Stratigraphic Study and Joint-Pattern Analysis in the Biwabik Iron Formation, Virginia Horn Area, Northeast Minnesota

Abstract

The Biwabik Iron Formation of the Mesabi Range in northeastern Minnesota is comprised of chemically precipitated minerals. Cherty (granular) iron-formation formed on a shallow, tidally-influenced shelf, while slaty (fine-grained) iron-formation formed in deeper water. Thirty-nine sections from a 311-foot long drill core taken from north of the EVTAC pit area were studied to determine the mineralogy and paragenesis of the formation. Growth textures and crosscutting relationships indicate the primary minerals are chert (cement), greenalite, hematite and siderite. Very fine-grained carbonaceous material was deposited simultaneously. Secondary minerals include ankerite, magnetite, minnesotaite, and stilpnomelane. There were multiple generations of chert, minnesotaite, and siderite. Minnesotaite and magnetite replaced chert, iron-silicates, and ironcarbonates. Ankerite replaced chert and iron-silicate and grew in close association with magnetite. Magnetite was not replaced by any mineral, and grew abundantly in the cherty iron formation. Stilpnomelane grew abundantly in the slaty iron-formation. A study of joint orientations was undertaken on the premise that the data would be of value in planning the orientations of blasting grids at taconite pits. There are common joint orientations in seven taconite pits near the Virginia Hom area. The joints do not appear to vary with the strike of the iron-formation in the anticline/syncline structure of the Virginia Horn, and dips are generally vertical. A dominant joint direction found in Archean rocks of the Virginia Horn area (280°) is close to the dominant fault strike direction (285°) in the Virginia Horn, suggesting some control of the iron-formation jointing by underlying Archean basement rocks.