Most prospectors, when they think of hard rock gold deposits, think of quartz containing particles of gold. While gold deposits can be things besides quartz veins, gold is certainly found in the quartz of veins cutting through various types of rocks. Gold in quartz veins occurs as particles and scales scattered through the quartz, often filling cracks and openings in the vein material. The gold can be such small particles as to be invisible to the naked eye, or as larger aggregates of easy to see blobs, leaves and crystals in cavities in the quartz. The gold fills cracks and spaces in the quartz as the ascending gold-bearing solutions infiltrate the existing quartz vein, filling in the last remaining openings.
Gold in quartz can be really beautiful stuff, but there are times when it’s hard to know what you have. Sometimes it’s very easy to recognize gold, but that’s not always so. Various sulfide minerals like pyrite and chalcopyrite (copper-rich pyrite) can fool people, especially when the crystals are very small. While gold normally looks different from sulfides, at times it can take some experience to spot native gold in hard rock ores, especially when pieces of gold are small. It can be even more difficult if the gold and pyrites occur together, which they commonly do. All I can recommend in these situations is to practice looking at different types of free gold ore and to gain experience seeing the difference between gold and pyrite minerals.
One or more sulfides—such as pyrite, arsenopyrite or chalcopyrite—are almost always associated with the gold in quartz veins. Extracting gold from within these minerals can be more difficult. On surfaces exposed to the effects of weather, pyrite minerals rust out under the influence of water and oxygen, and the iron stains the quartz, sometimes leaving voids or a porous structure where the pyrites were originally placed. Small particles of gold can be disseminated through pyrite (FeS2) and sometimes other sulfide minerals. When iron sulfides are oxidized by weathering, the gold is not affected and is left behind in the metallic state and much easier to recover. If the chemistry is right, a portion of the gold will be dissolved and carried downward and precipitated, enriching the deposits below.
Quartz veins often fluctuate widely in value, with the richest gold ore occurring in pockets or “bonanzas” as they are sometimes called. In some mining districts the bulk of the gold is found in these rich pockets and the rest of the vein may be nearly barren. Where the normal gold concentration might be a tenth of an ounce per ton or even less, a rich pocket might contain 20 or even 100 ounces per ton—and sometimes even more. In some cases the locations of these pockets can be predicted, but in most places the miner can only drift along on the vein hoping to encounter a “bonanza” without knowing that the next pocket could be two feet or two thousand feet away. Pocket mining can be incredibly frustrating!
It is very possible to miss rich concentrations of gold quartz when mining underground. As an example, I recently had the experience of testing a mine famous for its rich gold quartz pockets, when a possible gold pocket was pointed out in the hanging wall of the vein. I did not discover this pocket, but could see it was in an unexpected spot that was missed by the old timers. It later yielded a considerable amount of gold.
It is very common that gold will concentrate in one of three locations within a quartz vein. These are the hanging wall, the foot wall, or right down the centerline. Always be sure to check these three locations with a metal detector when in a hard rock mine searching for rich pockets.
Prospectors prize beautiful gold and quartz specimens and they are a rare find. When the gold is distributed through the quartz in a balanced way that appeals to the eye, the quartz can be cut into a gemstone and fitted into an exquisite piece of jewelry. The popularity of gold in quartz reached its height in Victorian times, perhaps 125 years ago. To cut a gem of this type, the quartz is cut into slabs to expose the gold. These slabs are evaluated by an artisan, and then the best areas for a gemstone are selected, cut and polished. These gems are set in jewelry to create a natural, one-of-a-kind piece. Because this type of gem is so rare, most consumers have never seen gold in quartz jewelry.
Because quartz and gold may be deposited together across a considerable range of temperatures, not all quartz veins are the same, and there are several different types of gold-quartz deposits. Those deposits formed of solutions that flowed at higher temperatures are distinguished by their own suite of gangue minerals such as tourmaline, apatite, garnet, biotite and amphiboles, in addition to the quartz. Good examples of this category include the gold-quartz of the Appalachian ranges of the eastern US and the veins of the Brazilian gold fields.
Those quartz veins formed near the surface often have cooler temperatures, typically between 100° and 200°C. Gangue minerals associated with quartz in these low temperature deposits include calcite, adularia, sericite, alunite and similar minerals. This category is typified by gold-bearing quartz veins like those of the Comstock Lode at Virginia City and at Tonopah in Nevada, at Bodie in California, and northwestern Hungary in Europe. These veins normally form in eruptive lava rocks like andesites and dacites of Tertiary age.
Between the two extremes stands the large and productive group of gold deposits whose characteristics point to a geological environment of formation at considerable depth, high pressures, and moderate temperatures of perhaps 200° to 300°C. The veins of these intermediate deposits are characterized by milky, coarsely crystalline quartz, in places associated with drusy and crystalline voids, but only rarely showing a comb-type structure. The gold content is found in the form of free gold and gold-bearing simple sulfide minerals such as pyrite or arsenopyrite. These mesothermal gold quartz veins occur in many places across the globe and well-known examples include the Motherlode gold-quartz veins of California, the Golden Triangle of eastern Australia, in Western Africa, and many localities in the Rocky Mountain region of North America.
Quartz veins can have a variety of differing structures depending on the environment of formation and the host rock. The most common type of quartz vein is the simple filled vein, which could have a thickness ranging from a small fraction of an inch up to monster veins of quartz with a thickness of 20 to 30 feet and sometimes even more. It is a sort of irony that most of the largest and widest veins tend to be low in grade. The old-time miners called these large masses “bull quartz” or “bull veins” because they were so poor in gold.
Another type is the composite vein or lodes in which zones of country rock are filled with a mixture of quartz and rock, with branching small veins and veinlets of quartz sandwiched between altered slabs of country rock. Sometimes these veins are broken and folded by movement along the fault zone that provided the conduit for the mineralizing fluids to flow. Another type of gold-bearing quartz forms when porous bodies of rock are changed by replacement into gold ores. These are often structurally controlled by their proximity to faults that provided the conduit for mineralized fluids. The gold-bearing quartz seams may follow joints formed in certain directions of shear within the large masses of rock; or in the case of limestone, the quartz may fill areas eaten away by the mineralizing solutions replacing the rock as a whole.
Extracting Gold from Quartz
Once the prospector has located a good deposit of gold-bearing quartz, the next goal is often extracting the gold from its host rock. Mankind has developed many ways of treating ores as there is no one universal method of extracting the gold. Gold has been recovered from some pretty lean ores, being profitably extracted from ores yielding less than two hundredths of an ounce in a ton of rock. However, this is the result of work done on a gigantic scale like the open pits of the Carlin district in Nevada. For individual prospectors and other small operators, gold-bearing quartz ores need to yield at least a quarter ounce to the ton to make the effort worthwhile. In many cases, the gold minimum content will be even higher, depending on the circumstances of the miner, the capacity of his equipment and the type of ore processing he needs to do to recover it. Normally the small-scale miner is seeking free gold and often does not have the capacity to recover gold from difficult to work sulfide ores.
Free gold in quartz veins comes in all sizes, with a full range of gold particles from obvious stuff easily visible to the eye down to gold that can barely be seen with a magnifying glass. The coarser stuff can be found with a metal detector, while the smaller sized gold requires that the rock be crushed and the resulting powder panned. The coarse and easily visible gold in quartz may be valuable as a specimen or for making gold-quartz gemstone jewelry. Knowing just how much gold is in your quartz is important, and I’ve written a sidebar article on estimating the gold in quartz (see "Determining the Amount of Gold in Rich Ores").
Ores that do not contain obvious visible gold or enough to sound off on a metal detector can still be quite valuable and well worth working. Crushing the ore can free the gold and allow recovery by gravity-based methods. In fact it’s very common that the gold in hard rock ores will range in the 30- to 200-mesh size range with most of the gold in the lower end of that range. The tried and true method for testing these ores in the field is hand crushing and panning. Pan testing for free gold was done extensively by the old- timers all over the world. Yes, there are some ores for which pan testing will not work, but with most prospectors using some form of gravity-based separation, anything that can be recovered by gravity methods can be test panned and will be visible even if magnification is required to see the gold. If it’s too small to see with a hand loupe, it’s going to need something like cyanide to recover it and cyanide is beyond the scope of most prospectors.
Free gold taken from hard rock ores always contains at least a little silver, with the average gold fineness typically being in the range of around 80 percent, but it does vary quite a bit. Any associated sulfides in the ore are likely to carry more silver in proportion than the native gold. Some types of quartz veins can carry a significant amount of silver, with the greatest amounts found in the epithermal veins deposited near the surface in Tertiary lavas in places like Nevada and other western states. Recovering any silver values from the contained sulfides would be a lot more work and beyond the capacity of most individual prospectors.
If you recover and want to process only a few pounds of higher grade ore now and then, you might use a simple cast iron mortar and pestle to grind up the rock and a gold pan to process it. However, if your ore amounts to hundreds of pounds or even a few tons, you will definitely need some sort of crusher and recovery system. The kind of equipment you might consider for crushing and processing your ore depends greatly on the nature of your ore, your budget, and the amount of rock you have to process. The difficulty with recovering free gold from hard rock ores has always been the cost of constructing a mill and operating it. With the portable chain and hammer mill crushers now available, the cost for the small miner can be pretty reasonable.
I own both a little jaw crusher and a small chain-based impact mill. My little jaw crusher can quickly break bigger chunks down to pieces of less than 1/8 inch, and the impact mill can grind the ore down to minus 20 mesh with the bulk of the ore being less than 50 mesh. There are a number of different systems for breaking rock out there, ranging from hammer mill crushers to rollers, cones and even small rod and ball mills. Normally one crusher is used for the initial primary crushing and another for secondary crushing down to a smaller size. (Silica quartz dust in large amounts can be a genuine health hazard, so take care to control the dust or use a respirator to keep down the amount of dust you inhale.)
When pulverizing ore, one of the most important questions for the operator to determine is how small should the miner grind the ore? The smaller you crush the ore, the more time, money and energy it takes to accomplish the task. In many gold ores, the majority of the gold is located in cracks and the space between the quartz crystals. This happens because gold is often deposited in the last phases of vein formation. The result is that the first crushing step will yield the most gold, with each pass after that yielding less. Each step is much more work and yields diminishing returns. It is also possible to crush too small and create slimes (minus 200 mesh) so fine that gravity methods cannot recover the gold within them. So, with every ore and equipment set up there is an optimum size range of crushing that will yield good gold and not require excessive work. Determining what that is sometimes requires some testing of the different size range possibilities.
When you are doing multiple steps in crushing, you want to screen the ore and remove the fines that have already been crushed sufficiently before moving on to the next crushing step. Usually most prospectors will screen to something around 30 mesh, but under special circumstances you may wish to go even finer. I recommend miners not go too much smaller—remember, the old stamp mills often had exit screens of around 20 mesh. As I have noted, it is possible to crush too fine and lose gold. I have known of prospectors who merely screened out the fines from old mine dumps and processed them, and they did fairly well for their efforts.
Once the crushing is complete, the final step is to process the material and recover the gold. Most prospectors will choose some gravity method based on the density of the gold. There are many types of equipment, and the selection depends on your budget and on the amount of ore you are planning to process. With the smallest amounts, a simple gold pan is normal, but with greater quantities of ore to be worked, processing equipment with a greater capacity like centrifugal bowls, spirals or shaker tables are better and provide the ability to process much more material. Some third world countries even use sluice box systems. All recovery equipment has its advantages and disadvantages. The gold concentrate is then dried and melted into bar or button form after which it can be sold to a refiner.
Commercial crushing and recovery systems come in all kinds of sizes from small to very large. The free gold extraction process can be scaled to pretty much whatever you would like and can afford to purchase. Depending on if it’s your intention is to process a few pounds per year or a few hundred tons per year or anything in between, the equipment you need is out there and available.
Ore processing equipment will be on exhibit at our Gold Prospecting and Mining Summit in Placerville in April—just one more reason to come and see this event. It is true that once you get to processing large enough amounts of ore you are likely to need environmental permits, etc., depending on the location you are working in. Most prospectors will choose to stay small, but even on a small scale, you can be processing gold-quartz ores recovering some very good gold and getting into real hard rock mining.
“Much of the ground where Ms. Hollingshead found her diamond is made of unweathered volcanic rock. When it rains, flowing runoff often leaves loose gravel, and sometimes diamonds, on the surface in these areas.”
Black diamonds, also known as carbonado, are extremely rare, and are found naturally only in Brazil and Central Africa. The cosmic origin theory is based on their carbon isotopes and high hydrogen content.
While recovery rates are important, they must necessarily be secondary to the volume of material processed. Running more material at lower recovery rates is generally preferable to increasing the efficiency of the system.
October 2012 I decided to focus my attention on some exposed bedrock that was along a little stream only about 100 yards from our cabins. My very first pan full of gravel showed several nice small flakes that were just big enough to be picked up with my fingers. That first pan showed that indeed there was some very nice gold to be had here...