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Abstract

The isotopic composition of lead was investigated in and around Kuroko deposits of the Hokuroku district, Japan. Although the ore leads of these deposits were found to occupy a narrow isotopic range, each ore deposit has a characteristic isotopic composition. Within a given ore deposit, black ore has a uniform isotopic composition but is significantly higher in radiogenic lead than yellow ore. The differences between ore types are, however, smaller than those between ore deposits. The volcanic host rocks are in general lower in radiogenic lead than the ores, whereas the deeper, older formations, in particular the Sasahata Formation and the Paleozoic basement, have more radiogenic lead than the ores.

On the basis of the isotopic distribution we conclude that a major part of the lead in the Kuroko deposits was derived from igneous, probably volcanic rocks with an uncertain but significant contribution coming from the underlying pre-Nishikurozawa formations. The ore fluids reached the Sasahata Formation and most likely also the Paleozoic basement. Each ore deposit within the district was formed by a local hydrothermal system. The difference in isotopic composition between the yellow and black ores reflects a shift in the proportions coming from the two major sources due to the temperature evolution of the hydrothermal system. The yellow ore seems to have a greater igneous rock lead component than does the black ore.

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