Abstract

Mining operations at Steep Rock Lake since 1945 have yielded about ten million tons of high-grade iron ore and have cast new light on the geology of this long-famous Precambrian area. The productive length of this most-recently discovered of the Lake Superior ranges should exceed four miles and ore intersections have been obtained at depths down to 2,000 feet below the old lake bed, implying that potential reserves must be measured in hundreds of millions of tons.The Steeprock group lies unconformably upon a granitic complex ("Laurentian") and consists of:5. Interbanded flows, tuffs, and sediments4. "Ashrock" (a basic pyroclastic containing minor flows and tuffs)3. The "orezone"2. Limestone and dolomite (in part showing "algal" and other organic (?) structures1. Basal conglomerate (not everywhere present)All these formations (together with younger basic intrusives) have been steeply tilted, more or less altered, and show foliation or brecciation in appropriate rock types. Such features (plus sporadic pyritic replacement lenses along and near the hanging-wall of the "orezone," as well as recrystallization, modification, and minor migration of the iron ores) are ascribed to the activities of a post-Steeprock ("Algoman") batholith that outcrops two miles to the west. Several major faults cross the area, the largest of which has caused a strike separation of more than two miles in the Steeprock group.The "orezone" holds all the known iron ore-bodies, is up to several hundred feet in width, shows remarkable continuity and uniformity wherever explored, and is made up of two members. The stratigraphically lower part is chiefly manganiferous waste (averaging more than 2 percent Mn) with minor iron ore, resting residually in erosional disconformity on the underlying leanly manganiferous and ferruginous limestone and dolomite. The upper part of the "orezone" carries negligible manganese and is composed chiefly of goethite with a few relatively narrow bands of cherty, sandy, and aluminous sediments. The ores show a variety of breccia, vuggy, and colloform structures. The most common type of breccia has fragments of very fine-grained goethite surrounded either by coarsely crystalline goethite or specularite or by earthy hematite. A pisolitic facies locally present along the hanging-wall of the "orezone" contains gibbsite and resembles a ferruginous bauxite.The field relations, mineralogy, and paragenesis suggest that the ore-bodies represent sedimentary limonite of Steeprock age, brecciated by compaction and folding, with the more finely comminuted matrix recrystallized or modified by hydrothermal solutions, locally effecting migration of iron.

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