SECONDARY ZONING OF SULFIDE VEINS OF THE DARASUN DEPOSIT

A. D. MIKOV (TPI)

The Darasun deposit is located in the area of the upper course of the river of the same name, which belongs to the Nercha River system (Eastern Transbaikalia).

The ore bodies at the deposit are represented by gold-bearing sulfide veins. They are situated mainly in the near-contact zone of a granodiorite massif with ancient metamorphosed rocks, where a series of major disjunctive disturbances, predominantly of northeastern trend, passes through. The principal veins of Darasun are confined to these disturbances: the Main, Novo- and Yuzhno-Kuznetsovskaya, Futbolnaya, and others. Their dip is predominantly steep (80°) to the southeast, more rarely to the northwest. In addition, there are a number of veins located in zones of other trends: northwestern (the Karpaty vein, 4th Nagornaya, etc.), north-northeastern, and sublatitudinal, with generally steep dip angles (60–90°).

The veins are characterized by considerable extent along strike and dip, with small average thickness. The most persistent are the veins of northeastern strike. The confinement of the ore bodies at the deposit to long-lived tectonic zones could not fail to be reflected in their morphology. Their structure is rather complex. Despite their general persistence along strike and dip, pinches and swellings, as well as branching into separate veinlets that usually merge again, are frequently observed. Some pinches extend along strike for several tens of meters. More often, however, they are considerably smaller.

In the swellings the ore body reaches a thickness of 1.0–1.5 m. Along strike the ore shoots can usually be traced for several tens of meters, and down dip through several levels. V. M. Kreiter (Smirnov, 1961) believes that the main reason for this structure of the veins is their confinement to clearly fault fissures filled with thin, abraded clayey material, which hindered the development of thick ore accumulations and caused the lenticular character of the veins.

The ores of the Darasun deposit are characterized by a very complex mineralogical composition (Table 1). In terms of sulfide content,

in the ores the deposit may be assigned to the gold-sulfide formation, according to N. V. Petrovskaya (1960), or to the gold-sulfide-sulfosalt subtype, according to M. S. Sakharova (1966). At the deposit, seven stages of mineralization are distinguished (Zenkov, 1946; Timofeevskii, 1957): 1) quartz-tourmaline, 2) quartz-pyrite, 3) arsenopyrite, 4) galena-sphalerite, 5) chalcopyrite-bornite, 6) quartz-sulfoantimonite, and 7) carbonate. Not all of the indicated associations are distributed everywhere; they are characteristic only of particular veins (Sakharova, Nekrasov, 1964). The distribution of silver and gold in the primary ores, in the opinion of D. A. Timofeevskii (1957), is closely connected with the distribution of the arsenopyrite, galena-sphalerite, and, chiefly, chalcopyrite-bornite stages of mineralization. On the whole, quartz-pyrite ores are most highly developed at the deposit.

The prolonged peneplanation of the region of Eastern Transbaikalia during the Cretaceous and Tertiary periods led to the formation of a well-developed weathering crust. Thick zones of weathered intrusive rocks were formed. The presence of thin, steeply dipping ore bodies with varied mineralization had a favorable effect on the development of hypergene processes in the veins of Darasun. Their confinement to tectonic zones that had continued to exist, along which considerable filtration of vadose waters took place, also contributed to this. The manifestation of a whole series of favorable regional and local factors determined the presence of a clearly expressed secondary zoning. At the deposit the oxidation zone and the zone of secondary sulfide enrichment are well expressed.

The depth of development of oxidized ores is 15–27 m. At the outcrops the ore veins are usually represented by zones filled with breccia of the enclosing rocks, as if cemented by transported ocherous limonite. Sometimes weakly oxidized sulfide ores are also encountered at the surface. The gold content in these ores varies, but on the average it is considerably higher than in the primary ores, which should be taken into account in evaluating newly exposed ore bodies. The maximum content of the useful component is usually observed at a slight distance from the ground surface. Visible gold is often encountered in these ores, whereas in the primary ores of Darasun it is absent.

Study of the oxidation zone of Darasun and comparison of its section with those of other gold-ore deposits (Mikov, 1965) made it possible to conclude that at the deposit described only its lower part is represented. The upper part of the oxidation zone, owing to the sharp block uplift of the district, has been eroded to a considerable extent (Volarovich, 1947). This is also confirmed by the presence of formerly gold-rich placers in the valleys of the Uzur-Malakhai and Darasun rivers.

The zone of secondary sulfide enrichment manifested itself very clearly in the veins of Darasun. It can be traced in a number of veins to a depth of 100 m and more. This zone is characterized by the development of typical sooty ores, spreading in the plane of the veins in the form of large patches and deep “pockets.” These ores consist of chalcocite, to a lesser degree covellite, and other minerals. A chemical analysis of a sample of these ores from the Yuzhno-Kuznetsovskaya vein (Epov, 1962) determined the following composition: sulfur—16.4%, copper—74.4%, arsenic—0.8%, lead—0.47%, zinc—0.04%, iron—5.5%, alumina—0.7%, silica—2.0%, gold—80.4 conventional units, silver—1055 conventional units. Sooty ores, as is evident from the chemical analysis cited,

Fig. 1. Graph of changes in the average contents of precious metals and thickness along the apophysis of the footwall of the Karpaty vein.
1—gold, 2—silver, 3—thickness.

Labels in the figure:

• Content, Ag (conventional units)
• Content, Au (conventional units)
• Thickness, m
• Depth, m
• 1 — solid line
• 2 — dotted line
• 3 — dashed line

analysis, are characterized by a very high content of precious metals.

To confirm the fact of the increased gold content of supergene ores in comparison with hypogene ores, a statistical comparison was made of the average contents of the useful component for a number of veins of the deposit. For this purpose, graphs were constructed (Figs. 1, 2) of the change with depth in the average contents of gold and silver according to face-sampling data.

Fig. 2. Graph of the change in the average gold content and thickness along the Futbolnaya vein.
The symbols are the same as in Fig. 1.

chemical sampling. Similar graphs were also constructed for the thickness of the ore bodies. In doing so, all samples obtained during the mining of ore bodies by working horizons and production blocks were taken into account, either along the entire vein or for an individual ore shoot.

The calculation of the average grade and thickness was carried out by horizons and by 10–15 m layers along the levels of stoping workings. The average metal grade was calculated as the arithmetic mean. The use of this method of recording changes in the grade of precious metals made it possible to reveal certain regularities in their distribution in the ore bodies with depth. As examples, Fig. 1 presents graphs of the distribution of gold and silver along the apophysis of the footwall of the Karpaty vein (western sector), and Fig. 2 presents a graph of the change in gold content along the Futbolnaya vein (eastern sector). In the first vein the oxidation zone is clearly manifested, but a cementation zone is practically absent. Secondary sulfides are observed only sporadically, in the form of small nests. The graph presented (Fig. 1) shows that the maximum gold content is observed in the interval from 3 to 32 m. In the same interval, a reduced silver content is also observed, which fully corresponds to its behavior in the oxidation zone. In the primary ores, the content of the latter increases sharply. The average thickness of the vein varies within 0.1–0.25 m. No dependence is observed between changes in metal grade and changes in thickness.

In the Futbolnaya vein the oxidation zone is developed to a depth of 17–27 m. In the part of the ore shoot between shafts No. 1 and Vostochnaya, for which the calculation of changes in gold content was made, it is 25 m. The zone of secondary sulfide enrichment is clearly expressed. The primary ores of the vein, as was already noted by S. S. Smirnov (1961), are characterized by the broad development of copper minerals. On the graph shown in Fig. 2, two maxima of elevated gold content are clearly distinguished, at depths of 10–15 m (data from chemical sampling at the outcrop of the vein on the ground surface have not been preserved) and 80–140 m. The first maximum corresponds to ores of the oxidation zone, the second to those of the cementation zone. The average thickness along the vein varies from 0.1 to 0.3 m.

A similar pattern of vertical gold distribution has also been established for other veins of the deposit, which agrees well with data reported for other gold-ore deposits of the USSR [1, 4].

In conclusion, it should be noted that at the Darasun deposit there is a clearly expressed secondary zoning.

The oxidation zone is traced to a comparatively shallow depth (down to 25–27 m), which is due to the sharp uplift of the portion of the earth’s crust and erosion of an oxidation zone once developed to a considerable depth. The oxidized ores are characterized by the broad development of ocherous limonite and are considerably enriched in the useful component. The presence of visible gold is also characteristic. The increased gold content in oxidized ores must be taken into account when evaluating new veins of the district.

Ores of the cementation zone are developed in many veins and are traced to depths of 100 m and more. They are represented by typical sooty ores with an increased content of precious metals. The indicated ore types are widely distributed in the ore bodies and are of definite practical interest.

References

1. Al'bov M. N. Secondary zoning of gold-ore deposits of the Urals. Gostekhizdat, 1960.

2. Volarovich G. P. On the geological structure of the Darasun gold-bearing district of Eastern Transbaikalia. Transactions of the Nigrizoloto Institute, issue 16, 1947.

3. Zenkov D. A. Mine geology at the Darasun gold-arsenic deposit (Transbaikalia). In: Collected articles “Mine Geology,” Moscow, 1946.

4. Mikov A. D. On the question of secondary zoning in the gold-ore veins of the Mariinsk taiga (Kuznetsk Alatau). Proceedings of the Tomsk Polytechnic Institute, vol. 138, 1965.

5. Petrovskaya N. V. The character of gold-bearing mineral associations and formations of gold ores of the USSR. Materials of the XXI Session of the International Geological Congress. Reports of Soviet geologists, Gostekhizdat, 1960.

6. Sakharova M. S., Nekrasov E. M. Mineralogical-geochemical characteristics and localization conditions of new types of gold-sulfide ores of the Darasun deposit. Geology of Ore Deposits, no. 3, 1964.

7. Smirnov S. S. Polymetallic deposits and metallogeny of Eastern Transbaikalia. Publishing House of the Academy of Sciences of the USSR, Moscow, 1961.

8. Timofeevskii D. A. Geological-structural and mineralogical characteristics of the Darasun ore field and its peripheral parts. Transactions of Nigrizoloto, issue 24, 1957.

9. Timofeevskii D. A. Features of the geological structure and structure of the Darasun ore field (Eastern Transbaikalia), Transactions of TsNIGRI, issue 43, 1962.