Page 194 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 191
caused by the settling of the mass and the adjustment between blocks during cooling and shrinking.
ORE DEPOSITS
Gold is the chief metal in the deposits of this area, and the known veins are typically of the Tertiary epithermal type. Silver accompanies the gold in varying proportions, and small amounts of lead and zinc are associated with the gold-silver ores in places. In the lower part of the southwest slope of the range, traces of copper are found in the andesite breccia, and many prospect pits have been dug in these outcrops. So far as known, however, no shipments other than a few sacks of hand-picked ore have ever been made, and within a short distance from the surface the veins pinch to knifeblade seams and disappear.
WORKINGS
The Goldsborough district lies within the southern foothills of the San Mateo Mountains and is about 28 miles from Hot Springs, the nearest town. At the time of the writer's visit only one group of eight claims had the appearance of being held by bona fide annual assessment work. These claims were originally located by H. A. Hanley and his associates, who had been grubstaked by N. S. Sweeney, a merchant of Hot Springs. The claims are now held by Mr. Sweeney and some associates. Ap- parently, none of the claims of the district have been patented.
The area is one of andesite and rhyolite flow rocks, which have a general strike of N. 80° E. and a dip of about 10° SE. The rhyolite has flowed out over a gently undulating and partly dissected surface of andesite, the depressions in which had been partly filled with loosely consolidated beds of detrital material. Rhyolite tuffs and breccias containing coarse quartz crystals and much-broken feldspars are a prominent part of the sequence. These rocks are interbedded with the rhyolite flows and in part overlie them. Erosion since the last of the rhyolite intrusions has again laid bare the underlying andesites and breccias in the canyons and on the low slopes of the foothills. On the claims mentioned the rhyolite is probably at no place over 100 feet thick, although it is much thicker farther to the south, where the flows dip below the floor of the plains and are protected from erosion. On the east side of the property, near one of the shear zones, a shallow prospect pit is said to have penetrated into the andesite, but as this pit and its dump were covered over by the dump from a later shaft, this could not be verified.
Crossing the property with a strike of N. 10° W. is a zone that dips 68° SW. and has a width of 3 to 10 feet, in which the rhyolite is sheared and brecciated and slightly altered. Two hundred feet east of this zone is a second zone with a strike of N. 15° W. and a dip of 81° SW., and similar in nature to the first. Numerous cross fractures in the rhyolite show evidence of slight