Nogales Mining District (Gold Hill Mining District), Santa Cruz County, Arizona, USAi

Regional Level Types
Nogales Mining District (Gold Hill Mining District)	Mining District
Santa Cruz County	County
Arizona	State
USA	Country

A Au-Pb-Ag-Cu-W (Mo) mining area located in T23S, R14E, G&SRM, in the Mount Benedict area, between the Santa Cruz River and Nogales Wash, on the international border. It adjoins the Palmetto and Patagonia districts on the west and extends from the border 10 miles northward to Sonoita Creek, and from Nogales Wash on the west to the Santa Cruz River on the southeast. At the border it has a width of 10 miles. Most of its country is less than 4,000 feet in elevation. The highest point is Mt. Benedict at 4,500 feet.

Mineralization is varied: (1) Irregular fissure veins, with spotty, often banded, pyrite, cerussite, galena and minor copper carbonates and sulfides, in Jurassic quartz monzonite that has been strongly sliced by sheeted fracturing and intruded by aplite, granite porphyry, diorite, lamprophyre, and diabase dikes; (2) Irregular and lensing, comb-structured, quartz-wolframite veins with minor scheelite associated with lamprophyre dikes cutting Jurassic quartz monzonite; (3) and, Spotty gold placer deposits.

The oldest formation is the quartz monzonite of Mount Benedict, which, beginning at the south base of the mountain and occupying a belt about 2 miles wide, extends northwestward for 5 miles to a point within 1½ miles of Calabasas. It is locally called "bull granite" and is coaser in texture than the similar rock in the Washington-Duquesne area. The rock is sliced by a dominant sheeting that dips to the NW. It is also traversed by a system of east-west fissures in which the ore deposits at Mount Benedict occur. It is intruded by dikes of aplite, granite porphyry, diorite, and lamprophyric and diabasic rocks, with which the ore deposits are mostly associated. The granite porphyry forms the summit of Mount Benedict and the crests of the highest ridges, notably on the west and the south.

The quartz diorite is the next youngest rock, which occupies a semicircular area on the south, extending from Nogales eastward along the border for 2 miles and seemingly forms but the northern portion of a large area of the same rock, most on the Mexican side. It is a dark iron-gray, fine-grained rock, composed principally of oligoclase-andesine in prisms, biotite, and hornblende in short, prismatic laths in about equal amount, and some associated augite, but the rock is low in dark silicates. Some orthoclase and considerable magnetite are also present.

Next younger is the reddish bedded volcanic tuffs which occupy an irregular area of about 3 square miles in the vicinity of Nogales, extending several miles northward down Nogales Wash and northeastward up the Proto Canyon road. The tuffs are in many places covered by the Quaternary gravels and are generally best exposed in the bottoms and sides of the drainage ways. In the southwestern part of the district, at Nogales, the beds overlap the diorite on the hillside already mentioned.

At 3 miles east of Nogales and 1/3 mile north of the Mexican line, black basalt occurs in a small north-south area, and intrudes the Quaternary gravels.

The lode deposits occur principally in the middle of the western part of the district in the quartz monzonite area, mainly at Mount Benedict, abiout 4 miles north of Nogales, and 2½ miles NW of Mount Benedict, at the reagan camp. The Mount Benedict deposits are principally gold-bearing, and those of the Reagan camp area carry tungsten.

Workings include mostly shallow shaft, tunnel and open cut operations. Worked as early as the 1850's and sporadically up to 1967. Estimated and recorded production would amount to some 1,100 tons of ore containing about 1,019 oz. of gold, 95 tons of lead, 14,200 oz. silver and 9 tons of copper. About 1,500 pounds of 50% WO3 concentrates may have been shipped. Output from gold placers is unknown but was probably relatively small.

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