Data obtained during recent detailed mapping of intrusive areas in the eastern Great Basin combined with an examination and review of 107 mining districts in the Basin and Range Province of the United States containing hydrothermal deposits of Au, Ag, Pb, Zn, Cu, Mo, W, Kg, and U has led to the recognition of significant differences between "productive" intrusives, (those associated with hydrothermal mineralization) and "barren" intrusives, (without significant mineralization). For 86 percent of the deposits of Cretaceous and younger age there is a significant amount of associated intrusive porphyry or aphanitic rock present as large dikes, sills, and breccias with aphanitic matrix or more commonly as stocks or pipes occurring with or without granitoid textured rock. These rock types with few exceptions, are exposed within 2 miles of the ore deposit. The great majority of barren intrusives are of granitoid rocks only, except for the presence of smaller porphyry dikes and sills. Deposits of tungsten show an association preference with granitoid intrusives. These statistics indicate that in finding or developing a new ore deposit of the above metals other than tungsten, it is necessary to have a large intrusive aphanite porphyry existing within 2 miles of the exploration area in order reasonably to expect (for 86% of the cases) the existence of a significant ore body. Because barren intrusive porphyries do exist in the Basin and Range Province, further investigation of the differences and similarities between productive and barren porphyry plutons is contemplated.

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