Alto Mine group (Alto vein swarm), Alto Hill, Salero area, Tyndall District, Santa Rita Mts, Santa Cruz Co., Arizona, USA

‡Ref.: Stevens, J.H. (1904) The Copper Handbook, Vol. IV.

Schrader, F.C. (1915) Mineral deposits of the Santa Rita and Patagonia Mountains, Arizona, with contributions by J.M. Hill: U.S. Geological Survey Bulletin 582, 373 p., 3 sheets, scale 1:125,000: 197-203.

Galbraith, F.W. (1947), Minerals of Arizona, Arizona Bureau of Mines Bull. 153: 15.

US Atomic Energy Commission Prelim. Reconnaisance Report A-P-360 (1955).

Keith, Stanton B. (1975), Arizona Bureau of Mines Bull. 191, Index of Mining Properties in Santa Cruz County Arizona: 83 (Table 4).

U.S. Bureau of Mines field notes, PB20.

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 101.

Arizona Bureau of Mines card file Santa Cruz County.

MRDS database Dep. ID file #10234992, MRDS ID #M030411, MAS ID #0040230095; and, Dep. ID file #10037106, MRDS ID #M030411.

A former underground Pb-Ag-Zn-Cu-Au (Sb-Bi-As-Ba-U) mine located on a group of 21 claims, located in the S½S½ sec. 12, and the North ½ sec. 13, T.21S., R.14E., 2¼ miles nearly north of Salero, at an elevation of about 5,400 feet, on land of mixed status. It is in Alto Hill (named El Plomo by the Spaniards for the lead minerals found here), which rises 900 feet above the valley. Owned/operated at times, or in part, by Mr. Mark Lully of Nogales, who re-discovered it as the Gold Tree (1875-1880); Albert Steinfield & Co. of Tucson ( -1902); Alto Consolidated Mines, Smelting & Transportation Co. (1902-1915); Alto Copper Co. (1907-1913); Southwest Development Co. (1911); Henderson (1924); Bradford (1924); Bond (1929); Laguna (1931-1932); Moreno (1928); Long Contact Manufacturing Co. (1940-1942); B. & R. Mines; Alto Mines Co. (1930); Stone (1935); Miller (1938); Griffith (1947); Mrs. Eva Henderson (1952-1954); S. Ceanne and N. Mazer (1956); and the Fortuna Mining Co.

Claims include: Steinfelt, Steinfelt West, Donali, Great Eastern, El Plomo, Excelsior West, Ophir No. 1 & 2, Gold Tree, Long Contact No. 1 & 2, Mineral West, Albert, Salero, Buena Vista, Excelsior, Alto East, Grand Prize, Hillside, Albert No. 2, Oak, Albion, Record, B and R.

Mineralization is a series of roughly parallel, strong, quartz-barite fissure veins carrying lenses and pods of sulfides and sulfosalts, oxidized and supergene enriched near the surface. Wall rocks are Cretaceous rhyodacite welded tuff with interbedded arkose, underlain by Jurassic granite. Sparse fine-grained uraninite crystals occur in a cross-fracture. 6 major veins transect the property: Mineral, Mineral No. 2, Albert, Alto, Excelsior, and the Hillside. The majority of workings are on the Alto vein, which averages 3-7 feet wide. NW to SE zonation of sulfides (galena) to baryte with deeper erosion to the NW.

The area is the rough, hilly ground of Alto Hill that is a short, truncated, spur-like piedmont ridge about ½ mile in diameter at the base and but a few hundred feet wide at the top.

The geology of Alto Hill is complicated. In the upper western part of the hill the country rock is mainly the quartz latite porphyry, with probably a core or base of granite porphyry, which seems to be the dominant rock in the eastern part of the hill, and is soon succeeded by the belt of diorite and monzonite rising in the mountains on the east. The granite porphyry, and probably also the diorite, prior to the eruption of the latite, apparently formed the east side of the valley or piedmont front on which the latite was deposited. Dacite and rhyolite rocks and andesite are also present. The latite is disposed in massive sheets or flows, which at the top of the hill dip 35ºE. and are agglomeratic. About 1,200 feet in from the mouth of the Alto tunnel, it gives way to granite porphyry, boulders of which 2 feet or more in diameter occur as inclusions in the latite on the east near the top of the hill. At 950 feet into the tunnel, the latite is intruded by dense dark reddish-brown andesite.

The Alto property contains 6 veins, mainly in the latite and granite porphyry. They strike a little north of west and in general have a steep or vertical dip. Beginning on the north they are the Mineral vein, Mineral No. 2 vein, Albert vein, Alto vein, Excelsior vein, and the Hillside vein.

Workings include extensive tunnel and shaft operations. A 217 foot deep shaft was sunk 1905-1907. Tunnels, drifts, shafts and stopes aggregate 10,000 feet or more of workings. The longest tunnel is the Alto at 1,632 feet long.

Discovered and worked originally by Jesuits of the Tumacacori mission about 1687, and they worked it rather steadily up to 1857. It was later worked by others, intermittently, since the 1680's. Total estimated and recorded production probably was more than 3,500 tons of ore averaging about 12% Pb, 14 oz. Ag/T, 3% Cu, and minor Zn and Au.