Abstract The western Upper Peninsula of Michigan is well known for hosting significant concentrations of copper in copper-dominated deposits. Most of the copper is hosted by rocks of the Mesoproterozoic Midcontinent Rift. Copper deposits in the western Upper Peninsula can be subdivided into two overlapping world-class copper mining districts. The Keweenaw Peninsula native copper district produced 11 billion lbs of copper and a lesser unknown but significant quantity of silver. Native copper deposits in this district are stratiform and hosted by tops of rift-filling subaerial basaltic lava flows and interflow coarse clastic sedimentary rocks. These deposits are interpreted to be the result of mineralizing hydrothermal fluids derived from rift-filling basaltic volcanic rocks that migrated upwards, driven by late Grenvillian compression of the rift some 40–50 million years following cessation of active rifting. The Porcupine Mountains sediment-hosted copper district produced or potentially will produce 5.5 billion lbs of copper and 54 million ounces of silver. These stratiform/stratabound deposits are hosted in rift-related black to gray shale and siltstone and dominated by chalcocite rather than native copper. Chalcocite is interpreted to be the result of introduction of copper-bearing fluids during diagenesis and lithification of host sediments. At the now-closed White Pine Mine, the chalcocite mineralizing event was followed by a second stage of native copper deposition that demonstrates a spatial and temporal overlap of these two world-class mining districts. While these two districts have been dormant since 1996, favorable results from recent exploration at Copper-wood suggest a revival of the mining of copper-dominated deposits in the western Upper Peninsula of Michigan.

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Abstract Nicholson. S. W., Cannon, W.F. and Schulz,'K.J ., 1992. Metallogeny of the Midcontinent rift system of North America. In: G. Gaal and K. Schulz (Editors). Precambrian Metallogeny Related to Plate Tectonics. Precambrian Res., 58: 000-000. The 1.1 Ga Mtidcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ~1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection proflles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments: (2) sediment-hosted copper sulfide and native copper: (3) copper sulfide veins and lode.s hosted by rift-related volcanic and sedimentary rocks: and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Mtagmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include Cu-Ni-PGE and Ti-Fe (V) in the Duluth Complex. U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks: the interaction between magma and country rocks was important in generation of the magmatic Cu-Ni sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. Mtore than 1 million km 3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift. providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30-40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift. or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep ponions of the rift.

The Keweenaw Peninsula segment of the Midcontinent rift system 1s host to the world's largest native copper mining district (Fig. 1). Cumulative production between 1845 and 1968 was about 5 billion kg of refined copper (Weege and Pollack, 1971) . Although about 96 % of the production came from a 45 km long belt in vicinity of Houghton and Calumet, small uneconomic amounts of native copper occur in basalt flows throughout the exposed Midcontinent rift, including Isle Royale (Michigan), Wisconsin, Minnesota, and Ontario (Canada) (see Cannon and McGervey, 1991). Lenticular, blanket-like orebodies, locally designated as lode deposits, are found along brecciated and amygdaloidal tops of lava flows and interflow conglomerate units within the Portage Lake Volcanics. Minor copper production was from vein orebodies. Economic deposits of native copper are stratigraphically restricted to the Portage Lake Volcanics, but uneconomic amounts of native copper do, occur in younger rift formations. In the White Pine area significant native copper occurs within the Nonesuch Shale (Mauk et al., this volume).

The White Pine copper deposit is hosted by sedimentary rocks that accumulated in a major intracontinental rift and appears to have formed fron1 large-scale processes that occurred within the rift basin. This deposit is one of the best-studied examples of sediment-hosted stratiform copper mineralization that appears to have spatial if not genetic associations with the development and evolution of an intracontinental rift basin (Boyle et al., 1989).

Abstract This road log runs from the Portage Lake area of the Keweenaw Peninsula, north and east to Copper Harbor located in the eastern end of the peninsula (Fig. 1). The road log is designed to provide coverage of all major lithologic units within the Keweenaw Peninsula segment of the middle Proterozoic Midcontinent rift system (Fig 2). Many of the stops include examples of the effects of late compressional faulting and of native copper mineralization and associated burial metamorphic alteration. Stops include roadcuts, lakeshore outcrops, old mine waste rock piles and a private mine. The Michigan Tech Earth Science Laboratory and Experimental Mine and the Caledonia Mine are described separately. The present problems of access to stops described here are minimal, but please respect private property and use low profile outdoor principles. Also note that old mine dumps can be hazardous where bad ground occurs. The user should refer to introductory articles for stratigraphic nomenclature, geologic background, and discussion of native copper deposits. Several stop descriptions used here are revised from previous guidebooks first authored by Bornhorst (Bornhorst, 1989; Bornhorst et al., 1986; Bornhorst et al., 1983). Extra stops have been added beyond the formal SEG field trip for the benefit of users of this guidebook.

Abstract The Caledonia Mine, owned by Red Metal Explorations, is located near Mass, Michigan (Fig. 1). The Caledonia Mine is southwest of the Mass and Adventure Mines (Fig. 2) within an area of native copper deposits outside of the major deposits of the Keweenaw Peninsula native copper district, some 40 km southwest of a major producer, the Baltic Mine (see Fig. 3 of Bornhorst, this volume). The mines of the Mass area worked the Evergreen, Ogima, Butler, Mass, South Knowlton and Knowlton mineralized flow top lodes, termed the Evergreen Series, over a total strike length of about 5 km. The Evergreen Series produced about 33 million kg of copper at grades between 0.5 and 1.25 % (Weege and Pollack, 1971) . The Mass Mine, largest producer in the area, produced 23 million kg of refined Cu while the Adventure Mine produced 5 million kg from 1851 to 1923 (Butler and Burbank, 1929). The Butler, Evergreen, and Knowlton lodes were the most productive of the flow tops; the Butler was the largest. The other flow tops yielded only small amounts of copper.

Abstract In the past, we have not prepared an official "road log" for visitors to the White Pine mine. Aside from the fact that it requires a substantial amount of time to prepare such a guide, we have avoided the chore because often, we are not sure where we will take visitors when a tour arrives. A working mine is a dynamic entity, one that constantly changes shape and character as new workings are opened up and older workings are abandoned. Therefore, any underground guide will be obsolete before it is even published. Nonetheless, we have chosen to present a set of stops which reflect the geology of the White Pine mine as we understand it in 1992.

Abstract This field trip examines the geology of rocks of the Keweenawan Supergroup (1.1 Ga) and related intrusive rocks of the Midcontinent rift system (MRS) in the western part of the northern peninsula of Michigan. The combination of stops includes all formations of the Keweenawan Supergroup in this region. Examination of all described localities requires more than a single day and participants are encouraged to use this guidebook on their own to supplement the localities that will be visited on our one-day trip. Stops are numbered in stratigraphic order, from oldest to youngest, not in the order in which they will be visited.

Abstract Nicholson. S. W., Cannon, W.F. and Schulz,'K.J., 1992. Metallogeny of the Midcontinent rift system of North America. In: G. Gaal and K. Schulz (Editors). Precambrian Metallogeny Related to Plate Tectonics. Precambrian Res., 58: 000-000. The 1.1 Ga Mtidcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ~1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection proflles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments: (2) sediment-hosted copper sulfide and native copper: (3) copper sulfide veins and lode.s hosted by rift-related volcanic and sedimentary rocks: and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Mtagmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include Cu-Ni-PGE and Ti-Fe (V) in the Duluth Complex. U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks: the interaction between magma and country rocks was important in generation of the magmatic Cu-Ni sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. Mtore than 1 million km 3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift. providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines.