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Platinum, gold, and asteroids

5 Dec 2012 by Jim Fickett.

Most of the earth's available platinum and gold is probably derived from asteroids, and there is now a serious plan to mine the asteroids directly. Any influence on markets is a long ways off, but it is possible that precious metals could become significantly less precious in the long run.

Here's an interesting tidbit: Although there were heavy elements in the original dust cloud from which the earth formed, essentially all them sank into the core. So the gold and platinum now found in the crust had to come from somewhere else. There are several sources of evidence that the platinum and gold we currently see in the crust came from the asteroids.

First, the ratio of elemental concentrations differs in different regions of the solar system, and what we find on earth matches some of the asteroids:

During a 12-year study, Dr Schmidt and colleagues have analysed the concentrations of HSE [a group of metals including rhenium (Re), osmium Os), iridium (Ir), ruthenium (Ru), rhodium (Rh), platinum (Pt), palladium (Pd) and gold (Au)] at meteorite impact sites around the world, as well as in the samples from the Earth’s mantle and crust. …

Although HSE were present in the nebula from which the Earth formed, as the young planet evolved and heated up they were stripped, along with other heavy elements, from the silicate mantle into the iron and nickel-rich metallic core. The presence of HSE in the mantle is still a matter of debate. However, a widely accepted theory is that HSE were added by meteorite impacts as a veneer of material over the Earth’s surface after the core had formed, about 20-30 million years after the planet’s accretion. This could have been by the collision with a Mars-sized impactor that led to the formation of the Moon.

Different classes of meteorites have characteristic elemental ratios of HSE that give indications where in the Solar System they formed. However, the characteristic ratios of HSEs in the Earth’s upper mantle (for example the ruthenium/iridium element ratio of about 2) match up with theoretical predictions for asteroids formed in the Mercury-Venus region.

Second, older lava flows have less platinum, showing that the platinum in the mantle accumulated over time, and hence was not in the original material from which the earth formed:

Report author CSIRO Minerals Down Under Flagship researcher Dr Stephen Barnes said the study group collected a large body of data on the platinum content of lava flows called komatiites, which host some of the world’s major nickel deposits.

“We found that the oldest komatiites have the lowest platinum content,” Dr Barnes said.

“The platinum content gradually increases from about 3.5 billion years to 2.9 billion years ago.

“This tells us that the deep source where the komatiite came from, down near the boundary between the Earth’s core and mantle, was gradually gaining platinum over time”.

The paper’s authors now think they know why.

“When the Earth’s core formed, it took all the available platinum with it, leaving the mantle and crust with none,” Dr Barnes said.

“Following that, a steady rain of meteorites created the so-called Late Veneer – a thin surface layer of meteorite debris rich in platinum.”

With time through large-scale convection processes, which now cause plate tectonics, this material was stirred down into the interior of the Earth.

Third, the ratio of different isotopes of tungsten differs from ancient to modern rocks, and this is consistent with modern tungsten, and other heavy elements, having arrived at the earth in meteorites:

Many precious, ‘iron-loving’ metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet’s deep interior. One explanation of their over-abundance is a ‘late veneer’—a flux of meteorites added to the Earth after core formation as a ‘terminal’ bombardment that culminated in the cratering of the Moon. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio 182W/184W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon (more than four billion years ago) or core–mantle interaction, do not explain the W isotope data well. The decrease in mantle 182W/184W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in 142Nd/144Nd. We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.

Okay, so all that is intellectually interesting, but what does it have to do with investing? This: there are asteroids whose current orbits bring them very near to earth, and that have platinum content ten times higher than any ore on earth. Some investors believe such asteroids could be profitably mined and might provide platinum as a lower price than current mining.

First, here are some of the key facts from a May 2000 analysis by Brad Blair, professor of metals economics at the Colorado School of Mines, suggesting that mining asteroids could be profitable:

Two classes of asteroid contain high concentrations of platinum group metals (PGMs): Metallic Asteroids and LL Chondrites. Members of both groups have been identified in orbital trajectories close to Earth, having the potential for significant transportation cost leverage. …

Platinum, rhodium, iridium, palladium and gold are found in significant concentrations (total PGM content exceeds 50 grams per ton) in meteorite samples attributed to the LL Chondrites …

Estimated reserves [in the largest earth deposits have] an average grade of 0.25 ounces per ton. [i.e. about 10 times lower than the asteroids mentioned above] …

Many asteroids lie much ‘closer’ to Earth than the Moon. The orbital alignment of the near-Earth asteroids brings them alarmingly close to Earth’s orbit on a regular basis. During the period of alignment, the amount of energy required to reach Earth orbit can be as little as 1/1000 the energy as an equivalent spacecraft launched from Earth’s surface. …

Given the estimated population density of LL chondrite NEAs, there are over 8000 candidate bodies less than 100 meters that are high enough in grade to host platinum production. It is important to note that a 100-meter sphere of rock weighs in at almost 1.4 million metric tons, and could contain over 40 tons of platinum at grades measured in meteorite samples. …

Sonter (1997, p.144) estimates the equipment mass to move a 20 meter “arjuna” type asteroid (very low orbital transfer energy) into low-Earth orbit to be less than two tons. This would include mining equipment, a power supply and simple thrusters to maneuver the asteroid into Earth orbit using water extracted from the asteroid. A simple calculation using the Advanced Missions Cost Model (Cyr, 1988 – developed to estimate costs for human planetary exploration missions) yields an estimated cost of between $500 Million and $1 Billion to construct a two-ton prototype spacecraft.

Obviously there are many uncertainties here, but some investors have been convinced, and an effort to mine the asteroids is now underway:

Google Inc executives Larry Page and Eric Schmidt and filmmaker James Cameron are among those bankrolling a venture to survey and eventually extract precious metals and rare minerals from asteroids that orbit near Earth, the company said on Tuesday.

Planetary Resources, based in Bellevue, Washington, initially will focus on developing and selling extremely low-cost robotic spacecraft for surveying missions.

Clearly any influence on the precious metals market is a long ways off. But there is at least a small chance that gold and platinum might, in the future, become much less rare, changing market dynamics and lowering prices significantly.