Page 35 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

34 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
these rocks outcrop, are quoted from Gordon's description 19 of the Chloride district.
The rhyolite resting upon the andesite is largely tuffaceous and rarely shows indication of flow structure. Some miles west of Chloride the canyon cuts across an old channel in the andesite tuffs which was sub- sequently filled by the eruption of rhyolite tuffs. The deposits of rhyolite on this side of the range are far less extensive than those of andesite, and stream erosion has dissected the sheet, leaving only cowlings on the ridges or the filling of old depressions in the surface of the andesite.
In this section the rock is found to be notably porphyritic, showing large phenocrysts of corroded and broken quartz crystals along with more numerous but smaller fragments of crystals of orthoclase, plagioclase (an- desine), and a few of biotite. The groundmass is glassy, in places spher- ulitic, and here and there partly crystalline. In some places the pheno- crysts predominate over the groundmass, giving the rock in hand specimen a pseudogranular structure.
In surface exposures the rhyolite flows are dark brown in color, while in fresh specimens they vary from deep grayish- purple to light purplish-gray. The breccias are of a lighter shade than the flow rocks, while the tuffs are characteristically cream colored or white. Sericitization of the orthoclase and plagioclase has been the chief alteration of the original constituents, with probably considerable local development of kaolin due to surface weathering. Alteration to an extremely low-grade bentonitic material has occurred in the rhyolite tuffs locally, but no com- mercial beds of this material have yet been found in the county.
During late Quaternary time large areas of the Quaternary gravels of the region were covered with thick sheets of basalt. Scattered remnants of these flows cap the buttes and mesas at a number of places in the central and eastern parts of the county. The prominent basalt mesa near San Marcial in Socorro County is the largest of these ; it, extends some miles into the northeast part of Sierra County. Other important occurrences are near the Elephant Butte reservoir, on the mesa northeast of Hillsboro, and at several places between Hermosa and Hot Springs.
INTRUSIVE ROCKS
At a number of places in Sierra County, intrusive masses of porphyry have penetrated the earlier rocks in the form of stock- like bodies, sills, and dikes. These rocks are probably of early Tertiary age, younger than the andesite extrusives, older than the rhyolites, and contemporaneous with the latites, which are con- sidered by the writer to be apophyses from the main mass of monzonitic magma. At Kingston a dikelike mass of monzonite porphyry about 400 feet wide cuts the sediments for a consider- able distance north and south of the town. Similar outcrops to the south in the Tierra Blanca and Carpenter districts are thought to be parts of the same general mass, although in the Carpenter district the intrusive rock is a granite porphyry rather
19 Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), p. 262.