2
$\begingroup$

This question already has an answer here:

Is it possible to locate potential gold deposits using maps that describe dominant rock type, altitude, vegetation, or geological formations?

(I realize that the location would probably still have to be verified by traveling to the actual site. I am only looking for information that serves as an indicator of potential gold reserves.)

For the sake of this question I am assuming of course that these maps would not directly list gold deposits or existing or past gold mining operations.

What kind of information would a map need to contain in order to derive an indication of gold deposits from it?

$\endgroup$

marked as duplicate by Gimelist, Tactopoda, arkaia, Jan Doggen, bon Jul 15 '17 at 8:08

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

5
$\begingroup$

On of the issues with geological maps is they have all required humans to walk over the ground and mark on a piece of paper the surface expressions of different types of rock and geological structures, such as faults and folds.

Because of this process and the time constraints placed on people when they mapped the land there will always be some level of detail missing.

Geological maps will show the different rock types and geological structures on the surface but they may not necessarily show what is at depth.

Geology has advanced to a state where most well trained geologist will know the potential for certain rock types to host certain type of mineral deposits, including gold. However, pinpointing where a gold deposit, or any other mineral deposit is located will always require a human to walk the ground and inspect it in more detail and to take samples and have them analyzed in a laboratory.

Initially those samples may be geochemical samples, others will be rock samples and chips taken from the surface or trenches dug into the surface, culminating in drill samples being taken.

To further assist where to look, geophysical testing methods may be used, such a magnetic anomaly testing or ground electrical resistivity testing.

It is very unlikely that anyone will be able to look at a geological map, and with any degree of certainty say "there is a gold deposit" or "there is a gold deposit that is economic to mine". More likely, a person will look at a geological map and say something like, "there is a greenstone belt, this may have potential to host a gold deposit, further investigation is needed".

There is an adage amongst some geologists that states "gold is where you find it". There is no altitude preferable for finding gold deposits. It's found in mountains, in deserts, in forests, under salt lakes, even dissolved in the oceans. The oceans contain the largest quantities of gold, but it is uneconomic to extract it from sea water because the concentration are so low.

Biological indicators, such a vegetation may work for some base metals, such as copper, but not necessarily for gold.

Also, gold can occur as a deposit of just gold, but it can also occur in association with other metals in the form of polymetallic deposits which contain gold, silver, copper, lead or zinc, even uranium.

Finally, not all deposits of minerals, gold included, are reserves. To be classified as a reserve and an orebody, a deposit of mineralization must be economic to mine.

$\endgroup$
  • $\begingroup$ Do you perhaps mean "Gold is where I find it"? I don't understand the statement as put. $\endgroup$ – JeopardyTempest Jul 11 '17 at 4:30
  • $\begingroup$ @JeopardyTempest "Gold is where you find it" was a 19th/early 20th century put down of geologists. The geologists might have theories about where the gold was but the old prospectors would ignore them and look everywhere. The 'you' in the sentence is the idiomatic English way of specifying a general unspecified person. But that is a discussion for a different StackExchange. $\endgroup$ – verisimilidude Jul 11 '17 at 5:51
  • $\begingroup$ @verisimilidude Had not heard it before, but not being familiar much with such topics, perhaps no surprise. Meant to write it with you instead of I... but the point is that wasn't what he wrote, and so I was left uncertain if that's what was intended :-) I'll change it given your information, thanks $\endgroup$ – JeopardyTempest Jul 11 '17 at 5:58
4
$\begingroup$

If you were an exploration geologist tasked by a mining company with finding a new gold deposit you would start with a geologic map. In this sense, yes you would be using a map to locate potential gold deposits. But you would also do a lot more research reading geological survey reports about the geologic history of the area, mine permit and mine history records of the area, and satellite images of your area of interest trying to come up with a hypothesis about why mineralization was found in particular areas (where old mines are) and not in other areas. If you are looking in particular for gold you would decide whether you are going to concentrate on placer gold, deposited in stream valleys after weathering from source deposits elsewhere, or if you are going to look for hidden geothermal mineral veins that may hold some gold. The history of mining in an area is probably a good guide to which will be productive. Working for a company, you would probably also purchase field notes, drilling records, geochemical surveys, etc. done by previous companies working in the area.

And all this research is done before doing any field work.

So if I expand the meaning of your question to "Is it possible to locate potential gold deposits through research" the answer is yes. If you want to use just the map the answer is no.

$\endgroup$
-2
$\begingroup$

Most of the most productive gold mines are in locations where the mineral was brought up from deeper in the earth in extinct volcanic shafts and magma plumes. Gold is a heavy metal so when the earth was first formed and was much hotter and many of the elements and compounds that are now solid were in hot molten form.

At this early stage, it makes sense that the heavier and denser elements and compounds would have sunk deeper into the earth and lighter ones to float to the surface. If you pour oil into the water, the oil is less dense and floats to the surface. The same principle applies. Gravity pulls the denser material down closer to the center of the earth.

And that is what we see when we do seismic sonograms, and other analysis methods to reveal the earth's composition. Silicon has an atomic weight of 28 and is thus a lighter element so it is found in abundance in almost all surface rocks and sands in the earth's crust.

As we go deeper the composition shifts to heavier and heavier elements. The earth's core is mainly iron. Since gold is even denser and heavier than iron it should be found in much more abundance relatively speaking at earth's core than at the surface.

We must emphasize that gold is a rare element in the earth and the universe in general for the following reasons.

  1. All elements other than Hydrogen and Helium and counting up in atomic number to iron are formed from the nuclear fusion in stars. All elements heavier the iron, gold among them, can only be formed in the supernova explosion that occurs at the end of a massive star's life.

  2. Gold is not at any end of any radioactive decay chain from heavier unstable element radioactive isotopes. Uranium 238 will decay over billions of years into lead and some other elements but never into gold.

So gold, platinum, and other similar elements are rare because they are only formed in supernova explosions and no other process.

Getting back to answer your question, the only way to bring up the heavier and denser elements like gold back up to the earth's crust is through volcanic eruptions and magma plumes rising up from the interior.

Therefore, your best bet would be to investigate the igneous rocks in volcanic shafts, former eruptions, magma plumes, etc. The other surface characteristics you mentioned like surface vegetation are irrelevant.

$\endgroup$
  • 4
    $\begingroup$ The mass or density of a certain rare element has no effect on whether it "sinks" or "floats" in the Earth. Read my answer here: Why is uranium only in the crust, really?, particularly the second to last paragraph. $\endgroup$ – Gimelist Jul 13 '17 at 10:04
  • $\begingroup$ @Michael Not so much when it is a solid like now, but when all rocks and metals were very hot and in liquid form when the planet was first formed, then of course, its basic physics. $\endgroup$ – 0tyranny 0poverty Jul 13 '17 at 11:43
  • 1
    $\begingroup$ No, you are not correct. Basic physics say that you have liquid immiscibility between various major phases. In the earth - liquid Ni-Fe alloy and silicates dominated by Mg, Ca, Al, etc. Anything other than that follows the phases according to their geochemical affinity. Goldschmidt classification is a good place to start. It's not about density, it's about partitioning. $\endgroup$ – Gimelist Jul 13 '17 at 11:54
  • $\begingroup$ @Michael the source you cited does not compare how strong this geochemical affinity force is compared to buoyancy and gravity. Even if it were stronger, you are assuming monolithic layers with no variance through which buoyancy would still take place. Where is your evidence for this? $\endgroup$ – 0tyranny 0poverty Jul 13 '17 at 12:08
  • 3
    $\begingroup$ Trace elements are dissolved in major phases. Their buoyancy then depends on the buoyancy of the host phase. Noble metals go with iron and sink. Uranium and thorium, the heaviest metals go with silicates and float. Archimedes is correct, but it applies to bulk phases. Not individual atoms dissolved in a bulk phase. Dissolve potassium and sodium chloride in water: they are homogenously mixed, the Na doesn't float over the K. Same principle applies here. $\endgroup$ – Gimelist Jul 13 '17 at 12:35

Not the answer you're looking for? Browse other questions tagged or ask your own question.