Bedrock Geochemistry of Archean Rocks in Northern Minnesota

Abstract

The primary purpose of this project is the geochemical evaluation of Archean bedrock in northern Minnesota. The rocks included are both the greenstone/granite and gneissic terranes but exclude the Archean rocks of the Minnesota River Valley. The emphasis is on the characterization of the greenstone portions of the Archean subprovinces. A total of 15 counties with geochemistry pertinent to this project were included: Beltrami, Clay, Hubbard, Becker, Itasca, Cook, Lake, Lake of the Woods, Marshall, Koochiching, Roseau, St. Louis, Polk, Todd, and Norman. The first portion of this project consisted of the development of an archival geochemistry data base that contains all analyses available for Archean rocks through December 1, 1990. This data base consists of data from a number of sources, including published material, unpublished dissertations and theses, and data contained within the abandoned-lease files at the Department of Natural Resources - Minerals Division office in Hibbing, Minnesota. The data base contains 12,451 complete or partial analyses of drill core and bedrock outcrop rock samples that are divided into files by county and sorted by their township-range-section location. Planimetric digital county maps showing locations of samples and drill holes were constructed using AutoCAD software. An additional 219 samples were submitted to X-Ray Assay Laboratories for analysis as part of this project in order to cover areas lacking sufficient data and also to aid in current mapping projects. The second portion of this project consisted of evaluating the data base in terms of quality and completeness of the analyses. The total number of "quality" analyses was reduced to 1,477, and these were then separated into their respective subprovince or area. The divisions consist of the: 1) eastern Vermilion district (which includes Ranges 4 to 11 West of the Wawa-Shebandowan Subprovince); 2) western Vermilion district (which includes Ranges 12 to 17 West of the Wawa Shebandowan Subprovince); 3) western Wawa-Shebandowan Subprovince (Range 18 and west); 4) Giants Range batholith; 5) Wabigoon Subprovince; and 6) Quetico Subprovince. The analyses for each of these areas were then used to geochemically characterize the Archean subprovinces of Minnesota. The eastern Vermilion district is a greenstone terrane, largely volcanic in nature, and generally tholeiitic in composition. The western Vermilion district is more variable and contains both tholeiitic and calc-alkaline volcanics. The rest of the western Wawa- Shebandowan Subprovince has less exposure and is covered in most areas by thick glacial deposits. It appears to be largely tholeiitic in composition and also contains the Deer Lake Complex, ultramafic and mafic tholeiitic differentiated sills in Itasca County. The rocks of the Wabigoon Subprovince are predominantly sedimentary in origin. What little volcanic suites are present appear to be principally tholeiitic in composition, with a minor suite of calc-alkaline rocks. The majority of the samples from the Quetico Subprovince are calc-alkaline felsic intrusives or high-grade amphibolites and gneisses. Additionally, lamprophyres from all of these areas were broken out and considered as a separate package. They were scrutinized more closely due to the recent interest in the lamprophyre/gold association. The third part of this project involved a more intensive geochemical study of a portion of the Wawa-Shebandowan Subprovince described as the Soudan-Bigfork area (Plate 16). The purpose was to further characterize this area due to recent geological mapping projects by the Minnesota Geological Survey and also due to the high exploration activity in the area. This area is defined as Township 59 to 62 North and Range 14 to 27 West. It consists of 16 packages of rocks including: 1) granitoid plutons, scattered throughout the area and intrusive into the other packages; 2) the Giants Range batholith, marking the southern boundary of the area; 3) the Lake Vermilion Formation, generally sedimentary in nature; 4) iron-formations, interbedded within volcanic and sedimentary units throughout the region; 5) the Sherry Lake-Upper Ely Greenstone sequence, tholeiitic pillowed basalts; 6) the Bear Lake-Lower Ely Greenstone sequence, composed of both tholeiitic and calc-alkaline volcanics; 7) the Joy Lake sequence, with sedimentary and mafic to felsic volcanics; 8) the Deer Lake sequence, including differentiated gabbroic sills of the Deer Lake Complex and associated volcanics, both tholeiitic in composition; 9) the Thistledew Lake sequence, consisting of mafic volcanic rocks chiefly of Fe-tholeiite composition and a unit composed of graywacke, slate, and dacitic tuff; 10) the Wilson Lake sequence, with several units composed of mafic to felsic volcanic and sedimentary rocks; 11) the Cook sequence, composed of two units, a graywacke and argillite unit, and a unit consisting of mafic volcanic rocks tholeiitic in composition; and 12) mafic intrusions, a variety of separate intrusions that form dikes and sills in supracrustal sequences throughout the region. Of the original 1,477 "quality" analyses used in part two of this project, 572 samples fell within this area and were divided into these sequences for further detailed geochemical analysis that supports the tholeiitic-calc-alkaline descriptions given previously. Part four of this project involved a detailed geochemical analysis of the Virginia Horn area of northeastern Minnesota. The Virginia Horn area is located in the Virginia-Eveleth area of the Mesabi Range. It is comprised predominantly of metavolcanic and metasedimentary supracrustal rocks of Archean age that are similar to those found in the Vermilion district of northeastern Minnesota, from which it is separated by the Giants Range batholith. Both tholeiitic and calcalkaline suites of metavolcanic rocks occur here; the relationship between them is unclear. However, the occurrence of both suites of rocks without the presence of obvious structural or stratigraphic discontinuities may suggest a geologic setting similar to modern immature island arcs. The area additionally includes quartz-feldspar porphyries, probably altered dacites or rhyodacites, associated with anomalous gold values. These values are related to more intensely-altered zones within the porphyries.