On the Possibility of Applying Neutron Activation Analysis for Gold to Solve Practical Problems: The Case of the Eldorado Gold Deposit (Yenisei Ridge)
Abstract Generated abstract
This paper evaluates the use of neutron activation analysis for determining gold distribution at the Eldorado gold deposit in the Yenisei Ridge, where ore bodies are difficult to delineate by structural tracing alone. Based on 456 radioactivation analyses, with internal control tests indicating satisfactory accuracy, the study examines gold contents in host rocks, ore zones, altered schists, and rocks affected by progressive and retrogressive metamorphism. The results indicate lognormal gold distributions in many rock groups, elevated gold contents within and near ore zones, and higher values in rocks affected by stronger metamorphism, especially retrogressive alteration. The study concludes that neutron activation analysis can help identify anomalous areas, clarify gold redistribution, and support practical exploration for blind ore bodies.
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On the Possibility of Applying Neutron-Activation Analysis for Gold to Solve Practical Problems: A Case Study of the Eldorado Gold Ore Deposit (Yenisei Ridge)
A. M. Sazonov
(Presented by the scientific seminar of the Department of Petrography)
The Eldorado gold-ore deposit is located in the crystalline schists of the Gorbilok Formation. From the southwest, quartzites and schists of the Korda Formation adjoin the ore-field area, and from the northeast—schists of the Uderei Formation. All three formations belong to the Sukhopit Series of Upper Proterozoic age.
Structurally, the deposit is situated on the northeastern limb of a large anticlinal fold and is confined to a thick zone of crumpling of northwestern strike.
The mineralization is confined to 4 subparallel quartz-vein zones, representing a system of closely spaced quartz veins, lenses, and veinlets accompanied by sulfides (arsenopyrite, pyrite, pyrrhotite, sphalerite, galena), silicates (feldspars, micas, chlorite, apatite, tourmaline, amphibole), and oxides. Free gold is concentrated in veins, in the near-salband parts of quartz bodies, and often in fractures in the enclosing schists.
The formation of the deposit is genetically associated with the manifestation of two stages of metamorphism: progressive metamorphism, with the formation of schists of the epidote-amphibolite facies and the formation of gold-arsenic ores, and the subsequent overprinting of retrogressive metamorphism, with the formation of rocks of the greenschist facies accompanied by a new formation of gold-polymetallic ores.
As experience from geological exploration work at the deposit shows, the contours of the ore bodies are determined by sampling, rather than by tracing the vein zones along strike and dip.
To clarify the character of the behavior of gold during the formation of the deposit, to identify areas of localized mineralization, and to search for blind ore bodies by primary dispersion halos, we used a highly sensitive neutron-activation method for analyzing gold contents. Samples were taken along profiles spaced 450–500 m apart, with subsequent densification to 100 m. The distances between samples were 50–100 m, with densification to 10–5 m. Samples were collected from pits, trenches, underground mine workings, and boreholes.
The author has 456 results of radioactivation analyses. To determine the quality of the analyses, internal sampling control was carried out (37 analyses). Mathematical processing of the results
control samples showed a fairly high accuracy of the radioactivation analyses. The primary and control analyses are characterized by similar variability of the series. There is an almost direct correlation between the contents in the primary and control analyses; the probability of the occurrence of a systematic error is insignificant, and the possible mean absolute deviation between the primary and control analyses is only 4.3%.
The investigations carried out made it possible to establish the following regularities in the distribution of gold in the host rocks of the deposit (Table 1) and to outline a number of areas with anomalous content values that are recommended as prospective.
Table 1
Character of gold distribution in the rocks of the deposit
| Rock name | Number of analyses, $n$ | Mean content, $n \cdot 10^{-7}\%$ | Distribution law |
|---|---|---|---|
| Korda suite: | |||
| a) quartzites | 9 | 5.13 | |
| b) biotite (quartz schists) | 8 | 9.05 | |
| Gorbilok suite: | |||
| Products of the 1st stage of metamorphism | |||
| Garnet-biotite-quartz schists | 21 | 19.50 | lognormal |
| Biotite-quartz schists | 34 | 10.10 | lognormal |
| Garnet–two-mica–quartz schists | 82 | 6.10 | lognormal |
| Mica-quartz schists | 33 | 8.50 | lognormal |
| Schists outside ore zones | 321 | 10.50 | lognormal |
| Unaltered schists of ore zones | 68 | 22.0 | lognormal |
| Near-ore altered schists | 40 | 30.10 | lognormal |
| Products of the 2nd stage of metamorphism | |||
| Schists partially subjected to regressive metamorphism | 32 | 17.0 | lognormal |
| Chlorite-stilpnomelane-quartz schists | 34 | 20.10 | lognormal |
| Uderei suite | |||
| Phyllitoid schists | 28 | 8.50 | lognormal |
The background gold content in the rocks of the Gorbilok suite is equal to $10.5 \cdot 10^{-7}\%$. Within the ore zones and near them, the gold content in the host rocks increases by a factor of 2–3. The mean gold contents in crystalline schists, even without visible near-ore alterations near ore bodies, are $22 \cdot 10^{-7}\%$.
The gold content in schists that have undergone near-ore alteration is much higher and amounts to $30 \cdot 10^{-7}\%$.
In addition, a certain dependence is noted in the distribution of gold in rocks that have undergone different degrees of metamorphism. An increase in the degree of metamorphism of the rocks corresponds to an increase in the mean gold contents in them (Table 1).
Products of the regressive stage of metamorphism are also characterized by elevated gold contents. Moreover, for schists subjected to
…subjected to more intense retrograde metamorphism are characterized by higher gold contents.
On this basis, it may be concluded that, in the course of retrograde metamorphism, redistribution and accumulation of gold took place.
In conclusion, it may be stated that the studies carried out demonstrate the possibility of using the neutron activation analysis method to solve practical problems in geology.