231] J. T, Singewald 33 



sulphide ores are repeatedly duplicated in thin sections and 

 polished sections from all occurrences of titaniferous iron 

 ores in the United States. Titaniferous magnetite later than 

 the silicates and replacing them is seen in nearly every section 

 of the ores, though many of the contacts of the two sets of 

 minerals show what L. C. Graton and D. H. McLaughlin 

 have recently termed mutual boundaries, that is, boundaries 

 that give little evidence of the sequence of the minerals. 

 Only rarely is there unmistakable evidence of primary sili- 

 cates distinctly later than the ore. The replacement of the 

 silicates by the ore has not been accompanied by the forma- 

 tion of reaction silicates and in several instances hornblendi- 

 zatioirhas preceded the deposition of ore, phenomena in har- 

 mony with the nature of mineralization in the case of the 

 sulphide ores. If segregation takes place without the interven- 

 tion of mineralizers, one might expect the deposit at Iron 

 Mountain, Wyoming, to afford such an example. The ore 

 body there occurs as an almost pure mass of titaniferous 

 magnetite .cutting the anorthositic country rock as sharply 

 as any igneous dike ever pictured. Yet the numerous oliviny 

 crystals which occur locally in the ore are rounded and em- 

 bayed without the formation of reaction silicates in exactly 

 the same manner as in other occurrences. The deposit sug- 

 gests an injection from a basic magma analogous to a peg- 

 matite from a more acidic. In other cases, particularly at 

 Grape Creek, Colorado, the introduction of ore has been 

 accompanied by alteration of the feldspar so that the mag- 

 netite is separated from it by a band of hornblende, indi- 

 cating greater than usual activity of mineralizers. The Min- 

 nesota deposits conform for the most part to the general rule 

 that the ore is later than the silicates and afford some exam- 

 ples of hornblendization preceding or contemporaneous with 

 ore deposition, but also instances of feldspar and pyroxene 

 later than ore. 



An excellent example of an iron ore deposit in a basic 

 igneous rock giving unmistakable evidence of active partici- 

 pation by mineralizers in the formation of the ore is afforded 



3 



