Damascus Mining District (Sunny South Mining District), Placer Co., California, USAi

Regional Level Types
Damascus Mining District (Sunny South Mining District)	Mining District
Placer Co.	County
California	State
USA	Country

A former Au-Ag-Pt-Cu-Zn-Fe-Pb-In mining area located in secs. 1 to 4, T14N, R11E & in secs. 10 to 15, 21 to 28, 33 to 36, T15N, R11E, 10 miles NE of the town of Foresthill.

Location: This district is in cast-central Placer County at the site of the old town of Damascus, about seven miles southeast of Dutch Flat. It includes the lode mines of the Pioneer-Humbug Bar area on the north and the extensive placer deposits that extend from Damascus south through Forks House to the Sunny South-Gas Hill area.

History: The streams in this area were originally mined during the gold rush, and drift mining began in the late 1850s. The Hidden Treasure drift mine was discovered in 1875 and was worked on a major scale through the early 1900s. There was some mining activity in the district again during the 1930s, and there has been intermittent prospecting since. This area has
been quite productive; the drift mines alone have had a total output of more than $12 million and the lode mines several million dollars more.

Geology: An early Tertiary gravel channel extends from Damascus south to Gas Hill and eventually southwest to Michigan Bluff. An intervolcanic channel that apparently eroded away portions of the earlier channel enters the area from Westville to the east. The lower earlier "white" channel is more than 300 feet wide. It contains abundant quartz. The gravels in this early channel yielded 50 cents to $1.75 per ton at the old price, and the gold was coarse. The upper channels are commonly known as "blue" channels. Bedrock is slate and schist, and the gravels are capped by rhyolite and andesite. The channel has been mined by drifting almost continuously from Damascus south to Hidden Treasure, a distance of more than four miles. The quartz veins, which occur in slate, range from two to eight feet in thickness and contain free gold and often abundant sulfides. The ore usually is low to moderate in grade, but the ore shoots had stoping lengths of up to several hundred feet. The Pioneer Mine was developed to a depth of 1400 feet.

Rocks of the Damascus district can be divided into four units consisting of basement complex, Eocene auriferous gravels, interbedded volcanics and gravels of the Valley Springs Formation, and an uppermost Mehrten Formation volcanic cap.

The main body of basement rocks within the district consists of sandstones, siltstones, slates, and schist of the Shoo Fly Complex within the Sierra Nevada Eastern Belt. In the extreme southwest corner of the district, the Volcano Canyon Fault, within the Melones Fault zone, juxtaposes partially to completely serpentinized peridotites against the Shoo Fly Complex rocks.

Basal Eocene Auriferous Gravels:

The primary Damascus placer deposits were deposited by a north-south-trending early-Tertiary tributary channel, which flowed southward into the adjacent Michigan Bluff District where it joined the ancestral Middle Fork of the American River. This channel can be traced from Damascus to Gas Hill, a distance of 4-1/2miles, and it has been drifted almost continuously for over 4 miles from Damacus to the Hidden Treasure Mine. The ancestral Middle Fork flowed northwest-southeast following the same general course as the modern Middle Fork of the American. Gravels from this large channel are also important producers in the famous Forest Hill District 10 miles to the southwest.

Both the tributary channel and the main channel incised deeply into the underlying bedrock. The bedrock surface is irregular with ridges, swales, and potholes conducive to trapping placer gold. In contrast to the gravels in the Forest Hill District, which are generally a blue gray due to a concentration of slate and metamorphic rock, the Damascus and Michigan Bluff district gravels are almost exclusively white quartz gravels with occasional rounded white quartz boulders of up to 20 tons. This difference is attributed to their location upstream of Calaveras Complex bedrock. The tributary quartz gravels are generally less cemented than many Eocene gravels in the Northern Sierra Nevada and generally don't require crushing.

Deposits within the tributary channel can be traced for several miles within the district to a southern limit at the Gas Hill Mine. North of this mine, the channel is truncated by a deeper intervolcanic channel, but reappears 1 mile farther north in the Hidden Treasure Mine (Sec. 35, T15N-R11E), where it was called the "White Channel" and produced over $4 million. From the Hidden Treasure Mine, the tributary can be traced almost continuously to Damascus (Lindgren, 1911), a distance of almost 4 miles. Upon entering the Michigan Bluff District, its deposits have been largely removed by erosion in El Dorado Canyon.

The channel is a wide (> 300 feet), flat depression in soft swelling clayey slate bedrock, which required substantial timbering to keep the tunnels open. The channel was filled with almost 200 feet of uncemented quartz gravel, sand, and clay with some quartz boulders. The gravel is markedly finer and more quartzose than that of the main American River Channel into which it drained. Breasting could be done with pick and caving and blasting was limited to the removal of some large boulders. In places the channel widened to 800 feet with rims rising gradually to 16 feet above the thalweg. The width of the gravel breasted was 250 feet, and 4-7 feet of the lowermost gravel, including 1 foot of bedrock, was extracted. The gold was generally coarse, with gravels yielding only $0.50 to $1.75/ton. Only the unusually low cost of production allowed this mine to profitably produce this grade of material via drift mining.

Valley Springs Intervolcanic Channels:

Overlying the Eocene channels are varying thicknesses of intercalated rhyolite tuffs and intervolcanic channel gravels (often called "cement" or "blue" channels) of the Valley Springs Formation. Towards the top of the formation the tuffs become progressively more andesitic. The thickness of the sequence is highly variable. Thicknesses of up to several hundred feet of gravel, sand, and pipe clay, can extend well beyond the limits of the lowermost bedrock channel depression. Little information is available regarding specific thicknesses within the Damascus or neighboring Michigan Bluff districts, but in the Forest Hill District, exposed thicknesses of rhyolite tuff and intervolcanic gravels range from 40 to 130 feet.

Many intervolcanic channels eroded deeply into older auriferous gravels either partially or wholly destroying them. In these cases the intervolcanic channels can be locally very rich, having cannibalized the gold content of the older channel. Clay beds are common in the upper portions of intervolcanic gravels. Petrified and/or lignitized cedar and oak tree trunks are not uncommon. Some gravel layers have become highly cemented by percolation of siliceous and calcareous waters and colors range from gray, blue, reddish brown, to white depending on the source material and oxidation of the gravel and/or cementing material.

The Valley Springs intervolcanic channel sequence is capped by andesites of the Mehrten Formation along the crest of the Foresthill Divide, which traverses the district in a northeasterly direction.

Gold particles tended to be flat or rounded and ranged from fine to coarse gold and nuggets, although large nuggets were unusual. A little fine or flour gold was found in the upper sands and clays that covered the gravels. The gold particles are in some places associated with platinum and almost invariably associated with black sands composed of magnetite, illmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentine.

Lode Gold Deposits:

The Damascus District also contains lode gold mines, the most important being the Pioneer Mine, which was developed to a depth of 1,400 feet and produced $1 million (period values). The gold quartz veins occur in slate and are of limited extent and thickness. Veins trend northwest and northeast dipping steeply to the east. They range from 2 to 8 feet in thickness and contain free gold and often abundant sulfides. The ore is usually low to moderate in grade, but the ore shoots had stoping lengths of up to several hundred feet.

Mineral occurrence models information: Model code: 119; USGS model code: 39a; BC deposit profile: C01. C02; Deposit model name: Placer Au-PGE; Mark3 model number: 54; Model code: 273. The ore bodies are genreally irregular, tabular, lenticular.

USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27.

Host and associated rocks:

Host: unconsolidated Tertiary gravels; Devonian-Ordovician Shoo Fly Complex slate; Devonian-Ordovician Shoo Fly Complex schist

Controls for ore emplacement included mechanical accumulation on irregular bedrock riffles and within river and stream channel lag gravels, bars, and point bar deposits. Ore shoots within mesothermal gold-bearing quartz veins. Local rocks include Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range).

Regional geologic structures include the Gills Hill Fault, Foresthill Fault, and the Melones Fault Zone. Local structures include the Melones Fault Zone.

Commodity Information: Placer deposits: fine to coarse gold to large nuggets. Lode deposits: free-milling, gold-bearing quartz veins. Ore materials: Native gold - fine to coarse gold and nuggets (.900 fine). Gangue materials: Quartz and metamorphic gravels; accessory minerals: magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite.

Mines: Lode: American Eagle, Black Hawk, Central, Dover, Floyd, Lynn, Mars, North Star, Pioneer ($1 million), Rawhide ($300,000+), and the Southern Cross. Placer: Bullion, Cameron, Comet, Gas Hill, Golden River, Hermit, Hidden Treasure ($4 million), Mountain Chief ($700,000+), Mountain Gate, Rainbow, Tickell.

References

Other Databases

Link to USGS MRDS:	10310606

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