Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Economic Geology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40Linked to original sources
USGS1963

Studies of fluid inclusions iii: Extraction and uantitative analysis of inclusions in the milligram range

A method has been developed for the extraction and chemical microanalysis of individual fluid inclusions , or groups of inclusions , in the milligram range . Usable quantitative analyses for Na, K, Ca, Mg, CI, B, and SO* have been obtained of mineral samples containing several milligrams of inclusion fluid , and with increased experimental errors, on fractional milligram samples. The steps involved are: 1) concentration of inclusions by sample selection and cutting; 2) electrolytic cleaning; 3) crushing in soft copper sample tube in vacuo; 4) conversion of emitted water to hydrogen and determination of its volume; 5) mass spectrometric determination of the D/H ratio if desired; 6) leaching of the crushed mineral to dissolve soluble salts remaining; 7) microanalysis of the filtrate by sensitive colorimetric and flame photometric methods. The method has been applied to determine the composition of fluid inclusions in mineral samples from several types of deposits, with special attention to a series of samples from Mississippi Valley-type ore deposits.

Economic Geology↗
USGS1963

Distribution of fluorine in unaltered silicic volcanic rocks of the western conterminous United States

An investigation of more than 170 samples of glass-rich volcanic rocks of rhyolitic or rhyodacitic composition shows marked regional variations in the fluorine content. The uniformity of the class of rocks was controlled by index of refraction determinations on fused beads and by chemical analyses of about one-seventh of the total number. The fluorine content ranges from 20 to 4,900 ppm. The frequency distribution closely approaches a log-normal one; the median value is 520 ppm, the 90 percentile 1,820 ppm. The mean for 167 samples is 820 ppm. These correspond fairly well with earlier averages for similar types of rocks . The highest values are predominantly from a belt in central Colorado and west-central New Mexico, from the Big Bend region of Texas, and from southeastern Idaho and western Utah and northeastern Nevada. The distribution of the fluorine in these rocks correlates well with the distribution of significant fluorspar deposits and somewhat less well with areas where volcanic rocks are prevalent and ground waters carry large amounts of fluorine .

Economic Geology↗
USGS1964

Nickeliferous laterites in southwestern Oregon and northwestern California

Deposits of ferruginous nickeliferous lateritic soils formed by weathering in place of ultramafic rocks occur at several places in northwestern California and southwestern Oregon, mostly in the Klamath Mountains province. Most of the deposits have been derived from relatively fresh peridotite, although at least one deposit was formed on serpentinite. The accumulations of lateritic soil are on flat-lying to gently sloping surfaces in a terrain that has been extensively dissected and deeply entrenched.The thickness of the deposits ranges from a few feet to more than 50 feet, and may vary widely in any given deposit, for the bedrock surface is commonly highly irregular. The deposit at Nickel Mountain, Oregon, is unique, because it is the only one in which the nickel silicate, garnierite, is plentiful. At Nickel Mountain, garnierite, accompanied by abundant microcrystalline quartz, occurs in veinlike bodies and boxworks that lie at the base of the soil zone and persist in depth along joints and fractures into the underlying slightly weathered or fresh peridotite.Mineralogical studies and chemical analyses show that weathering of the ultramafic rocks destroyed olivine and much of the orthopyroxene and serpentine minerals. Secondary minerals formed are predominantly hydrated ferric oxides (goethite); minor amounts of a montmorillonite mineral, chlorite, and talc were also identified. The kaolin group and bauxite minerals were not found. Except for garnierite at the Nickel Mountain deposit, no nickel-bearing mineral was identified. Nickel may occur, however, in montmorillonite, chlorite, and talc where it substitutes for Mg to a limited extent; in serpentine minerals that were not destroyed by weathering; and possibly in combination with ferric hydroxides. Cobalt may also occur in the clay minerals and probably is associated with trace amounts of manganese oxide or hydroxides.The lateritic soils in northwestern California and southwestern Oregon are similar to soils formed by lateritic weathering of ultramafic rocks in Cuba, the Philippines, New Caledonia, and other tropical regions of the world. They have, however, a higher content of SiO 2 and MgO, and a lower content of Fe 2 O 3 than deposits formed in tropical climates. The California and Oregon deposits are considerably smaller and lower in average metal content which, because they are widely scattered in a rugged, relatively isolated terrain, makes them unpromising as likely reserves for commercial exploitation in the foreseeable future.Most of the deposits are regarded as having been formed by chemical weathering in a climate having alternating wet and dry seasons, probably similar to that prevailing today. Their age is uncertain but may range from post-Miocene to Pleistocene.

California, Oregon↗
USGS1964

Chemical aspects of hydrothermal alteration with emphasis on hydrogen metasomatism

The ratio of cation to hydrogen-ion concentration is used as a basis for coordinating many observed varieties of silicate rock alteration. Low ratios drive alteration from feldspar through sericite, montmorillonite, or paragonite, toward kaolinite or pyrophyllite. Principal controls of ratio are reactions with wall rock, pressure-temperature changes, mixing of solutions, and oxidation of H 2 S; their interactions govern the assemblage attained.

Economic Geology↗
USGS1964

Mineral paragenesis and zoning in the central Kentucky mineral district

The Central Kentucky mineral district occupies the greater part of the Blue Grass lowland of Kentucky , which lies astride the Lexington dome and the Cincinnati arch. The oldest exposed rocks of the mineralized area are limestones of Middle Ordovician age. The paragenesis throughout the district is constant and traceable, although the relative abundance of minerals varies from vein to vein. The mineral association, areal distribution of the ore minerals, and comparison with other epithermal deposits indicate that the Central Kentucky veins are epithermal. The ore deposits have a district -wide lateral zoning in a concentric pattern. Near the middle of the district , and within the Lexington dome, are two centers of fluorite-calcite-dark sphalerite veins that form Zone 1. Surrounding these centers is a much larger Zone 2 in which nearly all the veins contain fluorite, barite, calcite, and two-colored (yellow-black) sphalerite. Galena is absent or present in very minute quantities in Zones 1 and 2. In Zone 3, which surrounds Zone 2, barite, galena, and bright-colored sphalerite are the principal minerals. Fluorite is absent, or present only in traces. Textures of the mineral veins change from bands of coarse comb crystals in Zone 1 to mostly colloform bands in the outer zones. Structural control of the ore is related to faults and fractures transecting the collapsed Lexington dome and is an important factor in the distribution of mineral veins. The district has many of the mineralogic, zonal, and structural features of the Central Tennessee, Kentucky -Illinois, and Pennine, England, mineral districts, which suggest that the origins of these districts were similar. The ores in the Central Kentucky district were probably deposited from ascending thermal solutions of slightly lower temperatures than those that formed the deposits in the Kentucky -Illinois district . The solutions are believed to have been derived from a deepseated magmatic source centered beneath the Lexington dome, but they may have been greatly diluted by ground waters within the dome. The innermost zone probably is nearest to the emanative centers.

Kentucky↗
USGS1964

Experimental determination of the reaction chalcopyrite + sulfur = pyrite + bornite from 350 to 500° C

The fugacity of sulfur along the curve representing the univariant equilibrium assemblage, chalcopyrite + bornite + pyrite + vapor, has been determined by the electrum tarnish method (1). The conditions of the equilibrium may be expressed by the equation (TP) - /°s 2 (T, 1 atm) = -50,730 + 56.95T (cal.) ( 350 - 500° C ) (T in °K). With gentle curvature, the line may be extended upward to the invariant point at 568° C , which represents the assemblage chalcopyrite + bornite -j- pyrite + liquid + vapor (8).

Economic Geology↗
USGS1964

Supergene iron ores of minas Gerais, Brazil

The iron ores of Minas Gerais , Brazil , fall into two categories: (1) hypogene hematite ore averaging 66 percent or more Fe, and (2) lower-grade supergene ores . Most ore now extracted is high-grade hypogene ore ; lower-grade supergene ores will be of much future value. All supergene ores formed by weathering of itabirite, a metamorphosed oxide-facies iron formation averaging about 38 percent Fe and 44 percent Si02. The Caue Itabirite crops out for about 540 linear kilometers in central Minas Gerais . Supergene ores fall into three intergradational categories: (1) enriched itabirite, averaging 49 percent Fe, easily con-centratable, with reserves about 25,000 million tons; (2) intermediate grade ores , averaging perhaps 63 percent Fe, with indicated and inferred reserves more than 600 million tons; and (3) canga, averaging between 57 and 62 percent Fe, with reserves in the hundreds of millions of tons. Disaggregation of hard and brittle itabirite by solution primarily of quartz and secondarily of other soluble constituents causes residual enrichment in iron with minor hydration of hematite. As weathering continues, most of the quartz is removed and more of the hematite is hydrated, producing intermediate grade ore . Secondary enrichment by limonite is important. The final weathering product is canga. In canga, almost all the iron is hydrated, and the rock is still further impoverished in Si02 and residually enriched in A1203 and P. Canga also forms by cementation of iron -rich detritus by limonite. Four factors control the supergene ore -forming process: (1) physiography, for ores occur on ridges and plateaus; (2) climate, for seasonal rainfall is apparently needed for the formation of extensive canga blankets; (3) grain size of the original itabirite, for quartz with a grain size much greater than 0.1 millimeter is not readily soluble; and (4) composition of the iron formation. The chemically inert and physically resistant canga blanket is essential to supergene ore formation because soft weathering products would otherwise be removed as fast as formed. Thus, canga gives time for the formation of other ore types. Iron fixed as cementing limonite in canga and as enriching limonite in intermediate grade ore was derived by leaching and hydration of hematite from itabirite. It probably moved in the ferrous state and was precipitated as insoluble hydrous ferric oxide primarily by oxygenation of the solutions, to a lesser extent by their evaporation at or near the surface, and to a still smaller degree by pH changes. The data show that the high-grade hematite ore , 66 percent Fe or higher, cannot have formed directly by supergene action. Geochemical processes resulting in supergene concentration of iron also concentrated alumina and phosphorus. The high-grade ore contains about the same low percentages of these materials as unweathered itabirite.

Minas Gerais↗
USGS1964

Paleo-channels at the Guayacan copper mine, Cabildo District, Aconcagua Province, Chile

An uneven erosion surface separates calcareous sedimentary rocks above and vesicular andesite porphyry below at the Guayacan mine of central Chile . Channel -like depressions filled with sedimentary rock flank elongate, stratiform bodies of disseminated copper ore which impregnate the vesicular crests of andesite lava flows. The " channels " lie parallel to and mark the margins of tongues of lava. Erosional features such as scour surfaces, conglomerate and crossbedding mark some of the channels . Other channels show no evidence of erosion, suggesting that some, at least, represent depressions or spaces that were not filled during extrusion of the lava. The flows strike east. Their crests are vesicular and impregnated with copper sulfide minerals, mainly bornite, to depths as much as 3 m below the contact. Bornite fills vesicles and fractures, impregnates the andesite ground mass, and replaces andesine phenocrysts, mainly along cleavage planes. Chalcopyrite is mixed with bornite in the upper, more vesicular part of the flows and occurs with fine-grained pyrite as disseminations and replacing fossils in the base of the overlying sedimentary rock. The channel axes also strike roughly east and, in most places, are underlain by barren ground. These mappable features, when recognized, are valuable in determining the size, shape, and orientation of adjacent ore bodies and, thereby, assist in the evaluation, exploration and mining of these stratiform copper deposits.

Economic Geology↗
USGS1964

Veins of hypogene manganese oxide minerals in the southwestern United States

Characteristic minerals are psilomelane, hollandite, cryptomelane, and coronadite, more rarely ramsdellite and pyrolusite. Host rocks are Mn-deficient; 80 percent of examples are middle to late Tertiary layered volcanics. Though deposits are shallow, mostly mined to only 100-200 feet (maximum 500 feet), a hypogene origin is indicated by their persistent association with barite and fluorite, a peripheral position in the zonal pattern of some metal-mining districts, alteration of plagioclase to K-spar, and abundance of W, Pb, Cu, Mo, Ti, As, Sb. They represent the subzone of Mn-bearing epithermal vein deposits lying nearest the surface, succeeded in depth by four other subzones: barite, fluorite, gold-silver, and base metals.

Arizona, California, Colorado, Nevada, New Mexico,↗
USGS1964

Principal features and origin of podiform chro-mite deposits, and some observations on the Guleman-Soridag District, Turkey

Podiform chromite deposits occur in alpine peridotite and mafic complexes and fundamentally are tabular, pencil-shaped, or irregular in form. The chromite characteristically is anhedral and commonly shows effects of granulation and magmatic corrosion. Flow-layering, foliation, and lineation are parallel in most chromite deposits and peridotite host rocks, and normally pass through major rock units, including chromite ; locally, foliation and lineation may cross layering. Most podiform deposits are oriented with their longer dimensions essentially parallel to layering or foliation in the host peridotite, but some are crosswise. In all podiform deposits , whether parallel or crosswise, internal flow structures are, to use Hans Cloos' terminology, harmonious with those in the country rock, except where disturbed by postmagmatic faulting. The relations between podiform and stratiform deposits are analogous to those between metamorphic and sedimentary rocks. Though relict structures formed by crystal settling are preserved in some massive ore, most features in podiform deposits are due to extensive flowage during re-emplacement, under magmatic conditions. As the rules governing sedimentary rocks apply in exploration of stratiform deposits , so metamorphic principles apply to podiform deposits . Evidence is presented to show that the chromite deposits in the Gule-man-Soridag district , Turkey , are podiform ; and that the Golalan ore body is closely related to nearby gabbroic rocks of normal alpine type. The Soridag deposits are regarded as originally podiform rather than as fault segments of stratiform deposits .

Economic Geology↗
USGS1965

Nature and origin of the high-grade hematite ores of Minas Gerais, Brazil

The high - grade hematite deposits of Minas Gerais , Brazil , are those averaging more than 66% Fe and less than 1.5% H20+. They occur in the Caue Itabirite, a metamorphosed oxide-facies iron formation of Pre-cambrian age. This formation was intricately folded during an orogeny that metamorphosed pelitic rocks to the greenschist and almandine-amphi-bole fades, accompanied by the formation of granitic rocks. The folding produced much plastic rock flowage toward the axes of many folds, thickness of the itabirite ranging from a few to more than 1,400 m. Of 40 ore bodies on which enough information for judgments is available, 11 probably contain more than 100 million tons, 22 between 10 and 100 million tons, and 7 less than 10 million tons. At least two are thought to be larger than 400 million tons. The great majority of these ore bodies are associated with folded structures in the host itabirite, a few are associated with preore thrust faults, and some with abrupt steepening of regional linear structure. Most of the ore occurs in the lower, more siliceous two-thirds of the itabirite; some occurs in the upper third, which is locally dolomitic. Some of the ore is foliated, some unfoliated; the unfoliated cuts the foliated and heals ore breccias. The ore mineral is dominantly hematite . Ore was formed during the metamorphic cycle, after the folding. It is localized in relatively low-pressure, hence higher permeability, parts of folds. Ore replaced pre-existing itabirite, preserving structures. Regional metamorphism indicates that the metasomatizing fluids were at high temperature and under high confining pressure. Increasing solubility of quartz with gradually rising temperature caused metasomatic replacement of quartz by hematite picked up from iron formation by throughpassing fluids during the rising temperature cycle of metamorphism. Source of fluids is thought to have been first the metasedimentary rocks, then granitic gneiss and intrusive granite; the source of heat to have been the metasomatizing fluids and intrusive granite. The unfoliated iron ore healing brecciated foliated ore was deposited during the waning metamorphic cycle. Gold ore with associated tourmaline and cassiterite is found in porous but not massive iron ore . The abundant soft high - grade hematite ore formed from the more porous parts of the hard ore bodies, where metasomatism had not been complete, by supergene leaching of very minor quantities of hematite from crystal boundaries.

Minas Gerais↗
USGS1965

The system arsenic-antimony

The one atmosphere isobaric phase relations have been determined for the system As-Sb, using a simple experimental arrangement where a low melting temperature alkali halide melt acts as a reaction vessel. The one atmosphere univariant temperature, where a liquid of composition 45.5 mol % As, plus a solid of composition 62 mol % As and a gas of composition 89 mol % As coexist, is 666° ± 3° C. The one atmosphere sublimation point of As was measured as 611° ±2° C. Arsenic and antimony are hexagonal isostructural compounds with a complete solid solution series between them. The a cell edge and volume of the unit cell vary linearly with composition in mol percent. The c edge changes in a markedly nonlinear manner with composition. The measured unit-cell parameters at 25° C are: As, a = 3.760 ± 0.001 A, c = 10.555 ± 0.003A, V = 129.2A3; Sb, a = 4.310 ± 0.001 A, c - 11.279 ± 0.003A, V = 181.4A3.

Economic Geology↗
USGS1965

Gamma-ray spectrometer studies of hydro-thermally altered rocks

The uranium, thorium, and potassium content of hydrothermally altered rocks in the vicinity of several copper and copper-lead-zinc deposits in Arizona was determined by chemical analysis. Potassium in the more intensely altered zones is about twice that in unaltered areas. There is no corresponding increase in thorium, so a higher potassium/thorium ratio also results from alteration; abnormally high uranium, locally in secular disequilibrium, was observed at the Bagdad porphyry copper deposit. A truck-mounted gammarray spectrometer was used to obtain a spectrum of gamma emission at each sample location. Equating the spectrometric with the chemical data gave standard errors of 0.6 percent potassium, 3 ppm thorium, and 10 ppm uranium for the outcrop analyses with the spectrometer . The amount of potassium introduced by hydrothermal alteration is thought sufficient in quantity to be detected by either surface or aerial spectrometry; spectrometric detection of anomalous uranium is probably marginal, owing to the disequilibrium. The potassium/thorium ratio may provide a means of distinguishing, in such surveys, between hydrothermally altered high-potassium zones and unaltered extrinsically high-potassium intrusives.

Economic Geology↗
USGS1965

Electrochemical geothermometer: A possible new method of geothermometry with electro-conductive minerals

It was demonstrated with artificially processed sulfides that the minimum temperature of equilibration of a pair of electro - conductive minerals can be determined electrochemically. The procedure consists of setting up an electrochemical cell with the mineral electrodes and a suitable electrolyte, changing the temperature of the cell slowly in a furnace, and finding the temperature at which the emf of the cell becomes zero. The experimental results strongly suggest that this electrochemical method could be used as a new method of geothermometry .

Economic Geology↗
USGS1965

Report on S.E.G. Symposium on the chemistry of the ore-forming fluids August-September,1964

Sessions included presentations and discussions of the following topics: mineral stability and phase assemblages at ore - forming temperatures and pressures; high-temperature solution chemistry of sulfides, carbonates, and silicates, with consideration of complexing and stability, and the generation of ore fluids ; hydrothermal fluids as deduced from wallrock alteration, from fluid inclusions and hot springs, from isotopic studies, and from mine studies, zoning, and so forth.

Economic Geology↗
USGS1966

The system Cu-Ag-S

Compositions on the join Cu 2 S-Ag 2 S were studied with X-ray diffractometer heating stage. Low-temperature phases, which break down at temperatures between 67 degrees and 119 degrees C, are chalcocite, stromeyerite, Cu (sub O.8) Ag (sub 1.2) S, jalpaite, and acanthite. High-temperature phases, all cation-disordered, are hexagonal close-packed (high chalcocite), face-centered cubic (high digenite), and body-centered cubic (argentite) in order of increasing Ag. Above 593 degrees C, the face-centered cubic phase occupies the whole field of the join, extending also to more sulfur-rich compositions. Although reaction rates are extremely rapid in the system, and the high-temperature phases are unquenchable, pseudomorphs and textures in natural specimens can be informative.

Economic Geology↗