R.N. Ivanova, O.A. Maslova

National Research Irkutsk State Technical University

Irkutsk, Russia

Structural peculiarities of Albazinskoe deposit

(Khabarovskiy kray)

Abstract: Ores of Albazinskoe gold deposit were formed due to hydrothermal vein and hydrothermal metasomatic processes. From the position of the tectonofacial analysis ore bodies and adjoining rocks belong to typical mesozonal formations [1]. The degree of rock deformity on the deposit has predetermined the character of its development.

Key words: ore formation, tectonofacies, degree of rock deformity, alpine hydrothermal veins.

Albazinskoe gold deposit is located in the area named after Polina Osipenko in Khabarovskiy kray. From the structural point of view the deposit is situated in Ulbanskaya structural-formation zone of Amuro-Okhotskaya fold system of the fold area, which is represented by complex dislocated Jurassic terrigenous and in a less degree volcanogenic-siliceous structures. On the given territory there could be defined three structural levels based on structural-material parameters of geological complexes.

The lower structural level consists of strongly dislocated sedimentary deposits of Triassic-Jurassic age. They are represented by four units (bottom-up): siliceous and sandstone strata, Demyanovskaya and Elgonskaya suites.

Siliceous stratum is composed of two layers. The lower one is made by siltstones and argillites with isolated thin (1-5 cm) interlayers of fine-grained sandstone. The upper layer of the stratum is more mixed by composition and is made of siliceous and siliceous-clayey materials, siltstones and spilites. Alteration of siliceous materials and siltstones is every 1-10 cm, thickness of spilite interlayers varies from 2 to 20 meters.

Sandstone stratum is more homogenous and composed mainly of dark-grey fine-grained sandstone, more rarely of medium-grained materials. Single interlayers of siltstones up to 1 (seldom more) meter thick are strongly attached to the bottom of the stratum where thin interlayers of sediment breccias could be found as well.

Demyanovskaya suite is 60-70 % composed of thin- and medium-grained sandstone of distinctive pinky-grey color with thin interlayers enriched with phytodetritus. Siltstones and clay shales could be found as bundles of rhythmic alterations with sandstones.

Elgonskaya suite consists of three layers.

The lower layer is 720-900 meters thick and is composed of roughly alternating medium- and coarse-grained sandstones, gravelites and fine-pebbly conglomerates.

The middle layer of the suite is composed by fine-grained sandstones and siltstones rhythmically alternating every 5-20 cm. Siltstones distinguish themselves by presence of unclear stratification and “stirring-up” structures. The middle layer of the suite is 450-500 meters thick.

The upper layer of the suite is mainly composed of sandstones; in its lower part bundle of gravelites and fine-pebbly conglomerates could be found while in upper parts gravelites are located as thin (up to 2 meters) single interlayers.

Terrigenous sediment deposits of the region are crushed into the folds of north-eastern sublatitudinal trend (azimuth 60-80°) which in whole form the structure of the first order, and namely host-anticlinal fold. The limb spread of smaller folds is from 100 to 500 meters.

The middle structural level is represented by late cretaceous and Eocene sediments which take up to 20% of the area. Three complexes of magmatic rocks could be defined:

1. Late cretaceous andesite-rhyolite complex which is represented by volcanic and subvolcanic rocks;

2. Late cretaceous granodiorite complex;

3. Eocene basaltic complex which is represented by single basalt dykes.

In most cases dykes of rhyodacites of late cretaceous complex are hydrothermally modified into sericite-quartz metasomatites. Hydrothermalites of later periods are veins of quartz and calcite 1-10 (seldom more) mm thick. Zones of silicificated rocks are located both in metasomatites and sediment rocks. More seldom vein silicification could be found in endocontact parts of microdiorite dykes. The thickness of zones of modified rocks is from several up to 100 meters. Industrial ore bodies of Albazinskoe deposit are spatially connected with fracturing zones within the limits of propagation of late cretaceous andesite-rhyolite complex.

The upper structural level is composed of Quaternary loose depositions. The following modern and late Quaternary alluvial depositions are represented: shingles, sands, silts, and clays.

Based on the results of exploration works Albazinskaya ore-bearing structure is clearly defined. With different detail degree the structure is traced to the distance of 5 km within the stripe of 500 meters wide. The structure has north-western trend (azimuth 330-350) and north-eastern dip at an angle of 30-60 degrees.

Hydrothermally altered rocks with which mineralization is connected are developed on sandstones, rhyodacites, dacites and more rarely microdiorites. Zones of altered rocks are from 20 to 100 meters thick. Sandstone and siliceous layers are bearing strata for dykes. Industrial ore bodies are mainly located in the sandstone layer.

Ore bodies are made of different hydrothermally and tectonically altered rocks and have 400 meter trend and 180-230 meter dip. Detailed petrographic mapping of ore zones has identified as follows: tectonic breccias of mixed composition consolidated by sericite-carbonate-quartz aggregate (“drags”); brecciated and streak-silicificated sericite-carbonate-quartz metasomatites on rhyodacites; sericite-carbonate-quartz metasomatites on rhyodacites; fine streak-silicificated sandstones or siltstones, sometimes with thin zones of brecciation and more intensive silicification; brecciated silicificated sandstones.

“Drags” with imposed streak silicification have the richest content of gold. In general “drags” and brecciated silicificated metasomatites are characterized by average content.

Ore genesis resulted from hydrothermal vein and hydrothermal metasomatic processes. Petrographic analysis of samples has shown that all of them were touched by hydrothermal process: hydrothermal metasomatic and hydrothermal vein. Hydrothermal metasomatism appears as spacious pelitization and sericitization of aluminum silicate and silicate rocks (pic. 1). Hydrothermal vein process resulted in juxtaposition of quartz, dolomite on the rocks altered by metasomatism (pic. 2).

Main ore minerals are as follows: pyrite and arsenopyrite, more rare - gray copper ore, chalcopyrite, pyrrotin, galena (gold in crush samples). The size of sulfide extractions doesn’t exceed a fraction of a millimeter, rarely – 1-2 mm [2]. The overall quantity of sulfides is 1-5%, rarely up to 10%.

Vein minerals are represented by two generations of quartz (chalcedonic and crystal), carbonates (ankerite and calcite), feldspar. Judging by the composition of ore bodies and their mineragenic characteristics, Albazinskoe deposit is a typical representative of poor-sulfide quartz-gold-ore-bearing formation of mineralized zones.

Disjunctive dislocations are wide spread in the deposit area and play an essential role both in distribution of products of magmatic activity and ore bodies and post-ore deformations.

The majority of disjunctive dislocations are long-living ones; they were laid in the time of folding and more than once renewed in the period of magmatic activity.

Industrial ore bodies are attached to disjunctive dislocations of north-western submeridional trend. Mineralization is also connected with dislocations of north-eastern trend, though they are insufficiently studied.

Post-ore deformations are most clearly registered on disjunctive dislocations of north-eastern sublatitudinal trend. Slips on small-scaled breaks which are often healed by granite-porphyry make up 3-20 meters. Slips on larger faults of north-eastern trend reach the first hundreds of meters. Based on these dislocations the territory of the deposit is divided into the number of elementary blocks of horst or graben type. Ore bodies in these blocks are characterized by different level of erosion.

On the southern side in the area of limiting north-eastern fault a disjunctive dislocation was found which deforms the ore-bearing zone. The disjunctive dislocation has north-western trend (azimuth 290-300) with north-eastern dip at an angle 45 degrees. Displacement on horizontal plane makes up 10 meters. Shatter zone is 5-10 cm thick and made by tectonic clay gauge. To the south of this dislocation there were detected several crevasses of sublatitudinal trend displacing dacite dyke located to the west of microdiorite dyke. Displacements on these crevasses on horizontal plane are 1-2 meters.

From the position of tectonofacial analysis ore bodies and adjoining rocks of Albazinskoe deposit belong to typical mesozonal formations [1, 3]. It is indicated by insignificant level of metamorphism not exceeding the initial stage of green shale facies development [2]. The following types of formations are widely-spread on the deposit: alpine hydrothermal veins and streaks; multiple brecciation zones often representing rich ore bodies.

By the degree of rock deformity deposit formations belong to the lower (A) and medium tectonic structures (B), tectonic facies of III to VI.

From the regional point of view the degree of rocks deformity on the deposit has mainly predetermined the character of its formation – pre-ore structural workup displayed in the formation of adjoining rocks fracturing zones allowed concentrating of ore substance whose source is late cretaceous andesite-rhyolite complex of magmatogen structures of the deposit.

References:

Ivanova R.N., Kochnev A.P. Tectonofacies of Olkhon crystal complex: monograph.- Irkutsk: ISTU publishing house, 2012. – 130 p.

Maslova O.A., Semeikin I.N. Geology, mineral exploration, methods of geological research: proceedings of All-Russian scientific and technical conference “GEOSCIENCES” dedicated to the 80^th anniversary of the faculty of geology, geoinformatics and geoecology. – Issue 10. – Irkutsk: ISTU publishing house, 2011. – p. 107-112

Patalakha E.I. Tectonofacial analysis of Phanerozoic folded structures (grounds, methodology, application). – M: Nedra, 1985. – 169 p.