A List of Rock Types and the Gold They Contain (Part 2)
They say "knowledge is power." Nowhere is this more true than it is with prospecting and gold mining. That knowledge coupled with some good experience is a one-two punch when it comes to gold recovery for small-scale miners. Now here's part two of this series of posts.
Gabbro is a dark (grey, black, or sometimes greenish) intrusive igneous rock that's composed of calcium plagioclase, pyroxene, olivine, but never quartz. It is sometimes found in gold mineralization areas but is not noted as a host rock for gold. Average gold content: 5.4 parts-per-billion (ppb).
(Granite rocks: top row rocks have quartz inclusions; bottom row carbonate [CO3] inclusions.)
(Quartz veins in a mass of California granite.)
Granite is a very common type of intrusive igneous/felsic (containing feldspar) rock. It can be grey, white, or even pinkish in color. Aside from feldspar, granite's other constituents include quartz, muscovite, biotite, and hornblende. Average gold content: 7.1 ppb.
Here's where things get interesting. Although granite itself contains gold in the (ppb) range it's real importance for small-scale miners and prospectors lies in the fact that it's often a host rock for gold-bearing quartz and carbonate veins. The main sources of gold in the California Motherlode Region were rich mineralized quartz and carbonate veins locked into granite. These quartz veins varied from less than a inch in width (so-called "stringers") to massive veins that were measured in feet. In fact, the entire Sierra Nevada Mountain Range is one huge granitic batholith. Again, although granite is common throughout the world rarely do we see it containing gold-bearing gold veins like those seen in California. Any time you're out in the field quartz or carbonate veins or stringers in granite are ALWAYS worth a closer look. Those veins can contain gold in "sugar" quartz (has the look of powdered or confectioner's sugar) or heavily iron stained veins or stringers containing pyrites. And if that gold is there in quantity we aren't gonna be talking about ppb, but grams, ounces, and even pounds!
Granodiorite is a deeply intrusive igneous (volcanic in origin) rock that contains about 20-25% quartz and the remaining percentage some sort of feldspar. It also contains mica and hornblende. It's not a typical gold host rock but is often found in or near gold areas. It's also frequently found in gem mineral locations. Average gold content: 3.5 ppb.
Granophyre is a sub-volcanic (formed at medium or shallow depths) rock whose name comes from granite and porphyry. It's composed primarily of quartz, feldspar, and alkali and has a very tightly bound textural body. It can be associated with gold and gold veins and at times it can act as a host rock for gold (for example, Iceland is known for granophyric gold host rock). Average gold content: 16.3 ppb.
(Simple diagram of a Kimberlite "pipe.")
Kimberlite is the main source of diamonds throughout the world, so forget about gold. (One small but high clarity diamond can be worth many ounces of gold.) Kimberlite is an igneous rock that is pushed to the surface through volcanic (magmatic) intrusions known as "pipes." This type of diamond-bearing rock is made up of about 30%-35% olivine with the rest of its constituents being mica, serpentine, and calcite. Kimberlite doesn't contain any quartz or feldspar, by the way. Average gold content: 2.6 ppb.
Nepheline (or nephelene) is reminiscent of quartz but in reality it's an alkaline plutonic rock containing aluminum silicate/syenite. Prospectors who aren't well-versed from a geological standpoint often misidentify it as iron-stained (mineralized) quartz but despite this mistake their intuition is correct in the broader sense since nepheline is often found in association with certain gold deposits. Average gold content: 1.8 ppb.
That's it for this round. See you around the next bend.
(c) Jim Rocha 2018
Questions? E-mail me at firstname.lastname@example.org