Klaus J. Schulz

The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA

A layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the Paleoproterozoic iron ranges of northern Michigan, in the Lake Superior region of the United States. Radiometric age constraints from events predating and postdating depo si tion of the breccia are ca. 1875 Ma and 1830 Ma. The major bolide impact that occurred at 1850 Ma at Sudbury, Ontario, 500–700 km east of these sites, is the likely causative event. The Michigan sites described here, along with previously described sites in Minnesota and Ontario, defi ne an extensive ejecta-bearing deposit throughout the Paleoproterozoic iron ranges of the Lake Superior region that we refer to as the Sudbury impact layer. The layer at the sites in Michigan exhibits a range of thicknesses, lithologic characters, and sedimentary settings. The diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented, but the detailed investigations needed to document them are incomplete. Many of the sites had been described and interpreted previously as products of common terrestrial processes, but the presence of relict shock-induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact. At most localities, this layer also contains relict fragments of altered devitrifi ed glass and/or accretionary lapilli. One immediate use of the impact layer is as an ultraprecise time line that ties together the well-known stratigraphic sequences of the various geographically separated iron ranges, the correlation of which has remained controversial for many decades. The Sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region. The impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks, commonly black shales. The impact may have produced regional, if not global, changes in the environment that resulted in this widespread synchronous change in sedimentation style.

Incompatible-element–rich andesitic amphibolites from the Archean of Montana and Wyoming: Evidence for mantle metasomatism

The eastern and central Beartooth Mountains of Montana and Wyoming are composed of 2,800-m.y.-old granitoids that intruded older rocks of variable age and composition. The most abundant rock type among these older units is a 3,000-m.y.-old amphibolite with a major-element composition similar to modern andesites. The concentrations of the incompatible trace elements, particularly the light rare earth elements, however, are much higher than those of most modern or ancient andesites. In addition, the range in concentration of many major and trace elements is large, and strong correlations for both mobile and immobile elements are evident. This coherence suggests that the observed element-abundance patterns in these rocks are not the result of metamorphic or other postcrystallization, nonisochemical processes. The compositional features of these andesitic rocks seem to be explained most easily as the result of variable degrees of partial melting of a source enriched in incompatible elements, followed by variable extents of crystallization of the resulting magmas. The mafic compositions of these magmas and limited Sm-Nd isotopic data suggest that the magmas were of mantle origin and that they were generated with the aid of incompatible-element–rich or metasomatic fluids.

Metallogeny of the midcontinent rift system of North America

Abstract The 1.1 Ga Midcontinent rift system of North America is one of the worlds 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 profiles. 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 lodes 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. Magmatic 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. More than 1 million km3 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 portions of the rift.

The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition—Implications for the rifting of Ganderia

The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the westernmost of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia. U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ± 0.8 Ma) and overlying Castine Volcanics (503.5 ± 2.5 Ma) are Middle Cambrian in age. Two tholeiitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava fl ows and as sills in both units, has depleted La and Ce ([La/Nd] N = 0.53‐0.87) values, fl at heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantle‐ normalized diagrams. In contrast, type Tb-2 basalt, present only in the Castine Volcanics, has slightly enriched LREE ([La/Yb] N = 1.42‐ 2.92) values and no Th or Ta anomalies. Both basalt types have strongly positive e Nd (500) values (Tb-1 = +7.9‐+8.6; Tb-2 = +5.6‐+7.0) and relatively enriched Pb isotopic compositions ( 206

Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America

A deep seismic-reflection profile in northern Lake Michigan, midcontinent North America, provides a cross section of the crust across the 1850 Ma Penokean orogen, in which an Early Proterozoic island-arc complex was deformed between two converging Archean continental masses. The island-arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base, above which it is variably reflective to transparent. The Archean terranes have thicker crust, as much as 50 km, the lower 20-25 km of which is strongly reflective. Abrupt offsets of Moho near terrane boundaries may have been preserved since accretion during the Penokean orogeny.

The magmatic evolution of the Vermilion greenstone belt, NE Minnesota☆

Abstract The Vermilion greenstone belt, a nearly linear belt of steeply dipping, low grade, metavolcanic and metasedimentary rocks and adjacent areas in northeastern Minnesota compose a typical Archean (∼2.75 Ga) greenstone—granite association. In the western part of the belt, four formally designated units have been defined, which together form a complex volcanic—sedimentary pile that apparently formed through the coalescence of petrologically distinct volcanic centers. The oldest supracrustal unit, the lower Ely Greenstone, consists of calc-alkaline basalts, andesites, heterolithic agglomerates and tuffs deposited in a relatively shallow subaqueous environment. The lower part of this unit apparently was destroyed by emplacement of a granitic batholith. Following a period of volcanic quiescence, during which the Soudan Iron Formation and associated rocks were deposited, basaltic (tholeiitic) volcanism commenced and produced a subaqueous shield volcano (upper Ely Greenstone); the basalt interfingered laterally with calc-alkaline, dominantly dacitic material. The termination of basaltic volcanism was followed by a period of eruption of largely dacitic volcanic material accompanied by rapid erosion and turbidite deposition represented by the Lake Vermilion Formation and Knife Lake Group. This period was followed by renewed volcanism that changed laterally from dominantly calc-alkaline lava, tuff and breccia (felsic member of Newton Lake Formation) to dominantly basaltic lava and mafic—ultramafic intrusions (mafic member of Newton Lake Formation). Within the mafic sequence five petrologically distinct basalt types were erupted. Assuming that the present erosion surface in the Vermilion greenstone belt is a representative cross-section of the original supracrustal sequence, the volume of calc-alkaline, dacitic volcanic material appears to exceed that of basaltic volcanic rocks. If current petrologic models proposed for these rocks are valid, a dynamic tectonic environment capable of generating and rapidly recycling large volumes of volcanic and derived sedimentary material is required to account for the development of the Vermilion greenstone belt.

Rare earth element mineralogy, geochemistry, and preliminary resource assessment of the Khanneshin carbonatite complex, Helmand Province, Afghanistan

The Khanneshin carbonatite is a deeply dissected igneous complex of Quaternary age that rises approximately 700 meters (m) above the Neogene sedimentary rocks of the Registan Desert, Helmand Province, Afghanistan. The complex consists almost exclusively of carbonate-rich intrusive and extrusive igneous rocks, crudely circular in outline, with three small hypabyssal plugs of leucite phonolite and leucitite outcropping in the southeast part of the complex. The igneous complex is broadly divisible into a central intrusive vent (or massif), approximately 4 kilometers (km) in diameter, consisting of coarse-grained sövite and brecciated and agglomeratic barite-ankerite alvikite; a thin marginal zone (<1 km wide) of outwardly dipping (5°–45°) and alkali metasomatized Neogene sedimentary strata; and a peripheral apron of volcanic and volcaniclastic strata extending another 3–5 km away from the central intrusive vent. Small satellitic intrusions of biotite-calcite carbonatite and rare leucite phonolite, no larger than 400 m in diameter, crop out on the southern and southeastern margin of the central intrusive vent. A zone of prospective light rare earth element (LREE) enrichment was delineated by Soviet geological teams in the mid-1970s. The area of LREE-enrichment is situated in extensively veined and dike-intruded barite-ankerite alvikite in the outer part of the central vent near its northeast contact with Neogene sedimentary rocks. In addition to having very high concentrations of LREE, the barite-ankerite alvikites are also highly enriched in barium and strontium. Three reconnaissance scoping missions to the Khanneshin carbonatite were led by scientists of the U.S. Geological Survey (USGS). Two of these were to LREE area of interest which is the primary subject of this report. Two types of LREE mineralization occur. Type-1 LREE mineralization consists of semiconcordant, symmetrically banded veins and discontinuous seams, as much as 0.5–0.7 m thick and several tens of meters long. These occur throughout a vertical thickness of at least 150 m. Type-1 banded veins and seams are yellow-weathering zones, symmetric about a dark central zone, that are enriched in khanneshite-(Ce), barite, strontianite, and secondary LREE minerals (synchysite-(Ce) and parisite-(Ce)). The dark central zone, consisting primarily of ankeritic dolomite, barite, apatite, and strontianite, also has trace khanneshite-(Ce). These type-1 veins and seams alternate with dark, meterthick layers of ankerite-barite alvikite (wall rock) over a vertical distance of approximately 150 m. In some veins LREE carbonate minerals form dense spherically shaped aggregrates (100 micrometers diameter), presumably crystallized from immiscible droplets, which constitute as much as 30 percent (by volume) of the vein. Type-1 veins and seams average 19.92 weight percent (wt. percent) Ba, 3.61 wt. percent Sr, and 2.78 wt. percent total LREE. The values of ∑ LREE (∑ LREE is the sum of La, Ce, Pr, and Nd) for eight average whole-rocks range from 6.23 to 1.83 wt. percent.

APPLICATIONS OF A GLOBAL MINERAL-RESOURCE ASSESSMENT FOR ADDRESSING ISSUES OF SUSTAINABLE MINERAL RESOURCE DEVELOPMENT 1

The future of mining depends on balancing global demands for minerals with societal demands for sustainable development. The U.S. Geological Survey, in collaboration with a variety of international cooperators, is assessing the undiscovered global resources of copper, platinum-group elements, and potash at a scale of 1:1,000,000. Assessment products include maps that show significant identified deposits and permissive areas for undiscovered deposits, as well as probabilistic estimates of contained metal. Derivative products applicable to sustainability issues include maps showing the spatial relationship of permissive areas to infrastructure development, protected areas, threatened ecosystems, seismically active areas, and watersheds.

The global mineral resource assessment project in the southeast Asia region

In response to the growing need for minerals information, the U.S. Geological Survey (USGS) is conducting a cooperative international project to assess the world’s undiscovered nonfuel mineral resources. The Global Mineral Resource Assessment Project (GMRAP) has met with member countries of the Coordinating Committee for Geoscience Programs in East and Southeast Asia (CCOP) to assess undiscovered copper, PGE and potash resources of the region. Tracts permissive for undiscovered porphyry copper and sedimentary-hosted copper deposits have been delineated and probabilistic estimates of the amount of copper resources for each tract made. Preliminary data also have been collected for PGE and potash in Southeast Asia.