Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
Establishing the age of Paleoproterozoic mafic magmatism in the Black Hills crystalline core would constrain the timing of rift-basin sedimentation and related supercraton break-up. Accordingly, zircon from the tholeiitic Bogus Jim sill (Nemo, SD) has been dated by U-Pb ion-microprobe methods. Twenty-five nearly-concordant microanalyses have yielded an upper-intercept 207Pb/206Pb date of 2011.8 ± 3.2 Ma (2σ, MSWD = 1.7) and a weighted-mean date of 2012.2 ± 2.3 Ma (2σ, MSWD = 0.77). We interpret this 2012 Ma date as indicating the age of mafic sill intrusion and associated listric normal faulting during regional crustal extension. Moreover, intrusion of this sill heralds the beginning of a ~2015-1885 Ma interval of regional rifting in the Black Hills and local deposition of the Poorman, Homestake, and Ellison Formations in the Lead-Deadwood area. The Homestake Iron Formation (BIF) hosts the world-famous Homestake gold deposit, which was mined from 1876 to 2001 and accounts for 10% of total U.S. gold production. Yet, the precise depositional age of the Homestake BIF remains unclear. However, the Bogus Jim sill is lithostratigraphically equivalent to the basal Poorman Formation, which implies a maximum depositional age of 2012 ± 3 Ma for the overlying BIF. Coupled with a known age of 1974 ± 8 Ma for the overlying Ellison tuff, we can now pinpoint a ~2012-1974 Ma depositional age for the Homestake Iron Formation. Regionally, the 2012 ± 3 Ma intrusive age of the Bogus Jim sill is also equivalent within error to the known 2011 ± 1 Ma age of the Kennedy mafic dike swarm (SE Wyoming), thereby broadening the known extent of ~2.01 Ga rifting along the Archean Wyoming cratonic margin. This event potentially corresponds to the breakup of ~2600-2100 Ma supercraton Superia (or Kenorland) and correlates with worldwide mantle superplume event(s) known at ~2.0-1.9 Ga.