Page 137 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

136 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
the direction of drag of the fault gouge may determine the direc- tion in which to crosscut.
Within the veins of all types in the andesite, the ore occurs in shoots and pockets. The veins proper pinch and swell at more or less regular intervals, and in general the richer ore is in the narrower parts of the veins. In not a few places the wider parts of the vein have been filled with the early quartz, leaving a cen- tral narrow opening continuous with the narrow part of the original opening, and this has been filled with the later ore min- erals. The original fracture surfaces were slightly curved. Later shearing has brought concave areas of the walls opposite each other to provide lenticular openings, while convex parts opposite have constricted the openings to a small width. Ore shoots in general are of short length along the drift, but they may extend vertically for several hundred feet. Exceptions to this general statement are the Bonanza, Rattlesnake and Oppor- tunity mines, where several shoots of ore on each vein were so nearly adjacent that they were mined in one stope with a hori- zontal length of several hundred feet along each vein, but with a somewhat smaller vertical dimension. (See figure 13.)
Disseminated Deposits in Monzonite Porphyry.—The south- ern nose of the monzonite porphyry stock in Copper Flat near the Sternberg shaft contains a considerable area of sheared and fractured rock in which the oxidized copper minerals, malachite and azurite, are abundantly present, together with limonite and the secondary sulfide, chalcocite. In the Sternberg workings much cuprite, most of which is the variety chalcotrichite, was found in vesicular cavities in basic dikes, and in the wallrock, pyrite and chalcopyrite coated with chalcocite were encountered. Water stands at 60 feet below the collar of the Sternberg shaft and at about the same depth in other short shafts in the district, indicating that erosion within this basin has been operative at a slightly greater rate than has oxidation. In specimens of ore obtained from the dumps of several shafts 50 to 60 feet deep in the flat below the Sternberg shaft, minute specks of pyrite and chalcopyrite are disseminated through partly sericitized monzon- ite porphyry, and veinlets and small seams of these minerals traverse the rock in all directions. Along the major stringers these primary minerals have been largely converted to chalco- cite, while the finer seams and the disseminated specks are coated with only a thin film of this sulfide. At the Chance mine the monozonite has a distinctly granitic aspect, and it is cut by stringers of pegmatite consisting principally of biotite, with quartz, orthoclase, pyrite and chalocopyrite. In the Copper King shaft, 80 feet deep, there are pegmatite stringers of this same type, but the sulfide content is higher and the surrounding rock is replaced with much pyrite and with chalcopyrite which in turn has been partly replaced by chalcocite.
Replacement Deposits in Limestone.—These deposits are