Baley, Balei, Taseevskoe

Zabaikalsky Kray, Russia

Main commodities: Au Ag
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The Balei (Baley) - Taseevskoe epithermal gold deposits are located adjacent to the Unda River, within and up to ~2 km south of the town of Baley, 56 m south by paved road from the Trans-Siberian Railway station at Priiskovaya, and 320 km SE of Chita in the Zabaikalsky Krai of Eastern Siberia (#Location; 51° 33' 11"N, 116° 38' 55"E).

The Taseevskoye deposit was discovered in 1941 and between 1948 and 1994 produced 200 t of gold from 16.3 Mt @ 12.2 g/t Au from a swarm of epithermal gold bearing quartz fissure veins located within a circular area 1000 m in diameter. The predominantly underground mining operation concentrated on three principal vein systems, leaving un-mined a large lower grade mineralized envelope contained within an argillic alteration halo surrounding the fissure veins. Remaining JORC compliant Indicated + Inferred Mineral Resources at 31 December 2019 were 31.063 Mt @ 5.1 g/t Au for 158 t of contained gold, of which 25.785 Mt @ 4.9 g/t Au were Indicated (Highland Gold Mining Limited website, viewed December, 2020).

The Balei gold field lies within the Early Cretaceous Balei Graben, which is bounded on its NW margin by a regional fault, with mineralisation concentrated at the intersection of sets of NE and NW trending faults sets. The basement of the graben is composed of Palaeozoic granites, overlain to the south and west by lavas and tuffs of the Upper Jurassic Talangu Group that have, in turn, been intruded by an east-west belt of dykes and stocks of diorite porphyry in the south of the gold field. The graben was filled with up to 650 m of Upper Jurassic to Cretaceous continental clastic sedimentary rocks of the Ust'Kara Group, composed of a lower, principally conglomeratic suite (the Tergen Sub-group) and and upper sequence of sandstone and conglomerate (the Balei Subgroup). Near the top of the upper suite, bodies of secondary quartzite, possibly the product of silica alteration, frequently define the upper limit to economic mineralisation. The same sequence is also cut by dyke-like injections of explosion breccias and associated silicification (Smirnov, 1997).

The gold field is surrounded by and bounded by NE and NW trending faults, whilst NW and near north-south faults divided it into the Western, Central and Eastern blocks that have been raised of depressed by different amounts. The bulk of the economic ore is concentrated in the Central Block, which has also been subjected to the maximum brecciation (Smirnov, 1997). Within this Central Block, gold mineralisation is concentrated in the southern and central parts of the graben.

The Balie mineralisation is located in the north of the mineralised part of the graben. The northern section of the graben is more mobile. Its structure is controlled by a combination of near east-west, predominantly flat-lying crush zones and steeply dipping fractures associated with east-west faulting. The intersections of pre-mineral faults are accompanied by intense fracturing and are concentrated in the internal part of the graben where the host is the conglomerates, with lesser dacitic to andesitic volcanic rocks, sandstone and siltstone of the Tergen Subgroup, although mineralisation also extends into sections of the Unda Granodiorite that adjoins the graben. Ore veins fill both steeply dipping fractures and flat crush zones forming a complex stockwork of steeply dipping and flat veins. Individual veins are 0.2 to 2.0 m or more in thickness (Smirnov, 1997). Veins within the flat lying zones tend to be lenticular, short and thin and composed of quartz, whilst those in the steeper structures have a more complicated morphology. The veins within the granitoid in the north of the deposit have a variety of directions, producing a vertical stockwork that has a plan area of ~1 km2. Mineralisation has been intersected in drill holes to a depth of 0.8 to 1 km (USGS).

The Taseeva deposit is in the southern part of the mineralised section of the graben, hosted by almost horizontal clastic sedimentary rocks of both the Balei and Tergen subgroups. These rocks have been dislocated by extensive, steeply dipping sets of near north-south to northeasterly striking faults that are interpreted to be influenced by reactivation of basement structures. These faults host all of the major ore veins, although minor fractures in both the hanging and footwall of these faults control lesser veins and disseminations. The results in a pattern of short, steep, sub-parallel veins grouped as closely spaced vein series, accompanied by inter-vein intervals and aureoles of disseminated and veinlet mineralisation that together form near north-south elongated shoots. The larger veins reach 1 to 2, and sometimes 3 to 4 m in thickness (Smirnov, 1997). The deposit is a multi-root system, with separate and coalescing bodies located at different depths, composed of orebodies that extend over strike lengths of 1 to 2 km, vertical intervals of 700 to 1000 m and a thickness of up to several tens of metres (USGS).

Ore veins within the gold field comprise 90 to 99 vol.%, fine-grained, complexly banded chalcedonic quartz, with minor thin-platy carbonates, adularia and kaolinite. Sulphides constitute <1 vol.% of the veins and include pyrite, marcasite, arsenopyrite, tennantite-tetrahedrite, pyrargyrite, miargyrite (AgSbS
2) and Te minerals. In the gold rich sections of the veins, tennantite-tetrahedrite and silver sulphosalts are of more significance, whilst pyrargyrite commonly accompanies gold and is typical of the Balie deposit (Petrovskaya et al., 1961), such that at the junctions of flat-lying and steeply dipping fractures, there are high-grade ore shoots containing native gold, tennantite-tetrahedrite and pyrargyrite (Smirnov, 1997). The bulk of the gold occurs as a finely dispersed greenish variety characterised by 20 to 30% Ag with traces of Sb, Hg, Se and Te. There is also less common coarser grained 0.2 to 0.5 mm aggregates of yellow gold. The overall fineness of the gold is 650 to 750 (Smirnov, 1997).

Hydrothermal alteration is widespread within the gold field, commencing with early sericite and kaolin, followed by a separate phase of silicification, concentrated around the groups of closely spaced veins. Subsequent complexes of further silica, adularia, pyrite and late kaolin and carbonate are closely related to the introduction of gold mineralisation. Adularia accompanies the richest gold (Smirnov, 1997).

Gas-liquid inclusions show that the deposits were formed at 270 to 150°C and 0.03 to 0.04 kb, at an estimated depth of 150 to 200 m below surface (Lyakhov, as quoted by Smirnov, 1997).

The most recent source geological information used to prepare this summary was dated: 2010.    
This description is a summary from published sources, the chief of which are listed below.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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