Recent development and geologic characteristics of the Nurukawa kuroko deposit

概要

論文の詳細を見る
The Nurukawa kuroko deposit was discovered in 1984 and commenced its production in 1987. Current monthly production is 5, 000 tons of ore grading 6.8g/t Au, 123g/t Ag, 0.74%Cu, 3.86%Pb, and 7.92%Zn. The deposit consists of five discrete ore bodies; Nos 5 and 3 ore bodies are under mining operation and the rests are in exploration stage. Among them, No. 5 ore body is the largest in size and highest in ore grade. It is characterized by gold and silver rich black ore (massive sulfide) and gold-bearing siliceous ore which develops in footwall tuff breccias. The latter ore is subdivided into two types, namely, stratiform and stockwork types, on the basis of its mode of occurrence and distribution. The stratiform type siliceous ore is composed of fragments of gold-bearing network ore of quartz-chalcopyrite-pyrite and interstitial siliceous matrix rich in lead and zinc. On the contrary, the stockwork type siliceous ore is rich in chalcopyrite and pyrite and is overprinted by lead and zinc mineralization. No.3 ore body consists of bedded clayey black ore which is considered to have replaced tuff breccia, fine-grained massive black ore contaminated with muddy material, and underlying gypsum ore. No. 2 ore body is composed of massive sulfide ore which is zoned vertically into black and yellow ores, and ore composed of fragmented and trans-ported massive sulfides. Those are underlain by gypsum and siliceous ores. The wall-rock alteration zoning around the Nurukawa deposit is; sericite-chlorite, mixed-layer clay, montmorillonite, and zeolite zones, in ascending order. Kaolinite and kaolinite-montmorillonite mixed-layer clay mineral are observed above the black ore of the Nos. 3 and 5 ore bodies. The mineralization age determined by K-Ar method on sericite samples has a range between 10.7 and 12.5Ma with an average of 11.7Ma. Homogenization temperatures of fluid inclusions in quartz cluster around 265-281°C. It is suggested that no boiling had occurred during the formation of the deposit. The δ34S data on mineral separates and bulk sulfide ores range from +4.8 to+6.8% except one sample. All of these geochemical data coincide with "ordinary" values of the kuroko deposits of the Hokuroku area. Therefore, it is not easy to explain the reason why the Nurukawa deposit has extremely high gold content compared to other kuroko deposits. However, positive relationship between salinity and filling temperature of the fluid inclusions and geologic reconstruction of the area both suggest that the formation of the deposit appears to be happened near shore where ore-forming solution was diluted by meteoric water. This mixing might effectively cause the precipitation of gold.
資源地質学会の論文

Recent development and geologic characteristics of the Nurukawa kuroko deposit

スポンサーリンク

概要

資源地質学会 | 論文

スポンサーリンク