2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 16
Presentation Time: 8:00 AM-4:45 PM

U-Pb Geochronologic Study of Magmatic Zircon in Paleoproterozoic Granitic Pegmatite and Associated Metapelites, Black Hills, South Dakota: Implications for Gold Petrogenesis and Sedimentary Provenance

GHOSH, Amiya K., Department of Geology, Kent State University, Kent, OH 44242, FREI, Robert, Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, Copenhagen, DK-1350, Denmark, WHITEHOUSE, Martin J., Swedish Museum of Natural History, Box 50007, Frescativägen 40, Stockholm, SE-104 05, Sweden and DAHL, Peter S., Department of Geology, Kent State Univ, Kent, OH 44242, aghosh3@kent.edu

Precambrian crystalline rocks from the northern Black Hills (NBH) preserve a rich history of sedimentation, thermotectonism, magmatism, and mineralization. This study is part of an ongoing effort to unravel this history. Specifically, we report a new intrusive age for mid-crustal, Harney Peak-type pegmatitic granite (HPG) and the ages of detrital components in older metapelites. High-U (~1,800-22,000 ppm) zircon from the Crook Mountain pegmatite (CMG) was spot-dated by U-Pb ion-microprobe methods. Despite extensive metamictization, 16 of 32 zircon analyses yielded upper- and lower-intercept ages of 1718 ± 22 Ma and 51 ± 17 Ma (207Pb/206Pb, 2σ), respectively. This ~1718 Ma date, interpreted as the intrusive age of the CMG, represents the first HPG-equivalent, magmatic age reported from the NBH. Other workers have linked CMG-related hydrothermal activity and gold mineralization in the nearby Homestake Mine. Assuming this link, we infer a 1718 ± 22 Ma age for the gold deposit and interpret the ~51 Ma date as indicating an episodic Pb-loss event associated with coeval Tertiary hydrothermal activity and regional exhumation. Also, five detrital populations of magmatic (i.e., Th/U=0.4-0.8) and age-zoned zircon (mostly with older cores and younger rims) were characterized from 60 spot-analyses of 27 grains from two metapelites known to have been deposited between ~2.01-1.88 Ga. The weighted-mean ages of these populations (from >90 % U-Pb concordant data, n=33) are: ~3830 ± 8 Ma (n=1, 2σ); ~3450 ± 7 Ma (n=5, MSWD=1.5); ~3370 ± 5 Ma (n=14, MSWD=2.1), ~3240 ± 4 Ma (n=5, MSWD=0.72), and ~2970 ± 11 Ma (n=8, MSWD=4.1). We infer that the Archean Wyoming craton was the most likely source of these detrital zircons, that ~3.83-2.97 Ga magmatic crust was exposed there during the ~2.01-1.88 Ga interval of Black Hills shale (metapelite) deposition, and that this domain was part of the Wyoming craton by that time.