RA 
ECONOMIC GEOLOGY 
which have been intruded by an andesite-porphyry that 
grades into quartz-diorite. This intrusion was fractured 
and invaded by a superheated boric acid solution which 
altered the rock into a mosaic of quartz, tourmaline, sericite, 
and rutile, and deposited large quartz-veins containing 
sulphides and magnetite. While the andesitic diorite was 
still charged with superheated water it was invaded by 
dykes of alkaline dacite-porphyry. The consequent rise of 
temperature, perhaps combined with release of pressure 
owing to fracturing, exploded the superheated water and 
blew out a crater 3000 feet in diameter and 6000 feet deep. 
This crater was occupied by a lake and gradually filled by 
the Braden Tuff being washed into it. Solutions rose along 
fractures around the crater, and deposited quartz-veins with 
tourmaline and pyrite, and large bodies of ore containing 
about I per cent. of copper. The less permeable tuffs re- 
ceived lower-grade ores with 4 per cent. of copper. An 
alkaline porphyry breccia was upthrust between the tuffs 
and the crater wall, and tourmaline was deposited both in 
this breccia and the Braden Tuff. As the volcanic activity 
waned, the rising solutions were cooler and did not carry 
boric acid; they deposited bornite and other sulphides, 
sulpharsenites, sulphates, carbonates, and tungstates. Later 
still cooler solutions deposited chalcopyrite and bornite, 
with quartz and gypsum, which in some of the cavities grew 
into crystals 10 feet long. After the volcano had become 
wholly extinct, descending meteoric waters leached the ores 
from the surface to the depth of 150-300 feet and redeposited 
the copper as secondary enrichments of chalcocite. The 
Braden Mine therefore illustrates all stages from pneumato- 
lytic and contact ores to ordinary quartz-pyrite veins and 
secondary enrichments. This association of the different 
types is due to volcanic action being local and intense; 
in most mining fields the subterranean conditions vary less 
quickly. 
Lopes or Cornwarr—>Solutions that acted at less depth 
and at a lower temperature than those that produced the 
pneumatolytic ores and had silicic acid as the predominant 
acid, have formed the quartz-copper lodes, which were for 
long the chief source of copper. The lodes of Cornwall and 
Devon show a transition downward into pneumatolytic tin