Quite a few of you out there seem keenly interested in gold ores and gold occurrences. If you ever want to move away from placer mining and into the hard-rock or vein mining venue then this series of posts will definitely increase your knowledge in that direction. Actually, the info contained in this series is good for placer miners as well since there's always a beneficial knowledge overlap to learning everything you can about the myriad contexts that gold appears in.
What I'm going to do here is list types of gold ore occurrences and then discuss each of them briefly. I'll do my best to explain terms that may be somewhat "alien" to you because of their scientific roots or the simple fact that you may not have ever heard of them before. A lot of this info will be highly detailed so if you're looking to seriously increase your overall knowledge it might not be a bad idea to print off these posts and combine them together into a folder or notebook of some sort. Also note that I won't be discussing gold placer occurrences since they're something most of us have been dealing with for some time now.
Free-Milling and Refractory Ores
Before we get started let me remind you once again that there are two main types of gold ore bodies, free-milling and refractory. Free-milling ores typically contain gold that is in metallic form ranging in size from large chunks or inclusions right on down to microscopic or sub-microscopic particles. This type of ore is what most of us are used to or wish we could find. With the exceptions of ores containing microscopic or sub-microscopic gold particles, free-milling ores contain visible gold that is either apparent on the surface of the ore or contained within once it's broken open or crushed. Carlin-type gold ores (named after the region and huge open-pit mine in Nevada) also contain free-milling gold but the metal is infused throughout the host rock in tiny particles that are virtually invisible to the naked eye. You could walk over a ton of this stuff and probably not realize what's underfoot unless you're a trained geologist or do some serious sampling and assaying.
(A small example of free-milling gold in mineralized quartz.)
Refractory ores can contain free-milling gold on occasion but in most instances the Au contained in the host rock or carrier is in a chemical state, usually some sort of sulfide. Telluride gold ores like many of those found in Colorado are very difficult to process and usually require special treatment to extract the gold they contain. Some refractory ores also contain gold in microscopic or sub-microscopic particles, so despite their interesting appearance (heavy oxidation, rainbow or "peacock" coloration) you're not going to be able to see the gold refractories contain without the type of magnification that scientific instrumentation is capable of, including electron microscopes.
(A chunk of "peacock ore" containing chalcopyrite.)
TYPES of GOLD OCCURRENCES
1. Quartz Vein Ores or Lodes
When most people think about hard-rock gold they'll zero in on quartz 99.9% of the time. Gold in quartz host rock or stringers, veins, reefs, ledges, or blow outs is probably the most common gold carrying matrix out there. However, as you'll find out in this series of posts, quartz is NOT the only carrier of natural gold. Based on my own knowledge and e-mails from many people over the years, many folks continue to believe that gold only occurs in quartz matrices. This is dead wrong and I hope to set things straight in this series of posts.
(Just because it's quartz doesn't mean there's gold in it.)
OK, that said, let's move forward. In quartz gold occurrences the Au is usually locked into the quartz rock itself and is derived from veins, reefs, or stocks that can be highly mineralized (multi-colored or oxidized with iron sulfides) or alternately, devoid of any coloration or apparent sulfide presence. An example of the latter are those pieces of ore you've probably seen in specimen photos where visible gold is on or in a white quartz that is reminiscent of powdered sugar in both texture and coloration. In fact, this type of quartz matrix is often called "sugar quartz." Usually the gold found in quartz matrices or carriers is free-milling, but there are instances where it's in microscopic or chemical form. Quartz containing tellurides, aurostibite, or maldonite are good examples of this, although the latter two are infrequent carriers of gold. (Aurostibite is what geologists call a "gold antimonide" which means that it contains some antimony as well as pyrites. It is usually associated with hydro-thermal gold deposits. Maldonite is dark-colored mineral that contains bismuth and sometimes, silver or gold.)
(A good example of a non-quartz gold ore occurrence.)
Any quartz that catches your eye out in the field in a known, gold-bearing area should be subjected to further examination. This is especially true if it shows evidence of iron oxidation or the presence of sulfides such as iron pyrites (FeS2). Even if visible gold isn't present in that chunk of oxidized quartz you're holding, it doesn't mean gold isn't present within it or in microscopic or chemical form. Now hold up pard! That doesn't mean every piece of interesting looking quartz you pocket is a gold carrier. No sir or madam. All it means is that further examination may be warranted. For most of us, the best way to determine if gold is present in that quartz is to actually see it with your naked eye. Barring that, crushing it into a fine powder and panning it out (or using a small gold concentrator like a Spiral Wheel or Blue Bowl) to see if gold shows up or sending samples off for fire/chemical assay are, once again, your best bets for determining if that quartz contains gold. The latter move can be costly so don't go off half-cocked with quartz samples you've picked up in the field. Most of them are just going to turn out to be pretty rocks. That's the long and short of it.
("Peacock ore" from Nevada containing microscopic gold particles.)
There's much more to come. This series may turn out to be lengthy but I guarantee you'll learn something valuable if you stick with me.
Best to all!
(c) Jim Rocha 2016
Questions? E-mail me at firstname.lastname@example.org