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Abstract

The hydrothermal system responsible for Miocene massive sulfide mineralization in the Hokuroku basin of northern Japan is analyzed by combining simple heuristic calculations with critical field data, and by incorporating redistribution of anhydrite, silica, and δ18Or into a finite difference model of the hydrothermal system and comparing model chemical predictions to field observations. Kinetic constraints are taken into account. The exercise suggests that the Kuroko massive sulfide deposits were formed as the result of multiple small (1 × 3 km in cross section) intrusive pulses that cooled by convection and formed individual sulfide and sulfate lenses in less than 5,000 years and probably ~100 years. The general formation permeability was at least a few millidarcies and probably a few hundred millidarcies. Sulfate and silica deposition preceded sulfide deposition, performing the necessary function of isolating hot deeply circulating solutions from cool shallowly circulating solutions so they could vent uncooled into the sea. Surface discharge was controlled by major fractures. Calculated δ18Or anomalies are in excellent agreement with those observed near the Fukazawa massive sulfide deposit. The model deduced for the Knroko hydrothermal system is compatible with and supported by observations that have been made on ophiolite suites, and on hot and Warm springs at mid-ocean ridges. The correspondence between δ18Or profiles calculated here and those measured in the complete composite section through the Semail Ophiolite, Oman, is particularly striking. Insights are gained into the nature of deep (greater than 2 km) equilibrium oxygen isotope alteration. Exploration implications are indicated and a confirmatory test suggested.

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