ISSN 1062-7391, Journal of Mining Science, 2013, Vol. 49, No. 2, pp. 273–278. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.A. Ermakov, A.M. Burakov, 2013, published in Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, 2013, No. 2, pp. 123–131.
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Complex-Structure Gold Placer Mining in Yakutia S. A. Ermakov and A. M. Burakov N.V. Chersky Institute of Mining of the North, Siberian Branch, Russian Academy of Sciences, pr. Lenina 43, Yakutsk, 677980 Russia e-mail:
[email protected] Received May 21, 2012
Abstract—The gold placer in the valley of the River Bolshoi Kuranakh occurs in complicated geological and mining conditions and holds gold of highly nonuniform quality. Modeling shows the reserves occur in clusters distributed all over the productive sand extension. High clay content and liberal share of fine gold impede processing and reduce the output. The article offers a combined processing technology for the given placer, based on pre-concentrating of commercial mineral, and specifies further research. Key words: Placer, nonuniformity, clay content, quality of gold reserves, processing, mining situation modeling.
Although it has long ago been necessitated to develop and extend underground gold mining, half of gold production in Yakutia and 30% of gold production in Russia is contributed to by placers [1]. Placers are mainly situated on the east and south of Yakutia. For better than 400 placers analyzed in Yakutia, geological and mining as well as technical and economical conditions are nonuniform: productive sand thickness and gold content can vary greatly (3–5 times), occurrence boundaries and depths are complicated, gold is fine, loss in extraction grows, quantity of processed rock mass per unit of extracted gold increases. These factors place exclusive standards on surveying and extraction technology and on gold pre-processing. Nearly 60% out of the studied placers occur at depth of 10 m, 40%—at 10–20 m, the deepest placers occur at 22 to 60 m below surface. Extension of 75% of the placers is 5 km, 25%—5 km and longer. The most extended places are from 15 to 30 km. Sands and overburden rocks make up hundreds of thousands to million and tens of millions of cubic meters in volume. One of the largest fine and very fine gold placers is the placer in the valley of the River Bolshoi Kuranakh in the Central Aldan gold-bearing area. This placer is one of unique placers as far as dimension, overburden quantity and gold content are concerned. The placer extends to 22 km, overburden makes up 300 Mm3, including 75–80 Mm3 of dumps formed by the upper productive layer extraction. Geology features large occurrence depth (50–60 m below ground water level), nonuniform rock mass with coarsely clastic rock inclusion, high content of tough clay (to 60%). The placer gold content is medium (242 mg/m3), prevailing gold content of geological samples is 0.5 g/m3. At different times, gold was extracted by dredging, continuous flow process technology with rotorand-conveyor and rotor-and-bucket machinery, shovel and truck technology and bulldozer method. Geological parameters of the productive contour are highly variable, as well as percentage of gold sizes grade: content of 250–100 mm is 5–6%, 100–16 mm is 15–27%, 16–4 mm is 10–20%; clayey different-grain gold is 18–23%, clayey material is 40–60%.
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Description of the placer in the River Bolshoi Kuranakh valley Index Prospective area Extension, km Thickness, m: minimum maximum Width, m: minimum maximum Gradient, ‰ Gold content, g/m3 Gold reserves, t (as of 2013)
Total
By extraction sites 2 3 84–140 140–196 5.8 5.1
30–248 21.7
1 30–84 4.2
4 196–248 6.6
6.1 55
8.1 29.1
20 32
14.3 47.8
6.1 55
130 1600 0.004 0.28 62.81
130 400 0.002 0.23 1.43
350 950 0.0035 0.26 7.65
350 950 0.0034 0.21 15.26
650 1600 0.002–0.006 0.33 38.47
The placer thickness varies from 30–40 to 60–80 m along the strike. In larger part of the valley, 20–30 m thick alluvial underlies the upper quaternary alluvial 8–10 m thick, previously containing gold and now being mining dumps. It is seen from the table that maximum/minimum thickness ratio changes 2.5 to 9 times, maximum/minimum width changes 2.5 to 3 times. Profitable cut-off occurs in 95 geological blocks, including 64 primary blocks and 31 incidental blocks. Mostly the blocks cover area 10–20 hectares, the largest area is above 50 hectares. Overburden thickness changes from 5 to 15 m, gold-bearing sand thickness varies from 46–52 to 10–15 m. Stripping ratio is rarely above 1. Based on many-years experience gained in extracting the placer gold by various technologies, except for the fine and very fine gold extraction, exhibited issues associated with features of the placer structure and its quantitative and qualitative nonuniformity. Irrespective the mining equipment, average gold content persistently changes. For instance, in mining season 2011, actual gold content was under 115 mg/m3, far under 100 mg/m3 in dredging, and 450–500 mg/m3 in shovel-and-truck extraction. That was observed in the previous mining seasons, too. The research carried out by the Institute of Mining of the North showed that the placer gold occurs in clusters [2]. The clustered structure, when clusters of standard quality gold occupy smaller portion of the prospected bocks but contain major share of gold, is typical of the absolute majority of different placers and underground deposits. The Bolshoi Kuranakh buried gold placer contains different-age clusters traced for hundreds meters in the placer profiles at different levels. The clusters are determined based on average gold content calculations by the exploration profiles. Gold is concentrated in the modern alluvium, which is completely worked by dredges, and occurs as an upper gravel layer and an underlying fine dump waste product layer. Gold content is commercial down the depth of the placer to the bed rock and lengthwise the placer strike, to 20 m long. Gold distribution in old alluvium is of the same character as in its new layer. High or low content of gold does not depend on clay content, lithology or occurrence depth. Up to the present day, current mine planning is usually based on constant standards set up by detail exploration and then refined using operational exploration and mining data. The Standard Procedure on using extracted hard mineral standards [3] says that persistent standards of reserves evaluation are usable in current mine planning. But current planning requires estimating reserves by sites of a deposit rather than the total deposit reserves. In this case, evaluation of gold reserves based on persistent standards may result in inefficient utilization of known reserves, erroneous estimate of average gold content, etc. So, it is required to have a unified procedure of evaluating reserves of separate extraction sites with mining and processing conditions different from the conditions set by the persistent standards, or where reserves are beyond the persistent standards. JOURNAL OF MINING SCIENCE Vol. 49 No. 2 2013
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Fig. 1. Gold distribution in geological profile.
Estimation of structure of a placer, commercial and waste mineral distribution evaluation, multivariant calculations and mining sequence optimization require automated calculation and numerical modeling methods. To date, these theoretical and practical geological, surveying and mining tasks are solved using information technologies which accelerate data processing, three-dimensional modeling, etc. Using digital geological-surveying databases, geological-mining information system MainFrame (Mining Institute of the Kola Scientific Center RAS), BTSOD program (Institute of Mining of the North) and standard statistics processing packages, the Kuranakh gold placer mining was modeled. The modeling showed high qualitative–quantitative variability of the main parameters of productive bed (thickness, gold content, clustered gold areas and their sizes) by geological profile and by different sites of the placer (see Fig. 1). More than a half of gold reserves (63%) occurs in less than 20% of the total volume of the placer. We accepted a “gold/alluvium” coefficient to assess the reserves nonuniformity: this is a ratio of gold percentage with content higher than 0.3 g/m3 to the respective quantity of alluvium. By extraction sites (see the table) this coefficient is: extraction site 4—2.5 (Fig. 2a), extraction site 3—2.9; extraction site 2—3.3; extraction site 1—5.3 (Fig. 2b). It is seen that the nonuniformity of the reserves grows down the strike of the placer due to change in the gold content and occurrence parameters. The highest gold content, more than 0.3 g/m3 is on extraction site 4 (69%) and extraction site 2 (66%). Besides, the nonuniformity coefficient varies within an extraction site: from 1.8 to 2.2 on extraction site 4; 3.2 to 5.9 on extraction site 3; 2.2 to 2.7 on extraction site 2 and from 3.7 to 7.2 on extraction site 1.
Fig. 2. Structural nonuniformity of gold reserves: (a) extraction site 4; (b) extraction site 1. JOURNAL OF MINING SCIENCE Vol. 49 No. 2 2013
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By geological data the highest gold content and overburden quantity is on extraction site 4: out of 29 blocks, 14 blocks contain 1000 kg of gold and 5 blocks contain 500 kg of gold. Out of 29 blocks of extraction site 3, 6 blocks contain 1000 kg of gold and 4 blocks contain 500 kg of gold. On extraction site 2, out of 17 blocks, 2 blocks contain more than 1000 kg of gold and 5 blocks contain more than 500 kg of gold. Extraction site 1 has the lowest gold content: out of 20 blocks a few blocks contain 100 kg of gold. The gold/alluvium coefficient changes depending on the quality of gold reserves of an extraction site, but the clustered behavior of gold is consistent. Gold content sharply varies in a wide range (0.07 to 2 g/m3 and the clustered gold areas greatly differ in size. Besides, gold distribution nonuniformity is traced using geological exploration holes. Based on the analysis of the local gold distribution, an indication of placer gold reserves by sites has been expressed as the known Lorentz curve (Fig. 3), where horizontal axis shows share of gold sand with growing gold content, and vertical axis shows gold share by decades with incremental output, which allows setting a limit gold content and assessing share of offgrade gold sand in their total quantity. The diagonal in Fig. 3 shows uniform distribution: the higher the nonuniformity of the gold distribution, the lower goes the Lorentz curve. The statistic frequency curve of the gold content, р(С), is expressed in terms of derivative of original spatial distribution set by the never-increasing function C = f 0 ( x) (Fig. 4). Analysis of the actual original gold content distribution yields an estimate of efficiency of selective gold extraction and allows selecting optimum mine-engineering decisions, mining sequence and advance and combinations of mining equipment. The modeling of the Kuranakh gold placer conditions and the analysis of the quality characteristics of gold content in different calculation domains showed correlation between the average and limit gold contents. The correlation describes the change in the average content when the calculation domain is added with additional volumes of gold sands with lower or higher (limit) content (Fig. 5).
Fig. 3. Lorentz curve for an extraction site.
Fig. 4. Original spatial distribution of gold contents.
Fig. 5. Average versus limit gold content by exploration profiles.
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For example, the minimum commercial content of chemically pure gold extracted by gravity in an extraction block is 232 mg/m3. Using this value and the plots in Fig. 3, the calculated limit values of gold in the area adjacent to line 112 is 58 mg/m3; to line 140—63 mg/m3; to line 172—87 mg/m3; and to line 217—14 mg/m3. Having this done, it is possible to differentiate gold quality standards in a complex-structure placer. It is evident that mining method and mining and processing machinery are selected based on mineral occurrence quality, morphology and grain-size classification. The processing characteristics and grain-size classification of the Kuranakh gold were analyzed by the IRGIREDMED and TSNIGRI. The analyzed gold contains much silt-and-loam. The content of sizes smaller than 0.074 mm is 40% on the average, and the content of sizes smaller than 10 µm is 16–50%. Samples with clay content from 16 to 67% prevail, the clay content of rocks increases downstream. The upper layer of high-clay alluvium is foxy clays with inclusions of pebble stones (to 5%), gradually changing to lurid yellow and yellow clay with low content of coarse material (under 1%). On the whole, material smaller than 3 mm is nearly 7% in the clayey layer, and clay fraction (under 0.003 mm) is 30 to 40%. The change in the clay content shows no regular behavior, and as a consequence, in 2008–2011, dredging in high-clayey sites of the placer resulted in drop in production and in higher gold loss. The analysis of the Kuranakh placer gold composition and properties showed that free gold is very fine. Nearly 60% free gold is sizes under 250 µm, including 24% gold of finer than 20 µm. The prevailing sizes are – 0.25 + 0.1 and – 0.1 mm. No particles larger than 1 mm were observed. The gold rate is 890. The gold processing is recommended using gravity (bull jigs, concentration tables, centrifugal apparatuses) and magnetic concentrations, with treating separately the grain and clay fractions of the gold sands. The laboratory extraction of gold was 95–96%. The estimated extraction of free gold (by amalgamation) was 79.8% in dredge conditions and 82.9% on gold washing machine. With increase in mass fraction of clay and fine gold, gold extraction decreases [5]. With high content of fine and very fine gold and high content of silt-and-loam, the mining and processing equipment capacity drops while commercial component loss grows. The Institute of Mining of the North offers a new approach to the Kuranakh gold placer treatment based on preconcentration of the commercial component [6, 7]: —quality separation by setting the upper and inner boundaries of the production area with different commercial component contents; —selective excavation with varying (if necessary) an extraction block’s boundaries and differentiating gold quality standards; —prime hydraulic screening on extraction site; —gold-bearing fraction separation on concentration machines; —control of sluicing and settling in an intermediate container or a tailings settler to create layers with higher commercial component content. The developed combined processing method for gold sands [8] includes prime quality separation, excavation using equipment allowing choosing optimum gold size to be fed to washing, for instance by adjusting cutting parameters in blast-free shovel excavation of gold sands, and hydraulic screening on site. The slurry with high gold content is fed directly to concentration; the low gold content slurry goes to intermediate container for density classification. Some elements of this method have already been commercially approved in Yakutia gold mines. In order to improve the surface mining technology at the complex-structure clustered Kuranakh gold placer, it is required to select the optimum combined extraction scenario toward complete and selective recovery of commercial component in the following stages: —accumulation of electronic test databases for geostatistics analysis, calculation of quality characteristics, modeling orebody (bed) contours, solution of technological and organizational problems of placer mining; JOURNAL OF MINING SCIENCE Vol. 49 No. 2 2013
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—analysis of gold reserves; —calculation of quantity and quality parameters of gold distribution in a placer; — determination of optimum parameters and characteristics of an extraction site; —differentiated analysis of commercial component distribution consistently with expansion of boundaries of an extraction site. CONCLUSIONS
1. Gold placers in Yakutia feature nonuniform geological, mining and economical conditions, which imposes higher standards on surveying, preparation and processing of gold sands. 2. The largest gold placer in the valley of the River Bolshoi Kuranakh is represented by fine and very fine gold, and the local geological and mining conditions greatly vary across the production area. The placer gold occurs in clusters that occupy small area of extraction blocks but contain major part of gold reserves. 3. The placer mining modeling based on the digital geological and surveying database showed gold distribution nonuniformity by extraction sites and geological profiles. The authors propose the gold/alluvium coefficient to estimate gold concentration and the placer gold concentration indicator expressed as the Lorentz curve. 4. Once the Kuranakh gold placer gold is mainly fine and very fine gold that is nonuniformly distributed across the placer, the authors offer a new approach to the gold processing using preconcentration of the commercial component and a new combined method of gold processing, some operations of which have already been approved in gold mines in Yakutia. REFERENCES 1. Braiko, V.N. and Ivanov, V.N., Gold Mining Industry Results in 2009, Zolotodobycha, 2010, no. 136. 2. Burakov, A.M., Ermakov, S.A., and Blinov, A.A., Nourishment Source of the Kuranakh Buried Placer and Its Influence on the Choice of Gold Extraction Technologies, Problemy osvoeniya i perspektivy razvitiya Yuzhno-Yakutskogo regiona: cb. nauch. tr. (Development Problems and Prospects of the South Yakutia: Collection of Scientific Papers), Neryungri: IGDS SO RAN, 2001. 3. Tipovye metodicheskie polozheniya po primeneniyu konditsii na tverdye poleznye iskopaemye v protsesse razrabotki mestorozhdenii (Exemplary Provisions on Hard Mineral Quality Requirements in the Course of Mining), Moscow: IPKON AN SSSR, 1981. 4. Batugin, S.A. and Cherny, E.D., Teoreticheskie osnovy oprobyvaniya i otsenki zapasov mestorozhdenii (Theoretical Bases of Mineral Sampling and Reserves Estimation), Novosibirsk: Nauka, 1998. 5. Zamyatin, O.V and Man’kov, V.M., Current Placer Gold Processing Technologies, Gorny Zh., 2001, no. 5. 6. Ermakov, S.A., Burakov, A.M., Zaudal’sky, I.I., and Panishev, S.V., Sovershenstvovanie geotekhnologii otkrytoi razrabotki mestorozhdenii Severa (Improvement of Surface Mining Geotechnologies in the North), Yakutsk: SO RAN, 2004. 7. Ermakov, S.A. and Burakov, A.M., Application of Gravity Separation in Multi-Stage Processing of Placer Gold, Proc. 5th Sci. Conf. Current Material Resource Utilization Technologies, Krasnoyarsk: SFU, 2007. 8. Ermakov, S.A., Burakov, A.M., Panishev, S.V., Kasanov, I.S., and Ivanov, I.V., RF patent no. 2449126, Byull Izobret., 2012, no. 12.
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