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Iron Co., Wisconsin, USA

Iron County’s bedrock is dominated by a 35 km. thick sequence of Precambrian rocks tilted steeply by thrusting along the Marenisco Fault. These Precambrian rocks occur in NE striking belts, with the oldest at the southern edge of the county. These oldest rocks in the section are poorly exposed Archean meta-volcanic and meta-sedimentary rocks. These are overlain by a thick section of Proterozoic sedimentary rocks, including the Ironwood Iron Formation, which was the main formation extensively mined in the Gogebic Iron Range. These are overlain by a thick section of 1.1 billion year old (Keweenawan) volcanic rocks (Powder Mill Group and Portage Lake Lava Series) from the Mid-Continent Rift. These are intruded by the complexly differentiated Keweenawan–age plutons of the Mellen Complex. On top of these is a package of Proterozoic sedimentary rocks (Oronto Group) which further filled the rift after the cessation of volcanism. The youngest bedrock, separated from the older rocks by another large thrust fault, the Douglas Fault, is the little metamorphosed Proterozoic Bayfield Group sandstones.

The Keweenawan volcanic rocks are locally quarried for road metal. The Ironwood Iron Formation hosted some important mines, notably the Montreal and Cary mines, which produced world class specimens of goethite, hematite, manganite, manganocalcite, barite and other minerals. These were underground mines, which worked the oxidized zones in the iron formation down dip to depths of over 4,000 feet. The mines closed in the 1960's, but there has been renewed exploration in the area (2013).

Glacial drift and glacial lake sediments cover a lot of the county. The coarse tills and outwash can yield interesting minerals such as “drift” copper and Lake Superior agates.

Mineral List

Mineral list contains entries from the region specified including sub-localities
Actinolite
'Albite-Anorthite Series'
Almandine
Analcime
Aragonite
Baryte
'Biotite'
Braunite
Calcite
var: Manganoan Calcite
Celestine
Chamosite
'Chert'
'Chlorite Group'
'Clinopyroxene Subgroup'
Copper
Corrensite
Cryptomelane
Dickite
Dolomite
Epidote
Galena
Goethite
Graphite
Greenalite
Gypsum
var: Selenite
Hematite
var: Specularite
Hollandite
'Hornblende'
Kaolinite
'K Feldspar
var: Adularia'

Kutnohorite
Kyanite
Laumontite
Magnetite
Manganite
Marcasite
Minnesotaite
Muscovite
'Needle-ore'
Neotocite
'Orthopyroxene Subgroup'
Prehnite
'Psilomelane'
Pyrite
Pyrolusite
Quartz
var: Agate
var: Jasper
Rhodochrosite
Romanèchite
Siderite
Sillimanite
Staurolite
Stilpnomelane
Talc
Thomsonite-Ca
Vanadinite
Wollastonite


65 entries listed. 46 valid minerals.

Localities in this Region

USA
USA

The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.

References

Cannon, W.F., L.G. Woodruff, S.W. Nicholson & C.A. Hedgman (1996) Bedrock geologic map of the Ashland and the northern part of the Ironwood 30' x 60' Quadrangles, Wisconsin and Michigan: U.S. Geological Survey, Miscellaneous Investigations Series: Map I-2566, 1 sheet.

Dott, R.H. & J.W. Attig (2004) Roadside Geology of Wisconsin, Mountain Press Publishing Company, 346 p.

Cannon, W. , G. LaBerge, J. Klasner & K. Schulz (2007) The Gogebic Iron Range - a sample of the northern margin of the Penokean fold and thrust belt: U. S. Geological Survey Professional Paper 1730, 44 p. plus map.

Robinson, G. & G. LaBerge (2013) Minerals of the Lake Superior Iron Ranges: A. E. Seaman Mineral Museum: Michigan Technological University, 52 p.

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Copyright © Jolyon Ralph and Ida Chau 1993-2014. Site Map. Locality, mineral & photograph data are the copyright of the individuals who submitted them. Site hosted & developed by Jolyon Ralph. Mindat.org is an online information resource dedicated to providing free mineralogical information to all. Mindat relies on the contributions of thousands of members and supporters. Mindat does not offer minerals for sale. If you would like to add information to improve the quality of our database, then click here to register.
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