Patterns of Distribution of Gold and Minor Elements in Hydrothermally Altered Volcanogenic Rocks of the Belaya Gora Deposit (Lower Amur Region)

V. D. Melnikov, L. V. Melnikova (DVGI, Far Eastern Branch, Academy of Sciences of the USSR)

The Belaya Gora gold-ore deposit is located on the northern margin of the Primorye Upper Mesozoic–Cenozoic volcanic belt. Within the Belogorsk ore field, the Eocene andesite-basalt and basalt cover is cut by an Eocene–Oligocene extrusive body and by dikes of rocks of acidic composition, accompanied by a lava flow.

The rocks enclosing the extrusive body are represented by basalts and andesite-basalts, porous in near-surface parts and massive and amygdaloidal at depth. In chemical composition, the andesite-basalts are close to andesite-basalts of average composition after Daly. The rocks of the extrusion and flow are represented by felsite-porphyries, liparites, trachyliparites, and dacites. They are supersaturated with alumina and, in their numerical characteristics, are close to dacites, liparites, and trachytes after Daly, differing in a somewhat smaller amount of femic components and an increased alkali content.

The rocks within the Belogorsk deposit have undergone hydrothermal alteration of two types—pre-ore, areal alteration and fissure alteration associated with gold mineralization. Areal hydrothermal alterations are represented by silicification, hematitization, sericitization, kaolinization, pyritization, chloritization, and carbonatization. The alteration of rocks is especially intense at the contact of andesite-basalts with acidic rocks of the extrusion, increasing in zones of faults and disturbances. The andesite-basalts are intensely propylitized, with newly formed quartz, sericite, chlorite, carbonate, and epidote. In them, a successive downward change of the following facies is observed: a) kaolin–sericite–quartz, b) quartz–sericite–chlorite, c) carbonate–chlorite.

The acidic rocks have been transformed into sericite-quartz, hydromica–sericite–quartz, kaolinite-(dickite)-quartz rocks and monoquartzites. To a depth of 150–200 m, a zone of sericite-quartz rocks with kaolinite, dickite, and halloysite is widespread. In this zone there occur lenses of monoquartzites and rocks that are often so altered that they contain almost no relics of the primary structure. Below extends a zone of sericite–hydromica–quartz and hydromica–quartz rocks.

In all of the rock groups listed above, the content of gold and minor elements was determined. Gold was determined by fire assay and by a highly sensitive chemical–spectral method (Voskresenskaya et al., 1960). Copper, zinc, and lead were determined by the chemical method with dithizone; the remaining elements, spectrally. In all, 196 samples were analyzed.

Unaltered andesite-basalts contain gold from 0.02 g/t to 0.005 g/t, on average 0.01 g/t. In propylitized andesite-basalts the gold content rises to 0.02 g/t, and in sericitized andesite-basalts to 0.23 g/t. The gold content in kaolinite-sericite-quartz rocks developed after andesite-basalts reaches 0.06 g/t. Gold, in the form of microscopic particles, is included in sulfides; therefore andesite-basalts enriched in sulfides (for example, propylitized and especially sericitized varieties) contain gold in amounts above the clarke values. The sulfide content increases at the contact of sericite and carbonate-epidote-chlorite varieties of andesite-basalts to 20–30%. Here the gold content reaches 0.23 g/t. Among the sulfides, pyrite is most widely distributed; it often replaces femic minerals of the original rock. According to pan sampling and concentration of crushed material, the sulfide content in propylitized andesite-basalts reaches 30 kg/t. According to chemical analysis, the gold content in pyrite reaches 300 g/t. Spectral analysis also establishes the presence of gold in pyrite in an amount of 150–250 g/t. The most gold-bearing among the hydrothermally altered varieties of acid rocks are sericite-quartz and dickite-quartz rocks, with gold contents from 0.3 to 0.8 g/t. The minimum gold content (0.2–0.3 g/t) was established in hydromica-quartz and in sericite-hydromica-quartz rocks.

In hydrothermally altered rocks, a relationship is established between gold and their silica content. In sericitized andesite-basalts containing 0.23 g/t gold, the amount of silica reaches 83.83%, as against 55.26% in propylitized varieties containing 0.02 g/t gold. In sericite-quartz and dickite-(kaolinite)-sericite-quartz rocks containing the greatest amount of gold, the amount of silica is 73.02–81.58%, as against 71.86% in sericite-hydromica-quartz rocks (Table 1). In addition, in hydrothermally altered acid rocks a relationship is observed between gold and their potassium content. Rocks containing the greatest amount of potassium (sericite-quartz, dickite-sericite-quartz) also contain more gold. Enrichment of these rocks in potassium is observed owing to the development of adularia and sericite, which possess high sorptive properties with respect to gold and promote its precipitation. Dickite also exerted a precipitating influence on gold-bearing solutions. At Belaya Gora, gold is observed to be confined to dickite. Some part of the gold in hydrothermally altered acid rocks is associated with pyrite, which forms fine disseminated impregnation in them.

Silver is observed in all hydrothermally altered acid rocks (from 1 to 3 g/t), and also in sericitized and in kaolinite-sericite-quartz varieties of andesite-basalts (4–5 g/t). Propylitized and unaltered andesite-basalts contain silver in insignificant amounts (less than 0.1 g/t). All hydrothermally altered rocks that contain gold also contain silver. Only for a small part of the elements (gallium, beryllium) is the average content in all distinguished rock groups more or less constant. The content-

The concentrations of the remaining elements undergo considerable fluctuations. The maximum contents of nickel, cobalt, vanadium, and chromium were found in andesite-basalts. The content of these elements changes, decreasing from propylitized varieties to sericitized ones, with a minimum content in kaolinite-sericite-quartz rocks developed after andesite-basalts. In acidic rocks these elements were found in very small amounts. The presence of nickel, cobalt, vanadium, and chromium in andesite-basalts and their propylitized varieties is probably related to the presence in these rocks of a large amount of rock-forming dark-colored minerals and epidote. The barium content decreases from 210 γ/g in propylitized andesite-basalts to 43 γ/g in kaolinite-sericite-quartz rocks; strontium, respectively, from 218 to 70 γ/g. Among the acidic rocks, monoquartzites contain barium in an amount of 114 γ/g and strontium 113 γ/g; in the other groups of rocks the barium content is 40–60 γ/g, and strontium 60–96 γ/g. The strontium content increases sharply in sericite-quartz rocks (up to 230 γ/g). The increase of barium and strontium in these rocks, as well as in propylitized andesite-basalts, can probably be explained by the presence in them of potassium feldspar.

The content of titanium and manganese in propylitized and unaltered andesite-basalts is higher than in their kaolinite-sericite-quartz varieties. In thin sections of rocks from the Belogorskoe ore field, among titanium minerals, sphene, leucoxene, rutile, and ilmenite were encountered; their amount is greater in andesite-basalts and their propylitized varieties than in the other groups of rocks. In addition, titanium, like manganese, is present as an isomorphic admixture in many accessory minerals and in sulfides. The amount of titanium and manganese increases especially in the transition zone from kaolinite-sericite-quartz rocks to sericitized andesite-basalts. Here the manganese content reaches 1000 γ/g and titanium 5000 γ/g, compared with average contents in propylitized andesite-basalts of 518 and 3840 γ/g, respectively.

Lithium is contained in sericite-quartz rocks and in monoquartzites. The other groups of hydrothermally altered acidic rocks and andesite-basalts contain lithium in insignificant amounts. The lithium content increases especially (up to 300 γ/g) in sericite-quartz rocks and in individual samples of kaolinite-sericite-quartz rocks. These data indicate the occurrence of lithium in the micaceous minerals of these varieties of rocks. Lanthanum is present only in the transition zone from sericitized andesite-basalts to propylitized ones, and in acidic rocks only in some samples of kaolinite-sericite-quartz and sericite-hydromica-quartz rocks (up to 100 γ/g). Apparently, the rare earths, in the form of isomorphic admixtures, are contained in some accessory minerals of these rock varieties. Zirconium enters into the composition of the accessory mineral zircon, the amount of which, as shown by panning studies, is greater in hydrothermally altered varieties of acidic rocks. In them the average zirconium content is 80–100 γ/g, compared with the average content in andesite-basalts of 23–46 γ/g.

For andesite-basalts, the values of ratios of indicator elements were calculated. For unaltered andesite-basalts and basalts these values are: Cr : Ba = 0.12; Cr : Sr = 0.14; Cr : Ti = 0.007; Ni : Ba = 0.08; Ni : Sr = 0.066; Ni : Ti = 0.005. Almost identical values of these ratios were also established for propylitized andesite-basalts: Cr : Ba = 0.13; Cr : Sr = 0.13; Cr : Ti = 0.007; Ni : Ba = 0.10; Ni : Sr = 0.10; Ni : Ti = 0.006, while in sericitized andesite-basalts these ratios decrease: Cr : Ba = 0.13; Cr : Sr = 0.08; Cr : Ti =

Scheme of the Staging of Hydrothermal Alteration

Table 1

Note.
1 — andesites, basalts;
2 — dacites, their tuffs;
3 — lithic-crystal porphyry tuffs, their tuffs;
4 — trachytes, granitoids;
5 — granite porphyries;
6 — low-temperature propylitization;
7 — formation of secondary quartzites.

= 0.004; Ni : Ba = 0.05; Ni : Sr = 0.037; Ni : Ti = 0.0015. The values of the ratios of indicator elements for unaltered andesite-basalts are almost equal to the Clarke values for rocks of average composition (Vinogradov, 1962). This indicates a change in the basicity of the andesite-basalts during the process of hydrothermal alteration of the rocks.

In propylitized andesite-basalts the average copper content is 32 γ/g, lead 22 γ/g, and zinc 50 γ/g (as compared with the average contents in other groups of rocks, respectively, 11–20 γ/g, 13–20 γ/g, 18–20 γ/g). In the sericite–hydromica–quartz varieties of acid rocks, which show the greatest development from depths of 100–200 m, the copper content is 22 γ/g, lead 28 γ/g, and zinc 35 γ/g (as compared with 8–15 γ/g, 7–25 γ/g, and 0–20 γ/g).

Andesite-basalts and their propylitized varieties are characterized by a Cu : Pb ratio of 0.9–2.6 (Table 1), which for propylitized andesite-basalts nearly coincides with the value for average rocks of the Earth’s crust, equal to 2.3 (Vinogradov, 1962). The other hydrothermally altered varieties of andesite-basalts are characterized by lower values of the Cu : Pb ratio relative to the Clarke value. It decreases from propylitized to kaolinite–sericite–quartz varieties, approaching the average Clarke value for acid rocks of the Earth’s crust. Hydrothermally altered acid rocks are characterized by Cu : Pb ratios of 0.6–1.1, which is almost equal to the average value for acid rocks of the Earth’s crust, 1.0 (Vinogradov, 1962). This ratio decreases from sericite–hydromica–quartz (0.8) to kaolinite–sericite–quartz rocks (0.6) and corresponds to the intensity of hydrothermal alteration.

Thus, all the identified facies of hydrothermally altered rocks are characterized by significant differences in the contents of gold and minor elements, which makes it possible to use them in geochemical prospecting for subvolcanic gold deposits and in assessing their erosional level.

REFERENCES

1. Vinogradov A. P. Average contents of chemical elements in the principal types of igneous rocks of the Earth’s crust. Geochemistry, No. 7, 1962.

2. Voskresenskaya N. T., Zvereva N. F., Rivkina P. L. Spectrochemical determination of gold in silicate rocks and minerals. Zh. Analit. Khimii, vol. 20, issue 12, 1965.