CLASS 40: ORE DEPOSITS; GEOLOGY OF GOLD INTRODUCTION People have used and treasured metals for thousands

of years. Classically history was organized in terms of metals: none (stone age), copper age, bronze age and iron age. These successive stages are based on increasing ability to separate metals from their ores (exploitable mineral deposits). The earliest metals used were the native metals. These include most gold, some silver and copper and a little bit of iron from meteorites. The oldest artifacts of native metals date from over 20,000 years ago. The next step, about 5,000 years ago, was learning how to separate metals such as copper, tin, lead and zinc from sulfide and oxide minerals. In addition, mixing of metals produced alloys such as bronze (=copper plus tin or zinc) that had more useful properties than the individual components themselves. Iron was more difficult to separate from common iron minerals (hematite, magnetite, pyrite). Aluminum, although the most common metallic element in the crust, was not even discovered until 1825 and was considered a rare and precious metal until little over 100 years ago. Napoleon III supplied his dinner guests with sets of aluminum cutlery, costlier than gold. DISTINGUISHING GOLD FROM PYRITE AND MICA Pyrite, fools gold, is readily distinguished from gold. Gold is much, much denser, has no cleavage, has a gold streak, is malleable and will not burn. Pyrite has cubic cleavage, a black streak and burns in air with a blue flame and sulfurous odor. Weathered mica flakes may have a gold or golden brown color. Their density, however, is very low so the flakes are easily separated from gold dust using a prospectors pan. GEOLOGY OF GOLD Gold occurs in many geologic settings of which three are the most important. Epithermal gold deposits are basically hydrothermal veins associated with subduction zone volcanoes. The magmatic heat sets up a convective system in which hot water rises in the central part of the system and cool water descends around the edges. It is not altogether clear if most of the gold comes from the magma or is leached out of the surrounding country rock. As the gold-bearing fluid rises and cools, quartz, sulfide minerals and gold crystallize in veins. Gold is commonly found associated with rhyolite and andesite volcanoes. The epi in epithermal means that the gold-bearing veins were emplaced at shallow depths (less than a few km underground) and have been exposed by erosion. As the name implies, mesothermal gold deposits were emplaced at greater depths and thus deeper erosion is needed to expose them. Thus it is no surprise that most of these deposits are found in very old rocks of the continental shields (Archaean=older

than 2.5 billion years). The deposits are associated with granite-greenstone belts. These consist of sea-floor volcanics (basalts and komatiites) and marine sediments (largely volcanoclastic turbidites) that have been modestly metamorphosed (to greenstone facies). These rocks were then folded and faulted and intruded by younger granites. The gold occurs in veins intruded into both lithologies but usually near the granite/greenstone contacts. Unlike these two kinds of quartz vein deposits, the third common kind of gold deposit is sedimentary. Placer deposits consist of particulate gold laid down in stream and beach sands. The gold comes initially from vein deposits that then follow the rock cycle (weathering, erosion, transportation, deposition). The gold, being very dense, tends to settle to the bed of the stream wherever the velocity decreases. One puzzling fact is the apparent concentration of gold at the very base of the placers. In some fashion the gold must be able to work its way down to the lowest levels of the deposits. SEPARATION OF GOLD FROM ORE Gold forms only a tiny fraction of most deposits. To get the gold we must separate it from the quartz-rich ore. There are three common processes for doing this. The oldest (the patio process) is to crush the ore and then soak it in mercury (Hg). The liquid mercury dissolves the gold. The gold-bearing liquid is then distilled to drive off mercury vapor and to concentrate the gold. The potential for mercury pollution is vast. A newer process uses cyanide (KCN, a deadly poison) to leach gold from huge piles of ore (heap leaching). The liquid leachate is then mixed with aluminum whereupon the gold is precipitated. The newest processes use gold-loving bacteria to extract the gold from big vats or even directly from streams. You can read more about this on the internet. FAMOUS GEOLOGISTS Many college majors in geology and related fields have achieved great success in other areas. Although their accomplishments probably are primarily due to their intrinsic abilities, it is possible that geological training helped prepare them. Geologic education stresses many different skills such as reading, writing, measuring, drawing and calculating. Computer skills are important. Much student work is done in teams so interpersonal skills are essential. Geology students also deal with land owners, government officials, civic groups and even people in other countries. Geologists have to work with incomplete and imprecise data. This breadth of theory and practice provides a strong background for success in many other fields as the list below will demonstrate. Colin Powell (geology): Chief of Staff of the Army, Secretary of State.

Bruce Babbitt (geophysics): Secretary of the Interior. Harrison Schmidt (geology): First scientist on the moon, US Senator. Harry Wu (geology): Revealed secrets of Chinese gulag. Emil Constanescu (geology): President of Rumania. Morgan Fairchild (geology ??): TV actress. Herbert Hoover (mining engineer): President of USA. Bob (?) Farrelly (geology): movie producer (Dumb and Dumber, Something About Mary). Wen Jiabao (geology): Primier of China.