(Close up of gold ore from South Dakota.)
This is the 4th installment in this series of posts about gold occurrences. I'm providing some very good information in this series so why not make the most of it?
6. Arsenic Sulfide Ores
In these types of ores gold is usually found as small, microscopic, or even sub-microscopic particles. The main host rocks of arsenic sulfides are what are known as arsenopyrites (FeAsS). Arsenopyrite is silverish-white to nearly grey in color and on rare occasions will display some "peacock" coloration when tarnished (exposed to the natural elements). If you're prospecting in geological zones where the primary country rock is igneous or metamorphic in nature then aresenopyrite can be a likely find. There's also some evidence for arsenic sulfide ores in sedimentary rock but I wouldn't put tons of faith in that one, despite what the geologists say. These types of ores are hard to process and it usually takes extremely large amounts of them to make things profitable economically speaking. That's why large-scale open pit mining is often the approach used to get the gold out of arsenopyrites.
7. Antimony Sulfide Ores
Antimony is very brittle and silverish-white to grey in color. In the ancient past it was used in home-brewed medicines and as a cosmetic. It's main form is stibnite (Sb2S3). These types of ores can be a very good thing if you happen to stumble across them since they can contain visible, free-milling gold in quantity. Stibnite is usually associated in gold deposits that are hydrothermal in origin where lots of pressure and super-heated chemical solutions were present in the geologic past. However, most antimony sulfides are found in small, very localized areas. Large, commercially viable deposits are hard to come by if not totally absent altogether. But for small-scale prospectors, coming across a rich little stibnite deposit could be a VERY good thing.
8. Bismuth Sulfide Ores
Like antimony sulfides, bismuth sulfides can contain visible, free-milling gold. Bismuth sulfide ores are usually associated with something called "maldonite" (Au2Bi). If you look at that elemental equation the first thing that should catch your eye is the "Au." Maldonite is a mixture of bismuth and gold that is often found as part of quartz veins, sometimes very rich ones like Australia's Nugget Reef. Bismuth sulfides ahve been known for producing large masses of free-milling gold, including large nuggets (hence the Aussie "Nugget Reef). Bismuth sulfides range in color from a copperish-red to black, and then even silverish-white. So it takes a practiced eye to spot these types of ore out in the field. That said, however, it won't take much of an eye to spot bismuth sulfides containing gold!
(Bismuth sulfide. Image courtesy Dakota Matrix.)
9. Telluride Ores
Yes, I know. The first thing that comes to mind here is Telluride, Colorado (USA). Well ladies and gents, Telluride got that name for a reason, mainly because of the telluride ores found in that vicinity in the late 1800s and early 1900s. Tellurides usually have a distinct metallic look about them and are sometimes a dull light to dark grey like galena, or lead ore. However, one of the richest forms of telluride is something called "calaverite," which was named after Calaveras, California where in the mid-to-late 1850s Gold Rush Argonauts working in the southern Motherlode mines came across large calaverite veins rich in free-milling gold. Calaverite is usually light yellow to a pale, pinkish white in color. Another important gold carrier is sylvanite (AgAuTe2) which contains both silver and gold. It looks a lot like calaverite but is lighter in weight and density and usually is silvery white. Some tellurides can be difficult to process because the gold they contain is in sulfide (chemical) form and not free-milling. Tellurides have been responsible for producing some staggering amounts of gold in the American West, so learning to identify them is a weapon you can wield to good effect in the field.
(Close up of calaverite.)
10. Carbonaceous Sulfide Ores
As their name suggests, these ores contain carbon in one form or another. Sphalerite and cobaltite are the most common forms of carbon sulfides, with our old friend chalcopyrite bring up the rear. Sphalerite is usually a zinc ore, but it can also contain iron, copper, silver, and gold. It's usually light or dark yellow in color or sometimes a mixture of the two. Cobaltite is composed of cobalt, sulfur, and and arsenic (a dangerously potent composition under certain circumstances). Structurally it looks a lot like pyritic ores and is dark grey or blackish in color with occasional iron staining on the surface. Cobaltite is very metallic looking, so that's a give away. Don't expect to see or find easily visible free-milling gold in carbonaceous sulfides. The gold they contain is usually in the form of fine-grained particles or sub-microspic in nature. Bummer, huh?
That's it for this round. I trust you are all well and staying happy.
See you next time.
(c) Jim Rocha (2016)
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