Dealing with Black Sands:
Capturing the Gold

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Every placer miner has to deal with it in one form or another, and some locations have it far worse than others, but nearly every placer has at least some of it: black sand. It collects in our concentrates and gets in the way of recovering our fine gold. Some prospectors worry about getting every speck of gold and save it in buckets until they have tons – others take all the gold they can conveniently remove and then toss out the rest. So what is this mysterious stuff and what should we do about it? Lets take a look at black sands and the best ways to extract all the gold that is reasonable to get, and our options for recovering that gold.

The sand, rocks and gravels in the placer deposits we work have a density of about 2.5 to 3. Gold on the other hand, has a density of 19.3 when pure. That difference is what makes all the gravity methods of capturing gold work – whether it’s a simple pan, or a sluice box, dredge, dry washer, table or jig, that big difference in density is what makes working with placers pretty easy. However, these gravity-based methods are not just specific to gold – they capture all the heavy materials in the placer and concentrate them, including black sands. The frustrating thing about black sand is that while it is relatively easy to separate the normal sand and gravel from the gold, it much more difficult to separate the small sized gold from the black sand and other concentrates. There are many methods to accomplish this job – some a lot more efficient than others.

In addition to magnetite and hematite, black sands may also contain small amounts of a lot of different heavy minerals depending on the rock types in which you are working. Heavy concentrates may or may not contain at least some of the following: chromite (an ore of chromium), Diamonds, platinum, gold bearing amalgam (lost by early day miners), lead (from fishing weights and bullets), floured mercury, cinnabar (a natural mercury ore), sheelite (tungsten ore), zircon (a rare gem with a density of 4.7), garnets, pyrite and barite. The proportions that you may find of these minerals or man-made products depend on the specific location where you are prospecting. Some of these things, like diamonds, platinum and amalgam, are valuable in their own right. Mercury and amalgam may contaminate the gold you recover in your sluice box.

The amount of black sand and other heavy minerals, which you find associated with your placer gold, varies a lot – there is not a simple direct correlation. There are places that are loaded with black sand but have no gold. Conversely, there are also places that have good quantities of gold but very little black sand.

It seems like at least a little black sand is found practically everywhere, so perhaps the next question is “where is all this black sand coming from?” As I noted iron is the fourth most common element in the earth’s crust, so some iron-bearing minerals are found in most rock types. They are especially common in metamorphic and igneous rocks. Since gold deposits are often associated with igneous and metamorphic rocks, it’s easy to see why we nearly always find at least some black sands in our placers. So now that we know a bit about this stuff and where it comes from, what are we supposed to do to get the gold out of it?

First, many new prospectors, believing that it somehow contains large quantities of “hidden” precious metals, find black sand mysterious and incomprehensible. Often this leads to the promotion of expensive and highly questionable treatments to “unlock” the “invisible” values of black sand – including “special” assays, strange leaching techniques, and “magic” fluxes. The truth is that most of the values in black sands are simple free gold particles, which are recoverable by standard gravity recovery methods. Any values truly locked into the black sand, surrounded by minerals such as magnetite so that it cannot be recovered by normal gravity separation systems, are typically very small. The amount of any truly “locked in” gold can be scientifically determined by very meticulously recovering all the free gold possible from a sample of black sand, making sure to carefully examine it under 10x magnification to see that all the free gold has been removed, and then take that clean black sand and run a standard fire assay on it. Do an assay before you invest money in any expensive treatments, because in nearly all cases, this assay will show it is not economically worthwhile to pursue any further treatment of the black sands, especially for the small operator.

The answer as to how to best deal with black sand depends on how much you are generating. If you are an individual prospector who dredges or dry washes during weekends or while on vacation, you might generate 20 to 200 pounds per year. If you just snipe with a pan or use a hand operated sluice box, the amounts will be even less. On the other hand, if you are a large commercial operator, you may produce tons of black sand, and the use of more sophisticated equipment such as shaker tables will be justified. Lets take a look at both possibilities:

Dealing with Small Quantities of Black Sands: This is the category into which most prospectors and weekend or seasonal miners fall – including me. I normally spend a week or two per year dredging, and combined with my other prospecting, I generate something around 20 to 40 pounds per year of black sands that need to be processed. Here are the techniques I use to go from concentrates fresh out of the sluice box, all the way to clean gold.

At the end of each dredging day, I clean up the front section of my 3 stage sluice box. About every other day I clean out rear top section, and about once per week I clean out the undercurrent section of the sluice box. Many new miners, worrying about loss of gold, clean up too often and this takes away from the time they could be dredging and finding more gold. Sluice boxes on newer dredges are well designed, and if your sluice is set up so that the gravel is not accumulating so much that it buries the rifles nor running so fast that it sweeps the rifles and carpet clean, then once a day clean up is all you should need on most store bought dredges. When operating properly, your sluice will have about one inch of carpet showing between the gravel of the middle riffle of the sluice and the one below it.

There are of course, a number of ways to go about doing this final cleanup. Some folks use mercury to get the small gold. I have done this in the past, but with the advent of the super magnet and the method outlined above, in my opinion, mercury is simply no longer necessary for the individual prospector. I like to stay away from the use of mercury, because the regulatory agencies go so crazy about it. All in all, it is faster not to use it. There are spiral panning wheels, micro sluices, tables and other devices for final clean up, they can do the job and all have their proponents, but I use a blue bowl, mostly because it is cheap and it works.

Sometimes (actually fairly often) my gold needs to be treated to remove mercury stains that either happen in the river or during recovery and processing. I prefer to use nitric acid for this, but some guys also remove mercury by heating. Mercury vapors are toxic, so do this outside and be careful – also be sure all the lead has been removed before heating, or you will have to use nitric acid to separate the melted lead from the gold.

Dealing with Larger Quantities of Black Sands from Commercial Operations:  Even small commercial operations may generate more than 50 pounds of black sand per day, and processing equipment beyond the scope of the individual prospector is normally well justified. These operators commonly make use of shaking tables, large spiral wheels, mercury amalgamation and other commercial equipment to handle these larger volumes of black sand concentrates.

It is always important for the commercial operator to keep close tabs on his recovery equipment and do regular testing to make sure it is working properly. If this is not done, the operator may well be loosing significant fine gold. During the early 1980’s I had the privilege to inspect and evaluate the operations of St. Joe Minerals’ bucket line dredge operation at the Yuba Goldfields near Yuba City, California. At the time, they delivered their entire finished black sand product to the property owner after they had extracted all the gold that they believed was present in the material. It turned out that they were not keeping good controls on their processing. The property owner simply had one of his employees re-run the black sand over a finishing style shaker table, and he recovered additional gold, in the amount of about 3 ounces per day.

Large operators who generate many tons of black sand each year may wish to consider additional steps beyond tables or spiral wheels such as reprocessing after crushing the black sands or leach extraction by cyanide.  However, these options should be considered only after assaying the finished tailing material to see if cost and effort of any additional processing is justified. With the continued regulatory concern over mercury, it has lost much of its traditional place as a method of recovering fine gold, but it is still used, especially in less economically developed countries. For commercial operators in the US, mercury can be a significant environmental liability, as even comparatively small spills may be quite expensive to clean up.

Mercury is still used by some small prospectors, and if you do choose to use it be sure to take all necessary precautions. The use of mercury potentially involves some inherent risk to your health if not done properly. In the final step, the mercury is distilled in a retort in order to recover all the dissolved or amalgamated metals. The fumes are toxic, and mercury chemicals can be absorbed through the skin. Once Mercury is absorbed into the body, it is retained for a long period of time and it attacks the central nervous system. I used to use mercury to recover my fine gold, but the advent of the rare earth super magnet has convinced me its unnecessary for my efforts to separate my beautiful placer gold from those nasty black sands. For reference, see my page on using Mercury Amalgamation to Recover Gold.

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