ORE DEPOSITS IX MANSFIELD SLATE. 69 



Metallic iron, 64.80; phosphorus, 0.037; silica, 3.70.^ According to Dr. 

 N. P. Hulst,^ those ore deposits in the Menominee range which ha^-e poorlv 

 defined walls carry a minimum of phosphorus. This body, however, shows 

 that the same conditions do not exist at the Mansfield mine, since, while it 

 has both sharply defined foot and hanging walls, it contains but a low 

 per cent of phosphorus. From an examination of the analyses from which 

 the above average was obtained I find that the percentage of phosphorus 

 shows a marked increase in the lower levels of the mines over that of the 

 higher, and there is also a slight corresponding decrease in the content of 

 metallic iron. Increase of phosphorus with depth is also found in the 

 adjoining Menominee range, as noted by Messrs. E. F. Brown,^ of -^he 

 Pewabic mine, and Per Larsson,* of the Aragou. It is impossible to state 

 whether or not this distribution is due to the action of percolating water, as 

 suggested by Hulst,* Larsson,* and other Michigan mining- engineers. Only 

 a large number of good analyses from carefully selected ores and asso- 

 ciated rocks and a detailed study of conditions of occurrence could lead to 

 any accurate determination of the reason for such distribution, and a dis- 

 cussion of these reasons is by no means warranted by the few and imper- 

 fect analyses of the Mansfield ores, which I have been able to obtain. The 

 ore body changes in composition to the south of the shaft, as shown by the 

 di'ifts in this direction. The ore in this part of the mine contains more phos- 

 phorus, alumina, and calcium, and less iron. This low-grade lean ore then 

 passes over into the banded chert and ore mixed with the lime and quartz 

 rock mentioned above. 



MICROSCOPICAL CHARACTER OF THE ORES AND ASSOCIATED CHERT BANDS. 



The ore varies from a soft limonitic hematite to a moderately hard 

 hematite. It is for the most part opaque under the microscope, l3ut in 

 places shows bright-red to brownish-red color in transmitted light. In 

 incident light the ore for the most part shows a dull-brown or reddish color, 

 though in places it has a bright metallic reflection. In places in the ore 



' An average of 62 per cant metallic Iron and .030 per cent phosphorus is reported in Report of 

 Commissioner of Mineral Statistics of Michigan (G. A. Newett) for 1896, p. 85. 



-The geology of that portion of the Menominee range east of the Menominee River, by N. P. 

 Hulst : Proc. Lake Superior Inst. Min. Eng., Vol. 1, 1893, p. 28. 



^Distribution of phosphorus and system of sampling at the Pewabic mine, Iron Mountain, by 

 E. F. Brown : Proc. Lake Superior Inst. Min. Eng., Vol. Ill, 1895, p. 49. 



*0p. cit.,p. 52. ^Op. cit., p. 28. ''Op. cit., p.53. 



