Page 49 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

48 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
Deposits of Type 3 are known in the county only at Hills- boro, where mineralized veins occur in the monzonite at Copper Flat and are genetically related to it.
The copper deposits due to oxidizing surface waters are rep- resented by the disseminated mineralization in the Copper Flat area near Hillsboro, but the actual amount of enrichment by this agency has been of slight importance. At the Silver Monu- ment mine in the Black Range, enrichment of the ore shoots has occurred, but the mine is not entirely dependent on the material thus enriched for commercial operation.
Veins and replacement deposits in limestone. exclusive of contact-metamorphic deposits, have been the principal source of the metal production of the county. Such deposits occur at Lake Valley, Kingston, Hermosa, Tierra Blanca and Carpenter, with minor mineralization of this nature near the main Hillsboro area and in the Sierra Caballos.
Of the veins connected with volcanic rocks of Tertiary age, the Chloride and Hillsboro districts afford the outstanding ex- amples. Other deposits of this type include the veins in the San Mateo Mountains and in the Macho district, the tin deposits in the Taylor Creek district and in the Sierra Cuchillo, and veins in several areas in the Fra Cristobal Range and the Sierra Ca-
ballosL.eadandcopperveinsofdoubtfulorigin,accordingtoLind- gren's classification, are not known to occur in the county.
Placers in the Hillsboro area have been derived from the veins connected with the volcanic rocks of Tertiary age. At the Pittsburg (Shandon) placers, the original source of the gold was in veins in the pre-Cambrian complex at the base of the Sierra Caballos. Whether these veins are of pre-Cambrian age or were formed in the pre-Cambrian rocks at some later time is not known. To date no veins have been discovered in this region with a gold content sufficient to warrant development
work.Copper deposits in sandstone, more commonly known as "Red Beds" deposits, occur sporadically along the eastern slope of the Fra Cristobal range, the Sierra Caballos, the Sierra Cu- chillo, and throughout the length of the Black Range in Sierra County. Mineralization of this type in the known deposits of Sierra County is insignificant.
Throughout the county, in every case except the Pittsburg (Shandon) placers, the ore deposits are clearly related to the period of Tertiary igneous activity, which was so widespread, not only in New Mexico, but over the entire southwest. Nearly all of the mineral deposits in this region are considered to have been formed by solutions that came from the same magma chamber following the intrusion of the stocks, dikes, sills and flows of monzonite, latite and latite porphyry. Among these are the con- tact-metamorphic deposits of iron, copper, and lead-zinc ; the