Gold-bearing siliceous ore of the Nurukawa kuroko deposit, Akita prefecture
1987 Volume 37 Issue 202 Pages 109-118
Details
1987 Volume 37 Issue 202 Pages 109-118
The Nurukawa deposit is of Kuroko type lately discovered in the northeastern extension of the Hokuroku district. The first discovery was made by surface diamond drilling in 1984 under thick cover of Quaternary pyroclastics about 2 km west of the lake Towada. An extensive exploration program has been carried out by combining construction of underground drifts and subsequent underground drillings.
The No. 5 and No. 3 ore bodies are now under exploration and exploitation. The characterisitcs of the deposit are nearly the same as those of known Kuroko deposits except its high gold content. It consists of polymetallic massive sulfide layer (black ore) and underlying silicified tuff breccia with network and dissemination of sulfides (siliceous ore). The siliceous orebody is classified into two types, namely, layered and stockwork, based on their shape and mode of occurrence. The latter type has cylindrical shape with a diameter of 20 to 40 meters and the vertical length of more than 50 meters. It has been revealed that gold content in the siliceous ore is extremely high especially in its core which ranges from 15 gram/ton to 100 gram/ton.
Gold and silver mineral in siliceous ore is identified as electrum which has 50-70wt.% gold and 50-30 wt.% silver. In layered and stockwork siliceous ores, the electrum is concentrated in the chalcopyrite-pyrite-quartz veinlets as tiny grains about 10 microns in their diameter. These veinlets occur both on the margins of sphalerite and galena-bearing sulfide veinlets and within breccias in sphalerite and galena-bearing silicified pyroclastic rocks.
Statistical analysis of assays on gold, silver, lead, zinc, iron, and barium (sulfate) in the siliceous ore indicates that gold has strong affinity with copper and iron. Whereas, lead, zinc and barium which represent the elements enriched in black ore show good correlation each other. It is concluded, therefore, that the gold mineralization took place prior to that of the black ore.
