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.   VIDEO   MORE INFO 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.