Genetic implications of fragmental ore texture in Japanese kuroko deposits

CIM Bulletin, Vol. 76, No. 849, 1983

LLOYD A. CLARK Chief Geologist, Saskatchewan Mining Development Corporation Saskatoon, Saskatchewan

The kuroko deposits, which are extremely variable in size, comprise both stratiform layers of clastic sulphide particles and cross-cutting vein and disseminated components. The ores lie on the flanks ofrhyolitic lava domes which have risen from a postulated subvolcanic magma chamber that provided cupriferous hydrothermal solutions and also acted as a heat source driving ground-water convecting cells. The polymetallic sulphides precipitated initially in explosion craters, pipe fillings and feeder veins as rising hydrothermal solutions mixed with sea water. As sulphides clogged the plumbing system, minor phreatomagmatic explosions produced layers of sulphide fragments on the adjacent sea floor. One to ten repetitions of this process produced multiple fragmental sulphide beds. Each deposit formed in a relatively short space of time, because only minor tuff beds are intercalated in spite of active volcanicity in the region. In this model, the heated ground waters convecting through thick piles of acidic pyroclastic rocks leach the zinc, lead and barium later deposited as kuroko ores. As the sub-volcanic pluton cools, the supply of silica and copper wanes and the minor counter-circulation cells above the pluton are replaced by main cell fluids bearing zinc, etc. This change in solution flow is abrupt rather than gradual, as the plumbing system is somewhat disrupted by the magmatophreatic explosions that create each ore layer
Keywords: Ore genesis, Fragmental ores, Kuroko deposits, Japan, Sulphides, Volcanogenesis, Breccia ores.