BIRKSU (RU: Бирксу) is the mercury deposit at the south of the Kyrgyz SSR. It is the mono-metallic (cinnabar) deposit of the carbonatic (limestone) type. It was intensively developed during the 5-11 centuries by the ancient miners, who extracted the cinnabar paint and metallic mercury (within the ceramic retorts). It has been re-discovered during the 1908. It was explored during the 1942-46. It has been operated by the prospecting method during the 1944-48. The mineralization is associated with the zones of crushing and paleness within the massive and laminated limestones. Judging on the configuration of the ancient mine workings (approximately 250 ones within the area of 10x2 kilometres), the exhausted ore bodies were represented mainly by the small nests of the lumped (more than 1% of Hg) ores, by the large, but poor (less than 0.2% of Hg) stockworks, and by the thin (up to 0.5 metres), but rich (more than 1% of Hg) veins.