This paper analyzes mass transfer and skarn ore-forming processes in the metasomatic zones between the marble and the quartz diorite stock in the Nippo deposit of the Kamaishi mine. The zoned skarns are more than 50 m in width and consist of the following zones, listed from the quartz diorite to the marble side: skarn Ia, hornblende-clinopyroxene-plagioclase skarn; skarn Ib, clinopyroxene-plagioclase skarn; skarn II, plagioclase-garnet-clinopyroxene skarn; skarn III, garnet-clinopyroxene skarn. Sulfide minerals are concentrated mostly in skarn III, particularly at the contact with the marble, and secondly in skarns Ib and Ia. These skarns were formed at about 550 degrees C, where total pressure was assumed at 2 kb. A quantitative reaction model for the formation of the zones was formulated on the basis of mass balance calculations by using the petrological characteristics of the zones. The reaction model is made up of five interfacial reactions at the zone boundaries. Thus, skarn II was formed as an end product, whereas skarns Ia, Ib, and III were intermediate products which changed to the end product with progressive development of the zones. Mass transfer can be divided into diffusive and infiltrative contributions. Si, Fe, S, Cu, Zn, Ni, and Co were largely introduced from external sources by infiltrating hydrothermal fluids; Al and a considerable proportion of Ca and Mg migrated within the skarn by diffusion; CO 2 , H 2 O, Na, K, and a portion of Ca were removed by infiltration.Hydrothermal fluids flowed primarily at the reaction sites where CO 2 or H 2 O was released. There was a positive correlation between mass addition and the loss of CO 2 or H 2 O. About 90 mole percent of the mass addition was concentrated at the contact of the skarn with the marble. Release of CO 2 or H 2 O promoted the introduction of hydrothermal fluids, because production of volatiles temporarily increased fluid pressure, which caused pores and fractures, thus increasing permeability. The hydrothermal fluids followed the release of volatiles from the wall rocks, forming a single flow system at the reaction sites. Furthermore, Ca or alkalies were liberated and sulfide minerals were precipitated at the same devolatilization sites. This supports the conclusion that crystallization of the sulfide minerals was promoted by the neutralization of acid hydrothermal fluids reacting with carbonate minerals or feldspars. Thus, devolatilization, especially decarbonation, dominated the process of skarn ore formation from the introduction of hydrothermal fluids to the precipitation of ore minerals. The reaction model explains the location of ore pods within the Nippo skarn.