Paper No. 9
Presentation Time: 4:10 PM


LUND, K., U.S. Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225 and ALEINIKOFF, J.N., U.S. Geological Survey, Denver Federal Center, MS 963, Denver, CO 80225,

Location of the Neoproterozoic-Paleozoic Cordilleran rift margin is constrained by mapping and dating of Idaho batholith roof pendants. Fourteen SHRIMP U-Pb dates provide evidence of discrete magmatic events, including (1) volcanic rocks of the Edwardsburg Formation at ~685 Ma, (2) three syenite-diorite suites and a tuffaceous diamictite at 665-650 Ma, and (3) seven syenite-diorite complexes at 500-470 Ma. These ages delimit 200 m.y. of recurrent alkalic magmatism that occurred along the southeastern two-thirds of a 650 km-long, northwest-striking, upper-plate continental-rift domain. This domain was bounded by northeast-striking, offsetting structures in northeastern Washington-southeastern British Columbia (St. Maries-Moyie) and southern Idaho-northern Nevada (Snake River).

The U.S. Cordilleran margin formed as northwest-striking rift segments offset by northeast-striking transform or transfer zones, including the St. Maries-Moyie transform, Washington-Idaho upper-plate rift, Snake River transfer, southeast Idaho-Utah-Nevada (Great Basin) lower-plate rift, and Mina transfer. This U.S. margin had similar geometries to those interpreted for the Canadian margin, such that the Cordilleran rift and passive margin had a zigzag geometry of reentrants and promontories. It is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains where transform-transfer zones localized subsequent deformation.

Sediment-hosted ore deposits also follow the rift margin: Cambrian–Early Ordovician and Devonian extension-related sedimentary exhalative (SEDEX) Zn-Pb-Ag ± Au and barite ore deposits in continental-slope rocks and Late Devonian–Mississippian and Mesozoic Mississippi Valley type (MVT) Zn-Pb deposits in continental-shelf rocks during structural reactivation. Distribution of the sediment-hosted deposits was controlled by polarity, kinematics, basin depth, and internal structure of each rift or transcurrent segment. Locally, discrete mineral belts parallel internal structures such as rotated crustal blocks at depth that produced sedimentary subbasins and caused hydrothermal-fluid conduits. Where overprinted by Mesozoic and Cenozoic deformation and magmatism, igneous rock-related, epigenetic ore deposits prevail.