I've heard that gold ore is often associated with quartz veins. What geological processes enable gold particles to naturally cluster together like this? What characteristics/properties should I look out for on the surface of a rock that are likely to yield gold within it? Are there remote sensors that I use to detect the presence of gold in a rock?
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$\begingroup$ maybe use a gravimeter ? I think this question is on-topic but i'm not really sure we want to encourage this type (I didn't downvote, btw). I am worried that this type of question will be repeated / common with other precious earth materials. $\endgroup$– NeoCommented Apr 21, 2014 at 17:00
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$\begingroup$ Perhaps the initial question could be refined to something more like "In which geological situations is gold found in", so to prevent snark answers such as "Fort Knox"? I also find the rock formations are more likely... line troublesome, as I think they're intrusive, so it's not especially relevant. $\endgroup$– SivCommented Apr 21, 2014 at 17:01
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$\begingroup$ @Neo: Good point. Could 'gold' be generalized in some way to avoid repetition with other precious materials? $\endgroup$– PaulCommented Apr 21, 2014 at 17:09
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1$\begingroup$ Perhaps rather than the formations bit, a question regarding the processes which would be likely to naturally concentrate weathered-out native gold in the environment, now and in the past? $\endgroup$– SivCommented Apr 21, 2014 at 17:44
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$\begingroup$ @Siv: thanks for the suggestion. I updated the question accordingly. $\endgroup$– PaulCommented Apr 21, 2014 at 20:14
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2 Answers
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I think Azzie's answer describes the situations where gold is formed and found as Lode deposits. But native gold in lode can then be subject to weathering when it is exposed, which releases the gold into the environment as flakes, grains and nuggets. This gold is then subject to erosive and sedimentary processes.
As gold is denser than most minerals it can often be concentrated by sedimentary processes as Placer Deposits, where the gold is concentrated by gravity. Dense gold quickly settles out of a water column, and lighter minerals are winnowed away. These accumulations of gold can then be economically mined using Placer Mining techniques such as Gold Panning. The Witwatersrand Basin in South Africa is an important palaeo-placer deposit from which nearly 50% of all of the gold ever mined has been obtained.
Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further.Typical locations for gold placer deposits are on the inside bends of rivers and creeks, in hollows, at the break of slope on a stream or the base of an escarpment, waterfall or other barrier.
The vast majority of the Earth's gold and other heavy metals are locked up in the Earth’s core, being a dense material most of the gold is thought have sunk to the centre of the Earth as the core formed. Though there is 1000 times more gold in the Earth's silicate mantle than would be expected. Evidence from tungsten isotope studies indicates that most of the gold in the mantle and crust (which we can access) is derived from gold which arrived during the Late Heavy Bombardment some 3.9 billion years ago.(BBC Article 1) Which has been called the Late Veneer Hypothesis.(BBC Article 2)
Future mining of asteroids for gold and other metals has been proposed,(Reuters Article) with estimates that there are up to 20,000 million tonnes of gold in the near-Earth asteroid Eros, with an estimated value of $1,000 billion (1999).(BBC Article 3)
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1$\begingroup$ I really like this answer and it is very detailed and has many supporting links, but it doesn't seem to answer the original questions. (Also, you might want to switch 'the answer above' to 'Jaxon's answer' because your answer has more votes than mine at the moment.) ;) $\endgroup$ Commented Apr 24, 2014 at 23:43
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$\begingroup$ I wasn't seeking to address the parts you had already covered in your answer, merely to broaden out the geological situations where gold could be found. $\endgroup$– SivCommented Apr 25, 2014 at 0:13
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$\begingroup$ I understand, so I guess that means we're working together on this one then... :) +1 on your answer by the way, it's very well done. $\endgroup$ Commented Apr 25, 2014 at 0:23
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$\begingroup$ "20,000 million tonnes of gold in the near-Earth asteroid Eros, with an estimated value of $1,000 billion" wow, only 5 cents for a kilogram of gold! $\endgroup$– DavePhDCommented Nov 13, 2014 at 17:11
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I've heard that gold ore is often associated with quartz veins. What geological processes enable gold particles to naturally cluster together like this?
First of all, gold is mostly found in quartz veins in and around igneous intrusions.
It happens with a very hot hydro-thermal fluid of silica, oxygen and concentrated gold that is created during incidences of compressional forces acting on the crust. It is then released at the end of magma crystallization and fills cracks and voids in the surrounding rock. This fluid then solidifies to form gold-rich (and also other metals like silver) quartz veins.
What characteristics/properties should I look out for on the surface of a rock that are likely to yield gold within it?
I'm not sure of any way to tell on the outside of a rock except maybe if it is igneous and you see signs of a quartz vein.
Are there remote sensors that I use to detect the presence of gold in a rock?
There are many metal detectors that have a frequency you can set them to that can detect gold.
answered Apr 24, 2014 at 0:13