Page 33 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

32 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
along the Black Range appears to have exceeded 4,000 feet. This rhyolitic material in many places filled the old canyons eroded in the surface of the older flows, and as in the canyon of the Rio Cu- chill° south of Chloride, it flowed out over the sediments de- rived from the more basic flows and accumulated in basins on these older surfaces. After the rhyolite was extruded, erosion again profoundly carved and dissected the terrane, until at present the rhyolite exists in extensive areas only over the main massif of the Black Range and the San Mateo Mountains. Over the remainder of the area where these flows extended, only scattered remnants are to be found. In the Hillsboro dis- trict only two or three small patches of rhyolite remain, and even the andesite has been deeply eroded and in places cut through, exposing the underlying sediments. Between Kingston and Hillsboro great masses of rhyolite and tuff still cover the faulted blocks of that section, and between Kingston, Tierra Blanca and Lake Valley a persistent layer of obsidian marks the base of the rhyolite series. Rhyolite and basic dikes cut the andesites in many exposures throughout the county, and small sills of rhyo- lite occur in the andesites and older sediments near Hillsboro, at Tierra Blanca and elsewhere.
The normal andesite of the region is dark colored, fine grained to aphanitic, and at the surface usually appears brown- ish-gray from weathering. In the fine-grained varieties, white plagioclase and black augite grains are present in about equal proportions, as seen in the hand specimen. Porphyritic varieties show phenocrysts consisting of tabular crystals of plagioclase in a dense groundmass, and under the microscope these are shown to be enclosed in a groundmass of smaller crystals of plagioclase, with hornblende, augite, biotite, orthoclase, magne- tite and apatite. A dioritic variety, apparently intrusive into the earlier flows of andesite as small sills and dikes, consist of a dark-colored fine to coarse grained porphyritic rock, in which the larger grains are equally divided between plagioclase and augite. The groundmass is similar to that of the extrusive an- desite. Near the base of the series the andesites are brecciated and much altered. Here the rock is of a light greenish-gray color, with blotches of purple or light green representing the breccia fragments. The constituents of these altered andesites are plagioclase and augite in a groundmass composed chiefly of calcite and chlorite, with some epidote, serpentine. magnetite and apatite. The magnetite is often altered to pyrite, and veinlets of calcite, pyrite and epidote, with occasional adularia, are pres- ent. This type of rock alteration is known as propylitization and is a characteristic alteration of the andesites throughout the county. Near Hillsboro, on the south slope of Animas Peak, a fairly fresh andesite contains many spherical segregations of epidote which vary in size from half an inch to several inches in diameter. Where the rock is worn away these segregations col-