The single most common gold-bearing matrix in the world as a whole is the quartz vein. I'm no geologist, but as a hard-rock or placer gold miner I do know that having a basic understanding of the geological characteristics of auriferous quartz veins is important. So here goes:
Sulphides and Chemical Oxidation
Most gold-bearing quartz veins and stringers (Note: Small or very thin veinlets of quartz. J.R.) will contain at least some minor sulphide minerals as part of their composition. One of the most common sulphide materials is iron pyrite, or FeS2, a form of iron sulphide resulting from the chemical oxidation of the iron inherent to this material.
Quartz veins that contain iron sulphides or oxides are pretty easy to spot since they will display a yellow, orange, red, or reddish-orange coloration. This "rusty" look is just that and it is very similar to the colors found when when old iron is left outside in the elements to rust and oxidize.
"Country Rock" or Host Rock
Typically (but not always) these sulphide quartz veins are found along or near major geological faults or in areas where large tectonic processes may have occurred in the distant past. The quartz veins themselves will often be fractured or "crosscut" in any number of directions and it is at these junctures, fractures, or crosscuts that the greatest amounts of gold will be found.
You've heard me talk elsewhere about "country rock" or host rock, which is the most prevalent underlying rock (including bedrock) in any given gold-bearing locale. In areas where auriferous quartz veins are found some of the most common host rocks are:
Schist (especially green schist);
and various forms of Metamorphic volcanic rock.
Let's Talk About This Last One
Let's talk about this last one for a moment. Many newcomers to gold mining or those with little knowledge about the processes involved in gold mineralization automatically assume that any area displaying volcanic activity (lava beds, etc.) will contain gold.
WRONG! Areas or locations with recent (geologically speaking, that is) volcanic activity seldom contain gold in any concentrations at all. The term metamorphic means that some sort of significant chemical and/or geological changes took place over time that altered that original volcanic host rock into something altogether different. (Note: Metamorphics, by the way, form some of the best gold ground in the American West and Southwest. J.R.)
Shale, Limestone, and Carbon
Geologists also say that areas containing shale, carboniferous (carbon-bearing...coal is carbon but I don't think they mean coal per se), and limestone country rock can also host gold-bearing quartz veins. Okay, they are the experts and I have great respect for the science of geology and those who practice it, but I will tell you something here and now.
Dickies Work Clothes
In over three decades of small-scale gold mining I have yet to find a single grain of gold in areas containing these types of country rock. However, I have prospected areas here in New Mexico where rich Metamorphic gold ground exists just a few miles away from fairly large areas of shale, limestone, and carboniferous host rock. So, the geologists are on to something.
That's it for this round. There's more to come so stay tuned...
If you liked this post, you may want to read: "Hard Lessons: What I've Learned in 30+ Years as Small-Scale Gold Miner (Part 5)"
(c) J.R. 2011
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