27 



ORIGIN OF ORE DEPOSIT x 



\\ ' may consider one type in the segregation of ore 

 I'm n i MM ultra-basic magma traversing the Appalachian belt 

 I'miii Alabama to Nova Scotia, appearing often in the Carolinas, 

 Maryland, Pennsylvania, Vermont and Quebec in a belt of 

 Paleozoic rocks pierced here and there by peridotite. The 

 segregation in the Thetford and Black Lake districts, Quebec, 

 is as follows, as shown in Fig. 20. 



(1) The metals, chromium and iron, become oxidized and 

 form lens-shaped masses of chromite and magnetite. There 

 is a tendency for the chromite and magnetite to migrate toward 

 the periphery of the mass and to appear often as lens-shaped 

 ore bodies in the midst or in the border of the basic peridotite. 

 The peridotite, rich in ferro magnesian minerals was the second 



FIG. 20. Section to illustrate the segregation of chromite and magnetite as 

 lens-shaped ore bodies, Troy, Vermont. A, Perdotite; B, sericite schist. 



mass to solidify. The peridotite consists almost entirely of 

 the ferro magnesian mineral olivine. 



If a difference existed in the constituents of the magma the 

 pyroxenites would appear third in the process of solidification. 

 The next product in the order of the solidification was acidic in 

 character, light in color, of lower specific gravity, and the product 

 a granite containing crystals of the potash feldspar, orthoclase, 

 with granules of some ferro magnesian minerals, usually horn- 

 blende. The final product as a rock mass from solidification is 

 the most acidic and ribbed by veins of aplite, in which the feld- 

 spars, micas and amphiboles are apparently absent. The final 

 product through the differentiation of the ultra-basic magma 

 gives free silica or quartz. 



