The Jiama copper and polymetallic deposit, located in an inter-arc sedimentary basin, occurs as stratiform and stratoid bodies in the transitional zone between the carbonate rock (J3d) and sandy slate (K1l). Hydrothermal altered skarn and hydrothermal-sedimentary diopside-feldspar rocks relate to the formation of the deposit are ore-bearing rocks of the Jiama deposit. On the basis of studies on isotopic, REE, components of fluid inclusions in the minerals from the deposit, this article holds that ore-forming metals of the deposit came from arc-volcanics, basement of the Jiama intra-arc basin; ore-forming fluids originated from the old seafloor hydrothermal circulatory system; sulphur of the deposit are characterized by biogenetic sulfur; and water-rock interaction (metasomatism) is an important mechanism of ore-formation in the Jiama deposit. Thus, this paper argues that the deposit belongs to the products of old seafloor hydrothermal metasomatism, and the deposit is a special exhalation one bound up with the old seafloor hydrothermal circulatory system.