Alteration Minerals and Mineralization in the Shakanai Kuroko Deposit, Akita Prefecture

Abstract

The modes of occurrence, associated mineral assemblages, and chemical compositions of minerals from the No.11 ore deposit and from the samples of bore holes in the north area of the Shakanai mine, are described on the basis of microscopic observation, X-ray diffraction analysis and electron probe microanalysis. As a first step to estimate the activity ratio of Mg ⁺⁺ to Fe ⁺⁺ in hydrothermal solution, exchange isotherm between chlorite and chloride solution has been made.
Though principal constituent ore minerals of the No.11 ore deposit are silmilar to those of other kuroko deposits, they have wide varieties in mode of occurrence, texture and chemical compositions. Complexmixtures of black ore and tuff breccia, and a paragenesis of various sphalerites support that the No.11 ore deposit has formed at various stages.
Chlorite minerals and interstratified sericite/montmorillonite are extensively distributed around the ore deposits. Contribution of magnesium-rich hydrothermal solution is primary factor for the formation of Mg-chlorite. It is possible that the mixed layer clay zone closely associated with the ore has formed in a different process from the formation of the mixed layer clay zone regarded as a transitional zone. This zone associated with the ore is considered to be influenced by the deposition of sulfide minerals with decreasing of temperature.