Coeur d9Alene District Research Articles

Constriction fracture flow; a mechanism for fault and vein formation in the Coeur d'Alene District, Idaho

Transection cleavage, faults, and veins outside the Terror fault in the Middle Proterozoic sedimentary rocks of the Coeur d9Alene district, Idaho, contain steep stretching fabrics. These structures formed during metamorphism in the lower greenschist facies. Cleavage formed in sequential plane strain and Late metamorphic faulting and veining followed, and local mylonite overprinted cleavage synchronously with mineralization. High-angle reverse and normal faults and veins constitute constrictionlike arrays with widely spaced slip planes which formed during non-Andersonian strain. Bulk strain consisted of plastic slip parallel to the line of fracture intersection, along spaced anastomotic shears. The proposed deformation type is termed constriction fracture flow. Inhomogeneous shear or cross trend shortening may account for synchronous reverse and normal slip in different parts of the rock.

Seismicity and tectonic stress in the Coeur d'Alene mining district

Abstract The Coeur d9Alene mining district lies within a minor zone of seismicity in northern Idaho along the Lewis and Clark Zone, a 400-km-long system of west-northwest-trending faults. Rockbursts (seismic events that cause damage in mines) have become increasingly common in the Coeur d9Alene district during recent years. Our epicenter map of the district shows only 52 seismic events since 1982. However, magnitude-frequency analysis suggests that regional seismograph coverage is complete only for events above magnitude 2.8, and that 90 per cent of the seismic events above magnitude 1.5 are not located by the regional seismograph network. A fault-plane solution for the 1 August 1988 magnitude 4.1 tectonic earthquake that occurred 15 km northeast of Mullan, Idaho, shows, together with previous in situ stress measurements, that the Coeur d9Alene mining district lies within a regional northeast-southwest extensional regime with the maximum compressive stress directed northwest-southeast. This orientation is deleterious for mining, as it favors right-lateral slip on the steeply dipping west-northwest-trending faults and bedding planes common in the district. Low-level seismicity in the immediate vicinity of the Lucky Friday Mine is driven in the short term by mining rate, as is evident from the decreased levels of seismic activity on weekends when mining does not take place. In the longer term, cycles of tectonic stress build-up and release may be important. Large rockbursts at the mine seem to be followed by several months of seismic quiescence.

A regionalized multivariate approach to target selection in geochemical exploration

Characteristic analysis is proposed as a multivariate technique for comparing regional unknown cell data with data for explored areas. Original data are first converted into binary arrays according to whether or not each measured variable at a given location is considered to be favorable (1) or of indeterminate favorability (0) in terms of mineralization. A characteristic vector is calculated for the model selected, and each region cell is compared with the model vector. Region cells are ranked according to the measure of association with the model, and the results to be interpreted are plotted on a map. An interactive computer program was written to perform the analysis.Twelve separate mineralized areas were selected as models in the Coeur d9Alene district, Idaho. The area comprising1 all twelve mineralized areas was selected as a thirteenth model. Geochemical data for Hg, Te, Cu, Pb, Zn, Ag, Cd, As, Sb, Na, and K in 4,000 rock samples were used as the variables. The map derived for each model shows the distribution of degrees of association of all cells in the district. A structural restoration of the aggregate mineralization model shows that nonmodel cells located between major models have high degrees of association with the aggregate model, implying continuity of mineralization. Because these cells would be the logical potential target areas considering the known geology, it is concluded that characteristic analysis is a powerful analytical tool in exploration.

Origin of Slaty Cleavage in the Coeur d'Alene District, Idaho

Deformational features in argillaceous rocks of the Belt Supergroup in the Coeur d9Alene district indicate that the slaty cleavage was produced by mechanical reorientation of original clays during an early period of folding. Furthermore, this regional folding event probably occurred soon after the deposition and burial of the younger units in the district. Regional folds and slaty cleavage are closely related geometrically. The cleavage is parallel to the axial planes of the folds in their hinges, but fans divergently along their limbs; that is, cleavage dips in the same direction as the bedding, but more steeply. Motion within the argillites during folding produced dimensional preferred orientations of detrital quartz and muscovite. Low-grade metamorphism produced crystallographic preferred orientations in quartz in the Prichard Formation but not in the shallower Wallace rocks. Metamorphism was synchronous or later than folding. In the Wallace Formation, injection structures of argillite intruded the sandy layers at the time of folding. The slaty cleavage was enhanced locally near the injection structures where the greatest amount of motion in the argillite occurred. Molar-tooth structures were folded both by compaction during burial and by shortening perpendicular to cleavage during folding. Mechanical reorientation of platy minerals is suggested by the following evidence: the strongest cleavages were formed in zones of greatest movement of argillite; a dimensional preferred orientation was produced mechanically in other detrital grains; the degree of recrystallization in the argillites was quite low; and there was no correlation between quality of cleavage and degree of recrystallization. The deformational environment is more difficult to deduce. The possibility that the environment was more diagenetic than metamorphic is suggested by the probable soft-sediment origin for the argillite injection structures, the lack of extensional deformation features in molar-tooth structures oriented parallel to the cleavage, and the presence of the bulk of small tight folds in the Wallace Formation rather than in deeper stratigraphic units.

Deformation Lamellae Parallel to (10macr13) and (0001) in Quartz of the Coeur d'Alene District, Idaho

Deformation lamellae oriented parallel to the (10¯13) and (0001) crystallographic planes in quartz have been considered to be deformation structures unique to shock metamorphism induced by meteorite impact. Rocks of the Belt Supergroup of the Coeur d9Alene district, Idaho, contain quartz with deformation lamellae parallel to both (10¯13) and (0001). All available evidence indicates a geotectonic rather than an astrotectonic origin for the deformation lamellae of the district. Therefore, the uniqueness of these orientations to astrotectonic deformation is doubtful.

Graben structure in the Coeur d'Alene district, Idaho

Information published in U.S.G.S. Professional Papers 445 and 478 indicated to the writer that the structure of the Coeur d9Alene District in northern Idaho were developed by slumping along a rift graben that formed during late Precambrian time while the sediments were more or less semi-consolidated. Deep synclinal folds with adjacent anticlines over-turned toward each other, minor slump features, the St. Regis Breccia resembling a quickstone breccia and abundant evidence of past hydrothermal activity all seem to support the theory of an ancient rift graben.

Correlations and Problems in Belt Series Stratigraphy, Northern Idaho and Western Montana

A continuous strip of geologic maps has recently been completed along the Idaho-Montana state line between Clark Fork, Idaho, and Superior, Montana. New stratigraphic and petrographic information provides the basis for stratigraphic correlations and for the interpretation of facies changes in this part of the basin of deposition of the Precambrian Belt Series. Identification of facies changes is aided by the recognition of siltite (low-grade metamorphosed siltstone) as a valid rock type, in addition to quartzite and argillite, to classify most of these rocks and to establish mappable units. The older Belt rocks (Prichard through Wallace Formations) were deposited in a trough whose axis trended northwestward, perhaps approximately through Libby, Montana. Subtle facies changes suggest an ancient shore line southwest of the Coeur d9Alene district, Idaho, perhaps near the present exposed edge of the Idaho batholith. The younger Belt formations of the Missoula Group, however, thicken markedly southeastward toward Superior. Thus, the younger Belt rocks were deposited in a trough whose main axis was about at right angles to that of the older trough and perhaps was near Missoula, Montana. Deposition in the Clark Fork area was scant and often interrupted in Missoula time. The young major cross-warp in the old Belt geosyncline undoubtedly has contributed complexities to Belt correlation farther north. Detailed petrographic studies indicate that the Belt rocks from the Pend Oreille area, Idaho, are remarkably uniform in the mineralogic composition of similar rock types throughout 40,000 feet of strata. Further mineralogic studies are required to determine whether this uniformity is local or widespread.

Glaciation of the Coeur d'Alene District, Idaho

Cirques and glaciated valleys are present on northern slopes of peaks exceeding 5500 feet in elevation in the Coeur d9Alene District, Idaho. Glacially striated rock surfaces at the tops of cirque headwalls and on the divides above indicate that ice carapaces were present when the cirques were overfilled during glacial maxima. Freshness of landforms and deposits indicates a Pinedale age for the last widespread glaciation. Small ice masses may have developed within the most sheltered cirques during Temple Lake time. Till of Bull Lake age is present on benches and spurs near the head of the South Fork valley. Striated cobbles in coarse gravel as much as 1250 feet above present drainage levels are believed to be evidence of Buffalo glaciation. Following deposition of the gravel, streams eroded laterally before rejuvenation resulted in excavation of new valleys and exhuming of former valleys. It is possible to identify cirques that were occupied during one, two, or more glacial episodes. Continental ice of Spokane (Wisconsin) age impounded Glacial Lake Coeur d9Alene to an elevation of about 2600 feet. Water level was stabilized by a bedrock spillway at Setters on the western side of the lake. Numerous boulders of granite, plucked from bedrock near Pend Oreille Lake, were rafted into the district on ice bergs. Positions of these erratics indicate that postglacial erosion has been extremely slight.

Geochemistry of sphalerite from the Star Mine, Coeur d'Alene district, Idaho

Forty sphalerite samples from the Star mine, Coeur d9Alene district, Idaho, and 19 sphalerite samples from other Coeur d9Alene mines have been quantitatively analyzed by spectrographic methods for Fe, Cu, Pb, Cd, Ga, Ge, In, Mn, Co, and Hg. The samples from the Star vein were collected over a strike length of 2,000 feet and a vertical distance of 5,300 feet.It is believed that Star mine sphalerite formed in equilibrium with Fe; consequently, following Kullerud9s work, the Fe content may be used as a relative temperature scale. Relative Fe temperatures suggest that the lower portions of the Star ore body formed at lower temperatures than the upper portions of the ore body.Distribution of the minor elements in sphalerite is random within the ore body. Also, there is no correlation of Fe with minor elements other than Mn; therefore we believe temperature has had no influence on the distribution of minor elements, except Mn, in this ore body.The principal reason for this seems to be that equilibrium concentrations of the minor elements were not reached. Minor-element concentrations in sphalerite from the Coeur d9Alene district are similar to those in other districts throughout the world; consequently, the writers suggest that in general the minor-element content of sphalerite, other than perhaps Mn, cannot be an indicator of formation temperatures because equilibrium concentrations are rarely, if ever, attained.The average minor-element content in a district is probably that of the ore-forming solutions. Specific minor-element content in sphalerite is random.

Comments on minor elements in pyrrhotite [Idaho

The cobalt and nickel content of pyrrhotite from the Highland-Surprise mine, Coeur d9Alene district, Idaho, was determined by spectrographic methods. The Co and Ni values fall within the ranges of values found by other investigators. Comparison of the distribution of Co and Ni in the Highland-Surprise ore shoot with the distribution of these elements in the Silberberges, Germany, and Noranda, Canada, ore shoots suggests that caution must be used in the determination of relative temperature conditions in ore shoots by means of studies of minor elements, principally because it seems likely that equilibrium concentrations are commonly absent.

The occurrence of cobalt and nickel in the Silver Summit Mine, Coeur d'Alene district, Idaho

Cobalt and nickel of possible commercial interest have been discovered in the Silver Summit mine, Coeur d9Alene district, Idaho. Assay data indicate that an contained about 0.40 percent cobalt and about 1.0 percent nickel in addition to the copper and rich silver ores. The cobalt-nickel mineral gersdorffite occurs in tetrahedrite-siderite veins. Gersdorffite is widespread in small amounts and appears to have been the first of the ore shoot minerals. The known occurrence of either cobalt bloom or gersdorffite in other properties of the Coeur d9Alene district Silver Belt suggests that other shoots may exist that contain commercial quantities of cobalt and nickel.

Indicator minerals, Coeur d'Alene Silver Belt [Idaho

Deeply buried, high-grade oreshoots in carbonate-quartz veins are found in the Silver Belt of the Coeur d9Alene district by the application of various geologic criteria. Exploration is costly, and additional indications of the proximity of ore are highly desirable. The purpose of this study is to examine significant mineralogical features of the district in an attempt to develop additional helpful criteria.A search is made for indicator minerals; i.e., minerals, the spatial distribution of which may be indicative of the position of ore. In this respect, spatial distribution patterns of most of the vein and country-rock minerals are discussed, and the genesis and time relationships of the minerals are interpreted as an aid in the evaluation of these patterns.Arsenopyrite is believed to be an indicator, apparently forming envelopes around oreshoots. Similarly, late hydrothermal chlorite is considered to be an indicator. Sericite and carbonates of pre-hydrothermal-vein origin have a negative significance as indicators; i.e., little ore has been found in areas where these minerals are concentrated. Since beds rich in detrital quartz are the best ore horizons, such quartz is considered to be an indicator of limited practicability.An attempt is made to clarify the genetic classification of the various minerals, and within genetic types a few minerals not previously described in the district are listed. Six genetic types of chlorite are defined.The hydrothermal-vein history of the district is divided into three stages. In chronological order these are: (1) the bleaching alteration stage, (2) the carbonate-quartz stage, and (3) the sulfide stage. The hydrothermal bleaching alteration of large areas of the country-rock is largely the destruction of the rock pigments, and no strong sericitization is involved as previously thought.

Uraninite in the Coeur d'Alene district, Idaho

Trace amounts of uraninite occur in the lower levels of two silver-base-metal producers in the Coeur d9Alene district, Idaho. For the most part, the uranium-bearing veinlets follow small structures in the quartzite wall rock of the major ore bodies. The radioactive veinlets are bordered by a zone of red stained quartzite.The ores are characterized by tetrahedrite, pyrite, arsenopyrite, and chalcopyrite in a siderite-quartz gangue. Uraninite forms small spherulites that replace vein quartz, quartzite wall rock, and (to a lesser extent) tetrahedrite. Observations to date suggest that uraninite was deposited in the late stages of ore deposition.Ore deposits of the Coeur d9Alene district have certain features in common with the great pitchblende producing veins of the world.

A copper deposit of the Ducktown type near the Coeur d'Alene district, Idaho

A copper deposit about 40 miles southwest of the Coeur d9Alene district, Idaho, shows a marked structural and mineralogical resemblance to the copper deposits of the Ducktown type. It occurs as a replacement along bedding plane fractures and fissures in quartzitic beds of the Belt series and its ore consists chiefly of pyrrhotite, chalcopyrite, and lesser cubanite in a silicate and carbonate gangue. Fracturing repeated several times during mineralization caused deposition in three distinct mineral stages, shown by the partial replacement of one group of minerals by later ones. Minerals deposited during the first stage include the silicates (diopside, tremolite, actinolite, biotite, epidote, tourmaline, microcline, and muscovite), and a little quartz; during the second stage, the carbonate (ferriferous dolomite); and during the third stage, the sulphides (pyrrhotite, chalcopyrite, arsenopyrite, and cubanite), and a little quartz and calcite. Except for its mode of occurrence, the deposit possesses most of the characteristic features of a pyrometasomatic deposit but is, nevertheless, a hypothermal deposit formed probably under exceptionally intense temperature conditions.