208 



ECONOMIC GEOLOGY 



According to Van Hise the orginal iron ore formation was a 

 cherty iron-bearing carbonate which in some localities closely 

 approached a siderite. The upper part of the Negaunee formation 

 in Inter-Marquette times was transformed into ferruginous slates 

 and ferruginous cherts. In the early Upper-Marquette time de- 

 trital material from the Negaunee formations accumulated. This 

 material, formed at the base of the Upper-Marquette series, con- 

 sisted of iron oxide and quartz. Thereafter the original rock 

 masses, the weathered rocks in situ, 'and the detrital material 

 were all buried beneath Upper-Marquette sediments and igneous 

 rocks of Upper-Marquette and Keweenawan age. 



While these rocks were deeply buried they were intensely 

 folded. Where the original carbonate of iron remained and es- 

 pecially where it was intruded by abundant igneous rocks it was 

 partly transformed into a griinerite magnetite schist. The iron 



FIG. 111. Generalized section in the Marquette district showing the 

 relation of all classes of ore deposits to the associated formations. (After 

 C. R. Von Hise, U. S. Geological Survey.) 



oxide of the ferruginous slate and ferruginous cherts was dehy- 

 drated and these rocks were transformed to jaspilites. The de- 

 trital ores were converted into hematite and jasper quartzites and 

 conglomerates. Subsequent to this period of folding, but prior 

 to Cambrian time, the region was folded anew. Denudation 

 cut through the Upper-Marquette formation and again exposed 

 the Negaunee terranes to the processes of weathering. Where 

 the unaltered cherty iron carbonate was abundant it was trans- 

 formed into ferruginous slates and cherts. It was at this period of 

 alteration that the ore deposits were developed. 



According to E. T. Hancock the workable deposits of the 

 Negaunee formation may be divided into three classes. (1) 

 Ore deposits at the base of the iron-bearing formation. (2) 

 Ore deposits within the iron-bearing formation. (3) Ore de- 

 posits at the top of the Negaunee formation and at the base of 

 the Goodrich quartzite. 



