Gold Prospecting in the New West
by Rodney Watkins
EDITOR'S NOTE: This article is reprinted from LORE magazine (vol 39, no.
2, (Summer, 1989), p. 26-30), a benefit of museum membership. ©1996 Milwaukee
Public Museum, Inc.
For most people, the mention of gold prospecting is likely to evoke an image of men in slouch hats and western garb, armed with gold pan, pick and shovel and
accompanied by a trusty burro packed with supplies for several months in the wilderness. As we know from film and novels, these hardy fellows spend much of their lives combing the hills, for that vein of precious yellow metal which will make them rich. In reality, the modern western prospector typically works for a salary, discovery or not, and he is likely to wear a baseball cap with a company logo. He uses a four-wheel- drive vehicle instead of a burro, usually stays in motels, and his prospecting tools include topographic maps, a plastic garden trowel, cloth bags, compass and tape measure. Even more incongruously, the modern prospector almost never sees the gold he is searching for, even when his
work result in a major discovery and the opening of a mine.
Gold prospecting, or to use the preferred term in the business, gold
exploration, is currently booming in the western states. Two major factors account for this activity. Int he early 1970s, the government deregulated the price of American gold. Long held at $35 an ounce, the price of gold was allowed to float on the open market, reaching a high of over $800 an ounce by 1980. gold has been fluctuating around $400 an ounce for the past few years, down from its
all-time high but still at a healthy enough level to encourage production. The second major factor to boost the American gold industry has been the development of a gold recovery technique called heap leaching. In this process, crushed ore
is spread on concrete pads and drowned in solutions which literally leach the gold from the rock. When operated on a large scale, heap leaching recovers large amounts of gold which cannot be economically obtained by conventional smelting methods.
These two factors have revolutionized the definition of economically minable gold ore. Rich veins with visible gold at the ground surface have virtually all been discovered and mined over the past one hundred and forty years. In contrast to these veins, which carried a half-ounce or more of gold per ton of rock, the deposits sought by today's prospector (or explorationist, as he prefers to be
called) contain less than one tenth of an ounce of gold per ton and are usually concealed beneath the ground. The gold in such deposits occurs as microscopic particles, and millions of tons of rock must be mined to obtain economic results. The search for these large volume, low grade gold deposits has produced a type of prospecting which is quite different from that of years gone by.
The first step in modern gold prospecting is the selection of a target area with gold potential. Unlike the old-time prospectors who travelled patiently from one stream or mountain to another, the modern explorationist tries to cut his time and expenses by narrowing the field of search. This is done partly by following the footsteps of the old-timers, for abandoned mining sites are
attractive targets for further exploration. The reason for the attention to historical workings is by no means sentimental. High grade gold veins, discovered and mined out long ago, are often an indication of large, low grade gold deposits in the same area.
The other method of selecting a target area is more sophisticated and involves the use of geologic theories of ore formation. Gold deposits are formed in far too many ways to review in this article, but one important theory, the volcanic hot springs model, serves as a good example. some twenty years ago, a handful of economic geologists began investigating gold occurrences in relation
to volcanic activity. Volcanos are fed by magma combers at depth. Magma is molten, semi-liquid rock material, and magma deep in the each is both a heat source and a "chemical soup" of many elements, both rare and common. Not all magma reaches the each's surface as lave, and much of it forces its way into subsurface rocks to cool and harden underground. In the process of doing this,
the magma can produce heated, mineral-laden fluids called hydrothermal solutions. One element in some hydrothermal solutions is gold, and under the right conditions of pressure and temperature, hydrothermal solutions disseminate large amounts of microscopic gold in the rocks through which they pass. This
activity occurs hundreds to thousands of feet below ground, but it often has a surface expression as volcanic hot springs.
Armed with this theory in the late 1970s, geologists from Homestake Mining Company, a major American gold producer, began exploring a target area in northern California with evidence of ancient volcanic hot springs. This was an area passed over by the forty-niners, and no gold had previously been found
within it. In this case, the theory proved correct, and Homestake found a low- grade gold deposit which has been developed into one of the largest gold miners in the country. The successful test of the volcanic hot springs model was a break-thorough in gold exploration,a nd areas with similar geology are being
actively explored throughout the west.
Once a target area for exploration has been selected, the real work on the ground begins. The first step in this work is geochemical sampling. Gold deposits, even when buried below the surface, can leave their signature as trace amounts of metal in the overlying soil. Geochemical soil sampling is a standard entry- level job in the mining industry. A small hole is dug and about a pound
or so of dirt is placed in a new cloth sample bag. A plastic garden trowel is used for digging,a nd the person taking the sample allowed to wear no metal rings of any kind. Such care is necessary to prevent contamination of the sample. The bag of soil is sent to an assay laboratory, and using a technique called neutron activation analysis, which identifies gold from its atomic properties, the laboratory determines the amount of any gold in the sample. Gold
content of the sample is measured in parts per billion or parts per million of he total soil. gold content of, for example, 20 parts per billion is obviously
a very small amount, but even this result can be a significant sigh of buried ore. Other elements which are associated with gold deposits, such as silver and mercury, are often analyzed as well.
Besides taking the sample, the modern prospector also plots its location precisely on a topographic map. The first phose of geochemical sampling often covers areas of one hundred square miles or more. Based ont he initial survey, those areas with the most promising soil values are visited again, and a second
phase sampling program is begun. In this second phase, tape and compass are used to lay out a grid upon the ground, and soil samples are taken every one hundred feet at the corners of the grid. Geochemical soil sampling is unglamorous work, often conducted in extremes of weather and topography, and it requires the collection of thousands of samples to be effective. The end result of a geochemical sampling program is a map on which all sample values are plotted and areas of highest soil values for gold or other elements are identified. Such areas, called soil anomalies, usually cover a few acres or less, and they often identify the location of gold ore bodies at depth. Rock outcrops around the
anomalies are also samples and mapped in detail.
These prospecting activities ar conducted by a team of geologists. Target areas and application of geologic theories are determined by a senior company geologist. A crew chief is employed to coordinate field activities, a specialized geologic mapper (my own job while employed int he business) is used as needed, and several young geologists, typically fresh out of college, are hired to take the soil samples. Another aspect of the work is the staking of federal mining claims on favorable areas. Claims are usually staked between the
first and second phases of geochemical sampling, long before any decision to actually mine an area had been made. Depending on the size of the target area, from one to three field seasons are required for the prospecting and claim staking.
All of this activity is directed toward the final and most critical phase of gold exploration, which is drilling. No legitimate mining company ever begins a mine without drilling, a fact which is often unknown the investors who fall prey to gold scams. Drilling, of course, provides a direct sample of the subsurface rock, and it is the ultimate test of the presence or absence of minable gold
ore. Drilling is also, however, the most expensive part of an exploration program, and the whole purpose of the initial geologic investigation and geochemical sampling is geared to picking the most favorable drill sites. Like other business people, gold explorationists work within the limits of a budget,
and before the drill rigs arrive on a property, great care must be taken to see that available funds for drilling are efficiently spent.
Two basic forms of drilling are used in gold exploration. The least expensive of the two is called rotary drilling, and a single hole is drilled to depths of anywhere from 100 to 800 feet, depending on geologic features of the target area. Rotary drilling crushed the rock as the drill bit moves through it, and the material brought to the surface consists of rock ships one half inch or less in size. Chips are collected for every five to ten foot interval of the hole,
and samples are sent to an assay laboratory for the same type of gold analysis performed on soil samples. Rotary drilling is relatively fast (a good crew can drill through several hundred feet of rock per day), but downhole contamination of chips is sometimes a problem, and the geologic information which can be obtained from chips is often limited. Core drilling is much more expensive, and
generally less that one hundred feet of drilling is accomplished per day. Core drilling, which yields excellent geologic information, produces solid rock cores from one to two inches in diameter. The core is split into sections which are also analyzed for gold content each five to ten foot interval of the hole.
A modern gold exploration program is an expensive proposition, and well over a million dollars are generally spent from the time of target area selection to drilling. As a rule of thumb, a drilling program is considered a success if several holes penetrate rock with gold values of five one-hundredths of an ounce or more per ton. Even at this stage of exploration, the gold particles are
usually too small to be visible, and assay results determine if the rock is minable ore. All serious gold exploration culminates in drilling, but fewer than one in ten projects result in discovery of an ore body. A successful drilling program, the dream of every new west gold prospector, is followed by development
of a large open pit mine, which is another story in itself.
2, (Summer, 1989), p. 26-30), a benefit of museum membership. ©1996 Milwaukee
Public Museum, Inc.
For most people, the mention of gold prospecting is likely to evoke an image of men in slouch hats and western garb, armed with gold pan, pick and shovel and
accompanied by a trusty burro packed with supplies for several months in the wilderness. As we know from film and novels, these hardy fellows spend much of their lives combing the hills, for that vein of precious yellow metal which will make them rich. In reality, the modern western prospector typically works for a salary, discovery or not, and he is likely to wear a baseball cap with a company logo. He uses a four-wheel- drive vehicle instead of a burro, usually stays in motels, and his prospecting tools include topographic maps, a plastic garden trowel, cloth bags, compass and tape measure. Even more incongruously, the modern prospector almost never sees the gold he is searching for, even when his
work result in a major discovery and the opening of a mine.
Gold prospecting, or to use the preferred term in the business, gold
exploration, is currently booming in the western states. Two major factors account for this activity. Int he early 1970s, the government deregulated the price of American gold. Long held at $35 an ounce, the price of gold was allowed to float on the open market, reaching a high of over $800 an ounce by 1980. gold has been fluctuating around $400 an ounce for the past few years, down from its
all-time high but still at a healthy enough level to encourage production. The second major factor to boost the American gold industry has been the development of a gold recovery technique called heap leaching. In this process, crushed ore
is spread on concrete pads and drowned in solutions which literally leach the gold from the rock. When operated on a large scale, heap leaching recovers large amounts of gold which cannot be economically obtained by conventional smelting methods.
These two factors have revolutionized the definition of economically minable gold ore. Rich veins with visible gold at the ground surface have virtually all been discovered and mined over the past one hundred and forty years. In contrast to these veins, which carried a half-ounce or more of gold per ton of rock, the deposits sought by today's prospector (or explorationist, as he prefers to be
called) contain less than one tenth of an ounce of gold per ton and are usually concealed beneath the ground. The gold in such deposits occurs as microscopic particles, and millions of tons of rock must be mined to obtain economic results. The search for these large volume, low grade gold deposits has produced a type of prospecting which is quite different from that of years gone by.
The first step in modern gold prospecting is the selection of a target area with gold potential. Unlike the old-time prospectors who travelled patiently from one stream or mountain to another, the modern explorationist tries to cut his time and expenses by narrowing the field of search. This is done partly by following the footsteps of the old-timers, for abandoned mining sites are
attractive targets for further exploration. The reason for the attention to historical workings is by no means sentimental. High grade gold veins, discovered and mined out long ago, are often an indication of large, low grade gold deposits in the same area.
The other method of selecting a target area is more sophisticated and involves the use of geologic theories of ore formation. Gold deposits are formed in far too many ways to review in this article, but one important theory, the volcanic hot springs model, serves as a good example. some twenty years ago, a handful of economic geologists began investigating gold occurrences in relation
to volcanic activity. Volcanos are fed by magma combers at depth. Magma is molten, semi-liquid rock material, and magma deep in the each is both a heat source and a "chemical soup" of many elements, both rare and common. Not all magma reaches the each's surface as lave, and much of it forces its way into subsurface rocks to cool and harden underground. In the process of doing this,
the magma can produce heated, mineral-laden fluids called hydrothermal solutions. One element in some hydrothermal solutions is gold, and under the right conditions of pressure and temperature, hydrothermal solutions disseminate large amounts of microscopic gold in the rocks through which they pass. This
activity occurs hundreds to thousands of feet below ground, but it often has a surface expression as volcanic hot springs.
Armed with this theory in the late 1970s, geologists from Homestake Mining Company, a major American gold producer, began exploring a target area in northern California with evidence of ancient volcanic hot springs. This was an area passed over by the forty-niners, and no gold had previously been found
within it. In this case, the theory proved correct, and Homestake found a low- grade gold deposit which has been developed into one of the largest gold miners in the country. The successful test of the volcanic hot springs model was a break-thorough in gold exploration,a nd areas with similar geology are being
actively explored throughout the west.
Once a target area for exploration has been selected, the real work on the ground begins. The first step in this work is geochemical sampling. Gold deposits, even when buried below the surface, can leave their signature as trace amounts of metal in the overlying soil. Geochemical soil sampling is a standard entry- level job in the mining industry. A small hole is dug and about a pound
or so of dirt is placed in a new cloth sample bag. A plastic garden trowel is used for digging,a nd the person taking the sample allowed to wear no metal rings of any kind. Such care is necessary to prevent contamination of the sample. The bag of soil is sent to an assay laboratory, and using a technique called neutron activation analysis, which identifies gold from its atomic properties, the laboratory determines the amount of any gold in the sample. Gold
content of the sample is measured in parts per billion or parts per million of he total soil. gold content of, for example, 20 parts per billion is obviously
a very small amount, but even this result can be a significant sigh of buried ore. Other elements which are associated with gold deposits, such as silver and mercury, are often analyzed as well.
Besides taking the sample, the modern prospector also plots its location precisely on a topographic map. The first phose of geochemical sampling often covers areas of one hundred square miles or more. Based ont he initial survey, those areas with the most promising soil values are visited again, and a second
phase sampling program is begun. In this second phase, tape and compass are used to lay out a grid upon the ground, and soil samples are taken every one hundred feet at the corners of the grid. Geochemical soil sampling is unglamorous work, often conducted in extremes of weather and topography, and it requires the collection of thousands of samples to be effective. The end result of a geochemical sampling program is a map on which all sample values are plotted and areas of highest soil values for gold or other elements are identified. Such areas, called soil anomalies, usually cover a few acres or less, and they often identify the location of gold ore bodies at depth. Rock outcrops around the
anomalies are also samples and mapped in detail.
These prospecting activities ar conducted by a team of geologists. Target areas and application of geologic theories are determined by a senior company geologist. A crew chief is employed to coordinate field activities, a specialized geologic mapper (my own job while employed int he business) is used as needed, and several young geologists, typically fresh out of college, are hired to take the soil samples. Another aspect of the work is the staking of federal mining claims on favorable areas. Claims are usually staked between the
first and second phases of geochemical sampling, long before any decision to actually mine an area had been made. Depending on the size of the target area, from one to three field seasons are required for the prospecting and claim staking.
All of this activity is directed toward the final and most critical phase of gold exploration, which is drilling. No legitimate mining company ever begins a mine without drilling, a fact which is often unknown the investors who fall prey to gold scams. Drilling, of course, provides a direct sample of the subsurface rock, and it is the ultimate test of the presence or absence of minable gold
ore. Drilling is also, however, the most expensive part of an exploration program, and the whole purpose of the initial geologic investigation and geochemical sampling is geared to picking the most favorable drill sites. Like other business people, gold explorationists work within the limits of a budget,
and before the drill rigs arrive on a property, great care must be taken to see that available funds for drilling are efficiently spent.
Two basic forms of drilling are used in gold exploration. The least expensive of the two is called rotary drilling, and a single hole is drilled to depths of anywhere from 100 to 800 feet, depending on geologic features of the target area. Rotary drilling crushed the rock as the drill bit moves through it, and the material brought to the surface consists of rock ships one half inch or less in size. Chips are collected for every five to ten foot interval of the hole,
and samples are sent to an assay laboratory for the same type of gold analysis performed on soil samples. Rotary drilling is relatively fast (a good crew can drill through several hundred feet of rock per day), but downhole contamination of chips is sometimes a problem, and the geologic information which can be obtained from chips is often limited. Core drilling is much more expensive, and
generally less that one hundred feet of drilling is accomplished per day. Core drilling, which yields excellent geologic information, produces solid rock cores from one to two inches in diameter. The core is split into sections which are also analyzed for gold content each five to ten foot interval of the hole.
A modern gold exploration program is an expensive proposition, and well over a million dollars are generally spent from the time of target area selection to drilling. As a rule of thumb, a drilling program is considered a success if several holes penetrate rock with gold values of five one-hundredths of an ounce or more per ton. Even at this stage of exploration, the gold particles are
usually too small to be visible, and assay results determine if the rock is minable ore. All serious gold exploration culminates in drilling, but fewer than one in ten projects result in discovery of an ore body. A successful drilling program, the dream of every new west gold prospector, is followed by development
of a large open pit mine, which is another story in itself.
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Cored section of low-grade gold ore from the Mohave
Desert of California. This rather unspectacular rock is rhyolite tuff breccia, and the microscopic gold which it contains can be detected only in an assay laboratory. Large volumes of such low grade ore form the foundation for modern western gold mining. (Core diameter is approximately 3 inches.) |
High-grade gold ore from Colorado. such occurrences of
visible gold flakes in quartz veins -- the dream of the old-time prospector -- are now so rare as to be economically negligible in western mining. Phot of museum speciment enlarged to show detail. |
Rotary drill rig at work on a gold exploration project in Nevada. note lack of rock outcrops in immediate area; this is a typical example of the type of exploration target located by geochemical soil sampling. |
