Types of Gold Occurrences (Part 3)
(You damn well better pick up something like this in a known gold area.)
This is the third post in this series and it includes new information about various types of gold occurrences, plus whatever incidental information I think may be valuable to you as a small-scale gold prospector or miner. So read on if you're interested in learning something new that can help you out in the field.
TYPES of GOLD OCCURRENCES
4. Copper Sulfide Ores
Copper sulfides are considered to be economically important, not only because of their copper content but because they often contain smaller amounts of gold or other metals. Typically gold is contained in copper sulfides as small or very fine particles. The three main types of copper sulfides are chalcocite (Cu2S), bornite (Cu5FeS4), and covellite (CuS). Although many ores containing copper or copper sulfides will exhibit green coloration as part of their mineralization, chalcocite is typically dark-grey to black in color with a very pronounced metallic luster. We talked about "peacock" gold ores previously in this series, and of all the copper sulfides bornite is the most likely to have this sort of multi-colored appearance...almost a rainbow coloration. However, before it is oxidized by the elements bornite will appear as brownish to red in color, a bit like the color of natural copper. Bornite's "peacock" look only occurs when the ore is exposed to rain, snow, salts, and so on. If you look at the elemental formula for bornite it's composed of copper (Cu) and iron sulfides (FeS) whereas chalcocite and covellite are primarily sulfides of copper minus the iron. The gold content of copper sulfides varies, but rarely exceeds the 10-15% range and is often much less than these percentages, although Ma Nature makes exceptions in certain instances. But gold is gold, right? The three photos below are examples of each. If you come across anything similar to them out in the field they're worth further investigation or assaying.
(Chalcocite. Notice the metallic luster of this sample.)
(Bornite after oxidation. Note the rainbow or "peacock" coloration.)
(Close up of a covellite sample.)
5. Iron Sulfide Ores
These type of sulfide ores are probably the best known and most commonly encountered by small-scale miners and prospectors. The gold's host rock typically contains quartz (although again, many iron sulfide ores are not quartzitic). The host rock or quartz will frequently have an iron-stained or rusty looking appearance ranging from pale orange to a very deep, or dark reddish orange. Other colors signifying mineralization can also be present but an iron oxide coloration will be the most prominent. Gold in iron sulfide ores can be easily visible with large chunks or masses of Au present right on down to small or microscopic particles. Many iron sulfides will also have pyrites present. Pyritic ores like this have been responsible for some very rich strikes in the American West and Southwest, as well as in many other locations throughout the world. Although less-frequent producers of natural gold, related sulfides like marcasite and pyrrhotite can be important field clues for you prospectors out there. Marcasite is a semiprecious stone containing iron pyrite but unlike pyrite, marcasite's crystals are tabular in form, not cubic. Pyrrhotites are very often found in metamorphic rocks, including those that are volcanic in nature (igneous). But remember, the volcanic rock has to have undergone great change (metamorphosis) to have any chance of it containing gold. I don't want you newbies rushing out and gathering pick-up loads of lava or volcanic basalt thinking you've hit the "big one." I've written about metamorphics in the past in Bedrock Dreams, so you might want to use the site's search function to take a look at that info. By the way, some of you have sent me pictures and samples of some pretty juicy looking iron sulfide ore in the past. In one instance (and the finder will remain unnamed), those iron sulfides were chock full of visible, free-milling gold.
(Small chunk of iron sulfide gold ore. Note the presence of quartz.)
(Sample of pyritic iron sulfide gold ore. Both gold and small, cubic crystals of pyrite are present along with slight iron staining.)
(One example of pyrrhotite.)
Again, your eyes are your best tools when prospecting out in the field. Any rock that catches your eye in a known, gold-bearing area deserves further examination. Now I'm not talking about "pretty rocks" here (although some gold ores are striking in appearance). I'm talking about rocks or samples that display characteristics like I've been describing in this series of posts. I recommend that some of you (especially if you're new to the gold mining/prospecting game) buy one of those rock and mineral field guides containing high-quality, color photos and carry that thing with you in your backpack when you're out and about in the field. One or two good pieces of copper sulfide or iron sulfide ore could contain more gold than you could pan or sluice in a week.
That should be a good motivator, methinks...
(c) Jim Rocha 2016
Questions? E-mail me at email@example.com