Browsing by Subject "Keweenawan"
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Item Bulletin No. 28. The Geology of the Anorthosites of the Minnesota Coast of Lake Superior(Minnesota Geological Survey, 1939) Grout, Frank F.; Schwartz, George M.The northeast corner of Minnesota, lying north of Lake Superior and comprising Cook and Lake counties and part of St. Louis County, is underlain by Keweenawan rocks except for a narrow strip along the Canadian border. Early studies of these rocks were made principally by Winchell and by Irving, as shown by the list of publications given below. Because of the size and inaccessibility of much of the region, these early studies were confined to a narrow strip along the shore of Lake Superior. Later Grout and other members of the Minnesota Geological Survey made detailed studies of the Duluth gabbro. Between the areas covered by these workers lies a region in which only reconnaissance work had been done previous to the mapping presented in this report. It is planned eventually to map in detail this entire area, but as it embraces approximately 4,000 square miles of forested or brushy country, much of it not easily accessible for detailed work, it will take many field seasons to complete the task. The particular region mapped in this survey was selected because of unusual plagioclase feldspar masses of very high purity, for which it is hoped uses may be found, but the results are also significant as an example of the geology of a great Keweenawan area. The Keweenawan of this region consists almost entirely of igneous rocks. and even the few sedimentary rocks known are closely connected with extrusive igneous activity. It is about centrally located in the Keweenawan area of the north shore of Lake Superior and lies above the thickest part of the Duluth gabbro, but probably is not connected with it by continuous intrusive masses below the drift.Item Geology and Mineralization in the Dunka Road Copper-Nickel Mineral Deposit, St. Louis County, Minnesota(University of Minnesota Duluth, 1990-03) Monson Geerts, Stephen D; Barnes, Randal J; Hauck, Steven AThe Dunka Road Cu-Ni deposit is within the Partridge River Intrusion (T. 60 W., R. 13 W.), which is part of the Duluth Complex, and is approximately 1.1 b.y. (Keweenawan) in age. Relogging of 46 drill holes at the Dunka Road Cu-Ni deposit identified four major lithologic units and several internal ultramafic subunits that can be correlated over two miles. The ultramafic subunits (layers of picrite to peridotite) exhibit relative uniform thicknesses and are present at the same relative elevation within the major lithologic units. The major lithologic units, the same as delineated by Severson and Hauck (1990), and upward from the basal contact are: Unit I, a fine- to coursegrained a sulfide-bearing troctolite to pyroxene troctolite (450 ft. thick) with associated ultramafic layers I(a), I(b), and I(c); Unit II, a medium- to coarse-grained troctolite to pyroxene troctolite (200 ft. thick) with a basal ultramafic layer II(a); Unit III, a finegrained, mottled textured troctolitic anorthosite to anorthositic troctolite (150 ft. thick) with one minor ultramafic layer III(a); and Unit IV, a coarse-grained troctolite/pyroxene troctolite to anorthositic troctolite with associated ultramafic layers IV(a) and IV(b). Most sulfide mineralization occurs within Unit I. Within Unit I the sulfide mineralization is both widespread but variable in modal percentage (rare to 5%), continuity and thickness (few inches to tens of feet). Sulfide mineralization is somewhat related with proximity to: hornfels inclusions, the basal contact with the footwall Virginia Formation, and some of the internal ultramafic layers within Unit I. Precious metal mineralization (Pd+Pt+Au) is associated with fracturing and alteration of the host rocks. The alteration assemblage is chlorite, bleached plagioclase, serpentine and uralite. Pd+Pt values range from 100 to >2400 ppb over 10 foot intervals. These intervals can occur independently as 10 to 50 foot zones, or as part of a larger correlatable occurrence/horizon. Two mineralized subareas within the Dunka Road deposit are: 1) an area which is peripheral to a highly anomalous Pd occurrence (reported by Morton and Hauck, 1987; 1989) herein termed the "southwest area", and 2) the "northeast area" which contains several drill holes that have near surface intercepts of >1% Cu. There are four somewhat large mineralized occurrences within the study area that carry >300 ppb combined total Pt+Pd+Au. These mineralized zones appear to be stratigraphically controlled by the ultramafic subunits within Unit I. Three of the four correlatable zones are found within the southwest area, and range from 40 to 130 feet thick. High Pd values within these zones range from 10 to 20 feet thick with values of 800 to 1650 ppb Pd. In the northeast area, the fourth mineralized zone appears continuously throughout Unit I. This zone ranges from 120 to 300 feet thick. High Pd values within this zone range from 10 to 40 feet thick with values of 800 to 1500 ppb Pd. Many 5 to 30 foot intersections of >1 ppm Pd+Pt+Au occur throughout the mineral deposit. Geostatistical analysis based on 72 vertical holes and 12 angle holes suggests: 1) the base of the complex is a critical datum with the higher grade intercepts located between 100 and 400 feet above the base; 2) high inter-element correlations support local redistribution/concentration of primary mineralization by a secondary hydrothermal process and thus, polymetallic mining selectivity is possible; 3) the available drilling gives a spacial range of geologic influence at 400 foot centers, but sufficient angle drilling is not available to assess the potential of high grade, steeply dipping mineralized zones; 4) additional vertical in-fill drilling will almost certainly not discover any additional quantity of ore within the volume of rock studied; but 5) additional angle drilling to assess the potential of high grade, steeply dipping, mineralized zones would benefit a more complete geostatistical analysis.Item Midcontinent Rift System Bibliography(University of Minnesota Duluth, 1995-12) Hauck, Steven AThe co-chairs of the IGCP Project 336 field conference on the Midcontinent Rift System felt that a comprehensive bibliography of articles relating to a wide variety of subjects would be beneficial to individuals interested in, or working on, the Midcontinent Rift System. There are 2,543 references (>4.2 MB) included on the diskette at the back of this volume. PAPYRUS Bibliography System software by Research Software Design of Portland, Oregon, USA, was used in compiling the database. A retriever program (v. 7.0.011) for the database was provided by Research Software Design for use with the database. The retriever program allows the user to use the database without altering the contents of the database. However, the database can be used, changed, or augmented with a complete version of the program (ordering information can be found in the readme file). The retriever program allows the user to search the database and print from the database.