Hard iron ore at the Cliffs Shaft Mines, Ishpeming, Michigan

Gair, Jacob Eugene · 1974 · Report

At the Cliffs Shaft Mines, for many years the premiere hard iron ore mine of Michigan, bodies of ore typically are localized in the upper part of the

This 1974 document, Hard iron ore at the Cliffs Shaft Mines, Ishpeming, Michigan, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

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figillogioopp Hard Iron Ore at the Cliffs Shaft Mines, Ishpeming, Michigan by Jacob E. Gair 1-Z 200) n0.74-227

rhis report be en edit -d rwr r -N conforrn;t v or hard iron Ore at ',he Jc. c: 4.: (ec At t e Cliffs Shaft for many years the prier hard iro,. ore mine of :achi,-,an, bodies of cry. typically are localized in e upper part of the ::eir.au:ee ­ close 1)(.-::eati, Goodrich (.7;uartzite in the axial part of t. e :J.rquette synclinorium. Individual ore L may be as much as 100 feet thick and extend laterally for mar:y hundreds of feet. The cluster of ore bodies making up the Cliffs Shaft deposit has an overall lenP:th of about 4,500 feet from east to west parallel to the axis of the syncliorium and a width of 1,500 feet from north to south. the deposit extends ben eath much of the City of Ishp, mil:,„;, from a short distance west of the city limits to almost the east ede of tne - oreeari ng zone Fenerally does not reach more than 150 feet stratiraphicall.v into the iron-formation from the bast, of the Goodrich Quartzite. Folding of the ore-bearing7. zone and the overlyinr7 and under­ lying rocks took place as part of the folding of the synciiorium and carries the ore to varying7 depths 1-elow the land surface. Foldi::p: has placed the ore-hearing zone in anticlinal, synclinal, and monclinal configurations. Hard ore is compact, massive, and non-porous, and varies in color from blue black to reddish black, steel gray, or reddish dark blue. Over tie years of mining (approx. 1880-1965), the ore has had an iron content of 58 to 62.5 percent, silica of 5 to 7 percent, phosphorous of .09 to .13 percent, and man:7anese of .2 to .8 percent. The iron mineral is mainly specular hematite, but magnetite octahedra are sprinkled through

the Lematite r-Las:es in places. Grici:.a:t lamfi preserved soave hard ore a:A remains of red c:iert jas-L7r; r C ► chart ;:e'is are 1.,corported in t:.e ore in places. ..arr3 ore n:1 met.anorphic recry!-,LallIze.1 fabrics in whirh secular or hematite and quartz rains are locked tore .her and have revL!inr,d virtually intact since the end Precal:rian time. Bodies of hard ore occur almost exrdusiv6ly in jaspilite ironformation, hut ir. a few places. with appropriate Chan ;es in onmposjt.iori and texture, may extend into overlyiria Goodrich Quartzit,2. :'31:.ce the early days of inr-in the I.:arquette distric-,,, the Cocdric:1-.e::aunee contact has ,eer. rcco,inized as a ,:aide in the search for additional deposits of hard ore. The Goodrich innediately overly': 2:eaunee iron-formation is quartzite, con:lonerate, or an ari7;illaceous slaty rock. Basal Goodrich conloerote generally is quart_zite and contains fragments of the underlying jaspilite iron-ford.ation. Hard ore deposits that extend into the Goodrich become con:-lonerate ore--a mixture of typical detritus of the Goodrich quartzite and a hematite or hematite­ nw;netite matrix (derived from erosion of the underlying iron-formation). The Goodrich-::er,aunee contact in the Cliffs Shaft mine thus is an erosio:: surface, but one along which there is little or no angular discordance. The virtual lack of an angular discordance sw-gests that the anticlines and synclines orebodies are now distributed did not exist prior to deposition of the Goodrich.

the irre. -ularly shaped ar.d orE—o'iles, typically specular hematite and red chert (jasper; but includ(:s some interlv,fred maf-n iite-jasper and martitetiasper. ,Tespilite is -w.derlai.. in 1 Jr sideritic iron-formatio!'. The zone of jaspilqte 1-eween hart ore a. di sideritic.iron-formation is o: irregular thickness, 10 to 100 feet, or more. In some places in the mine. the selva:e of jaspilite between hard ore and sideritic iron-formation is only inches thick. Jaspilite beds have been seen in a few places in the mine to pass lori strike i!.to chert-carbonate beds, and small "islands"--unoxidized remnants—of cert-carborlate iron-formation in a few places are surrounded by jaspfilite. Sideritic iro:.-formation, below jaspilite is notably free of alteration and veiniL-. It is clar that at least some jafTilite was derived by alteration of iron-formation., and tliat alteration probably moved downward from above the contact of jaspilite and carbonate iron-formation. Lumerous irregular intrusive ladies and dikes of chloritized EryeensLone—metaabbro and metadiabase--cut iron-formation and odies of hard ore in the mine. ?;lost such bodies appear to predate noodrich Quartzite, but some bodies that lie along or extend across the contact of the two formations are ouriir.er in age than the Goodrich. Most intrusive bodies in contact with hard ore, other than thin dikes, dip under the ore, so that the ore generally occurs between the greenstore and the Goodrich hanging wall. The irregular-shaped areas of iron formation caught between the intrusive bodies and the Goodrich contact

make for a corxlex co! aLA dis:Iv.]:Y- io of in:?1:111A bodies of :ard ore, but nla. proxinitT the folder! -oolri(' -c" tar''. The ma nett cont,.!tt or ore a..d iror-formation typical:,- increases percepti' 1: of rock a'.d 1:1 places a septum of nearly solid mL,-Letite, an inch or so t'lick, :orders Inc contact with intrusive rock. Iron-formatio near co:.tats with greenstore may be mainly interlaminated ma7netite and iray yellow sulfide also occurs in a few places alonEr, contacts 1-clac-n sive ?;reerstone and iron-formation. In the axial part of the 11arquette syncliLorium at the Cliff:, Shc deposit, the Feaunee and Goodrich formations. as well the erosional contact between them, and the adjacent ore-:tearing zone, are I e: .t. into a number of west-trendjn: folds. The zone of jaspilite and enclosed Yodies of hard ore follow the foldeil erosion surface up at.d down. The folds are more or less disrupted by faults that F.enerally also have an east-west alinement. Fold plunges are westward in the western part of the deposit in rTeneral conformity with the plun'e of the synclinorium. In the central 2,500 feet or so of the deposit, beneath the western and central parts of the city of Ishpemins7, fold axes flatten and then reverse to a gentle eastward plun:re in the eastern part of the deposit. Almost at the very east end of the deposit, fold plunges reverse again and the ore hearing zone bends upward to the east. The deepest ore at the west end of the mine is about 300 feet above sealevel and 1,100 feet below the land surface. A corresponding part of the structure and ore zone rising', up plunge to the east reaches to about 850 feet above sea

level in the oY te d,-,ro:-- 1L. a

east port of .' 2 d-flp-to -en Feet a (-.2 erc:z,r, ('Pn 'e seen to 'J'eet. Ire on the cre of t1 reaches a Uttle core 1,20", feet n:ove sea le',e1 below the land surface. Ore ::..00ts in old just eriJsi_ of 4.*f., Clirfs Shaft deposit proper. conti:iued up plut -e to the east a .:6 the surface at Jasper Knob near the southeast corner c: is. The aforeme:itioned o servations tend to support eri -­ metamorphic theory of C. R. Van Eise for the origin of the hard erg,: and some, if not all, of the jaspilite in the Cliffs Shaft mire. The pr3r01­ pal considerations are as follows: 1) The sprit/3l association of jaspilite and hard ore slests that they are related in ori,Tin, and that the relationship is controlled in some way by the contact erosion surface) separatirthe Goodrich Quartzite and T:er7aunee Iron-forma­ tion. Relict iron-formation laminations in the hard ore indicate that the ore-pTade concentration of iron was accomplished by a direct replacement of iron-formation, especially of the cherty laminations. 2) At least some jaspilite in the Cliffs Shaft mine waf3 derived by alteration of carbonate iron-formation, and the proximity of jaspilite to the pre-Goodrich erosion surface sw.c-ests that the alteration was the result of weathering.

3) An oxide iro:-orion corr, ,:-Tos..11 L: and ore-,- tc I ,x7e colIctrat:to:.s of , present bodies of hard ore ex:Ls1.,H ror to th Penokf2an meta:lorphis!-, and. pr or presence in nasal Goodri(: ( of jaspilitehard ore indicate r!ro essentially pre-Goodrich ir! of jnspilite and hard ore sows tat icir ir: sition was already established when neorpIlimn took pla(!e. 4) The position of intrusive :-reer!stone bc ion dirrir:;T under eIoreodies. suPi7ests an artie-y wi 1 b soe lodi(;s of soft iron ore formed from enrto:_at,J which uderlyinp., intrusive rock provide a t:barrier to the downward movement of i-round water that led to a of concentrations of secondary iron oxide above the barrier-­ between the harrier and the erosion s-,Irfe. 1%:asses of maric intrusive rock emplaced in the iron-horatio beFore the postNeauee, pre-oodrich erosion interval could have had the same effect durin:i; that interval. Hower, not all intrusive bodies are clearly pre-erosion interval in ar,e; some appear to have come later. The ma,7netite present in many bodies of hard ore adjacent to contacts with intrusive ;7reenstone apparently is a product of contact metamorphism, so is not readily explained unless the intrusive rock was emplaced after the ore-PTade concentration of iron had taken place. The simplest explanation is that mafic intrusive bodies were emplaced before and after the erosion interval, and before reE7ioal

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Plates & figures from the original