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At least 415 records · Page 23Linked to original sources
USGS1938

The origin of the iron ore deposits in the Bull Valley and Iron Springs districts, Utah

A summary of the geology of the Bull Valley district, southwestern Utah, and a description of the occurrence of the magnetite and hematite deposits is given, as well as brief descriptions of the ore pits at Iron Mountain and Desert Mound 20 miles to the northeast, in which deposits formed under similar geologic conditions have been developed. Neither the ores nor the country rock show the features characteristic of contact-metamorphic deposits. The following explanation of the origin of the deposits is offered. During the intrusion of monzonite porphyry stocks, the hoods and overlying rock were fractured, causing a sudden release of pressure and liberating FeCl 3 and H 2 O gas from the stocks which were still at high temperature. These gases found ready egress to the surface along tension fissures without appreciably heating the country rock. They deposited magnetite and hematite in the fissures and replaced the contiguous fractured limestone. The period of gas emanation was brief and the hydrothermal stage probably evolved during the later magmatic history never reached this area. The deposits were formed under a cover of less than 5,000 feet.

Utah↗
USGS1989

Undiscovered lode tin resources of the Seward Peninsula, Alaska

The United States is a net importer of many important minerals, including tin. Consumption of primary tin in the United States is about 36,000 metric tons per year. Identified U.S. tin resources consist of about 40,000 metric tons. Although such figures provide insight about vulnerability to supply disruptions in the short term, they do not provide information about the potential of a country to meet its needs for a commodity from undiscovered domestic sources. Recent developments, including the preparation of models of mineral deposits and their grades and tonnages, and the application of computer simulation techniques to the estimation of metallic mineral resources, make it possible to estimate the magnitude of undiscovered resources, by deposit type, for relatively small areas such as the Seward Peninsula. This paper uses these developments and geophysical data to estimate undiscovered lode tin resources on the Seward Peninsula of Alaska. The assessment is based on a three-step methodology that (1) identifies the types of tin deposits that may be present in the region, (2) identifies the geophysical characteristics of unroofed granites and shallow granitoids, and (3) estimates, on the basis of various combinations of geologic and geophysical conditions, the number of undiscovered deposits present within or near exposed or concealed granite plutons. Computer simulation was used to combine the estimates of the number of deposits with available grade and tonnage models. Simulation experiments were designed to estimate the amount of tin in undiscovered deposits under a variety of limiting conditions. Results of simulation experiments indicate there is a 90 percent chance that the Seward Peninsula contains at least 51,000 metric tons, a 50 percent chance that it contains at least 390,000 metric tons, and a 10 percent chance it contains at least 1,100,000 metric tons of tin in undiscovered greisen, vein, and replacement deposits that have average grades of at least 0.5 percent tin.

Economic Geology↗
USGS1994

Evolution of volcanic rocks and associated ore deposits in the Marysvale volcanic field, Utah

A geological account on the igneous activity and associated mineral deposition in the volcanic field of Marysvale in Utah is presented. Three episodes (34-22 Ma, 22-14 Ma and 9-5 Ma) involved in the volcanic rock eruption and associated mineralization are described. The first episode is believed to have occurred during the time of tectonic convergence when two contrasting suites of rocks, Mount Dutton Formation and Bullion Canyon Volcanics, erupted concurrently. Mineralization during this period was sparse. In the second episode, change from intermediate to bimodal volcanism occurred. During the third episode, basaltic compositions did not change. Although major element constituent had rhyolites similar to that of the second episode, rhyolites had a marked radiogenic isotope characteristic difference.

Utah↗
USGS1994

Chemical reaction path modeling of ore deposition in Mississippi Valley-type Pb-Zn deposits of the Ozark region, US midcontinent

The Ozark region of the U.S. midcontinent is host to a number of Mississippi Valley-type districts, including the world-class Viburnum Trend, Old Lead Belt, and Tri-State districts and the smaller Southeast Missouri barite, Northern Arkansas, and Central Missouri districts. There is increasing evidence that the Ozark Mississippi Valley-type districts formed locally within a large, interconnected hydrothermal system that also produced broad fringing areas of trace mineralization, extensive subtle hydrothermal alteration, broad thermal anomalies, and regional deposition of hydrothermal dolomite cement. The fluid drive was provided by gravity flow accompanying uplift of foreland thrust belts during the Late Pennsylvanian to Early Permian Ouaehita orogeny.In this study, we use chemical speciation and reaction path calculations, based on quantitative chemical analyses of fluid inclusions, to constrain likely hydrothermal brine compositions and to determine which precipitation mechanisms are consistent with the hydrothermal mineral assemblages observed regionally and locally within each Mississippi Valley-type district in the Ozark region. Deposition of the regional hydrothermal dolomite cement with trace sulfides likely occurred in response to near-isothermal effervescence of CO 2 from basinal brines as they migrated to shallower crustal levels and lower confining pressures. In contrast, our calculations indicate that no one depositional process can reproduce the mineral assemblages and proportions of minerals observed in each Ozark ore district; rather, individual districts require specific depositional mechanisms that reflect the local host-rock composition, structural setting, and hydrology.Both the Northern Arkansas and Tri-State districts are localized by normal faults that likely allowed brines to rise from deeper Cambrian-Ordovician dolostone aquifers into shallower carbonate sequences dominated by limestones. In the Northern Arkansas district, jasperoid preferentially replaced limestones in the mixed dolostone-limestone sedimentary packages. Modeling results indicate that the ore and alteration assemblages in the Tri-State and Northern Arkansas districts resulted from the flow of initially dolomite-saturated brines into cooler limestones. Adjacent to fluid conduits where water/rock ratios were the highest, the limestone was replaced by dolomite. As the fluids moved outward into cooler limestone, jasperoid and sulfide replaced limestone. Isothermal boiling of the ore fluids may have produced open-space filling of hydrothermal dolomite with minor sulfides in breccia and fault zones. Local mixing of the regional brine with locally derived sulfur undoubtedly played a role in the development of sulfide-rich ore runs.Sulfide ores of the Central Missouri district are largely open-space filling of sphalerite plus minor galena in dolostone karst features localized along a broad anticline. Hydrothermal solution collapse during ore deposition was a minor process, indicating dolomite was slightly undersaturated during ore deposition. No silicification and only minor hydrothermal dolomite is present in the ore deposits. The reaction path that best explains the features of the Central Missouri sulfide deposits is the near-isothermal mixing of two dolomite-saturated fluids with different H 2 S and metal contents. Paleokarst features may have allowed the regional brine to rise stratigraphically and mix with locally derived, H 2 S-rich fluids. The Viburnum Trend and Old Lead Belt ores are galena rich with lesser amounts of sphalerite; they replace the most permeable dolostone facies in the Bonneterre Dolomite. Hydrothermal dissolution of host dolostone was concurrent with sulfide deposition, but dolomite deposition occurred episodically between periods of sulfide deposition. The important ore controls in these districts are a variety of sedimentary and geologic features that allowed cross-stratigraphic fluid flow and provided opportunities for fluid mixing. The reaction path which best reproduces the broad features of the Viburnum Trend and Old Lead Belt ores is one in which a dolomite-saturated, lead-rich, zinc- and H 2 S-poor brine mixes with a less saline, H 2 S-rich fluid. The brine became enriched in K, Mg, and Pb and depleted in H 2 S as it flowed through sandstone and redbed aquifers prior to entering the district. This mixing model is consistent with the abundant fluid inclusion and stable isotope evidence for fluid mixing in the districts. Small amounts of cooling associated with the mixing may have contributed to sulfide deposition.

Alabama, Arkansas, Illinois, Indiana, Kansas, Kent↗
USGS1946

Sedimentary and volcanic processes in the formation of high alumina clay

In the West, where volcanic materials are abundantly distributed among the rocks of the geologic column, the importance of sedimentary processes in the formation of high-alumina clays has not been fully appreciated. At Ione, Calif., Castle Rock, Wash., Whiteware, Mont., Hobart Butte and Mollala, Oreg., where the Geological Survey has been investigating clays jointly with the Bureau of Mines, U. S. Department of the Interior, high-alumina clays formed by sedimentary processes are more important than those formed by all phases of volcanic activity. Clays derived directly from volcanic materials are composed dominantly of montmorillonite; but these clays, with the exception of the beidellite-nontronite varieties, have relatively low percentages of available alumina. In contrast, clays derived by thorough leaching of various aluminous rocks under conditions favoring thorough drainage are composed dominantly of kaolinite and when sorted by sedimentary processes form high-grade deposits; furthermore, the depositional structures of these sedimentary clays have favored the derivation of gibbsite through weathering, resulting in an increase in the alumina content of the clays. Kaolinite, gibbsite, and dickite all have relatively high percentages of available alumina.

Economic Geology↗
USGS1985

Geology of the Brick Flat massive sulfide body, Iron Mountain cluster, West Shasta district, California

The Brick Flat massive sulfide body is one of a group of 8 individual bodies that constitute the Iron Mountain cluster in the S part of the West Shasta district. Before they were separated by postmineral faulting, 5 of the 8 sulfide bodies formed a single large deposit about 1375 m long with a mass of some 23 million metric tons. The pyritic Brick Flat sulfide body is one of the 5 faulted segements of this deposit. The Brick Flat massive sulfide lies within medium phenocryst rhyolite that is characteristic of the ore-bearing middle unit of the Balaklala Rhyolite. It is interpreted to be downfaulted a vertical distance of 75 to 85 m from the Old Mine sulfide-gossan orebody along the N-dipping Camden South fault. It is bounded in turn on its N side by another parallel fault, the Camden North, which drops the orebody down another 75 m to the level of the Richmond orebody. -from Author

Economic Geology↗
USGS1973

Iron-formation in South America

Except for recent studies by certain South American governmental and quasi-governmental companies and agencies, little effort has been devoted to study of the iron-formations from which the great iron ore deposits of South America formed. Great gaps in basic information exist. Iron-formation is found in the Guayana and Brazilian Precambrian Shields as a common rock type and also occurs in Chile and astride the Bolivian-Brazilian border. Only the carbonate and oxide facies are known, the former being quite rare. The dominant oxide facies occurs in major units averaging more than 100 m in thickness and extending over hundreds of square kilometers, generally in a miogeosynclinal or intra-cratonic basin environment. The relation of such deposits with volcanism is tenuous and obscure, if indeed there is any direct relation. Smaller units of oxide facies iron-formation occur in many minor beds from widely varying geologic environments. The carbonate facies is found in a eugeosynclinal suite in Minas Gerais, Brazil, and is of the Algoma type. The deposits range in age from about 3,200 m.y. to late Precambrian or early Paleozoic; although the major epoch of deposition is debatable, it probably was about 2,000 m.y. ago. The South American oxide facies iron-formations are richer than many in the Northern Hemisphere, those of early and middle Precambrian age averaging about 40 percent in Fe and the same in SiO 2 . Younger iron-formations are still richer, averaging perhaps 50 percent Fe. Scanty trace element data do not indicate volcanic affiliations. The iron is present as magnetite, hematite, and martite; most rocks have been metamorphosed, and accordingly it is not known how much of the magnetite is metamorphic and how much is diagenetic or depositional in origin. Hematite and martite are dominant in most iron-formations. South American iron-formations are quite similar in lithology and occurrence to the major deposits in Africa and India and possibly formed when these continents were contiguous. These formations differ from those in the Northern Hemisphere in having a narrower range in lithologic facies and a generally higher iron content. In few areas can any direct relation with volcanism be demonstrated. © 1973 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1958

The relation of phosphorites to ground water in Beaufort County, North Carolina

Recent ground-water studies undertaken by the U.S. Geological Survey in cooperation with the North Carolina Division of Mineral Resources have delineated phosphorite deposits, tentatively regarded as being of middle Miocene age, in Beaufort County. These deposits lie unconforma-bly on limestone of Eocene age and are unconformably overlain by late Miocene marl. The phosphorites, buried beneath strata ranging in thickness from 45 to 250 feet, underlie an area approximating 450 square miles. The total thickness of the phosphorite column throughout the area ranges from several feet to nearly 90 feet. The phosphorite column consists of phosphatic sands and intercalated shell limestones. The sands, composed of pellets of brown sand-size collophane (probably carbonate-fluorapatite) and sand-size, flat-sided angular quartz with some silt, clay, and organic material, have a median diameter between 0.50 and 0.25 mm. Chemical analyses of representative samples of the raw sand show a variation in PaOs content from 8 to 31 percent. The P2O5 content is apparently proportionate to the collophane content throughout the area. Reconstruction of the geologic history suggests that the phosphorites were deposited as chemical precipitates and as in situ replacements in a restricted marine basin where the pH of the water acted as the primary depositional control. Reconstruction of the hydrologic history of the phosphorites indicates that the deposits were preserved under artesian conditions; they were never subjected to alteration under water-table conditions. The absence of postpositional alteration makes this area a potentially classic one for studies of phosphorite genesis. Chemical analyses of artesian waters from the phosphorites and from limestones overlain by phosphorites reveal significantly greater concentrations of iodide and bromide in solution than are present in water from overlying sediments or from underlying limestones not overlain by phosphorites. Such anomalies, if present in other similar terranes, may indicate the presence of buried phosphorites. The depth and size distribution of the material suggest a recovery method utilizing wells constructed to "pump sand."

North Carolina↗
USGS1985

Regional setting and new information on some critical geologic features of the West Shasta district, California

The West Shasta massive sulfide district is in the easternmost of a series of accreted island-arc and oceanic crust terranes that comprise the Klamath Mountains. A sequence of submarine volcanic rocks of predominantly Early Devonian age is the principal component of the island-arc terrane in which the sulfide deposits are hosted. The Copley Greenstone, consisting mainly of andesitic and basaltic pillow lavas and breccias totaling at least 1,800 m in thickness, is the oldest rock unit in the sequence. It is overlain and also intruded by dikes of the Balaklala Rhyolite. Northeast of the West Shasta district, greenstone also overlies the Balaklala Rhyolite, suggesting that a major greenstone unit may overlie potentially mineralized rhyolite east of the district. However, recent studies have shown that the complex relations in that area can be explained by folding.The Balaklala sequence consists of silicic flows, breccias, and tuffs having a maximum thickness somewhat in excess of 1,000 m. The Balaklala has been divided by Kinkel et al. (1956) into three units: a lower nonporphyritic to slightly porphyritic unit containing large amounts of breccia and tuff; a middle unit characterized by rhyolite containing quartz phenocrysts 1 to 4 mm in diameter but also containing a complex assortment of tuff, breccia, and pyritic massive sulfide bodies in its upper part; and an upper unit typically containing dark quartz phenocrysts in excess of 4 mm in diameter. Much of the lower part of the upper unit is pyroclastic material, whereas most of the upper part of the unit appears to be a massive volcanic-flow rock. At least half a dozen eruptive centers for the Balaklala Rhyolite are identified, three of them within an area measuring 17 X 3 km that constitutes the limits of the former mining district.The youngest unit in the arc sequence is the Kennett Formation consisting of black shale and chert containing radiolarians, which indicate fairly deep-water deposition in its lower part, and limestone containing a shallow water fauna of probable Middle Devonian age in the upper part. A 400-m.y.-old trondhjemite stock, named the Mule Mountain stock, intrudes the Copley Greenstone and Balaklala Rhyolite and is considered to be essentially coeval with these volcanic units.The pyritic massive sulfide deposits occur in clusters of individual bodies owing in large part to disruption by postmineral faults. The deposits are stratigraphically confined to the upper part of the subhorizontal middle unit of the Balaklala, and their horizontal distribution is here interpreted to have been controlled by an extensional tectonic regime that prevailed during Early Devonian time. The major geologic evidence for such a regime is the marked preferred elongations exhibited by the distribution of the afore-mentioned eruptive centers, the eight or nine massive sulfide clusters in the district, and the geometrically similar distribution of the clusters in relation to each other. The preferred directions of elongation in both the detailed and broader senses may be generalized to N 20 degrees to 25 degrees E, N 37 degrees E, N 60 degrees to 80 degrees E, and N 40 degrees to 60 degrees W. The three northeast trends dominate throughout the district, whereas the northwest trend is evident mainly in the northern part and is more speculative. The major deposit clusters fall at intersections of the major trends, some of which may have been grabens. Trend intersections having no known deposits may be good exploration targets in localities where the stratigraphically favorable middle unit of the Balaklala is still present.

Economic Geology↗
USGS1996

Significance of tourmaline-rich rocks in the North Range group of the Cuyuna iron range, east-central Minnesota

Concentrations of tourmaline in Early Proterozoic metasedimentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water alepositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a reevaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sediment-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins.In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

Economic Geology↗
USGS1996

Age of supergene oxidation and enrichment in the Chilean porphyry copper province

Twenty-five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees S in northern Chile yield K/Ar ages ranging from about 34 to 14 Ma. Therefore supergene oxidation and enrichment processes were active from the early Oligocene to the middle Miocene, a minimum of 20 m.y. Supergene activity at individual deposits lasted for at least 0.4 to 6.2 m.y. The early Oligocene supergene activity affected deposits in the Paleocene porphyry copper belt, whereas early and middle Miocene supergene processes are documented in the Early Cretaceous, Paleocene, and late Eocene to early Oligocene porphyry, copper belts. Middle Miocene oxidation also affected the oldest epithermal gold-silver deposits in the Maricunga belt farther east. Supergene activity commenced no less than 11 m.y. after generation of each porphyry copper deposit because of the time required to unroof the copper-bearing parts of the system. Supergene activity throughout northern Chile ceased at approximately 14 Ma. The geologic features of deposits and prospects and their morphotectonic positions, present latitudes, and present elevations display no obvious correlations with the supergene chronology. Exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels (+ or - ignimbrites) of Oligocene through middle Miocene age unless their accumulation is demonstrably late in the documented history of supergene activity, or in porphyry copper provinces, such as those of central Chile and northwestern Argentina, which formed after approximately 14 Ma. The uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern Chile and contiguous areas unless their effects were much more prolonged. Intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern Chile, and tectonic uplift was its most probable cause. However, more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role.

Economic Geology↗
USGS1985

Geologic setting, petrology, and geochemistry of stratiform sphalerite-galena-barite deposits, Red Dog Creek and Drenchwater Creek areas, northwestern Brooks Range, Alaska

Similar stratiform sphalerite-galena-barite deposits occur in the Red Dog Creek area, De Long Mountains quadrangle, and in the Drenchwater Creek area, Howard Pass quadrangle, northwestern Brooks Range, Alaska. The deposits, located approximately 180 km apart, are hosted by Mississippian and Pennsylvanian strata of the Kagvik structural sequence of late Paleozoic and early Mesozoic age. This sequence is situated in the lowermost structural plate of a series of predominantly east-striking, shallow-dipping thrust plates that characterize the regional structure. The deposits occur in shale, tuff, tuffaceous sandstone, altered chert, and quartz exhalite. These units are in places interlayered with or adjacent to bedded or nodular barite or submarine keratophyre flows. Chert and shale commonly contain abundant highly altered, fine-grained feldspar, pumice lapilli, and mafic volcanic rock fragments.The main types of occurrences at Red Dog Creek are: (1) disseminated and stratiform sulfides thinly bedded in organic-rich Mississippian and Pennsylvanian shale and chert, (2) massive sulfide veins and breccia fillings in silicified Mississippian and Pennsylvanian shale, and (3) stratiform sulfide-bearing lenses subdivided into (3a) quartz-exhalite with up to several percent sulfides and barite, (3b) massive sulfide-quartz lenses with up to several percent barite, and (3c) barite-quartz lenses with up to several percent sulfides.At Drenchwater Creek sulfides and barite occur as: (1) disseminations in shale, chert, tuff, and tuffaceous sandstone; (2) disseminations to aggregates in quartz exhalite; and (3) occurrences in veins crosscutting cleavages in brecciated chert and shale.The stratiform sulfides occur in zones up to several thousand meters long and several tens of meters thick. Preliminary grades at Red Dog Creek are 17.1 percent zinc, 5.0 percent lead, and 2.4 oz silver per ton contained in at least 85 million tons.The range of sulfide sulfur isotope values at Red Dog Creek is -16.6 per mil to +3.6 per mil. Sphalerite averages 0.8 per mil. Pyrite is isotopically lighter than coexisting sphalerite in quartz exhalite and massive sulfide lenses. This isotopic relationship is compatible with paragenetic relationships which show pyrite crystallized both earlier and later than other sulfides. Galena is isotopically lighter than coexisting sphalerite. Three groups of geologically and isotopically distinct barites exist including isotopic values appropriate for derivation from late Paleozoic seawater sulfate.Early in the depositional sequence isotopically light pyrite formed from organically reduced seawater sulfate. Sphalerite, galena, late pyrite, and some barite precipitated from two sulfur-bearing solutions that mixed at the mineralization site. Isotopically light barite associated with sulfides precipitated from oxidized H 2 S without seawater sulfate contribution. Lead isotope data from Red Dog and Drenchwater Creeks are compatible with generation of lead in a Phanerozoic mature island-arc setting, a typical orogene, or possibly a back-arc type intracratonic basin.The stratiform sphalerite-galena-barite deposits probably formed in an incipient island-arc environment near a continental margin, containing magmas of keratophyric and andesitic composition, where metal-laden hydrothermal fluids discharged into a deep (?) water, low-energy environment. Later intense deformation, associated with plate docking and thrusting, disrupted and partly remobilized and stratiform deposits.

Alaska↗
USGS1981

Physical factors that could restrict mineral supply

Stages in the metal supply process are affected by each of the following physical (geologic) factors: (1) geographic distribution of concentrations of potential ore minerals, (2) depth of these concentrations, (3) mineralogy, (4) grain size of the minerals, and (5) grade and (6) tonnages of the concentrations. For mineral deposits of each type in each geologic and political environment, the lowest cost metal will tend to be produced first because: (1) the largest deposits tend to be found first, (2) the few largest deposits should reap the benefits of economies of scale and will tend to account for the lion’s share of total metal, (3) higher grade deposits that require substantially less energy per ton of metal than do lower grade deposits will be mined first, and (4) deposits that have the mineralogy and grain sizes that are relatively cheap to process will be produced first. Shallow deposits that are relatively cheap to find and mine and deposits near existing infrastructure will also be produced first. Because stages in the supply process are sequential, problems at any stage caused by the physical factors are reflected in subsequent stages and adversely affect production costs. In the long term, effects of such problems will be not physical shortages of metals but a rapid or persistent increase in real prices of metals.

Economic Geology↗
USGS1953

Iron deposits of the congonhas district, minas Gerais, Brazil

Various origins have been proposed for the itabirite and associated hematite ores of Minas Gerais, Brazil. The U.S. Geological Survey, in cooperation with the Brazilian Departamento Nacional da Produqao Mineral, has undertaken a comprehensive program of mapping of these Precambrian deposits, which bear many similarities to other Precambrian iron formations. Itabirite is a finely laminated quartz-iron oxide rock with more or less dolomite, corresponding to James's oxide facies. It occurs principally in the middle group of the Minas series-a eugeosynclinal assemblage of quartzite, schist, chemical precipitates, graywacke, and volcanic rocks. Iron and silica were precipitated rhythmically in a shallow restricted basin under somewhat acid conditions that inhibited the precipitation of carbonates. Erosion of moderately deformed rocks of the Minas series furnished sediments for the overlying conglomeratic Itacolumi series. Severe post-Itacolumi deformation folded and thrust-faulted rocks of both series; the accompanying regional metamorphism recrystallized the chert and iron oxide to quartz, specular hematite, and minor amounts of magnetite. Ultramafic intrusions antedate this diastrophism; granodiorites and various basic dike rocks are younger. Hydrothermal replacement of breccia zones in itabirite and of dolomitic beds associated with the iron formation produced high-grade specular hematite ore nearly free of impurities. The areal distribution of the deposits indicates that the solutions followed fault zones. Heated meteoric water may have been responsible for the replacement, as minerals of obvious magmatic origin are absent. Since early Tertiary time the area has been uplifted several thousand feet. Leaching and cementation related to the present surface have modified both the iron formation and the hematite deposits, giving rise to various types of commercial ore. © 1953 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1992

Manganese carbonate mineralization in the Molango district, Mexico

The Molango manganese deposit in Mexico is by far the largest known Mn deposit in North America. Mn carbonate mineralization is hosted by a finely laminated Upper Jurassic marine sedimentary sequence that forms the base of the Chipoco facies of the Taman Formation (Kimmeridgian). Manganese enrichment occurs over a total stratigraphic thickness of approximately 50 m, and a length of over 50 km along strike and serves as the protore for supergene Mn oxide ore. The carbonate ore zone comprises the basal 1 to 10 m of the Chipoco facies.The carbonate ore bed consists of fine-grained rhodochrosite and dispersed organic matter, magnetite, and maghemite but generally only trace quantities of pyrite. Fine laminations and clotted textures suggest deposition in a restricted marine environment. Manganese content is highest at the base of the mineralized interval, drops abruptly to subore grade, Mn-rich carbonates, and then gradually decreases upward to background levels (i.e., trace Mn in calcite). Chemical and mineralogical studies indicate that the predominant mineralogy changes upward from rhodochrosite in the ore zone, to mixed carbonates (Mn calcite, kutnohorite, + or - rhodochrosite), to Mn calcite.Several geologic and geochemical processes resulted in the formation of Mn carbonate by the early diagenetic reduction of Mn oxides through the oxidation of organic matter and iron sulfide. Mn oxides were concentrated at the margins of a dysaerobic to mildly anoxic stratified basin and became an important oxidizing agent of organic matter in the sediment pile. Effects of organic matter oxidation reactions included production of manganous ion (Mn (super +2) ), MnCO 3 precipitation, oxidation of iron monosulfide (precursor to pyrite), and formation of Fe oxide. The latter two processes explain the anomalously low pyrite and abundant magnetite content in the ore zone. The observed vertical variation of Mn carbonate mineralogy and the mineral compositions reflect variations in the concentrations of reduced manganese in the pore waters. Thus, Mn carbonate precipitated from pore water and did not form as a chemical sediment in the water column. The ultimate source of the manganese is uncertain but may have been fluvial-sediment loads or hydrothermal activity associated with the rifting of the Gulf of Mexico.

Economic Geology↗
USGS1946

Relations of bauxite and Kaolin in the Arkansas bauxite deposits

Investigation of the Arkansas bauxite deposits by the U. S. Geological Survey during the years 1942 to 1945 has brought out the fact that secondary kaolinization or silication of the bauxite is much more common than has been generally recognized. On the other hand no direct evidence was found of the development of bauxite from kaolin. These facts have led us to reconsider the genetic relations of bauxite and kaolin. We do not mean to deny that there has been direct kaolinization of the nepheline syenite; but in view of the above observations and of the character of the kaolin derived from bauxite, we believe that the relations of bauxite and kaolin in the Arkansas deposits are more readily explained on the assumption that the bauxite is derived directly from the nepheline syenite, rather than from kaolinized nepheline syenite. The silica causing its subsequent kaolinization may be derived from overlying sediments or from remnants of unbauxitized nepheline syenite within or adjacent to it .Our conclusions about the relations of the kaolinized and bauxitized nepheline syenite are based entirely on rock weathered in place.

Arkansas↗
USGS1972

Geology of the massive sulfide deposits at Jerome, Arizona; A reinterpretation

The massive sulfide deposits at Jerome, consisting largely of pyrite, chalcopyrite, and sphalerite, are concordant stratabound lenses in massive quartz-bearing crystal tuffs of Precambrian age and overlying bedded tuffaceous rocks. The crystal tuffs were eraplaced as submarine pyroclastic flows. The evidence permits the interpretation that the massive sulfide lenses are essentially syngenetic and related to hydrothermal brines that discharged into a submarine basin. The host rocks and sulfide lenses were folded and metamorphosed. Two periods of folding can be recognized; during the second period, the older folds were deformed along vertical axes, and some of the chalcopyrite in the major lens migrated downward to form shoots of intersecting veins in the crystal tuff and chloritized tuff (black schist). Fluid inclusions in volcaniclastic and vein quartz appear to be related to deformation and metamorphism and provide no direct evidence on the nature of the primary fluids that presumably introduced the ore metals to the volcanic rocks accumulating on the sea floor. Liquid CO2-bearing inclusions are restricted to rocks and veins near the United Verde deposit and associated chalcopyrite ore shoots. Fluids at that time were CO2-rich, with temperatures of approximately 235 ø C and confining pressures of about 700 bars or less.

Arizona↗
USGS1986

Ages of igneous and hydrothermal events in the Round Mountain and Manhattan gold districts, Nye County, Nevada

About 60 new isotopic age determinations help clarify the geologic history of the Round Mountain and Manhattan gold districts, Nevada. Cretaceous granite of Shoshone Mountain, southeast of Round Mountain, was emplaced about 95 m.y. ago and was domed, metamorphosed, and mineralized with tungsten-bearing quartz veins 80 m.y. ago. The granite of Pipe Spring, south of Manhattan, was emplaced probably at about 80 m.y. ago. Mineralization associated with the Pipe Spring body occurred about 75 m.y. ago. A granodiorite stock and associated rhyolite and andesitc dikes intruded the granite southeast of Round Mountain 36 m.y. ago; apparently base and precious metals mineralization accompanied their intrusion, but mineralization has not been dated isotopically. Ash-flow tuff at Round Mountain was emplaced about 26 m.y. ago, the Manhattan caldera formed about 25 m.y. ago, and gold mineralization at Round Mountain occurred at about 25 m.y. ago. Gold mineralization at Manhattan took place 16 m.y. ago.The isotopic data allow interpretation of the cooling histories of some of the igneous rocks and suggest possible thermal events that reset mineral ages.

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