San Cayetano Mining District, San Cayetano Mountains, Santa Cruz County, Arizona, USAi

Regional Level Types
San Cayetano Mining District	Mining District
San Cayetano Mountains	Mountain Range
Santa Cruz County	County
Arizona	State
USA	Country

A Cu-Ag mining area located in T21-22S, R12-14E, G&SRM, in the San Cayetano Mountains. It almost adjoins the Nogales District on the north and the Tyndall District on the west. It has a length of 6 miles and a width of 3 miles. It extends from Josephine Canyon on the north nearly to Sonoita Creek on the south.

The periphery of the district lies at a general elevation of about 4,000 feet, from which the surface rises to about 6,000 feet along the middle part of the crest of the San Cayetano Mountains. The mines are near the central part of the district on each side of the range.

The geologic structure is monoclinal, the rocks dipping to the east. The formations, except a small north-south area of sedimentary rocks on the middle south slope of the range at an elevation of about 5,000 feet, are all igneous. The sedimentary rocks are composed mainly of thin-beddedd red and green quartzites and hornfels, which are regarded as of Paleozoic age. They dip 40ºE. and are underlain by intrusive quartz diorite, and overlain by rhyolite. On the west the lower and middle slopes of the mountains are occupied by the quartz diorite rising to an elevation of about 5,000 feet, and on the east bedded volcanic tuffs rise to about the same elevation, while the intervening belt, composing the upper slopes, crest, and axis of the range, consists of rhyolite.

The quartz monzonite is a greenish-black intrusive rock of Mesozoic age. It is composed mainly of labradorite or a related basic plagioclase, with some orthoclase, brown biotite, hornblende, a small amount of quartz, and accessory magnetite and apatite.

The rhyolite, which overlies the diorite on the east, is dark greenish and generally vitreous. It is composed mainly of a felsitic to glassy groundmass of quartz and orthoclase with a little biotite and acidic plagioclase and has in places a few small phenocrysts, especially of quartz and orthoclase. It belongs to the Tertiary lavas. Both the rhyolite and the diorite are intruded by quartz monzonite porphyry, a light gray, coarsely porphyritic rock in which quartz, plagioclase, orthoclase, and hornblende can be distinguished in hand specimens, some of the crystals being about ¼ inch in longer axis. The groundmass is microgranular and is apparently composed of quartz and feldspar. The phenocrysts, which are unequal in size, are, in the order of their abundance, andesine, orthoclase, hornblende, quartz, and biotite. Secondary minerals are calcite, chlorite, and epidote, and the rock contains a little accessory magnetite.

The tuffs which cap the rhyolite on the east occur in cream-white to yellow beds and are rhyolitic. At and beyond the base of the mountains and the eastern limit of the district they are capped by a younger blackish scoriaceous andesite.

Mineralization is quartz-calcite veins and veinlets containing spotty and weak argentiferous copper mineralization in Laramide quartz diorite and in a strong fault zone cutting Cretaceous and Tertiary rhyodacite volcanics.

The mineralized veins occur in the rhyolite, in which they stand vertical, and in the quartz diorite, in which the dip is rather flat and about parallel with the sheeting structure of the rock. All the veins occur in immediate vicinity of intrusive masses of quartz monzonite porphyry. Considerable shearing is evident along the veins, which were deposited from solution, as is indicated by their banded structure.

Workings include shallow shaft and tunnel operations. Worked in the early 1900's but only a relatively few tons of copper-silver ore were ever shipped.

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