Report of Investigations

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Browsing Report of Investigations by Author "Boerboom, Terrence J."

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    RI-51 Geologic Setting and Descriptive Geochemistry of Archean Supracrustal and Hypabyssal Rocks, Soudan-Bigfork Area, Northern Minnesota: Implications for Metallic Mineral Exploration
    (Minnesota Geological Survey, 1998) Southwick, D.L.; Boerboom, Terrence J.; Jirsa, M.A.
    Geochemical data are presented for a suite of samples chosen to represent as nearly as possible the compositions of unaltered rocks within the Archean greenstone belt of the Soudan-Bigfork area in northern Minnesota. These data are interpreted within the stratigraphic and structural framework provided by geologic mapping, and are used to deduce probable depositional and tectonic environments for the volcanic and sedimentary sequences described. In addition, the data indicate regional differences in intrinsic mineral potential, as judged from the presence or absence of geochemically favorable rock types associated with economic deposits of metallic minerals elsewhere in the Superior Province. The greenstone belt as a whole has attributes consistent with development in an oceanic volcanic-arc setting that is broadly analogous to modern arcs of the western Pacific basin. Supracrustal rocks in the Soudan belt (southern portion of the Soudan-Bigfork area) include a lower volcanic cycle that consists of the upper parts of a calc-alkaline volcanic edifice which evolved with time from a submarine basaltic pile into an emergent or nearly emergent dacitic eruptive center, and an upper cycle that represents submarine fan deposits mingled in time and space with tholeiitic basalt of possible back-arc origin. The Newton belt (northern portion of the Soudan-Bigfork area) consists of several fault slices made up chiefly of tholeiitic basalt and pyroclastic and epiclastic dacitic rocks. Because of structural shuffling and imbrication, the rock sequences within the Newton belt cannot be reliably correlated from slice to slice, which handicaps paleotectonic interpretation. Geochemically they are consistent with a back-arc origin. Basaltic sequences in the Soudan belt are evolved to iron-enriched compositions and exhibit very low background values for gold. Basaltic sequences in the Newton belt are moderately more magnesian and auriferous, and therefore may be somewhat better candidates for harboring economic gold deposits. The dacitic rocks of the Soudan belt fall into the least favorable geochemical category for hosting massive- sulfide deposits, based on empirical associations throughout the Superior Province, whereas the geochemical attributes of dacitic rocks in the Newton belt remain inadequately characterized in this respect. Differentiated gabbro-pyroxenite-peridotite sills are absent from the Soudan belt but are characteristic of the Newton belt. Peridotite sills within the Deer Lake Complex contain subeconomic amounts of magmatic Cu-Ni sulfide minerals and are petrologically reasonable, but untested, candidates for hosting platinum-group minerals. On the basis of these broad lithogeochemical criteria, the mineral potential of the Newton belt would appear to warrant serious further investigation.
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    RI-70, Characterization of the Franklin Peridotite and Other Similar Intrusions in East-Central and Southwestern Minnesota
    (2014) Boerboom, Terrence J.
    Outcrops of peridotite adjacent to the Minnesota River near the town of Franklin in Renville County were sampled and petrographically characterized as part of a small study funded by the Minnesota Department of Natural Resources in 1997. That study obtained mineral separates with the intent of examining them for kimberlite indicator minerals. The results were not formally published, but rather summarized in an unpublished final report to the Minnesota Department of Natural Resources titled "Mineral Investigations of Franklin Kimberlites." In addition to petrographic and geochemical characterizations, ground magnetic traverses were made across the outcrop area in order to quantify the size and shape of the peridotite body. Based on simple ground magnetic surveys, the peridotite body is approximately 1 square kilometer (0.4 square mile) in area. The peridotite in the outcrops is extensively silicified, most likely by low-temperature alteration associated with lateritic weathering beneath Cretaceous sedimentary strata. Peridotite that is not silicified is composed of olivine (serpentinized), orthopyroxene, hornblende, magnetite, and minor spinel, phlogopite, ilmenite, and sulfide minerals; all the silicate phases are Mg-rich. The silicified peridotite contains abundant secondary quartz and chalcedony, but the silicification did not affect the Fe/Mg ratio, as both silicified and unsilicified peridotite have Mg numbers of 87 to 88. The Franklin peridotite is similar to ultramafic peridotite and pyroxenite bodies in east-central Minnesota, as well as the Cottonwood peridotite body intersected by drilling in northern Lyon County, 55 kilometers (34 miles) west–northwest of the Franklin peridotite and south of the Minnesota River valley. The peridotites in east-central Minnesota are between 1,770 and 1,791 Ma in age, whereas the age of the Franklin and Cottonwood peridotites is unknown.