BY PETE H. McLAUGHLIN, Assayer
In the study of any subject, it is often helpful to step back and get the whole field in perspective before you try to sort out the details. Looking for lode or placer deposits is that field of study which locates the current resting place of gold, and raises the question:
“How did the gold get there?” The trail, when followed back, will take you some 10 billion years!
Current theory of the beginnings of this universe state that some 15 billion years ago, there was a single super dense point of energy which was incredibly hot and smaller than a proton. It exploded; and in a flash, produced photons which are particles of light. The photons collided and produced electrons, protons and neutrons of matter and anti-matter. The matter and anti-matter collided and produced photons.
This cycle continued as the universe expanded and cooled until it became too cool to support these reactions. This caused a final reaction between the matter and anti-matter particles that turned most of the mass in the universe back to photons and left the slight excess of matter which we now see as the planets and stars in the universe today.
At this point, four minutes after the start of the “Big Bang,” the mass of the universe is 76% hydrogen, 24% helium with trace quantities of lithium. The temperature is about a billion degrees and the universe is still rapidly expanding. None of the heavier elements had formed in this first “Big Bang,” because the conditions were not right. About a million years go by, and the hydrogen and helium clouds condense in space where chance caused slightly denser clouds of gas, gravity takes over and makes the cloud still denser. The pressure builds up and so does the heat; until finally, hydrogen fuses to helium and a star is born.
These first stars are called Population-III stars. As they burned up their hydrogen, they became denser and hotter, until they reached the temperature of 200-million Kelvin. Now, helium begins to react to produce beryllium and then carbon and oxygen. This is important, as all the oxygen you and I breathe, and the paper this story was written on, are made of the oxygen, hydrogen, and carbon that were formed in a Population- III star two to three million years after the beginning of the universe!
The carbon and oxygen reactions in a star are very rapid; and in a couple of’ centuries, the star is again heating until it reaches a temperature of around 800-million Kelvin, and new reactions take place that produce neon, magnesium, and sodium. A little more heat and you have silicon. A little more pressure, and the silicone becomes chlorine; and magnesium becomes aluminum.
The Population-III star now becomes so dense that the gamma rays which have supported its mass react with each other, and the star collapses and goes super-nova, spewing its mixture of chemicals into the interstellar medium. There are no Population-III stars left in our galaxy. All of them exploded some three to four million years after the “Big Bang.”
Population-II Stars, or how to make gold in your own nuclear furnace
The matter spewing out of Population-III stars floods the universe, so that stars forming four million years after the Bang are contaminated by heavier elements than were available for the Population-III stars. The Population-II stars bum their hydrogen to helium as the Population-III stars did, and begin to burn the heavier elements until they produce iron; at which point they reach the end of the line. If their mass were about that of our sun, they would become red giants; and as the outer surface cools, it can condense at a distance too far to be pulled back to the star, which is now a white dwarf. If the mass is heavier than our sun, things get very lively indeed.
More and more, matter in the core is converted to iron and cannot go any further. The reactions in the core cease; and as this happens, there is less and less radiated energy to support the surface of the star, and it collapses. For a brief period, measurable in minutes, the density of the star and its temperature rise beyond that needed to fuse elements heavier than iron. There is a flash of light brighter than the output of a whole galaxy of stars, and a super-nova occurs that will flash, expand and cool to nothing in a few-day’s time. In the middle of this maelstrom, small amounts of bismuth, lead, uranium and gold are formed and then scattered out between the stars in the resulting explosion.
After the super-nova, the Population-II star will become a black hole if it retains enough mass, or a neutron star. If the star is about seven times the mass of our sun or less, it will become a white dwarf.
Population-I Stars or, Home Sweet Home!
Now we get down to current history. Ten billion years after the beginning of the universe, most of the Population-II stars have super-nova’ed, and the interstellar medium is loaded with small amounts of the 96 elements available in nature today. A cloud of these materials, still containing mostly hydrogen, condenses and forms a swirling disk. The center pulls most of the mass into itself because centrifugal force is weakest here, and the mass heats up until fusion starts and our sun is born. The radiation from this new sun does the first job of concentrating gold for you, by driving off the lighter elements in the disk of matter around it. As the planets condense and develop a gravity field that pulls in more mass, the mass becomes richer in heavy elements, and lacks in the preponderance of hydrogen that the sun has. The closer to the sun the planet is, the more the heavy elements appear to be predominate.
At first, the Earth is relatively evenly-mixed and composed of hydrogen, helium, carbon, nitrogen, oxygen, neon, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, argon, calcium, iron, and nickel and a trace of the rest of the 96 elements, including gold. As its mass increases, pressures and temperatures rise. The natural decay of thorium and uranium contribute to the temperature rise, and the core of the planet becomes liquid, allowing the heavy elements (mostly iron) to drain to the center of gravity. The lighter elements hydrogen, helium, oxygen, nitrogen, neon, and much of the sulfur boil off and are lost because the gravity of the planet is too weak to hold them.
As more matter is captured by the gravity of the forming Earth, the gravity-field strengthens and an atmosphere is retained. The molten iron in the core begins to flow, in rising and falling currents that act like a generator to produce electricity and the magnetic field of the planet. The silica, iron, calcium, magnesium, sodium and potassium to form the rocks of the mantle and crust.
Gold Deposits in the Mantle and Crust
Present theory is that gold and other heavy elements would have been evenly spread in the forming Earth. However, the percent of gold present in interstellar matter is extremely minute. As the core melted, gold would amalgamate along with iron and nickel. The silicates floated above the core and formed the mantle which extends from a few miles below the present surface to about 2,000 miles down. It is composed of iron and magnesium silicates called Olivine with minor amounts of other impurities like gold. The surface of the planet is called the crust and is about 20 miles thick under the continents. Below the oceans, the crust is much thinner or completely absent. The crust differs from the mantle in that the rocks are made up of the elements sodium, magnesium, aluminum, and calcium.
Gold is present in the crust and mantle in very low concentration. Tests on California granite show an average of 0.103 parts per million, which works out to 0.003 ounces per ton. T.K. Rose, in his book, “The Metallurgy of Gold,” cites a number of assays on rock samples taken from locations remote from known gold deposits. The values range from 0.03 ounces to 0.003 ounces of gold per ton.
High grade lode gold sites which have been worked in the past are the result of hydrothermal concentration or organic deposition.
Hydrothermal concentration is the leaching of deep mantle rock by water from the surface of the planet. Under the ocean, those geysers found during the last few years are evidence of this process continuing into the present time. Seawater seeps down into the mantle where it is heated and makes contact with the olivine rock. This rock contains small amounts of gold, manganese, and cobalt and larger quantities of iron and sulfur. These are leached from the rock and the heated seawater begins to rise until it finds a vent where it returns to the sea and deposits the sulfides of these metals as it cools. The process is the same on land; and many of the world’s gold deposits were created from this process. The hydrothermal solutions at work on land are evident at Yellowstone National park where “Old Faithful” spews its lode of mineral-rich water on the hour, and hundreds of hot springs and mud holes smell of the sulfur-rich mineral they are bringing to the surface.
Organic deposition is similar to the carbon recovery from leach solutions which is used so much today. Ocean water contains gold in varying amounts; but the average is something like 120 to 130 tons to the cubic mile of sea water. If this solution is passed through previous rock or gravels containing organic matter, some of the gold will be deposited on the carbonaceous material. The gold reefs of South Africa were probably derived from this process. T. K. Rose theorizes in his book that much of the gold deposited in sedimentary rocks was by the process of adsorption on carbonaceous material.
Where to Look
Gold is still where you find it. However, with the carbon heap leach process, the average value of profitable ore is now down to the .03 to .05 area under good conditions, and this is only 10 times the concentration of gold in average crustal rock. This makes it a little more likely that if you look diligently, you’ll be successful.
It is amazing that all the material of this planet, including the gold, was once the heart of a star; and it had to be blasted from that star by a super-nova before you and I could come along and make use of it as a planet to live on. But, these are the facts as best as the scientific community can work them out at this time.