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Relationship between Native Copper Deposits and Tectonic Development of the North American Midcontinent Rift System

1995; Springer Nature (Netherlands); Linguagem: Inglês

10.1007/978-94-017-0831-9_5

0928-267X

Theodore J. Bornhorst,

Geochemistry and Geologic Mapping

The North American Midcontinent Rift System (MRS) is host to the world’s largest native-copper mining district in the Keweenaw Peninsula (Fig. 1). Beneath Lake Superior the rift is filled by more than 18 km of subaerial basalt lava flows with minor interflow sedimentary beds overlain by up to 12 km of dominantly red clastic sedimentary rocks (Cannon et al. 1989). Magmatism (passive rifting) lasted from 1109 to 1087 Ma while sedimentation (post-rift thermal subsidence) is likely older than about 1040 Ma (Davis & Paces 1990; Hutchinson et al. 1990; Nicholson et al. 1992;). Progressive syn-depositional downwarping of rift-filling strata resulted in 40° of steepening of dip down section (towards the Keweenaw Fault) (White 1960, 1968). During the last phase of development of the Midcontinent Rift System, original graben-bounding normal faults were transformed into high-angle reverse faults at about 1060+/−20 Ma (Cannon et al. 1990; Nicholson et al. 1992). Continued movements on the Keweenaw fault with multiple kilometers of reverse displacement, caused further steepening of dips to present attitudes. Within rift-filling strata, the faults, fractures, and broad open folds are related to this compressional event. The reverse faulting played a role in development of a rift-flanking basin filled with the red Jacobsville Sandstone (Kalliokoski 1988; Hedgman 1992).