Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
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.