New occurrence of Ni-rich horomanite from the Kouyama gabbro, Hagi city, Yamaguchi Prefecture, Japan

Abstract

Ni-rich horomanite is found from the chalcopyrite-bearing (1.0-1.5 mode%) layer in the Kouyama gabbroic body, Hagi city, Yamaguchi Prefecture, western Japan and as second occurrence in the world. It occurs as inclusions in chalcopyrite interspaced with silicate minerals, vanadium-bearing magnetite and ilmenite. It is often associated with siegenite and is secondarily replaced by violarite. Chalcopyrite associating with Ni-rich horomanite commonly shows the polysynthetic twin. Analytical data for horomanite obtained by EPMA are Cu: 0.56-2.19, Fe: 23.01-25.32, Ni: 37.45-41.35, Co: 1.56-4.03 and S: 32.85-33.32 wt%. Their variations are small for inner grain or another grain. The atomic ratio of (Cu + Fe + Ni + Co): S correlates well with ideal formula of 9-8 for horomanite. In addition, Ni content in metal ratio for (Cu + Co): Fe: Ni (at%) ranges from 52 to 59 and is Ni-rich than that of original horomanite from the Horoman peridotite. Horomanite might be considered to be continuous solid solution ranging from 3.0 to 5.5 in terms of Ni(+Co) content. Therefore, general formula for horomanite is thought to be (Fe + Cu)6-x(Ni + Co)3+xS8(0 < x < 2.5).