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Where Does Placer Gold Come From? – Part 3 Placer

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Where Does Placer Gold Come From? – Part 3 Placer

Placer gold mining has been practiced for thousands of years with evidence dating back as far as 2600 BC in ancient Sumeria and Egypt.  The technology required is minimal with only a gold pan you can refine gold in a placer deposit.  The word “placer” comes from the spanish word meaning “pleasure”.  Perhaps an allusion to the delight of finding precious metal in a river bank.  The word was spread as gold bearing gravels were discovered in parts of North America colonized by Spain.  In fact the discovery of gold the primary motivation for Spanish explorers to dig deeper and deeper into the newly discovered continent.

Big Al Jig

As we discussed in the part 1 and part2 gold is created in fantastic cosmic explosions.  It has traveled across the universe and made up a small part of the material that the earth formed from.  Tectonic and volcanic forces collected gold in concentrated lode deposits where it can be mined.  The concept of how gold transfers from lode deposits to placer deposits is pretty straightforward.  Rock holding the gold bearing veins or ore is slowly chiseled and broken by weathering and erosion.  The erosive forces of water, wind, and ice transport rock fragments into drainage systems such as streams and rivers.  Gold and other heavy minerals will settle out in areas in the stream where the water loses momentum or creates a trap.  These traps form into placer deposits over time.

Erosion

Placer deposition is driven by gravity.  Gold is very dense, meaning that compared to another substance of the same volume it experiences a stronger pull of gravity.  There are other principles of physics that apply to placer deposition.  The property of inertia is the resistance of any physical object to any change in its state of motion.  Less dense objects require less force to move them and in the case of a stream will travel farther and faster than heavy objects.  Gold has a density that is twenty times that of water and about 8 times the density of sand.  Another factor in the formation of placer deposits is Archimedes’ principle which states that the force of buoyancy on an object is  equal to the weight of the displaced fluid.  This principle was historically used to measure density of gold by Archimedes himself.  As gold is many times more dense than water the force of buoyancy on submerged gold particles is much less than the force of gravity.  So gold in a stream is held in place by gravity and resists movement due to its weak buoyancy and strong inertia.
Nuggets

There are several types of placer deposits.  There are alluvial placers, eluvial placers, beach placers, eloian placers and paleo-placers.  For each type of deposit there are miners who specialize in that type of deposit.  All placer deposits have concentrated gold from its source in some kind of trap.  The vast majority of the placer gold that is mined in the world is of the alluvial variety.

Alluvial placer deposits are formed in watercourses such as creeks, rivers, streams and deltas.  The gold is eroded from lode deposits and carried into the watercourse through rains and melt.  Once into a stream it can be moved great distances.  Gold does not move easily in a stream due to the inertia and buoyancy forces described above.  It takes many years for gold to make its way into a stream and to travel within it.  The gold will move along the bottom of the stream until it reaches a point where the water loses velocity or it is physically trapped.  Typically gold will accumulate on the inside bends of a river where the water velocity is lower.  Large rocks or outcrops can create a natural riffle or eddy where the water slows down and dense material will accumulate.  Waterfalls are another great trap for gold.

Alluvial placers can be broken into several groups.  Flood gold is placer gold that moves during annual floods or other flood events.  Gravel bars and upper sections of stream sediment are where flood gold is usually found.  This type of deposit generally consists of small flake and flour gold since they move more rapidly than nuggets.  Flood gold is actively being deposited and will replenish year after year

StreamSediment

Streambed placer deposits are essentially the same as flood deposits except that they no longer move.  Streambed placers are found in a current watercourse.  These deposits typically consist of gravel that is settled in the stream bed.  To produce a streambed placer you have to mine under the water.  Techniques that can be used are sniping, suction dredge, or diverting the water using a dam such as a wing dam.

The third type of alluvial placer deposit is a bench placer.  Bench deposits are part of the old stream bed before it cut into a deeper channel.  Benches can contain huge amounts of gold if the river carried gold at that time.  A bench is typically flat on top and may appear like steps coming down the valley side.  Benches can be mined using conventional mining equipment since they are usually high and dry above the current river.

OldSchool

Eluvial placer also known as residual placer deposits are formed before any water transportation has taken place.  These deposits form close the source of hard rock gold.  Eluvial placers will contain much large particles of gold than other types because it takes a lot of energy to move large nuggets.  Often quartz will be found with gold still attached in Eluvial placers.  These types of placers are formed by weathering and decay of the host rock that holds gold.  Areas where there is a lot of iron can break down rapidly as the iron oxidizes.  The lighter overburden is generally washed away and unsorted gravel and heavy material is left in place.  These deposits are generally small and very attractive to small miners they also are close to gold bearing veins which can be very exciting.

Beach placers are deposits that occur on the edges of large lakes or the ocean.  The wave action on the beach is the mechanism that concentrates gold and other dense minerals.  Gold can either be carried to the beach by an alluvial system or eroded directly by waves.  A famous beach placer is the deposit in Nome, Alaska which is featured in the TV reality show “Bering Sea Gold”.

CreepSaltSusp

Eolian placers form in areas where wind is the main mechanism of erosion and not water.  Eolian placers are similar to Eluvial placers in some ways, they occur close to the hard rock source, and are poorly sorted.  Wind does a terrible job of moving gold.  In Eolian placers the overburden is swept away by strong winds and leave the heavy ore behind.  They occur primarily in desert regions such as the arid regions of Australia.

The last type is paleo-placer deposits.  These are any of the above placer types that happened a long time ago.  By a long time we are talking about millions of years.  Paleo-placers were once placer deposits but over time they have been hidden and covered in sediment.  There is often no sign at the surface of ancient river systems below.  Paleo-placers can be ancient river channels, benches or sedimentary rock formed from old placers such as quartz pebble conglomerate.  This kind of deposit can amount to huge quantities of gold and make you very rich.  The largest known gold deposit in the world in Witwatersrand, South Africa is one of these.  Over 1.5 billion ounces of gold has been mined in Witwatersrand.  Deposition occurred approximately 3 billion years ago in Witwatersrand, and it is estimated that 50% of all the gold mined on earth came from that deposit.

Witwatersrand

That’s the story of where placer gold came from.  It was created in incredibly powerful explosions from dying stars.  It made up the earth as it formed and was squeezed into concentrated deposits by volcanic processes.  The veins eroded into river systems and hopefully made its way into your gold pan.  Gold’s unique properties of density and its resistance to corrosion and most other chemical reactions allow it to build into the kind of deposits that we can find and mine.

 

Where Does Placer Gold Come From? – Part 2 Deposits

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Where Does Placer Gold Come From? – Part 2 Deposits

In part 1 of “Where Does Placer Gold Come From?” we discussed the origins of gold and how it appeared on earth.  Now we’ll discuss how it moves into mineable deposits.

LodeGoldwide

Gold is spread relatively evenly throughout the crust of the earth at approximate concentrations of 1 part per billion.  To put that into contrast, low grade mineable gold deposits need to be concentrated to at least 1 part per million which is about 1000 times more concentrated than the background.  High grade gold deposits are in the order of 20-100 ppm.  Gold concentrations are usually expressed in grams per ton, which is interchangeable to ppm.

So if gold is evenly spread through the crust how does it become concentrated?  There are several natural processes that allow this to happen and they are all driven by the same force, plate tectonics.  Plate tectonics is the force that moves continents, creates mountains and most volcanoes, and of course earthquakes.  The image below shows the current tectonic plates and their names.

Tectonic Plates

The plates are constantly moving, crashing into each other and subducting, they are pushed by convection currents in the mantle.  In the distant past there have been several supercontinents where all the continents had come together to from one.  Past supercontinents have names like Rodinia, Godwana and Pangea, it is predicted that a new supercontinent will occur in the next 250 million years.

Subduction

At the boundaries of these plates is where the excitement happens.  It is at these areas such as the West coast of North America where volcanic processes squeeze gold into veins.  There are two main ways that this happens.  Orogenesis (mountain building) takes place as the force of two plates hitting each other forms mountains as the edges of the plate buckle and slide.  In the mountain forming process rock is squeezed to the point of breaking and creates fissures and faults that extend deep into the crust.  These cracks allow heated and pressurized water to come up the cracks.

The other way is driven  by volcanoes forming from the subducted plate.  When the edge of the plate is far enough below the surface it re-melts and the newly molten rock builds up pressure.  This pressurized molten rock is what forms the volcanic chains inland from the subduction zone.  As volcanoes form they crack and fissure the surrounding rock and contribute to the same epithermal process.

HotSprings

Imagine the rock as a sponge and when it is squeezed the water is expelled through the cracks.  It is actually the same way that hotsprings work but with more squeezing.  This kind of gold mineralization often takes place near volcanic or geothermal activity such as hot springs or geysers.  When the mineralized water cools it leaves behind the minerals in solid form which we then call a vein.  Typically we are looking for quartz veins.  Vein deposits are often called lode deposits in artisanal miner vocabulary.  Placer miners will often refer to the “mother lode” that is the quartz vein or veins that broke down into rich placers.

 

There are other hard rock gold deposits other than epithermal lodes.  There are Greenstone, volcanic massive sulphides, porphyry and Calrin trend deposits.  All of these depend on volcanism as well and occur in similar ways as described above.  Areas high in volcanoes and seismic activity are good places to look for gold.  The Pacific ring of fire is an area surrounding the boundary of the Pacific tectonic plate.  This area contains 3/4 of the worlds volcanoes and is responsible for 90% of the world’s seismic activity.  In the gold rushes of the 1800’s prospectors envisioned a world wide gold belt.  It wasn’t until the 1950’s that plate tectonics became an accepted scientific theory and decades later we mapped out that gold belt.RingofFireROFdepositsOf course not all gold is found in the ring of fire.  The largest known gold deposit on earth is in Witwatersrand, South Africa.  It is estimated that 50% of the gold mined on earth has come from this mine.  Witwatersrand is actually a huge placer deposit from 3 billion years ago.    In my next post we’ll finally get to the formation of placer gold deposits.  Stay tuned.

Check out Part 3 on how gold placers form:

Where Does Placer Gold Come From? – Part 3 Placer

Where does placer gold come from?

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Where does placer gold come from?

If you are trying to find gold it helps to know where it came from.

gold-panning

To start with there is only one kind of gold.  Placer gold and lode gold both come from the same place and are made of the same stuff.  Gold is not actually formed on earth it was formed millions of years ago in distant stars.  In large stars, much larger than our sun elements are combined together in their cores through the process of nuclear fusion.  Our sun like all stars runs on fusion too but it does not have enough mass to produce atoms larger than carbon or oxygen.  Larger stars can generate the gravitational force and heat in their cores necessary to produce elements as heavy as iron.  To create things like gold even more energy is required and that takes place in a supernova.


Supernova

When a large star runs out of light matter the fusion reaction is no longer sustainable and the star begins to collapse on itself very rapidly.  The supernova collapse takes place in a matter of seconds.  While the star is collapsing it produces heat very rapidly and explodes in what is essentially a humongous nuclear bomb.  Supernova events are so bright and powerful that they are brighter than then entire galaxy that hosts the star.  This nuclear explosion allows for higher energy fusion reactions that can produce heavy elements like gold.  The explosion also scatters the newly created material over great distances.
SolarNebula

So how did the star dust make it into the mountains and rivers on earth?  When our solar system began approximately 4.6 billion years ago it was a cloud of dust and gas called a nebula.  This nebula was composed of the remains from older stars that had spread their guts around the universe in supernova explosions.  The molecules of the nebula naturally pulled on each other by the force of gravity growing more and more dense.  As the nebula was collapsing in on itself it also started to spin faster and faster.  The condensing and spinning action formed the nebula into a disk, much like you spin dough into a pizza.  In the center where the force of gravity is the strongest a new star was created, our sun.  The swirling mass around the sun clumped together into the planets, moons, asteroid and comets that we see today.
Early Earth

The early solar system was different that it is today.  The big planets did not form all at once, it was a gradual process.  Small plantoids formed first and crashed and coalesced into each other to form larger planets.  In theory the distribution of gold was basically even in all the rocky material that made up the early solar system.  In the early earth, while it was still completely molten the heavy material (such as iron and precious metals like gold) all sunk to the center of the planet to form the core.  The process is similar to the way that dense material sinks to the bottom of your gold pan.  If you could mine the core you would be very rich but it would be very difficult with current gold mining equipment.  Current scientific theories estimate that there is enough gold in the core to cover the surface of the earth with a 4 meter thick layer of pure gold.


earth-core

We can only reach gold that is trapped in the crust of the earth.  The precious metals in the crust were put there by meteor bombardments that took place after the crust had formed.  As these meteorites crashed into the surface of the earth they disintegrated and mixed their material into the upper mantle.  The meteorite guts had the effect of enriching the amount of precious metals in the crust.

 

So we know where gold came from and how it was formed.  Stay tuned for a future post to learn how the gold formed into deposits in the mountains and streams that we mine.

 

Check out Part 2 & 3 here:

Where Does Placer Gold Come From? – Part 2 Deposits

Where Does Placer Gold Come From? – Part 3 Placer