Silver Hill Mine (Gold Coin Mine), Cerro Colorado District, Cerro Colorado Mts, Pima Co., Arizona, USA

A former small underground Ag-Au-Pb-Cu-Zn-Hg mine group located in East-central sec. 17, T.20S., R.10E., 7¾ miles N or Arivaca. Operated 1880-1939. Owned/operated by C.M. Cotton & C.B. Flynn; A.C. Lowell; Jose Ahumada; and, MacFarlane.

Mineralization is irregular, lensing, oxidized quartz-fissure veins with argentiferous base metal sulfides in a stock of Laramide dioritic intrusive and bordering andesite porphyry. Minor tetrahedrite, galena and cinnabar are present. The copper oxidation products (green staining, coatings) appear to eminate from the irregular blebs of mercurian and argentian tetrahedrite and often surround these blebs. The Hg in the tetrahedrite is also the apparent source of the cinnabar, which is sparse. The galena occurs as relatively small, rounded blebs in the vein quartz and appears to oxidize into anglesite and cerussite. Cavities and fractures in the veins often contain apparent iron oxides, dark green botryoidal material and bright yellow coatings. Although the stated value for Ag in these ores is high, the ore is very lean in appearance, with small blebs and irregular masses of sulphides and sulphosalts plus thin coatings of secondaries in massive "bull" quartz.

Workings include shallow shaft operations. Worked sporadically from the 1800's to 1930's. It produced an estimated 50 tons of ore averaging about 120 oz. Ag/T, 2.7 oz. Au/T, 19% Pb and 2% Cu.

Several openings on and around Silver Hill constitute the mine group. The principal group of openings lie about half way up the hill on the north side. These openings are a series of closely spaced shallow shafts (about 10 to 15 feet [3 to 5 meters] deep). The waste material around these openings appeared barren.

Another opening lies low on the north side of the hill (North opening) and consists of a shallow tunnel with openings in a north-south direction, which entend for a distance in both directions. The "roots" of an apparent small old ore pile lie on one side with waste dumps on the other sides of the workings. This opening was unofficially dubbed the "north opening."

Yet another opening lies on the south side of the hill (South Opening), part way up. The actual opening is a trench into a vertical, or nearly so, vein. It is about six feet (2 meters) wide and perhaps 15 to 20 feet (5 to 6 meters) high at the face. Several "roots" of ore piles are located near the opening. This working was unofficially dubbed the "south opening."

Several additional openings, including one very small prospect pit on the upper north side, and an opening high on the south side (not visited), plus several openings off the hill to the south complete the set of workings in this group. There is no evidence that any of these openings interconnect. Overall, the size of the openings is consistent with the total stated production of only some 50 tons of ore. It would appear that the majority of this production came from the south opening (C. Lemanski, et al observatons, February 2008).

References:
Keith, Stanton B. (1974), Arizona Bureau of Geology & Min. Technology, Geological Survey Branch Bull. 189, Index of Mining Properties in Pima County, Arizona: 114 (Table 4).

Arizona Bureau of Mines file data.

MRDS database Dep. ID file #10109613, MRDS ID #M050479; and, Dep. ID #10162275, MAS ID #0040190403.