Principal Zone Research Articles

The J-M platinum-palladium reef of the Stillwater Complex, Montana; I, Stratigraphy and petrology

Exploration of the Stillwater Complex by the Johns-Manville Corporation has led to the discovery and delineation of an approximately stratigraphic zone rich in platinum and palladium comparable to the Merensky Reef. This zone, here called the J-M reef, is generally 1 to 3 m thick and has been traced for 40 km, essentially its maximum possible length within the outcrop area of the intrusion. It is distinguished by small concentrations (0.5-1.0%) of chalcopyrite, pyrrhotite, and pentlanditc in which are contained tiny grains of various platinum-group minerals (mainly moncheite, braggite, cooperite, kotulskite, and Pt-Fe alloy). One segment, 5.5 km long, averages 22.3 g (0.65 troy oz) Pt and Pd per short ton through 2.1 m. The ratio of Pt to Pd typically is about 1:3.5.A new stratigraphic classification of the Stillwater Complex is presented in which three long-standing, major divisions are renamed the basal, ultramafic, and banded series. The first comprises basal norite and basal bronzitite zones totaling 50 to 150 m in thickness; the second, harzburgite and upper bronzitite zones totaling 800 to 1,200 m. The banded series is a 4,500 m succession of plagioclase-rich cumulates with the J-M reef located 400 to 450 m above its base. It is divided into six megacyclic units (I-VI, from bottom to top) in which the principal zones are: (I) norire, gabbro; (II) norite, gabbro; (III) anorthosite, troctolite-gabbro; (IV) anorthosite-troctolite, troctolite-gabbro; (V) anorthosite, troctolite-gabbro; and (VI) gabbro, pigeonite gabbro. Two thin troctolite-anorthosite zones, I and II, occur at the top of units I and II, and troctolite-anorthosite zone I is host to the J-M reef.Where troctolite-anorthosite zone I is best developed in the west-central part of the complex, it is about 100 m thick and is associated with ten relatively continuous olivine-bearing members. These members are generally 1 to 5 m thick and are composed of one or more of peridotite, troctolite, and olivine norite. Members 1 to 4 alternate with varied, discontinuous successions of gabbro, norite, pyroxenite, and anorthosite that are collectively assigned to a gabbro subzone. Members 5 to 10 occur with anorthosite and norite in the anorthosite 1, norite, and anorthosite 2 subzones, and they can each be identified with a cyclic unit. The type rock sequence in five of the cyclic units is anorthosite-peridotite-troctolite; in the sixth, it is anorthosite-olivine norite-norite. The unit associated with olivine-bearing member 5 contains the J-M reef and extends to both ends of the complex; the others have variously been traced for distances of 15 to 36 km.Mineralogical data and 92 whole-rock analyses are presented for troctolite-anorthosite zone 1 and the adjoining zones. Plagioclase becomes gradually less calcic upward through this zone, and normatire anorthite/albite decreases sympathetically, but no cyclic mineralogic or chemical trends are evident. The mafic silicates tend to have higher Fe/Mg within and above the reef than below--a difference that may in part be primary but that is largely a postcumulus effect relating to the larger ratio of trapped (high Fe/Mg) intercumulus liquid to cumulus (low Fe/Mg) mafic minerals in the anorthosite 1 and 2 subzones as compared with the gabbro subzone. Similar variations are shown by MnO and NiO in the mafic minerals. The whole-rock variations of S and Cu indicate that the parental magma(s) of the banded series first reached continuous saturation with immiscible sulfide liquid at the level of the J-M reef, and from the compositions of other sulfide zones at higher levels it appears that the Pt and Pd contents of the magma(s) were effectively exhausted by the formation of the reef.Through the whole of the Stillwater Complex, two principal crystallization orders are indicated for the cumulus silicate minerals. The earlier order, defined in the ultramafic series and in megacyclic units I and II, is olivine, bronzite, plagioclase, augite. The later order, evident in units III through VI, is plagioclase, olivine, augite, bronzite (or hypersthene). The complicated stratigraphy of the troctolite-anorthosite zone I appears to represent an aborted transitional stage. Because both orders are manifest through cumulate successions thousands of meters thick, it would appear that two parental magmas were involved, the first with ultramafic affinities and the second with anorthositic affinities, and indications are that the Pt and Pd in the J-M reef came from the first, and the S from the second. A mechanism for forming the reef by mixing these liquids will be described in the second paper of this series.

Roosevelt Hot Springs Geothermal System, Utah--Case Study

The Roosevelt Hot Springs geothermal system has been undergoing intensive exploration since 1974 and has been used as a natural laboratory for the development and testing of geothermal exploration methods by research organizations. This paper summarizes the geological, geophysical, and geochemical data which have been collected since 1974, and presents a retrospective strategy describing the most effective means of exploration for the Roosevelt Hot Springs hydrothermal resource. The bedrock geology of the area is dominated by metamorphic rocks of Precambrian age and felsic plutonic phases of the Tertiary Mineral Mountains intrusive complex. Rhyolite flows, domes, and pyroclastic rocks reflect igneous activity between 0.8 and 0.5 m.y. ago. The structural setting includes older low-angle normal faulting and east-west faulting produced by deep-seated regional zones of weakness. North to north-northeast-trending faults are the youngest structures in the area, and they control present fumarolic activity. The geothermal reservoir is controlled by intersections of the principal zones of faulting. The geothermal fluids that discharge from the deep wells are dilute sodium chloride brines containing approximately 7,000 ppm total dissolved solids and anomalous concentrations of F, As, Li, B, and Hg. Geothermometers calculated from the predicted cation contents of the deep reservoir brine range from 520 to 531°F (271 to 277°C). Hydrothermal alteration by these fluids has produced assemblages of clays, alunite, muscovite, chlorite, pyrite, calcite, quartz, and hematite. Geochemical analyses of rocks and soils of the Roosevelt Hot Springs thermal area demonstrate that Hg, As, Mn, Cu, Sb, W, Li, Pb, Zn, Ba, and Be have been transported and redeposited by the thermal fluids. The geothermal system is well expressed in electrical resistivity and thermal-gradient data and these methods, coupled with geologic mapping, are adequate to delineate the fluids and alteration associated with the geothermal reservoir. The dipole-dipole array seems best suited to acquire and interpret the resistivity data, although controlled source AMT (CSAMT) may be competitive for near-surface mapping. Representations of the thermal data as temperature gradients, heat flow, and temperature are all useful in exploration of the geothermal system, because the thermal fluids themselves rise close to the surface. Self-potential, gravity, magnetic, seismic, and magnetotelluric survey data all contribute to our understanding of the system, but are not considered essential to its explorati n.

Focal depths and moment tensor representations of shallow earthquakes associated with the Great Sumba Earthquake

Focal depths in the range 8 to 24 km below the seafloor were determined for seven of the larger shocks associated with the Great Sumba earthquake of August 19, 1977. Depth control was achieved by modeling digitized long‐period P waves recorded by the World‐Wide Standard Seismic Network. If the base of the rupture at the time of the mainshock were no greater than the depth of the deepest aftershock, the stress drop would be nearly 500 bars. Amplitude and first motion data from the P waves were inverted for point source moment tensors. The inversion of Rayleigh and Love wave spectra computed from digital seismograms recorded by the Seismic Research Observatories (SRO) and the Abbreviated Seismic Research Observatories (ASRO) broadened the source study to include earthquakes with moments as small as 1024 dyn cm: too small for either traditional first motion solutions or P wave modeling. The inversion procedure was applied to data in the passband from 30 to 120 s. The propagation phase for the Love and the Rayleigh waves was calibrated with signals from two earthquakes for which the source phase was inferred from the P wave solutions. Agreement with the corresponding P wave solutions is nearly the same whether or not the smaller eigenvalues of the inner product matrix are retained in the inversions. The double couple components generally account for more than two thirds of the total moment. Normal faulting along an east to northeasterly strike characterizes the mechanisms in the principal zone of aftershock activity. Strike slip mechanisms consistent with horizontal compression normal to the arc prevail in the delayed aftershock zone located 200 km northwest of the mainshock. Of the earthquakes subjected to this surface wave analysis the foreshock stood out as having felt a significantly higher apparent stress, perhaps as much as one half an order of magnitude higher than the others. These results suggest that accurate source parameters for moderate to small magnitude earthquakes can be determined from surface waves recorded by the SRO and ASRO. In particular, the mechanisms of small magnitude foreshocks could be obtained in this way.

The relationship between the vitrinite reflectance and "principal zone of oil formation" in northeastern Japan

The relationship between the vitrinite reflectance and "principal zone of oil formation" in northeastern Japan

Holographic Investigations of the Human Tibiofibular Interosseous Membrane

The interosseous membrane contributes to the function of the fibula by neutralizing the freedom of movement in the upper tibiofibular joint and by keeping the fibula from being passive. The main role of the membrane is indicated by its shearing capacity. As a result of dislocation and bending of the tibia (during axial loading), and the passivity of the fibula, the membrane slides in the direction of the leg's axis causing the zone of maximal shearing in its upper central part. This is the principal zone of strain and divides the upper portion of the membrane into the tibial and fibular area. Damage of the upper part of the membrane, perpendicular to the main fiber bundles, causes an irregular deformation of the talus, while complete damage of the membrane unburdens the fibula in load transference by more than 30%. The interosseous membrane is important in the normal function of the lower leg by maintaining the axis of balanced loading of the foot.

The space-group determination of GaS and Cu3As2S3I by convergent-beam electron diffraction

Space-group determinations were carried out on GaS and Cu3As2S3I single crystals using convergent-beam electron diffraction data. The space groups found were P63/mmc and Cmcm, respectively. The effectiveness of the specific test for centrosymmetry was examined. In both structures the test was made clearer by use of the inclined-axis technique, in which the incident beam is at an appreciable angle to the principal zone axis. It was concluded that the test was superior to optical methods in structures such as GaS which have a high dislocation density.

Sedimentary Facies Relations and Inferred Dynamics of a Single-Barred Nearshore Environment, Atlantic Coast of Eastern Long Island, New York: ABSTRACT

Facies relations in a conglomeratic single-barred nearshore environment on the glaciated Atlantic Coast of eastern Long Island, New York, were determined by examination of 42 can cores, bed-form distributions, and sediment textures. Two principal subtidal zones are recognized. The shoreface zone (0 to 10 m water depth) is composed of clean fine to coarse sand and consists of the planar parallel to megaripple-laminated upper shoreface facies, the massively bedded longshore trough facies, and the megaripple to parallel-laminated longshore bar facies. The inner shelf zone (> 10 m water depth) is composed predominantly of organic-rich fine sand and consists of the parallel-laminated transitional facies and bioturbated offshore facies. Scattered throughout this zone are coarse sand outcrops of megaripple-laminated inner shelf lag facies. By deployment of dye tracer and by inspection of bed forms in the longshore trough (during low wave-energy, fair-weather conditions) little evidence of longshore or seaward flow could be found. Landward migration of bar deposits over trough deposits was observed over a 6-week period during fair-weather conditions. A core in the trough facies adjacent to the distinct bar-trough contact revealed an underlying bar facies. An equilibrium model of near-continuous fair-weather landward migration of the bar facies interrupted by high wave-energy pulses of seaward movement of trough sediment is believed to account for the stable position of the longshore bar. End_of_Article - Last_Page 783------------

Functional properties of pulvinar-lateral posterior neurons which receive input from the superior colliculus.

Focal electrical stimulation of the superior colliculus induced short latency responses and/or a long duration suppression of spontaneous activity in single cells isolated in the pulvinar and lateral posterior (LP) nuclei of the cat. Cells which responded with one or more spikes at short latencies were relatively few in number, and these tended to be encountered in or near the principal tectorecipient zone of LP. In contrast, a suppression response was observed in many more cells which were widely distributed throughout the pulvinar-LP complex. Stimulation of the superficial layers of the colliculus was more effective in driving or suppressing pulvinar-LP cells than that of intermediate and deep layers. There was also an indication of a coarse topography in this projection. On the basis of their sensory response properties the cells responsive to tectal stimulation, at short latency or with a suppression, could not be differentiated from the overall sample of pulvinar-LP neurons.

Effects of Uranium Mining on Groundwater in Ambrosia Lake Area, New Mexico: ABSTRACT

The principal ore-bearing zone in the Ambrosia Lake area of the Grants uranium district is the Westwater Canyon Member of the Morrison Formation of Jurassic age. This is also one of the major artesian aquifers in the region. Significant declines in the potentiometric head within the aquifer have been recorded, and digital modeling shows that declines may locally approach 3,000 ft (900 m) in the vicinity of a mine. Loss of potentiometric head in the Westwater Canyon Member has resulted in the interformational migration of groundwater along fault zones from overlying aquifers of Cretaceous age. This has resulted in local deterioration in chemical quality of the groundwater. End_of_Article - Last_Page 690------------

An entomologic and parasitologic survey for onchocerciasis in western El Salvador.

Abstract Despite similarities of topography and climate in the coffee-producing regions of the two countries, human infection with Onchocerca volvulus has rarely been reported in El Salvador, whereas onchocerciasis has long been endemic in the coffee plantations of neighboring Guatemala. The apparent near absence of the infection in El Salvador was investigated in a coffee-growing area in the western part of the country some 50 km from Guatemala's principal onchocerciasis zone. Collections of black flies attracted to humans and, for comparison, to horses were made for 1 yr at elevations from 300–1,330 m, and a skin biopsy survey was conducted among area residents ranging in age from 10 yr to over 60 to estimate the prevalence of microfilarial skin infection. Simulium metallicum and S. callidum accounted for 77 and 21%, respectively, of the flies collected from humans and 87 and 11%, respectively, of those from horses. Both species showed a marked preference for horses as a source of blood. When coming to feed on humans, they chose the legs more often than the shoulders as a feeding site. Other species, which together made up only 2% of the 5,507 flies collected, were S. downsi, S. pulverulentum, S. mexicanum, and S. haematopotum. The highest landing rates recorded for both S. metallicum and S. callidum on humans were at collecting sites from 600–940 m elevation during the October-February period which spans the annual transition from wet to dry seasons. The absence of S. ochraceum from the collections is noteworthy. This anthropophilic black fly frequently attains very high densities in Guatemala, where it is regarded as the principal vector of O. volvulus. No microfilariae were detected in skin biopsies from 941 study area residents, indicating that no significant reservoir of infection exists there and, together with the entomologic data, supporting the assumption that the region is free of endemic human onchocerciasis.

Anisotropy of damage productions in electron irradiated molybdenum

Anisotropy of damage productions in crystals due to high energy electron bombardment are caused from two different origins. One is an anisotropic displacement threshold energy, and the other is an anisotropic distribution of electron flux near the atomic rows in crystals due to the electron channeling effect. By the n-beam dynamical calculations for germanium and molybdenum we have shown that electron flux at the atomic positions are from ∽4 to ∽7 times larger than the mean incident flux for the principal zone axis directions of incident 1 MeV electron beams, and concluded that such a locally increased electron flux results in an enhanced damage production. The present paper reports the experimental evidence for the enhanced damage production due to the locally increased electron flux and also the results of measurements of the displacement threshold energies for the <100>,<110> and <111> directions in molybdenum crystals by using a high voltage electron microscope.

THE PERIODICITY OF DROUGHTS IN THE GRAIN-GROWING AREAS OF THE USSR

On the basis of a chronology of droughts in the grain-growing regions of the USSR, compiled from historical sources and the more recent period of instrumental observations, the author shows that the occurrence of drought may be regarded as a random process characterized as a quasi-stationary Poisson series of events. Most of the grain regions in the USSR follow this regularity. In the case of simultaneous drought throughout the principal grain zone of the USSR, comprising the European part and the Midland region (West Siberia and Kazakhstan), the sequence of events assumes a nonstationary character. Statistical parameters are developed for the time series of drought years and nondrought years, and are compared with extreme values in grain-yield fluctuations. Over the last 140 years, drought occurred in 76 years in at least one of the three major grain regions of the USSR (European part, West Siberia, Kazakhstan) while 64 years had no drought and corresponding normal or higher yields. A catastrophic drough...

Localization of electron current density and its effect on damage production in a molybdenum crystal

Abstract Three-dimensional, fine distributions of electron current density in a molybdenum crystal are examined by the many-beam dynamical theory of electron diffraction for electrons incident parallel to the principal zone axes. It is found that the two-dimensional distribution in the transverse plane oscillates along the depth direction, and that a highly localized distribution around the rows of atoms and an almost flat distribution appears alternatively with definite periods of depth. The effect of the localized current density on the damage production is discussed by introducing the “effective current ratio”, J eff which is defined as the ratio of the local current density at the rows of atoms to the mean current density for electrons incident parallel to the zone axis. The values J eff for the incidence of 1 MeV electron are found to increase in the order of the (111), (001) and (110) directions. The energy dependence J eff is also examined for the (001), (110) and (111) axial cases respectively.

Metabolism of [ 35S]-thiosulphate and [ 35S]-thiocyanate by human saliva and dental plaque

Chromatographic and radioisotope techniques were employed to characterize the mechanisms involved in the metabolism of thiosulphate and thiocyanate by human saliva and plaque. The criteria for determining the utilization of substrates were based on measurements of 35S-binding by salivary proteins, formation and identification of volatile and non-volatile 35S compounds, and production of inorganic [ 35S]-sulphate. Through selective 35S-labelling at the beta ( 35S.SO 3 2−) and alpha (S. 35SO 3 2−) positions, it was demonstrated that the two S portions of thiosulphate are metabolized through different pathways. Whereas the alpha S yielded only traces of intermediate volatile S products and was completely recovered as sulphate, the beta S was first converted to the volatile fraction which was subsequently reabsorbed and oxidized by the incubated reaction mixtures to several non-volatile products. Gas chromatographic (GC) analyses of the head-space of the 35S.SO 3 2− systems detected a single prominent peak which corresponded to the retention time for H 2S. Paper chromatographic analysis of the non-volatile components showed that the salivary metabolism of 35S.SO 3 2− gave rise to 6 principal zones of radioactivity which were identified on the basis of R F values and staining reactions as: protein, inorganic sulphate, trithionate, tetrathionate, pentathionate and hexathionate. In addition to these components, a very prominent 35S peak was also obtained for thiosulphate when H 2 35S was employed as the sole source of 35S. It appears that H 2S and S 2O 3 2− are intermediate products of oral organic sulphur metabolism leading to the formation of inorganic sulphate and polythionates. The GC and 35S analyses showed that 35SCN − was not a precursor of volatile S compounds and yielded only traces of [ 35S]-sulphate in putrescent systems. Similarly, the paper chromatographic studies showed that, except for the uptake of approximately 1 per cent of 35SCN − radioactivity by the salivary proteins, the remainder of 35SCN − appeared inert. The addition of SCN − had no discernible effect on the normal putrefactive activity or oxidation of l-methionine, l-cysteine and l-cystine.

Near-bottom geophysical study of the Mid-Atlantic Ridge median valley near lat 37° N: Preliminary observations

Near-bottom geophysical observations made over the median valley of the Mid-Atlantic Ridge near lat 37°N show that the median valley can be divided into four provinces: the outer walls, terraces, inner walls, and floor. Block faulting along normal faults with as much as 600 m throw is the dominant mechanism for creating median-valley relief, particularly at the inner and outer walls. Most of these faults dip toward the median-valley axis an average of 45°. Many of the block tops slope 3° to 10° away from the valley axis. Some of the major fault scarps are linear for more than 20 km. Large-scale block faulting occurs less than 1 km from the axis of spreading. Inversion of near-bottom magnetic anomalies suggests that the principal zone of intrusion may be no wider than 1 to 2 km and is along the valley axis. This spreading center is marked by either a central ridge or central depression, which suggests either discrete magma sources or discrete taps of a linear source beneath the median-valley axis. The central ridge is a volcanic construction, although faulting and (or) volcano-tectonic uplift may contribute to its relief.

On the Macroterritorial Structure of the Economy in the European Grouping of Comecon Countries

AbstractThe economic integration program of the European group of Comecon, the Council for Mutual Economic Assistance, is analyzed in spatial terms as involving major zones of interaction that are linked by multifunctional transport arteries (comprising railroads, highways, pipelines and power transmission lines). In the USSR, the principal zone is centered in the southern heavy-industry belt (Dnieper bend, Donets Basin), which is the source of ores, coal, iron and steel for the Comecon countries. This Soviet zone extends northward through the potential Kursk iron and steel complex to the Moscow manufacturing complex. A less developed Soviet zone runs closer to the western frontier from Moldavia to the Baltic republics. In contrast to the north-south zones in the USSR, the main industrial belts in the northern tier of East European Comecon countries extend east-west. They are primarily the industrial belt running from Thuringia east through Saxony, Bohemia and Upper Silesia, with scattered industrial comp...

Emerging picture of pleistocene Australians.

Man was well established in Greater Australia between 25,000 and 30,000 years ago and by at least 20,000 BP was occupying almost all of the principal ecological zones of the continent.

Hydrothermal alteration at the Middle Fork copper prospect, central Cascades, Washington

The Middle Fork copper prospect is in the central Cascade Range of Washington State, near the southeastern margin of the Miocene, composite Snoqualmie batholith. The principal zones of chalcopyrite mineralization and related wall rock alteration occur in medium-grained, equigranular, main-phase Snoqualmie granodiorite. The mineralized zone is characterized by en echelon, northeasterly trending shearing. Significant sulfide concentrations appear to occur in linear breccia zones commonly near intersections of northeasterly trending shears with steeply dipping, north to north-westerly striking cross faults.Geologic mapping, petrographic data, and whole rock chemical analyses suggest the following sequence of alteration and mineralization:(1) A northeasterly trending zone of unmineralized, quartz-deficient biotite-plagioclase breccia and peripheral quartz-stable, biotitic granodiorite breccia formed from main-phase Snoqualmie granodiorite during a period of presumed deuteric alteration. The removal of quartz from the quartz-deficient zones resulted in a volume loss of approximately 30 percent.(2) Subsequent introduction of solutions rich in potassium, iron, sulfur, copper, and minor molybdenum was restricted primarily to the quartz-deficient biotite-plagioclase breccias. The alteration took place at constant volume and resulted in an assemblage of hydrothermal orthoclase and secondary biotite associated with relict igneous plagioclase and primary biotite. Subequal amounts of disseminated chalcopyrite (with pyrrhotite blebs) and pyrite occur within this alteration zone. Retrogressive alteration of biotite to chlorite and plagioclase to sericite and carbonate minerals commonly is superimposed on the earlier assemblage.(3) A late-stage assemblage of sericite, pyrrhotite, quartz, carbonate, and chalcopyrite is especially well developed along northwesterly striking discontinuous shear zones. The sericite-pyrrhotite alteration occurred at constant volume and probably represents deposition from the same solutions responsible for the orthoclase alteration, but at decreased K (super +) /H (super +) ratios and lower temperatures.(4) Northerly to northwesterly striking post-mineralization faults cut the mineralized zone into discrete blocks.The Middle Fork deposit has hydrothermal alteration assemblages similar to southwestern United States porphyry copper deposits, but differs from them in that it (1) occurs in medium-grained quartz dioritic-granodioritic intrusive rocks of batholithic dimensions, (2) has linear, structurally controlled zones of alteration and mineralization, (3) has potassium silicate alteration assemblages remarkably deficient in quartz, (4) lacks a peripheral propylitic zone, (5) contains pyrrhotite as a major sulfide mineral, and (6) has virtually no supergene enrichment. Despite these differences, the Middle Fork deposit appears to belong within the broad spectrum of porphyry copper deposits although diverging widely from the unifying model of Lowell and Guilbert (1970).

"Geolog System" for Computer and Manual Analysis of Geologic Data from Porphyry and Other Deposits

The Geolog System is a computer-oriented method of recording geological field observations, primarily from drill holes, in porphyry-type and other mineral deposits. The System also facilitates manual analysis and interpretation of the data.Data are compiled on GEOLOG, an 80-column format describing principal zone (cap, supergene, hypogene, fresh rock, fault), interval, rock type, fractures, alteration mineral assemblages, limonites, and ore mineral assemblages. Prescribed headings on GEOLOG are keyed to logical word abbreviations and simple codes to give concise, consistent, and readable descriptions. Because column names are easily modified and various codes and scales are available, GEOLOG is flexible; it therefore facilitates description of many types of deposit, accommodating a variety of logging techniques that may emphasize theoretical examination, routine survey, or rapid reconnaissance. Collecting data with Geolog System appears better than existing or standard logging techniques because information is recorded to be: keypunched directly; retrieved readily; consistent with respect to routine description of presence or absence of particular features; specific by use of prescribed headings, codes and scales; related to an existing porphyry deposit model. Time and effort involved in collecting drill data on GEOLOG are comparable to common logging procedures.Interpretation of the large amount of complex data on GEOLOG is aided by using a computer to provide manipulation of data and appropriate output. Because GEOLOG is keyed to recently published models of porphyry deposits that show ore location with respect to zoning of mode and degree of dispersion of mineralization, alteration mineral assemblages, and ore mineral assemblages, application of the System has important future use in predicting ore locations from even preliminary data.

Microearthquakes at The Geysers Geothermal Area, California

Microearthquakes in The Geysers area of northern California were recorded for 3 weeks with a radio-telemetry array of eight seismograph stations in order to examine their distribution with respect to the area developed for geothermal power. Locations were determined for 53 earthquakes within about 10 km of The Geysers. Most epicenters lie in a zone about 4 km long and 1 km wide passing through the geothermal field along a principal fault zone. Focal depths in this trend range from near surface to about 4 km. A composite fault-plane solution indicates dextral strike-slip faulting on a NNW-striking plane subparallel with the regional fault pattern. The results of this study suggest that accurate mapping of microearthquakes can be useful in the exploration for geothermal resources, if the earthquakes studied here are not somehow caused by development of the field.