A Guide to Overlooked Gold Deposits—Pt 4

A Guide to Overlooked Gold Deposits--Pt 4

(Series started with September 2003 issue)

by Lawrence Dee

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Trends

Trends are geologic situations in which the geological structure of an area is conducive to the deposition of minerals. These structures are generally fault systems that can be small and localized or can run for miles. Take a look at any map showing gold mines and you will notice that they often fall into straight lines. This is because the minerals are deposited within the lines of weakness in the rocks known as faults. Most mineralization occurs along faults as they present openings in the rocks for mineralization to take place. These intricate systems of broken rock are responsible for some of the great mining districts of the West. The Nevada trend shows prominently how important trends can be in locating valuable mineral deposits.

In addition, the intersections or crossings of these trend lines often represent an increase in the amount of mineralization resulting in extremely rich mining camps such as Goldfield, Nevada. You might ask, “How can this help me as a prospector?” The answer is by walking out the fault zone along which the mines are aligned, it may be possible to locate buried veins that were overlooked. An example is the residual placers that were formed from the decomposition of the veins but were not discovered by the early miners. Also the deposits at the intersection of major trends, such as the Nevada trends, can be very rich and may be worthwhile pursuing as nuggetshooting targets where not already under claim.

The Trans-Challis Fault Zone, a recently discovered northeast-trending zone, stretches from Montana into southern Idaho and has along its length most of the rich gold and silver mines in the state. The very rich Boise Basin deposits are directly related to this zone. It is interesting to consider how many valuable districts may remain along this trend that have yet to be discovered. Rough terrain and a lack of roads has discouraged some prospecting but these are the areas that may contain the undiscovered high-grade deposits. Again, much of the research done by the USGS and the state geological surveys describes these trends.

Mountainous Areas

Have you ever noticed that in many of the mining districts the mines are high in the mountains? The placers, if there are any, are situated down toward the valley. But what about in between the mountain slopes and the valley? Quite often you see no sign of placer work below the lode mines that supplied the placers to the valley, but you know that gold had to shed from the veins and be taken to bedrock in these situations.
Most of it headed downstream where it was deposited near or just beyond the major slope break. But this was in active stream valleys. What about in the desert where there were no streams in later geologic time? The early miners were largely uneducated and their only prospecting skills were learned from other prospectors. So they generally went for what they knew were the possible sites for gold deposition.

A good example was a placer on Jordan Creek near Stanley, Idaho. Here was a very rich placer just mined ten years ago, with many one-ounce and larger nuggets recovered that the old timers had completely missed. Why? Probably because the stream had cut into the far bank and created a high-bar deposit. Because the high-bar was covered by trees and vegetation, the old timers did not think to look so far from the present stream. I suspect a good detector could have located some of that very coarse gold and thus the deposit that was only a few feet below the surface.

Rich Hill and Similar Sites

Rich Hill, Arizona, near the town of Congress, was discovered by early pioneers who reportedly picked up large nuggets on the surface, filling buckets with them. Today, nuggetshooters are still finding coarse gold on Rich Hill and the area around it. Is there only one Rich Hill or could there be others?

There are possibilities for other Rich Hills, but it would take some dedicated research to pin the areas down and then to test them for values. The gold deposit on Rich Hill was very likely deposited by an early river system that may or may not have been the present Hassayampa. Rivers generally meander over large areas over long periods, thus the area of deposition could be much larger than just Rich Hill but not recognizable because of changes in the topography over geologic time. What we see today often bears little resemblance to the situation in the geologic past and only an expert can divine what has actually occurred. Considering the fabulous gold recovered from Rich Hill, it could well be worth some study and field work to test the possibilities.

Another Rich Hill-type deposit was one called “the potato patch” near Globe, Arizona. In this deposit large silver nuggets were found on the surface and gathered up like potatoes.

The Planchas de Plata is a well-authenticated discovery made by the early Spaniards in Mexico just south of Nogales. They found planks of silver on the surface that were so large they had to be sawed up in order to smelt them. Similar clumps of silver were found in some of the mines north of Kingman, Arizona—too large to be taken up the mine shaft! These types of extreme surface/near-surface enrichments could be easily found today with the modern metal detector. The question remains, how many other Rich Hill type deposits are yet to be found in some of the very rough desert country of the southwest?

Important Considerations

An important consideration in evaluating gold is its fineness. Pure gold is 1000 fine by definition. Any number less than 1000 means that part of the value is commonly taken up by silver. If your gold is only 800 fine, as is much of the gold in Nevada, then it is only 80% gold. Once you become familiar with the appearance of gold you can get a fairly good idea of the fineness just by looking at the color of the gold specimen. If it has a lighter color or silvery hue as compared to your gold ring, then it is much less than 1000 fine. If your ring is 24kt gold, it is 1000 fine; if 18kt, it is 750 fine. You can determine the fineness of a gold specimen, but it must be placer gold without quartz or other minerals attached.

Pure gold has a specific gravity of 19.3 and is 1000 fine. Thus, if you can measure the specific gravity of your specimen, you divide the number you get by 19.3, move the decimal point in the answer two places to the right, and that is the percentage of gold in your specimen. Any good book on minerals will describe how specific gravity measurements are made and there are some prospecting supply companies that sell devices specially designed to measure specific gravity.

The Size of The Gold

You have undoubtedly seen many ads for “micron gold” or “colloidal gold” recovery devices. It does not take a genius to determine how much of that “micron gold” you would have to process to make a dollar, even if the process worked. The ones making the largest profit are the ones selling the books, chemicals, and equipment. Recovering micron gold is okay as long as your deposit also contains enough larger gold particles to make it profitable.

The famous Snake River gold of Idaho and the Green River gold in Utah is so fine that it floats on water, yet prospectors continually attempt to mine it. Historically, miners using blanket sluices did make a living mining this elusive gold, but that was when you could live on less than a dollar a day. In more modern times, the only ones that have made a profit on Snake River gold are some of the gold dredges of the early 1900s and the sand and gravel company that currently recovers the gold with sophisticated Reichert cones while selling the gravel commercially.

Lastly, there is an enterprising gentleman who recovers the gold and places it in small vials to sell to the pilgrims. The point is that gold must have weight to have value. It may look great spread out in a gold pan, but as far as value you are probably not making wages. It may be fine for the hobbyist, but for the prospector who would like to make a profit, coarse gold is a necessity.

The serious hardrock miner will probably need ore that runs at least one ounce per ton to make a profit. When you consider that underground mining can cost $100 per foot and up, it is a very expensive business.

Research

This one topic may be the most important as far as locating mineralized areas that have not been stripped of their values. The US Geological Survey and the many state geological surveys are constantly researching the geology and mineralization of areas in the US. Their information is readily available, much of it for free or on library loan. This is important—you do not have to be a geologist or engineer to understand most of these reports. You are just looking for information that will help you tie down an area of interest. Some examples of what you are looking for:

Gold producing districts. What values did the mines of the district produce? Most important, were there high-grade mines in the district and were there placers? What was the gold like in the placers? If it was fine gold you are probably better off looking elsewhere. If large nuggets were common, as they were in many western placer districts, then you will want to check it out. What is the extent of the dry placer areas? Does the author give any ideas of other areas to prospect and does he give any recommendations?

This applies to both placer and hardrock. A bulletin on an Idaho mining district mentioned an early mine where the miners found high-grade gold in quartz that had rolled down the mountain when it eroded off the vein. All they had to do was dig up the ore and throw it into wagons. What did they leave behind that may have been buried by thousands of years of erosion?

The Custer Slide at the ghost town of Custer, Idaho, is an interesting situation. The Custer vein high on the mountain was so exposed by erosion that the miners built a trail so they could drive wagons under the vein and pull the high-grade down into their wagons. A woman who wrote a history of the area told me she and her husband would climb up the steep slide, probably in the 1930s, and pick up high-grade that had fallen from the vein and was never recovered.

The Custer Slide in Idaho -- photo courtesy of Lawrence Dee

Hardrock mines. What type of rock did the gold occur in? You are probably more interested in quartz veins than gold in other rocks, but it just depends on the situation. If the other rocks showed high-grade you will want to read the description of them. What did they look like? What is the color, rock type (granite, gneiss, etc.), and what were the signs of mineralization? These could include bright colors resulting from hydrothermal alteration, presence of jasper or agate, gossans, and silicification, which is another sign of hydrothermal alteration. In the case of silicification, the country rock has been invaded by the hot acidic solutions and the minerals in the solution have case-hardened the rock, often leaving it streaked with bright colors. If your rock hammer bounces off of this colorful rock, it is probably silicified. These situations will be described in the reports on the mining districts and can be found by skipping over the technical material and just looking for the parts that have meaning to you.

Geochemical prospecting. The more modern reports generally have pages devoted to the results of geochemical sampling programs. These are studies that sample rocks and stream sediments to determine what minerals are present. In the reports they will list the minerals tested, with gold and silver almost always two of the metals listed. In the lists will be sample numbers and the result of the assayed sample. You will be looking for “kicks” in the samples—big numbers that may indicate the presence of placer or lode gold or other minerals. If you find numbers that look interesting, find out if a map of the area was made to accompany the report. If available, look to see if a sample number is plotted on the map. You should be able to find that sample site on the ground and do further checking. The number will generally be in parts per million [ppm] or parts per billion [ppb]. Pay no attention to numbers in ppb as that is strictly reconnaissance sampling, but you should be aware that 34,286 ppb equals one ounce. It is far more likely that you will be seeing tables with ppm in which 34.3 ppm equals one ounce.

Historical Mining. There can be some very worthwhile information tucked away in the history of the mines and mining districts. One of the situations I always look for is the early presence of arrastres.
As you probably know, the arrastre was a Spanish invention in which flat stones were laid into the ground to form a circle. Stones were placed around the outside of the circle to keep the ore from bouncing out. One or more drag stones were chained to the end of a right-angled arm attached to an upright post that could be rotated by mulepower, waterpower, or in the case of the Spaniards, enslaved Indians. The ore was thrown into the arrastre and the stones were drug around and around until they crushed the ore and liberated the gold.

Why would I be interested in this contraption? Because the only way an arrastre made any sense was if the ore was very high grade. It was such a crude process that it did not pay to mill low-grade ores. Thus you have a clue to the presence of high-grade in this mine and if someone has not beaten you to the punch you can dig around the bottom stones in the arrastre and often find high-grade that slipped into the cracks. Arrastres were used into the 1930s because they were cheap and easy to build. I saw one at an Arizona mine that had a Model T differential driving a steel encased round stone. Some were made of poured concrete but all had some sort of dragging crushing device associated with them.

The book, Lost Treasures on the Old Spanish Trail, a very interesting treatise on the Spanish mines in America, has a picture of a Spanish arrastre on the back cover. I knew when I saw that picture I had seen that arrastre before. There is an arrastre in the Kofa Mountains south of Quartzsite that looks exactly like the one pictured in the book. The Spanish apparently had a particular way of building their arrastres with large upright stones forming the outside of the circle. It is interesting to consider where they were getting the gold as it could not have been too far from their mill and the water they needed to mill the ore. The Kofas had some very high-grade mines such as the King of Arizona and many small high-grade veins are likely scattered throughout the rhyolitic rocks that make up the range. Sorry, but unfortunately prospecting is forbidden in most of the Kofas as it is a federal wildlife refuge. There is some patented ground in the Kofas where you might be able to prospect with permission.
In my article, "Gold Deposits of the Stanley Area" (April 1989, CMJ), I inserted a statement I found in an early Idaho Inspector of Mines report. The Inspector of Mines made yearly trips to inspect operating mines and report on their production and progress. Many states had an Inspectors of Mines, and their published reports contain some very worthwhile information. The Inspector in his report stated: “A prospector accompanied by a young boy, packed into Hailey last summer two horseback loads of ore amounting to 475 pounds in weight containing substantial values of gold and silver. The assay results on the lot yielded at the rate of 1020 ounces of gold [per ton] and 450 pounds of silver per ton.”

The Inspector was certainly being conservative when he said “substantial values,” but in the early days, mining 1000 ounces of gold per ton of ore was not that unusual in handcobbed ores. Today, a backpack load of that ore would be worth a small fortune not only for the gold and silver, but for the collector or lapidary value of the visible gold and silver in the ore.

The miner knew better and did not divulge where he mined the ore other than to indicate it was somewhere in the Stanley area. But there was a clue found in the assaying of the ore that could lead to the site. Tellurides, which are relatively rare gold compounds, were found in the ore. Find the area where tellurides were found in one of the Stanley area mining districts and you may have the location. These are the kinds of clues you will be looking for in historical data that may lead to little-known mineralized areas.

Here is another example of possibly worthwhile information that was found in a recent publication on Nevada mining and geology: “Possible placers may occur northwest of Aurora buried beneath Quaternary [typically, the latest sediments on the surface] gravels. The deposits are localized along a lineament [a topographic feature that is believed to reflect geologic structures such as faults]. This lineament is believed to have channeled major stream drainage out of Aurora. The best placer site lies at the junction of the lineament with a possible buried topographic high of volcanic rocks that could have acted as a barrier to the streamflow.” Hopefully they would show on a map where they thought the best site was, but in any case a map would likely show where the buried volcanic rocks were located. This is typical of some of the information to be found in the geologic literature.

Other Minerals and Values. Gold is not the only valuable mineral to be found in mining districts. Do not overlook fine crystals that can be sold to collectors, or gemstones such as sapphires, gem garnets or aquamarines that can be found with placer gold. The same applies to lode gold. Heavy gold in quartz can be acidized by an expert so that the wires and sheets stand up as a fine and very valuable specimen. It is worth many times the value of the gold. This is also true of gold in quartz or other rocks than can be cut and polished for lapidary uses. If you were to find native gold in a black rock that could be polished, the material would be extremely valuable. Do not discount the other minerals and other values of gold.
Contrary to what you have may read, black sands have little value by themselves. When you see someone advertising that they wish to purchase your black sands, what the buyer is looking for are the gold values that you failed to recover. The black sands themselves could only be valuable in large quantities to a company that is equipped to process them for other minerals.

Trends. Trends were described earlier. The geology and mining bulletins often go into detail describing trends and showing them on maps. Remember that it is possible to find gold anywhere within these trends by detecting, by looking for the signs of mineralization, and then by sampling.

—to be concluded next issue

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Lawrence Dee is a retired geologist, and former District Geologist with the BLM, turned prospector. He’s a lecturer and instructor in placer mining for gold and local area geology.