(Area in red shows the presence of sulfides in a crystalline carrier sandwiched between the host rock.)
Now that I've gotten the previous post's core message off my chest, it's time to move forward once again. You know, the information contained in Bedrock Dreams tends to slant toward the placer side of small-scale gold mining but in this series of posts I want to bring a few things forward about gold-bearing veins.
Can We Agree?
OK, my view is this. The more you know about gold in general the better off you're going to be as a miner and the greater your ability to locate and recover that precious yellow metal. Even if you never do any hard rock prospecting during your entire small-scale mining career, knowing something about how gold forms in veins encased in host rock gives you a leg up over the competition because this sort of knowledge can only enhance what you already know about placer mining and increase your level of understanding. Additionally, I've always thought that small-scale miners should be multi-dimensional in terms of their knowledge, not one dimensional. Can we all agree on that point? I thought so.
You've heard me refer to veins, stringers, blow outs, outcroppings, ledges, and reefs numerous times over the years. I'll quickly run through my definitions of these gold mining terms once again for you newbies and greenhorns out there and provide visual examples of each:
Vein: A distinct linear or lens-like body of crystallized minerals with a host rock (sometimes called "country rock"). One of the most common forms of crystallized veins in gold mining is the mineralized quartz vein. It is this vein material (quartz or other crystallized minerals) that contains precious metals like gold or silver, either in a chemical state (sulfide form) or as physical metal ("free-milling").
(Heavily oxidized country rock with a crystallized vein running through it.)
Stringer: Gold veins can be many feet thick at times. Stringers, on the other hand, are very narrow veins that rarely exceed six inches in width, although their lengths are not necessarily similarly affected. I personally think that the six-inch width interpretation is OK, but I usually define stringers as vein material three or four inches (or less) in width...sometimes only an inch or two in width. You've probably seen quartz stringers in host rock many times, even in non-gold bearing regions.
(Two stringers intersect in this photo...one of quartz and the other of pure gold!)
Blow Out: When a localized area of heavily mineralized country rock containing vein material breaks the earth's surface through erosion or geological forces, this is known as a blow out. Most blow outs I've come across over the years were contained within a 10-30 yard radius and I've come across a few that were much more localized than that. The blow-out descriptor tends to refer to the scattering of eroded ore on the ground around the blow out and/or the fact that powerful geological forces such as buckling, pressure, magma flow, etc. forced mineralized material to pop up or blow out above the earth's surface.
(I've used this photo before but this a blow out accompanied by a radius scatter of ore.)
Outcropping: An outcropping is a visible exposure of country rock (including bedrock) projecting above the surrounding earth's surface. Rocky outcrops in gold-bearing regions can be heavily mineralized and shot through-and-through with vein material so you should always give them closer examination out in the field. Outcrops are usually fairly large in scale and in some instances can cover quite a bit of ground.
Ledge: A shelf-like formation or ridge that is usually found on the crest of hills or along their upper slopes. Some ledges can exhibit stringers, veins, or even significant amounts of sulfides or oxides. Historically, some very rich ledges have been found in the American West and Southwest, and a few ledges have become the subject of famous treasure legends.
(One type of ledge.)
(Quartz stringers encased in an unusual-looking section of Aussie country rock.)
Aside from quartz, other crystallized carriers of gold (Au) in veins include calcite, galena, pyrite (FeS2 or "fool's gold"), and chalcopyrite (or copper iron sulfide, CuFeS2). Now here's where it gets a bit messy. Again, the gold contained in these sorts of carriers can be present as free-milling metal or it can remain in sulfide or chemical form. These latter forms are known as refractory ores and they require specialized treament to leach, process, and recover the gold within them. You should also note that many gold sulfide veins are identified by the presence of calcite as well as small amounts of silver (Ag). Carbonates can also make an appearance in sulfide gold ores and some of the largest hard-rock gold deposits in the state of Nevada are based on extremely rich carbonaceous ore bodies containing sub-micron particles of the precious yellow metal.
This should get things going for. Some of you already know this stuff and some of you don't. Either way, by the time I complete this series of posts all of you will have learned something new.
That's a promise.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org