Abstract: The aim of this master thesis was to create a geological map of a section in the III horizon in Idrija Mercury Mine and determine mineralogical, petrological and geochemical characteristics of the mapped rocks. During the preparation of the geological map a total of 10 rock samples and two samples of material from the fault zoneswere taken for further analysis. On these samples we performed mineralogical analysis using the X-ray diffraction analysis, petrological analysis using the polarizing microscope (micropetrographic determination) and analysed the main elements, trace elements and rare earth elements (REE). By using a polarizing microscope we determined micropetrographic characteristics of mapped rock types as follows: siltstones to laminated siltstones/shales, medium grained quartz arenite, early diagenetic dolomite which is classified as dolomicrite with fossils and dolobiosparite, metamorphosed breccia sandstones/mylonite breccia and early diagenetic dolomite breccia. The main mineral phases in samples of Carbon shale, sandstones and basal sandstones are quartz and kaolinite. Secondary mineral phases in these rocks are micaceous minerals and pyrite. In samples of Permian Groden sandstone in addition to above mentioned mineral phases we also found dolomite as a main mineral phase. C ontrary t o A nizian dolomite, early diagenetic dolomitecontains kaolinite as a minor mineral phase. In the fault zone betweenPermian Groden sandstone and Anizian dolomite we found gypsum and rozenite; minerals that originated from weathering of primary mineral phases. Mineralization with cinnabar is located in: (a) fault zones, (b) the contact zone between Anizian dolomite and basal sandstones and (c) early diagenetic dolomite breccia. The highest value of Ba, Co, Ni, Rb, V, W, Mo, As, Cu, Pb and Zn were determined in samples from the fault zones. We found increased values of As in samples in which the main mineral sulphide phase is cinnabar. Above mentioned implies a different mobility of elements along fault zones.Early diagenetic dolomite breccia has up to 10 times higher ΣREE compared to Anisian dolomite. This clearly shows that during the mineralization there has been a remobilisation of REE. In relation to the composite of North American shales Permian Groden sandstones, basal sandstones and all dolomite samples are enriched in heavy lanthanides. So far we have no clear explanation why these rocks have been enrichedin HREE.