[H-RE13] 資源地球科学

[HRE13-07] Petrographic and geochemical characteristics of the Okohongo copper-silver deposit, Kaoko Belt, Kunene region, Namibia

*Abner Nghoongoloka^1,2、Ryohei Takahashi¹、Pearlyn Manalo ¹、Andrea Agangi¹、Hinako Sato¹、Manuel Nopeia³、Nigel Blamey⁴、Rob Bowell^5,6、Tavis Enno⁴ (1.Graduate School of International Resource Sciences, Akita Univ、2.Geological Survey of Namibia、3.Center for Regional Revitalization in Research and Education, Akita Univ、4.Department of Earth Sciences, Western Univ、5.Department of Earth Science Miller Hall, Queens Univ、6.SRK Consulting)

キーワード：Okohongo copper-silver deposit, hydrothermal deposit, fluid inclusion gas compositions

The Okohongo deposit, situated in the Opuwo district, Kunene region of Namibia, has an estimated resource of 10.2 million tonnes with copper and silver grades of 1.12 % and 17.7 g/t, respectively. The deposit is hosted by the lower part of the Devede and Beesvlakte Formations of the Ombombo Subgroup, Otavi Group in the Kaoko Belt. The ore body is sitting along N-S striking structures, characterized by N-S disharmonic folds and NW-trending shear zones. The copper and silver mineralization occurs as copper oxide and sulfide-bearing stratiform quartz-calcite veins, semi-massive copper-oxides and malachite, and chalcocite-bearing quartz-calcite-plagioclase stockwork. The quartz-calcite veins containing chrysocolla, malachite, shattuckite and chalcocite which are concordantly hosted by shale have been observed from the surface to a depth of 50 m in the drill core. The semi-massive copper-oxides and chalcocite ores in the drill core occur at depths ranging from 114 to 131 m, while the stockwork ores containing chalcocite associated with hematite and goethite occur at approximately 200 m depth. Both the semi-massive and stockwork ores are hosted in the brecciated, interbedded dolostone and grey shale of the Beesvlakte Formation. The Post-Archean Average Shale (PAAS)-normalized REE and Y patterns of the quartz-calcite veins show a flat trend, while those of the semi-massive and stockwork ores are characterized by a depletion of light REE, and the stockwork ores have a distinct positive Y anomaly. On the other hand, the brecciated schist and dolostone host-rocks exhibit a flat PAAS-normalized REE and Y pattern with a trivial light REE enrichment. The sulfur isotope ratios (δ³⁴S_CDT) of chalcocite from the semi-massive and stockwork ores are +5.6 ‰ and +8.3 ‰, respectively. The primary fluid inclusions in quartz from the quartz-calcite veins consist of two phases, liquid and vapor, with salinity and homogenization temperatures of 13.9-26.8 wt% NaCl eq., and 127-148 ^oC, respectively. The weighted mean of bulk fluid inclusion gas compositions is 98.98 mol% H₂O, 0.75 mol% N₂, 0.25 mol% CO₂, and trace amounts of CH₄, Ar, H₂S and He. The fluid inclusion gas compositions suggest shallow meteoric and magmatic signatures