U-Th-Pb and 40Ar/39Ar Dating of the Mountain Pass Carbonatite and Alkalic Igneous Rocks, Southeastern California
Published:January 01, 1987
Ed DeWitt, L.M. Kwak, R.E. Zartman, 1987. "U-Th-Pb and 40Ar/39Ar Dating of the Mountain Pass Carbonatite and Alkalic Igneous Rocks, Southeastern California", Proterozoic Ore Deposits of the Southwestern U.S., Ed DeWitt
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The age of the Mountain Pass carbonatite, the largest source of rare-earth elements in the world, is an important constraint in the search for similar bodies. Emplacement of the carbonatite was the last event in the intrusion of a 1400-Ma alkalic complex. Based on field relationships, the intrusive sequence was 1) shonkinite, 2) syenite and granite, and 3) carbonate bodies, dikes, and veins. Late shonkinite dikes cut syenite and granite, but not the carbonate bodies.
Alkalic igneous rocks were emplaced between 1410 and 1400 Ma. Apatite from the shonkinite has a U-Pb date of 1410±2 Ma. Phlogopite from the shonkinite has a 40Ar/39Ar plateau date of 1400±8 Ma. Arfvedsonite from the syenite has a 40Ar/39Ar plateau date of 1403±7 Ma. Zircon from the syenite are highly discordant and have Pb-Pb dates as old as 1330 Ma; their U-Pb dates were not made appreciably older by abrasion techniques.
Monazite from the carbonate body have Th-Pb dates of 1375±7 Ma and imply that the carbonatite was emplaced about 25 Ma after syenite and granite. Bastnaesite and parisite from the carbonatite contain considerable common lead and suggest that the body was open to lead migration after emplacement. Clear parisite has an older Th-Pb date (1332±7 Ma) than coexisting bastnaesite, but that date is anomalously young if compared to the Th-Pb dates from monazite. The least radiogenic common-lead ratios of galena from the carbonatite and potassium feldspar from alkalic igneous rocks (204:206:207:208 = 1:16.08:15.23;35.61) indicate that the parent magma for both rock types was
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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.