Page 150 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 149
pletely through the mountain, a distance of 3,500 feet, but is badly caved at a distance of 2,200 feet from the south portal. The lower tunnel is blocked by a cave 1,800 feet from the portal, but it is reported to be approximately 2,000 feet long.
The ore consists of pyrite, chalcopyrite, and small amounts of sphalerite and galena in a gangue of quartz and a little calcite. The sulfides carry gold and silver. As in the other mines of the district, the upper portion of the vein is oxidized and contains free-milling gold. The zone of oxidation and enrichment is not regular, however. Considerable free-gold ore has been mined from the lower tunnel level, and sulfide ores have been shipped from residual patches and upward-projecting spurs of primary ore in the upper and intermediate tunnels. The secondary min- erals are chalcocite, argentite, limonite, calcite and silica. In the shallow workings the gold is in limonite and silica box-work in the spaces formerly occupied by pyrite and chalcopyrite. The free gold seen by the writer was quite coarse, and several sacks of hand-sorted sulfide ore gathered during operations in 1931 contained numerous pieces in which broken pyrite crystals were prevented from falling apart by heavy wires of gold. A small amount of bismuth is present, probably as the sulfide.
The Bonanza vein strikes N. 37° E. and dips 80° to 90° NW. In places it consists of a fracture zone 2 to 8 feet wide. In other places the vein more nearly resembles a shear zone of about the same width, and in still other places the ore is localized in lenses arranged en echelon in a shear zone as much as 80 feet wide. This shear zone is probably a direct extension of the shearing that is prominent in Copper Flat. The material between walls is fractured and sheared andesite. In places both walls are ande- site, the dike being many feet away from the ore. Usually a seam of gouge 1 to 15 inches wide makes the footwall of the vein. In parts of the vein a prominent seam of mineralized quartz varying from one to several inches in width forms the high- grade streak, with minor veinlets of mineralized quartz and dis- seminated mineralization scattered fortuitously throughout the remainder. In other places the chief quartz seam is absent, and the ore consists of a branching system of thin seams scattered through a fractured zone from 2 to 7 feet wide. In the lower tunnel the main vein averages 3 to 4 feet wide between well-de- fined walls in the andesite and with the usual gouge-filled fault zone on the footwall. At short intervals thin seams of ore lead out into the footwall of the vein, and occasionally also into the hanging wall. The vein on the upper levels apparently does not project downward into the position of the vein in the lower tun- nel, but is in the footwall at a distance of 30 to 40 feet. The stringers of ore making into the walls are possibly connecting links between lenses of ore occupying the shear planes within a wide zone, and it is felt that thorough prospecting of this mine