US Crust Thickness

Geology: A belt of gray to black slate of the Mariposa Formation (Upper Jurassic) one to two miles wide extends north through the central portion of the district. Greenstone and amphibolite are to the west, and schist and slate of the Calaveras Formation (Carboniferous to Permian) and granodiorite lie to the east. The Tertiary South Fork of the American River, which has numerous tributaries, entered the Placerville basin from Newtown. In places the Tertiary gravels are overlain by thick beds of rhyolite tuff and andesite.

While the Placerville District includes some Mother Lode quartz lode mines, it is best known for its highly productive Tertiary channel deposits, the most important being at Coon Hollow, Diamond Springs, Smith's Flat, Texas Hill, and White Rock.

The Placerville District straddles the Melones Fault zone and hence bedrock varies from east to west across the district. A belt of Mother Lode Mariposa Formation gray to black approximately one to two miles wide extends northward through the district. To the east, bedrock is schist and slate of the Calaveras Complex and granodiorite. To the west, bedrock is amphibolite and greenstone (Clark, 1970).

The district is situated on a ridge between the South Fork of the American River (American River) and Weber Creek, on which occur remnants of channel gravels deposited by southward flowing tributaries to the ancestral South Fork of the American River. Gravels deposited within the main channel of the American River, which approximately followed the course of Weber Creek, have been largely lost to erosion.

The gravel deposits are largely overlain by overlain by thick beds of rhyolite tuff and andesite. In the Placerville area, the Valley Springs Formation coinsist of generally flat lying beds of rhyolite tuff. The tuff usually is fine grained and contains small crystals of black biotite. Smaller amounts of breccia, conglomerate, and siltstone are present (Clark and Carlson, 1956). The overlying Mehrten Formation occupies many of the interstream ridges and consists chiefly of andesitic volcanic debris composed of boulders, cobbles, and pebbles.

Placerville District gravels are generally composed of pebbles and boulders of quartz, chert, granitic and volcanic rocks interbedded with clay and sand. Considerable amounts of placer gold is present at or near the bottom of these deposits (Clark and Carlson, 1956). Generally, the gold on bedrock was smooth and coarse. Associated with the gold in the placer deposits are magnetite and other heavy resistant minerals such as ilmenite, garnet, rutile, zircon, and platinum-group metals.

The richest gravels in the Placerville District were those at Coon Hollow, just southwest of Placerville, and just west and downstream of the Mother Lode bedrock quartz veins (Lindgren, 1911). Lindgren (1911) describes a number of the individual deposits in greater detail.

The ancestral Tertiary American River and its tributaries entered the district from the northeast. The most productive tributaries from east to west across the district were the Deep Blue Lead, Spanish Hill-Green Mountain Channel, and the Coon Hollow Channel.

Deep Blue Lead Channel: Of the numerous American River tributaries in this district, the best known was the Deep Blue Lead. It entered the northeast corner of the district and extended southward for about 2 miles from White Rock Canyon to Smith's Flat and then west-southwest through Texas Hill to Cedar Ravine.

The Blue Lead channel consists of a deep, fairly straight channel flanked by broad sweeping and curving benches. This tributary followed a bedrock slate-granodiorite contact for several miles and is believed to have derived much of its gold from many small veins in the contact zone. The general course of the Deep Blue Lead was from north-northeast to south-southwest. At White Rock, the exposed gravels were hydraulically mined, producing about $5 million. The bedrock channel at White Rock was 30 feet wide with 12 feet of gravel. Two benches 40 feet high also yielded paying gravel (Lindgren, 1911). For about 2 miles from White Rock to Smith Flat, where the gravels were overlain by the Valley Springs and Mehrten formation rocks, the channel was almost continuously drift mined. From Smith Flat, the Deep Blue Lead trended west-southwest for another 2 miles to Texas Hill where it was again mined hydraulically. For most of this stretch, the channel was developed by the Bendfelt, Hook & Ladder, Lyon, Kumfa, Try Again, Texas Hill, Clark, Rivera, and Linden drift mines (Lindgren, 1911). The Lyon Mine produced $1.4 million. At the Benfield Mine, a gravel deposit some five feet thick and 50 to 120 feet wide yielded $2 to $8 per ton. Blue Lead gravels were generally not as rich as those to the west in channels crossing the Mother Lode quartz belt. Blue Lead gravels were deposited before the beginning of the later rhyolitic flows, for cobbles of rhyolite are contained in all of them (Lindgren, 1911).
Coon Hollow Channel

The Coon Hollow channel was the richest of the Placerville district's placer deposits, having produced about $10 million. This channel is in the western part of the district and its deposits are thought to have been enriched by erosion of bedrock Mother Lode gold veins. The Coon Hollow deposits contained 3 gravel pay streaks, the first on bedrock, the second 25 feet above, and the third 60 feet above. The first and third were very rich (Lindgren, 1911). Coon Hollow gravels were much richer than the gravels of the Blue Lead or Spanish Hill - Green Mountain channels. Overall, the entire section of gravel was at least 100 feet thick and averaged about $1 per yard (Lindgren, 1911). A single claim of 20 acres (Excelsior claim) produced $5 million from gravels containing as much as $5 per cubic yard.

Total width of the Coon Hollow bedrock channel was 2,000 feet. The third streak was reportedly 300 feet wide. Part of the gravel was cemented. Unlike the gravels farther east, there were no volcanic pebbles in the Coon Hollow gravels (Lindgren, 1911).

Spanish Hill-Green Mountain Channel: The Spanish Hill-Green Mountain Channel, in the south central part of the district, formed a well-defined tributary to the main river channel and was separated from the Deep Blue Lead to the east and the Coon Hollow channel to the west by pronounced intervening bedrock ridges (Lindgren, 1911). Several mines exploited these channel deposits including the important Spanish Hill hydraulic mine, which produced $6 million, Cedar Springs, and Green Mountain drift mines. In the Cedar Springs Mine, the bedrock channel was 300 to 600 feet wide with pay gravel 4 to feet thick.

Ore Deposits: Of the numerous tributaries of the main Tertiary channel in this district, probably the best known and one of the richest was the Deep Blue Lead. This channel extended south from White Rock to Smith's Flat and then west-southwest through the Texas Hill area. The lode-gold deposits arc massive quartz veins as much as 20 feet thick with numerous parallel stringers. The ore bodies are low to moderate in grade (y, to y. ounce of gold per ton), but the veins have been mined to depths of 2,000 feet. The ore contains finely disseminated free gold and small amounts of pyrite. The veins occur chiefly in slate.

Workings: The Placerville District gravel deposits were generally worked by small- to medium-sized drift mines with access through shallow shafts or through adits generally less than 2,000 feet long, and/or large scale hydraulic mining operations. Consequently, there is meager information regarding specific mine workings. Limited information about some of the districts drift mines is contained in unpublished historical files of the California Geological Survey and other published sources (Noble, 2002). Specifics of the hydraulically mined deposits are even more scarce. Generic discussions of these mining techniques follow:

Drift Mining: Drift mining involved driving adits and tunnels along or close to the lowest point in the bedrock trough of an ancient channel and following it upstream along the bedrock surface. Some deeply buried drift mines were originally accessed through vertical shafts requiring timbering, headframes, hoisting, and pumping equipment. Larger shafts were seldom over 3 compartments Smaller mines often had single compartment shafts as small as 2 x 5 feet. Since considerable water was associated with the gravels, it was a serious problem in deeper shafts and costly pumping was required. By the 1890's, due to drainage problems and the expense of hoisting, most major drift mines were accessed through tramway and drain tunnels driven into bedrock below the channels.

Channels were usually located by gravel exposures on hillsides and terraces. Exposures of upstream and downstream gravels were called "inlets" and "outlets," respectively. Where a ravine or canyon cut into, but not through an old channel, the exposure was called a "breakout."

The preferred method of developing an inlet was to tunnel through bedrock under the channel at such a depth and angle as to break through into the bed of the channel providing natural drainage. The overlying gravels could then be accessed directly through the tunnel or by periodic raises and drifts. Development of an outlet involved following the bedrock channel directly into the hillside, the incline of the bedrock providing natural drainage. The tunnel entrances were usually in or near a ravine or gulch to aid in waste-rock disposal.

Prospecting and developing a breakout was more difficult, since the exposed gravel could be in the basal channel or hundreds of feet up on the edge of the channel, making it impossible to locate a prospect tunnel with any certainty. The surest method of prospecting was to run an incline on the pitch of the bedrock. Another method was to sink a vertical shaft on the presumed channel axis. The former method proved superior since it involved less subjectivity and often uncovered paying bench gravels on edges of the old stream. Once the bed of the channel was located, it was prospected by drifts and cross cuts to ascertain width, direction, grade, and the location, extent, and quality of pay.

Access tunnels were driven in bedrock to minimize timbering and ensure a stable roof, through which upraises were driven to work the placer gravels. Tunnels were generally run under the lowest point of the bed of the channel in order to assure natural drainage and to make it possible to take auriferous gravels out of the mine without having to hoist it.

The main drifts were kept as straight as possible and in the center or lowest depression of the channel. To prospect the width of the channel, crosscuts at right angles to the drift were driven on each side to the rims of the channels or the limit of the paying lead. These were timbered and lagged in soft gravels, but not to the extent of the main drift. In wide pay leads, gangways paralleled the main tunnel to help block out the ore in rectangular blocks. In looser gravels, timbering was required and the main difficulty was preventing caving until timbering was in place. The looser gravels were excavated with pick and shovel. Up until the late 1800's, most workings were driven by hand, then later by machine and pneumatic drills.

Working drifts in the gravel beds and pay leads themselves were larger than the bedrock tunnels and usually timbered due to their extended and long-term use. In wide gravel deposits, as a precaution against caving, gravel pillars from 20 - 40 feet wide were left on each side of the drift. When the main access tunnel was in bedrock following the line of the channel, pillars were not required, as the tunnel in the gravel was only for temporary use in mining the ground between its connections with the bedrock tunnel. Raises to access the gravel were made every 200 - 400 feet as necessary.

The breaking out of gravel (breasting) was done from the working faces of drifts. Usually, 1-2 feet of soft bedrock and 3-4 feet of gravel were mined out to advance the face. When the gravels were well-cemented, blasting was required. Otherwise the material could be removed with picks. Boulder sized material was left underground and only the gravels and fines were removed from the mine.

Bedrock swelling was a frequent problem. Tunnels on and within bedrock were sometimes affected by the upward swelling of the bedrock. In these cases, heavy timbering was required and the tunnel floor had to be periodically cut and lowered to keep the tunnel open.

Soft or fractured slates were the most favorable bedrock. The surface was usually creviced and weathered enough that gold could be found to a depth of 1 foot in the top of the bedrock. Where sufficiently weathered and soft, this upper bedrock layer could be easily removed. If the surface of the bedrock was too hard to be worked, it was cleaned thoroughly, and the crevices and surface were worked with special tools to remove every particle of gold.

According to the gravel's hardness, they were either washed through sluices or crushed in stamp mills. Much of the gravel was so highly cemented it had to be milled several times. Stamp mills with coarse screens were also found to be suitable for milling cemented gravel. For soft and uncemented gravels, a dump, sluices, and water supply under generally low pressure comprised the entire surface workings.

Ventilation of mines was accomplished by direct surface connection through the use of boreholes and the mine shafts and tunnels. It relied on natural drafts, drafts by fire, falling water, or blowers. Within the mines, arrangements of doors were often used to direct the flow of air through the tunnels, drifts, and breasts.

Ore was removed by ore cars of various capacity determined by available power and tunnel size. In smaller mines, small cars were often pushed by hand. In larger mines using horsepower or trains, larger two ton cars could be brought out in trains of 5-10 cars.
Hydraulic Mining

Hydraulic mining methods were first applied in the Placerville District in 1854 at Spanish Hill. Hydraulic mining allowed the bulk processing of large volumes of low yield that would otherwise be unprofitable by other methods of mining. Hydraulic mining involved directing a powerful stream of high pressure water through large nozzles called monitors or "giants" at the base of a gravel bank, undercutting it and allowing it to collapse. Large gravel banks several hundred feet high were mined in this manner, but larger banks were often hydraulicked in two or more benches. In some cases, adits were driven into the exposed face and loaded with explosives to help break down the exposure. The resulting slurry of clay, sand, gravel, and gold was washed through sluice boxes to trap the gold. The sluice boxes were generally four feet wide and deep and often over a thousand feet long and lined with riffles or over devices to mechanically trap the gold. Mercury was added to amalgamate the finer gold. The remaining debris was indiscriminately dumped in the nearest available stream or river.

One of hydraulic mining's highest costs was in the ditches, flumes, and reservoirs needed to supply sufficient volumes of water at high pressure. A mine usually needed its own system of ditches and flumes to deliver water from distant and higher reservoirs or rivers. A mine might have 10-20 or more miles of ditches as well as dams and reservoirs, flumes, and tunnels. At the mine site, the water fell through large iron pipes into the monitors. Hydraulic mining flourished for about 30 years until the mid-1880's when the Sawyer Decision curtailed debris disposal.

Another expensive undertaking was often finding an outlet for the debris. As the gravels were washed lower and lower in the ancient channel beds, it was often necessary to drive a tunnel through the bedrock channel rim to drain the workings into a nearby valley.

Production: The value of the total output of the Placerville District is unknown, but the placer mines are estimated to have yielded at least $25 million, and lode mines at least $2 million.

Mines: Lode: Elliott, Epley ($100,000+), Griffith, Guildford ($200,000+), Harmon ($100,000+), Larkin ($125,000), Margurite, Oregon ($100,000+), Pacific ($1,486,000), River Hill, Sherman ($136,000), Superior, True Consolidated ($100,000), Van Hooker ($100,000+).Placer: Coon Hollow ($10 million), Diamond Springs, Green Mountain, Negro Hill, Sacramento Hill, Spanish Hill ($6 million), Smith's Flat ($2 million+), Texas Hill, White Rock ($5 million). Drift: Benfield, Cedar Spring, Clark, Kumfa, Landecker, Lyon, Pascoe, Rivera, Texas Hill, Try Again, Union.