More on Gold Prospecting: Sulphides and Gold

("Peacock" rock; in this case copper sulphide ore containing small percentages of silver and gold as well.)

No Dummies or Fools

The oldtimers were no dummies or fools, despite their lack of formal education. One important piece of knowledge that most old-time gold prospectors gained over time was an intrinsic understanding of the significance of iron, copper, or arsenic sulphides (FeS, CuS, As) in the formation of gold (Au) ores.

Placer Gold Locations

Typically the oldtimers would keep a practiced eye out for various signs of sulphides, including heavy iron staining of nearby country rock, especially if this staining was widespread in reddish, rusty looking rock called "gossan." Additionally, the presence of iron pyrite (FeS2) crystals or multi-hued "peacock" rock (see image above) were solid indicators of the presence of gold, silver, or copper in various combinations and amounts.

Some of the Largest and Richest Deposits

Some of the largest and richest gold vein deposits in the American West (as well as in other parts of the world) were formed in sulphide geologic "zones" where pyrites or chalcopyrites (Cu2O) predominated. In the former, gold values could run into multiple troy ounces to the ton of material processed and in the latter, a quarter troy ounce per ton.

For large-scale lode or open-pit mining operations, these are economically feasible gold values where money can be made. But we are individual miners, not large mining corporations. The important fact to carry away here is an understanding that sulphide ore bodies may lead us to gold in one way, shape, or form.

Au, Ag, and Cu in Combination

Another important aspect of sulphides is that they are often associated with silicates like quartz or other forms of host rock containing quartzitic elements. But don't get stuck on this idea of quartz being the only host for sulphides, because it isn't. Keep your eyes peeled for the main sulphide "aspects" (pyritic crystals, iron staining, "peacock" rock, etc.) when you are out and about in the field.

Wolverine Boots
Dickies Work Clothes

Quite often, sulphide ore deposits are polymetallic, meaning simply that they can contain a variety of metals in combination. These can include precious metals like gold, silver (Ag), and copper in all sorts of percentage combinations with any one of these three metals (Au, Ag, Cu) predominating.

"Free-Milling" and Refractory Sulphides

Sulphide ores can contain gold in various forms, including those that are "free-milling" and the harder-to-process refractory types of ores and ore bodies. As small-scale or recreational gold miners we are most likely to spot and exploit "free-milling" ores where the gold particles are visible to the naked eye or under slight magnification.

Small motorized rock crushers or even a strong mortar and pestle can be used to crush pieces of free-milling sulphide ore for their gold content (although really nice examples of free-milling gold ore bring higher prices as specimens). On the other hand, refractory sulphide ores contain gold in chemical form and require leaching or other types of chemical processing to extract gold values. They also require more than just your eyes or a jeweler's loupe to identify.

One last item to remember about sulphides. They are usually found in areas or locations where very strong mineralization has taken place as a result of major geologic alterations in the earth's crust.

Good luck out there!

If you liked this post, you may want to read: "More on Gold Prospecting: Modern Methods (Part 2)"

(c) J.R. 2010

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