Metal Provinces of the Western United States

By plotting to scale and by value every metal mine or district in the United States west of the Great Plains, it has been possible to draw outlines of metal provinces, both for the separate metals and for combined metals. The separate metal provinces show distinctive, but generally similar shapes; however, the points of greatest concentration of the different metals (except for lead and silver) are quite distinct and are widely separated. The combined metal provinces, and some of the separate metal provinces, are very large, hundreds of miles long, and they make a crude raylike pattern around the Colorado Plateau, localized at four thick piles of Tertiary volcanic rocks.

Within a few hundred miles of the Pacific coast, all features (coast line, crustal tectonics, and metal provinces) are about parallel, striking north-northwest. Farther inland the metal provinces are almost wholly independent of crustal structure, with some trends in north-northeast and northeast directions, but are not well marked.

There are a few important districts of Precambrian age, none of Paleozoic age, a few of Mesozoic age, and a tremendous surge of deposition of Laramide (Late Cretaceous-Eocene) age. Since Eocene time metallization has continued, but in diminishing intensity. The piles of volcanic rocks around the Colorado Plateau, with which the combined metal provinces are associated, are younger than the main Laramide period of metallization, but are older than the succeeding stages.

The western border of the North American continent is outlined by a series of bulges, like linked sausages, in which the continent protrudes westward into the ocean and deformed areas protrude eastward into the undeformed continent. The metal provinces described here occupy the largest of these bulges, but there are similar features in Alaska, in Mexico, and perhaps in British Columbia, each surrounding complexes of metal provinces. From British Columbia through the United States to Mexico, the main period of metallization is progressively younger: Mesozoic in British Columbia, Laramide in the United States, and Miocene in Mexico. The volume of Cenozoic volcanic rocks increases progressively southward from Alaska through British Columbia and the United States to a tremendous culmination in Mexico.

Many new geophysical data show that the area of this study in the United States is anomalous in many respects: thin crust; high heat flow; low Bouguer gravity data; and low P_n velocities in the upper mantle, with considerable lateral heterogeneity of physical properties in the mantle.

It is concluded that the metals that make up the metal provinces must have come from the mantle, and that the role of igneous intrusion was probably that of a structural control rather than a source. The metal province maps are believed to outline a primitive heterogeneous distribution of metals in the upper mantle.