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Kazuyo Hirose, Daizo Ishiyama, Toshio Mizuta
GEOLOGY AND GEOCHEMICAL CHARACTERISTICS OF THE KAOLINITE-BEARING GOLD-RICH NURUKAWA KUROKO DEPOSIT, AOMORI PREFECTURE, JAPAN

Hirose1,D. Ishiyama2, T. Mizuta2

1Nikko Exploration & Development CO., LTD., Tokyo, Japan

2Akita University, Akita, Japan

 

GEOLOGY AND GEOCHEMICAL CHARACTERISTICS OF THE KAOLINITE-BEARING GOLD-RICH NURUKAWA KUROKO DEPOSIT, AOMORI PREFECTURE, JAPAN

                                                   

  1. Introduction

The Nurukawa Kuroko deposit is located in the northeast margin of the Hokuroku district, northeastern Japan, an area in which many Kuroko deposits are distributed. The Nurukawa Kuroko deposit is one of the gold rich volcanogenic massive sulfide deposits and also the only example
of gold rich Kuroko deposit among Kuroko deposits in Hokuroku district. The average Au content
of typical Kuroko deposits in the Hokuroku district is 1.3 g/ton [Tanimura et al., 1983]. Tonnage of ore of the Nurukawa Kuroko deposit is about 1million ton and the average gold grade is 6.8 g/ton [Yamada et al., 1988] that is much higher than typical Kuroko deposits. We describe the mode of occurrence of ores and the geochemical characteristics of Au and Pb-Zn mineralization of the Nurukawa deposit and estimate hydrothermal system forming gold-rich Nurukawa deposit.

  1. Outline of geology and ore deposits

The Nurukawa Kuroko deposit was formed in Middle Miocene time [MMAJ, 1988]. The Nurukawa deposit consists of five orebodies, Nos. 1 to 5. The largest of them is No. 5 orebody. It lies in the uppermost part of acidic tuff breccia of the Lower-Hayasemori Formation. It is covered at the top by pumice tuff of the Upper-Hayasemori Formation [Nishitani et al., 1986]. Some dacitic crypto-lava-domes, intruded into the acidic tuff breccia of the Lower-Hayasemori Formation, can be found under No. 5 orebody [Yamada 1988]. No. 5 orebody is composed of an Au-bearing stockwork siliceous orebody, an Au-bearing bedded siliceous orebody, and a massive base-metal bedded orebody in ascending order. The Au-bearing stockwork siliceous orebody is funnel-shaped, the Au-bearing bedded siliceous orebody is dish-shaped, and the massive base-metal orebody is thinly lenticular in shape [Yamada et al., 1988].

  1. Mode of occurrence of ores and mineral assemblage

The ores of No. 5 orebody are divided into four types: Au-bearing siliceous ore, Pb-Zn-bearing siliceous ore, massive black ore and brecciated black ore. Au-bearing stockwork and bedded orebodies consist of Au-bearing and Pb-Zn-bearing siliceous ores. Massive base-metal orebodies consist of compact and brecciated black ores. Au-bearing siliceous ores are cut by Pb-Zn-bearing siliceous ores showing network structure. The Au mineralization is earlier than the Pb-Zn mineralization associated with the formation of Pb-Zn-bearing siliceous ores and compact and brecciated black ores.

Au-bearing siliceous ores consist of major amounts of pyrite, chalcopyrite and quartz, and lesser amounts of electrum, sphalerite, galena, hematite and kaolin and sericite. Pb-Zn-bearing siliceous ores include major amounts of quartz, sphalerite and galena, and small amounts of chalcopyrite, pyrite and sericite. The main constituent minerals of compact and brecciated black ores are sphalerite, galena, pyrite, chalcopyrite and barite, with lesser amounts of tetrahedrite, pearceite, pyrargyrite and sericite and rarely electrum and bornite. In the black orebody composed of brecciated black ores, there is alternation of brecciated black ores, layers rich in barite, layers composed of kaolinite and layers composed of sericite/smectite mixed layer mineral. The clay-bearing layers show a banded texture. Based on the fact that the presence of kaolin is in altered rocks of Au-bearing siliceous ores and clay layers in brecciated black ores, acidic hydrothermal activities took place during gold and base metal mineralization stages intermittently.

  1. Homogenization temperature and salinity of fluid inclusions

Quartz of gold-bearing siliceous ore and sphalerite of black ore contain liquid-vapor two phase fluid inclusions. Homogenization temperatures and salinities of fluid inclusions in quartz of Au-bearing siliceous ores and in sphalerite and barite of brecciated black ores were found to be 253 to 286 °C with a peak of 280 °C and 3.3 to 5.3 wt % NaCl eq. for quartz, 210 to 252 °C with a weak bimodal distribution and 2.7 to 4.1 wt % NaCl eq. for sphalerite, and 145 to 262 °C and 1.5 to 2.7 wt %, NaCl eq. for barite, respectively. Homogenization temperatures and salinities of fluid inclusions in the Au-bearing siliceous ores are higher than those of fluid inclusions in the brecciated black ores. The salinity of fluid inclusions in the brecciated black ores is similar to the salinity of seawater.

  1. Hydrogen and oxygen isotopic ratios of hydrothermal solution forming Nurukawa deposit

The oxygen isotopic ratios of quartz in Au-bearing siliceous ores and Pb-Zn-bearing siliceous ores range from +9.2 to +10.2 and +9.0 to +10.0 per mil, respectively. The oxygen isotopic ratio of quartz crystals in druses of brecciated black ores is +10.5 per mil [Yamada et al., 1988]. There is no significant difference among these oxygen isotopic ratios.

The oxygen isotopic ratios of fluid responsible for the formation of Au-bearing siliceous and black ores were estimated on the basis of oxygen isotopic ratios of quartz presented above, the formation temperatures and fractionation factor of quartz-water by Matsuhisa et al. [1979]. The ranges of the calculated oxygen isotopic ratios of hydrothermal solution responsible for Au-bearing siliceous ores and brecciated black ores are +0.4 to +2.8 per mil and –0.5 to +1.7 per mil, respectively.

The hydrogen isotopic ratio of sericite in brecciated black ores ranges from –37 to –41 per mil. The hydrogen isotopic ratio is very close to the hydrogen isotopic ratio of sericite associated with other Kuroko deposits in Japan. The hydrogen isotopic ratios for hydrothermal solution responsible for brecciated black ores were estimated on the basis of the hydrogen isotopic ratios of sericite, the formation temperature, and the fractionation factor of sericite-water by Marumo et al. [1980]. The estimated hydrogen isotopic ratios range from –14 to –10 per mil.

Hydrogen isotopic ratios of kaolin of Au-bearing siliceous ores range from -62 to -53 per mil. The hydrogen isotopic ratios of kaolin are about 30 per mil lower than those of kaolin that is associated with typical Kuroko deposits in Japan. The hydrogen isotopic ratios are also different from those of kaolinite equilibrated with meteoric water from vein-type deposits and modern geothermal area in Japan. Hydrogen isotopic ratios for hydrothermal solution responsible for Au-bearing siliceous ores were also calculated using the hydrogen isotopic ratios of kaolin, the formation temperatures, and the fractionation factor of kaolinite-water by Sheppard and Gilg [1996]. The calculated hydrogen isotopic ratios range from –47 to –33 per mil. The hydrogen isotopic ratios of hydrothermal solution directly extracted from fluid inclusions in quartz for Au-bearing siliceous ores range from –55 to –45 per mil. The hydrogen isotopic ratios of hydrothermal solution forming the Au-bearing siliceous ores is distinctly smaller than those of hydrothermal solution forming the brecciated black ores, although there is a difference of about 10 per mil between the calculated hydrogen isotopic ratios of fluid and those measured in fluid extracted from fluid inclusions in quartz.

Considering the higher salinity of fluid inclusions of Au-bearing siliceous ores and the relationships of hydrogen and oxygen isotopic ratios of hydrothermal solution, it is thought that the Au-bearing siliceous ores at No. 5 orebody of the Nurukawa deposit were formed by hydrothermal solution containing fluid of magmatic origin. Based on the geological relationships around the Nurukawa deposit, there is a possibility that the generation of fluid of magmatic origin was caused by the emplacement of dacitic crypto-lava-domes under the Nurukawa deposit. When the Au-bearing siliceous ores were formed, the contribution of fluid of magmatic origin to hydrothermal solution of seawater origin would have been large. The style of circulation of hydrothermal solution changes from the hydrothermal system associated with great contribution of magmatic water to the seawater dominant hydrothermal system according to the decline in activity of dacitic magmatism.

 

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