Page 117 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

116 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
trending displacement, rather than that the contact is along the face of a cuesta. During the period of faulting the Magdalena rocks both above and below the monzonite sill were gently warped and folded along eastward-trending axes. The extent of this folding is small, the folds being elevated not more than a foot or two at the high points of most of the arches, and the width of the folds is often but a few feet. They may die out along the axis within a few feet, or they may continue without noticeable change for several hundred feet. Bending was sufficient, however, to crack the more brittle beds along the crest of some of these arches, thus creating channels of migration for solutions that were given off from the solidifying magma of the sill.
It is believed that the block faulting and related tear faults and folding, which formed the Sierra Cuchillo, were later than the similar events that outlined and elevated the other ranges in the county. In the Sierra Cuchillo, the Tertiary andesite and latite lie in accordant attitudes over the Paleozoic sediments, indicating that in a large measure at least, block faulting did not occur until after this phase of Tertiary volcanic activity. The sill and laccolith of monzonite porphyry is considered to have been in- truded between the period of the andesites and the rhyolites, at a time corresponding to the period of latite porphyry flows, sills and dikes throughout the county. Extreme faulting of the range could not have occurred until after the intrusion of the sill, as it was clearly cut by the main fault and its eastern part elevated to its present position. Near Willow Springs in the Sierra Cuchillo, the latite porphyry within the andesite is very similar megascop- ically to the underground specimens of monzonite in the Dictator mine, with the exception that in the monzonite, hornblende is the predominating dark mineral, while in the latite porphyry bi,otite is locally more abundant.
ORE DEPOSITS
The ore deposits of the Sierra Cuchillo consist of contact-
metamorphic deposits of iron in the extreme northern end of the range, contact-metamorphic deposits of lead and copper in the region of the Hot Springs-Fairview highway, and lead-zinc con- tact-metamorphic and replacement deposits on the east slopes of the range southeast of Cuchillo Peak. Very little gold and silver occur in these ores, and they are in general a partly oxidized com- plex mixture of sulfides near the surface.
HISTORY AND PRODUCTION
The deposits in this range were discovered at about the same
time as those at Chloride and Fairview, but owing to the presence of hostile Indians in the region, prospecting and development were carried on with difficulty and were often interrupted. Some shipments were made during these times to the Chloride and Fairview smelters from the Rifle Shot, Dictator and Black Knife