Old Baldy Mining District, Pima County, Arizona, USAi

Regional Level Types
Old Baldy Mining District	Mining District
Pima County	County
Arizona	State
USA	Country

A Cu-Ag-Au-Mo mining area located in T19S, R14-15E, G&SRM. The Old Baldy District, formerly a part of the Tyndall district, lies in the west-central part of the Patagonia quadrangle, adjoining the Greaterville District on the southwest. It is about 3½ miles wide and extends from the McCleary Ranch and Sawmill Canyon 6 miles southward across the Pima-Santa-Cruz County line to Mount Hopkins and Old Baldy Peak, at its southeast corner. It lies on the northwest slope of the Santa Rita Mountains.

This is a northerly continuation of the Old Baldy District from Santa Cruz County.

Ore was first discovered in this district in the late 1870's or earlier, but there was not much production.

The topography is for the most part very rugged, especially on the east, where it is of the volcanic rock type.

The western fourth of the district is underlain by coarse granite, which contains northward-dipping fault planes, joints, and associated quartz veins and is cut by dikes of aplitic rock aqnd rhyolitic porphyry. Granite occurs also on the southeast in the saddle between Old Baldy and Mount Hopkins. On the southwest the northeastern part of Mount Hopkins is composed of a coarse, dark quartz diorite that intrudes the granite and is traversed by northwest-southeast faults which are the sites of mineralization.

Old Baldy and the high ridge descending from it to the north, between Stone Cabin and Jackson canyons, are composed of dense gray Tertiary rhyolite which crops out in the saddle south of the peak at an elevation of 7,630 feet and is stained red by iron. A few knobs on the north ridge are apparently capped by red andesite. The rhyolite extends to the latitude of the McCleary Camp. It occupies the upper part of Stone Cabin Canyon and forms the high, broad ridge adjoining the canyon on the east.

Between the rhyolite on the east and the granite on the west is a narrow belt of rock 3½ miles long and a ¼ mile wide, which corresponds in the main to intrusive diorite but whose principal mineral is locally albite.

Between the southern half of the diorite belt on the east and the granite of Madera Canyon on the west is a belt one-sixth of a mile wide of dark epidotized garnetiferous micaceous quartz schist of supposed Cambrian age, in which the schistosity trends NNW. Quartz veins associated with monzonite porphyry strike NE across the schistosity. On the northeast the schist passes beneath a small belt of dark bluish limestone. In the northeastern part of the district, in Stone Cabin Canyon, the volcanic rocks give way to a belt of underlying Paleozoic limestone and shale which dips 60ºSW., and in the eastern part of the district all the rocks are cut by a NE-SW dike of whitish rhyolite, which seems to be the continuation of a dike 5 miles to the NE, in Box Canyon. It is conspicuous in the granite ridge west of Jackson Canyon and in Sawmill and Stone Cabin Canyons, where it has a width of 20 feet. As it is younger than the rhyolite glows and all except possibly the very latest of the volcanic rocks, it is a good register of late geologic disturbance shown by offsets or lateral faulting occurring at numerous points along its course. On the northwest the granite is overlain by heavy alluvial cone deposits of Quaternary gravels discharged from Madera Canyon.

Mineralization is varied: (1) Scattered, small, and relatively weak, partly oxidized copper and local molybdenite mineralization in quartz fissure zones in Mesozoic volcanics and Laramide intrusives; and, (2) Gold placer deposits in an alluvial cone in the Madera Canyon.

The district contains a few small mines and a dozen or more prospects. There are no deep workings, and few are more than 100 feet in depth (circa 1915). The properties are situated principally in Madera, Jackson and Stone Cabin canyons.

Workings include scattered, small, and generally shallow prospects and mines worked intermittently since the 1880's. Total estimated and reported production is not more than a few hundred tons of sorted, high-grade copper ore. Considerable placer gold was recovered in the late 1880's.

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