The Kobui Project
- The Kobui Project area contains two large epithermal, advanced argillic alteration (AAA) lithocaps.
- The two Kobui lithocaps may have formed above telescoped porphyry systems. The larger Kobui (western) lithocap is of particular interest and shows prominent alteration zoning and associated strong gold and multi-element anomalies. Based on the alteration and metal zonation patterns interpreted by Japan Gold’s consultants, the causal porphyry intrusions associated with the Kobui lithocaps are interpreted to lie between 1 to 2 km depth below the present-day land surface.
The Kobui Project comprises eight contiguous Prospecting Rights Application blocks (2,738 hectares) in the Kameda Peninsula of southwestern Hokkaido.
Interest in the mineral potential of the Kameda Peninsula followed the discovery of gold in the active hot springs of the Osorezan geothermal field on Shimokita Peninsula in northern Honshu by the MMAJ in 1988.
Reconnaissance work undertaken by the 1Nissho Iwai-Austpac Gold NL JV (NI-APG) in late 1988 identified extensive areas of hydrothermal alteration at Kobui and returned encouraging results of up to 0.13 ppm gold, 27 ppm silver, 2020 ppm mercury and 200 ppm tellurium in rock-float samples.
The MMAJ undertook extensive geological mapping and metallogenic studies over the Kameda Peninsula in the 1990’s. Their work included detailed geological mapping, petrological studies, radiometric age dating and limited scout diamond drilling; this work was the first to define the distribution and character of the AAA lithocap occurrences at Kobui and the nearby Minamikayabe area. The two major lithocaps at Kobui are known as Kobui (4.1 km2) in the western part of the project and Tokiwamatsu-Furube (1.2 km2) in the east. The 2MMAJ XRD studies confirmed the presence of advanced argillic alteration minerals in the lithocaps (alunite-pyrophyllite-diaspore rich with subordinate topaz, zunyite and outer kaolinite). Radiometric K-Ar age dating on alunite sampled from both lithocaps returned different formation ages, characterized by 2.9 to 1.8 Ma in the western lithocap and 1.3 to 0.2 Ma in the eastern lithocap.
Geology and Mineralisation
The Kobui area is situated in the Kameda peninsula in southwest Hokkaido, the northern part of the Northeast Japan Arc, where epithermal base and precious metal deposits cluster in association with the Neogene igneous activity. In the Kameda peninsula, Mesozoic sedimentary rocks (Toi Formation) underlie Middle Miocene submarine andesitic volcanic rocks (Kunnui Formation) and Middle to Late Miocene submarine sedimentary rocks (Shiodomarigawa Formation) (Hata and Tsushima, 1971; Hata and Uemura, 1983). These Miocene rocks are intruded by NNW-SSE trending basaltic dikes of Middle Miocene age, and WNW-ESE trending dacitic intrusions of Late Miocene age (Hata and Tsushima, 1971; Hata and Uemura, 1983). Andesitic rocks of Pliocene and Quaternary age cover these older rocks, forming a NW-SE trending volcanic chain (3Watanabe et al., 1996). Major Quaternary volcanoes in the chain include Komagatake, Yokotsudake, Maruyama and Esan from northwest to southwest. The Kobui area is located near to the southeastern end of the volcanic chain (referenced from Watanabe et al 1996).
The advanced argillic alteration in the Kobui area is hosted by the Pliocene Kobui andesite lavas and is divided into Kobui (KAA) and Tokiwamatsu-Furube advanced argillic alteration areas (TFAA). KAA formed during the period from 2.9 to 1.8 Ma and TFAA from 1.3 to 0.2 Ma.
The alteration is divided into pyrophyllite-diaspore-alunite, alunite-minamiite, alunite, dickite-kaolinite and smectite zones from inner to outer alteration zones. KAA is characterized by the presence of pyrophyllite, diaspore, topaz and zunyite, whereas TFAA is lacking in these minerals, but contains tridymite and cristobalite.
The hydrothermal activity in the Kobui area began with acidic and higher-temperature fluids which formed the advanced argillic alteration, followed by crater-forming eruptions, and ending with sulfur-dissemination in crater-filling lacustrine sediments by H2S dominant, more reduced solutions. The loci of the activity has shifted to the east since 2.9 Ma (referenced from 2Watanabe et al 1997).
Japan Gold Exploration Work and Results: Japan Gold conducted a short program of reconnaissance field work in August-September 2017. The aim was to validate the occurrence, conduct preliminary geological mapping and assess the prospectivity of the Kobui AAA lithocaps for copper and gold. The work program included:
- SWIR spectrometer alteration survey (Plus Minerals) – 165 spectral analyses;
- Surface rock geochemical survey – 136 gold and multi-element analyses (ALS);
- Surface rock geochemical survey – seven whole-rock lithogeochemical analyses (ALS);
- Petrological study and lithogeochemical analysis – seven rock samples from the lithocap and less-altered host-rocks (Dr Tony Crawford); and
- Geochemical metal zoning analysis (Dr Steve Garwin).
This work program confirmed the occurrence of a two large AAA lithocaps hosted by Plio-Pleistocene andesitic-dacitic volcanic rocks separated by a younger volcanic center (Mt Maruyama). The alteration systems exposed at Kobui surround a series of generally eastward-younging, andesite-dacite domes that lie along an east-southeasterly trend that extends toward the active Esan volcano complex, which is located about eight km east of Kobui.
The larger western lithocap (Kobui) is exposed between about 525 and 225-m ASL; it is alunite-pyrophyllite-diaspore dominant and contains several massive and vughy silica ledges. Selective grab rock samples return the following gold and multi-element results; characterized by maxima of 0.15 ppm gold (range 0.01-0.15 ppm), 2.1 ppm silver (range 0.01-2.1 ppm), 287 ppm copper (range 0.3-287 ppm), 164 ppm molybdenum (range 0.32-164 ppm), 1,115 ppm arsenic (range 1.4-1,115 ppm), 55 ppm antimony (range 0.35-55 ppm), 47 ppm bismuth (range 0.01-47 ppm), 102 ppm selenium (range 1-102 ppm) and 5.8 ppm tellurium (range 0.05-5.8 ppm).
The smaller eastern lithocap (Tokiwamatsu-Furube) is exposed between about 450 and 150-m ASL; it is alunite-dickite dominant and contains several massive and vughy silica ledges with tridymite and cristobalite present. Selective grab rock samples return weaker gold and multi-element results; including maxima of 0.07 ppm gold (range 0.01-0.07 ppm), 0.27 ppm silver (range 0.01-0.27 ppm), 83 ppm copper (range 0.3-83 ppm), 5.0 ppm molybdenum (range 0.40 -5.0 ppm) , 226 ppm arsenic (range 1-226 ppm), 27 ppm antimony (range 0.3-27 ppm), 100 ppm mercury (range 0.007-100 ppm), 4.8 ppm bismuth (range 0.01-4.8 ppm), 15 ppm selenium (range 1-15 ppm) and 4.3 ppm tellurium (range 0.05-4.3 ppm) .
The two Kobui lithocaps may have formed above telescoped porphyry systems. The larger Kobui (western) lithocap is of particular interest and shows prominent alteration zoning and associated strong gold and multi-element anomalies. Based on the alteration and metal zonation patterns interpreted by Japan Gold’s consultants, the causal porphyry intrusions associated with the Kobui lithocaps are interpreted to lie between 1 to 2 km depth below the present-day land surface.
The porphyry copper potential at Kobui is supported by a 2014 report published by the USGS that assesses the porphyry potential of Japan, as described in the summary of the Minamikayabe Project.
1 Austpac Gold NL. Company Reports, 1988 to 1994.
2Watanabe, Y., Aoki, M., and Yamamoto, K., 1997. Geology, age and style of advanced argillic alteration in the Kobui area, southwestern Hokkaido, Japan. Resource Geology, vol. 47(5), pp.263-281.
3Watanabe, Y., Aoki, M., and Nakajima, N., 1996. Age and Style of Epithermal Gold Mineralization in the Minamikayabe Area, Southwestern Japan. Resource Geology, vol. 46 (6), pp.317-326.