The Eboshi Project
- Eboshi lies about 50-km south of the Osorezan volcanic complex where a large geothermal system is actively depositing high-grade gold in hot spring precipitates and in veins in the subsurface.
- The Eboshi advanced high sulphidation alteration lithocap represents an encouraging exploration target for shallow-level epithermal gold and deeper porphyry copper-gold deposits.
- The porphyry copper potential at Eboshi is highlighted by a recent report published by the USGS1 in 2014 assessing the porphyry potential of Japan. Eboshi lies on a Late Miocene segment of the Northeast Japan Arc in calc-alkaline andesitic-dacitic volcanic arc rocks; this and similar arc segments containing preserved advanced argillic alteration systems are considered permissive for undiscovered porphyry copper deposits at depth.
- Prospective shallow epithermal gold and associated (deeper) porphyry copper-gold mineralization is proposed to be tested by a follow-up work program of airborne magnetics/radiometrics survey, more detailed surface exploration (mapping, systematic soil and rock samplings, ground IP-Resistivity surveys). The results from which would be used to plan a follow-up scout diamond drilling program.
- The southwest of the lithocap shows shows prominent high temperature alteration zoning and associated strong gold ± multi-element anomalies suggestive of a near surface mineralized porphyry intrusive.
The Eboshi Project comprises nine contiguous Prospecting Rights blocks (3,080 Ha) near the coastal town of Noheji in northern Honshu. The project area contains a large (exposed) high-sulfidation epithermal (HSE) alteration lithocap, one of several discovered along the Northeast Japan Arc by the Metal Mining Agency of Japan (MMAJ) during regional metallogenic surveys conducted in the 1980’s and 1990’s.
The Eboshi HSE lithocap was first evaluated by the MMAJ during a regional geological and exploration survey of northeastern Honshu in the mid-1990’s (2MITI, 1995-1999). The MMAJ work at Eboshi included geological mapping, geochemical rock-chip sampling, XRD alteration studies, MMI & conventional soil geochemical sampling, CSAMT geophysical surveys and two scout diamond drill holes (aggregating 954.7 m). The MMAJ work highlighted extensive low-level gold and associated arsenic-antimony-mercury anomalies in wide-spaced rock-chip and soil samples that partly coincided with the lithocap alteration area, which was interpreted to have overprinted a Late Miocene andesitic stratovolcano complex. The lithocap-style alteration zone is lobate in shape and extends over 4 km by 1.0 to 1.5 km. Within this zone, a north-northeasterly trending corridor of massive and vughy-silica, 750 m long by 350 m wide, forms Mt Eboshi and the adjacent ridge crests. Minor epithermal quartz-calcite-sulfide mineral vein occurrences were found in the valleys that lie along the eastern flank of the Mt Eboshi lithocap.
Surface rock sampling of epithermal veins reported gold values up to a maximum of 0.55 g/t Au and 0.63 g/t Au (no range or statistical distribution of sample values stated in literature) from two separate epithermal veins, each less than one meter wide. The two scout diamond holes were targeted on these vein occurrences. The first hole 7MATK-1 was drilled at a low-angle to 651 m depth and intersected a few narrow, low-grade quartz veins in quartz-sericite altered andesite, which included the best gold-silver intercept of 15 cm at 0.48 g/t Au and 57 g/t Ag at about 300 m vertical-depth below surface. The second hole 11 MATK-1 returned no significant gold intercepts. Neither drill hole tested the Eboshi advanced argillic alteration lithocap and its associated gold & multi-element soil and rock-chip anomalies.
An historic sulfur working, mined in the 1940’s, occurs in a valley off the eastern edge of the Eboshi lithocap. Its exact location was not clearly identified. However, small stockpiles of quartz-pyrite vein cobbles found by Japan Gold in the general vicinity of the recorded occurrence suggests that the working was developed on a quartz-pyrite vein of unknown dimensions.
Geology and Mineralisation
The results and interpretations of the alteration study done by Plus Minerals have significantly advanced our understanding of the Eboshi HSE lithocap. They interpret it to be related to an eroded Late Miocene stratovolcano about 10-km in diameter and containing three nested craters (each about 2-km diameter) aligned NNE.
The Eboshi HSE lithocap is hosted by Late Miocene rhyolitic breccia and andesitic volcanic rocks. It occurs on the highest ridge crests of the project area between elevations of 720 and 250-m ASL. The distribution of altered rock outcrops and residual boulders define a NE-SE elongated zone up to 1-km wide extending for about 5-km along a curvi-linear ridgeline.
The advanced argillic alteration assemblage defining the lithocap is pyrophyllite-dominant with varying proportions of vughy quartz, K-alunite, diaspore, zunyite, topaz, dickite and native sulfur . The lithocap is strongly oxidized at surface with abundant secondary hematite developed, probably after the oxidation of mainly pyrite. No secondary copper or arsenic minerals were observed.
A NE structural control on the lithocap is inferred from its elongated orientation. Minor intermediate-sulphidation epithermal quartz veins cutting andesitic volcanic rocks off the edges of the lithocap have a similar orientation
Japan Gold conducted a program of reconnaissance field work in June-July 2017 involving several highly respected industry consultants. The aim was to validate the occurrence, characterize and assess the prospectivity of the Eboshi HSE alteration lithocap for gold and copper. The work program included SWIR spectrometer alteration survey, surface rock geochemical survey, surface whole rock lithogeochemical analyses, petrological study & lithogeochemical analysis of lithocap, epithermal vein and host rocks; and geochemical metal zoning analysis.
Japan Gold’s initial reconnaissance work program produced encouraging results3. They have confirmed the occurrence of a large zoned HSE alteration lithocap centered on Mt Eboshi. The lithocap is hosted by a package of Late Miocene andesitic-dacitic volcanic rocks that form part of an eroded stratovolcano of about 10-km in diameter. The lithocap is exposed between about 720 and 250-m ASL; it is pyrophyllite-dominant and encloses several massive and vughy silica ledges. Selective grab rock samples taken within the lithocap returned anomalous gold and multielement (porphyry-pathfinder) anomalies; with values ranging between 0.11-0.71 g/t gold, 1.0-5.25 g/t silver, 50.2-769 ppm arsenic, 101-667 ppm copper, 4.01-361 ppm antimony, 5-108 ppm selenium, 5.05-90.8 ppm molybdenum, 1.08-53.7 ppm tellurium and 1.05-48.3 ppm bismuth.
Japan Gold infers that the large advanced argillic alteration zone at Eboshi is a high-sulfidation epithermal alteration lithocap at the top of a potential telescoped porphyry system. The project area shows potential for disseminated gold-silver within the lithocap, associated porphyry copper-gold beneath the lithocap, and low sulphidation epithermal gold-silver veins surrounding the lithocap alteration zone.
The porphyry copper potential at Eboshi is supported by a recent report published by the USGS in 2014 that assesses the porphyry potential of Japan. Eboshi lies on a Late Miocene segment of the Northeast Japan Arc in calc-alkaline andesitic-dacitic volcanic arc rocks; this and similar arc segments containing preserved advanced argillic alteration systems are considered permissive for undiscovered porphyry copper deposits at depth.
Prospective shallow epithermal gold and associated (deeper) porphyry copper-gold mineralization is proposed to be tested by a follow-up work program of airborne magnetics/radiometrics survey, more detailed surface exploration (mapping, systematic soil and rock samplings, ground IP-Resistivity surveys). The results from which would be used to plan a follow-up scout diamond drilling program.
1Hammarstrom, J.M., Bookstrom, A.A., DeMarr, M.W., Dicken, C.L., Ludington, S., Robinson, Jr., G.R., and Zientek, M.L., with contributions from Cox, D.P., Manipon, C.J.C., Rollan, L.A., Sakimoto, T., Takagi, T., and Watanabe, Y., 2014, Porphyry copper assessment of East and Southeast Asia—Philippines, Taiwan (Republic of China), Republic of Korea (South Korea), and Japan: U.S. Geological Survey Scientific Investigations Report 2010–5090–P, 241 p.
2MITI (1995). Report on the regional geological structure survey in northeastern Honshu during the fiscal year Heizei 7, 37p. (in Japanese).
2MITI (1996). Report on the regional geological structure survey in northeastern Honshu during the fiscal year Heizei 8, 32p. (in Japanese).
2MITI (1997). Report on the regional geological structure survey in northeastern Honshu during the fiscal year Heizei 9, 50p. (in Japanese).
2MITI (1998). Report on the regional geological structure survey in northeastern Honshu during the fiscal year Heizei 10, 70p. (in Japanese).
2MITI (1999). Report on the regional geological structure survey in northeastern Honshu during the fiscal year Heizei 11, 28p. (in Japanese).
3EBOSHI Project – SWIR Alteration Mapping Report & Presentation (Plus Minerals, 2017)