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Geometry of the Neoproterozoic and Paleozoic rift margin of western Laurentia: Implications for mineral deposit settings

¹ U.S. Geological Survey, MS 973, Denver Federal Center, Box 25046, Denver, Colorado 80225, USA

The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian–Devonian intracontinental rifting that formed the western margin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest-striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary–Moyie transform, eastern Washington–eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform-transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin.

Sedimentary exhalative (sedex) Zn-Pb-Ag ± Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian–Mississippian and to a lesser degree, during the Cambrian–Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley–type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian–Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the polarity and kinematics of the rift segment. Locally, discrete mineral belts parallel secondary structures such as rotated crustal blocks at depth that produced sedimentary subbasins and conduits for hydrothermal fluids. Where the miogeocline was overprinted by Mesozoic and Cenozoic deformation and magmatism, igneous rock–related mineral deposits are common.

Keywords: U.S. and Canadian Cordilleran miogeocline • Laurentia • Neoproterozoic–Paleozoic rift • sediment-hosted mineral deposits • sedimentary exhalative • Mississippi Valley type