96 ECONOMIC GEOLOGY 



great fissure vein several hundred feet in width and four miles 

 long with branching ends. The fissure follows a fault line, and 

 at the center where the displacement is the greatest the width is 

 300 ft. The mine reaches a depth of nearly one mile. All the 

 veins were originally opened for silver, for they contain silver at 

 the surface. As the veins were worked to lower depths copper 

 ores appeared. The silver soon became refractory and the per- 

 centage too small for profitable extraction. 



The ore occurs in true fissure veins bearing native silver 

 and the silver sulphides, associated with zinc and manganese. 

 The gangue consists of rhodonite, rhodochrosite, and quartz. 

 Probably there were no open fissures before the deposit occurred 

 for the ore is deposited along fractures or cracks impregnating and 

 partially replacing the wall rock, so that there is a gradual joining 

 of the vein and the wall rock with no sharp line of demarcation 

 between them. 



The surface ore is black due to such manganese compounds as 

 pyrolusite, MnO 2 , resulting from the breaking down of manganese 

 minerals. At the lower depths the mineral remains pink, the 

 natural color of rhodonite and rhodochrosite. A conical peak 

 2000 ft. above the valley is cut by a pure white vein of quartz 

 containing ruby silver in little red specks with traces of pyrite, 

 galenite and sphalerite. This locality is remarkable for the depth 

 of the oxidation of the ore reaching 1400 ft. on the sides and 1000 

 ft. in the center of the mound. This was a very important field 

 in the production of silver before the decline in the price of the 

 metal. The country rock is basic, diabase and diorite. 



In the Eureka district, oxidized lead and silver ores, auriferous 

 to a considerable degree, occur in a brecciated Cambrian lime- 

 stone and shale. 



New Mexico. In the Lake Valley district there occur galenite, 

 cerussite and embolite in Paleozoic limestone. At Silver City in 

 the Breman mine, argentite and cerargyrite occur at the contact 

 of shale and limestone impregnating both. At Lone Mountain 

 cerargyrite, bromyrite and embolite occur in a gangue of quartz. 



In Wardner County and Bitter Root Mountain, Idaho, galenite 

 occurs in quartzite and mica schist in large chutes impregnating 

 the fissured hanging walls. This is one of the most productive 

 regions of the world. 



In the Thames district the gold-silver lodes consist mainly of 

 quartz, in. which both metals are present in threads/foils and 



