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1.65 to 1.78 b.y.-old volcanogenlc massive sulfide deposits that are related to centers of rhyolitic activity in central Arizona were initially mined for enriched gold and silver contents of oxidized ores. Although the precious metal contents of unoxidized (protore) ores are considerably lower than those of oxidized counterparts, many unoxidized deposits today would be considered of economic value only because of their gold and silver, not just for their base metals.

Massive sulfide deposits in Arizona are located, from west to east, in the Hualapai, Bagdad, Mayer-Prescott, Jerome, and Payson areas. Production statistics for the first four areas indicate that gold grades in the oxidized ores ranged from 1.1 to 0.2 oz/ton, and in the protore from 0.7 to 0.001 oz/ton. Silver grades in the oxidized ores ranged from 8 to 3 oz/ton, and in the protore from 4.5 to 0.2 oz/ton. Total silver/gold ratios varied from 600 to 15. Gold and silver grades were highest in the Mayer-Prescott area (0.068 oz Au; 2.37 oz Ag), and decreased in the Jerome (0.043 oz Au; 1.55 oz Ag), Hualapai (0.004 oz Au; 0.61 oz Ag), and Bagdad areas (0.002 oz Au; 0.39 oz Ag). This spatial variation of precious metal grades is believed to be related to the duration of volcanism and type of associated volcanic rocks in the four areas; the variation suggests that gold and silver mineralization was controlled by magma chemistry and evolution.

Precious metal contents appear to be independent of the overall copper, lead, or zinc grade

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