Page images
PDF
EPUB

annealed, or erased, by reheating.

igneous rock mass associated with Because different minerals anneal at mineralization at the mining camp. The different rates, scientists can use

heat source can be sensed more than 3 thermally sensitive minerals, like

miles away in rocks that are not apatite, which anneals at 105 degrees changed visibly by using the apatiteCelsius, to record a heating event

derived paleothermal data. farthest from the heat source. Fission Organic material is also a sensitive tracks from several samples can be recorder of thermal events. Conodonts, used to determine the direction of the the microscopic hard parts of certain thermal gradient. A recent Geological marine worms (fig. 4D), exhibit a Survey study demonstrated that a major systematic color change as they are 5-million-year-old paleothermal anomaly heated through the temperature range was present east of the old mining camp of about 60 degrees to 550 degrees of Rico, Colorado. This anomaly is Celsius. This conodont color alteration believed to be related to a concealed index, which has been used for regional

[graphic][graphic]

studies of hydrocarbon generation and preservation, is being tested as a paleothermal measure for mineral exploration. Solid organic matter that is dispersed in rocks matures as a function of time to become a glassy, coallike substance called vitrinite. Increased reflectance, as measured by microscope studies, indicates increased maturation. Vitrinite reflectance is applicable to rocks of a wider geologic time span than the conodont index and also is being applied to the search for paleothermal anomalies.

sible the use of relatively inexpensive bulk-mining techniques. The majority of known disseminated gold deposits in the United States are located in Nevada and Utah (fig. 5). Gold occurs in the deposits as micrometer-sized particles generally disseminated in either silty to sandy, organic-rich dolomites and limestones or in organic-rich limey shales. The origin and many geologic characteristics of these deposits are not well understood. The Geological Survey currently is conducting research to determine the zonal alteration patterns of these deposits, their geochemical and geophysical characteristics, regional and local structural controls for the ore-forming processes, the age of mineralization, the role of organic matter in the origin of the deposits, the evolution of the oreforming fluids, and the source of the gold. The research includes study of the mineralogy of the deposits to determine genetic relations and age and study of the fluid inclusions in quartz within the deposits to determine the evolution of the ore fluids. The resulting model can be used to help industry in its exploration efforts.

[merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small]
[ocr errors][merged small][ocr errors][ocr errors][merged small]

Jerritt Canyon
Getchell
Pinson: Dee

Bootstrap
Preble

Goldstrike

Blue Star do Carlin
Florida Canyon

Bullion Maggie Creek/Gold
Monarch Rain

Quarry i
Standard

Tenabo
Relief Canyono Gold Acres Horse Canyon

Cortez' 'Tonkin Springs

Alligator Ridge

Mercur

------

- Alligator Ridge

Windfall

[blocks in formation]

100 MILES

100 KILOMETERS

Figure 5. Disseminated gold

deposits in Nevada and
Utah.

CONCLUSIONS

The examples described here demonstrate the Geological Survey's commit ment to research that addresses several objectives. Tied closely to this research is an acute awareness of the need for exchange of information. This exchange is accomplished by publication and presentation of results and by cooperative programs with other research groups. Results of completed studies are available for use; reports about ongoing research activities encourage assistance by and suggestions from others.

A new annual forum for the dissemination of Geological Survey research results and information about current research activities is to be held in Denver in February 1985. The first annual McKelvey Forum on Mineral and Energy Resources is titled USGS Research on Mineral Resources1985. The subject matter of the forum will alternate yearly between research on mineral resources and research on energy resources. The forum, which honors Vincent E. McKelvey, Director of the U.S. Geological Survey from 1971 to 1978, provides an opportunity for oral

presentations, poster sessions, and laboratory tours that will augment communication among scientists from the Geological Survey, industry, and academic institutions. Directing all sectors of the Nation's scientific strength towards discussion of the multifaceted Geological Survey research effort on mineral resources will help maintain a research agenda that is responsive to present problems, draws on a past record of scientific achieve ment, and plans ahead for future demands for mineral-resource information.

Geological Survey research on mineral resources includes programs that are focussed on present national problems of maintaining the availability of reasonably priced mineral supplies and of using today's mineral-resource information to set tomorrow's land-use priorities. Accessible systems for storage and organization of information from past studies are essential for efficient research. Continued strong, basic research in fundamental aspects of geological science is essential to provide the tools to solve tomorrow's problems.

Significant Accomplishments of Research
Programs: 1984

This chapter presents a selected number of U.S. Geological Survey research accomplishments for 1984. More complete discussions of the investigations summarized here are available in other chapters of the Yearbook and in other Geological Survey publications on specific topics.

MINERAL-RESOURCE SURVEYS

formation of granitic magma, provided water-rich fluids that leached gold and other elements from the host rocks and then deposited them in fractures that

now form the gold-bearing veins. Fluid-inclusion, isotope, and

geochemical studies demonstrated that the silver- and base-metal veins of the Coeur d'Alene district of northern Idaho were formed from metamorphic fluids produced during late Precambrian metamorphism of rocks of the Belt Supergroup. The fluids leached the metals from the typical Belt rocks and deposited them in fractures in the intensely and complexly

deformed sections of the Belt rocks. • A study of the nitrate deposits in the

• New isotopic dating research,

chemical analyses, and structural
studies have shown that gold veins
in the Chugach Range, Prince
William Sound region, southern
Alaska, formed at the same time
that the collision of large blocks of
oceanic crust was forming what is
now Alaska. The heating caused by
the collisions, and the resulting

[graphic]

region of Death Valley, California,

Gold crystals, about 1 mm

in diameter, from a placer deposit near Sacramento, California.

[graphic]

The Carlin Mine, northeastern Nevada, one of the largest disseminated gold mines in the United States.

shows them to be analogous to the famed Chilean nitrate deposits, the major sources of nitrate for fertilizer and explosives for the 100 years before World War II. The deposits in California consist of wind-transported saline material that accumulated in near-surface layers in the so-called clay hills, which consist of relatively impervious montmorillonite-rich lake beds up to 63 million years old. These deposits do not contain exploitable resources of nitrate, but they do provide a new insight on the mechanisms of accumulation of

saline minerals in deserts. Geochemical studies of rhyolitic ash

flow tuffs in southern Bolivia, carried out by the Geological Survey in cooperation with the National Geological Survey (GEOBOL) and National Mining Company (COMIBOL) of Bolivia, reveal anomalously high values of tin and lithium in some tuff units. The high tin values indicate that source

magmas were enriched in tin and that tin mineralization did not end necessarily before eruption of these rocks as had been assumed previously. It is now believed that tin deposits may be associated with buried intrusive bodies within the volcanic complex. Also, the high lithium values indicate that these rocks are now the major source of lithium found in the widespread and abundant lithium-rich brines in

salt pans of this region.
• Previous studies have shown that,

during fractional crystallization of
silicates from tholeiitic magmas, the
residual liquids are depleted
greatly in nickel, depleted slightly
in cobalt, and enriched in copper.
Tholeiitic suites from pre-Triassic
rocks that illustrate this fraction-
ation trend (Palisades sill, New
Jersey, Dillsburg intrusion,
Pennsylvania, Great Lake sheet,
Tasmania, and Aloe lava lake,
Hawaii) show an increase of
copper with a decreasing nickel-

« PreviousContinue »