
2 — district ore-bearing coefficient;
3 — nodes with a high ore-bearing coefficient;
4 — nodes with an average ore-bearing coefficient.
for prospecting for industrial gold deposits. It is necessary, in this connection, to bear in mind that the method of quantitative assessment of the degree of ore-bearing potential of areas is applicable only to districts of mixed, ore and placer gold-bearing potential. The Kuznetsk Alatau, Gornaya Shoriya, the Salair Ridge, and Northeastern Altai, in terms of the features and character of the metallogeny of ore and placer gold, largely meet this condition. Here a close spatial and genetic relationship is established between placers and primary sources, owing to which nodes of placer gold-bearing potential act as reliable prospecting indicators for ore gold.
The available materials on the metallogeny of gold in the territory under consideration give grounds to assume the presence of deposits with very fine, finely dispersed gold, which are expected within numerous and poorly studied fault zones, accompanied by intense crushing of rocks mineralized with sulfides. It is quite obvious that such deposits, in the process of destruction, form weak mechanical halos of gold dispersion or do not produce them at all. If one proceeds from these positions, then the quantitative assessment of the ore-bearing potential of areas of development of rocks mineralized...
with sulfides, by the method of using ore-bearing coefficients, becomes unacceptable. In this case, when assessing the prospects for the ore-bearing potential of areas, it is necessary to proceed from a favorable combination of geological-metallogenic signs of mineralization. If these signs are present, such areas, in terms of their degree of prospectivity, must be equated with nodes that have high and medium ore-bearing coefficients.
Methodology for Calculating Forecast Gold Reserves
Ore-bearing coefficients were also used to estimate the forecast reserves of gold in areas prospective for ore gold. The initial data for this were: a) corrected ore-bearing coefficients of the nodes; b) the area occupied by prospective sectors; c) the depth to which the calculation of reserves extends.
The areas of prospective sectors that were included in the calculation of forecast reserves are delineated according to the same principle as in the calculation of ore-bearing coefficients. The depth to which the calculation of forecast gold reserves extended was taken as approximately equal to the actually attained depth of gold-ore deposits currently being exploited. For each gold-ore district it was taken differently, but in all cases it did not exceed 400–1000 m. In other words, the depth of extension of forecast reserves was limited by the economic contour of the ore bodies, which determines the profitability of mining the deposits.
The values of the ore-bearing coefficients calculated for the purpose of assessing the degree of ore-bearing potential of prospective areas cannot be used in calculating forecast gold reserves. To calculate reserves it is necessary to introduce a correction to the ore-bearing coefficients for the amount of gold that is carried out from primary sources but does not accumulate in placers. Otherwise, we would far from fully reflect the overall scale of ore-bearing potential of prospective areas. Indeed, studies by I. N. Plaksin (1958) showed that in gold-ore deposits the main mass of gold (about 75%) belongs to the fine, very fine, and submicroscopic size class (100–0.1 microns), which, as is known, is not concentrated in placers. The amount of fine and small gold in ores is usually very large, and in many primary deposits there is almost no coarse “visible” gold. The deposits of Western Siberia are no exception to this rule, as is evident from Table 1 cited.
Determination of the numerical value of the correction to the ore-bearing coefficient for the amount of gold that is carried out from primary sources but does not accumulate in placers was carried out on the basis of a comparison of the granulometric composition of gold from ore (Table 1) and placer (Table 2) gold deposits, and also according to the results of extracting gold from ores by amalgamation (Table 3).
A comparative analysis of Tables 1 and 2 showed that in primary deposits fine and very fine gold predominates, with a grain size of less than 0.147 mm. On average it amounts to about 56%.
In placers, on the contrary, gold coarser than 0.147 mm sharply predominates (92–96.7%). It is very noteworthy that the concentration of gold coarser than 0.589 mm in placers exceeds its concentration in primary deposits by approximately 12–14 times. A different ratio is observed in the behavior of gold of grain size +0.104 and 0.208 mm,
Placer gold is somewhat smaller than that in primary deposits. The established ratios of gold sizes are a possible indicator of the process of abrasion and comminution of gold in placers, coming from primary sources. Is not the absence in them of gold of sizes 0.295 and 0.417 mm also an indication of the amount of comminuted gold in placers, whereas its content in primary deposits reaches 30%? It is quite possible that these size classes of gold are characterized by an increased capacity—
Table 1
Sieve analysis of ores with fixation of gold by size classes.
| Deposits | +0.589 | +0.417 | +0.295 | +0.208 | +0.147 | +0.104 | +0.074 | −0.074 |
|---|---|---|---|---|---|---|---|---|
| Komsomolskoye (quartz-sulfide ores) | 17.0 | 15.47 | 15.38 | 9.46 | 8.15 | 6.65 | 17.9 | 4.56 |
| Gavrilovskoye (quartz-sulfide ores) | 2.49 | 6.05 | 5.95 | 5.33 | 4.77 | 9.1 | 46.8 | |
| Natalyevskoye (gold-skarn ores) | 19.1 | 8.0 | 12.5 | 8.9 | 6.5 | 10.6 | 40.0 | |
| Average | 5.7 | 18.8 | 9.8 | 9.3 | 7.5 | 6.0 | 12.5 | 30.4 |
| Urals (sulfide-quartz) | (Zelenov, 1961) | (Zelenov, 1961) | (Zelenov, 1961) | (Zelenov, 1961) | (Zelenov, 1961) | 24.4 | 75.6 | |
| Southern Urals (Kolychevo data) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | Gold sizes range from 0.004–0.3 mm; 0.02–0.08 predominate (Bergelson, 1961) | |
| Leninogorsk group (complex gold-polymetallic) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | Gold sizes range from 0.1 to 0.08 mm; 0.05–0.08 predominate (Plaksin, 1943) | |
| Maikain, Darasunskoye, Artemyevskoye (Kolychevo data, sulfide) | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). | Gold sizes range from 0.004 to 0.3 mm; 0.02–0.08 mm predominate (Bergelson, 1967). |
Table 2
Sieve analysis of gold from alluvial placers.
| Areas of placer gold-bearing potential | +5 mm | −5 +3 | −3 +1 | −1.168 +0.589 | −0.417 +0.295 | −0.295 +0.208 | −0.208 +0.147 | −0.147 +0.104 | −0.104 | Number of analyzed placers |
|---|---|---|---|---|---|---|---|---|---|---|
| Gornaya Shoria | 1.5 | 16.9 | 40.1 | 15.4 | 24.5 | 0.2 | 0.06 | 2.1 | 0.15 | 14 |
| Kuznetsk Alatau | 0.8 | 32.38 | 58.4 | 5.6 | 2.0 | 0.3 | 0.03 | 28 | ||
| Salair Ridge | +2 mm — 2+0.5 / 6.3 | +2 mm — 2+0.5 / 35.0 | 38.2 | 10.2 | 5.2 | 4.5 | 0.6 | 6 |
—with respect to re-comminution, the transformation of it into finer size classes. The revealed ratios of the sizes of placer and primary gold indicate that along the migration paths the primary gold is abraded and re-comminuted and, evidently, with an inevitable tendency for its coarse classes to pass into fine ones.
The material presented shows with complete persuasiveness that, for the territory under consideration, more than half of the gold from primary sources, because of its fine and very fine particle size, does not accumulate in placers. It is carried by water flows far beyond the limits of the source areas and therefore cannot be taken into account when calculating ore-bearing coefficients. The formation of placers should thus be regarded as the result of concentration of the coarsest gold from ore deposits, as a kind of natural concentrating laboratory. It has already been noted earlier that some part of the gold coming from primary sources is abraded and further comminuted during transport to particle sizes that preclude the possibility of its accumulation in placers. For this reason, the total amount of gold concentrated in placers is still smaller.
The sources of the placers known at present are primary deposits whose ores contain sufficient reserves of “placer” gold, i.e., gold coarser than 0.1 mm. Such reserves in the primary deposits being mined can be established from the amount of gold recovered by amalgamation. These data, accumulated over many years of work by gold-extraction plants, are given in Table 3.
Table 3
| Method of extracting gold from ores | Gold-ore deposits: quartz-sulfide — Komsomolskoye | Gold-ore deposits: quartz-sulfide — Centralnoye | Gold-ore deposits: quartz-sulfide — Berikulskoye | Gold-ore deposits: quartz-sulfide — Uralskiye | Gold-ore deposits: gold-skarn — Natalyevskoye | Gold-ore deposits: gold-skarn — Lebedinskoye | Gold-ore deposits: gold-skarn — Sinukhinskoye | Salair group (gold-barite-quartz) | Uralskiye (pyritic) | Darsun, Leninogorsky, Maikain (gold-polymetallic) |
|---|---|---|---|---|---|---|---|---|---|---|
| Amalgamation | 29,7 | 25,32 | 31,08 | 24,4 | 47,6 | 59,0 | 50,0 | 17,5 | 39,8 | 17,25 |
| Cyanidation, flotation | 67,98 | 74,55 | 61,8 | 48,3 | 35,0 | 47,9 |
The average percentage of gold recovered by amalgamation from ores of various morphogenetic types of mineralization is on the order of 30–40%. Of particular note is the fairly close order of magnitude of the percentage of gold recovered by amalgamation and the amount of coarse gold (0.208 mm) in primary deposits—precisely the grain size of gold that is preferentially concentrated in placers. Analysis of gold recovery by amalgamation indicates that about 60% of the gold reserves of primary deposits do not accumulate in placers.
An exceptionally vivid indication of the amount of ore gold that goes into the formation of placers is provided by the ratio of gold production from several of the best-studied ore deposits and the alluvial placers associated with them: Centralnoye — 4 : 1, Berikulskoye — 5 : 1, Natalyevskoye — 10 : 1, Udarninskoye — 3 : 1, Urskoye — 9 : 1, and Salairskoye — 17 : 1. Thus, the reserves of ore deposits that have undergone erosion exceed the reserves in gold-bearing placers by a minimum of 3 times and a maximum of 17 times.
The material presented makes it possible to determine the value of the correction to
the ore-bearing coefficients of the nodes. The ore-bearing coefficients used in calculating forecast reserves of gold must be increased by a minimum of 2 and a maximum of 4 times. Only under this condition is the greatest possible reliability achieved in estimating the forecast gold reserves of prospective areas and their overall scales of ore-bearing potential.
Conclusions
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The introduction into the practice of forecast-metallogenic studies of node ore-bearing coefficients, in combination with the identified regularities in the distribution of mineralization, makes it possible to approach more objectively the assessment of the degree of ore-bearing potential of prospective areas. An exceptionally high contrast is achieved in subdividing areas according to the degree of their prospectivity with respect to lode gold.
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The corrected ore-bearing coefficients of nodes for the amount of gold that is removed from primary sources and not concentrated in placers are used in calculating forecast gold reserves, which forms the basis for subdividing nodes according to the scale of ore-bearing potential and for determining the order of their development by prospecting and exploration work.
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Forecast-metallogenic maps containing data on the geological-quantitative assessment of the degree of ore-bearing potential of areas and forecast gold reserves acquire a complete form. They constitute a reliable basis for prospective planning of prospecting, exploration, and geological surveying work for gold.
LITERATURE
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Plaksin I. N., Yukhtanov D. M. Hydrometallurgy. Metallurgizdat, Moscow, 1949.
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Plaksin I. N. Metallurgy of Precious Metals. Moscow, 1958.
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Syrovatskii V. V., Shaikher E. D. Gold losses in placers and their influence on the assessment of placers. Production-Technical Bulletin “Kolyma”, No. 8, 1968.
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Shatalov E. T. Terms relating to the general definition of metallogeny, metallogenic zoning, and metallogenic maps. Publishing House of the Academy of Sciences of the USSR, 1963.
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Sher S. D. On the ratio of the scales of primary and placer gold-bearing potential in various gold-bearing provinces of the globe. Soviet Geology, 3, Nedra Publishing House, 1965.