アラスカ州ストーンボーイ地域ポゴ地区における金鉱床の発見について

Sumitomo Metal Mining Arizona, Inc. and three other international companies formed the Stone Boy joint venture in 1991 and started a regional grassroots exploration program east of Fairbanks (Stone Boy area) focusing on SEDEX type lead-zinc and gold deposits.<BR>The region is underlain by highly deformed, amphibolite-grade gneiss of the late Proterozoic to mid-Paleozoic Yukon-Tanana terrane, intruded by Cretaceous granitoid bodies. In 1994, the first diamond drill hole on the Pogo claims intersected the high grade gold mineralization (15ft, 27g/t Au) which was later confirmed to be a part of the Liese gold deposit. The Liese Au de-posit has an estimated gold resource of 162 metric tonnes with an average grade of 17.8 g/t Au as of December 1998. Exploration to date on the Pogo claims has focused on the Liese Zone in the northwest part of the claims. Mineralization in the Liese Zone consists of at least two large, tabular gold-quartz bodies, designated L1 (Liese main ore body) and L2 (Liese lower ore body). These quartz bodies contain approximately 3% sulphides, including arsenopyrite, pyrite, pyrrhotite and loellingite. Other minor metallic minerals include various Au, Bi, Te, Cu, Pb, Zn, Ag, W and Mo minerals. Au-Bi has the most significant positive correlation.<BR>In addition to the Liese gold-quartz mineralization, two other styles of gold mineralization, stockwork and gold sulphide dissemination were discovered in the Pogo area. The Liese gold-quartz mineralization has some similar earmarks to the mesothermal gold quartz veins in the Fairbanks mining district, however its formation temperatures (300°C to 600°C) and trapping pressures (1, 700 to 2, 000 bar) are higher than those veins, and microscopic studies have revealed that the Liese gold-quartz is not vein but meta-quartzite due to its equigranular granoblastic texture. Arsenopyrite geothermometry indicates temperatures of 380°C to 600°C for the Liese deposit and 520°C for the North Zone stockwork gold mineralization. Fluid inclusion studies have suggested that the Liese deposit has homogenization temperatures of 290°C to 580°C, and was formed under ductile conditions at about 6km below the paleosurface. Stable isotope analysis of the Liese deposit indicates that the δ³⁴S values of sulfide minerals are±0 (-1.4 to +2.8%) suggesting mantle origin, δ¹⁸O values of gold-quartz are +12.3 to +15.7‰ and estimated δ18O values of hy-drothermal fluid are +5.07 to +10.40‰, which are similar to those of magmatic or metamorphic water. Evidence suggests that the Liese deposit is a metasomatized quartz or a tectonized quartz vein, or a combination of both formed within tectonic shear zones under high P/T conditions.<BR>The radiometric dating of the alteration minerals at Pogo suggests that the Pogo gold mineralization took place between 97.7Ma, and 91.2Ma, and well coincides with the ages of granodiorite (96.6Ma to 91.9Ma) and diorite (97.9Ma to 91.1Ma).<BR>The genesis of the Liese deposit still remains enigmatic and controvertial. Underground exploration, envirofimental studies, and final feasibility, including all engineering work and plant scale metallurgy will be completed by the end of the year 2000 with the intention of bringing the Liese deposit into production.