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Polymetallic mineralization in the Bayanlig area, Aimak Bayankhongor, Mongolia

Łydek P., Wnęk P.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

496--497

Since 2007, exploration works have been carried out for the metalliferous deposits in SW Mongolia (Bayankhongor province), close to the north edge of the Gobi desert. Mongolia is subdivided into 44 fault-bounded terranes (Badarch et al. 2002). The studied area is a part of the Gobi Altai Terrane, which is a long and narrow belt, located in the south of the Main Mongolian Lineament. The terrane is genetically classified as backarc/forearc basin and mainly consists of greenschist facies metamorphosed sandstones, shallow-marine limestones, volcanic rocks and granite/plagiogranite intrusions (Badarch et al. 2002). The presented research is based on the mineralogical samples collected during the exploration works conducted in 2012. During the fieldwork several types of rocks were identified, such as mafic and ultramafic rocks which are serpentinised to various degree and also more felsic lithologies. Both mafic and felsic rocks revealed macroscopically visible metalliferous mineralization. The aim of the study was to identify ore minerals in all types of rocks and to determine succession of the ore minerals occurring in the serpentinised mafic and ultramafic rocks. Microscopic observations in transmitted and reflected light were conducted in the Ore Deposits Geology Laboratory of the Mining and Economic Geology Department at the AGH University of Science and Technology in Krakow. Macroscopic observations revealed that magnetite is a dominant ore mineral in the mafic lithologies, whereas malachite and copper sulfides in the felsic rocks. On the basis of microscopic observations two main types of ore minerals were identified: oxides and sulfides. Samples form mafic rocks revealed chromite, magnetite, other Cr-Fe spinels, hematite, ilmenite, rutile, millerite, polydymite and pentlandite. Oxide minerals dominate in these rocks. Ore minerals identified in samples from felsic lithologies are represented by chalcopyrite, pyrite, idaite, covellite, malachite, pyrrhotite and molybdenite. Moreover, electrum and native gold have also been recognized in one set of samples from felsic rocks. As a result of the conducted research an attempt of determining precipitation stages of ore minerals occurring in the serpentinised rocks was made. On the basis of ore structures, textures and mineral assemblages, three main phases of ore minerals succession have been distinguished. The first phase is represented by primary ore minerals like ehromite and ilmenite. The second one is closely related to serpentinisation process and characterized by the occurrence of minerals like magnetite and Fe-Cr spinels, and also pentlandite, millerite, polydymite. The last stage is related to the oxidation process and represented by hematite. The presented results are part of the research which is still in progress and is focused on better understanding of metallogenic processes which occurred in the studied area.