4IO E. C. HARDER AND R. T. CHAMBERLIN 



mentioned above. Dr. Derby^ has made a study of the genesis of 

 the mineral and ore deposition at this mine and has estabhshed 

 three successive stages of minerahzation. He beheves that the 

 original quartz-oligoclase deposition was of a pegmatitic nature, 

 the material being derived from some intrusive igneous mass which, 

 however, has not yet been discovered. For although nowhere in 

 the district has any evidence been found of igneous intrusions into 

 the sedimentary series, the nature of the mineralization is such as 

 to leave no doubt as to its magmatic origin. During the deposition 

 of the quartz and oligoclase it appears that garnet (andradite), 

 biotite, cyanite and staurolite were developed locally along the 

 contact, these minerals occurring in the country rock along the 

 border of the vein. Crystals of apatite are associated sparingly 

 with the garnet. Graphite also is associated with the contact 

 minerals, being found along shearing planes. 



The second stage of mineralization, according to Dr. Derby, con- 

 sisted in the introduction of tourmaline along cracks and as impreg- 

 nations into the earlier minerals.^ This stage was very pronounced, 

 tourmaline being one of the principal gangue minerals. Its intro- 

 duction was closely followed by the third stage, namely the sulphide 

 mineralization, by which were introduced arsenopyrite, pyrrhotite, 

 and pyrite. During the introduction of both the tourmaline and 

 sulphides, the alteration of the oligoclase to calcite and white mica 

 took place. Both tourmaline and sulphides occur with the white 

 mica and calcite in the decomposed oligoclase masses, their abund- 

 ance depending on the degree of alteration. There is evidence that 

 the tourmaline and sulphide solutions attacked the contact minerals 

 as well as the vein material. The biotite is decomposed in places, 

 and tourmaline and sulphides are commonly associated with the 

 contact minerals. 



In the vicinity of the gold-bearing lode, but not directly con- 

 nected with it, are certain geodes with the following mineral 

 association: calcite, siderite, albite, quartz and muscovite. The 



I O. A. Derby, "On the Mineralization of the Gold Bearing Lode of Passagem, 

 Minas Geraes, Brazil," Am. Jour. Sci., XXXII (1911), 185-90. 



^ Op. cit., p. 190. 



