Tracing the Mother Lode
Ross Smith, Consultant—June 13-14, 2009
Report and photos by Anne Sanquini, from the November 2009 newsletter
Download the field-trip guide here
Over twenty five “NCGS miners” headed for the Gold Country June 13th and 14th, 2009, to trace the Mother Lode, a zone of gold-bearing quartz veins, from Placerville to Mariposa.
The Mother Lode, which runs along beautiful Highway 49 in the Sierra foothills, is known geologically as the Melones Fault. It is considered to be a crustal suture separating continental rocks of the Shoo Fly and Calaveras Complexes to the east from oceanic rocks, including the Mariposa Formation, to the west.
Our trip leader, Ross Smith, enjoyed a 30-year career in international oil exploration. For the past 10 years, he has been a precious minerals consulting geologist. Ross provided each of us with a 35-page field trip guide that includes several maps, a road log with descriptions of each feature and ideas for subsequent visits to the area.
Day One: Gold Bug and Kennedy Mines
We left a drizzly bay area, and headed east on Highway 80, then Highway 50 to Placerville, home of the Gold Bug mine. The mine entrance is a drift, indicating that this mine is drilled horizontally. A self-guided tour took us 350 feet into the drift.
After quartz ore is drilled, blasted and taken out of the mine, it goes to stamp mill, the next stop on our tour, for crushing. The local guide started up a small-scale version of the mill for us, and even that was loud enough to be able to understand how miners often lost their hearing working under these conditions.
Figure 1. NCGS “miners” at the Kennedy Mine. Trip leader Ross Smith is in the center on the lawn. The mine headframe is in the background.
Following the tour and optional gold panning in the local trough, we had lunch at picnic tables by the creek. The sun came out and weather for the rest of the trip was lovely.
The NCGS miners chased the quartz vein south on Hwy 49 through the towns of El Dorado, Drytown (which apparently was anything but), Amanor City and Sutter Creek. Along the way, we noted geomorphic evidence of the Melones fault, and the locations of old mines.
At the Kennedy Mine near Jackson, famous for being one of the deepest mines in the country at almost 6,000 feet, we were given a very informative presentation and tour by one of the volunteer docents, Al Sklensky.
Al walked us through a diagram of the mine that includes the fault location and the date each level opened as the mine operators pursued gold-laden quartz veins deeper and deeper into the earth. Beneath the 125-foot steel headframe, there are large skips used to transport men and equipment down into the mine. The same skips brought out the ore and also carried out about 70,000 gallons of water each night from lower mine levels.
We overlooked a field of huge stamp mills. Here, quartz ore is pounded into fine particles that are sent over mercury-coated plates. The mercury binds with the gold, forming an amalgam. Next was a trip up to the mine office, where the retort process is done, converting the amalgam to gold ingots.
Figure 2. Inspecting a “jaw crusher” at the Kennedy mine.
The Kennedy mine was operational until 1942, when all mines were closed to free up labor for the war effort. During its operation, it produced $34,280,000 of gold. See www.kennedygoldmine.com for more information.
Figure 3. The retort room, where the amalgam of gold and mercury was converted into solid gold ingots for shipment to San Francisco.
Barbecue dinner, origin of gold
Just past Angels Camp, the group pulled into the Glory Hole Recreation area. Dinner was barbecued chicken, grilled corn on the cob and macaroni salad. Thank you to Rob Nelson’s mom for marinating the chicken and making the salad. All scrumptious.
After dinner, as twilight was falling, Ross gave us a talk on the origin and occurrence of gold.
Figure 4. BBQ at the campsite. Perfect weather, view over the New Melones Lake, on the Stanislaus River.
How gold formed: Shortly after the big bang, about 13.5 billion years ago, hydrogen was formed. Stars process hydrogen as fuel towards heavier elements, up to iron. Since iron is at the peak of nuclear binding activity, the fusion process stops there. However, big stars can end their lives with a supernova explosion and during such an explosion, heavy elements such as gold are made.
Where gold is found: Along mid-ocean ridges, “black smoker” vents discharge mineral-laden very hot water. These goldbearing fluids migrate into rocks of the oceanic crust. The crust travels via plate tectonics until it meets its fate and is subducted. As the down-going oceanic plate reaches great depths, dewatering occurs, releasing mineral-rich fluid. These fluids move upwards, following pathways formed by faults and fractures. When this fluid reaches the level of 13 kbars and cools to 300 to 500°C it is at the precious minerals horizon, where gold and other minerals precipitate along with quartz. Gold mining along the Mother Lode essentially consists of chasing these quartz veins.
Day Two: Quarries, Placer Deposits and the California State Mineral Museum
The group was on the road by 7:30, headed for Carson Hill, one of the richest areas along the Mother Lode. Here, gold is found in auriferous schist on the sides of the quartz veins. We stopped at the Carson Hill Mine. Reopened about 20 years ago, the mine operates as a “decorative rock” quarry that also happens to have ancillary gold production.
South of Tuttletown, the group pulled off the road at what appeared to be simply small fields of big limestone boulders. In fact, this is the exhumed Columbia limestone karst topography of the Calaveras Formation. Gold is heavy, durable and chemically stable. It erodes from the surrounding mountains and is transported by water. In this area it made its way to the potholes and crevices of irregularly eroded limestone. Between 1853 and 1870, about $55,000,000 in gold was excavated by hand within an area of about a one mile radius from this field stop.
Piles of rounded boulders curiously line some sections of Highway 49. We stopped at Moccasin Creek to understand why. The specific gravity of gold is 19.3 g/cm3; for sand and gravel only 2.6. Gold migrates to the lowest point in an area, often a streambed, where it settles at the bottom. The most obvious way to get at it is to redirect the stream if needed, then manually take out every single rock, dig up all the sediments, and scoop up the thin layer of gold.
Figure 5. The author confronts the mystery of the displaced rounded rocks.
At the Mariposite Quarry, owner Louis Bickford greeted us. The mineral mariposite is a green chromian mica formed by the alteration of serpentine by hydrothermal fluids. The rock of the same name includes quartz and brown ankerite, an iron carbonate.
Louis led us on a beautiful hike about a mile up the mine road past a drift mine and large serpentinite outcrops. We were allowed to climb all around the area, rock hammers swinging, and collect hand samples.
Figure 6. Mariposite Quarry specimen
Our mariposite treasures safely stowed, the successful miners headed to Coulterville for a stop at the Fielding Trading Post, owned by Louis and his wife, Karin. They carry an interesting offering of native jewelry, hats and gold-mining pans.
Figure 7. NCGS miners prepare to hike up to the Mariposite Quarry.
The town of Mariposa was our final stop. The California State Mining and Mineral museum is here, having been moved to this location in 1983, after 100 years in San Francisco. It includes thousands of mining artifacts, displays of mining processes, and stunning gem and mineral specimens. Behind glass in a vault is the Fricot Nugget, a 13.8-pound mass of crystalline gold. The trip wrapped up with optional stops at the History Museum in Mariposa and Dial’s Rock Shop at Cathey’s Valley on Highway 140.
Figure 8. View to the north of where we came on Hwy 49, along strike of the Mariposa Formation. Note serpentinite in roadcut to the east just past the Bagby Bridge over the Merced River.
The Northern California Geological Society thanks Ross Smith for designing and leading this wonderful trip. He mentioned that he and his wife drove this area four times, talking with mines owners and deciding the best route and stops for us. We sincerely appreciate the thoughtfulness and effort that was put into the trip, including tips for finding gold! We also thank Rob Nelson for organizing the trip.