Native gold and the minerals of the Fe-Co-Ni-As-S system from the Nippo ore deposit of the Kamaishi mine

Abstract

The Nippo ore deposit, formerly called the Rasa Omine copper deposit, is one of the Kamaishi contact-metasomatic iron and/or copper deposit genetically relating to the Cretaceous granitic activities. Ore minerals of the Nippo deposit consists mainly of chalcopyrite, cubanite, less amounts of pyrrhotite and bornite. The ore contains about 1.0-1.5 grams per ton of Au and 10.0-12.8 grams per ton of Ag.
The writer obtained the native gold and the minerals of the FeAsS-CoAsS-NiAsS and FeAs ₂-CoAs ₂- NiAs ₂ systems, by means of panning of minerals which had been accumulated at the bottom of the classifier in the Nippo mill plant. This mill plant had treated the crude ores from the Nippo ore deposit alone.
The native gold from the Nippo ore deposit are classified into two systems, the Au-Ag, and the Au-Cu-Ag. The gold of the Au-Cu-Ag system is far less in amount than that of the Au-Ag system. The gold of the Au-Ag system ranges in composition from Au ₈₅Ag ₁₅ to Au₆Ag₉₄, and mostly abounds in Ag than Au₅₀Ag₅₀ In the gold of the Au-Cu-Ag system, the minerals of Au ₅₃Cu₄₆Ag (1:1 type cuprous gold) and Au₇₈Cu₁₉Ag₃ (4:1 type cuprous gold) seem to be the stable or metastable phases. Moreover, the lattice intergrowth texture of pinkish gold (cuprous gold) and yellow gold (Ag-poor gold), resulting from the exsolution of the solid solution of these two minerals, can be observable.
The minerals of the FeAsS-CoAsS-NiAsS system from the Nippo ore deposit were divided into two areas, when the composition of minerals was plotted in the ternary diagram of arsenopyrite-cobaltite-gersdorffite (Fe-AsS-CoAsS-NiAsS). However, the following relation is acceptable covering the above two areas:
y=0.60x+0.36
(Correlation coefficient being 0.91, provided that x=As/S and y=Fe+Co+Ni/S)
The Fe-Co-Ni diarsenides from the deposit are of loellingite and Fe-rich safllorite, while Co-rich safflorite and rammelsbergite (pararammelsbergite) are not detected. The ternary Fe-Co-Ni diarsenide compositions fall on the area of the more Fe-rich range (safliorite).
This paper has reported about the chemical compositions of minerals rarely occurring from the Nippo ore deposit, and also some considerations and discussion on the minerals have been given.
Analyses were preformed using the Hitachi Energy-Dispersive Type Electron Microprobe Analyzer, Model-X-560.