A List of Rock Types and the Gold They Contain (Part 1)
One of the constant queries I experience here at Bedrock Dreams is...yep, you guessed it again..."Do my rocks contain gold?" This series of posts may be an exercise in futility but in an earnest attempt to answer this question I'm providing a list of generic rock types and the average gold they contain. This list is based on scientific studies of samples taken from various rock types.
The Only Sure Way
Before I begin this list I must stress the following premise yet again: The only sure way of knowing whether your rocks contain gold and just how much is by submitting those rocks (or samples thereof) for chemical or fire assay. A good assay will not only determine if your rocks contain gold and the quantity therein, but can determine the other metal (and mineral) constituents contained in them such as copper, silver, iron, and so on. Yes, assays cost money but they will answer your basic question with finality. X-Ray diffraction (spectroscopy) is also a good means of determining rock constituents and you can purchase a hand-held unit for field use but be prepared to pay big bucks (thousands) for a really good unit. Alternately, you can do things the old-fashioned way and crush (using a rock crusher or iron mortar and pestle) your rock samples into a fine powder then simply pan them out or run the remnants through a fine gold recovery system. One note here, however. If you do things this way it will only tell you if your rocks contain free-milling gold...not gold in chemical stasis. So sending me a digital photo or two of your rocks and asking me to tell you if they contain gold is, in essence, you pissing into the wind (to put things bluntly). The only thing I can tell you in these instances is if I see visible gold in your rocks or whether or not they look "juicy" from a gold mineralization standpoint. OK?
(Hmmm...what's in my rock?)
Pay Close Attention
Let's get back to the list now. Before your heart starts beating fast and your palms get sweaty, understand that when scientific analyses are performed the greater the number of samples analyzed the more accurate the data is. In other words, more samples equal more accurate data. This list is based on a multitude of samples taken over time (years and even decades) and then averaged out. Pay close attention to the following two sentences: When large-scale sampling and analyses are performed under strict scientific methodology the end data is usually expressed in the greatest terms possible. So the gold amounts in the following list are expressed in parts-per-billion (ppb). This is the average gold content in these rocks based on their entire composition of one billion parts. Also note this content data is based on the fact these rock types are non-mineralized from a gold standpoint. In other words, they're just basic rocks minus any gold mineralization. Thus, don't expect to grab one of these and go rushing out the door to sell it to a collector, jeweler, or coin shop owner. You'll end up being the brunt of cruel jokes for quite a while in that establishment. Now onto the list:
Andesite is an extrusive ("forced" out or extruded) igneous rock composed of micas, plagioclase, orthoclase and at times: quartz and pyroxenes. It's usually a dull grey in color but can be pink or green in certain locations. Geologically speaking it falls in between granite and basalt. It's not typically known as a host rock for gold. Average gold content: 5.2 ppb.
Basalt is an extrusive igneous rock rich in iron and magnesium. It's very dense and usually solid reddish-brown or dark grey/black in color. It is not a known gold host rock but can be found in certain gold areas. Average gold content: 3.6 ppb.
Diabase is a very common field rock that's usually dark grey to black in color. It's very similar to gabbro and basalt but actually falls in between those two rock types. It's often used in cemetery monuments and statues. Average gold content: 14.7 ppb.
Diorite is an intrusive (forced into or held within) igneous rock composed primarily of silicate minerals like feldspar, hornblende, pyroxene, and biotite. This is a very hard rock that's extremely difficult to break or carve. It usually has a "speckled" black and white color pattern. Diorite is not typically a gold host rock. Average gold content: 4.3 ppb.
Dunite is an intrusive igneous rock that's related to the peridotite group and is usually a light yellowish-green. It contains olivine and pyroxenes had has a very low silica (quartz) content. Gem minerals are often associated with dunite, including peridot and amethyst. It's not a gold host rock. Average gold content: 8.2 ppb.
Felsite is a fine-grained igneous rock containing quartz and feldspar. It's usually light in color and can be found in certain gold areas. Average gold content: 5.5 ppb.
There's more to come so stay tuned.
(c) Jim Rocha 2018
Questions? E-mail me at firstname.lastname@example.org