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lost by diffusion; this closure age is interpreted to be the age at which the mineral last cooled below a temperature at which argon diffusion is nil (approximately 200 °Celsius in this case). The 175 m.y. apparent age of the feldspars is interpreted as being due to a heating event (>200 °Celsius), which affected these and probably the other basins. The most likely source of heat is hydrothermal fluids, possibly migrating in response to regional deformation. The 175-m.y. argon closure age may reflect the formation age of some of the vein and stratabound base-metal deposits described above.
by the Stillwater Mining Company, a joint venture of Chevron Corporation, Lac Minerals Limited, and Manville Corporation (fig. 4). The J-M Reef was discovered in 1973 and has been extensively explored since then. This reef is similar to the Merensky Reef of the Bushveld Complex in South Africa, which is the major supplier for the annual U.S. consumption of about 3.9 million ounces. Annual production from the mine is expected to be approximately 2.5 percent and 7.5 percent of the annual U.S. consumption of platinum and palladium, respectively.
A cooperative study by the USGS and the Stillwater Mining Company was initiated to examine the spatial geometry and geochemistry of platinum-group-element (PGE) mineralization of the J-M Reef by using drillhole and underground information from the Minneapolis Adit mine. These investigations are in part continuations of many earlier investigations in various parts of the Stillwater Complex during the last century by the USGS, industry, and academia. PGE mineralization in the J-M Reef appears to be very irregularly distributed. Characterizing and understanding the type of distribution are critical to mining and thus are being investigated by use of threedimensional display systems and statistical analysis.
Detailed mineralogical studies of the residence of the platinum-group elements,
Platinum Mining in the Stillwater Complex, Montana By Norman ) Page
The only mine for platinum-group metals in the United States, the Minneapolis Adit mine, shipped its first concentrate in March 1987. The ore to produce the concentrate comes from the J-M Reef of the Stillwater Complex, Montana, and is mined
using the scanning electron microscope in combination with other techniques, demonstrated that minerals containing tellurium, arsenic, sulfur, antimony, bismuth, tin, and mercury are important in controlling the distribution of the platinum-group elements. In order to examine the relations of platinum-group minerals to their stratigraphic setting, it was necessary to develop rapid, precise, and extremely sensitive analytical techniques. Two techniques were evaluated that determine tellurium down to 10 parts per billion in solid geological samples. One of these techniques has an ion exchange preconcentration step, and the other has a solvent extraction preconcentration step. Both of these techniques were followed by a graphite furnace atomic absorption measurement. Techniques to determine tin and bismuth at similar low levels are being developed based on the ion-exchange procedure. Procedures have also been developed for determining the lower limits of arsenic and antimony. The development of these techniques with lower levels of determination is necessary because the platinum-group elements that are being mined occur at the part per billion to part per million level. Preliminary evaluation of a limited data set indicates correlations between the trace elements and platinum-group elements.
In addition to providing the data required for understanding the setting of the J-M Reef, these new techniques will increase the accuracy and reliability of other assessment programs for precious metals.
properties, to determine crustal structure, and to better understand the generation of earthquakes along the San Andreas fault. A joint effort of the National Science Foundation (NSF) and the U.S. Geological Survey (USGS), the well was begun in November 1986, and is one of the first projects to be undertaken in the new national Continental Scientific Drilling Program. This program aims to enhance our knowledge of the composition, structure, dynamics, and evolution of the continental crust and of how these factors affect the origin and distribution of mineral and energy resources and natural phenomena such as volcanic eruptions and earthquakes.
Most of the world's earthquakes occur in relatively narrow bands that mark the boundaries between sections of the outer shell of the Earth. These sections, known as plates, are driven by the internal heat of the Earth and move slowly and inexorably with respect to one another. The boundary between the Pacific Plate and the North American Plate is located in California and forms the San Andreas fault system (fig. 5). The relative motion between these two plates is horizontal, and the rate of movement is about 2 inches per year. In other words, each year Los Angeles moves about 2 inches closer to San Francisco. Along the San Andreas fault the plates move past each other at a steady rate, but at many places in the upper 5–10 miles of the crust the brittle character of the rocks causes the fault to lock, preventing motion and producing a buildup of stresses. When the accumulating stress exceeds the strength of the rocks bounding the fault, the rocks suddenly slip. This slipping action allows the surface rocks to snap to a new position, in effect catching up with the subsurface rocks and, at the same time, producing the strong shaking that constitutes an earthquake.
Though field and laboratory studies of fault slip have been underway for years, the mechanics of the slip process are still not well understood. For instance, a longstanding question exists concerning the significance of low levels of heat flow along the trace of the San Andreas fault where friction associated with fault motion might be expected to produce relatively higher geothermal levels than those measured from the rocks in the surrounding region. Generally referred to as the stress-heatflow paradox, the measured levels of heat
Answers from Deep
Drilling of a 12,000-foot-deep scientific well has been completed at Cajon Pass in southern California to measure crustal
tific questions, NSF-supported university scientists and USGS scientists and engineers are collaborating in a two-phase program of deep drilling into rocks adjacent to the San Andreas fault at Cajon Pass, California, northwest of San Bernardino (fig. 5). This location was chosen because of the availability of a site close to the fault with a relatively unfractured and unaltered body of rock in which to make important measurements and because of its critical location as the end point of the fault rupture during the last great earthquake in southern California (1857).
Funds for the drilling and overall project management were provided by Deep Observation and Sampling of the Earth's Continental Crust, Inc. (DOSECC), a nonprofit university consortium under contract to the National Science Foundation. The principal investigator for the project is Dr. Mark D. Zoback of Stanford University; co-investigators are from the USGS and other universities.
The first phase of drilling of the Cajon Pass well was completed in April 1987 at a depth of 6,938 feet. Samples of rock cuttings were collected every ten feet, and 33 rock cores, totaling 269.5 feet in length, were recovered (fig. 6). The USGS has provided sample and core handling, distribution, and curatorial facilities and personnel. Working in an on-site laboratory constructed especially for this project, USGS technicians have marked and tested samples and photographed the rock core. Under the direction of the California Institute of Technology, scientists have prepared a comprehensive descriptive log of the cuttings and core. Geophysical logs have been obtained through both commercial contract and university and USGS researchers.
The logs and rock cuttings show that the well first penetrated 1,625 feet of sandstone, siltstone, and claystone of Tertiary age (2–63 million years old), followed by a complex assortment of crystalline basement rocks composed largely of sheared granite, granodiorite, and gneiss. Analyses of these rocks, of the geophysical logs, and of seismic-reflection profiles are central to broadening our understanding of the history and interrelationship of basement terranes, particularly the nature and extent of
flow would suggest that the fault slips at low-shear stresses, on the order of 200 bars (1 bar is approximately equal to 1 atmosphere) or less, thus implying a weak fault. Basic faulting theory, laboratory experimentation, and in-situ stress measurement by the hydraulic fracturing method suggest, in contrast, that the average shear stress resisting plate motion is about 750 bars, suggesting a strong fault.
Resolving this paradox is important to the fundamental understanding of the physics governing fault mechanics, the magnitude of the stresses that drive the crustal plates, and the generation of earthquakes. To address these and other scien
compressional regime at depth. This stress The USGS will assume control of the orientation is quite different from that well sometime in the future and, in collabexpected for a fault that slips horizontally, oration with university scientists, will but it is consistent with the orientation of begin to develop equipment to establish a stress-induced borehole elongations identi- permanent deep Earth observatory primafied in the well and with the orientation of rily for earthquake prediction purposes. stresses that have produced many of the This monitoring will provide a long baseline recent earthquakes in the region. The of reliable scientific data valuable for idenstress orientation also accounts for the tifying precursors of the next large southwidespread occurrence of geologicallyern California earthquake. youthful folds and reverse faults that parallel the San Andreas fault. Scientists propose that crustal compression perpendicular to the fault is the result of extremely low shear strength of the San Andreas fault, consistent with the observation of low heat flow in the area of the fault. Temperature and heat-flow measurements have been made in the well and do not show a geothermal anomaly. Stress and heatflow measurements will provide critical information for establishing data trends necessary to resolve the stress-heat-flow paradox.
By David Sutphin In addition to stress and heat-flow studies, hydrologic and geochemical studies are being conducted on fluid samples In March 1987, the Minneapolis Adit collected in the well. Preliminary meas mine began producing platinum-group meturements are being made of rock pore als from the Stillwater Complex in Monpressure and permeability. Analyses of the tana. This mine, which is the first major fluid samples indicate that the well has platinum-group-metals mining operation in penetrated two separate fracture systems the United States, could eventually supply that are only slightly interconnected. Mea about 10 percent of annual U.S. consumpsurement of fluid flow through the rocks tion of approximately 3.9 million ounces. will provide the data needed to determine The U.S. currently relies on foreign the role of water in reducing the frictional sources for 98 percent of this annual conresistance of rocks to faulting and in dissi sumption. The platinum-group metals, pating subsurface heat generated by earth platinum, palladium, rhodium, ruthenium, quakes.
iridium, and osmium, are of strategic Phase two drilling of the Cajon Pass importance to the Nation because their well deepened the drillhole from 6,938 to unique chemical, physical, and mechanical about 12,000 feet. About 10 percent of the properties are essential to many industrial well will be cored at specified intervals, and processes. The current U.S. dependence on more rock cuttings and fluid samples will foreign sources for these much-needed be collected. Samples will be examined for metals makes domestic resource research evidence of subhorizontal faults and will be and production an important national analyzed to reconstruct the area's geologic priority. and tectonic history. Hydrofracture and The U.S. Geological Survey and the holographic stress measurements will be U.S. Bureau of Mines participated with made to calculate the amount of stress and earth-science and mineral-resource agento determine whether or not the orienta cies of Australia, Canada, Germany, South tion of the stress field changes with depth Africa, and the United Kingdom in a coopor has been affected by the presence of erative effort that reviewed worldwide nearby shallow faults. Geological, hydro- platinum-group-metal resources and prological, and geophysical measurements will duction. This study was undertaken by the continue to be made to resolve the many International Strategic Minerals Inventory remaining scientific questions.
(ISMI), which has as its goal the collection,