Page 78 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 77
The silver-copper ores occur generally to the west of the main mineralized belt described above and nearer the crest of the range. They are in veins that trend more to the east than those of the other system, although a much greater variation in strike is to be noted here. They are also less persistent than the gold-silver veins, ranging from half a mile to 11/0 miles in length. Many of them terminate against one of the main north-south faults of the range. This irregular system of veins is concen- trated largely around the area which includes the Silver Monu- ment mine at the head of Chloride Creek. It is thought that these veins have been localized over a deep-seated mass of in- trusive rock, which in approaching the surface arched and frac- tured the overlying extrusive rocks and opened channels along which the mineralizing solutions were able to penetrate and de- posit their load. They were formed under higher temperature conditions than the north-south system. A similar concentration and arrangement of veins occurs near Roundyville, and these also contain silver-copper ores. The veins carry but a small amount of quartz, being composed principally of bornite, and subordinate chalcopyrite, tetrahedrite and pyrite, which occupy shear zones in the andesite. The minerals have been deposited in these zones, partly as a filling of open fractures but principally as a replace- ment of the crushed rock along the sides of the fissures. The hanging wall is usually well defined, with fracturing and brec- ciating gradually diminishing into the footwall. Secondary en- richment of the silver and copper minerals has been of some im- portance locally, and although primary sulfides are found at sur- face in the ore shoots, there is a tendency toward a concentration of gold in the upper levels, and an increase of silver-copper con- tent at distances of 200 to 500 feet from surface. At greater depths a somewhat lower grade primary ore prevails in the veins. The ore shoots do not persist for long distances on the levels, but are apt to continue for several hundred feet vertically. They pitch to the east at angles of about 45° as may be seen from the section of the Silver Monument mine, figure 5.
PRODUCTION
The figures on the production from this district can be dis- tributed among the mines only in part, and data on total produc- tion from the area are not available locally. Mr. Ed James of Chloride very kindly aided the writer in compiling the accom- panying tabulation which, although not complete, may be of in- terest as an indication of the size of the mines, the value of the ore produced, and the distribution of the mineralization in the district.