Precambrian tectonics and mineral deposits, Lake Superior region

Abstract

IntroductionDURING the past 15 years, studies of the major iron ore districts and copper districts in the Lake Superior region were completed and abundant new .data on the geology and geochronology of the Precambrian rocks became available. During this same period, there was renewed interest in the base metal potential of the .region, stimulated largely by the discovery of the famous Kidd Creek massive sulfide orebody in Ontario. Accordingly, there has been a gradual shift away from traditional studies of the classic mining areas to studies of broader geologic environments and of the sequential events of the geologic history of large areas. I am pleased to have had a part in these later studies.An outcome of the recent work (Morey and Sims, 1976) has been the recognition that the Lake Superior region is underlain by t•vo fundamentally different basement rock terranes--an older gneiss terrane (3,000 m.y. or more old) and a younger greenstone terrane (2,700-2,750 m.y. old). In this paper, I will discuss the contrasting effects these two types of basement rocks had on later geologic events, with emphasis on the tectonic evolution of the middle Precambrian basin in which the great iron-formations were formed. Finally, I will compare the middle Precambrian basin of this region with the Labrador trough, a Proterozoic basin of approximately equivalent age and tectonic setting that also contains valuable ore deposits.The subdivision of the Precambrian used in this paper is given in the explanation for the geologic map (Fig. 2). The three-fold subdivision--lower, middle, and upper Precambrian--which is based on orogenies, is widely accepted in .the Lake Superior region (James, 1958; Goldich, 1968; Lumbers and

An early Proterozoic metagreywacke-slate turbidite sequence: The Tampere schist belt, southwestern Finland

“…They described in detail a portion of a metaturbidite sequence with Bouma intervals, similar to the turbidites of the Tampere area. A somewhat analogous complex of about the same age is found in the early Proterozoic of the Lake Superior region where volcanism and sedimentation culminated with the Penokean orogeny, as reviewed by Sims (1976).…”

“…This terrane could have existed side-by-side with active explosive volcanoes during a continuum of volcanism, uplift, erosion and repeated volcanism. Such a scenario has been suggested for the Vermilion district of northeastern Minnesota where Archean volcanism, plutonism, and sedimentation were encompassed by an estimated 50 Ma time span (Sims, 1976).…”

Contributions to mineral resources research, 1984

“…At least 1,250 kg of gold has been produced from the Ropes gold mine in northern Michigan (Rossell and Kalliokoski, 1983), and a small· amount of gold was produced from a mine on an island in Rainy Lake (Sims, 1972).…”

“…A few deposits known to occur in adjacent Ontario, Canada, but not in the United States are mentioned because of the possibility that such deposits could exist in this country. This report is a revision of an earlier one (Sims, 1976), as well as an elaboration of an oral presentation to a Society of Economic Geologists symposium at the 113th annual meeting of SME-AIME in Los Angeles, Calif., February 1984. …”

“…Naldrett (1981) has likened the CuNi deposit in the Duluth Complex to the Norilsk-Talnakh Cu-Ni deposit in the U.S.S.R., which also lies within a major continental rift along the northwestern margin of the Siberian platform. The volcanic-and sedimentaryhosted copper deposits of the Keweenaw Peninsula are secondary concentrations resulting from the leaching of country rocks and the transport and deposition of the metal in favorable lithologic or structural traps (White, 1968;.Major aspects of the evolution of the Early Proterozoic rocks (:::::2,100-1,850 Ma) in the region have been controversial (Sims, 1976;Van Schmus, 1976), but there now is a convergence of opinion, if not general agreement, that these rocks formed in a plate tectonic regime, probably analogous to modern tectonic environments.The recent model proposed by Schulz and others (1984), a refinement of an earlier one by Cambray (1978), suggests early rifting of the Archean continental margin, spreading, subsequent subduction and formation of a complex volcanic arc, and collision of the arc, first with an Arche~n microcontinent(?) on the south and then with the continental margin sequence and Archean crust (of northern Michigan) on the north.…”