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Abstract

The lead of Mississippi Valley deposits shows anomalous isotope ratios and great variability in these ratios within the separate districts. The anomaly consists of an excess of both Pb206 and Pb208 in comparison to ordinary lead. These facts suggest that the isotopic evolution of lead in this mineral province has been modified from that prevailing in most districts by some unusual variation of the geologic history that allowed evolution for a considerable period in an environment richer in uranium and thorium than that in which ordinary lead has evolved. The hypothesis is here presented that this special environment was the Precambrian granites of the Central craton; and that the remobilization was an igneous phenomenon closely tied to the evolution of alkaline and, particularly, potassic igneous rocks.

Post-Precambrian igneous rocks of the Mississippi Valley are widespread, though not closely related in space to the ore deposits. They are predominantly alkaline, rich in potash. Such rocks are believed to have evolved in continental areas from a hybrid magma produced through partial assimilation of crustal granitic rocks by primary olivine basalt from depth. Basalt contains uranium, thorium, and lead approximately in ratios suitable for evolution of ordinary lead. Granites, however, contain higher ratios of uranium and of thorium to lead. From the time of emplacement of the granite in Precambrian time, its contained lead evolved in an environment exceptionally rich in radiogenic lead. Absorption of this granite into a basaltic magma should produce a hybrid alkaline magma whose lead is isotopically anomalous.

Alkaline magmas are evolved typically in tectonically stable platform areas of the earth's crust, characterized by simple fracturing without pronounced orogenic deformation. These magmas contain up to several times as much radioactive material as the subalkaline rocks, and this may have contributed much of their magmatic heat. Individual magmas could be isolated below a thick competent crust where they could undergo a long period of quiet assimilation of country rock and of differentiation. Their structural setting lacks the orogenic pressures that would tend to propel them toward the surface, but ore deposits given off from such bodies could occur in overyling crustal blocks with little evidence of igneous association.

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