The Kadara gold ore field is located in the Mogochinsky district of the Trans-Baikal Territory, between the Shilka River and the Amazar River, the Yerofey Pavlovich railway stations are 50 km away and the Amazar railway station of the Trans-Baikal Railway is 65 km away. The field area is linked with the Amazar and Yerofey Pavlovich settlements by dirt roads.
The Mogochinsky district is located in the north-eastern part of the Trans-Baikal Territory. The district lies in UTC+08 time zone, and the time lag between Moscow and Mogocha is six hours. The Mogochinsky District includes a town of regional subordinance, 5 urban settlements (Amazar, Davenda, Itaka, Klyuchevskiy, Ksenyevka) and 30 rural settlements. The district center is the town of Mogocha. In the east, the Mogochinsky district borders on the Amur region, in the south – on the Sretensky district and China along the Argun and Amur rivers, in the west – on the Chernyshevsky district and Gazimuro-Zavodsky District, in the north – on the Tungiro-Olyokminsky district and Tungokochensky district.
The district has a severely continental climate. Mostly, the weather of the Mogochinsky district is similar to the weather in the northern regions of the Trans-Baikal Territory. Winters are harsh, long and cold, and average January temperature ranges from -28 to -32°C (at the absolute minimum being -53°C). The amount of annual precipitation ranges from 400 to 600 mmpa. Snow cover period lasts for 164 days; and the average snow cover depth is 16 cm. Summer is short and relatively warm. The average temperature in July ranges from+14 to +18°C (the maximum being + 37°C). Spring is windy, arid; and autumn, as compared to the springtime, is cold and wet. The vegetation period lasts for 120-150 days. The bulk of precipitation (90-95% of the annual amount) occurs during the warm period, from April to October. The wettest month is July, due to rains and frost melting.
The region belongs to the taiga mining belt, which is characterized by larch moss-and-shrub and rhododendron forests with occasional pines and birches. Continuous and insular permafrost soils are typical for the area. Soils are predominantly orogenic taiga cryogenic gley podzolized, soddy and modal. Humus-gley permafrost soils can be found in intermountain depressions. Larch forests with a ledum storey on drained soils are widespread, as well as sparse forests with a dwarf birch understorey, mountain larch-and-pine forests.
The terrain is predominantly of mountain-taiga, locally saturated type, with a composite orohydrography. The main ridges are: Borshchovochnyy Kryazh, Amazarsky, Tungirsky, Cheromniy, Sobachkin, West Lundor, and the spurs of Khorkovoy. The ridges are considerably dissected, have rather steep slopes and wide gentle intermountain basins.
The stream system of the Mogochinsky District includes the large Shilka River, the Amazar (the left feeder of the Amur River), Black Uryum, Mogocha, Big and Small Chichatka, Kadara, Boguzia, Zheltuga rivers, etc. The Shilka River is the largest river in the area. It is navigable and its confluence with the Argun River originates the Amur River. The valleys of the Amazar, Mogocha, Lower and Middle Olongro rivers are much swamped. There are almost no lakes in the area. The rivers freeze in late October/early November; and break up in late April/early May. The average period of ice cover lasts for about 180 days with occasional flood ice. During winter, the feeds of Amazar (Kadarа, Kaltagay, etc.) freeze to the bottom. The stream speed of the Amazar River is 1.8-2.0 m/s, with alternating bars and reaches. The rivers are unsteady; the rise of flood along the Amazar River during the rainy season reaches 5-6 m, and the stream speed increases up to ≈ 5.5 m/sec.
The natural ecological system of the area has been slightly affected by technogenic and anthropogenic processes: felling of timber forest, exploration ditch drilling, and well drilling.
Drinking and service water for the works is supplied from drilled wells or from surface water sources during the summer period.
Railway accounts for the bulk of cargo and passenger operations. Prior to the 1990s, aviation played a significant role in the delivery of passengers, mail and urgent cargo. The cross-district transportation is difficult, as there are no permanent motorways. Winter time, free vehicle traffic is possible only along the Trans-Siberian Railway, and summertime - only within short sections of wood-transport and prospectors’ roads. The available dirt roads are sparse and need repair badly. In winter, ice roads are driven along the Amazar River. Permafrost sometimes occures on the slopes of the northern exposure.
The settlements are located mainly along the railway line and in the Shilka river valley. The urban population prevails. The Trans-Baikal railway and the mining industry play the leading part in the economy of the Mogochinsky district. Railway (the Mogochinsky branch of the Trans-Baikal railway), gold-mining (mining enterprises: Davendinskoye mining administration, Klyuchi, Rudnik Klyuchi OJSC) and timber-processing enterprises (including Amazar Timber Industry Enterprise, OJSC, Mogochinsky Forestry, Taptugorsky Timber Industry Enterprise OJSC) are the ‘cornerstones’ of the district economy. Mogochinsky district is one of the Trans-Baikal leaders in gold reserves. In 2016, one-fourth of the total territorial gold output was provided by the district. Energy and heat is supplied by a number of small-capacity power plants.
The prospective area of the Kadara River basin is a part of the Kholodzhikano-Ilichinskoye ore zone, which adjoins the Gonzhinsky ore area from the west, and is located between the Stanovoy folded and the Mongolian-Okhotsk geosynclinal areas of the Upper Amur zone. In addition to the Kholodzhikanskoye ore field, the Borgulikanskoye, Pionerskoye, Pokrovskoye and Burindinskoye ore fields with gold deposits were discovered within the ore district. The allocated ore district is deemed to hold a strong potential for discovery of gold deposits, gold-silver and gold-sulphide ore formations.
The licensed area features an exceptionally challenging geological structure due to its location in the conjugation of large structural elements of the earth's crust, within the area of continental crust extension and lasting effect of subduction processes. This results in considerable displacement and alteration of rock, and occurrence of upthrusts. The stratigraphic breakdown was made according to the typical legend of the district. The following rock units form the geological structure of the district:
The area surface exposure is uneven within the district. The bedrock is exposed on the slopes of the valleys, and bedrock exposure and remnants are scattered on the watershed divides. There are many water-logged areas, where mapping and sampling is possible by drilling only.
The interpretability of the territory is satisfactory (25%) and poor (15%) due to forestation and focal frost resulted in solifluction processes that mask characteristics of rocks used for interpretation. Remote surveys can interpret numerous fractures, linear zones of dislocation metamorphism and some elements of their internal structure.
The Ordovician-Karaganskaya suite (Okr). The suite is composed of rock units of volcanic-terrigene origin. Vulcanites of the suite are of basalt-andesite-rhyolite origin. The suite base is not confirmed. The upper age limit is evaluated based on data of the faunistic Omutninskaya suite (S1-2om) with the basal level at the base overlapping the Karaganskaya suite, as well as the intrusions of the Doptuganskiy complex (Odp) penetrating the two. The lower age limit was evaluated by availability of fauna sediments in the suite as old as Ordovician age. Therefore, the age is assumed to be Ordovician. A section of the Karaganskaya suite mainly includes siltstones, metasiltstones, or, occasionally, meta-sandstones, quartzite-like sandstones and limestones. The incision is abundantly (up to 50% of the volume) saturated with stratal bodies of subvolcanic intrusions of gabbrodiabases, microdiorites of the Uteninsky complex. It is difficult to represent the composite section of the Karaganskaya suite sediments due to the mentioned circumstances, diversity of the strata and extensive brachyfolded structures. Probable thickness of the Karaganskaya suite is about 700 m. The rocks of the Karaganskaya suite underwent regional metamorphism in greenschist facies. They have mostly been changed into para- and orthoschists of somewhat uncertain origin, with small “islands” of slightly altered rocks. The secondary geochemical field under the sediments of the Karaganskaya suite is rich of copper, tin, lead, and features higher level of molybdenum, and lithium.
The Ordovician-Uteninskaya suite (Out). The composition of the volcanic rocks of the Uteninskaya suite is similar to that of the lower Karaganskaya suite, but the basal structure of conglobreccia and rotten stones on the suite borderline prevents their inclusion into a single structural unit, although the break was, probably, short. The basal level of the Uteninskaya suite lies directly on the volcanic-terrigene formations of the Karaganskaya suite. The Uteninskaya suite consists of volcanic mafic-to-acidic rocks, their lavers, clastolavas, tuffolavas, tuffs, and occasional tuff-conglomerates and tuff-sandstones. In its section, the Uteninskaya suite is clearly distinguished by the lower, more mafic rocks including basalts, trachybasalts, basaltic andesites, trachyandesite basalts, andesites, trachyandesites, their lava-breccia, and tuffolava. The upper section is more acidic and includes dacites, rhyodacites, trachyrhyodacites, rhyolites and frequent agglomerate lavas, while some problematic tuffs are typical of the middle section. Most of the Uteninskaya suite formations were probably corroded in the pre-Silurian period, so the suite rocks are infrequent in the Early Paleozoic section. The exposed thickness of the suite formations is insignificant (100 to180 m). Small outcrops of the Uteninskaya suite (0.5-2 km2) are isolated within the described area, in the upper reaches of the Orochenka fold, where they were previously exposed by excavations. The Ordovician sediments in this area are thrust over the continental sediments of the Neogene; north dip of the contact at an angle of 35-40º.
The Silurian-Omutninskaya suite (S1-2om) is a transgressive rhythm complex, consisting of two rhythm series. The lower rhythm series is determined as the Lower Omutninskaya, the upper - as the Upper Omutninskaya subsuites.
The suite breakdown into the lower and the upper one is made according to the lithostratigraphic principle. Since no index fauna forms have been established, the lower subsuite is faunistically undercharacterized. The suite rocks are found throughout the entire southeastern part of the licensed area. The predominance of quartzite-like sandstones, resistant to weathering, in the sections caused the contrast of the relief forms within their occurence area.
The Lower Omutninskaya subsuite (S1om) is composed of 80% consertal quartz-like sandstones of feldspar-quartz, quartz-feldspar composition, or, less often, of polymictic sandstones, siltstones, silty sandstone (≈ 10%). The suite bottoms usually include conglobreccia, rotten stones, crushed stony sandstones, and calcareous sandstones. The basal level is almost constant at the base of the subsuite. Its habitus usually depends on the composition of the bedding rocks: the basal level of the Silurian base lying on the granitoids of the Doptuganskiy complex usually features a coarse arkose composition, and is saturated with fragments of effusives, greenschists on the border with the Ordovician igneous-sedimentary rock. The basal level forms the first few meters. The thickness of the Lower Omutninskaya subsuite is 180-200 m.
The Upper Omutninskaya subsite (S1-2om2). Unlike the lower subsuite, it is more fine-detrital and diverse, the interbeds of quartzite-like sandstones are thin, siltstones, silty sandstones, polymictic and calcareous sandstones, and top limestones are more widespread. A sharp increase in the calcareous rocks is observed at the top of the subsuite. Most of the suite consists of quartz-feldspar, feldspar-quartz, quartz, and arkose sandstones. They are quartzite-like and form about 60% of the suite rock volume. These are light-gray massive fine- and coarse-grained rocks. Siltstones are dark gray and black, fissile, differ in the metamorphism intensity, and feature well preserved debris. Relics of quartz fragments survive more often (up to 3-5%). Microlipidogranoblastic, granolepidoblastic, blastoaleuritic structures are typical. Mineralogical composition is as follows: quartz, sericite, albite, chlorite, leucoxene, and rarely biotite. Calcareous rocks are represented by gray fine-grained and transition limestones, including silty, sandy limestones, calcareous siltstones, and sandstones. The admixture of clayish-terrigenous material is wide ranged (5-70%). The rocks are regionally metamorphosed in the greenschist facies. The thickness of the upper subsuite is 160-220 m. The total thickness of the Omutninskaya suite is 350-420 m.
Jurassic (Upper Series) – Cretaceous system
The lower series. The Kholodzhikanskaya suite (J3 – К1hl). The Kholodzhikanskaya suite is composed of coarse-detrital and fine-detrital sediments forming the same depression. They are exposed in the western part of the work site. The Kholodzhikanskaya suite includes:
The sedimentation fields of the Lower and Upper Kholodzhikanskaya subsuites feature different forms of relief. The lower, fine-detrital one is characterized by flat, slightly dissected, often waterlogged watershed areas; rocks are poorly exposed, the sediment structure is uninterpretable.
The sediments of the upper subsuite are characterized by a rigid relief; splits of boulder conglomerates form cuestas and valley cliffs. The suite deposits show material difference by gamma-rays intensity comparing to the sedimentary rocks of the Phanerozoic, with their radioactivity is by 3-7 µR/h lower than the main level of the latter.
The Lower Kholodzhikanskaya subsuite (J3-Khl) is composed of sandstones (70%), siltstones, gritstone and rotten stones with layers of pebble conglomerates, argillites; single horizons of tuffs, tuff lavas, tuff-conglomerates and tuff-sandstones.
The lower bench (J3-K1hl11) is mostly represented in the southeastern and eastern sides of the depression, where its wide (3-5 km)sub-meridional extension covers the distance from the estuary part of the Kholodzhikan Creek, the headwaters of the Andreevskiy Creek, along the banks of the Doptugan River to the upper reaches of the Kadara River. The eastern side of the depression irregularly overlaps the Ordovician foundation. The deposition there was gentle and tectonic, therefore, unlike other sections of the Kholodzhikanskaya depression, its eastern part fratures more siltstones, fewer coarse-grained crushed stony sandstones, gravelites and its sorting index is increasing. The thickness of the lower bench deposits is estimated to be about 800 m.
The upper bench (J3-K1hl12) stretches to the south of the Kadara River basin. Its structure is more mottled, with gradual upsection coarsening. The lower boundary of the bench is conditionally determined by a thick, rather monotonous bench of black siltstones (the upper reaches of the Andreevskiy Creek) in the section. The thickness of the upper bench (J3-K1hl12) is about 550-600 m. The total thickness of the Lower Kholodzhikanskaya subsuite is about 1,400 m.
The Upper Kholodzhikanskaya subsuite (J3-K1hl2) is composed of rough terrestrial molasses, including pebble and boulder conglomerates, nongraded sandstones with pebbles, gritstone, interlayers of silty sandstone, and siltstones. The lower boundary of the subsuite is marked by sharply emerging thick boulder conglomerates in the monotonous sandstone stratum of the lower subsuite. The upper subsuite includes three benches, featuring materially different lithology and grading:
The lower bench (J3-K1hl21) is composed of nongraded material, especially in the lower section, where the saturation of the sandstones with boulders and pebbles reaches 50%, the conglomerate levels are fuzzy, and gradual transitions from conglomerates to sandstones saturated with “float” pebbles, are common. Sandstones, usually coarse-grained ones, are crushed stony. Up along the bench, the grade of sedimentary material increases, the sandstones have scant “float” pebbles, gravelites, and conglomerates with well-defined boundaries appear. The total thickness of the lower bench deposits in the upper subsuite is about 580 m.
The middle bench (J3-K1hl22) shows mottled rock units, including silty sandstones, sandstones, rotten stones, siltstones, crushed stony breccia on silty-sand or sand cement, and conglobrecchia. The feature of the bench is a dark-gray color of the rocks (“dark bench”), which is due to the presence of thick horizons of black siltstones, rotted rocks, and gravel of black siltstones in rotten stones and conglobrecchia, as well as abundance of silty sand cement.
The “dark bench” is the centerpiece in the section of the Kholodzhikanskaya suite, playing an important part in the formation of a chaotic stratum, which is a complex of the syndepositional olistostrome. This is due to its sharply different granular composition compared to the overlapping and underlying strata. The presence of the “dark bench” with siltstone interlayers on the boundary of the sandy-conglomerate lower and monotonously bouldery conglomeratic upper bench resulted in heterogeneity in strength and viscosity at the bench boundary, which, under the pressure of the northwest overthrusts, resulted in formation of numerous tectonic failures, thrust slices within the “dark” middle bench. The “dark” bench rocks, a result of mechanical disintegration and mixing of the deposits of the Kholodzhikanskaya suite, formed a kind of melange at the base of tectonic covers. The expected thickness of the middle bench is about 150-200 m.
The upper bench (J3-K1hl23) features a monotonous composition and includes conglobrecchia, boulder-pebbled and occasional pebble-bouldery conglomerates. The rocks are gray or greenish-gray and densely cemented; the cement is gruss-sandy or sandy (5-10%). The size of the boulders reaches 0.5-1.0 m, with an average size of 10-20 cm. Its boundary on the underlying middle bench is sharp. Probable bench thickness is 200 m or less.
Chaotic stratum J3-K1hl). It is extended along the rocks of the Kholodzhikanskaya suite in the olistostrome and tectonic melange zones. It includes complex genesis rock units located in the northwestern edge of the basin, where the regular sedimentary deposits of the Kholodzhikanskaya suite are found along with olistostromic redeposits, mixed sediments of the Kholodzhikanskaya suite. The chaotic stratum formed in the frontal part of numerous thrusts is a product of a mechanical disruption of sedimentary rocks from almost the entire section of the Kholodzhikanskaya suite, found along with regular sedimentary rocks.
The so-called “black” benches are clearly distinguisged in the chaotic stratum. Such “black” bench includes spatially extended layers and horizons of lithologically similar rocks – “monomictic” bouldery and pebbled conglomerates, wherein pebbles and boulders are represented by dark gray “anorthosite-like” granitoids. Conglomerates are formed of black siltstone, silty sandstone cement. In addition, there are benches of black rotten stones, crushed stony sandstones, crushed stony breccias with the rotten stones of the Kholodzhikanskaya suite rocks. We can also distinguish benches of greenish-gray conglobrecchia, rotten stones with limestone olistholiths stuck in them. The “black” bench has been tracked to be 18 km long, and its outcrop width ranges from several meters to 2-3 km. All the rocks of the bench are dark-gray and black due to the black aleuritic cement, clastic sandy and siltstone material.
Pebbles and boulders of “monomictic” conglomerates are also dark-gray. The composition and age of the pebbles and boulders varies from gabbroids to granites.
The dark-gray color of the pebbles is newly formed due to their finest impregnation with a carbonaceous substance formed during grinding with carbonaceous siltstones in the tectonic melange zone, and due to presence of a carbonaceous metasomatism.
Dark-gray veins and veinlet are widely spread in the conglomerates. Boulders of conglomerates are literally saturated with them. This is due to the cracking of granitoid boulders in the melange zone and impregnation of numerous microcracks of granitoids with a carbonaceous substance. The stronger fracture resulted in a “vein” formation. A small network of microfractures smeared outlines of “metasomatites”. In addition, the dark-gray “veins” are a kind of reference for mapping thrust slices in monotonous conglomerate strata. Thus, it was possible to reliably determine the boundaries of the extended thrust slices and accompanying fault zones during their geological mapping (1987, 1995).
Saturation of the melange and olistostrome (chaotic stratum) zones by olistolites of the Paleozoic rocks is uneven. Olistolites cannot be found in the southern and middle parts of the zone, but they are widespread in the northern part of the depression, where they saturate a 1-2 km wide rock belt of the chaotic stratum, from the upper Kholodzhikan to the Vosmaya fold. The largest and most extended, though intermittent, horizons of the olistoliths are represented by Early Paleozoic limestones, which can be tracked for several kilometers. Olistoliths are several meters or dozens of meters long, have an oval shape and sharply folded. The olistolites are located close to each other in a single row, forming a single horizon, similar in attitude to the under- and overlying strata. Occasionally, the olistoliths are represented by mafic and acidic effusives, quartz sandstones or metasiltstones. The shapes and compositions of the rocks comprising the olistoliths are similar to those of the Solontsovskaya and Karaganskaya suites.
In the chaotic stratum, the melange zone is also represented by tillite-like, mostly semi-loose rocks, the product of mechanical mixing disruption of the Kholodzhikanskaya suite rocks, located usually on the boundary of the lower allochthon stratum and the lower melange (subautochthon). Tillite-like rocks are black, their argillite-siltstone substrate is saturated with pebbles and rotted rock of the Kholodzhikanskaya suite rocks, psammitic grains, they are difficult to detect, as their tectonic boundaries are not easily defined. The metallogenic structure of the chaotic stratum is gold-sulphide.
The age of the chaotic stratum is defined by the rocks of the entire section of the Kholodzhikanskaya suite, including the upper bench of the upper subsuite, and penetrated by the early Cretaceous dykes of Tymager. The interlaced facies of the limnic, swampy and deltoid origin and proluvial deposits, alluvial cones, piedmont facies, along with a trace impurity of volcanic sedimentary facies allows us to classify the sediments of the Kholodzhikanskaya suite as the suite of continental molasses. Early metagenesis is characteristic of the suite deposits. The rocks are closely welded, the newly formed leprose greenish-brown biotite and the feldspar sericitization is typical.
Hydrothermal-metasomatic transformations of the suite rocks, including silicification, sericitization, carbonatization, kaolinisation, epidotization, chloritization, propylitization, carbonization, are widespread within the olistostromea and melange zones.
The age of the Kholodzhikanskaya suite is well determined in faunistic terms and dates back to the late Jurassic-Early Cretaceous period. The absolute age (potassium-argon method) of tuffs sampled from the Lower Kholodzhikanskaya subsuite is 135 million years.
Neogene deposits of the Kadara River basin include different ratios of pebbles, boulder-pebbles, boulders, gravel, sands, loams, sometimes grading into clays. They form the disjointed Koltagayskiy and the Upper Kadarinskiy (Doptuganskoye Plateau) outcrops. The Koltagayskiy outcrop covers watershed areas on the right bank of the Utena River, in the middle reaches of the Big Ilichi River and the Small Ilichi River, the upper reaches of the Orochenka creek. The outcrop is a north-east oriented strip, about 17 km long and 1-4 km wide. The total area is about 40 km2.
The Upper Kadarinskiy outcrop (Neogene section) covers the watershed area of the rivers Kavykhta, Doptugan, and Kadara. It is extended to the north-west direction; its area is about 10 km2, the length is 4.5 km, and the width is 2.2 km. The area of the Neogene sediments is satisfactorily interpreted due to the light gray tone of the photo image and the features of the forest cover. Weak negative magnetic and lower gamma fields are observed above the Neogene deposits. Neogene sediments are level or flatly inclined, depending on the bed relief. The conformality of the boundary line of the complex base and contours indicates a near-level occurrence:
The neogene sections are studied from inside the boreholes and pits. They are described in the geological mapping reports. The section of the Koltagayskiy outcrop of the Neogene sediments includes two salient deposit complexes:
Lithological variations observed at their boundary reflect the change in the paleoclimatic conditions of the sedimentation and, consequently, tectonic restoration.
The Sazankovskaya suite (N1-3sz) is not exposed on the surface; it is opened by boreholes in the Koltagayskiy outcrop. The basal horizon (formed by boulders) lies horizontally on the structural eluvium of the Ordovician sediments. Upwards, there are poorly-defined two-component rhythms (boulders-pebbles) of different thickness. A layer of small-pebble conglomerates with an ocher-brown aggregate ends the section and begins the section of the Belogorskiy suite. The thickness of this section in the Sazankovskaya suite is 90 m. The sediments are fascially unstable. This is a “network” of different facies, including alluvial-proluvial, proluvial and fluviolacustrine ones. The debris is nongraded and has a varied abrasion degree, the bulk of pebbles are saturated with boulders and gravel, and boulders are saturated with pebbles and gravel. There are no clean sands and clays.
The first ones are clay and silty, the second ones are silty and sandy. Both contain gravel and pebbles. Volcanics – andesites, dacites, rhyolites, their lava-breccias, sparse quartzites, gneisses, granitoids, quartz – predominate in the pebble-boulder material. Their abrasion is varied, the rocks are mostly well-abraded. The pebble material mostly features an uneven corrosion. The aggregate of boulder-pebbles is loamy at the bottom, and upwards it is sand-loam and sandy gray and greenish-gray in color. Sandy aggregate is polymictic by composition. Sands, as well as the sandy aggregate of the gravel, are polymictic, consertal, ungraded, mostly admixed with gravel and pebbles. Their color is gray to greenish-gray. There are no shows of clean clays, they are admixed with sand (up to 20%), gravel or pebbles and form loamy horizons of gray to greenish-gray color. Clay minerals are hydrous micas. They include such heavy minerals as zircon, ilmenite, rutile, tourmaline, sparse garnet, sphene and gold. The Sazankovskaya suite sediments are referred to the upper Miocene.
The Belogorskaya suite (N23bl) unites chalky loose deposits of all the Neogene areas, including the upper complex of the Koltagayskiy and Kadarinskiy outcrops, as well as the alluvium of the paleoplain remains of the Amazar River fourth terraces. The sediments of the Belogorskaya suite overlie all older rocks, including pebble bed of the Sazankovskaya suite and feature stratigraphic and structural discordance. The boundaries of the suite with the basal complex are mostly regular stratigraphic, delineated by a distinct bench, 0.5-10 m high, with the outcrop of the bedding rocks in the basement (left bank of the Zolotonosha creek, right and left bank of the Orochenka creek). The northwestern border of the Koltagayskiy outcrop is tectonically thrust (0.0-10° azimuth, 300 angle, K-17). The structure of the Belogorskaya outcrops suite was studied from inside the pits and boreholes. In general, it is a two-component multipartite macro-rhythm of chalky, light-gray sediments intermittent with “red” horizons of loose sediments. Its bottom half is saturated with boulders and pebbles, while the top half is smaller, with fine pebbles along with gravel consertal sandy and loamy horizons. The thickness of the sediments ranges from 80 to 90 m.
High concentration of lead, copper, arsenic, tin, and molybdenum is typical for the section of the Neogene sediments. Several horizons are gold-bearing. The sediments have a complicated genesis, they are multipartite. In general, they are alluvial-proluvial or proluvial formations of merged subaerial deltas of waterholes with slope and slope-proluvial deposits. They are referred to the upper Pliocene.
In the section of the Quaternary system the following formations are distinguished:
In the buried cut the following formations are distinguished:
The sediments are dated based on spore-pollen complexes and general geomorphological situation.
Lower Pleistocene (QI). These sediments are exposed by drilling in a buried cut lying in the 18-20 m bottom of the second supra-floodplain rock-defended terrace on the spit of the Amazar and Bagadzha rivers. The sediments are represented by two facies: floodplain and channel. The floodplain facies, 1.5 m thick, are composed of bog-silty, silt-loam sand-silty sediments with sparse fine pebbles, fine- and medium-grained sands of quartz-feldspathic composition. The channel facies, thick 3.0 m, are represented by sand-gravel-pebble deposits of light-gray color. Pebbles are mostly fine and medium, sometimes – coarse, well-abraded. The content of pebbles in sediments is up to 50-60%, of clay – up to 10-20%, of quartz-feldspathic sand – about 20%. Quartz, quartzitic, granitoid pebbles are prevailing. Fine pebbles have a round shape. Medium and coarse pebbles are oval, slightly elongated. Fine sand is saturated with ilmenite, zircon, ilmenorutile, monazite, and gold.
Middle Pleistocene (QII). The sediments were found in the same place, adjacent to the above, early Quaternary ones. They are represented by two facies. The 1.4 m thick floodplain facies are light-gray, greenish-gray or gravel-pebble with sandy and clayey aggregate. Pebbles are medium-sized, well-abraded, have the granitoid, quartz, quartzitic and effusive composition; the content is up to 40%. The sand is medium- and coarse-grained, and fill about 20% of the total volume. Clay content is up to 15-20%. Mineralogical composition of the fine fraction is as follows: zircon, ilmenite, monazite, ilmenorutile; natural-resource base is represented by gold - the first tens and hundreds of mg/m3.
Upper Pleistocene (QIII). The Upper Quaternary formations include alluvial depositions in the second (Т2) and third (Т3) terraces above the flood-plain of the Amazar River and its feed, the Utena River.
The alluvial deposits in the third terrace above the flood-plain feature a classical binary structure. The upper portions of 1.5-2.5 m-thick sections represent flood-plain facies. They consist of sandstones, sandy material with gravel, scarce inclusions of small pebble, interlaced by silts and humified varieties. The lower portions, which are 1.5-3.0 m thick and represent the river bed facies, consist of medium- to large-sized pebble bed including small boulders, lenses of sands and clays up to 0.2 m thick. The size of the pebbles grows from the upper portions of the section downwards. The content of pebble in the deposits is up to 50-60%, boulders - 1-5%, clays represent approximately 15%. The pebble is well-rounded. Its roundness ratio (RR) is 1.95-2.0 points. The total thickness of depositions does not exceed 5-6 m.
Considering mineralogical composition of the black sand, the deposits are classified as a garnet and amphibolic association with a small quantity of epidote, ilmenite, rutile, hematite, zirconium silicate. Gold ranges from tens up to 400-500 mg/m3.
The deposits in the second terrace above the flood-plain (Т2) are represented by flood-plain facies. Those are mostly sandy loam and loamy, argillaceous and sandy formations, frequently including humified horizons and depositions of land- and rock waste up to 10 meters thick. In the section of the second terrace above the flood-plain, near the Koltagayskoye widening, along the line of the test pits I, the deposits are represented by two facies. The flood-plain facies (approximately 1.5-2.0 m thick) is represented by clays, sandy and argillaceous formations including scarce pebble, rock waste and debris.
The river bed facies (up to 12 m thick) features pebble beds and boulders sized up to 30-50 centimeters with a sandy and argillaceous filler, interlaced by sandy and argillaceous, sandy material with scarce fine pebble up to 1.0 m thick.
Increase in size of pebble and boulders is observed downward the section represented by such stable rocks as quartzites, quartz, effusive and pyroclastic rocks. The roundness ratio (RR) of the pebble is 2.0-2.2 points.
The difference in series (thicknesses) and sections may be explained by fluctuations of hypsometric depths and unevenness of the basement. Due to these reasons, the nature and thickness of the terrace deposits may vary. Considering mineralogical composition of the black sand, the deposits are classified as a garnet-ilmenite-epidote association with a small inclusion of hornblende, sphene and zirconium silicate. The mineral resources include gold in the amount of up to 4,000-5,000 mg/m3.
According to the historical survey data, the second terrace above the flood-plain is the most productive one in terms of alluvial gold.
Upper Pleistocene-Holocene (QIII-IV). Formations of this age form the first terrace (Т1) above the flood-plain. Alluvial depositions are represented there by two facies. The first, flood-plain one, 0.5-3.0 m thick, is formed by sandy and loamy materials, which include gravel and scarce fine pebble, interlaced by silts, sand and humified horizons. The river bed (second) facies, 2.0-5.0 m thick, consists of pebble-beds and small- to medium-sized boulders, lenses of sands up to 0.5 meters thick. It is filled by sandy-gravel and argillaceous mixture. The diameter of boulders measures up to 30 centimeters, the diameter of pebble is up to 10-15 centimeters. The contents of pebble and boulders is 50-60%. The pebble is well-rounded, its RR = 1.90 points.
Its petrographic composition is diverse: effusive rocks, quartz, granitoids, quartzites, as well as occasional gneisses, diorite gneisses, sandstones and shales. It is filled with inequigranular medium-to-coarse grained sand (20%) and clay (10-15%). According to the mineralogical analysis of the black sand, the deposits can be classified as amphibolic and epidote mineralogical association with a small quantity of sphene, garnet and ilmenite. The mineral resources include gold in the amount starting from traces up to several hundred mg/m3.
Holocene (QIV). Present-day quaternary formations include the depositions of lower and higher flood-plains, the beach and water stream beds of all types. In the water channels of higher orders (VI-VIII), several levels of flood-plains can be singled out: 2.5-3.5 meters; 4.5-6.5 meters; 7-8 meters high. The higher and lower flood-plain is represented by two facies: the flood-plain one, which is 1.0-4.0 m thick, and the watercourse bed, which is 3.0-6.0 m thick. The flood-plain facies consist of silty and argillaceous, sandy-silty-argillaceous material, including lenses of sands, inclusions of scarce fine pebble, gravel, with occasional peaty loams and peat bogs. The watercourse bed facies consist of boulder and pebble material filled by sand, gravel and clay. The pebble is well-rounded, with a diameter of up to 12-15 centimeters. The maximum boulder diameter is 30 centimeters. According to the mineralogical analysis, the thin facies is classified as an amphibole – an epidote mineralogical association; amphibole – 45-65%, epidote 16-25%, garnet – 12%, sphene, apatite ≈ 6%.
The present-day depositions in the beach and watercourse beds of large water streams are represented by boulder and pebble material filled with sand, gravel and clay. The diameter of boulders measures up to 50 centimeters, the diameter of pebble is up to 15-20 centimeters. The content of pebble and boulders is 50-70%, gravel – 15-20%, argillaceous, sandy and clayey particles account for 5-10%.
Deposits in the flood-plains and watercourse beds of smaller water streams are represented by a regular alluvium. Two facies are singled out in the flood-plain depositions: a flood-plain one (1.0-3.0 m thick) and a watercourse bed one, which is up to 4.0 meters thick. A detailed description of the their features compared to deposits of larger watercourse flows in terms of their petrographic composition and the size of clastic and rounded material is given in the historic reports and in the schematic sections of drilling lines within the Amazar River area. In general, it should be noted that the watercourse bed alluvium and the terraces above the flood-plain become more concentrated and include stable rocks downstream, resulting from wear of softer rock fragments, and, probably, the increasing impact of re-washed ancient alluvial formations. Non-segmented quaternary formations include various formations of mountain slopes and interfluve space between rivers: eluvial-fluvial, deluvial, deluvial-solifluctional, colluvial mixed types thereof.
Intrusive formations in the basin of the Kadara River are represented by Paleozoic, mostly granite intrusions and occasional Late Mesozoic middle-to-major intrusions of the Middle to Late Jurassic (?) age as well as the Early Cretaceous granitoids.
Doptuganskiy intrusive complex (Odp). In terms of formation sequence, petrographic composition and relative relationship, it is divided into three independent phases. Phase one includes gabbro, gabbro-diorites, gabbronorites, subalkalic gabbro, diorites. Phase two includes granodiorites, quartz diorites. Phase three (the principal one) includes leucogranites, subalkalic leucogranites, granites, plagiogranites and dykes of granite porphyrites and plagiogranite- porphyrites.
The intrusions of the Doptuganskiy complex are chemically similar to the volcanites of the Uteninskaya suite, comagmatic to the rocks of Uteninsky complex. Their relationship in terms of space, similar composition and common structural control is clearly distinguished. Thus, we mayconsider them a single volcano-plutonic association. The formation mechanism of intrusions within the Doptuganskiy complex is not quite clear; they feature signs of both magmagene and metasomatic origin. In strongly deformed, cataclastic zones, kalifeldsparization and silification are observed. Granitoids of a less modified composition are close to regular granites or plagiogranites, but the latter account for no more than 10% of the total volume of intrusions in the complex; the remaining portion has passed a potassic metasomatism under the dislocation metamorphosis.
The Doptuganskiy complex forms the Mogochinsky massif of total area > 60 square kilometers located within the described area. The massif is partially included in the boundaries of the licensed area; its main part is located to the south. Primary relief of the massif is of erosive-denudation type with crested spurs.
The massif also features boulder streams and slides, which are highlighted in a lighter photo shading, the area of their presence exceeds the massif size.
According to the results of quantitative analysis of physical fields, the massif represents a thin plate. A higher level of gravity field almost completely coincides with the central part of the massif, which leads us to an assumption that the central portion of the massif has a more basic composition (closer to granodiorite). The central portion also features a lower radio activity as compared to the marginal zones. Granitoids of the massif indicate a higher radioactivity (up to 30 micro roentgen/h, the content of thorium is up to 30-35*10-4%, uranium - up to 6-7*10-4%, potassium - up to 4.5%). Such level of radio activity is characteristic of subalkaline granitoids, which is confirmed by its petrochemical parameters.
The massif is framed by a zone of alternating magnetic field up to 50-100 nT. This zone includes both exo- and endo-contact and is related to hornfelsing, and there are many bodies of gabbroids of phase I in the marginal part of the massif. The massif is multi-phase. Gabbroids of phase I are developed insignificantly, the breadth of outcrops measures several meters, rarely dozens of meters. Gabbro is mostly cataclastic, greenstone transformed, and amphibolized. Very scarce are the relics of slightly transformed massive gabbro with remaining diopside, hypersthene, olivine. Gabbroids are hornfelsed at the contact with granitoids of phase III.
Normal hybridism is a typical feature of the Mogochinsky massif: the assimilation of gabbro-diorites and diorites by acid magma produces specific hybrid ocellar rocks, which are close to quartziferous diorites and granodiorites by composition. The ocelli are made of bluish quartz rimmed by amphibole, sized 2-5 mm. Such hybrid rocks occur in endo-contacts of large xenolith diorites and gabbro-diorites in the central part of Mogochinsky massif.
Multiple facies are typical of the phase III granitoids of the massif, manifesting itself by a change of composition thereof – subalkaline leucogranites, granites as well as in their structural and textural specifics. Massive and porphyroblastic varieties are prominent, ranging from a fine- to coarse-grained structure; another typical feature is a contrast color ranging from rose-gray to dark-gray facies. Phase III granitoids of are very cataclastic and schistose. Massive, non-cataclastic granitoids are not encountered. We observed porphyroblastic granitoids and newly-formed “granite-porphyric rocks” (felsic- and fluidal-like), resulting from tectonic dislocations with subsequent metasomatic transformations. Areas of the most intensive potassium metasomatosis are distinctly marked by the areal and local anomalies of potassic and uranium nature.
Atypical (dark-gray or anorthite) color of granites is due to two reasons:
Petrographic study of the complex rock is complicated due to their location in the areas of intensive dislocations. The geochemical specialty of Doptuganskiy complex granitoids is characterized by a higher content of tungsten (W), which is 10 to 15 times higher compared to the content of clarke and 4-7 times higher compared to the content of molybdenum; and the lower content of vanadium, chrome, lithium.
The general geochemical specialization of the Mogochinsky massif in tin, expressed by numerous tin ore aureoles, which coincide with the contours of the massif (as well as other granite intrusions in the same complex), does not match the content of tin in assays: 220 assays feature tin content which is lower than or equal to the level of clarke – 2.2-2.3*10-4. Among the factorial associations, only one is relatively stable – yttrium-ytterbium-lanthanum-cerium-beryllium.
Besides the Mogochinsky massif, the small linear areal intrusions (0.5-3 sq. km) are observed within the described area, including the basin of the Verkh-Koltagaichik creek, on the right bank of the Kadara River, in the Shilkinskaya zone of dislocational dynamo-metamorphism. There, prevailing rocks include cataclasites, blastomylonites of leucogranites, with occasional plagio-granites, granodiorites and gabbro. Granitoids are identified by the local gamma field anomalies, the principal rocks of phase I – by the areal peaks of magnetic field up to 2250 nT. Similar to the Mogochinsky massif, they feature a highly-flattened shape, which is also confirmed by geological observations. The principal rocks, from quartziferous diorites to pyroxenites, are mapped in the marginal parts of the small bodies of leucogranites.
The Doptuganskiy intrusive complex belongs to a gabbro-diorite-granite formation. Its petrographic composition depends on both the composition of replaced rocks and the extremely variable proportions between infiltration and diffusion processes during the magmatic replacement. The replacement of host effusive-sedimentary rocks is most clearly identified near the small linear intrusions in the zone of most intensive dislocations; blurred contacts of undoubtedly magmatic formations are identified with the rocks of exo-contact zones, in each specific case (subject to the composition of the replaced rocks), with their own characteristic set of newly-formed minerals – quartz, feldspars, amphibole, biotite, typical granoblastic structures. Farther from the contact, the rocks only undergo recrystallization and are transformed into typical fornfelses; migmatite-like veins (streaks) are scarce. Hornfelses, hornfels knotty shales form a rather wide (up to 500 m) fields within the exo-contacts of intrusions in the Doptuganskiy complex. Less transformed hornfelses are preserved in the southern marginal part of the Mogochinsky massif. Calc-silicate hornfelses occur frequently and typically include garnet, andalusite, calc spar (calcite), pyroxene, epidote, actinolite, sericite and quartz.
Potassium-argon datings of granitoids and dioritoids of the Mogochinsky massif stay within the range of 340-140 million years. A younger age reflects the Late Mesozoic phase of the area activation stage. According to the geological criteria, the massif age is Ordovician.
Presence of Late Paleozoic granite intrusions within the studied area is doubtful. We believe that the Late Paleozoic granites mapped by earlier surveys and based on radiometric datings could be a product of rejuvenation of the Ordovician granites described above.
Middle to Late Jurassic intrusive formations comprise middle to principal intrusions, ranging from subalkaline gabbro to quartziferous monzodiorites. No exposure of the rocks of this complex has been recorded within the license area during the geological survey, but the geophysical operations identified a chain of hidden hearth structures, probably, deposited above the intrusions of this complex or the Tymagerskiy complex (area of the Verkh-Koltagaichik Creek, the Orachenka Creek). The exposure of the Middle to Late Jurassic intrusions nearest to the license area is found within the interfluve area of Utena, Kavykhta, and Buley rivers. The exposure is small (less than 0.05-0.2 sq. km) and coincides with the exposure of granitoids of the Tymagerskiy complex (K1t) and is, possibly, genetically related to it. Two injection phases of J2-3 intrusions are singled out. Phase one includes subalkaline gabbro, gabbrodiorites, gabbro-diabases. Phase two includes quartziferous diorites, diorite-porphyrites, and quartziferous monzodiorites. Dioritoids of age J2-3 are pierced and hornfelsed by the granitoids of the Tymagerskiy complex. Composition of hornfelses is as follows: zoisite-sericite-biotite-quartz-cordierite, andalusite-biotite-sericite-muscovite-quartz.
Post-magmatic transformations of dioritoids are related to the formation of intrusions within the Tymagerskiy complex and feature greysening. The greysened rocks contain substantial amounts of muscovite-biotite-quartz. The rocks of age J2-3(?) are characterized by alkalinity close to normal, and are rich in potassium. The geochemical specifics of quartziferous diorite-porphyries were studied in the Kovykhtinskiy section. They feature a higher content of molybdenum (two times higher compared to clarke), cobalt, and a twice lower content of nickel and manganese. Tungsten (W), silver, boron, phosphorus is not detected. The following associations (0.8-0.9 unit) are identified in the diagram of factorial associations: lead (Pb)-chrome, yttrium-ytterbium, cobalt-nickel. The position of J2-3(?) intrusions is controlled by the faults, which are apparently fractured and hypabissal. Smaller exposures, widespread development of hornfelses, often with no signs of outcropping intrusions, indicate a small erosive shear of exposures. The rocks belong to a gabbro-diorite-granodiorite formation. Their metallogenic specialization remains unclear.
Tymagerskiy complex (K1t). The complex comprises granites, subalkaline granites, subalkaline leucogranites, granodiorites, granitoids of the endocontactfacies – aplites, pegmatoid granites, rhyolites, granite-porphyries, diorite porphyrites, microdiorites, diabases, lamporphyres, veins of pegmatites, necks of explosive, eruptive breccia, hydro-thermalites and quartz veins. The major massifs include the Polosatikskiy and Tymagerskiy massifs located outside Sheets Nos. 87-Б, 76-В, 88-А.
Radiometric age of the granite-porphyries on the right bank of the Bulei River age (evaluated by K/Ar method using the bulk samples) is 135 million years (ref. Stetsyuk, 1977f), granites of Polosatikskiy massif – 110-107 million years, granite-porphyries deposited near by the massif – 125 million years (ref. Stetsyuk, 1977f). The absolute age of the Tymagerskiy intrusion is 122 million years.
The Tymagerskiy complex intrusions feature small areas of exposure (less than 0.5 km 2, in case of the exposures within the basins of the Utena River and the Bulei River), but much larger (up to 2-5 km 2) areas of hornfelsed rocks. Extensive fields of hornfelses without a sign of outcropping intrusions occur quite frequently. Thus, presence of large multi-phase (?) intrusions uncovered by erosion can be assumed, which is indirectly confirmed by the specifics of physical fields (including those in the licensed area). Such intrusions are clearly identified by local, areal, linear-radial zones of weak (70-150 Nt) magnetic field and gamma-field peaks.
Hydrothermal-metasomatic changes related to the final formation phases of the Tymagerskiy IC (for example, the manifestations within the Kovykhtinskiy complex outcrops), are manifested in greysening, silification, formation of secondary fine-grained quartzites in the carboniferous rocks, and formation of quartz lodes including gold-antimony-silver mineralization.
The dykes of the Tymagerskiy IC include diorite porphyries, microdiorites, lamporphyres (spessartites), and propylitized diorite porphyrites. Their distribution is uneven, and their concentration significantly increases near non-exposed large massifs (within the interfluve area of the Utena and Utenikan rivers, etc.). The dyke thickness is insignificant (0.1-10 m), their length ranges from several meters up to (occasionally) dozens of meters. These are fine-grained rocks of grayish-green and dark green color.
Explosive breccia are rocks of mostly dark gray color featuring a coarse-detrital breccia structure. The composition of debris is diverse, the debris of terrigenous rocks prevail. The saturation by debris is up to 90%, their size ranges within 0.3-4 cm. They are cemented by de-crystallized sour glass with a micro-felsitic structure, which is replaced by secondary minerals. The explosive breccia form isometric steeply dipping necks with an area of up to 50-120 m 2.
Fracturing changes of ryolitoids along the large preexistent faults (for example, the Uteninsky fault) are usually accompanied by wide (hundreds of meters) fields of hornfelsed rocks. The fields and ore manifestations of gold and antimony as well as gold and silver formations (Kholodzhikan, Davan, Kadara, Kavykhta, Andreevskoye, Bagadzha) are paragenetically related to the Tymagerskiy IC. The granitoids of the Tymagerskiy complex feature a higher alkalinity, which confirms their genetic affinity to the Middle to Late Jurassic (?) principal intrusions. The rocks of this complex belong to a granite-leucogranite formation.
The metallogenic specialization of the Tymagerskiy complex is gold. The granite-porphyries feature a higher content of tungsten (W)– 10 times, molybdenum – 5 times of clarke content. Tin, silver, cobalt, lithium, phosphorus, scandium were not found. The following strong correlations are identified: gallium-niobium-beryllium; yttrium-ytterbium-zirconium; manganese-titanium-vanadium-nickel-zinc.
The reported area is a part of the Amuro-Argunskaya structural and facies zone. This zone is an area of the Mid-Paleozoic and Mesozoic folding, where depositions of Mid-Paleozoic (Silurian-lower carboniferous), Triassic and Jurassic ages are widespread.
In terms of tectonics, the described area is a chain of the Mongolo-Okhotskiy folding belt. It lies in the junction area of the Trans-Baikal and Far-Eastern branches of this belt. The territory of the operations includes two distinct structural zones, different in terms of geological structure and development history, i.e. the Northern Yankanskaya and the Southern Amuro-Argunskaya areas. Their boundary stretches to the east and north-east from the upper reaches of the B. Davan River to the confluence of the Utena and Taganka rivers, and farther on – in the latitudinal direction along the valley of the Ogon River.
Two structural stages are singled out within the territory, i.e. the Mid-Paleozoic and the Mesozoic stages, featuring different composition of rocks, intensity and type of folding.
The lower Mid-Paleozoic structural stage is composed of Silurian, Devonian and Lower Carboniferous deposits, brought into a series of folds striking in sublatitudinal and north-east directions. The Uteno-Amazarskaya anticline is the largest one. The upper bend (crest) of this anticline can be traced from the Mogocha River to the Utena River in north-east direction and further on to the east, in the sublatitudinal direction, along the watershed divide of the Ogon and Kovykhta rivers. The core of this anticline is the rocks of Silurian age, and its legs consist of the Lower and the Middle Devonian depositions. Along with change in direction of the Uteno-Amazarskaya anticline axis, its crest dips in the south-west direction. This anticline is a normal fold within the Amazar river basin, featuring a more gently sloping to the north-west direction (the dipping angle of 40˚) and a somewhat steeper south-eastern leg (the average dipping angle of beds is 60˚). The southern leg of the anticline, located in the eastern part of reported territory, bounds on the Kovykhtinskiy fault; the northern leg of this fold is complicated by the Ogonskiy fault. The Utenkanskaya syncline is another large structure complicated by the Mid-Paleozoic depositions. The crest of this fold is stretched in the north-east and sublatitudinal direction, from the Lower Yapan creek and the Mid. Mangaley creek to the upper reaches of the Galgakan river in the east. The legs of this syncline are dipping towards each other at the angles of 40-60˚ and consist of the Middle Devonian rocks; the core consists of the Famennian and Tournaisian depositions. The crest of the Utenkanskaya syncline rises in the south-west direction. Besides the two Paleozoic structures described above, several smaller anticlines and syncline folds are observed within the Amuro-Argunskaya structural-facies zone. The folds are stretched in the north-east direction. In the eastern part of the region, they are replaced by a sublatitudinal symmetric structure with a somewhat gentler north-western sloping and a steeper south-eastern leg.
The Mesozoic structural belt comprises Triassic, Jurassic and Cretaceous rocks. The Triassic depositions, which form an independent substage, are widely spread in the northern part of the Amuro-Argunskaya structural-facies zone; they are not thick, and mainly form gently sloping folds, lying over the Paleozoic deposits. No Triassic deposits are found in the southern part of the Amuro-Argunskaya zone, where the rocks of the Jurassic age lie over the Paleozoic deposits.
The rocks of the Jurassic substage are widely spread in the southern part, depicted in Sheet No. N-51-XX. The Amazarskaya syncline, composed of lower Middle Jurassic seabed deposits is traced in the south-eastern part, along the Amur River. The described structure is a syncline fold overturned southwards. The legs of this syncline include the terrigenous rocks of Lias, and the core of the fold consists of shale beds of the Middle Jurassic age. The Jurassic depositions of the left bank of the Amazar River, which form the northern leg of the Amazar syncline, are in overturned bedding. Due to this fact, they feature a pseudomono-clinal dipping at the angles of 40-70˚ in the northern points of compass. The northern leg of the Amazarskaya syncline is cut by a large fault on the right bank of the Amazar River. The crest of this syncline gradually upheaves westwards; part of the Amazarskaya syncline, located within the Shilka river basin, consists of the Liassic deposits underlaid by Paleozoic deposits.
The Yankanskaya zone consists of large granitoid massifs of the Proterozoic, Paleozoic and Mesozoic ages, with deposits of Proterozoic metamorphic rocks in the form of xenoliths and relics in the roof. Considering the components Proterozoic deposits, the relics of a syncline structure are traced within the western part of the region. The core of this structure consists of the Motovinskaya suite deposits, and the legs – of the Arbinskaya suite deposits. The axis of this fold strikes in sublatitudinal direction within in the western part of the area, and in north-east direction – within the eastern part. The syncline also includes the second order folds and the two faults striking in latitudinal direction, which form a graben-type structure.
Spatial and statistical analysis provides a highly-complicated picture of folded formations within the Eastern Trans-Baikal territory. The parameters of folding feature a considerable dispersion. Still, the distribution of epicentral points turns out to be well-ordered: the folding structures distinctly run along the two mutually perpendicular branches: north-eastern (45-65°) and north-western (315-335°). The orthographic directions are highly subordinate. The Pre-Cambrian structural-substance complexes are the most deformed ones, while the axial surfaces of some folds are dipping south-eastwards, and others – north-westwards. Folds in form of folding zones are traced within the different blocks made of metamorphic AR and PR rocks, as if continuing each other. Some folding systems smoothly bend and stretch in either sublatitudinal or submeridional direction. Such deviations are accompanied by undulations of the crests, periclinal closure of the anticlinal structures, and flexures.
The Jegdochinskaya syncline is traced within the south-western part of territory, depicted in Sheet No. N-51-XX; it also consists of the Lower and the Middle Jurassic deposits. The crest of this structure rises in eastward direction; thus, the Middle Paleozoic depositions are exposed within the valley of the Shilka River. The fold strikes in sublatitudinal direction, and both its legs are bound on the faults.
The third and uppermost substage of the Mesozoic structural stage consists of the Cretaceous effusive rocks. These formations are located within the territory depicted in Sheet No. N-51-XX, and confined to the regional faults while lying over every sedimentary and magmatic Paleozoic rock. They feature gently dipping angles.
The South-Yankanskaya fault zone is critical for the formation of structures within the territory depicted in Sheet No. N-51-XX. It represents a chain of the Mongolo-Okhotskiy fault, which strikes along the northern boundary of the Mongolo-Okhotskiy folding belt for several thousand kilometers. The South-Yankanskaya fault zone is traced from west to east within the territory of the survey, depicted in Sheet No. N-51-XX. It first stretches in the sublatitudinal direction, and then turns east-north-eastwards. It splits within the Utena river basin. One branch of the faults in the form of a wide band of cataclasis is traced north-eastwards to the Yerofey Pavlovich station. Another branch stretches from the valley of the Utena River eastwards, in the latitudinal direction, along the Ogon River. Rocks within the South-Yankanskaya fault area are materially changed, transformed into cataclasites and mylonites, and sedimentary rocks are metamorphized into quartz-cericitic and chlorite-epidote shales. Width of the South-Yankanskaya fault zone ranges from 5 to 18-20 km.
A fragment of another deep-seated fault is located in the marginal southern part of the surveyed territory. It strikes in the sublatitudinal direction, along the valley of the Argun and Amur rivers. The zone of abyssal fault is represented by a band of intensively metamorphized sedimentary rocks of the Middle Paleozoic age, transformed into tremolite, hornblende-quartz-tremolite, hornblende-quartz-clinobasic, sericitic and epidote-chlorite rocks. The width of this band of intensively metamorphized rocks exceeds 5-6 km.
Thrusting structures play an important part in geology and minerageny of the area. The thrust surface of regional scale is confined to the «mid-band» of the Upper Kholodzhikanskaya subsuite (J3-K1hl22). This band is of a dark color, as the siltstones saturated by coaly material prevail in its composition. Due to subduction processes (sinking of the oceanic crust of the Pre-Pacific Ocean from South-West under the continental crust of the ancient North-Western Asia), subhorizontal stresses directed to the south-west emerged at the upper level of sedimentary mantle. These stresses caused a tectonic detachment fault at the level of «mid-band» of the Upper Kholodzhikanskaya subsuite (J3-K1hl22) made of plastic rocks, and a series of scaly thrusts occurred at the level of the overlying series. The chaotic stratum emerged in the base of these thrusts, which constitutes a tectonic melange containing debris of nearly all kinds of rock occurring within the thrust base, in different proportions.
The «mid-band» of the Upper Kholodzhikanskaya subsuite and the chaotic stratum (J3-K1hl) are quite different from the underlying rocks in terms of mineral composition and internal structure. Besides, both formations are saturated by coaly material. Thus, the “mid-band” of the Upper Kholodzhikanskaya subsuite and the chaotic stratum perform as a horizontal geochemical baffle, where conditions of vertical migration and chemical parameters of ore-bearing solutions were drastically changed. As a result, after reaching these strata, the solutions got saturated by ore minerals, and extraction of the minerals took place.
The structures and dislocations striking in north-eastern, sublatitudinal (near east-west) and north-western directions were the major factor of modern tectonic movements, which lead to formation of the current relief traits. The surveyed territory is located at the juncture of three morphostructural areas: Prishilkinskaya zone of small domes, Priargunskaya zone of horst-anticlinal ridges and the area of clumpy structures within the Upper Amur river region. It is deemed that portion of the Southern wall of Tungiro-Amazarskiy dome is located within the operating area, which is one of the largest and complicated domes of Prishilkinskaya zone in terms of structure. A portion of the right bank area of the Amazar river belongs to the Amazar-Shilkinskiy anticlinorium, which is a part of Priargunskaya zone of horst-anticlinal ridges of the Mongolo-Okhotskiy belt.
Detailed de-coding of the linear structures using the space photographs scaled to 1:2 000 000 and more detailed data of the Earth remote sensing along the sculptural relief forms, enabled to determine the key tensions (stresses), which influence the Earth surface block within the surveyed territory. Analysis of such tensions of the surveyed surface reveals presence of a reverse fault striking north-westward, at vertical amplitude of 75m, as well as a system of joint fissures striking north-eastwards. Computer simulations show that such joint set controls the location of gold metallization.
The de-coded elements enable to segregate the identified zones based on their common regional forms of relief. The latter were formed by action of regional tensions. The key compressing tension (σ1) strikes north-westwards and south-eastwards, and is defined by the tension tensors of such tensions forming the arched relief forms. The key stretching tension (σ2) strikes north-eastwards or south-westwards, the vectors of such tensions have a displacement component. According to Gzovskiy, the action of such tensions results in formation of the linear zones of gentle reverse faults and thrusts. The systems of associated diagonal strike-slips smoothing out towards the stressed contours are formed by longitudinal compression or bending. The echelon systems of joints strike diagonally. The separate reverse faulting displacements and their systems sloping at the angle of 45° occur along the linear areas flattening out as the curvature of bends increases.
The area is located at the joint of the Stanovoy folded and Mongolo-Okhotsk geosynclinical areas within the Kholodzhikan-Ilichinskaya ore zone. The deposits within the ore district are as follows:
The spatial and origin features of the industrial gold content are based on a basalt and rhyolitic formation of the cretaceous rock, which consists of effusions of the Talda rock formation with homodromous features.
The deposits’ environment is affected by its location within the lower part of the Kadara river valley slope, the lay of the land and the structural behavior of the bedding rock. The land within the deposit area is poorly broken. The bedding rock includes allochton cuesta-maker volcanic and terrigeneous rocks, coal shale and limestone. The autochthon includes graphitic shale of the Jurassic-Cretaceous age. The autochthon and the allochton are separated by a mass of tectonic melange originated from silty clay deposits of the Kholodzhikan suite; the melange includes rock debris of different composition and age. The melange has a relatively low strength compared to the material above and under the profile. The melange (chaotic stratum) is linked with primary features of the gold mineralization.
From November 2006 (after obtaining the licence) to March 2007, the Company collected, interpreted and analyzed the history of the licensed area and adjacent territories.
Such materials had been prepared based on work results of regional mining parties from 1970 to 2001 and included data on geological, survey and prospecting works, airborne geophysical survey of 1:25000 scale, geological mapping of 1:50000 scale, miscellaneous mining works and drilling results for individual wells.
In its interpretation and analysis, the Company followed traditional practices, as well as modern ones, such as a remote ground probing from space, geological information systems and other.
The ore-hosting section includes siltstones, sandstones, pudding rocks, unsorted arenas on a siltstone bed and conglomerates. The material age is ranging within O 1 - J 3.
The gold is mainly concentrated in packs of unsorted coal siltstones and arenas.
The materials in the ore-hosting mass are altered due to the hydrothermal effect – they are sericitized, kaolinized, carbonized, hematitized and epidotized. Sulphidic mineralization includes brassil, arsenic iron, galena, false galena and yellow copper. The morphological type of the ore is stringer porphyry.
The ore outline strikes in north-east and east-west direction, with decline in north-west direction at the angle of 5-25°. The major gold concentrations are linked with intersection of thrust slices and transverse the north-west faults. Thickness of the mineralized areas with scattered gold that contain ores of industrial significance ranges from 10 to 50 m. Length of the mineralized areas is the first few kilometers.
The top-cut of gold in the extracted ores is 15-42 g/t. The minimum production content is 2 g/t. The content based on which a primary outline of the ore area was made is 0.05 g/t.
Three ore areas with an average thickness of 20 m and length of more than 800 m have been outlined within the surveyed field by drilling down to 150 m.
Interpretation of aerospace photographs of the surveyed area led us to a conclusion on new data in the structural and tectonic structure of the area and allowed us to outline a unified development structure of the industrial gold and sulphide mineralization.
The obtained data allowed us to presume that a primary gold deposit of volcanic and hydrothermal type with a maximum estimated resource of 300 tons of gold and an average gold content in ores of 2-5 g per ton can be discovered within the area and the adjacent areas.
The Company’s licensed area covers the central and north-eastern part of the field. The probable resource of the licensed area was estimated at 170 tones at the time.
The quantitative resource evaluation per area and the conception of the size of the estimated fields, mineral composition and ore quality is based on a set of direct and indirect ore content indicators, materials of individual ore intercepts and data of the similar gold fields of the same formation type, discovered previously.
In early 2007, based on the history material interpretation, a model of the mining area of 12 km length was made, the total gold resources of the area under Category Р2 were estimated to be 177 t or 5.7 million oz troy. A prospecting and evaluating work project was developed for 2007-2010, which later, on July 19, 2007, was approved by the Trans-Baikal territorial branch of the Federal Agency for Mineral Resources (“Zabaykalnedra”).
Six most promising areas for ore discovery were outlined in the project for prospecting and evaluating works:
North-Eastern flank of the Mogochinsky Massif - 43 tons
Since Q2, 2007, the Company has been conducting geological survey works of the prospecting and evaluation stage within the two former areas. Within 2008-2016, a set of geological survey works was conducted within the licensed area, which determined the feature of the geological composition of the area and the pattern of primary gold field formation.
The geological survey works included geological field observation, surface trenching driving, drilling of prospecting wells along with a set of geological surveys works, analytical research and a unified analysis of the obtained data.
Results of the geological observation allowed to specify the geological structure of the area and showed that the primary feature of the geological structure is the presence of an allochton complex built of volcanic and volcanic-sedimentary rocks of the Silurian age (Uteninskaya and Omutninskaya suites) overthrust over younger (Jurassic Age) autochthon material. At the base of the allochton lie Malm graphitic slates (Kholodzhikanskaya suite), which contributed to the allochton movement and were transformed into melange, which contains debris of underlying and overlying strata of different compositions. The allochton complex is complicated by several overthrust faults.
|№||Work description||Unit of measurement||Quantity|
|3||Drilling operations||linear meter||1,705|
|4||Selection of channel samples||linear meter||5,400|
|5||Selection of core samples||pcs||826|
|6||Well core logging||linear meter||1,526|
|7||Mining works logging||linear meter||2 675|
|8||Processing of core samples||sample||826|
|9||Processing of channel samples||sample||4,128|
Table 9. Scope of Geological Survey Works in 2007-2016.
The ore mineralization development zone has a thickness of over 100 meters (its lower part could not be opened by drilling), it has been tracked for over 8 km from west to east with a visible width of openings ranging from 100 to 1,500 m, and goes down towards north-east with an incidence angle ranging from 5° to 25°, the incidence angle increases towards the thrust front. The ore zone is characterized by superimposed inclusions of metasomatically altered materials in graphite slates with a low-grade sulphide mineralization, split quartz veins of gold, sulphide and quartz. The gold content in the veins is up to 30 g/t.
The most probable areas of gold ore mineralization were discovered using the special methods of remote material analysis. These areas feature an explicit low-relief close-trend faults and geochemical anomalies of gold, silver, lead and other ore minerals.
A drilling conducted with a grid of 500*250 m confirmed availability of ore materials with gold content 100 m down from the surface.
The analysis revealed that all channel samples had some gold content. Gold content ranges from 0.1 g/ton to 15 g/ton. Bulk sampling showed the content fluctuations from 8 to 230 g/ton in heavy residue. The average gold content in the remnant ore is from 3 g/ton, with the remnant ore thickness of 25 m.
Thus, the developed geological model for localization of gold premineralization in the melange is validated based on results of the field works, and gold content is confirmed in the allocated prospecting areas. A sampling plan was prepared, which confirmed the content.
Completion of geological survey works in the licensed area will increase the prospective deposits of the category С1 + С2 and resources of the category Р1 will total to:
The work area is shown in the Sheet No. N-51-XX of map scaled to 1:200 000. This area has a developed state geodesic network (triangulation net) of the classes I-III (5 locations) and leveling of the class III-IV (4 locations). All locations are in a satisfactory condition. For all planned geochemical and geophysical works, drilling, trenching, and open pit backfilling, a land map of the licensed area was drawn, to the scale 1:10 000. The map comprises 6 sheets. The map limits match the boundaries of the licensed area at a side, and are limited by the lines along latitude 53о40″, longitude 121о25′’, 121о 0′’, 121о 35’′ at the other side.
Between the triangulation points, three progressions were made and based on their recording stations, the geodesic structure of the map was drawn up. For vertical and horizontal positioning of the geological survey objects, а class IV level run was made, which allowed fixing 6 leveling markers in the projected work area. First mapping topographic works were done by field survey of the licensed area along 11 routes using the corresponding survey equipment. The field survey area is 186 sq. km.
Transit GeoBox OT-05, optical leveling equipment Setl AL-20 and nos. 2 tacheometers Trimble M3 (5") were used for geodesic works. Land surveying works were done with GPS receivers Torcon GRS-1.
The data obtained by geological survey of the Kadara field allows us to classify the field type as a plutonic-volcanic-gold-adularia-quartz ore formation. The gold content in ore minerals ranges within 3-10 g/ton and in rare cases goes up to 30-40 g/ton. Ore materials include flat-lying stockwork-like area with thickness of 1.0-30 m and thrust seams. The open-cut method is proposed for this type of fields. The ore is referred to the free-milling technological type. The field capacity is 177 tons.
The geological survey data allow outlining a mineralized zone with length over 5 km developed in graphite slates and accompanied with quartz stockwork. The ore zone is developed within the tectonic contact area between terrigeneous-sedimentary roughly broken materials of Kholodzhikanskaya suite (approx. Lower Jurassic period) and metasomatically altered materials (secondary quartzites) of the Omutninskaya suite (approx. Ordovician period). The graphite slates observed in the contact area by different surveyors, were referred to different ages.
The industrial gold mineralization area was outlined based on geological and geophysical observations. Prospecting works were focused on 3 previously selected test sites, where signs of industrial ore mineralization were found during reconnaissance operations. Land geological observations in such areas showed alteration of the original composition of material, appearance of hydrothermal minerals, including quartz and yellow earth. The major signs of gold ore mineralization in the region are presence of graphite in bed rocks, tectonic dislocation of the material and position of a regional strip overthrust on graphite slates of the Kholodzhikanskaya suite, as well as a so-called chaotic stratum – melange in the foundation of thrusts and overthrusts.
Exploration mines (trenches) were made to open ore materials, determine their properties, boundaries, mode of occurrence, gold content and other properties of economic interest. Exploration mines were made in test sites where discovery of industrial-scale gold ore mineralization was expected based on geophysical, geochemical and geological criteria.
The exploration mine system in each test site included trenches outlining the test site, trenches running along longitudes and latitudes of geological structures and ore materials; and check trenches with a 45° angle towards the geological structures.
According to the plan of operations, drilling of prospecting wells was done along with core sample collection and subsequent pipe casing. Distance between the wells within the detailed works areas was within 250-1,500 m. For quick ground (talus, residual deposits, alluvia) drilling was done by longwall face without core selection; parent rocks were drilled by core drilling method with whole core samples. Design well depth was 100 m. The actual well structure was very similar to the design one. Recovery of core for parent rocks was above 95%, which is enough and satisfactory for correct deposit estimation.
The primary method of on-site material sampling was channel sampling. Samples were selected manually from the bottom and walls of the trenches. Each trench was divided into sections, each of 1 meter long. The sections were numbered – the number of section and samples corresponded to the trench number and their distance from the start of the trench.
Channel samples from a trench bottom were selected along a continuous channel stretching in the same direction with the channel, after the bottom was cleaned of contamination and falling material. As the ore bearing material is represented by graphite slates threaded with fine quartz streaks preventing from running a regular channel with section of 3×5 cm, the channel section was increased up to 10×15 cm. All channel material was collected from every meter of the trench individually, and was given a number based on the trench number and the meter where it was collected. The weight of samples from each meter of a trench was 25 to 35 kg. The sampling channels on the trench walls were vertical, their length was 2 m, and the section dimensions were 10×15 cm. The weights of such samples ranged from 50 to 80 kg. QC samples from trench walls were selected from each fifth meter of the trench.
After selection and drying, the channel samples were milled in 2 stages: first stage – down to ≤1 cm size; second stage – down to ≤1 mm size. After milling, the samples were reduced by quartering – after first milling – down to 5-6 kg; after the second one – down to 1-1.5 kg. The material separated during the channel sample reduction, of, supposedly remnant ore, was collected during the first stage and whole samples were formed out of it. During the last stage of the second quartering, two opposite fractions of a sample were united as a sample for analysis. And the other two held as a duplicate for storage.
The reduction was made according to the standard method, by quartering.
The reduced samples were milled down to 0.07 mm, and then were bolted through a grid of 100 µm. The material remaining on the grid and subsamples, which have passed through the grid, were subjected to a split spectrum analysis for gold, silver and associated elements and to an assay test for gold and silver. It is considered that the material that stayed on the grid describes the free gold content in the sample; and the material that has passed the grid – the content of fine gold in the sample and the material, as well as gold included into minerals as a foreign matter.
After obtaining the analysis results, separately for the material, which passed the grid and failed to pass the grid, they were recalculated according to a proportion to get the total gold content in the material as the result.
An assay test for gold and silver was performed in a laboratory according to the following procedure:
A quantity of subject (50 –100 g) was mixed with a feed composed of: a collector (РbО), fluxes (sodium borate and other), reducing agent (charcoal), and oxidants in some cases (Рb3О4, KNO3 etc.). The compositions and ratios of the feed components were based on the compositions of the analyzed material. The obtained mixture was melted in a refractory pot at 1000-1150°С. During this process, РbО was reverted to Рb, and a lead alloy with precious metals was formed, foreign inclusions were transformed into slag. The liquid alloy (bullion) was poured into casting forms and separated from slag after cooling down. Along with РbО, other metal oxides were partly reverted, too. In relation to the site, these are mostly copper, antimony and tin that interferes with further analysis.
The bullions were purified with a scorifying melting, which is an oxidizing and dissolving melting at 900-1050 °С in a scorifying dish – a shallow dish made of fire-resistant clay of 50-75 mm in diameter. The feed includes metal lead and sodium borate. During the melting, the inclusions were oxidized and transformed into slag. The oxidants were air coming through the open door of the furnace and oxygen formed during РbО disintegration. As with the crucible melting, the bullion was separated from slag and cupelled to disengage precious metals: it was subjected to an oxidation melting at 850-900 °С in porous shallow dishes (cupels) with 40-60 mm diameter, made of bone powder, magnesite or concrete. During the process, Pb was oxidized to РbО, and, for the most part (up to 98.5%), absorbed by the porous mass of the cupel and the rest was vaporized. The lead oxide oxidized the inclusion metals, dissolved their oxides and was absorbed into the cupel along with them.
Gold and silver alloy (dore) stayed on the cupel. This dore was weighted on an assay balance. Then the silver was dissolved in dilute HNO3; the remaining gold bead was flushed out, annealed and weighted. Quantitative separation occurred at maximum Ag to АAu ratio of 3:1; unless fine gold particles were lost. The weight of silver was determined as the difference of the dore and pure gold. The detection threshold of 0.1 g/t was limited by sensitivity of the assay balance (0.01 mg) and the material subsamples. The lower threshold of determined content was usually 1 g/t for gold and 3 g/t for silver.
The spectrum analysis (atomic emission spectrum analysis) was made in a laboratory according to the following procedure:
The sample milled in two stages down to 1 mm, was reduced by quartering down to 15 – 20 g. During the final quartering stage, the sample was divided into the analyzed and the test samples. The analyzed sample was milled down to 0.07 mm, and the test one was sent for storage.
The analyzed sample was mixed with graphite and then this mixture was infused into a radiating unit (AC electric arc), where the sample was vaporized at over 4000ºК, dissociated and atoms and ions were agitated.
The exited atoms and ions were resolved into a spectrum, where presence of lines specific of chemical elements were discovered, and intensity of these lines (by comparison with the reference sample) showed concentration of elements in the sample.
Evaluation error of the gold content in the ore was determined by statistical theory method. An arithmetical mean of gold content was calculated for each excavation and well and for each test site area selected for detailed survey based on analysis of geological, geochemical and geophysical data. The variance, mean square deviation, variance factor, ratio of asymmetry value to its error, kurtosis value ration to its error and mean deviation were calculated as well.