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A New Find of Barite Crystals With Phantoms From Michigan’s Copper Country

July 17, 2008

By Carlson, Shawn M Elder, Mark J

Mineral connoisseurs regard Michigan’s Copper Country as a mecca for world-class specimens of crystallized copper and silver. Lapidary enthusiasts prefer the area’s unique cutting rough, including Lake Superior agate, datolite, and greenstone (pumpellyite). However, the preeminence of these materials sometimes overshadows the many other collectible minerals that occur in the district, including such diverse species as powellite and ferberite, beautiful specimens that have perhaps failed to command much market attention because they have been discovered in only limited quantities. Barite is certainly one of the Copper Country’s underappreciated minerals. The White Pine mine produced some exceptional water-clear tabular barites associated with copper, silver, and chalcopyrite (Wilson and Dyl 1992). The Phoenix mine also produced rare accessory barite, typically as orange crystalline masses associated with native copper. Because of barite’s resistance to chemical weathering, there are several barite veins exposed throughout the Copper Country, though most of this material is fairly massive and unappealing to collectors.

In fall 2006, we stumbled across a large exposed barite vein in Ontonagon County during a traverse conducted for a mineral exploration company. The vein was first noticed as a slight depression, with weathered barite cleavages found intergrown with grasses and topsoil. The vein’s exact location is confidential (for now), but it is located within sandstone facies of the middle Proterozoic Copper Harbor Formation, associated with a northwest- striking fault (partially obscured by glacial boulder lag) that has been traced about 70 meters (in length) and ranges between 2 and 10 meters in width. In the middle of this fault lies the crystallized barite vein, infilled with dense reddish-brown clay (probably montmorillonite) that has protected the barite crystals from damage. Mineral collecting is accomplished by scooping out the clay with shovels, then washing the clay with water to liberate the barite crystals, a method similar to that described by Rosemeyer (2006) used at the Fivemile Point calcite vein in Keweenaw County.

The barite specimens (thumbnail to large cabinet in size) are composed of intergrown clusters of clear, tabular crystals to about 3 cm. The crystals invariably display a prominent {001} pinacoid, with a set of narrow {011} faces along the crystal sides and a pronounced set of {102} faces at the top and bottom of the crystals. Many crystals display additional {h0l} faces, too narrow to index by contact goniometry. Most crystals have bright orange-red or brown phantoms (single or multiple) due to microscopic inclusions of iron oxide. The brown phantoms are caused by concentrations of iron oxide spherules (about 7-200 [mu]m), some of which are nucleated on iron sulfide crystals, and the orange-red phantoms appear to be the result of diffuse iron oxide “dust” on early crystal faces that also have accumulations of microscopic disks (etch pits?) on their surface. Phantom morphology is similar to that of the external crystals, except that many phantoms display narrow {hk0} planes, a form not observed externally. Some barites are nucleated on breccia and wall-rock fragments, whereas others are floaters. The barite is nonfluorescent under ultraviolet radiation and is not anomalously radioactive (?-ray scintillometry). Associated minerals include calcite, chalcocite, chalcopyrite, galena (non-argentiferous), malachite, pyrite, and a botryoidal-to-vermiform “psilomelane” mineral (species undetermined). Nearly eight hundred specimens were recovered in 2006, and though most crystals collected at the surface display minor damage, crystals from deeper excavations are cleaner and more lustrous, suggesting better specimen potential at depth.

In recent years, northern Michigan has experienced a renaissance in mineral exploration activity, with several major companies searching for copper, nickel, gold, and unconformity-style uranium deposits. These activities have put more geologists into the field, increasing the odds of new discoveries that may benefit the mineral- collecting community. We anticipate more novel specimen discoveries from Michigan’s Upper Peninsula in the years to come.

AKNOWLEDGMENTS

We thank Drs. George Robinson and Robert Cook for carefully reviewing the manuscript and William Besse for preparing the map.

REFERENCES

Rosemeyer, T. 2006. News from the Keweenaw: Part 3-Recent mineral finds in Michigan’s Copper Country. Rocks & Minerals 81:260-76.

Wilson, M. L., and S. J. Dyl II. 1992. The Michigan Copper Country. Mineralogical Record 23 (2): 1-72.

SHAWN M. CARLSON

MARK J. ELDER

PO Box 234

Crystal Falls, Michigan 49920

shawncarlson@hotmail.com

markelder@juno.com

Shawn M. Carlson is a 1991 graduate (geology) of Michigan Technological University and a consulting diamond exploration geologist.

Mark J. Elder is a graduate of Bay de Noc Community College and a consulting geological technician.

Copyright Heldref Publications Jul/Aug 2008

(c) 2008 Rocks and Minerals. Provided by ProQuest Information and Learning. All rights Reserved.




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