MINERAL OCCURRENCES 135 
Since the discovery by Domeyko! a large series of papers” 
has already been published on these mineral findings. The veins 
set up in a porphyritic rock, which according to a verbal 
communication by dr Stappenbeck is of lower Gondwana age. 
The porphyrite sheet is crossed by numerous eruptive veins of young 
date, and these veins may be supposed to stand in genetic connec- 
tion with the selenide occurrences. The selenide minerals here have 
a very varying composition and all the specimens have different 
names (see Domeyko, 1. c.). 
It seems probable, that the Sierra de Umango and the other oc- 
currences of selenide minerals belong to the andine volcanic phase 
during which all the famous ores of the Cordillera and the Nevado 
de Famatina were formed. The straight cutting through of the 
Umango veins is also indicative of a very young age. 
Occurrences of nickel ores. 
Mineral occurrences of economic value, except the described sele- 
nide veins, have been found north of the Umango mountains, but 
in the same geological structure, where it reappears at the northern 
border of the alluvial plain of the Jagucl bolson. According to 
Brackebusch? there lies near to Potrero Grande a mine called 
„Solitaria“ and containing niccolite (NiAs) with 44 9/0 Ni, the only 
finding of this mineral in the Argentine. Annabergite |(AsO,);Ni, 
+ 8H, O] has been encountered also. The veins here cross an am- 
phibolite, associated with quartzitic and gneissic rocks. The mine 
was exploited during 1845 by two Germans, the Erdmann brothers. 
But the many political complications at that time in Argentine made 
it impossible for the miners to carry on their work. 
1 A. Domeyko: Anales de la Universidad Nacional de Santiago de Chile. 
Tomo XXIX. 1876. Pages 62-—68. 
2 See Stelzner (1876—85). Page 219. 
3 Ludw. Brackebusch: Die Bergwerkverhältnisse der argentinischen Repu- 
blik. Zeitschr. für Berg-, Hütten- und Salinenwesen im Preussischen Staate. Bd. 
E41. 1893. 
