The Dolaucothi gold deposit occurs in thin bedded turbidites of Upper Ordovician to basal Silurian age deposited near the southern margin of the lower Paleozoic Welsh basin. The host rocks were deformed during the Caledonian orogeny (390-410 Ma) into a series of inclined asymmetrical, commonly overturned, tight southeast-facing folds with associated cleavage. Reverse dip-slip faults accompanied the folding and the whole area is cut by late tensional faults. The mineralization is controlled by structures produced during the Caledonian orogeny, in particular a thrust fault which formed the host to the Roman lode, a flat-lying auriferous quartz reef up to 6 m thick.Gold occurs in a variety of hosts including pyritic shales containing both euhedral and framboidal pyrite, quartz-carbonate stringer veins, in steeply or gently inclined veins associated with shear zones and in planar quartz veins associated with the Roman lode. The mineralogy is simple, with pyrite and arsenopyrite being the dominant sulfides. Ankeritic carbonates, hydromuscovite, and the lithium silicate, cookeite, are also present.It is proposed that, during prograde metamorphism associated with early stages of the Caledonide orogeny, fluids circulated through the basement below the southeast margin of the Welsh basin and leached gold, and associated metals, from rocks of igneous or volcanic origin. These overpressured fluids were released to high crustal levels during later orogenic movements and uplifts, perhaps related to reactivation of such basement faults as the infra-Tywi fault. Geothermometric studies indicate that the host rocks experienced upper anchizone facies metamorphism whereas the temperatures of the mineralizing fluids were in the range of 345 degrees to 450 degrees C. Isotopic studies point to a common origin for the sulfides in both the shales and the veins and also indicate that the host rocks provided the source of the carbon now incorporated in the carbonates.