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
Figures & Tables
Proterozoic Ore Deposits of the Southwestern U.S.
This field conference of the Society of Economic Geologists, held in conjunction with the 1987 Annual Meeting of the Geological Society of America in Phoenix, Arizona, investigates various Proterozoic ore deposits in southeastern California and western and central Arizona. The trip starts in Las Vegas, Nevada, and ends in Phoenix, Arizona. Although Proterozoic ore deposits and geology are emphasized, the field trip traverses a variety of tectonic provinces, each containing ore deposits of various ages. Many geologic features and ore deposits that are not Proterozoic in age will be discussed during the course of the trip, but no stops will be made to investigate any of these, features.
The morning part of the trip starts in the Mesozoic foreland fold and thrust belt of Nevada and southeastern California, goes just east of the Mesozoic batholith belt of California, and ends in crystalline basement containing Early and Middle Proterozoic metamorphic and plutonic rocks. Within this basement is exposed the Middle Proterozoic Mountain Pass Carbonatite deposit, the largest source of rare-earth elements in the world. The Mountain Pass Carbonatite will be the focus of our morning stops.
The afternoon part of the trip crosses Early Proterozoic high-grade metamorphic rocks and plutonic rocks east of Mountain Pass. Ore deposits noted along the afternoon portion of the trip include middle Tertiary epithermal, gold-rich veins at Searchlight, Nevada, and disseminated gold deposits in Tertiary volcanic rocks near Hart Peak in the New York Mountains. The trip continues across the Colorado River trough along the border of California and Arizona, where crystalline basement and its cover of Tertiary volcanic rocks and plutons have been greatly extended along listric normal faults and low-angle detachment faults. The afternoon part of the trip ends in Oatman, Arizona, a district famous for its middle Tertiary epithermal, gold-rich deposits. The ore deposits at Oatman will be the subject of a slide show and talk after supper on the first night of the trip.
The first day ends with an evening drive across a middle Tertiary volcanic pile near Oatman. The first night will be spent in Kingman, Arizona.
Assemble at the Las Vegas Convention Center, south of the Hilton Hotel. Take any main street south to Tropicana Avenue. Turn west on Tropicana, past the Las Vegas Strip, and continue to Interstate Highway 15. Turn south on Interstate 15 toward Mountain Pass, California. Mileage starts at 0.0.