Prospecting & Detecting
Free / Prospecting & Detecting
Detecting at the Sixteen to One Mine
December 2012 by Chris
A while back I was given a very special opportunity to take my metal detector to one of the most famous gold mines in all of California, the Original Sixteen to One mine in Alleghany. I was asked to give a little demonstration and talk about metal detecting technology, how it has changed and improved, and to do some field testing of different types of detectors inside the mine.
I wasn’t really there to explore the mine workings and locate new deposits or even collect gold for myself. Instead I was there to help the miners and operators at the mine in their efforts to find the most gold possible because much of the gold within the Sixteen to One mine is found with the help of metal detectors. Basically, we were comparing the performance of different detectors under actual field conditions evaluating both PI (pulse induction) and VLF (very low frequency) types. Detecting for gold within this mine is a very high-stakes effort because there is little doubt that somewhere just beyond the walls of the mine workings are high-grade pockets with millions of dollars worth of gold. Of course, no one knows exactly where these rich spots are, but metal detectors have proven to be an excellent tool in finding gold pockets here.
I’ve known about this mine for years and even visited the outside grounds once before, so I was truly excited about doing a little metal detecting down inside one of the richest high-grade pocket mines ever discovered.
For those of you who aren’t familiar with this part of the California gold country, it would be worthwhile for me to go backwards with my story and delve a bit into the history and geology of this very famous mine. The Alleghany district is located in the southwestern part of Sierra County and has long been famous for big gold nuggets and rich pockets of gold-bearing quartz ore. The value of the total output of the district is unknown, but is estimated at more than 2.5 million ounces. The placer gravels of the recent streams, Tertiary channels and residual gold deposits were all mined in the early days and were spectacularly rich. Where the Tertiary channels dipped beneath thick layers of volcanic rock, these deposits were worked by drift mining, which for many decades was the chief source of gold after the initial surface placers played out. In the 1870s, hard rock mining began to play a larger role as a number of rich pockets were discovered by miners during this time. After the initial ores played out, the rediscovery of the Tightner vein in 1904 by H. F. Johnson led to a revival of lode mining operations that continued until 1965. The introduction of metal detectors in the 1980s led to renewed production that has continued to the present time.
Unlike most hard rock districts in California, the chief production at Alleghany has come from small but very rich pockets of gold ore that may contain anywhere from a few hundred to over 10,000 ounces of gold. In most hard rock districts, large bodies of low to moderate grade ore are the chief source of production in the mines, though sometimes a few higher grade pockets may be found. The Alleghany area is an exception to this rule and a considerable number of rich, small ore pockets or pocket shoots have been developed in the mines here.
Normally, pocket mines do not have enough rich ore to support production over the long-term, but again, this district is an exception to the rule. The Alleghany district in Sierra County is the richest and most famous of the high-grade pocket mine belts. The high-grade pockets usually occur within the veins and consist of a mixture of quartz vein material and free gold with minor amounts of sulfides. A considerable amount of jewelry-grade gold quartz is produced from these mines for use worldwide. The spectacular appearance of the gold in these high-grade pockets can fire up the imagination of any prospector.
The gold-quartz veins strike in a generally north direction following the trend of the enclosing rocks. They may dip either to the east or west and usually range from two to five feet in thickness, but may swell up to twenty feet in places. They occupy minor reverse faults, and occur in all of the rocks of the Calaveras Formation, as well as in the greenstone. The largest and most productive mines are found in the amphibolites of the Tightner Formation. The most characteristic features of the ore deposits here include their extreme richness, erratic distribution and small size. The rich pockets range in size from small to quite large. One ore body at the Sixteen to One mine, which had a pitch length of 40 feet, contained nearly 50,000 ounces, while another at the Oriental mine was about 14 feet long and yielded nearly 75,000 ounces.
The Alleghany gold district is underlain by north and northwest-trending beds of metamorphic rocks of the Calaveras Formation, serpentine, and greenstone. The Calaveras Formation here consists mainly of slate, amphibolite, chlorite schist, conglomerate, chert and quartzite. Throughout the gold-bearing area, these rocks have been cut by a number of basic and ultra-basic intrusions. The ultra-basic rocks have been largely altered to serpentine. Mariposite rock, a chromium mica-bearing quartz material is commonly found adjacent to the serpentine. Some fine to medium-grained dioritic dikes have also been intruded into the area.
The gold occurs in the native state commonly with arsenopyrite, but with only small amounts of other sulfides. In a few places pyrite is abundant. The numerous serpentine bodies and associated mariposite rock are structurally important in the localization of the ore bodies. The quartz veins tend to fray or bend near serpentine, and it is in these frayed or bent portions of the veins that the high-grade ore bodies are often found. High grade ore is also commonly found at vein junctions or in sheared portions of the veins.
Over time, a significant number of the high-grade pocket mines in and around Alleghany were consolidated into the Original Sixteen to One, Inc. group either through settlements of lawsuits or by property purchases. Formerly independent mines that have been consolidated into the Original Sixteen to One, Inc. group include the Belmont, Eclipse, Ophir, Osceola, Rainbow, Rainbow Extension, Red Star, Tightner, and Twenty One mines. The Tightner mine was the largest and most productive of these other mines. It was named for the tendency of its veins to pinch down and tighten.
When we arrived at the mine site we were given a brief safety overview. I assured them that I had my MSHA certification for mining work and shortly afterward we were taken down to meet with one of the miners who does a lot of the metal detecting searching for gold. Other miners were driving underground mine workings, drilling and blasting as in a normal underground mine. Access into the mine is through a long horizontal adit that was driven into the mountain just above Kanaka Creek to reach the old underground workings, thus avoiding the old access shafts that formerly were used to reach the old workings.
In the Sixteen to One mine most of the gold they find is leafy, forming thin sheets, in mossy accumulations, or in small wires. Because of the nature of the gold here, the underground miners use principally VLF metal detectors with very high frequency so that they can pick up very small gold pieces. On the other hand, pulse induction machines are used to detect larger pieces of gold at a slightly greater distance into the walls.
The pulse induction detector they had at the mine was a few generations old, and they wanted to take a look at the newest equipment to see how it performed in comparison to the VLF machines they use on a daily basis. The most recent generations of pulse induction detectors have become significantly more sensitive to very small gold. They are still not equal to VLF machines on the tiniest gold close to the coil, but the gap has been closed quite a bit in recent years. The pulse induction machines can still see a bit farther into the wall if the gold is large enough and that is an important advantage. Metal detectors are tools, and like any workers, these miners want to have the best tools available to them at their job. They use both types, but were interested in taking a look at the latest generation of pulse machine—and I own one.
One might think that within a short amount of time a person could run his metal detector over every inch of ground inside a mine. However, because the Sixteen to One is a combination of a number of old mines and the property has been worked on a more or less continuous basis for well over 100 years, there are many miles of old underground workings to explore, and one could spend a lot of time going over every bit of the vein that was exposed in the walls of the old workings. In exploring the old property, the miners have maps of the old workings including notations on the maps showing locations and amounts of gold recovered from each pocket. It was exciting to look on the maps of the underground workings and see notations that, for example, indicated that 10,000 ounces were recovered right next to where we were standing and another 2,000 ounces had been recovered a short distance down the drift. The Sixteen to One had been an extremely productive mine in its day, producing more than one million ounces of gold.
Once we got back into the depths of the mine, we began the actual testing of the detectors. A rock the size of a bowling ball with a small amount of gold had been found that morning and we both tested our detectors on it. Both the VLF and the pulse induction units had no problem seeing the gold in that rock.
We hiked around through various levels and stopes, testing the walls at spots where the VLF detector had indicated gold was present. It wasn’t my objective to find new deposits, but to see how the newest pulse machines compare with the high frequency VLFs. At just about all the known gold-bearing sites that we tested, the PI showed gold as well, but sometimes in slightly different spots. Our conclusion was that while there is no question that the VLFs still have an advantage on the tiniest gold, the gap is getting smaller. Each type of technology has its own place in prospecting for gold and the miners there use both types. I talked about the advantages and disadvantages of various detector technologies with the miners, but they were pretty well up on what is going on as far as gold-oriented metal detectors.
When I got into the mine and started testing my metal detector, one of the things I hadn’t counted on was the power lines that were virtually everywhere. In the underground mines where I worked years ago, we had power at certain points within the mine, but in general most of the mine had no power. In the Sixteen to One there are power lines down every drift and up every shaft. It was hard to find places where I could place the coil and not be within five or six feet of an AC power line. These detectors are not made to work in and around power lines without special adjustments and double D coils. Not expecting to find power everywhere, I did not bring a double D coil with me.
The comparison of metal detectors we did within this mine really was a site-specific thing and would not necessarily apply to other places—the conditions there were fairly unique. I personally do not do much underground metal detecting in old mines simply because of the dangers involved. In this case I was in a working mine accompanied by miners and employing all the necessary safety equipment, doing our work in compliance with federal MSHA standards. It was not the same as poking around some creaky underground mine that hadn’t been worked in 50 or more years. The prospecting I do related to hard rock mines is done on old mine dumps, testing the ores for the presence of free gold with my metal detector. I always recommend safety first when you’re out gold prospecting because there is no amount of gold worth dying for.
One thing that would not necessarily transfer well to other mines is that some ore minerals are commonly electrically conductive and will set off many metal detectors. Arsenopyrite is a good example of a conductive mineral that can cause problems in detecting hard rock ores for gold. When you prospect, you want to see an old mine dump where the only conductive material present is free gold. Commonly, there is a lot of trash including old nails and such, but when ore minerals themselves are conductive, detecting old mine dumps can be a very difficult task. Galena and cuprite are other minerals that can be conductive.
Note: While I was detecting inside the Sixteen to One mine, I was accompanied by Joey Wilson from the Prospecting Channel who shot video of the whole adventure. He is currently working on putting together a video version of this article. If you would like to see that video, it will be appearing online soon at prospectingchannel.com
I’m sure Joey will have it done and available for viewing in the next couple of months.
All photos courtesy of Joey Wilson of the Prospecting Channel website.
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