176 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
somewhat stained by iron, but not like the great iron gossan raps of 
other regions. This extends to a variable distance below the surface, 
200 to 400 feet in some instances, where it is replaced by partly oxi- 
dized and decomposed copper ores that form the upper limit of the 
remarkable glance, enargite, and bornite ore bodies of the district. 
Carbonates and Oxides are rare. 
The copper minerals occur in quartz-pyrite veins of remarkable 
width and extent. The Anaconda ledge is frequently Id) feet wide 
and will average half that width, as will also the Syndicate lode. 
The copper minerals of the Butte ores consist chiefly of chalcocite 
(copper glance), bornite (peacock copper), enargite (sulpharsenide of 
copper), and cupriferous pyrite. Govellite (cupric sulphide) occurred 
in considerable amount in one or two mines, but forms an insignifi- 
cant percentage of the total output. Tetrahedrite (gray copper) and 
chalcopyrite (copper pyrite) arc even rarer than the last-named min- 
eral. Until 1900 copper glance constituted the most important ore 
mineral of the veins, but it is now nearly equaled in quantity by 
enargite. In the great ore bodies of the upper levels of the Anaconda 
veins glance occurred in masses of nearly pure lead-like mineral 2| 
feet or more wide. In depth the mineral shows a more crystalline 
structure, and ii is found in all tin 1 mines in greater or less abundance 
and purity, but in the great bulk of the ores it forms small grains 
scattered through the ores. 
Bornite is less common than glance, and is practically restricted in 
occurrence to the veins in the western part of the copper area, where, 
however, ii occurs in great abundance, forming t lie chief ore of the 
( Original and Parrot mines. 
The gangue of all the veins is largely quartz, though there is also a 
large amount of altered granite with veinlets and bunches of ore. The 
vein walls are often defined by (day selvage, but these prove almost 
invariably to be due to post-mineral fracturing. More frequently 
there is a fading of ore into country rock, a feature characteristic of 
replacement deposits. 
THE ORES AND THEIR DEPOSITION. 
Cliaracter of tin ores. — The copper ores average 55 per cent silica 
and 16 per cent iron. About 15 per cent of the tonnage mined is 
first-class ore, averaging 12 percent copper; the remaining 85 per cent 
carries 4.8 per cent copper, and is treated in concentrating mills, the 
resulting product containing but 15 to 20 per cent of silica, while the 
copper is increased to 18 per cent. 
The ores contain gold to the extent of about -2\ cents to each pound 
of copper, with 0.0375 ounce of silver. Native gold has been found 
upon crystallized glance, but with this exception no gold or silver 
minerals are recognizable in the copper ores. It is estimated that the 
total production of copper ore has been about 31,000,000 tons, averag- 
