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Mapping stream sediments for resource exploration
The successes of the old-time and latter-day prospectors have diminished the likelihood for the discovery of additional mineral resources on the surface of our planet. Yet our national and global dependence on mineral resources continues to grow unabatedly, and recycling can only provide a fraction of our needs. By necessity, today s search for the many minerals vital to society is focused on ore deposits that lie beneath the Earth s surface.
Earlier in this Session (back by the illustration of the copper-molybenum porphyry cross section) we discussed the use of models to locate ore deposits . Another way of locating mineral resources is by identifying element-dispersion halos. Dispersion halos are abnormal levels of the metals that develop around deposits. This halo can extend for long distances from the deposit and, once recognized, can be used to trace down the source. The most familiar example of a halo is the dispersion of gold nuggets in drainages downstream from gold mother lodes.
Using today s technology, collected stream-sediment samples may be processed and analyzed for as many as 40 elements, giving an indication of very faint halos at some distance from a variety of deposit types. If elements of economic interest, such as gold, silver, copper, lead, or zinc, are present, they will be revealed in these analyses. This process is repeated for many samples until the entire study area is covered.
By evaluating our nation\'s mineral resources, we can determine tha appropriate use of Federal lands. Helicopters have little impact on the land and can be used in remote areas, such as Alaska, to efficiently gather samples for geochemical analysis.
Keeping track of the resources of our country
Congress has mandated the USGS to assess the mineral-resource potential of public lands, especially those lands set aside as wilderness or proposed wilderness. These assessments provide an inventory of mineral resources for future generations. In 1964, the Wilderness Act was passed, and a 20-year program to assess the mineral resources of U.S. Forest Service wilderness areas began. A large amount of this work involved the analysis of stream sediments to determine the presence or absence of halos. Subsequent laws have required mineral-resource assessments on additional public lands. The USGS also works with the Bureau of Indian Affairs and individual tribes to assess the mineral resources on Indian lands.
By relating ore-deposit models and geochemical data to geologic observations and plate tectonic theory, geologists can predict what types of ore deposits may be found in a given geographic area. The USGS supplies this information to the public and to other government agencies. Assessments are published by the USGS for use by land-use planners, Federal, state, and local government agencies, environmentalists, and private individuals. Many maps, such as the map of lead in stream sediments of Colorado, are useful for both resource evaluations and environmental assessments.
This map of lead in Colorado stream sediments was generated with existing data from the National Geochemical Data Base. It shows the presence of a geochemical halo from the Colorado Mineral Belt and also the lead caused by industry in some cities.
It is important to weigh the mineral-resource potential of a tract of land against other potential uses such as water resources, grazing, forestry, recreation, tourism, and scenic value. Chemistry plays a vital role in this assessment process. Return to this point in index.
Mobile laboratories
Looking for halos of mineralized areas or testing for pollution is like playing the game of hide-and-seek. The target can be found more easily if you are given hotter or colder clues. To provide these clues, analysts in mobile laboratories perform chemical analyses for geologists in the field. As a result, samples can be evaluated quickly. The use of mobile laboratories by the USGS dates back to the turn of the century. These pictures show a mobile laboratory used in Montana. It was a horse-drawn wagon that carried the necessary reagents, glassware, etc. that were set up in a tent.
You can find the halo of a deposit faster, if you are constantly aware whether you are getting closer or farther away. To rapidly determine the distribution of elements in the field in 1907, portable analytical equipment was used.
When the field area was reached, a tent was set up and wet chemical analyses were performed in primative conditions from a kneeling position.
Over the years, the mobile laboratories have become more refined. Since the 1960 s, these laboratories have provided USGS geologists and geochemists with over 1 million analyses, providing timely information for evaluating the mineral-resource potential of public lands.
Today, the principle of mobile laboratories is the same as the wagon of earlier times, but the equipment is significantly more advanced in technology. In addition to clean, relatively comfortable surroundings, which are protected from the weather, sophisticated electronic equipment shown here can be used to run a large number of sensitive analyses.
Exploration for covered ore deposits
Ore deposits covered by transported overburden, such as gravels, are more difficult to locate than ore deposits that are buried in the host rock in which they formed. New research using super-sensitive analytical techniques provide scientists with a way to see through that covering.
This research is based on the idea that buried ore deposits may release trace amounts of ore-related elements that are transported through the overburden. These trace elements that are found at the surface, however, may have been originally introduced with the transportation of the overburden and don t necessarily indicate the presence of a covered ore deposit. The ability to distinguish between the trace elements already in the overburden and those migrating from an ore deposit would provide a powerful tool for subsurface exploration. Two of the methods that are currently being researched by the USGS are ground-water analysis and selective chemical extractions of overburden samples for the loosely bonded migrating elements on the surface of the gravel fragments.
Ground water collected from wells, springs, and drill holes may provide clues to the presence of covered deposits. This water moves very slowly through the overburden until it discharges at the surface as a spring or seeps into a body of water. Subsurface flow rates vary from almost zero to over 100 feet per year. The slower rates cause water to have a longer contact time with the subsurface gravels, rocks, and, if present, ore deposits, permitting minute amounts of metals to be leached from the rocks.
Geochemists can sample water from previously drilled holes to detect the \"halo\" of an ore deposit.
Detecting gold in a ground-water dispersion pattern requires an extremely sensitive analytical technique. The USGS has developed a method for detecting gold in water at the one-part-per-trillion (ppt) determination level. One ppt could be represented by one marble on 20,000 football fields (almost 39 square miles) covered with marbles.
In this technique, gold ions are removed from relatively large-volume water samples by the use of anion-exchange resin, in a manner similar to the exchange of ions that takes place inside a commercial water softener. Later, the gold ions are stripped from the resin and analyzed using graphite-furnace, atomic- absorption spectroscopy. (AAS is discussed in the Maps of natural contamination section)
The USGS is working on a new method to gather information from nonproductive drill holes.
Using a simple device, a ground-water sample is recovered from the drill hole in hopes that it will show proximity to an ore deposit.
The relatively large dilute water sample is filtered and stabilized prior to being transported to a mobile laboratory for analysis.
Mineral scavengers provide a clue
There is another way of detecting the trace elements carried from a deposit by ground water. Ground water is drawn upward by evaporation at the surface. During this upward migration, trace elements in the water are affixed to minerals in the overburden. The affixation, or bonding, may range from weak to very strong. The strength of this bonding depends on the chemical nature of both the trace element and the host mineral. The differences in bond strength is comparable to the difference between the weak electrostatic attraction that holds an inflated balloon to a wall and a nail driven into a stud.
Minerals that are capable of scavenging trace elements from ground water with increasing bond strength include hydrated aluminum silicates (clays), secondary carbonates, amorphous (noncrystalline) oxides of manganese, and the amorphous and crystalline oxides of iron. Trace elements scavenged by these minerals are removed by treating samples of overburden with chemicals that react selectively with each mineral phase. Sequential selective extractions are used to release trace elements from the host minerals in the order of increasing bond strength such as clays first and crystalline iron oxides last.
The principal advantage of selective extractions is that they facilitate the distinction of elements that have migrated from other sources from those normally present in the overburden. Thus the presence of a gold deposit in Nevada may well be indicated by the occurrence of gold, or its associated elements, arsenic and antimony, in a specific mineral phase in the overburden. Return to this point in index.
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