Lesson 1 - Where Does Gold Come From? Technical Level: Basic

howdy folks I'm Keith Boland and this is Hard Rock University today's lesson is where does gold come from this is a gold-bearing quartz vein in southern Arizona and in a certain way this is where gold comes from however the story starts long before this vein was formed and much farther underground tens of millions of years ago in southern Arizona we went through a period long period of volcanic activity during this activity molten rock from deep in the earth called magma rise towards the Earth's surface and formed magma chambers when they got to weak areas in the rock as the magma chamber would fill the surface above it would rise most importantly for our purposes cracks look for if magma kept intruding into this chamber eventually we find one of these cracks and work its way to your surface and it would RuPt and form a volcano now here we have the cracks from which we form veins where did the minerals come from inside the magma kind of a steam dissolved in the molten rock is water and as the magma would cool that water would condense and liquefy and dissolve the substantial amount of minerals which could then be moved into and deposit it into the cracks in addition assuming you have a water table which we have almost everywhere on the planet ground water would work its way in from the sides get heated by the magma chamber and rise as it would rise up through these cracks near the bottom where it was really hot it would dissolve minerals and as it got cooler we would eventually start losing those minerals dropping them out a solution in a process called precipitation now as these minerals precipitated here is a crack in the rock with solution moving through it by the way these solutions are called hydrothermal solutions basically means hot water as the hydrothermal solutions we move through the cracks if it's a cooling zone it would start depositing like scale on the sides of the cracks just like scaleform's in a pipe except this is a lot different mineral scale and pipes usually calcium carbonate this would generally be quartz various metallic sulfides like pyrite chalcopyrite Galena things like that and gold and silver as time went on assuming the process continued didn't shut itself off somehow this crack would completely fill up with minerals and you now have a vein there's a structure that looks a lot like a vein called a dike basically one of these cracks gets filled with magma and then it cools but a dike looks substantially different than a vane they're not easily mistaken for one another the other way you get ore deposits from these hydrothermal solutions if they're moving through a porous bedrock the spongy enough to allow the water to move directly through it then it can start precipitating throughout the entire body of the rock and you get what's known as a disseminated deposit this will be much higher tonnage and much lower grade in general than a vein deposit most if not all large-scale mining operations today are in disseminated deposits many of which have veins in them I used to work at one where they they found a vein that was running about a hundred thousand dollars a ton in host rock that was running more on the neighborhood of $20 a ton but anyhow this is how your basic gold and silver and metallic ore body will form sounds simple doesn't it well as Yogi Berra once said in theory there's no difference between theory and practice and practice there is these magma chambers would lift the rock tectonic action would shift it sideways up and down and break it shatter it turn it over the mineral laden solutions called hydrothermal solutions would change over time sometimes their depositing one mineral sometimes another it's just a mess at first glance this appears to be just a quartz vein approximately a foot wide however upon closer examination you can see that this quartz right here is almost completely devoid of any other minute there's hardly any pyrites or anything in there whereas this layer down here only a couple inches thick it has a lot more mineralization in it these did not happen at the same time now whether this filled in and then this came in afterwards or whether this filled in completely and then ground movement created another crack at another time where different solutions came in it's impossible to tell but these are two separate events thing we can determine by looking at the other workings around here is that the old-timers were much more interested in this part right here than they were in this I call that a clue so if I was going to sample this this material would be one sample this material would be another sample I probably go ahead take some of this altered wall rock here it looks like a fair amount of mineralization I can't tell how much so take a sample while you're here take the sample once you get back you don't necessarily have to process it if you don't want to if this and that don't go very good you might not worry about it but anyhow sample everything separately so that you can tell exactly what's going on now that pretty much shows you what happens with hard rock but where does placer gold come from let's go outside and I'll show you here we are outside where the vein we were just looking at meets the surface as you can see the quartz vein is harder and more resistant to weathering than the surrounding host rock which is also called country rock or wall rock this leaves a substantial outcrop of large pieces of quartz still embedded in bedrock along with smaller pieces of loose rock called float that start at the vein and continue downhill from it if the courts had a lot of weathered sulfides in it it would generally be stained red to reddish brown and might be called a Scarn but outcrop is more common now crop is also more likely accurate because Scarn is technically the name given to a particular type of metamorphic rock Scarn x' may outcrop but outcrops are rarely Troost carts anyhow as the vein weathers and breaks down over time any golden it becomes part of the soil next to and downhill from the vein this gold in close proximity to the rock from which it is whether it is called a residual placer since the largest gold particles tend to move the least a residual placer is a good place to try metal detecting downhill from the residual placer until it reaches the first water course will be what is called an alluvial placer or hillside placer it can be rather arbitrary trying to draw a line between the residual and alluvial plasters but sometimes there is a market change in the concentration of gold in the long run it doesn't really matter what you call it so long as you know it's there once the gold reaches the nearest stream better River it becomes part of what is known as an alluvial placer or stream bed placer irregularities in the bedrock or channel can form a natural gold trap and concentrate large quantities of gold over many years any goal that doesn't get trapped will tend to move downstream during floods in a preferred path and when the water recedes the sediments will stop moving and those zones of gold along that preferred path are called pay streaks which are by definition surrounded by much poorer sediments there are many good websites videos that specialize on how to find and recover placer gold we are focusing on hardrock also called lode gold deposits so I will stop here on to the next lesson happy prospecting and keep it safe

Channel: Hard Rock University

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Keywords: Gold, Gold Mining, Gold Processing, Gold Prospecting, Prospecting, Mining, Gold Milling, How to make money gold *, gold ore bodies, where does gold come from, ore body formation, geology of ore bodies, hard rock gold, placer gold, gold deposits, hard rock gold deposits, hard rock ore bodies

Id: OHHnykFByzE

Length: 10min 34sec (634 seconds)

Published: Sun Jul 26 2015