Page 107 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

106 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
Valley, and the observations of the writer made during his visit
to the district.
LOCATION
The Tierra Blanca district is 6 miles south of Kingston, near the heads of Trujillo and Tierra Blanca creeks. It is about 10 miles by road southwest of Hillsboro and is 15 miles northwest of Lake Valley. The hills surrounding this area are covered with a prominent capping of white rhyolite tuff and related rocks, and from this circumstance the name Tierra Blanca has origi- nated. The district is in the rugged foothills on the east slope of the Black Range. Travel is by fair mountain roads and trails.
GEOLOGY AND STRUCTURE
The Tierra Blanca district includes a continuation of the same zone of folding, faulting and intrusion that is prominent in the Kingston district. As at Kingston, the east slope of the range is faulted to such an extent that pre-Cambrian granite is ex- posed at the base of the main escarpment. Paleozoic limestones form a long, narrow belt along the range east of the granite, although the continuity of this belt is much broken by faulting, and it is covered in many places by the usual series of andesite and rhyolite flows and tuffs. The beds dip to the east and south- east, much as they do at Kingston, and they are cut off in that direction near the mines by an intrusion of monzonite porphyry, which is probably an extension of the intrusion at Kingston. In addition to this stock, several dikes with north to northeast strike cut the sediments, and these grade from monzonite porphyry identical in appearance with the stock through a birdseye por- phyry to a fine-grained phase, with the same field relations to the parent mass of monzonite stock as have the dikes in the Hillsboro district.
Overlying the granitic basement rocks to the west, the lower Paleozoic sediments may be seen in places, while at others they are overlain and hidden by andesite, with white rhyolite tuffs capping the hills. Along the eastern foothills many of the faulted blocks are also covered with residual patches of the ande- sitic and rhyolitic flow rocks, and many buttes and mesas of these rocks constitute prominent features of the landscape. The contact between the latest flow of andesite and the first of the rhyolite flows in the district is an unconformable one that is char- acterized by a layer of obsidian at the base of the rhyolite flow. This relationship persists in the range all the way from Hermosa to Lake Valley.
Near the ore deposits the Lake Valley limestone has been arched into low folds having their axes parallel to the trend of the range, and along these arches north-south faulting has occurred, which has been accompanied by considerable cross fracturing and the division of the beds into many small fault blocks. The fracturing is generally inclined to the porphyry