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A return to the Hill Annex mine in August of 2003 (with
better photos on the left, 2000 photo below) shows some changes over
the 3 years. The water has risen more, the "telephone pole" with
the osprey nest is gone but the major geological features remain.
The unconformity between the glacial till and the iron formation is
still visible. |
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The photo above is what was called merch ore. It is a
block of conglomerate (many different sizes of rock cemented together)
containing rounded pieces of hematite. The rounding of the rock
indicates the weathering action in the caused before the formation of
the conglomerate. Left - Scouting the area produced some better photos of banded hematite (non magnetic iron mineral - the gray parts of this "rock") with chert (a micro crystalline form of quartz - in the red form caused by iron oxide it is called Jasper or sometimes bloodstone). The coin in the photo to give a size indication is a quarter. |
| Glacial Drift:
Sand, gravel, boulders and clay were left behind by glaciers.
This "overburden" glacial material covers most of the iron formations.
(top layer) Coleraine (Cretaceous) Formation: Not a band of ancient ore, but an unusual conglomerate layer of hematite and granite pebbles worn smooth by water action and cemented together with fine silt mixed with animal and plant life from 75-85 million years ago. The only layer that contains fossils. (second layer) Virginia Slate: A thin bed of gray to black rock found under Calumet and other locations on the Iron Range but not a the Hill Annex. Contains no iron. (third layer)  Pokegama Quartzite: An underlying, well-cemented, coarse-grained, quartz rock that contains no iron. Found at about 500 foot depth at Hill Annex. (eigth layer) Giants Ridge Granite: A medium to coarse grained rock lying under the quartzite. Contains no iron. (bottom layer) |
The
Biwabik Iron Formation (BIF) is the name given to the group of
layers of rock containing iron in the Mesabi iron range. This range is
at the north edge of the Animekie Basin. The photo is from the State
Park Center at the Hill-Annex mine. While the rocks are often banded,
BIF is NOT the acronym for banded iron formation but Biwabik Iron
Formation. From the text in the photo: Iron deposits were formed over 1.8 billion years ago (Precambrian Period) when volcanoes spewed gasses into an atmosphere devoid of oxygen and lifeless oceans covered much of the prehistoric land. For millions of years, grinding earthquakes, weathering, condensation of gasses, heat, sea water seepage, and evaporation contributed to the formation of various types of ore, which were embedded in layers of ancient rock. Iron ore deposits are located in nearly all areas of the Earth's crust, although few deposits are as commercially important as those of the Hill Annex and other mines of the Lake Superior region. (Iron containing layers in the "brown" section in the photo at the left) Upper Slaty Layer: Generally not a good source of ore but this fine grained layer may contain some bands of merch ores and wash ores. This layer and the next three layers below are part of the Biwabik Iron Formation. Upper Cherty Layer: This hard layer yielded some of the best ores. The richest were over 50% iron and were used directly as mined, hence the called "merch" (for merchantable or that needs no further processing). Lower Slaty Layer: A fine-grained layer containing mostly waste lean ore (less than 25% iron) and some iron pyrite (fools gold). The bottom of the lower slaty is the "marker layer" (it contains over 1.5% alumina) across the Range. Lower Cherty Layer: The best Hill Annex ores - merch ores and wash ores - are from this layer. Also contains some leaner ore (less than 50% iron) mined and processed as "heavy media" and very lean taconite. |
| The photo at the right shows a block of the merch ore that
was mined, crushed and directly shipped to the steel mills on the Great
Lakes. The high iron content of the hematite ore allowed it to be put
directly into the blast furnaces to make iron and steel. From the
display: Snails, clams and other fragments of ocean creatures in 75-85 million year old sediments are evidence that Minnesota and the Dakotas were once covered by a shallow, warm sea. A Common type of fossil found in the Cretaceous ores of Hill Annex is that of a marine animal with a flat, coiled shell called an ammonite. There were many different kinds of ammonites, they were related to the nautilus, squid and octopus. Shark's teeth are occasionally found. Their size indicates that sharks living here grew up to 15 feet long. Cretaceous rocks also contain plant remains such as carbonized wood and pine cones from the coniferous and deciduous trees that lived millions of years ago. Enough plant material accumulated into the eastern part of the range to form a thin deposit of lignite coal. |
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The enriched iron was formed during the Cretaceous Period is
right after the Jurassic Period (made famous by the movie Jurassic Park). These
Periods along with the Triassic Period make up
the Mesozoic Era, often referred to as the "Age of Reptiles", the
time period when Dinosaurs "ruled" the earth. A. Sharks teeth B. Pine Cone C. Ammonite D. Reptile Vertebra E. Snail F. Clam G. Oyster H. Fish Vertebra |
