THE RELATION OF BORNITE AND CHALCOCITE IN THE 

 COPPER ORES OF THE VIRGILINA DISTRICT OF NORTH 

 CAROLINA AND VIRGINIA. 



By Francis Baker Laney, 



Of the XI. S. Geological Survey . 



INTRODUCTION. 



The copper ores of the Virgilina district consist almost wholly of 

 bornite and chalcocite. The level of ground water is from 50 to 75 

 feet below the surface, and the zone of secondary alterations does not 

 appear to extend below 250 feet. The important mines of the dis- 

 trict are from 350 to 500 feet deep, and the ore from the deepest 

 levels contains almost as much chalcocite as bornite. If a specimen 

 from the upper levels of almost any of these mines be examined in 

 detail it will be found to consist of the two minerals in such rela- 

 tions that no hesitancy is felt in pronouncing much of the chalcocite 

 secondary and more recent than the bornite. If, however, a speci- 

 men be taken from the deeper levels the two minerals are seen to be 

 so intricately intergrown that no other conclusion than that they are 

 genetically contemporaneous seems possible. These facts have led 

 some observers, notably L. C. Graton, 1 to suppose that chalcocite 

 occurs in these mines as a primary mineral. Chalcocite has been 

 almost universally regarded as a mineral of secondary origin, i. e., 

 derived from some leaner copper-bearing sulphide. Its manner of 

 occurrence as heretofore observed left little doubt as to its secondary 

 nature, and there grew up a belief that the mineral is always second- 

 ary. The chalcocite of the Virgilina district therefore appeared to 

 offer an exception to the generally accepted conclusion, and with the 

 hope of throwing some light upon the genesis of this mineral, an 

 extended microscopical examination of these ores was undertaken. 



The field work upon which this investigation is based was done 

 while the writer was employed by the Geological Surveys of North 

 Carolina and Virginia, and a detailed report on the geology of the 

 district is now being prepared. The U. S. National Museum fur- 



i U. S. Geol. Survey, Min. Res. 1907, Pt. 1, p. 620. 



Proceedings U. S. National Museum, Vol. 40— No. 1835. 

 80796°— Proc.N.M.vol 40—11 33 513 



