Page 135 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

134 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
tant minerals are chalcopyrite, a small amount of bornite, pyrite and accompanying gold and silver. Quartz is the common gangue mineral, and calcite is subordinate. Molybdenite is an accessory vein mineral, and at one place in Copper Flat tetradymite, bis- muth telluride, was found in a small shoot in a vein at the sur- face, accompanied by pyrite and gold. The order of the deposi- tion of these veins minerals, as determined under the microscope and illustrated in figure 10, was quartz, pyrite and gold, followed by chalcopyrite, bornite, quartz and subordinate calcite. Fol- lowing this mineralization, movement in the veins occurred, and the openings formed were filled with barren quartz in which comb structure is well developed. In most parts of the veins the sequence of ore deposition is obvious.
The veins of this type are continuous over long distances, and are generally 2 to 8 feet between walls, although a few are considerably wider. The more-productive veins accompany dikes of latite. Some of these veins are located on one wall of the dike, while the other wall of the vein is a low-grade seam of gouge, or both walls are of gouge with vein quartz occupying irregular seams and fractures in the dike rock itself. In other veins the dike forms one wall of the vein, along which in the andesite country rock is a zone of brecciated andesite 2 to 4 feet wide, with a seam of gouge either next to the dike or in the andesite on the opposite wall. (See figure 10.) In places irregular stringers and veinlets of mineralized quartz pervade the mass of brecciated andesite, and in others a well-defined vein of quartz and other minerals 2 to 6 inches wide occurs on one of the walls, or at some place between walls. In many places pyrite in fine grains is distributed throughout the andesite or dike rock be- tween the walls, and the rock is much bleached and altered to sericite and kaolin or to chlorite. The andesite adjacent to the veins is propylitized in diminishing amounts for several feet.
Veins of another type occupy fractures or small shear zones in the andesite where latite dikes seem to be absent. This type of vein has so far proved to be of little economic importance. In the shear zone, the mineralized portions of the veins are apt to be discontinuous lenses distributed en echelon along the planes of shearing. The vein may follow one shear plane for a short distance and then pinch out, to take up again in a parallel plane that may be from a few inches to 20 feet away in either wall. Normally in this type of deposit, just before the lens pinches out entirely, a careful examination of the walls will show a small stringer of ore, from knife blade dimensions to an inch or more in width, taking off at an angle with the wall, and if this is fol- lowed, a new lens is very apt to be picked up. Failure to find the connecting stringers of ore entails crosscutting blindly into the walls at horizontal and vertical intervals. In places numerous cross faults interrupt the continuity of a vein or lens, usually with only a few feet of lateral movement, and close attention to