Rebellious Ores: Sulfides, Selenides and Tellurides

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The early day miners feared it, and it happened a lot more often than they would have liked: the easily worked rich surface ores they discovered on the surface gave way as they mined downward, to a vein full of sulfides, base metals and other nasty, rebellious and refractory ores.  In many locations where they had initially been recovering 90% of the assay values processing oxidized surface ore, with the rebellious sulfide ores they were recovering less than 50%.  The rest of the precious metals in their ore simply flowed uselessly out from the mill and into the tailings pond. Many rich and profitable mines suddenly became money losers when the miners got down to those unoxidized, primary ores. That's why the miners called them rebellious. 

This sad story was repeated all over the Western US and actually all across the world.   Over the years, miners worked to develop new ways to handle these difficult ores, and they made progress as time went on, but even cyanide doesn't normally do as well in processing sulfide ores without some special efforts.   Sure, today we've got some new technology, and there is no doubt we’re a lot better off, but sulfide ores still require a lot of extra treatment and handling to extract their values – which adds a significant cost. Let's take a look at some of those devilish minerals that make us work extra hard.

Sulfides

Sulfides are the most common member of this mineral grouping and most prospectors have seen them at one time or another. The familiar "fools gold" is iron pyrite. Pyrite, galena, sphalerite, and chalcopyrite are all common sulfide minerals that are found in many precious metal veins and other hard rock deposits.  When metals combine with sulfur, the resulting mineral is called a sulfide. These minerals are the most important sources of most base metals like copper, lead and zinc. Most have a reflective metallic sheen, like lead sulfide, also known as Galena. There are also sulfur minerals which contain more than one metal combined with sulfur such as parargyrite (ruby silver), bournonite (peacock copper ore) and tetrahedrite. Most sulfides contain an excess of sulfur more than what their chemical formula would imply. As an example, the chemical formula for galena is PbS. That means there is one atom of sulfur to each atom of lead. However in reality, galena crystals usually contain considerably more sulfur than a 1 to 1 ratio with the lead would call for. The same is true for pyrite and many others.

Sulfides are generally fairly dense and heavy, so they can be recovered by simple gravity methods.   Vibrating tables are sometimes used for this purpose and the sulfides will usually show up in the table middlings. However, in most commercial milling operations, sulfides are recovered by a method called flotation.   This technique employs the fact that sulfides are not easily wetted by water. As a result, in a froth of tiny bubbles, they will tend to stick to the air bubble.  The froth of tiny bubbles with their concentrate of sulfides can then be separated off and treated. Different minerals have a preference for different types of conditions in the flotation cell, so by planning the chemicals used to make the froth, the ph of the water and other factors, it is possible to separate differing sulfide minerals out of ores which contain more than one type of sulfide. This is extremely important in base metal ores which contain multiple valuable metals.

Selenides

Many selenide minerals have often gone unnoticed in years past, an in many cases, selenium is present as traces in normal sulfide minerals.  Only in recent times have mineralogists routinely recognized the difference between the standard silver sulfide acanthite (argentite) and the lesser known silver selenide, naumannite. In recent years, naumanite has been found to be an important mineral in low sulfidation epithermal bonanza grade ores in Nevada. The rich Nevada ore deposits at the Ken Synder mine at Gold Circle, the bonanza deposits of the Sleeper mine in Humboldt County and the deep epithermal veins at the Holister mine all contain naumannite as an important ore mineral.  Many silver bearing deposits in Nevada previously noted to contain sooty black deposits of silver sulfide, may actually be rich in silver selenides, especially naumannite, because of that mineral’s similar fine grained black appearance. While sulfur and its associated chemicals are toxic enough, selenium is even worse.

Tellurides

Tellurides are minerals containing tellurium. While it is rarely found in nature as the native element, tellurium is one of the very few elements that chemically combines with gold to form natural stable minerals. While not common on a world wide basis, locally rich telluride deposits have formed some very famous deposits. Gold bearing tellurides were important in some of the richest ores at Goldfield, Nevada – high grade ore that contained upwards of 10,000 ounces of gold per ton. If you have tellurides in your ore, it’s important that you are aware of that fact, because it affects how you will process it. Tellurides are leachable by cyanide but can take an extended long time period, as the telluride minerals have to be dissolved completely by the leaching solutions. There are a several telluride minerals that are of interest to prospectors and miners – here are a few of them:

Calaverite: This mineral is a combination of tellurium and gold. Calaverite is the name of a tellurium bearing mineral that occurs in certain low temperature hydrothermal veins, but is also sometimes found in higher temperature veins. It was first discovered in Calaveras County, California, but is also important at Cripple Creek and Telluride Colorado, as well as Kalgoorie, Australia. It is brittle and has a brassy gold to silver white metallic sheen. It’s appearance sometimes confounded old time prospectors who confused it with pyrite or arsenopyrite. It commonly occurs in the presence of sulfide minerals. Unlike Pyrites, Calaverite contains significant gold - about 40% gold by weight. With a 40 percent gold content by weight, you can see it wouldn’t take much calaverite to make for some very rich ore.
Sylvanite:   A rich telluride of gold and silver. After calaverite, sylvanite is probably the next most important telluride ore mineral. It can assay as high as 30% gold. It is found with other telluride minerals in some precious metal ore deposits.
Petzite: A gold and silver telluride that is mostly silver. Silver content is usually three times that of gold.
Nagyagite: This is a complex mineral containing gold, tellurium, sulfur, lead, antimony and bismuth that occurs in epithermal veins.
Hessite: This mineral is a silver telluride.
Coloradoite: A rare telluride of Mercury, but commonly occurs with deposits of precious metals.
Altaite:    This is a rare telluride of lead, but can contain a fraction of precious metals.

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