CHALCOPYRITE ON PYRITE

ABOUT THE SPECIMEN

Sulfide minerals from this region of Japan are both beautiful and very difficult to find on the market. This combination features sharp chalcopyrite crystals - many of them twinned - interspersed with traces of marcasite and cubic pyrite, all atop a pyrite matrix. The backside of the matrix has rounded negative casts that give it a neat and unique look. As far as Japanese localities go, the Kosaka Mine is a lesser known one, beginning operations all the way back in the Edo period and operating intermittently until the 1970's. This is a RARE example, likely dating all the way back to the 1800's and is a fine and important representation of both the species and of the sulfide minerals to come out of Akita Prefecture.

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The Kosaka Mine represents the type locality for Kuroko-type volcanogenic massive sulfide deposits, formed during mid-Miocene back-arc rifting approximately 15-13 Ma in the Hokuroku Basin. The deposit consists of stratiform massive sulfide lenses in rhyolitic volcanic rocks and associated sediments, with classic Kuroko zoning from barite ore through black ore (sphalerite-galena-rich) to yellow ore (chalcopyrite-pyrite-rich). Specimens display brassy chalcopyrite tetrahedrons reaching 1-2 cm, often associated with sharp pyrite cubes and spear-like marcasite crystals on quartz matrix. The sulfides crystallized from seafloor hydrothermal fluids in a submarine environment, with mineralization discovered in 1861 and mining operations running 1884-1974. What makes Kosaka historically significant is its role establishing the Kuroko model; these deposits became the global standard for understanding bimodal-felsic VMS systems and influenced interpretation of similar deposits worldwide. The mine produced over 30 million tonnes of ore grading 2.84% lead, 8.48% zinc, plus significant gold and silver. Authentic specimen-grade material from the classic mining era remains valued for both aesthetic quality and geological teaching purposes.