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Ashland Co., Wisconsin, USA
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Ashland 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. The 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 well exposed along the gorge in Copper Falls State Park. The Mellen igneous complex is seen in quarries and outcrops around Mellen, Wisconsin. The dark-colored anorthosites and gabbros of the pluton are quarried and marketed as “black granite”. The plutons also contain red granites that are quarried near Highbridge. The Ironwood Iron Formation and associated sediments are exposed on the ridges along Mt. Wittlesey. Recently (2013) a mining company is proposing to open a large open-pit iron mine in the Mellen area, exploiting these rocks.

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
Andalusite
var: Chiastolite

Ankerite
Anorthite
'Apatite'
Augite
'Biotite'
Bornite
Chalcopyrite
Chamosite
var: Thuringite

'Chert'
'Chlorite Group'
'Clinopyroxene Subgroup'
Copper
Cubanite
Dolomite
Enstatite
Fayalite
'Feldspar Group'
Forsterite
'Garnet'
Goethite
Grunerite
Gypsum
Hematite
var: Kidney Ore
'Hornblende'
Ilmenite
Kaolinite
Kyanite
Laumontite
'Limonite'
Magnetite
var: Titaniferous Magnetite
Malachite
Marcasite
'Mica Group'
Minnesotaite
Molybdenite
Muscovite
var: Illite
Nickeline
'Olivine'
Orthoclase
'Orthopyroxene Subgroup'
Pentlandite
Pigeonite
Pyrite
'Pyroxene Group'
Pyrrhotite
Quartz
var: Agate
var: Jasper
var: Lake Superior Agate
Siderite
Sphalerite
Staurolite
Talc
Zircon


68 entries listed. 39 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

Laybourn, D.P., 1979, Geology and Metamorphism of the Ironwood Iron Formation, Gogebic Range, Wi., Unpub. M.S. Thesis, U of M - Duluth, 223 p.

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.

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.

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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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