Abstract

The Globe-Miami mining district in south central Arizona is one of the oldest and most productive districts in the Arizona copper province. The district contains several small to large plutons, the K-Ar isotopic ages of which, with one notable exception, are Precambrian. The exception, the Schultze Granite, is early Tertiary in age and is the only pluton temporally and spatially associated with the copper deposits. Heat and other emanations from the Schultze Granite have reset the K-Ar isotopic ages of some of the older plutons so that their absolute ages do not indicate the age of crystallization. The Willow Spring Granodiorite appears to correlate with part of the Precambrian X or Y Madera Diorite and, if so, is about 1,600 m.y. old. The Lost Gulch Quartz Monzonite probably correlates with part of the Precambrian Y Ruin Granite and is about 1,400 m.y. old. The Solitude Granite is Precambrian, but its isotopic age has been reset, and there is no lithologic correlative of known age elsewhere. Diabase in the district is part of the extensive Precambrian Y diabases of south central Arizona; its K-Ar age is 1,079 + or - 30 m.y.The Schultze Granite is a composite pluton comprising at least three intrusive phases. The earliest is a granodiorite, the intermediate or main phase is a porphyritic quartz monzonite, and the youngest is a series of porphyries that were not all intruded at the same time. The copper deposits, of which there are at least eight productive deposits and several undeveloped prospects, are all related to the porphyries. The mean intrusive age of the main phase of the Schultze is 61.2 + or - 0.4 m.y. The isotopic age of the porphyry phase is uncertain because of extensive alteration and because of multiple periods of intrusion.The age of mineralization differs from place to place across the district and spans about 5 m.y. From oldest to youngest, the known periods of mineralization are as follows: Copper Cities orebody, 63.3 + or - 0.5 m.y.; regional quartz-sericite veins, 61.1 + or - 0.3 m.y.; Miami-Inspiration orebody, 59.5 + or - 0.3 m.y.; and Pinto Valley orebody, 59.1 + or - 0.5 m.y. The ages of mineralization of the Miami-Inspiration and Pinto Valley orebodies are indistinguishable, but the Copper Cities orebody and the regional quartz-sericite veins differ in age statistically from the Miami-Inspiration and Pinto Valley orebodies and from each other. There are, therefore, at least three statistically distinguishable isotopic ages of mineralization.

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