0. A. Derby — Gold-bearing Lode of Passagem. 185 



Art. XX. — On the Mineralization of the Gold-bearing 

 Lode of Passagem, Minas Geraes, Brazil; by Orville A. 

 Derby. 



Ix an article published in the Zeitschrift fur praJitische 

 Geologie for Oct., 1898, on the gold-bearing lode of the Passa- 

 gem mine near Ouro Preto, State of Minas Geraes, Brazil, Dr. 

 Eugene Hussak concluded from a detailed mineralogical and 

 petrographical study of the contents of the lode and of the 

 immediately adjacent portions of the country rock, that it is of 

 eruptive origin representing the ultra-acid phase of a pegrna- 

 titic apophysis from a neighboring mass of granite. Since the 

 lode is, in all essential respects, a typical quartz-pyrite one, 

 this conclusion has attracted considerable attention on account 

 of its bearing on the general question of the genesis of ore- 

 bearing bodies of this character. • 



With regard to the metalliferous portion of the lode (the ore 

 proper, consisting of a mixture in varying proportions of 

 arsenical, magnetic and common pyrites with traces of various 

 other sulphides, associated with a large amount of tourmaline) 

 Hussak expressed somewhat reservedly the opinion that it 

 should be classed among the primary constituents of the 

 lode. This opinion was based on the constant and character- 

 istic association of the metallic sulphides with the tourmaline, 

 which was regarded as a contact mineral and thus practically 

 •contemporaneous with the intrusion of the granitic apophysis. 

 A fine suite of specimens recently presented to the Geological 

 Service of Brazil by Mr. Arthur J. Bensusan, the able super- 

 intendent of the mine who is always on the alert for new 

 developments that promise to be of scientific interest, sug- 

 gested the hypothesis that the lode might possibly have become 

 mineralized by an introduction of sulphides and tourmaline 

 subsequent to its formation ; and to gather additional evidence 

 on this important point, an examination in situ of a recently 

 opened section of the mine that revealed certain features not 

 hitherto observed, was made by Mr. Jorge Ferraz of the staff 

 of the Geological Service. 



A drift now being driven through sterile quartz in search of 

 an ore shoot, begins to show on one of the walls and in the 

 heading a be^innino- of mineralization in the form of small 

 patches and streaks of massive tourmalinized quartz accompa- 

 nied by a small amount of arsenopyrite, and at the same time a 

 porphyritic appearance due to isolated polyhedral inclusions 

 of a greenish mineral sharply defined in a white or black 

 groundmass according as they occur in the sterile or in the 



Am. Jour. Sci.— Fourth Series, Vol. XXXII, No 189. — September, 1911. 

 14 



