Open-File Report 01-088 Geologic Map of the Ontonagon and Part of the Wakefield 30' x 60' Quadrangles, Michigan -- A Digital Version of Map I-2499 (1995) By William F. Cannon, Suzanne W. Nicholson, Laurel G. Woodruff, C.A. Hedgman, K.J. Shultz, and Shana G. Pimley INTRODUCTION The geologic map of the Ontonagon-Wakefield area portrays the geology of Middle Proterozoic rocks of the Keweenawan Supergroup along part of the southern limb of the Midcontinent rift as well as part of the Late Archean and Early Proterozoic crystalline basement on which the Keweenawan rocks were deposited. The Keweenawan Supergroup is notable for extensive sediment-hosted copper deposits including the White Pine deposit. Native copper also has been produced from both volcanic and sedimentary rocks. Early Proterozoic strata include extensive iron-formation of the Gogebic iron range, which has been an important source of iron ore. The geology of the region was mapped by previous workers, mostly between the late 1950's and early 1970's, at scales of 1:62,500 to 1:24,000. Most mapping was done by the U.S. Geological Survey (USGS) personnel in a cooperative program with the Geologic Survey Division of the Michigan Department of Natural Resources. Additional mapping was done by personnel of the Copper Range Company also in cooperation with the Michigan Geological Survey Division. Most of that mapping is available as USGS open-file reports or Miscellaneous Field Studies (MF) maps. This map compiles and synthesizes that older mapping and incorporates more recent data including our own work done from 1988 to 1993. STRATIGRAPHY AND TECTONIC SETTING Three principal stratigraphic sequences, separated by profound unconformities, are present in the map area. Each sequence was deposited by a distinctly different tectonic setting as the region evolved from a primitive oceanic terrane to a stable craton. Late Archean Archean strata are volcanic and sedimentary rocks typical of Late Archean greenstone belts of the Superior province. In the western part of the area the Ramsay formation is dominantly pillowed greenstones and less abundant felsic and mafic pyroclastic rocks. Metamorphism, probably related to the emplacement of the Puritan batholith at about 2.75 Ga, has converted some of these rocks to schists and gneisses (Ws). In the south-central portion of the area gneiss and amphibolite (Wga) derived from volcanic precursors probably equivalent to the Ramsay Formation, intertongue with metagraywacke (Wgw). The gneiss has a U-Pb zircon age of 2750 Ma (Sims and others, 1984). Early Proterozoic A sedimentary and volcanic sequence, the Marquette Range Supergroup, composes the Early Proterozoic strata of the region. The strata record cycle of passive margin sedimentation followed by deposition of graywacke and shale in a foreland basin. Rocks of the Menominee Group, including the Ironwood Iron-Formation, formed on a south-facing extensional passive margin near the transition with the more stable shelf to the north. In the region between Wakefield and Lake Gogebic the Menominee Group strata show prominent changes in depositional setting. Near Wakefield the rocks display a laterally uniform sedimentary stratigraphy typical of the central part of the Gogebic range and appear to have been deposited on a stable shelf. East of Wakefield, the sequence shows marked lateral facies changes, including the appearance of volcanic units of the Emperor Volcanic Complex intertonging with the Ironwood Iron-Formation. These changes and pronounced lateral changes in stratigraphic thicknesses of individual units probably resulted from deposition on a basement that was extending to produce normal-fault-bounded basins of deposition. In contrast, the Baraga Group, including the turbidite sequences of the Copps and Michigamme Formations, was deposited in a foreland basin and was derived from a southerly source, probably an accreting volcanic arc and related fold and thrust belt that marked the initial arc-continent collision of the Penokean orogeny (Barovish and others, 1989). Middle Proterozoic The Middle Proterozoic strata comprise a very thick sequence of rift-related volcanic and sedimentary rocks formed during development of the Midcontinent rift system. The lower part of the section is dominantly continental flood basalt, with lesser amounts of andesite and rhyolite, that was erupted during a 15 m.y. period of extension and copious subaerial volcanism. The upper part of the section is dominantly continental red beds with lesser lacustrine shales and siltstones deposited in a post-rift basin that subsided for at least 30 m.y. immediately following rifting. The lowest volcanic unit, the Siemens Creek Volcanics, is almost entirely subaerial flood basalt flows, which in part of the area were deposited over a thin blanket of the sandy protolith of the Bessemer Quartzite and elsewhere were deposited directly on older basement rocks. The Siemens Creek Volcanics is overlain by the Kallander Creek Volcanics, which is distinguished from the Siemens Creek by having a higher content of andesite, dacite, and rhyolite flows along with the flood basalts. These two volcanic units constitute the Powder Mill Group. Herein we change the former designation of Siemens Creek Formation and Kallander Creek Formation to Siemens Creek Volcanics and Kallander Creek Volcanics to maintain consistency with similar Middle Proterozoic continental volcanic sequences elsewhere in the Lake Superior region, all of which are referred to as volcanics rather than formations. Part of the Powder Mill Group is unconformably overlain by a blanket of Jacobsville Sandstone. Geophysical data (King, 1975; Klasner and Jones, 1989; Hinze and others, 1990) indicate that a northward-thickening wedge of volcanic rocks of the Powder Mill Group and possibly the Portage Lake Volcanics underlies the Jacobsville Sandstone as far north as the Keweenaw fault. North of the Keweenaw fault, the Portage Lake Volcanics is a sequence of mostly high-Al tholeiitic flood basalts interlayered with lesser amounts of rhyolite and conglomerate. The physical base of the exposed formation is the plane of the Keweenaw fault. Stratigraphically lower parts of the Portage Lake Volcanics probably lie beneath the Jacobsville Sandstone south of the fault as shown on section A-A'. Overlying the Portage Lake Volcanics is a sequence of rhyolite, andesite, and basalt that constitutes the Porcupine Volcanics. This unit pinches out near the eastern edge of the map area but reaches maximum thickness near the Porcupine Mountains. The Porcupine Volcanics is interpreted to be a volcanic shield formed by eruptions from a central vent located in the vicinity of the Porcupine Mountains and thus represents a distinct change in style of volcanism from the underlying flood basalts of the Portage Lake Volcanics. The overlying Copper Harbor Conglomerate has an inverse change in thickness with respect to the Porcupine Volcanics. This indicates that the Porcupine Volcanics formed a shield that was present during deposition of the Copper Harbor. Eruption of the Porcupine Volcanics and deposition of the Copper Harbor Conglomerate mark the transition from extension and voluminous subaerial volcanism to fluvial red bed sedimentation in a broad post-rift basin. The sedimentary sequence is made up mostly of the Oronto Group, which consists of the volcanogenic conglomerate and sandstone of the Copper Harbor Conglomerate at its base. The overlying Nonesuch Formation is black to gray shale, siltstone, and lesser amounts of gray to red sandstone, which probably were a lacustrine deposit. A return to fluvial sedimentation resulted in deposition of the red sandstone and siltstone of the Freda Sandstone, the uppermost formation in the Oronto Group. Although it contains an abundance of volcanic detritus, the Freda is distinctly more mature than the Copper Harbor. The youngest unit is the Jacobsville Sandstone, which fills a broad basin south of the Keweenaw fault where it is nearly flat-lying and lies with angular unconformity on the Siemens Creek Volcanics. The Jacobsville is entirely a fluvial red bed sequence. Near the Keweenaw fault the Jacobsville is folded and dragged by movement on the fault indicating that it was at least partly deposited before the 1060 Ma faulting. West of Lake Gogebic the Jacobsville dips moderately northward (as much as 35°) and is mostly conglomeratic. Clasts are largely derived from rocks typical of the Early Proterozoic and Archean rocks of the Gogebic iron range. This conglomeratic facies was interpreted to have formed during thrusting on the Marenisco fault at about 1060 Ma (Hedgman, 1992), which produced local uplift of the source rocks for the Jacobsville sandstone immediately south of the present Jacobsville basin. STRUCTURE Middle Proterozoic Structures within Middle Proterozoic rocks record a period of continental extension, which formed a deep rift basin, followed by a period of compression, which partly inverted the rift. Middle Proterozoic strata in the map area lie along the southern margin of the Midcontinent rift. Progressive northward tilting of the region toward the rift axis during volcanism and sedimentation produced a northward thickening wedge of rocks of the Keweenawan Supergroup in which dips diverge approximately 30° between basal and uppermost units. Later compression formed major thrust faults, principally the Keweenaw fault in this area, which accentuated northerly dips in the hanging wall north of the fault trace. The Marenisco and Pelton Creek faults are also thrust faults (Cannon and others, 1993) that cut Archean and Early Proterozoic rocks as well as Middle Proterozoic strata and produced a northward tilting of the hanging wall. These latter two faults separate an area to the east where flows of the Siemens Creek Volcanics dip about 10°N., from an area to the west where flows of the Siemens Creek dip about 50°N. Cannon and others (1993) showed that the Archean and Early Proterozoic rocks north of the Marenisco fault are similarly tilted so that southward thrusting on a listric fault appears to account for the unusually steep dips in the hanging wall block. Within the outcrop belt of the Jacobsville Sandstone the effect of the two faults is less certain. One or both of them is inferred to lie along the southern gradient of the middle range magnetic anomaly, a feature apparently caused uplift of normally polarized basalt (see cross section A-A'). The Jacobsville Sandstone was deposited during movement of these faults (Hedgman, 1992) so there may be little or no offset of near-surface Jacobsville units. On the basis of radiometric dates of coeval alteration minerals (Bornhorst and others, 1998) and thermal resetting of biotite ages (Cannon and others, 1993), the age of faulting is determined to be about 1060±20 Ma. The Cherry Creek fault, which is largely inferred from aeromagnetic data, may be of a different class. The aeromagnetic pattern of the region (King, 1987) shows a prominent south-sloping gradient along the inferred fault trace as well as truncations of numerous strong linear anomalies against the gradient. These anomalies are almost certainly produced by magnetic flows, hence, the gradient seems likely to be caused by a fault that crossed the stratigraphic section of the Portage Lake Volcanics and Porcupine Volcanics. An unusual aspect of this fault is that, whereas it causes substantial offset of volcanic units, it produces little or no offset of the contact between the Porcupine Volcanics and Copper Harbor Conglomerate. Thus, it appears to be a growth fault that was active only during volcanism. Although it could be a tectonically produced extensional fault, we prefer to interpret it as a fault related to the eruption of the Porcupine Volcanics because it is located near the Porcupine volcanic center and partly defines the elliptically shaped aeromagnetic anomaly that surrounds the center. It may be a normal fault that formed the southern boundary of a caldera into which parts of the Porcupine Volcanics were erupted. The same compression that produced thrust faulting also formed broad folds such as the Presque Isle and Iron River synclines and Porcupine Mountains anticline, and local tighter folds such as the anticline-syncline couplet near Greenland. The hook-shaped map pattern of units in the Porcupine Mountains is probably the result of broad folding of a roughly conical stratovolcano, which had been partly eroded before being buried by the Copper Harbor Conglomerate. The map pattern thus reflects volcanic and erosional processes as well as tectonism. Early Proterozoic Early Proterozoic rocks show the effects of both the Penokean orogeny at about 1.8 Ga and tilting in Middle Proterozoic time. Along most of the Gogebic iron range the steep northerly dips are the result of tilting related to the Midcontinent rift. The attitude of Early Proterozoic strata are nearly the same as basal flows of the Keweenawan Supergroup so the Early Proterozoic strata were, at most, only mildly deformed by the Penokean orogeny. In the eastern part of the Gogebic range, however, broad folding and faulting occurred during the Penokean orogeny and there is substantial structural discordance between Early Proterozoic and Keweenawan strata. Still farther east the Michigamme Formation and underlying volcanic rocks were rather tightly folded into upright folds during the Penokean orogeny. Archean Archean strata consist of volcanic rocks and graywacke and their metamorphosed equivalents that were folded prior to being intruded by the puritan batholith at about 2.75 Ga. The volcanics rocks are uniformly south-facing and steeply dipping and commonly show a steep lineation. They were intruded by the late- to post-tectonic Puritan batholith which is composed of massive to weakly foliated granitic rocks. South of the Gogebic range the Archean rocks were tilted northward 20° to 60° during Middle Proterozoic times as shown by southerly dips of once vertical diabase dikes of Early and Middle Proterozoic age (Cannon and others, 1993). ECONOMIC GEOLOGY Native Copper Volcanic-hosted deposits.- Native copper is a widespread hydrothermal alteration mineral in many of the volcanic rocks and some sedimentary rocks in this area. It was mined at numerous localities between about 1850 and 1920. The Caledonia mine was reactivated between 1951 and 1958. The principal mines were near the towns of Rockland and Greenland, where about 45 million kg of refined copper were produced. Seventeen individual companies reported production; details of production are given in Butler and Burbank (1929). The deposits in the Rockland-Greenland area are a direct extension of the more productive native copper district on the Keweenaw Peninsula to the northeast of the map area. Copper was deposited mostly in brecciated tops of basalt flows to form the so-called amygdular lodes of local terminology. Native copper and cogenetic alteration minerals, such as epidote, quartz, prehnite, and pumpellyite, fill spaces between basalt fragments or in vesicles, replace some fragments, or form veinlets in basalt. Less commonly, copper is in crosscutting veins, the so-called fissure veins of local terminology. The economically significant copper deposits are crudely stratabound, occurring in eight individual flow tops across a stratigraphic interval of about 150 m immediately above the Bohemia conglomerate of the Portage Lake Volcanics (Butler and Burbank, 1929). The deposits here are somewhat lower in the section than those on the Keweenaw Peninsula. Three lodes, the Knowlton, Butler, and Evergreen, accounted for most of the production. Most mines worked more than one lode but reported only total production, so an exact tonnage cannot be assigned to each lode. Grades were not routinely reported, but where known were typically in the range of 15 to 20 pounds of refined copper per ton (about 0.8-1.0 percent). Sediment-hosted deposits.- Native copper is fairly common in the uppermost Copper Harbor Conglomerate and lowermost Nonesuch Formation. It was mined in the late 1800's along the southern flank of the Porcupine Mountains at the Nonesuch, White Pine, and White Pine Extension mines. It is presently recovered at the White Pine mine along with the more abundant chalcocite discussed in the section on Copper Sulfide deposits. Native copper occurs as pore fillings in mineralized sandstone and as thin sheets along bedding and fractures in shale. Mineralization is concentrated near compressional faults and is believed to be coeval and cogenetic with native copper mineralization in the underlying basalts (Mauk and others 1992a). Most early production was from the original White Pine mine where about 8 million kg were produced at grades of about 1 percent refined copper. At the present White Pine mine, there are no accurate figures for production of native copper, but it probably exceeds 50 million kg. About 15,000 kg of native copper was produced at the Carp Lake mine on the northern flank of the Porcupine Mountains. Minor production also came from the Cuyahoga, Lafayette, and Union mines in this same region. These deposits are in a sequence of basalt flows and immediately underlying sandstone within the Copper Harbor Conglomerate. Copper Sulfide Deposits The map area contains very large sediment-hosted chalcocite deposits in the basal beds of the Nonesuch Formation. Two principal deposits have been well delineated by diamond drilling. White Pine deposit.- Chalcocite and lesser native copper are mined from the lower few meters of the Nonesuch Formation and, in some areas, from the uppermost sandstone beds of the Copper Harbor Conglomerate. Extractable reserves in 1992 were about 172 million metric tons of ore averaging 1.1 percent copper and about 0.2 Troy ounces of silver per ton (Mauk and others, 1992b). Mining is by room-and-pillar method, so proven in-place resources are about twice that amount. In 1992, the production rate was about 16,000 tons/day. Since the mine opened in 1955, it has produced about 1.4 billion kg of copper. The map shows both the approximate outline of proven ore and the area mined as of 1992. The mine workings are on a single level that follows the gently folded and locally faulted base of the Nonesuch Formation. Two stages of mineralization are recognized (Mauk and others, 1992a,b). In the first or main stage, chalcocite was formed mostly as a replacement of fine-grained diagenetic pyrite and is itself probably a late diagenetic product. Temperatures during sulfide mineralization were probably no higher than 100°C (Brown, 1971; Mauk and others, 1992b). The upper contact of the chalcocite ore with barren pyritic shale and siltstone, the fringe of local terminology, is a zone only a few centimeters thick. The lower contact is the sharp contact between the basal dark shale of the Nonesuch Formation and sandstone of the Copper Harbor Conglomerate. Second-stage mineralization involved formation of native copper and chalcocite, mostly near minor compressional faults, and probably formed from the same fluids that formed the extensive native copper deposits elsewhere in the region (Mauk and others, 1992a). Where most strongly developed, second-stage mineralization increased copper grade by 20 to 30 percent of the main-stage mineralization. (Note added in 2001: The White Pine Mine ceased production in 1995, and as of February 2001 is undergoing abandonment procedures and is flooding. Further production from this deposit is very unlikely.) Presque Isle deposit.- Mineralization very similar to that at White Pine is known at the base of the Nonesuch Formation throughout the Presque Isle syncline. The proven resource is about 86 million tons of ore averaging 1.27 percent copper (Cannon, 1985). Engineering difficulties encountered in early test mining have been among the factors that have prevented mining to date. Extensions of mineralization.- The basal units of the Nonesuch Formation are probably mineralized to some extent throughout the map area. Potentially significant copper deposits undoubtedly extend in all directions beyond the proven reserves at the White Pine deposit. The gently northwest-plunging syncline that contains the Presque Isle deposit continues beneath Lake Superior and is also undoubtedly mineralized. North of the Porcupine Mountains, the Nonesuch Formation lies only a short distance offshore beneath Lake Superior where significant mineralization is also likely. All of these factors indicate that the map area probably contains a very large resource of copper in as yet undeveloped and, in part, undiscovered deposits. Iron About 65 million tons of iron ore were produced from 14 mines along the Gogebic iron range in T. 47 N., R. 45 W., in the southwestern part of the map area. Mining activity began in 1885; the last mine closed in 1950. Ores were soft hematite and limonite, which formed as supergene concentrations within the Ironwood Iron-Formation. The ores most likely formed by weathering and consequent leaching of silica from the iron-formation during late Proterozoic or Cretaceous time. A large amount of iron-formation, in part magnetite-rich, remains in the Gogebic iron range and constitutes a resource of concentrating-grade material. The most promising area is in T. 47 N., R. 44 W., where a gently east-plunging anticline produces a broad outcrop belt of gently to moderately dipping iron-formation. REFERENCES CITED Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 1989, Nd isotopes and the origin of 1.9-1.7 Ga crust of the Lake Superior region: Geological Society of America Bulletin, v. 101, p. 333-338. Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619-625. Brown, A.C., 1971, Zoning in the White Pine copper deposit, Ontonagon County, Michigan: Economic Geology, v. 66, p. 543-573. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Cannon, W.F., 1985, Mineral-resources map of the Iron River 1° X 2° quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map I-1360-A, scale 1:250,000. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal river monocline-a 35-km-thick cross section of the Midcontinent rift in northern Wisconsin and Michigan: Tectonics, v. 12, p. 728-744. Fritts, C.E., 1969, Bedrock geologic map of the Marenisco-Watersmeet area, Gogebic and Ontonagon Counties, Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map I-576, scale 1:48,000 Hedgman, C.A., 1992, Provenance and tectonic setting of the Jacobsville Sandstone from Ironwood to Keweenaw Bay, Michigan: Cincinnati, Ohio, University of Cincinnati, unpub. M.S. thesis, 158 p. 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Sims, P.K., Peterman, Z.E., Prinz, W.J., and Benedict, F.C., 1984, Geology, geochemistry, and age of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area, northern Michigan: U.S. Geological Survey Professional Paper 1292-A, p. A1-A41. Trent, V.A., 1973, Geologic map of the Marenisco and Wakefield NE quadrangles, Gogebic County, Michigan: U.S. Geological Survey Open-File Map, 4 sheets, scale 1:20,000. Whitlow, J.W., 1974, Geologic map of the Greenland and Rockland quadrangles, Ontonagon County, Michigan: U.S. Geological Survey Miscellaneous Field Studies Map MF-596, scale 1:62,500.