Small Scale Mining Info

Generally, the beginning and the end of most small scale mining attempts, is the prospecting phase. This is because most prospectors are not geologists and have not had the experience and education required to comprehend the nature and origin of mineral and metal deposits in the earth.

I have seen too many so called "mines", particularly gold mines, that had a "lot of gold" based on a hand full of rocks sent to an assay lab. Generally, these "mines" turned out to be rock collections, and the assay lab a bunch of quacks or Frauds, pretending to be assayers. The second rule of prospecting is if the deposit is not economically viable, it is a Rock Collection. Economic viability simply means that it costs less to extract the valuable mineral or metal than the value of the mineral or metal extracted. Without the knowledge of a mining engineer and a metallurgical or process engineer, the determination of mining and processing a deposit is somewhat of a Wild Guess.

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Now for the first rule of prospecting, Finding a Ore Deposit. In the Gold Rush years, Geologists and Prospectors had similar results in finding gold and silver deposits, because for one, geologists of a hundred or so years ago, did not have the knowledge and understanding of today's geologists, and two, there were many, many more prospectors roaming the mountains and deserts. Quite often, a nugget, or a large rock with gold in it was discovered lying on the ground or in a stream, and Eureka, the rush was on. The land was covered with a swath of humanity, looking in every nook and cranny for gold or silver. No wonder they found some. However, we only hear about the five or ten that actually found the Motherload, not the tens and hundreds of thousands that found Rock Collections.

So, what does this have to do with prospecting today? Today, most prospectors actually think they can find gold the same way the gold rushers did. It will simply be lying at their feet. Some good advice, purchase a lottery ticket, it has better odds of hitting the Jackpot. If, however, you enjoy the outdoors and digging around in streams mountains and deserts, try to absorb as much geologic information and mining historical information in the area targeted for prospecting as possible. The Department of Mines and Geology of the state is a good place to look for information. Geologic maps published by the US Geologic Survey is another. Professional Mining organizations also generally have published books and material available for purchase. Stay away from "Bob's Gold Prospecting Book", because Bob probably never saw any gold either, and hasn't the foggiest notion of finding it, but is all too eager to "share" his knowledge, for a price, of course.

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If you are not discouraged already, then read on. If gold panning or recovering gold from a stream is what you are thinking of, then realize that water flows Downhill. If there is or has been gold mining and gold deposits at higher elevations in the watershed of the stream, then there is a good chance that some gold has washed down from those mountains, and will be in the stream. So you not only have to be concerned with the immediate area, but most likely, the areas upstream are more important.

If you are looking for Vein ore, then you have a much more complex problem. The rocks or the strata of the earth was mineralized in very many different ways, and how, or more importantly, If, the strata was mineralized, will determine where to look. Most prospectors do not possess the professional knowledge to delve into these scientific facts of geology, and should concentrate on a very oversimplified model. Gold, and silver were generally deposited by either volcanic, or molten metal laden material flowing up through the fissures of the earth, or in some cases by hydrothermal "steam" flowing from deeper in the earth. Both carried the metals and minerals towards the surface, or deposited them along the way. Hydrothermal deposits tend to have very fine particles of gold disseminated throughout a somewhat fractured or porous rock formation. You will find few, if any nuggets here.

How does this relate to looking for a vein? Well it is not easy, unless the vein actually "outcrops", or has a portion exposed at, or near the surface. Even then, most gold is not really visible to the eye, many geologists will admit to rarely seeing nuggets in the largest gold mines. So what do you look for? Well, most gold and silver is found in quartz or igneous deposits. Some gold is found in various sedimentary deposits, as well, including igneous intrusions into sedimentary deposits called "dikes", carbonaceous rocks, silts, even in limestone. However for the purposes of simplicity, ores have many other minerals, like magnetite, rutile, ilmenite, garnet, etc. in them, and these minerals tend to color the ore dark. Sulphide ores tend to be gray in color, while oxide ores tend to be tan to light brown in color. Visually, the gold bearing ore will generally be darker than the barren ore, due to the other minerals and metals present. These minerals could be called "indicator minerals" if their presence signals the presence of gold or silver.

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So generally speaking, the darker colored rock formations stand a better chance of having gold. Silver is found at much higher concentrations than gold, that it is usually visible to the eye (if it is native silver (Ag). However, silver can be found in high concentrations in a complex form and not be visible. The average prospector need not concern themselves with this, since they would not be able to identify it in the first place. One of the highest grade silver ore specimens I saw was from Australia, and it truly glittered (almost 100 ounces per ton).

Very High Grade Oxide Gold Ore, Assay 5-15 Oz/Ton 
When one locates a promising vein or mineralized formation that may be ore (as opposed to rock), the next step is to take some samples of the formation. Again, a prospector without professional training of geology will have difficulty in determining where to take the samples, because they will not have the fundamental knowledge of the rock formations, how they were mineralized, and the statistical theories of sampling to obtain a probable representative sample. Most prospectors will take a few samples from one or two spots and have them analyzed, if nothing is there, they will move on to another site. Most mineralized formations have barren areas (containing no material of value), and the mineralization can be in a pattern, widely distributed, or very random and sporadic. One method of reducing the chances of missing the valuable minerals in the formation, is to use a good sampling technique. For most small scale prospectors, drilling is out of the question due to cost factors, so they must rely on field samples from trenching, channeling, of chipping away a section of an exposed outcrop. In general, more samples from varied locations is better than fewer samples. The vein, or mineralized orebody should be identified, first, and uncovered or exposed so samples may be taken. Samples should be taken across the cross section of the vein of orebody, and they should be marked with the location (handheld GPS devices are ideal for determining the location of the sample spot) and placed on containers for processing later. Small scale prospecting means concentrating on a small area at a time. This requires less man hours and is manageable by a few people.

Trenches along a vein should be dug about every 50 feet, until the extent of the vein is uncovered. The dip of the vein (angle the vein is to the surface) and the general strike (general trend or direction the vein goes) should be determined as trenching progresses. This will help determine where the next trenching goes, and help locate the vein or formation on a map.

Core drilling is the best method to explore a formation for minerals or metals. This method is usually too expensive for small scale prospectors, however if trenching proves the deposit to have enough value, this should be the next step in exploration before mining is considered.

Once the samples have been collected, marked and stored, it is time to take them to either a commercial laboratory, or prepare them for assay in a field lab setting. They should be crushed in a jaw crusher, then pulverized, using a pulverizer to 100 mesh or finer. Then one of the most important steps is splitting the sample to obtain a 30 gram sample to send to the assayer for each sample of ore. The initial sample size may be 50 - 200 pounds. But assayers generally test 30 grams. All of the sampling is wasted if the pulverized samples are not accurately split, to obtain a representative sample for assay. Simple Jones type riffle splitters will give descent accuracy for field work. It is laborious, but the 50 pound sample is split 50-50, and one half is split again, reducing it to 25 pounds, then 12.5 and so on until a 30 gram sample is reached. It is probably desirable to only split the sample to 100 grams or so in the field and have the assayer split the 100 gram sample to extract the 30 gram for assaying, when working in the field. Once the assay splits are obtained, they should be sealed in plastic bags, marked with identifying information, and stored for sending to the assay lab.

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Selecting a Reputable Assay Lab is very important, since some studies have shown that about 50% of the assay labs in the West are either incompetent or dishonest. In either case, you pay the price. A lot of prospectors like to hear good news that they have 2 ounces of gold per ton and some labs will tell them this to keep them coming back, until they go broke, wasting their money trying to mine a rock garden. So, by all means, select a reputable assayer. If in doubt, generally the Department of Mines in the state can give some recommendations. I can give several:

ALS Chemex
994 Glendale Avenue, Unit 3
Sparks, Nevada, 89431-5730
Phone: (1) 775 356 5395
Fax: (1) 775 355 0179  
Nevada Bureau of Mines and Geology
Analytical Laboratory

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