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.

In the Minnesota River Valley, an epizonal, anorogenic granite that is often referred to as the granite of section 28 (lat 44°49.73′N, long 95°33.90′W) has been found to have a Pb-Pb age of 1.84 ± 0.05 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. The Th-Pb age of the feldspar-whole-rock pair is 1.9 b.y., which is in satisfactory agreement with the Pb-Pb age; but the U-Pb age is greater than the Pb-Pb age and probably indicates that uranium has been leached from the whole rock within the past several hundred million years, perhaps as a consequence of dilatancy resulting from uplift and erosion. Another granite, the mesozonal, late-tectonic Precambrian Sacred Heart Granite in the Minnesota River Valley (lat 44°41.3′N, long 95°21.5′W) is found to have a Pb-Pb age of 2.605 ± 0.006 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. Both the Th-Pb and U-Pb isochron ages are much older than the Pb-Pb age. An older age was expected for the U-Pb system as it had been previously found in the epizonal granite, but to also find it for the Th-Pb system was surprising. As is predicted for these kinds of granites in this type of tectonic environment on the basis of Mesozoic and Cenozoic analogues, the initial leads in the granites indicate that they were derived from source material having values of 238 U/ 204 Pb <9 normalized to the present day. This feature is common in Mesozoic and Cenozoic igneous rocks penetrating Precambrian terranes but is rarely observed in pre-Mesozoic igneous rocks. The Sacred Heart Granite is the oldest igneous rock known to show this effect and is the first representative of a mesozonal granite. The uranium depletion event appears to have been a granulite-facies metamorphism, but the age of that metamorphism cannot be determined from the available data. The model-lead-age information, however, suggests that it occurred before 2.78 b.y. ago. The source materials for both granites also underwent an earlier stage of extensive but unknown duration during which 238 U/ 204 Pb >9. In Phanerozoic rocks, such values are characteristic of ensialic tectonic environments. Similar development of ensialic environments was apparently occurring also in perancient times.