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Geological Setting
To date, within the area of the Avlayakan Project two gold deposits (Avlayakan and Kirankan) have been identified.
The Avlayakan Deposit
The Avlayakan deposit contains the following zones: Central, Northeastern, Southwestern I and Southwestern II. Most of the prospecting and surveying on the deposit carried out by Vostok focused on exploring the Northeastern mineralized zone with the remaining area evaluated by a wide-spaced grid of trenching and by sporadic drill holes.
The Central zone is located in the central part of the Avlayakan deposit and is represented by an extensive bedding plane from 5 m to 40 m wide filled by contiguous quartz, quartz carbonate veins, and zones of pervasive and pervasive metasomatic silicification within a halo of hydrothermally altered rocks. The Central zone strikes northwest and dips to the northeast at 60º to 80º.
The mineralized channels range in width from 0.1 to 10 m and frequently have lenses which branch out, and, less frequently, alternate with veins along strike. The contacts with the host rocks are sharp, steep, sometimes complicated by apophyses. Host andesite, its tuffs and tuffs of dacite are intensely propylitized, while rhyodacite and its tuffs underwent silicon potassic metasomatosis.
The Central zone is broken into blocks of various lengths by near-north south tectonic dislocations. Faults are filled either by shear zones and zones of mylonitization or by dykes of varying composition. The faults are largely throw and strike-slip types with horizontal offsets ranging between 0.5 m and 30 m. Some of the faults may be interpreted as synmineral with subsequent rejuvenations occurring during the deposition of the mineralization.
The basic mineral for the vein formations and metasomatites is quartz, with calcite, sericite, chalcedony, hydromica, chlorite, amphiboles, adular, fluorite and kaolinite being less common. Potentially economic minerals are not abundant, amounting to about 1% with only fine pyrite impregnation visually observed in the veins.
Gold is fine-grained (78% is less than 0.07 mm) and rarely reaches 2 mm. Morphologically large gold grains have a veinlet lamellar, arborescent appearance with isometric and scaly shapes predominating in fine gold grains.
Various mineral forms of silver have been found in the mineral zones: free silver - 50%, electrum - 20%, argentite - 28%, pyrargyrite and fahlore - about 2%. Free silver is typically fine (95% less than 0.07 mm).
Against the background of generally poor gold-bearing veins in the central section of the Central zone a mineralized zone was delineated. Laterally, a number of close spaced trenches explored the mineralized zone while vertically the zone has been explored by a number of drill holes and by the No. 1 adit. The adit was driven along strike on the 920 m horizon, and accessed a quartz vein with a width from 1 to 7 m; sampling results, however, proved that the vein was barren. Gold grade in most samples does not exceed 0.2 g/t and in single instances reaches 1 g/t. The highest gold grade consisting of 5 g/t over a width of 2 m was observed in crosscut No. 2.
As indicated by the drilling data, the Central zone on the 880 m and 890 m horizons and quartz and quartz carbonate bodies sitting subparallel to the basic structure are also barren.
In the Avlayakan River valley six lines of drill holes have studied the Central zone. Only one hole (No 6, on drill line 1) has revealed a gold grade of 9.3 g/t over a width of 2 m.
The mineralized section identified within the Central zone extends for over 250 m and ranges in width from 1.0 m to 24 m. Sampling has indicated that gold grades range in value from tenths of a g/t up to 551 g/t and silver grades range up to 2,376 g/t. Distribution of gold and silver is very variable and the mineralized sections are delineated only from the assay results generated by sampling the zone. The gold-to-silver ratio in the deposit is relatively consistent, varying from 0.21 to 0.46.
The gold grade in the mineralized zone is low overall but some intervals of up to 20 m in strike length reveal a grade of over 100 g/t gold. Similar intervals have been traced down dip for 20 m to 25 m, which makes it possible to assume an undulated distribution of gold in the wide portion of the deposit.
Tectonic deformation breaks down the mineralized zone of the Central zone into three blocks, which are significantly different in size.
In the western block of the Central zone, which extends over a strike length of 85 m, the width of the zone ranged from 1.3 to 9.1 m and averaged 4.6 m. The gold grades varied between 1.4 g/t and 18.4 g/t with an average grade of 6.5 g/t throughout the mineralized zone.
The eastern block of the mineralized zone is divided, in turn, into two unequal parts. The smaller part of the block on the footwall contact of the fault is comparable in grade and width with the western block, while the eastern part in the hanging wall contact sharply differs in parameters and contains the best mineralization within the Central zone. This part of the mineralized zone has been studied in more detail both from the surface and down dip at depth. The zone here has a simple structure: the central section is filled by a quartz vein 1.5 m to 4.5 m wide with the remaining portion of the zone comprised largely of quartz metasomatites with quartz veinlets. The total width of the zone here ranges from 6 m to10 m. Further to the west the width of the vein increases and breaks down into two branches, with the space between them intensely saturated by diversely oriented quartz veins. The total width of the zone in this western area reaches 30 m and, in the section where the faults join it, reaches up to 40 m. Gold mineralization in the area of this bulge is considerably higher than in the other parts of the zone. High gold and silver grades in the bulge are encountered both in the quartz veins and in the metasomatites containing vein-reticulated silicification. The average visible width of the mineralized zone in the bulge is 19.0 m, while average gold grades in the exploration trenches and drill holes over this interval range from 5.1 to 61.1 g/t.
The eastern and western flanks of the Central zone have been traced on the surface by trenches spaced at distances ranging between 20 m and 140 m and at depth by a number of individual drill holes. However, no assay results for the trenches have survived and the few assays from the preliminary exploration drilling which have survived indicate that the gold grade for the eastern and western flanks of the Central zone varies around 1 to 2 g/t.
The Northeastern zone is located in the northeastern area of the deposit, 400 m northeast of the Central zone. To date, the zone has been traced over a distance greater than 3 km by exploration trenches and the width of the zone ranges from 50 m to 120 m.
Host rocks of the Northeastern zone include andesite and its tuffs of the Magueysky and Motarinsky Suites. The Northeastern zone is represented by a series of 2-3 subparallel quartz and quartz carbonate veins, separated by sections of hydrothermally altered rocks, often veined and metasomatically silicified. Within the zone the veins are complicated by numerous apophyses which branch out and are connected between themselves by diagonal and oblique intersecting veins. Sometimes veins alternate along strike with zones of pervasive silicification. The width of the veins varies from 0.5 m to 10 m and the contacts with the host rocks are sharp, rectilinear and steeply dipping. Frequently the veins in the inside chill margins are sheared and from the surface are transformed into argillaceous tophaceous material, containing large round inclusions of vein material of up to 0.6 m in size and impregnated by iron hydromica.
In the central part of the Northeastern zone, located between Quartz Creek (left tributary of the Avlayakan River) and Blue Creek (right tributary of the Left Avlayakan River), three veins numbered No.1 to No.3 represent the zone. Of interest is vein No. 1, with the mineralization located in the extreme southwest of this series of veins. The main distinction between vein No.1 and veins No. 2 and No. 3, which are either poor in gold or barren, is its northeast dip at 55º to 80º, while veins 2 and 3 are sub-vertical. No direct relationship among the veins was observed and their age remains in question.
The veins are usually saturated (20% to 50%) by fragments of intensely altered host rocks and have a zonal structure. Their near-contact sections are usually composed of confluent fine-grained quartz, saturated, by fragments of the host rocks, up to the formation of breccia with a quartz cement. In the central sections the quartz is more coarse grained and vugs occur which are filled by coarse grained drusy quartz, carbonate and adular.
Besides quartz, adular (up to 5%), hydromica (up to 3%) and epidote and chlorite (up to 5%) have been observed. In the carbonate quartz veins, the material cementing the fragments is represented by quartz and an aggregate of white coarse crystalline calcite. Along the vein, intervals that contain prevalent calcite cement frequently alternate along strike and down-dip with sections containing little or no calcite.
The mineralized vein in the Northeastern zone has been explored vertically by drill holes down to between 50 m to 210 m from the surface. The lower boundary of the geological blocks of the deposit is currently identified as being between the 845 m to 920 m horizons. However, there is the possibility of identifying new areas of mineralization at greater depth. At present, the interval along the vein containing the higher grade gold mineralization has not been delineated along strike and only partially in the down-dip direction.
In the zone, the interval between trenches K-141 and K-507 the deposit is "blind", plunging 25 m down. On the surface, the gold (according to the trenching data) grades range from 0.1 g/t to 1.7 g/t.
Although no obvious regularity in gold distribution in the zone along either strike or down-dip has been noted, the following peculiarities were noted in the Russian reports:
- The highest gold concentrations were observed between the 930 m to 1,030 m horizons; with metal contents decreasing with depth practically over the entire extension of the studied zone interval.
- Mineralized shoots (bonanzas) occured in some sections of the zone, demonstrating consistent high gold grades fixed in two or more exploration workings (drill hole S-26 and S-32; trench K-517 and drill hole S-38).
The gold-to-silver ratio for the Northeastern zone of the Avlayakan deposit in the mineralized intercepts varies from 1:4 to 1:22. Average gold fineness (according to the results of sample analysis performed by Irgiredmet in 2004) is 614 (1,000 fineness being 24 carat gold).
The Southwestern zone I is located 400 m southwest of the Central Zone and has been traced by trenches spaced at 25 m to 300 m intervals over a distance of 2.5 km in the northwestern direction. On the right bank of the Avlayakan River the zone extends for 400 m and is represented by a 5 m wide quartz vein. The quartz vein alternates along strike with zones of pervasive silicification amidst sericite quartz metasomatites, whose width reach 20 m in some sections along the left bank of the Avlayakan River. The central section of the zone (350 m) has been explored from the surface by trenches spaced at intervals of 20 to 40 m and by three sections of drill holes down to 50 m. In the vein, gold in the trenches grades from 0.1 g/t to 1.0 g/t (persistent grades of tenths of gram) and is slightly higher in the drill holes: 0.2 g/t to 1.7 g/t (in drill hole No. 43 gold values are between 0.4 g/t and 1.7 g/t down to between 42 m and 47 m).
The Southwestern zone II is located at the mineralization boundary of the Avlayakan deposit, 250 m south of Southwestern zone I. The Southwestern zone II is represented by lenticular quartz and less frequently by quartz carbonate bodies spatially associated with pervasive silicified metasomatites. The veins are up to 3 m wide.
Gold grade in the trench samples is low, ranging from below 0.5 g/t and peaking at 1.9 g/t, with a maximal silver grade of 2.3 g/t.
The Kirankan Deposit
The Kirankan deposit is located on the right bank of the Kirankan River in the basins of Basovy, Promezhutochny and Tock Creeks. The deposit is confined to the eastern part of the Avlayakan Kirankan volcano tectonic structure, at the contact of the Lower Archean age formations and Cretaceous volcanics.
Numerous, spatially dissociated linear zones of hydrothermally altered rocks have been identified among metamorphic, effusive, subvolcanic and intrusive formations within the area. Overall, more than ten zones of quartz, sericite quartz metasomatites with superimposed veinlet silicification and quartz, quartz carbonate veins and tectonic breccia have been delineated. Metasomatites and quartz are slightly sulphidized. Almost all the zones and veins are controlled by northwest, near-north south and less frequently northeast faults. Northern trending faults dip steeply at 65º to 85º.
Potentially "economic" gold grades were found within two zones, the Basovy and Tock.
The Basovy zone extends over more than 400 m and was traced by trenches over a distance of 330 m. The zone exposed in trenches ranges from 15 m to 70 m wide and in plan resembles a stockwork.
The Basovy zone is confined to the tectonic contact between the dioritic porphyrites of the Jugjursky complex with cover andesite of the Motarinsky complex. The mineralized section of the zone was studied on the surface at 20 m intervals by trenching. Trenches have exposed sericite quartz, quartz metasomatites with minor zones of fine veinlet silicification, quartz (up to 9 m wide) and quartz carbonate (up to 5 m wide) veins with the latter very inconsistent in width.
The gold and silver mineralization in the Basovy zone is manifested in all types of the altered rocks, but the highest grades of gold and silver mineralization were observed in quartz, particularly quartz with sulphide mineralization. The gold grades are distributed extremely unevenly within the zone, ranging from 0.1 g/t to 40.6 g/t. Against the background of quartz veins with generally poor gold occurrences, two deposits with potentially "economic" mineralization were outlined in the middle of the zone.
In plan, deposit No. 1 is diamond shaped, has a 9.4 m by 20.0 m size, and is confined to two lenticular veins of quartz and quartz carbonate composition. The average gold grade on the surface is 15.6 g/t and the average silver grade is 12.6 g/t. The deposit has been traced by one fence of drill holes vertically and has not been delineated on the flanks of the zone. Gold grade in the deposit between the depths of 0.0 m to 47.1 m is extremely inconsistent and varies from 0.0 g/t to 25.0 g/t. At a depth of 71 m to 75 m in the deposit the gold grades are below 1.3 g/t.
Deposit No. 2 extends over a distance of 58 m and is located in the wallrock of the Basovy zone where it is confined to the quartz vein with a near-north south strike extent. The average width of the mineralization is 10.6 m, with an average gold grade of 9.7 g/t and an average silver grade of 9.0 g/t. The mineralization dips eastward at 70º to 80º. Vertically it has been explored by two fences of drill holes. Gold grades in the drill holes do not exceed 3.0 g/t down to between 29.6 m to 39.9 m.
The Tock zone with a length of about 600 m and a width of 150 m (trench length not mineralization) was defined and traced by trenches at 13 locations. The 440-m-long section, where trenches spaced at 10 m to 40 m intervals have defined the zone, is the most studied portion of the zone. Its morphology is similar to that of the Basovy zone but in appearance it resembles a linear stockwork. The zone trends from nearly a north south strike to a northeast strike direction with a steep 60º to 80º dip eastwards.
The host rocks are Archean age gneisses and subvolcanic andesite of the Motarinsky volcanogenic complex.
The Tock zone is comprised of quartz, quartz carbonate veins extending over a distance of 230 m. The width of the veins ranges from less than 1 m up to 7 m. Frequently the veins are accompanied by sections of metasomatites with a width of up to 10 m and a length of up to 80 m. Sericite quartz metasomatites comprise thin (several metre wide) zones along the quartz veins.
Sulphides in the veins are rare and form fine scattered impregnations and less frequently accretions.
Gold is present in all types of altered rocks with the grade varying from 0.1 g/t to 113.3 g/t, while the silver grade ranges from 0.1 g/t to 141.9 g/t. Nonetheless, potentially "economic" gold grades were found only in one interval with a strike length of 200 m (the Tock zone), which has been defined by nine trenches spaced at 10 m to 30 m intervals. The average width of the zone is 3.2 m while the average gold grade is 10.6 g/t and the average silver grade is 16.4 g/t.
Vertically the Tock zone has been studied by three fences of drill holes at 75 and 80 m intervals, in which potentially "economic" gold mineralization has been defined down to between 35 m to 40 m below the surface. At depths of 60 m to 80 m only narrow intervals (less than 1.0 m wide) with gold values of 1.6 g/t to 9.6 g/t have been defined in the drill holes. No delineating fences of drill holes have been done on the flanks of the deposit.
The gold-to-silver ratio within the mineralized intersections of the Kirankan deposit ranges from 1.35:1 to 1:3.
Based on geological structural and mineralogical similarity to the Avlayakan deposit, the Kirankan deposit has been classified as a gold and silver geological mining deposit with a Class 3 Complexity according to the classification of the Russian State Reserves Committee. The Class 3 Complexity category is defined as narrow to medium quartz veins with an irregular distribution of metal components.
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