GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 409 



quartz veins found throughout the district, while to the second class 

 belong the quartz-feldspar pegmatites and the gold-bearing quartz, 

 or quartz-sulphide veins. 



The quartz-hematite veins in the iron formation consist of quartz 

 in which flakes of specular hematite occur. The hematite may be 

 found in large, irregular, curved flakes along cracks, or it may be 

 imbedded within the solid quartz. These and various barren quartz 

 veins are of little or no importance as gold-bearing veins. They 

 are probably ordinary circulating water depositions, as the quartz 

 does not contain inclusions of monazite, zircon, rutile, garnet, or 

 xenotime, some of which are nearly always present in the quartz 

 in pegmatite veins and in igneous rocks.' It is possible, however, 

 that this evidence may not be conclusive and that by more detailed 

 study some of these veins may be found to be hot-water depositions. 



The quartz-feldspar pegmatite veins have already been men- 

 tioned in the description of the rocks of the basement complex. 

 They consist of quartz, feldspar, and muscovite with which other 

 minerals, such as beryl, columbite, tourmaline, etc., are sometimes 

 associated in minor quantities. They occur only in rocks of the 

 basement complex and are not gold-bearing. 



The gold-bearing quartz and sulphide veins are the important 

 sources of gold in the district. They occur in various formations 

 and differ somewhat in mineral composition in different localities. 

 In some places free gold occurs in quartz, sulphides being incon- 

 spicuous, while elsewhere the gold occurs in arsenopyrite, pyrrho- 

 tite, or pyrite associated with a variable amount of quartz. Other 

 minerals occurring in places in or near these veins are calcite, cyanite, 

 biotite, garnet, oligoclase, tourmaline, albite, siderite, muscovite, 

 and others. 



At the Passagem mine^ the principal sulphide minerals are 

 arsenopyrite, pyrrhotite, and pyrite, occurring in a gangue of 

 quartz, or quartz and decomposed oligoclase, strongly impregnated 

 with tourmaline. The oligoclase is altered to calcite and white 

 mica. With these occur in varying abundance the other minerals 



' Specimens examined by Dr. Derby. 



^ A. J. Bensusan, "The Passagem Mine and Works," Inst. Min. and Met., Twen- 

 tieth Session, October 19, 1910. 



