Page 166 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

164 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
nants of the upper portion of the cupola. The rock is very sim- ilar in appearance to the latite porphyry dikes that radiate out- ward from the cupola and that are considered to be direct off- shoots from it.
The mineralization within the basin of Copper Flat occurs as (a) veins filling fracture zones in andesite, (b) veins within the monzonite, and (c) deposits of copper in the monzonite. The veins filling fracture zones in the andesite country rock are along the walls of latite or monzonite porphyry dikes, in simple fractures, and in shear zones. They radiate outward from the mass of monzonite porphyry within the basin, and many of them can be traced directly into the important veins that occur on the outer slopes of the Animas Hills, such as the Richmond, Rattle- snake and Opportunity. With the exception of the Richmond, the northern part of the Bonanza workings, and the 85 mine on the north end of the Snake vein, these inside deposits have been of small importance as producers of commercial ore.
The veins within the monzonite mass of Copper Flat are quartz-filled fissures, which contain pyrite, chalcopyrite and gold, but very little silver. One vein, as developed in a surface trench, contains quartz, pyrite, tetradymite (bismuth telluride) and some gold. Molybdenite is sparingly present. These veins are due to a continuation of movement along the principal veins of the district, and a general shrinkage fracturing within the mass of the intrusive after it had solidified but prior to or contemporaneous with the vein-forming period. In general, they are of little importance, although some high-grade shipments have been made from them in the past, mostly as small hand-sorted lots of a few tons each.
The deposits of copper within the southern portion of the igneous mass consist of disseminated primary pyrite and chal- copyrite within the sheared and fractured mass of the monzonite porphyry near the Sternberg shaft. This disseminated ore has been enriched and converted to chalcocite, cuprite, azurite and malachite by oxidation and precipitation processes during the formation of the basin. Molybdenite is present with quartz in seams and stringers, and biotite is locally present with coarse feldspar crystals in small veinlets of pegmatitic appearance, both of these occurrences testifying to a local temperature much above what is considered normal for the formation of the chief ore deposits of the district. Erosion in the basin quickly followed oxidation and secondary enrichment, for within 50 feet of the surface in the prospect workings the secondary sulfides give way to primary pyrite and chalcopyrite.
The Sternberg underground workings have followed along a series of six trap dikes, which outcrop on the surface within a distance of 500 feet. These dikes have amygdaloidal cavities in which cuprite (including the variety, chalcotrichite), some black copper oxide, some native copper, malachite and calcite are