RI-17 Geology, Sulfide Mineralization and Geochemistry of the Birchdale - Indus Area Koochiching County, Northwestern Minnesota

Abstract

The rocks of the Birchdale-Indus area, in northern Minnesota, are part of a poorly-exposed volcanic-sedimentary sequence within the southwest extension of the Wabigoon Volcanic Belt of Canada. The bedrock is Early Precambrian (or Archean) in age and includes mafic to intermediate lavas and sub volcanic intrusive rocks; felsic dikes, agglomerate, tuff, and volcaniclastic rocks; iron-formation and associated metasediments; and granitic rocks of Algoman age. All rock types are cut by northwest-trending Middle Precambrian mafic dikes. The area is largely covered by Pleistocene glacial deposits. The volcanic-sedimentary sequence has been isoclinally folded and has steeply-dipping northeast-trending bedding and foliation. A doubly-plunging anticline and a syncline have been mapped within the area, and a second generation of folding on more northerly trending axes is suggested by some structural observations. Three sets of faults and fractures have been delineated. All the Lower Precambrian rocks were metamorphosed to amphibolite grade during the Algoman event. Synthesis of the geology of the Birchdale-Indus area with that of the better exposed Emo area in adjacent Ontario (Fletcher and Irvine, 1954-) provides a firm basis for interpretation of the development of the volcanic accumulation. Mafic and intermediate volcanic and intrusive rocks apparently constitute a lower stratigraphic unit. An explosive felsic volcanic center, marked by abundant agglomerate, developed upon the older, mafic platform in the vicinity of Birchdale, Indus, and Emo. Felsic tuff, volcaniclastic rocks, and iron-formation were deposited outward from this center. The Birchdale-Indus area and areas to the west and south have been actively explored for base metal sulfide deposits during the past decade. Thick zones of massive, submassive, and disseminated pyrite and/or pyrrhotite have been penetrated at several localities, but copper and .zinc have not been found in economic quantities. Several of the iron-sulfide bodies that were penetrated are associated with oxide iron-formation. To assist in further exploration, geophysical and geochemical anomalies have been located by ground survey methods by the Minnesota Department of Natural Resources.