THE source of the hydrothermal fluids vented in active volcanic areas on the sea floor(1-3) has been a matter of some debates(4-7); they may arise purely from the interaction of circulating sea water with the hot rocks through which it passes(1,3,8), or there mag be an admixture of a fluid escaping from magma at depth, as is seen in subaerial geothermal systems(9), The answer to this question also bears on the origin of the sulphide ores deposited by sea-floor hydrothermal systems, and their ancient analogues(8,10,11) preserved on land. Here we present direct evidence for the presence of magmatic fluid in the lavas that host an actively forming massive sulphide deposit in the eastern Manus back-are basin. We find high concentrations of chlorides and sulphides of ore-forming metals such as copper, zinc and iron in CO2-rich gaseous bubbles found both in melt inclusions frapped in the phenocrysts of the volcanic rocks, and in the matrix glass, We conclude that a metal-rich fluid was present in the magma before eruption, and probably exsolved as the pressure decreased. This finding suggests the possibility for the contribution of large quantities of ore- forming metals to a sea-floor hydrothermal system.