Finding the Mother Lode
Isn’t it great to dream of discovering buried treasure, hitting the jackpot, or winning the lottery? For miners, the big desire is to hit the mother lode.
Where exactly is the mother lode? Well, actually, it’s not a secret. According to the Hudson Institute of Mineralogy, the “Mother Lode gold belt carries the most productive and best-known mining districts in California’s gold country. Although the entire foothill region of the Sierra Nevada is sometimes loosely termed the ‘Mother Lode Country,’ technically the Mother Lode is a 120-mile-long system of linked or en echelon gold-quartz veins and mineralised schist and greenstone that extends from the town of Mariposa, north and northwest to northern El Dorado County.”
It’s not only NOT a secret, but the location has been known for many years. In fact, in a California State Mining Bureau bulletin written in 1900 by the state mineralogist, A.S. Cooper, described in detail the Mother Lode Region of California. Later, in 1934, Clarence A. Logan of the State of California Division of Mines, also published a bulletin on the Mother Lode Gold Belt of California. He described the discovery of the area as a result of a “search in the 1850s and 60s to find the so-called ‘mother’ vein or lode as the source from which the gold in the gravels had been derived by Nature’s process of erosion.” (You can read a summary of the history in Thermo Fisher Scientific’s previously published article Hitting the Mother Lode.)
There was at least one miner who did indeed hit the mother lode in those early years. In 1865, a miner by the name of William Russell Davis was mining approximately 200 feet underground when he hit a rich pocket of gold and exposed countless gold specimens. The California State Mining and Mineral Museum is currently displaying the largest specimen he unearthed, known as the Fricot “Nugget.” The museum claims that this spectacular 13.8-pound specimen is the largest remaining intact mass of crystalline gold from 19th century California, when these finds were more common but usually were simply melted down.
The Hudson Institute of Mineralogy — which is dedicated to the discovery, study and preservation of mineral species and their history, and to increasing public awareness and appreciation of the mineral kingdom — reports that the most productive portion of the Mother Lode has been the 1-mile segment between Plymouth and Jackson in Amador County. Other major sources of gold in the Mother Lode have been the Angels Camp, Bagby, Carson Hill, Coulterville, Georgetown, Greenwood, Jacksonville, Jamestown, Kelsey, Mount Bullion, Nashville, and Placerville districts.
If you’re looking for a mother lode of gold or other precious metals, you’d probably use more modern equipment than was used over 150 years ago.
Gold (Au) exploration is very challenging because its economic mining threshold is low, and it depends on the tonnage available. For example, Au concentrations at even less than 1 g/ton can be economic in some large deposits. Geophysical and geochemical methods of Au exploration have advanced enormously in the last decade, never mind century.
XRF (X-ray fluorescence) is a non-destructive analytical technique used to determine the elemental composition of materials. Portable XRF analysers can be used in any stage of exploration, mining, and ore processing of various metals from Au to silver (Ag), Cu, molybdenum (Mo), Pb, Zn, antimony (Sb), bismuth (Bi), etc. Detection limits for base metals in these instruments are low enough to allow even non-geologists to analyse any geological sample from outcrops to drill cores and soil specimens. In addition to precious and base metals, other elements, such as potassium (K), calcium (Ca), titanium (It), and light elements [magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), and sulphur (S)] can be assayed as well. This helps geologists in mapping the hydrothermal alteration of the exploration/mining area or with 3D modelling of the alteration and mineralisation.
Gold exploration is challenging because it is one of the rare metals found in the earth’s crust.