Occurrence and Paragenesis of the Nickel-and Cobalt-Bearing Minerals from the Nippo and Shinyama Ore Deposits of the Kamaishi Mine, Iwate Prefecture, Japan

Abstract

The ore deposits of the Kamaishi mine are of contact metasomatic origin and the mine is well known as the largest producer of iron-copper ores in Japan. The Nippo ore deposit, comprising five copper ore-bodies, is located at the western extremity of the Kamaishi mining area. These ore-bodies occur in "breccia skarn" consisting of biotite hornfels fragments and intersitial clinopyroxenes and garnets, and in massive clinopyroxenes or clinopyroxenes-garnets skarn. The Shinyama ore deposit, the largest one in the mine, consists of an iron ore-body and seven copper or copper-iron ore-bodies in massive garnets or clinopyroxenes skarn replacing porphyrite and limestone.
Nickel-and cobalt-bearing minerals are commonly found as minor consituents in sulfide ores from the copper ore-bodies. The major sulfide minerals are chalcopyrite, pyrrhotite, cubanite, bornite, and pyrite. The nickel-and cobalt-bearing minerals, identified by ore microscopy, X-ray micro-diffraction and electron micro-probe, are cobaltian or cobalt pentlandite, siegenite, nickel-and cobalt-bearing mackinawite, argentian pentlandite, millerite, and nickelian smythite, in decreasing order. Cobaltian or cobalt pentlandite occurs most abundantly in pyrrhotite and cubanite of the chalcopyrite-pyrrhotite-cubanite association as exsolution products and in chalcopyrite of the chalcopyrite-bornite association. Cobaltian or cobalt pentlandite is extensively altered to siegenite by hydrothermal solution probably related to the alteration of hexagonal pyrrhotite to monoclinic pyrrhotite. Mackinawite and argentian pentlandite are encounterd in chalcopyrite of the chalcopyrite-pyrrhotite-cubanite association. Occurrence of millerite is confined to the chalcopyrite-bornite ore in the Nippo ore deposit. Smythite is closely associated with monoclinic pyrrhotite which seems to have been altered to smythite during the last stage of mineralization.
Although there is much analogy in the paragenesis of sulfide minerals between Kamaishi and "Sudbury type" ores, the following facts in ores from the Kamaishi mine exhibit contrasts to "Sudbury type" ores. 1) extremely low exsolution temperature of pentlandite from Mss estimated from the Fe:Ni:S ratio of the ore; 2) very rare occurrence of pentlandite in chalcopyrite of the chalcopyrite-pyrrhotite-cubanite association; 3) no occurrence of pentlandite in direct conta t with pyrite; 4) high ratio of Co:Ni in the sulfide ores.