Darasun
Physico-geographical outline
The Darasun deposit is located on the South-East hillsides of the NerchinskoKuenginsky mountain ridge of Eastern Zabaykalye in the valley of the stream of UzurMalakhay of the right tributary of the river Darasun (the basin of the river Nerch).
The territory of the deposit is located in the central part of the systems of the basins of
the rivers of Ul’durga and Torcha and dated to the formation of the NerchinskoKuenginsky watershed mountain ridge. The peaks of the mountain ridge vary from 800
to 1200 metres in height. Relative excesses of the local watersheds over the floodplains of the currents vary from 100 to 350 metres.
The river network of the territory of the deposit is related to the water basin of the river
Nertch, the left tributary of the river Shilka. The largest rivers of the district are Ul’durga
with the tributaries of Zharcha, Uluntuy and Torcha with the tributary of the river
Darasun.
During the winter period the surface run-off ceases completely, due to freezing of the
river beds, and interstitial-lode waters reach the surface in iced formations. Surface runoff is generally observed during the first half of April.
The climate of the district is continental, and is characterized by a negative (from
-3° to -7°С) annual average temperature.
Winter is long and cold, with a small amount of precipitation. The average thickness of
the blanket of snow does not normally exceed 10 – 15 cm. The average temperature in
January is -30°С. Summer is short, and moderately hot. In July, the average
temperature is +20°С, the annual average amount of precipitation is 250 – 300 mm, and
during rainy years it can amount to 500 mm. Most precipitation occurs in the summer
months.
The area of the deposit is located within the outreaches of the summer boundary of the
expansion of permafrost strata and is characterized by their island-like development.
The locality of the deposit is dated to South-Siberian formation of mountain-taiga type,
where predominant flora are larch, mossy-brushland, and rhododendron varieties.
The main industries of the district include mining and wood harvesting. Agriculture and
fur production play a subordinate role.
The settlement of Vershino-Darasunsky, the site of the Darasun mine, is located 84 km
from Shilka, on the Zabaykal’skaya railway, with which it is connected by an unpaved,
gravel-coated motor road, which runs between Shilka and the settlement of Usugli.
Electrical power supply for the mine is provided by the “Chitaenergo” system, via the
Kholbonskaya substation (220/110/10 kV) and Vershino-Darasunskaya substation
(100/35/6 kV), via high-voltage transmission lines of 110 and 35 kV.
Timber for pit-props timber is supplied from local woods, 40-50km from the settlement.
The main source of technical and household-drinking water supply used to be the
Zharchinskoye water storage pond, with its 12-kilometer water supply line. An additional
source of technological water supply has been purified pit water.
Among the local construction materials, quarrystone and clays are available within the
area of the deposit.
Structure of the ore field
The Darasuns ore field is located within the boundaries of the DarasunoMogochinskaya structural-formational-metallogenic zone, which has been a part of goldmolibdenum metallogenic belt of the Easern Zabaykalye. From the North, the zone is
restricted by Voskhodsko-Mogochinsky fracture of the North-Eastern extension, from
the South by the Mongolo-Okhotsky boundary juncture. The Baleysko-Darasunsky
fracture is a transversal, with respect to the extension of the metallogenic zones of
Zabaykalye, hypogene structure of the North-Western extension, concordant with the
general structural plan of the substructure, runs through the ore field. An area within the
interfluve of the rivers Zhartcha and Darasun, in the shape of a rectangle 12 km long by
4km wide, is related to the Darasun ore field.
The South-Eastern boundary of the ore field runs within the zone of contact of
gabbroids, at a distance of 2.0 – 2.5 km from the contact. The South-Western boundary
runs inside the field of gabbroids and a distance of 1 km from its South-Western
protrusions. The North-Eastern boundary runs amongst granites, at a distance of 1.5 –
2.5 km from their contact with gabbroids.
Lower Paleozoic gabbroid strata and amphibolites, granodiorite massifs of middle
Paleozoic age, leucocratic massifs of middle Paleozoic age, leucocratic granites and
syenites of lower Mesozoic age, dikes of melanocratic strata and sub-volcanic
intrusions, and dikes of plagio-granite porphyries of middle-upper Jurassic age are
present in the geological structure of the Darasun ore field.
Extended disjunctive disturbances of the North-Eastern, North-Western, and sublatitudinal extension, in the nodes of intersection of which numerous gold-bearing ore,
polymetallic and silver deposits and ore manifestations are localized, are widely
developed in the district.
The ore field includes: Darasun, Teremky, Talatui (gold-bearing ore) deposits and a
number of ore manifestations. All ore formations with industrial concentration of gold or
silver have the same age and are para-genetically associated with middle-upper
Jurassic sub-volcanic intrusions and dikes of the strata of the Amudjikansky complex.
Brief geological characteristics of the deposit
The Darasun deposit is dated to the zone of development of the North-Eastern
disturbances, orientated approximately along the contact between the ancient Paleozoic
gabbroids and amphibolites with the middle Paleozoic granodiorites. With respect to the
Darasun ore field, the deposit occupies its South-Eastern part and is approximately 3.5
– 4.0 km in diameter.
In the central part of the deposit, in the North-Eastern direction, the contact between the
ancient Paleozoic gabbroids, amphibolites, and the late and middle Paleozoic granitoids
is located. The late Jurassic intrusion is presented by plagiogranite-porphyries, which
form, in the Southern part of the deposit, a rod-shaped body of a sophisticated (“spidershaped”) form with divergent dikes, of predominantly North-Western extension. Within
the boundaries of the rod, and partly within the containing strata, areal autometasomatic
changes of the type of beresitezation and development of dissemination of minerals
(pyrite, tourmaline, molybdenum, apatite, etc.), peculiar to the early stages of the
process of ore formation on the deposit are typical. Concentrically allocated isolines of
palaeotemperature field within the ore lodes, with respect to the rods of plagiograniteporphyries, also bear witness to the paragenetic association of the gold-bearing ore
formation with minor intrusions.
The structure of the deposit is determined by the availability of extended tectonic zones
and the cracks that fledge them, of predominantly North-Eastern and North-Western
directions.
The lodes of the North-Western extension are almost completely allocated towards the
South-West from the main shedding of the same extension, which divides the deposit
into two approximately equal parts. The lodes of the North-Eastern extension are
predominantly disseminated towards the North-East from the main shedding.
Industrial mineralization in the Darasun deposit is concentrated within the quartzsulphide lodes of small thickness. Their major part is dated to metamorphized gabbroid
strata, as well as to endocontactively-changed strata of granodiorite intrusion.
There are approximately 250 known lodes within the area of the deposit. Resources
were calculated on a base of more than 150 lodes. The thickness of these lodes varies
widely, from several centimetres to 1.5 – 2.0 metres. The lodes are on average 0.03 –
0.25 m thick.
Quartz-sulphide lodes are accompanied by the zones of hydrothermally changed strata.
Their thickness varies from 10 – 15 cm to 70 – 80 cm, reaching 2.0 – 2.5 m in several
cases. The character of the zone where change occurs is beresitezation. The zone of
beresitezation is clearly mapped in the mining documentation, and corresponds to the
conception of “phenocrysts”. The overall thickness of the ore fields (the lode part with
the “phenocrysts”) varies from 0.18 to 2.08 m, thus amounting to 0.91 m on average.
Approximately 59 – 60% of the industrial reserves are concentrated within ore bodies up
to 1 m thick. 28% of reserves are contained within ore bodies of 1.01 – 1.50 m thick,
and 12.4% within ore bodies of more than 1.5 m in thickness.
The lodes of the North-Eastern and North-Western extension are the most widespread,
amounting to approximately 97%. The absolute majority of the lodes have a dip into the
Southern rhumbs, at abrupt angles. 96.6% of the balance reserves are concentrated
within the lodes of the abrupt dip (> 50°), with 3.4% concentrated in the inclined dip (<
50°).
Among the gold-bearing quartz-sulphide lodes of the deposit, the following
morphological types are distinguished:
1) Considerably large (extended), having a length of more than 1 km and thickness of
up to 0.5 m, with an abrupt dip and vertical span of the mineralization of 700 – 1100 m.
Their extension is North-Eastern in direction. Most of these lodes have been
independently prospected and exploited. These include the lodes of NovoKuznetsovskaya, Novo-Electricheskaya, Glavnaya, Electricheskaya–2, Futbol’naya. The
role of these lodes within the current structure of the reserves is insignificant, as they
have now practically been worked-out;
2) Large, – more than 300 m in their extension, having a thickness of 0.1 – 0.2 m,
oriented sub-parallel or variously-directed, with vertical range of mineralization,
comparable with their length. Their role in the currently available structure of the
reserves is predominating, and they were, and they still are the main objects of
exploitation. To these lodes, the Sentyabr’skaya, 5th Elektricheskaya, Almaznaya, Iskra,
and others are attributed. A considerable part of these lodes have been worked-out,
however, during the process of prospecting, these are systematically being revealed.
The last example being the lode of “Ulybka”, bore 1 lode, 1460;
3) Small thickness (0.03 – 0.2 m), having medium (100 – 300 m) and short (less than
100 m) lengths, apophysises of large and considerably large lodes. such lodes
frequently form individual bunches within tectonic weakened zones, with converging (20
– 100 m) distances between them. The role of the above mentioned lodes in the
currently available structure of the reserves is substantially important.
The elements of bedding of the lodes generally vary insignificantly (5 – 10°), and though
the locations of bending have smooth shape, in total, the lodes consist of the sectors of
2 – 3 consecutively variating directions. As the result of relative displacement of the
blocks, bulges and pinches are formed in the lodes. However, the thickest parts of a
lode are not always the richest.
Changing of the sectors of ore bedding for the lode in the whole causes the formation of
concentrated ore columns. Vertical and abruptly inclining shape of the ore columns is
common.
In the ore bunches, localized within the lying sides of large lodes, sinking of the ore
columns in parallel to the dip of these lodes is common. The continuous, extended
along the dip, or close to isometric, shapes of the ore columns have the most common
distribution. The columns are known, which are stretched along the extension, divided
by horizontal pinches (the lodes of “Medvedevskaya”, “Lagunovskaya”). The shape of
ore columns is determined, to a significant extent, by the shape of the ore-containing
crack.
The Darasunskoye deposit is characterized by an extremely multifarious ploycomponent
composition with a substantial predominance of sulphides and has been a typical
representative of gold-sulphide formation.
The mineral composition of the ores is multifarious — there have been isolated 64 ore
and 23 lode minerals of hypogene origin out from them. The relationship between ore
and lode minerals, in volume, amounts to 3:2. The main minerals of the lodes are
represented by quartz (approximately 30%), pyrite (18 – 25%), arsenopyrite (14 – 20%),
chalcopyrite (8 – 10%), antimonial sulphosalts and carbonates (8 – 15%).
Gold is most frequently found in the form of microscopic segregation of 0.0005 – 2 mm.
The main bearers of the native gold include arsenopyrite, pyrite, chalcopyrite, pyrrhotine
and faded ore.
A complexity of mineral composition of ores and a long-continued history of the
formation of lodes causes the diversity of texture character of the lodes, where the
textures of open cavities implementation are predominant.
The most common distributed are streaky textures, caused by the interchange of the
stripes of the various mineral composition. In addition, massive, spotted, nested,
brecciated, vein, disseminated and other types of textures are developed. The zones of
pre-lode changes generally have disseminated texture.
The zone of oxidation of the ore bodies of the deposit is saw-shaped depth-wise, and
reaches 70 m. Oxidized ores, under the contemporary exploitation do not play any
substantial role, considering their small quantity and extremely localized distribution.
The reserves of these ores do not exceed 5% of the total reserves of the deposit.