MINERALOGICAL AND GEOCHEMICAL FEATURES OF THE ORE BEARING STRATA OF THE UGAKHAN DEPOSIT FROM BODAIBO ORE REGION, RUSSIA

Abstract

The relevance of the study is caused by the interest in the gold deposits located in the Bodaibо ore region. The study of this area is reflected in numerous publications, despite this there is no unified idea of the origin of gold deposits formed in the black shale strata. The Ugakhan deposit is the only gold ore object of the Bodaibo ore region localized in the rocks of the Buzhuikhta Formation, and the detailed mineralogical and geochemical study of this ore host strata can be useful to clarify genetic type of gold mineralization and the prospects of the Buzhuikhta Formation for the search for new gold objects. The main aim is clarification of the conditions of the lithogenesis in the concentration of metals in sulfide-bearing metasedimentary rocks of the Buzhuikhta Formation. Objects of the research is the Ugakhan deposit, located in the Irkutsk region of the Bodaibо ore region. Methods. The study of rocks was carried out using an Olympus BX51 optical microscope. The accessory mineralization was analyzed on a Tescan Vega 3 sbu scanning electron microscope equipped with an Oxford Instruments X-act energy dispersive spectrometer. The content of 12 basic oxides in rocks (9 samples) was determined by silicate analysis. The trace elements, including rare earth elements (6 samples, 40 elements), were studied by the ICP-MS method (inductively coupled plasma mass spectrometry) on the ICP-MS Agilent 7700x spectrometer (Japan). All mineralogical and geochemical researches were carried out at the Institute of Mineralogy of South Urals Federal Research Center of Mineralogy and Geoecology UB RAS (Institute of Mineralogy SU FRC MG UB RAS, Miass). Results. Ore-bearing rocks from the Ugahan deposit are represented by intercalation of metasandstones, metasiltstones and black shales. The detrital material in these rocks consists of weakly rounded quartz grains and feldspars (albite and orthoclase) and fragments of microfossils in the micaceous-chlorite-carbonate groundmass. The accessory minerals are tourmaline, zircon, rutile, apatite, titanite, numerous minerals of rare earth elements (monazite, xenotime, rare earth elements-bearing epidote, bastnaesite, ankylite, rare earth elements-bearing thorite, uraninite and coffinite). The sequence of mineral formation testifies to the redistribution of the initially accumulated matter and its concentration with the formation of mineral forms of rare earth elements, Th and U at the all stages of lithogenesis and the metamorphogenic-hydrothermal stage of primary carbonaceous sediment transformations. A comparative analysis of the chemical composition of metasediments shows that a decrease in SiO2 contents is accompanied by an increase in Al2O3, MgO, TiO2, FeO*, K2O, and P2O5 contents in the range: metasandstones → metasiltstones → black shales, In the same range, there is an increase in the contents of Sc, V, Cr, Zn, Ga, Rb, Sr, Y, Zr, Th, U and rare earth elements. A specific feature of metasediments of the Buzhuikhta Formation is the high content of V, Ba, Sr, rare earth elements, Th and U, which is caused by the participation of organic matter in diagenetic processes involving sulfides. The calculated petrochemical modules and their correlations testify to the common nature of sedimentary material and the possible influence of volcanogenic material in formation of sediments.

Keywords

• metasedimentary rocks
• mineral associations
• trace elements
• gold deposit
• bodaibo ore district