ILSG - Terrace Bay 2019

A full slate of pre- and post-meeting, one-day field trips is being planned to highlight the geology of the area. The trips will be held on May 14th and 17th.

Transect of the Quetico subprovince

Leaders: Mark Jirsa and Eric Nowariak, Minnesota Geological Survey, St. Paul, MN, USA

This trip will examine exposures of the metasedimentary, migmatitic, and intrusive rocks of the Neoarchean Quetico subprovince from north of Mountain Iron to near Crane Lake and along part of the Echo Trail. It will attempt to “unpack” the primary components of deposition, magmatism, deformation, and metamorphism that likely spanned 40 million years (~2700-2660 Ma). The latter is based in part on newly acquired geochronologic analyses (Jirsa, 2016; Salerno, 2017). The trip will also address the challenge of creating meaningful geologic maps of this and similarly complex terranes, and the apparent lithologic and temporal link between Quetico metasediments and those associated with successor basins in the region.

Neoarchean Alkalic intrusions in the Wawa and Quetico subprovinces

Leaders: Terry Boerboom (Minnesota Geological Survey, retired), Amy Radakovich (Minnesota Geological Survey)

A unique suite of Neoarchean alkalic plutons intrudes metamorphosed supracrustal rocks of the the Wawa and Quetico subprovinces in northern St. Louis and Itasca Counties, MN. Many of these plutons outcrop near the town of Cook, MN, while others are only intersected in drill core. The alkalic plutons have exceptionally high Ba and Sr contents, steep REE patterns, coarse-grained porphyritic textures, and strong aeromagnetic expressions. Though all are alkalic and relatively SiO2-poor, the suite can be compositionally divided into three groups: the syenitic clan, the monzodioritic clan, and the granitic clan. The syenitic plutons feature trachytic K-rich perthite in aegirine- or amphibole- groundmass and have strongly-developed trachytic fabric. The monzodioritic plutons are texturally similar to the syenitic clan, but contain Na-rich antiperthite and can vary from porphyritic monzodiorite to pyroxenite and granite. The granitic clan are vaguely to strongly porphyritic and trachytic, and have hornblende as the dominant mafic phase. While the granitic plutons are fairly uniform in composition, the syenitic and monzodioritic plutons commonly have abrupt phase transitions from mesocratic to either ultramafic or felsic compositions.

Moving from west to east, this fieldtrip will visit 6 of the alkalic plutons – 4 from the monzodioritic clan (The Side Lake, Morcom, Linden, and Idington plutons) and 2 from the syenitic clan (the Linden and Gheen plutons). None of the plutons from the granitoid group have outcrop exposure. Recent work has obtained U-Pb zircon ages of 2681.00 ± 0.29 Ma for the Linden pluton and 2675.1 ± 0.5 Ma for the Lost Lake pluton (Jirsa et al, 2022). Outcrops will demonstrate exceptional porphyritic, trachytic fabrics and unique alkalic mineralogy.

Unique Keweenawan inclusion (Colvin Creek) in the Duluth Complex

Leaders: Mark Severson (Natural Resources Research Institute, Teck, Retired), Allison Severson (Minnesota Geological Survey), and Lauri Severson (Earth Science teacher, retired)

The Colvin Creek inclusion is exposed in a remote area of the Partridge River intrusion (one of the intrusions that collectively comprise the 1100 Ma Duluth Complex). The inclusion is a very large raft that has been rotated to a near vertical orientation and consists of two magnetic metabasalt units and an overlying metasedimentary unit of gabbroic composition. The inclusion is about 2500 meters in strike length and 800 meters thick with stratigraphic tops to the northwest. Work by Patelke (1996) indicates that the metabasalt units can be correlated with intermediate olivine tholeiites of the North Shore Volcanic Group (NSVG); whereas the metasedimentary rock is more problematic. It is 350 meters thick, is extremely fine-grained, contains excellent sedimentary features (especially cross bedding), contains no quartz and is not obviously analogous to any of the typical interflow sediments of the NSVG.
This field trip will view several spectacular exposures of most of the units of the Colvin Creek inclusion including massive basalt, amygdaloidal basalt, and the “gabbroic” cross-bedded unit. The trip will involve mostly walking along a slightly improved logging road and some bush-whacking along flagged trails. Due to its remote location, spring road conditions will dictate if this trip will take place.

Drill Core from three Cu-Ni deposits of the Duluth Complex

Leaders: Mark Severson, Cullen Phillips (NewRange Copper Nickel) and Kevin Boerst (Twin Metals Minnesota)

Large resources of low-grade copper-nickel sulfide ore that locally contain platinum group element (PGE) concentrations are well documented by drilling in the basal zones of the Partridge River, South Kawishiwi and Bathtub intrusions. At least eleven occurrences of significant mineralization have been delineated in the basal 300 to 1000 feet of the intrusions. Of these eleven occurrences, three projects have currently undergone deposit definition drilling and are in various stages of early mine planning. These three projects include: the NorthMet and Mesaba deposits (NewRange Copper Nickel) and Maturi deposit (Twin Metals Minnesota). Mineralized drill core from all three deposits will be viewed at core facilities located at Babbitt and Ely, Minnesota. The drill core will be selected from different mineralization scenarios including both basal and PGE-enriched zones.

New Geological Insights into the Genesis of Iron Ores at Lake Vermilion – Soudan Underground Mine State Park

Leaders: George J. Hudak, Zsuzsanna P. Allerton, Annia Fayon

Massive hematite iron ores were extracted from the Soudan Mine from 1882 until 1962. The ores occur in proximity to and within shear zones that cut the Algoma-type banded iron formation units within the Soudan Member of the Ely Greenstone Formation. Several interpretations for the genesis of these iron ores have been proposed, yet their timing remained enigmatic for nearly a century. This one-day field trip will include several key outcrops that illustrate the stratigraphic and structural setting of the ores and incorporate an underground tour to investigate the massive hematite ores in the Montana stope at the 2700-level of the Soudan Mine. Discussions will focus on a new interpretation of the genesis and timing of these historical deposits.

Classic Outcrops of Northeastern Minnesota

Leaders: Dean M. Peterson and George J. Hudak

This field trip proposes to showcase some of the great diversity of Precambrian rock types in northeastern Minnesota using some of the most illustrative and readily accessible outcrops. The trip will revisit many of the outcrops upon which so many historical discussions, many of them heated, occurred. The planned outcrops include stops in the Archean (pillowed, sheet, and variolitic basalt lava flows; fragmental dacitic volcaniclastic rocks; greywacke-slate; shear zones, migmatites; and granitoids), Paleoproterozoic (Superior-type iron formation), Mesoproterozoic (Duluth Complex Cu-Ni-PGE bearing basal troctolites; sulfide-barren troctolites; layered troctolitic feeder dike; and Anorthosites), and Quaternary (remnant saprolite weathering and the Vermilion Moraine).

How do you make iron and/or manganese ores in Proterozoic Iron Formation?

Alex Steiner (Big Rock Exploration), Latisha Brengman (University of Minnesota, Duluth; Department of Earth & Environmental Sciences), Dean Peterson (Big Rock Exploration)

Iron formations are among the most important rocks for our modern industrial world. Their extraordinary iron content facilitates the manufacture of steel, while their manganese content is of crucial importance as a steel-alloy product and a critical component of battery technologies. Fueling modern technology requires efficient production of iron resources, exploration of manganese resources, and determination of enrichment processes that lead to ore formation. This field trip will explore ore-formation processes that turn otherwise uneconomic iron formations into valuable resources of iron and manganese. The trip may include an optional stop at the Hibbing Core Library where participants will examine drill core of the Biwabik taconite ores. Participants will explore sedimentary features, diagenetic reactions, and weathering reactions that contribute to iron grade and iron distribution within ore-horizons of the Biwabik. We will then travel to the North Star Manganese/Electric Metals core logging facility in Emily, MN where we will discuss the formation and subsequent redistribution of manganese within the Emily Iron Formation. Participants will have the opportunity to observe high-grade manganese oxide drill core, primary iron-manganese carbonate facies, and breccia horizons possibly associated with the Sudbury impact. The trip will then proceed to the Mary Ellen mine, a former natural ore pit, where the oxidation and weathering of the Biwabik was central to ore formation and early mining efforts on the range. Participants can observe primary features such as stromatolites and sedimentary structures as well as oxidation-weathering features. If time allows, we will wrap up the trip at the Biwabik outcrops in Virginia near the new bridge before heading back to Mountain Iron.

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