Carpathian Journal of Earth and Environmental Sciences

New geological data on the Orlea mining field, Roșia Montană ore deposit, Romania were obtained by fieldwork carried out in the underground level +730 m, where three types of mineralized structures hosted by the Vent Breccia formation were studied, i.e., (i) flatly dipping vein with rhodochrosite gangue, (ii) steeply dipping tectonic breccia dyke, and (iii) steeply dipping base metal vein, respectively. The Vent Breccia is a polymictic matrix-supported breccia with sedimentary (clay, sandstone), volcanic (dacite), and metamorphic (quartzite, garnet micaschist) clasts. Four types of hydrothermal alterations were identified, i.e., (i) K-metasomatosis (adularia I); (ii) phyllic alteration (sericite); (iii) silicification; and (iv) potassic alteration (adularia II). The ore mineral assemblage consists of electrum, pyrite, chalcopyrite, sphalerite, galena, and minor arsenopyrite and pyrrhotite. The higher-grade ore body is the intermediate-sulfidation flatly dipping vein with rhodochrosite gangue grading up to 101g/t Au. The tectonic breccia dyke is a low-sulfidation ore body illustrating the evolution from early banded vein structure to late/final open-space clast supported tectonic breccia dyke.