Sunday, January 31, 2016

Epigenetic or Syngenetic Deposits: Which is Easier to Find?



The following is a rather esoteric question – definitely not an elementary school question – but it provides a neat teaching moment or opportunity. 

Q: Between epigenetic and syngenetic deposits, which deposit would be easier to find?
- Rayon P.

A: Let's review the definitions of the two types of deposit first:

Syngenetic Deposit:
            This is a mineral deposit that was formed at the same time as its parent rock – and is always enclosed by it. There are two types of syngenetic deposits: igneous and sedimentary. Some examples of these kinds of mineral deposits include paleo-placer diamonds found in southern Venezuela. These diamonds are called “paleo-placer” diamonds because they weathered out of the ancient Tepuis, eerie platform-like mountains that are the inspiration for Aracnophobia and Avatar. There is also stratabound potash (e.g., pinkish salts found in the prairie provinces of Canada and in Central Asia, that are made of potassium, a critical element for agriculture and the “secret” of the “Green Revolution” of the past century. Other examples include the huge nickel deposits found around the huge, ancient asteroid impact crater near Sudbury, Ontario, Canada. There are many other examples, of course.

Epigenetic Deposit:
            This is a mineral deposit that formed following the development or emplacement of the enclosing or host rocks. These kinds of deposits might be found on top of the host rocks, or more commonly IN the host rocks. Some examples of these kinds of deposits include placer platinum group elements (e.g., palladium, rhodium, etc.), and Comstock-style placer gold deposits (think of what triggered the Gold Rush in 1849 in California). Another example are the huge porphyry copper deposits and sedimentary copper deposits found all over the world. Note: the latter example are secondary concentrations of copper in sedimentary rocks… the copper seeped in after the sediments were formed.

            For syngenetic deposits, especially those formed during sedimentary deposition, it makes sense that if you can find one economic outcrop of the mineral you’re looking for, then you could follow the strata it is found in to find more of that mineral of interest. 

            Epigenetic deposits, on the other hand, tend to be easier to see: by the definition above they disrupt the environment (the host rocks) that they are found in. Examples that come to mind are the low-sulfide gold-quartz deposits I found while working in the jungle in southern Venezuela. The gold there is found in thick mostly-quartz veins that fill faults and fractures in the geologic units that host them. Porphyry copper deposits on the other hand are huge things that I visited and occasionally worked in while living in the southwestern USA. These are the source of much of the wiring in your house that let you read this on a computer screen – and then walk out of the room later without stumbling in the dark.  Porphyry coppers, as they are often termed, form huge bulls-eye halos in the host rock. These haloes can usually be seen from the air, and can be detected with specialized instruments like a radiometric imaging system in an aircraft, or specialized electrical geophysical methods like induced polarization.

            In both cases, there are wide variabilities is how easy these deposits are to find, depending on the particular resource being sought. In virtually ALL cases, the “easy” deposits have already been found, and modern geoscientists must search beneath younger sediments or lava that have buried these deposits after they were formed. 

            In other words, the hard-to-find ones are what we are looking for now.

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