Page images
[merged small][merged small][graphic][ocr errors][ocr errors][table][ocr errors][graphic]
[blocks in formation]

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.

[ocr errors]
[ocr errors]

^Jerritt Canyon

PinsonJ Dee Bootstrap
Preble* VSoldstrike

Florida Canyon Bullion Maggie Creek/Gold

«*..„„.? Monarch 'Rain Quarry




m Alligator Ridge


[ocr errors]
[ocr errors]


#Shale Pit
*White Caps


[ocr errors]



>J Sterling

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


[ocr errors]


The examples described here demonstrate the Geological Survey's commitment 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 l/SGS Research on Mineral Resources—1985. 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 achievement, and plans ahead for future

demands for mineral-resource


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

• 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

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 region of Death Valley, California,

[merged small][graphic]

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 ashflow tuffs in southern Bolivia, carried out by the Geological Survey in cooperation with the National Geological Survey (GEOBOL) and National Mining Company (COMIHOL) 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 fractionation 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

The Carlin Mine,

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

« PreviousContinue »