Kuroko Deposit Modeling Based on Magmatic Hydrothermal Theory

Abstract

Three lines of evidence which support the magmatic hydrothermal origin of kuroko deposits are evaluated.
(1) Mineralization of kuroko deposits took place around 15 Ma, at the same time as several other geologic episodes including a major shift in the tectonic stress field, a peak of bimodal volcanic activity, and a maximum degree of subsidence. The style of eruption of rhyolite changed from lava flows, acid hyaloclastites, and lithic tuff breccias before mineralization to extensive post-ore pumiceous pyroclastic flows. In an analogy with the Yellowstone caldera, absence of pre-ore basalt directly below the known kuroko deposits suggests that contemporaneous acidic magma existed at a depth of a few kilometers with the potential to release magmatic fluid. Basalt lava is often seen in the hanging-wall sequence of kuroko deposits; it extruded after the acidic pluton had solidified subsequent to mineralization. The post-ore pumiceous pyroclastics could have been produced by violent submarine eruptions similar to those related to the formation of Valles type calderas from resurgent acidic magma. Apparently such an abrupt release of material from the magma chamber was not suitable for conditions to develop and form kuroko deposits. (2) Based on oxygen and hydrogen isotopic study, sea water cannot be the sole source of the kuroko ore solution. Metals cannot be leached from footwall volcanic rocks, because an outward flow of hydrothermal fluids within the volcanic units is deduced from wall-rock alteration study. In addition, the notable homogeneity of isotopic compositions of ore minerals rules out the possibility that the ore metals were largely derived from basement rocks by leaching. (3) The rhyolitic magma which was contemporaneous with ore deposition had a large potential to result in separation of a metal-rich aqueous solution during its crystallization. This aqueous phase would have been a very efficient ore-forming solution, even after dilution by convecting sea water. This magmatic hydrothermal model of kuroko deposits is more consistent with the observed geological features than other models.