SEARCH · USGS Science

Results for “Facies”

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

SourceResearch connectionFrom yearThrough year

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

At least 991 records · Page 55Linked to original sources

USGS1985

Mineralogy, textures, and relative age relationships of massive sulfide ore in the West Shasta district, California

The Devonian massive sulfide orebodies of the West Shasta district in northern California are composed primarily of pyrite, with lesser amounts of other sulfide and gangue minerals. Examination of polished thin sections of more than 100 samples from the Mammoth, Shasta King, Early Bird, Balaklala, Keystone, and Iron Mountain mines suggests that mineralization may be divided into six paragenetic stages, the last five each separated by an episode of deformation: (1) precipitation of fine-grained, locally colloform and framboidal pyrite and sphalerite; (2) deposition of fine-grained arsenopyrite and coarse-grained pyrite, the latter enclosing tiny inclusions of pyrrhotite; (3) penetration and local replacement of sulfide minerals of stages 1 and 2 along growth zones and fractures by chalcopyrite, sphalerite, galena, ten-nantite, pyrrhotite, bornite, and idaite; (4) recrystallization and remobilization of existing minerals, locally increasing their size and euhedralism and promoting their aggregation; (5) deposition of quartz, white mica, chlorite, and calcite; and (6) formation of bornite, digenite, chalcocite, and covellite during supergene enrichment of several orebodies at the Iron Mountain mine. Despite regional greenschist facies metamorphism and local heating by intrusive bodies, enough of the original depositional features of the ore remain to suggest that the deposits in the district formed by processes similar to those that formed Kuroko- and Besshi-type massive sulfide deposits. Mineralogic and textural evidence do not support a second major episode of massive sulfide mineralization during the Permian.

Economic Geology↗

USGS1986

Genesis of the Spar Lake strata-bound copper-silver deposit, Montana: Part I. Controls inherited from sedimentation and preore diagenesis

Mineable zones of the Spar Lake deposit occur where argentiferous copper sulfides and native silver formed cements and replaced certain earlier cements and clasts in the gently dipping middle quartzite beds of the upper member of the Revett Formation, middle Proterozoic Belt Supergroup. The copper sulfides and native silver are part of a large, zoned system of authigenic ore and gangue minerals at Spar Lake. Mineral zone boundaries of ore and gangue phases cross all five stratigraphic units of the upper member.Deduced depositional environments for the host sedimentary rocks include beach and near-shore slope environments for the lower quartzite beds and subtidal(?) channels for the middle quartzite beds. The deposit must be epigenetic because mineral zone boundaries cross every facies in the sequence of beach and nearshore slope sediments.Mineral zonation has been mapped, and seven major associations, each named for its most abundant sulfide and/or most characteristic gangue cement, are recognized. Zones that appear to be developed on a regional scale include, in spatial order, pyrite-calcite, chalcopyrite-ankerite, and the lavender (hematitic) zone. Minor concentrations of base and precious metals occur along boundaries between the hematite and chalcopyrite-ankerite zones, and between the chalcopyrite-ankerite and pyrite-calcite zones; however, at the major concentrations of metals in the Revett Formation, including the Spar Lake deposit, additional mineral zones are developed between the chalcopyrite-ankerite and pyrite-calcite regional zones. Mineral zones at the deposit are, from northwest to southeast: pyrite-calcite, galena-calcite, chalcopyrite-calcite, bornite-calcite, chalcocite-chlorite, and chalcopyrite-ankerite. Gangue minerals, including carbonates, Fe-Ti oxides, chlorite, barite, authigenic feldspars, and apatite, are zonally distributed with boundaries parallel to the sulfide-mineral zone boundaries. Bornite-calcite and chalcocite-chlorite zones form ore grades in certain, but not all, quartzite intervals.As observed at unmetamorphosed deposits where sulfide-mineral zonation is similar, some pyrite-calcite-zone minerals were probably replaced by galena-calcite-zone minerals, which were probably succeeded, in sequence, by minerals of the chalcopyrite-calcite, bornite-calcite, and chalcocite-chlorite zones. This inferred paragenesis suggests that the chalcocite-chlorite zone is more proximal to the source of ore solutions than the galena-calcite or pyrite-calcite zones. The subeconomic chalcopyrite-ankerite zone, found farthest to the southeast at the Spar Lake deposit, was apparently even more proximal to the source than ore. Ore deposition took place during diagenesis from solutions that migrated upward and laterally through the sediments from a southeasterly source.The distributions of mineral zones and ore grades were controlled by two factors, one inherited from sedimentation and the other from preore diagenesis. All mineral zones spread out within coarser grained portions of quartzite units, suggesting that lateral migration of ore solutions was controlled by primary permeability of the sediments. However, high grades of copper and silver are found only in certain of the coarser grained beds. The distributions of higher grades suggest that ore mineral abundances reflect the abundances of preore diagenetic phases which were involved in the ore precipitation reactions. Preore reactant phases were evidently more abundant in sandstones deposited in subtidal(?) channels (the ore-grade middle quartzite beds) than in sandstones deposited in beach and nearshore slope environments (the lower quartzite beds). The identity of the reactant phases and the processes that resulted in their concentration at the site of later ore deposition remain unknown, although a preore sulfide- and hydrocarbon-bearing pore fluid appears to be the best hypothesis as to the identity of the reactants.

Economic Geology↗

USGS1988

Mineral and whole-rock compositions of seawater-dominated hydrothermal alteration at the Arctic volcanogenic massive sulfide prospect, Alaska

The Arctic volcanogenic massive sulfide prospect, located in the Ambler mineral district of northwestern Alaska, includes three types of hydrothermally altered rocks overlying, underlying, and interlayered with semimassive sulfide mineralization. Hydrothermal alteration of wall rocks and deposition of sulfide and gangue minerals were contemporaneous with Late Devonian or Early Mississippian basalt-rhyolite volcanism. Whole-rock analyses of altered rocks surrounding the prospect indicate that strong chemical gradients exist in major and minor oxides and volatiles. These were not homogenized or overprinted by Late Jurassic (?) to Middle Cretaceous greenschist to glaucophane-schist facies metamorphism. Very low alkali contents and moderate to very high contents of Ba, F, and Mg are the major chemical characteristics of the pervasively altered rocks. The strong Mg metasomatism, high Ba contents, and limited extent of altered rocks suggest a rapid influx of relatively cold seawater into a hot hydrothermal vent system. Alteration developed asymmetrically around a linear fissure, suggesting fracture control of ore fluids rather than a point source. Micro-probe analyses of phyllosilicates from the Arctic area indicate two discrete mineral populations. Magnesian chlorite, barian white mica, and barian fiuorphlogopite in hydrothermally altered rocks have compositions distinct from similar minerals (chlorite, phengite, biotite) developed during high-pressure metamorphism in metapelitic and metavolcanic lithologies. These differences in mineral chemistry are the result of differences in protolith composition caused by hydrothermal alteration-metasomatism. Without mineral composition data, the contacts between some alteration assemblages and relatively unaltered metavolcanic and metasedimentary rocks with similar mineralogy cannot be determined.

Economic Geology↗

USGS1989

Constraints from fluid inclusions on sulfide precipitation mechanisms and ore fluid migration in the Viburnum Trend lead district, Missouri

Measurements on fluid inclusions in hydrothermal dolomite cements place constraints on sulfide precipitation mechanisms and on the thermal-hydrologic processes which formed the Viburnum Trend Mississippi Valley-type lead district. Homogenization temperatures and freezing point depressions were determined for fluid inclusions in Bonneterre Dolomite-hosted dolomite cements in mine samples, as well as drill core from up to 13 km outside of the district. A well-defined cathodoluminescent zonation distinguishes dolomite growth zones in the Vi-burnurn Trend as older or younger than main-stage mineralization (octahedral galena) and facilitates correlation with other dolomites outside the Viburnum Trend.Homogenization temperatures and salinities in samples from mines are not systematically different from those of samples outside of the district. Medians of homogenization temperature distributions differ by not more than 25 degrees C, so that a temperature gradient, if present, should not have exceeded approximately 25 degrees C within the study area. These observations are interpreted to indicate that the Viburnum Trend was not strongly thermally anomalous with respect to surrounding country rock and that fluid flow occurred on a broad scale through not only the Lamotte Sandstone but through the overlying Cambrian carbonates as well.The absence of a significant, recognizable decrease in temperature either vertically within the section or east-west across the district, coupled with the minor amount of silica in the district, argues against cooling as a primary cause of sulfide precipitation. Fluids whose primary aquifer was the Lamotte Sandstone, predominantly a quartz arenite, should have been in equilibrium with quartz. Quartz in the Viburnum Trend occurs as a minor, drusy, vug-lining phase, but the district lacks the intense silicification found in other Mississippi Valley-type districts such as Tri-State (Oklahoma, Kansas, Missouri). Quartz solubility is strongly temperature dependent and, under equilibrium conditions, a decrease of 10 degrees C or more should have precipitated at least as many moles of silica as galena (assuming a galena solubility of between 1 and 10 ppm). Clearly this is not the case, as galena is far more abundant than quartz in the Viburnum Trend.Ice final-melting temperatures (T m ) in fluid inclusions generally range from -14 degrees to -27 degrees C for primary dolomite-hosted inclusions. Using these T m values and cation ratios for the inclusion fluids, absolute concentrations for the individual cations and chloride were calculated using the thermochemical model of Spencer et al. (1990). The corresponding high but variable salinities, 3.9 to 5.9 chloride molality, are evidence for the presence of more than one distinct fluid during mineralization.In a reduced sulfur mineralization model with Pb carried as chloride complexes, dilution is also a possible sulfide precipitation mechanism. The difference in Pb solubility (for an equal quantity of reduced sulfur) in the extremes of the chloride concentration range, 3.9 vs. 5.9 molal, reaches 1 ppm only for pH values below approximately 4.5. Accepting 1 ppm as a minimum metal concentration for a viable ore-forming fluid, dilution only appears capable of precipitating sulfides in a fluid with pH near the lower limit of values considered geologically reasonable or attainable.Dolomite cements hosting warm (approximately 105 degrees -125 degrees C) saline fluid inclusions are ubiquitous in the porous dolomitic facies of the Bonneterre Dolomite. Based on stratigraphic reconstructions, however, it is unlikely that the Bonneterre was buried deeper than 1.5 km. The distribution of warm inclusions beyond the Viburnum Trend district implies that fluid migration was regional in scale. Fluid inclusion temperatures inconsistent with typical basement heat-flow-controlled geothermal gradients (25 degrees -35 degrees C/km) may be explained by long-distance migration of warm, basin-derived brines. Elevated temperatures observed in fluid inclusions at shallow stratigraphic depths are consistent with a gravity flow hydrologic system characterized by rapid flow rates and the capacity for advective heat transport.

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↗

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↗

USGS1994

Mineralogy, paragenesis, and mineral zoning of the Bulldog Mountain vein system, Creede District, Colorado

The Bulldog Mountain vein system, Creede district, Colorado, is one of four major epithermal vein systems from which the bulk of the district's historical Ag-Pb-Zn-Cu production has come. Ores deposited along the vein system were discovered in 1965 and were mined from 1969 to 1985.Six temporally gradational mineralization stages have been identified along the Bulldog Mountain vein system, each with a characteristic suite of minerals deposited or leached and a characteristic distribution within the vein system; some of these stages are also strongly zoned within the vein system. Stage A was dominated by deposition of rhodochrosite along the lower levels of the Bulldog Mountain ore zone. Stage B in the northern parts of the ore zone is characterized by abundant fine-grained sphalerite and galena, with lesser tetrahedrite and minor chlorite and hematite. With increasing elevation to the south, stage B ores become progressively more barite and silver rich, with alternating barite and fine-grained sphalerite + galena generations; native silver + or - acanthite assemblages are also locally abundant within southern stage B barite sulfide ores, whereas chalcopyrite and other Cu and Ag sulfides and sulfosalts are present erratically in minor amounts. Stage C in the upper and northern portions of the ore zone is characterized by abundant quartz and fluorite, minor adularia, hematite, Mn siderite, sphalerite, and galena, and major leaching of earlier barite; to the south, some barite and sulfides may have been deposited. Stage D sphalerite and galena were deposited in the upper and northern portions of the ore zone; a barite- and silver-rich facies of this stage may also be present in the southern portions of the vein system. Late in stage D, mineralogically complex assemblages containing chalcopyrite, tetrahedrite, polybasite, bornite, pyrargyrite, and a variety of other sulfides and sulfosalts were deposited in modest amounts throughout the vein system. This complex assemblage marked the transition to stage E. During stage E, the final sulfide stage, abundant botryoidal pyrite and marcasite with lesser stibnite, sphalerite, and sulfosalts were deposited primarily along the top of the Bulldog Mountain ore zone. Stage F, the final mineralization stage along the vein system, is marked by wire silver and concurrent leaching of earlier sulfides and sulfosalts; this stage may reflect the transition to a supergene environment.The sequence of mineralization stages identified in this study along the Bulldog Mountain system can be correlated with corresponding stages identified by other researchers along the OH and P veins, and the southern Amethyst vein system. Mineral zoning patterns identified along the Bulldog Mountain vein system also parallel larger scale zoning patterns across the central and southern Creede district.The complex variations in mineral assemblages documented in time and space along the Bulldog Mountain vein system were produced by the combined effects of many processes. Large-scale changes in vein mineralogy over time produced discrete mineralization stages. Short-term mineralogical fluctuations produced complex interbanding of mineralogically distinct generations. Fluid chemistry evolution within the vein system produced large-scale lateral zoning patterns within certain stages. Hypogene leaching substantially modified the distributions of some minerals. Finally, structural activity, mineral deposition, and mineral leaching modified fluid flow pathways repeatedly during mineralization, and so added to the complex mineral distribution patterns within the vein system.

Colorado↗

USGS1996

Metamorphism and gold mineralization in the Blue Ridge, southernmost Appalachians

Lode gold mineralization in the Blue Ridge of the southernmost Appalachians is hosted by metavolcanic rocks (e.g., Anna Howe mine, AL; Royal Vindicator mine, GA), metaplutonic rocks (e.g., Hog Mountain mine, AL), and metasedimentary rocks (e.g., Lowe, Tallapoosa, and Jones Vein mines, AL). Most gold occurs in synkinematic quartz + or - plagioclase + or - pyrite + or - pyrrhotite + or - chlorite veins localized along polydeformational faults that juxtapose rocks with significantly different peak metamorphic mineral assemblages.Mineralogy, chemistry, and O and H isotope studies suggest that the three types of host rocks have undergone differing amounts and types of alteration during mineralization. Limited wall-rock alteration in metavolcanic- and metasediment-hosted deposits, and relatively extensive wall-rock alteration in granitoid-hosted deposits, suggests that most deposits formed from fluids that were close to equilibrium with metavolcanic and metasedimentary rocks. Stable isotope compositions of the fluids calculated from vein minerals and vein selvages are consistent with a predominantly metasedimentary fluid source, but vary from deposit to deposit (-22 to -47ppm delta D, 4-5ppm, delta 18 O, and 5-7ppm delta 34 S at Anna Howe and Royal Vindicator; -48 to -50ppm delta D, 9-13ppm delta 18 O, and ca. 19ppm delta 34 S at Lowe and Jones Vein; and -22 to -23ppm delta D, 8-11ppm delta 18 O, 9-10ppm delta 34 S, and -6 delta 13 C at Hog Mountain). Silicate mineral thermobarometry of vein, vein selvage, and wall-rock mineral assemblages indicate that mineralization and regional metamorphism occured at greenschist to amphibolite facies (480 degrees + or - 75 degrees C at Anna Howe, 535 degrees + or - 50 degrees C at 6.4 + or - 1 kbars at Lowe, 530 degrees + or - 50 degrees C at 6.9 + or - 1 kbars at Tallapoosa, and 460 degrees + or - 50 degrees C at 5.5 + or - 1 kbars at Hog Mountain). Oxygen isotope fractionation between vein minerals and selvage minerals consistently records equilibration temperatures that are similar to or slightly lower than those estimated from silicate thermometry.Auriferous veins contain numerous fluid inclusions that were emplaced in several stages and can be subdivided into five compositional types based on salt and CO 2 concentrations. Fluid inclusion isochores for early formed inclusions from these veins intercept the pressure and temperature conditions estimated from silicate mineral thermobarometry and stable isotope thermometry, and are compatible with entrapment at those conditions. These fluids exhibit significant variation in salinity (X (sub NaCl equiv ) ) = 0.0-0.2) and CO 2 (X (sub CO 2 ) = 0.0-0.2), suggesting variation in fluid-wall-rock interaction that accompanied gold deposition during declining temperatures. Less abundant and later fluids within the veins are dominantly CO 2 .The association of gold mineralization with structurally controlled concordant and discordant quartz sulfide veins, and the temperatures and pressures of wall-rock alteration and regional metamorphism indicate that the present distribution of gold is a result of metamorphism during progressive D 2 -D 3 deformation. Isotopic data for alteration envelopes date this event as Alleghanian: 279 + or - 14 Ma (K-Ar whole rock) and 343 + or - 18 Ma (K-Ar biotite) at Lowe; and 315 + or - 18 Ma (Rb-Sr whole-rock isochron; 87 Sr/ 86 Sr i = 0.7061 + or - 0.0008) and 294 + or - 16 Ma (K-Ar whole-rock) at Hog Mountain. Available data are compatible with development of the lodes during early Alleghanian overthrusting of allochthons over sedimentary rocks of the autochthonous North American margin. The implication is that the fluids were derived from metasedimentary and/or metavolcanic formations in the lower parts of the crystalline thrust stack (or possibly from underlying autochthonous sedimentary formations), ascended along permeable fault zones, and were emplaced as veins into dilatent areas in and adjacent to the fault zones.

Economic Geology↗

USGS2003

Geochemistry of the furnace magnetite bed, Franklin, New Jersey, and the relationship between stratiform iron oxide ores and stratiform zinc oxide-silicate ores in the New Jersey highlands

The New Jersey Highlands terrace, which is an exposure of the Middle Proterozoic Grenville orogenic belt located in northeastern United States, contains stratiform zinc oxide-silicate deposits at Franklin and Sterling Hill and numerous massive magnetite deposits. The origins of the zinc and magnetite deposits have rarely been considered together, but a genetic link is suggested by the occurrence of the Furnace magnetite bed and small magnetite lenses immediately beneath the Franklin zinc deposit. The Furnace bed was metamorphosed and deformed along with its enclosing rocks during the Grenvillian orogeny, obscuring the original mineralogy and obliterating the original rock fabrics. The present mineralogy is manganiferous magnetite plus calcite. Trace hydrous silicates, some coexisting with fluorite, have fluorine contents that are among the highest ever observed in natural assemblages. Furnace bed calcite has ??13C values of -5 ?? 1 per mil relative to Peedee belemnite (PDB) and ??18O values of 11 to 20 per mil relative to Vienna-standard mean ocean water (VSMOW). The isotopic compositions do not vary as expected for an original siderite layer that decarbonated during metamorphism, but they are consistent with nearly isochemical metamorphism of an iron oxide + calcite protolith that is chemically and minerlogically similar to iron-rich sediments found near the Red Sea brine pools and isotopically similar to Superior-type banded iron formations. Other magniferous magnite + calcite bodies occur at approximately the same stratigraphic position as far 50 km from the zinc deposits. A model is presented in which the iron and zinc deposits formed along the western edge of a Middle Proterozoic marine basin. Zinc was transported by sulfate-stable brines and was precipitated under sulfate-stable conditions as zincian carbonates and Fe-Mn-Zn oxides and silicates. Whether the zincian assemblages settled from the water column or formed by replacement reactions in shallowly buried sediments is uncertain. The iron deposits formed at interfaces between anoxic and oxygenated waters. The Furnace magnetite bed resulted from seawater oxidation of hydrothermally transported iron near a brine conduit. Iron deposits also formed regionally on the basin floor at the interface betveen anoxic deep waters and oxygenated shallower waters. These deposits include not only manganiferous magnetite + calcite bodies similar to the Furnace magnetite bed but also silicate-facies deposits that formed by iron oxide accumulation where detrital sediment was abundant. A basin margin model can be extended to Grenvillian stratiform deposits in the northwest Adirondacks of New York and the Mont Laurier basin of Quebec. In these areas iron deposits (pyrite or magnetite) are found basinward of marble-hosted sphalerite deposits, such as those in the Balmat-Edwards district. Whether the iron and zinc precipitated as sulfide assemblages or carbonate-oxide-silicate assemblages depended on whether sufficient organic matter or other reductants were available in local sediments or bottom waters to stabilize H2S.

New Jersey↗

USGS2004

A geologic framework for mineralization in the western Brooks Range

The Brooks Range is a 950-km-long north-vergent fold and thrust belt, which was formed during Mesozoic convergence of the continental Arctic Alaska terrane and the oceanic Angayucham terrane and was further shortened and uplifted in Tertiary time. The Arctic Alaska terrane consists of parautochthonous rocks and the Endicott Mountains and De Long Mountains subterranes. The Endicott Mountains allochthon of the western Brooks Range is the setting for many sulfide and barite occurrences, such as the supergiant Red Dog zinc-lead mine. Mineralization is sediment hosted and most commonly is present in black shale and carbonate turbidites of the Mississippian Kuna Formation. The reconstructed Kuna basin is a 200 by +600 km feature that represents the culmination of a remarkable chain of events that includes three fluvial-deltaic and two or more orogenic cycles, Middle Devonian to Mississippian episodes of extension and igneous activity, and the emergence of a seaward Lower Proterozoic landmass that may have constituted a barrier to marine circulation. Mississippian extension and related horst-and-graben architecture in the western Brooks Range is manifested in part by strong facies variability between coeval units of allochthons and structural plates. Shallow marine to possibly nonmarine arkose, platform to shelf carbonate, slope-to-basin shale, chert and carbonate turbidites, and submarine volcanic rocks are all represented in Mississippian time. The structural setting of Mississippian sedimentation, volcanism, and mineralization in the Kuna basin may be comparable to documented Devono-Mississippian extensional sags or half-grabens in the subsurface north of the Brooks Range. Climate, terrestrial ecosystems, multiple fluvial-deltaic aquifers, and structural architecture affected the liberation, movement, and redeposition of metals in ways that are incompletely understood.

Alaska↗

USGS2004

Paleozoic sedimentary rocks in the Red Dog Zn-Pb-Ag district and vicinity, western Brooks Range, Alaska: provenance, deposition, and metallogenic significance

Geochemical analyses of Paleozoic sedimentary rocks in the western Brooks Range reveal a complex evolutionary history for strata surrounding the large Zn-Pb-Ag deposits of the Red Dog district. Data for major elements, trace elements, and rare earth elements (REE) were obtained on 220 samples of unaltered and unmineralized siliciclastic rocks from the Upper Devonian-Lower Mississippian Endicott Group (Hunt Fork Shale, Noatak Sandstone, Kanayut Conglomerate, Kayak Shale), the overlying Carboniferous Lisburne Group (Kuna Formation, unnamed drowned shelf facies), and the Pennsylvanian-Permian Siksikpuk Formation. Major base metal sulfide deposits of the region are present only in the Kuna Formation, which in the Red Dog district comprises siliceous black shale and black chert, minor limestone (calcareous radiolarite), and sparse lithic turbidite and bedded siliceous rock. Gray and rare black shales of the Kayak Shale and common black shales of the Kuna Formation are anomalously low in iron (avg Fe/Ti = 6.25 and 6.34, respectively) relative to other Paleozoic shales in the region (9.58-10.6) and to average shales worldwide (10.1-10.5). In contrast, the bedded siliceous rocks contain appreciable hematite (avg Fe/Ti = 35.0) and high U/Ti and REE/Ti ratios that are interpreted to reflect low amounts of detrital material and a major Fe-rich eolian component. Geochemical data (e.g., MnO <0.01 wt %; avg Cr = 317 ppm), sizes of framboidal pyrite grains, and limited bioturbation suggest anoxic and denitrifying depositional conditions for most black shales of the Kuna Formation; low Mo/Ti ratios argue against euxinic (sulfate-reducing) conditions. Organic-rich black shales of the Kuna Formation with up to 8.4 wt percent C organic and gray to black shales of the Kayak Shale with up to 4.1 wt percent C organic typically have only sparse pyrite (<1 wt % S) and very low iron-limited S/C ratios (mostly <0.2). Immobile element plots (e.g., Th-Zr/10-Sc) suggest that source terranes for all of the formations were dominated by one or more felsic-rich continental arcs; a small proportion of recycled sediments is present locally. A minor mafic igneous component also occurs in several shales of the Kuna and Siksikpuk Formations. High average values for the chemical index of alteration [Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O)] ∞ 100 for shales of the Endicott Group (76.4-81.5) imply moderate to intense chemical weathering in source areas of these sediments. A lower average for black shales of the Kuna Formation (73.7) does not require such weathering. Textural and geochemical data record the effects of diagenetic and/or hydrothermal fluid flow in some of the Paleozoic rocks. Mobility of P, F, U, and light REE is documented in black shales of the Kuna Formation by phosphate replacements of carbonate clasts and of matrix material surrounding the clasts. A relatively low average Ce/Ce* value of 0.73 for P-poor black shales of the Kuna Formation ( < 0.05 wt % P 2 O 5 ) and a similar Ce/Ce* value of 0.78 for a siderite concretion in Kayak Shale suggest that these diagenetic fluids were oxidizing. Many shales of the Kuna Formation have high (K 2 O ∞ 100)/(K 2 O + Al 2 O 3 ) ratios of 21.0 to 23.0, which contrast with low ratios of generally <18.0 for shales of the underlying Endicott Group. The high ratios in shales of the Kuna Formation reflect preferential reaction of smectite to illite during the Jurassic-Cretaceous Brookian orogeny, owing to high silica activities in pore fluids that were generated by the dissolution of abundant biogenic silica. The distribution and composition of Paleozoic strata in the western Brooks Range may have played a fundamental role in Zn-Pb mineralization of the Red Dog district. In our model, deposition and early lithification of biogenic chert and bedded siliceous rocks in the upper part of the Kuna Formation served as a regional hydrologic seal, acting as a cap rock to heat and hydrothermal fluids during Late Mississippian base-metal mineralization. Equally important was the iron-poor composition of black shales of the Kuna Formation (i.e., low Fe/Ti ratios), which limited synsedimentary pyrite formation in precursor sediments, resulting in significant H 2 S production in pore waters through the interaction of aqueous sulfate with abundant organic matter. This H 2 S may have been critical to the subsurface deposition of the huge quantities of Zn and Pb in the district. On the basis of this model, we propose that low Fe/Ti and S/C ratios in black shale sequences are potential basin-scale exploration guides for giant sediment-hosted, stratiform Zn-Pb-Ag deposits.

Alaska↗

USGS2004

Nature of hydrothermal fluids at the shale-hosted Red Dog Zn-Pb-Ag deposits, Brooks Range, Alaska

The Red Dog Zn-Pb-Ag district in the western Brooks Range, northern Alaska, contains numerous shale-hosted Zn-Pb sulfide and barite deposits in organic-rich siliceous mudstone and shale, chert, and carbonate rocks of the Carboniferous Kuna Formation. The giant Red Dog shale-hosted deposits consist of a cluster of four orebodies (Main, Qanaiyaq, Aqqaluk, and Paalaaq) that lie within distinct thrust panels that offset a single ore deposit during the Mesozoic Brookian orogeny. These Zn-Pb-Ag-barite orebodies contain one of the world's largest reserves and resources of zinc. Fluid inclusions in samples of vein sphalerite, which accounts for about 20 percent of the ore in the Main deposit, and quartz that composes the bulk of the extensive silicification in the ore deposit, were studied by microthermometry, Raman spectrometry, and ion chromatography. The study of fluid inclusions in the vein sphalerite was limited by the intense postore deformation of the ore deposits. However, four primary aqueous fluid inclusion assemblages in vein sphalerite yield temperatures of homogenization of 115° to 120°C, 123° to 127°C, 110° to 120°C and 175° to 180°C. More abundant final-melting temperatures indicate that the fluid inclusions in sphalerite have salinities of about 14 to 19 wt percent NaCl equiv. The fluid inclusion electrolyte data show that the ore fluid responsible for the vein sphalerite derived its salinity from the evaporation of seawater. Considering the salinity of the fluid inclusions together with the electrolyte data, it is possible that the evaporative brine was initially about 30 wt percent saline fluid and that it mixed with a more dilute fluid somewhere along its flow path. The temperature, salinity, and electrolyte composition of vein sphalerite in the Red Dog deposits are remarkably similar to those characteristics in sphalerite veins near the Century zinc deposit, Australia. Together, these data compose the majority of information on the temperature and composition of sphalerite in deposits of this type. On the basis of data describing fluid inclusions in sphalerite and the geologic setting of the ore deposits, a "reflux brine" model is suggested for the Red Dog deposits. In this model, brines were produced in evaporative environments in supratidal carbonate facies of the Lisburne Group less than 100 km from the Red Dog deposits. These reflux brines may have infiltrated the underlying rocks of Endicott Group or fractured metasedimentary basement rocks. In the absence of a local heat source at the Red Dog deposits, the temperature of the ore fluids (~100° to <200°C) requires that the fluids circulated at depths between ~ 2.4 and 7.4 km. In the Red Dog area, the metalliferous fluids ascended into the organic-rich rocks of the Kuna Formation, probably along zones of active extensional faults or breaches in the shale aquitards overlying the aquifers in the Endicott Group. Fluid inclusions were also studied in the abundant quartz that constitutes the majority of the silica rock in the ore deposits. This postore quartz extensively replaced barite and was traditionally thought to be part of the main ore event. Primary fluid inclusion assemblages contain two-phase aqueous inclusions, single-phase inclusions of dense methane, or both. Primary assemblages that contain single-phase, dense-methane inclusions together with two-phase aqueous inclusions yield consistent homogenization temperatures that provide unequivocal evidence for the coeval trapping of immiscible gas and aqueous fluids. The densities of the methane inclusions, together with the temperature of homogenization of coexisting aqueous fluid inclusions, show that these fluid inclusions were trapped between pressures of 800 and 3,400 bars and temperatures between 187° and 214°C. The pressures obtained provide unequivocal evidence that the quartz formed after ore deposition in the Carboniferous because such high fluid pressures could only have been produced from thrust loading during the Mesozoic Brookian orogeny. The observed large variation in pressure is best explained by transient fluid pressures from hydrostatic to lithostatic conditions during thrust loading. The 3,400 bars pressure corresponds with about 12 km of lithostatic burial, whereas the lower pressures (800 bars) correspond with about 8 km of hydrostatic pressure. Because of their low salinity (0-5 wt % NaCl equiv) the electrolyte compositions of the quartz fluid inclusions do not constrain their origin.

Alaska↗

USGS2003

Tectonic evolution of the Proterozoic Colorado Province, Southern Rocky Mountains: A summary and appraisal

The Colorado province is a major component of a >1000-km-wide belt of Paleoproterozoic ocean-arc rocks that occupies the southwestern United States. Known as the Transcontinental Proterozoic provinces, this belt of largely juvenile rocks was added to the southern margin of the North American craton during the interval 1.8-1.70 Ga by convergent tectonism along the Cheyenne belt. A growing body of data suggests that these rocks were deposited, at least locally, on older rocks of earliest Proterozoic and Archean ages, probably correlative with the Trans-Hudson and Penokean orogens. The volcano-plutonic and associated sedimentary rocks of the Colorado province record two major, regional orogenies: (1) an older, protracted thermotectonic episode (1.78-1.70 Ga), named the Colorado orogeny, which involved mainly amphibolite-facies metamorphism during piecemeal assembly of various ocean-arc terranes; and (2) a younger, Mesoproterozoic intra-continental orogeny, named the Berthoud orogeny, which involved associated regional heating and A-type plutonism chiefly during the interval 1.45-1.40 Ga. The term "Colorado orogeny" is proposed for the regional Paleoproterozoic dynamothermal deformation. To distinguish differences in the geodynamics and ages of deformation and facilitate comparisons from place to place, type localities are proposed for separate phases or events of the Colorado orogeny. The central Front Range is suggested as a type area for the Berthoud orogeny, because the character and orientation of structures there are readily distinguished from those of the older Colorado orogeny.

Colorado↗

USGS2009

Structural development of high-temperature mylonites in the Archean Wyoming province, northwestern Madison Range, Montana

The Crooked Creek mylonite, in the northwestern Madison Range, southwestern Montana, is defined by several curved lenses of high non-coaxial strain exposed over a 7-km-wide, northeast-trending strip. The country rocks, part of the Archean Wyoming province, are dominantly trondhjemitic to granitic orthogneiss with subordinate amphibolite, quartzite, aluminous gneiss, and sills of metabasite (mafic granulite). Data presented here support an interpretation that the mylonite formed during a period of rapid, heterogeneous strain at near-peak metamorphic conditions during an early deformational event (D 1 ) caused by northwest–southeast-directed transpression. The mylonite has a well-developed L-S tectonite fabric and a fine-grained, recrystallized (granoblastic) texture. The strong linear fabric, interpreted as the stretching direction, is defined by elongate compositional “fish,” fold axes, aligned elongate minerals, and mullion axes. The margins of the mylonitic zones are concordant with and grade into regions of unmylonitized gneiss. A second deformational event (D 2 ) has folded the mylonite surface to produce meter- to kilometer-scale, tight-to-isoclinal, gently plunging folds in both the mylonite and country rock, and represents a northwest–southeast shortening event. Planar or linear fabrics associated with D 2 are remarkably absent. A third regional deformational event (D 3 ) produced open, kilometer-scale folds generally with gently north-plunging fold axes. Thermobarometric measurements presented here indicate that metamorphic conditions during D 1 were the same in both the mylonite and the country gneiss, reaching upper amphibolite- to lower granulite-facies conditions: 700 ± 50° C and 8.5 ± 0.5 kb. Previous geochronological studies of mylonitic and cross-cutting rocks in the Jerome Rock Lake area, east of the Crooked Creek mylonite, bracket the timing of this high-grade metamorphism and mylonitization between 2.78 and 2.56 Ga, nearly a billion years before the 1.78-Ga Big Sky orogeny, which overprinted the basement rocks exposed in adjacent ranges of the Wyoming province.

Montana↗

USGS2016

Application of ground penetrating radar for identification of washover deposits and other stratigraphic features: Assateague Island, MD

A combination of ground penetrating radar (GPR) data, core data, and aerial photographs were analyzed to better understand the evolution of two portions of Assateague Island, Maryland. The focus of the study was to investigate the applicability of using GPR data to image washover deposits in the stratigraphic record. High amplitude reflections observed in two shore-perpendicular GPR profiles were correlated to shallow (<1 m) lithologic contacts observed in sediment cores. At these contacts, deposits consisting primarily of quartz sand overlie sediments with organic matter that include degraded plant root or stem material. The underlying organic matter likely represents the vegetated portion of the barrier island that was buried by washover fans deposited during hurricanes Irene (2011) and Sandy (2012), as indicated in high-resolution aerial photographs. The GPR data were able to delineate the washover deposits from the underlying stratigraphic unit; however, the radar data did not resolve finer structures necessary to definitively differentiate washover facies from other sand-rich deposits ( e.g., flood-tide deltas and dunes). Other GPR profiles contain reflections that likely correlate to geomorphic features like tidal channels and vegetated zones observed in historical aerial imagery. Burial of these features by overwash fluxes were observed in the aerial imagery and thus the resulting radar sequence is largely interpreted as washover deposits. Deeper, channel-like features that have been infilled were also observed in shore-parallel profiles and these features coincide with scour channels observed in the 1966 aerial photography. Additional sedimentological data are required to determine what role overwash played in the in-filling of these features.

Journal of Environmental & Engineering Geophysics↗

USGS2024

A methodology to estimate CO2 and energy gas storage resources in depleted conventional gas reservoirs

Depleted hydrocarbon reservoirs are subsurface geological structures capable of sequestering vast quantities of carbon dioxide (CO 2 ) as well as storing other energy gases for later usage, such as natural gas, and potentially hydrogen (H 2 ). Here we outline a methodology to quantify multi-gas storage resources in depleted conventional gas reservoirs for usage in assessments by the United States Geological Survey (USGS) at the scale of sedimentary basins. The methodology consists first of quantifying accessible pore volume in a depleted reservoir for natural gas storage using up to three equations. Input data are derived from commonly reported or estimated reservoir parameters and natural gas production volumes, and equations may be combined in linear models to improve pore volume estimates. Storage estimates from these equations are tested and validated for 31 reservoirs in the Michigan Basin Province, USA that were previously converted to underground gas storage facilities and have known (federally reported) natural gas storage capacities. Secondly, natural gas storage capacities can be transformed via fluid substitution calculations to estimate the storage resources for non-native fluids, applied here for, CO 2 , H 2 , and methane-H 2 blends, accounting for molecule-specific deviations from ideal gas behavior at reservoir pressures and temperatures as well as differing storage efficiencies. Importantly, the storage of non-native fluids may not be appropriate in all depleted gas reservoir settings due to potential risks like leakage, in particular in the case of H 2 storage, requiring additional knowledge of caprock sealing capacity. Given this caveat, we demonstrate the fluid substitution method for natural gas reservoirs of the Northern Niagaran Reef and Southern Niagaran Reef USGS plays in the Michigan Basin Province, as these trends of Silurian pinnacle reefs are capped with tight-sealing evaporite facies. The deterministic equations outlined from this methodology can be incorporated into future probabilistic USGS gas storage assessments for CO 2 , H 2 , and natural gas resources in the United States.

Michigan↗

USGS2017

Application of organic petrology in high maturity shale gas systems

Application of incident light microscopy techniques for organic petrology in high temperature thermogenic shale gas systems demonstrates that solid bitumen is the dominant organic matter. Solid bitumen is retained as a residual conversion product as oil-prone kerogen cracks to hydrocarbons or occurs from the cracking of once liquid oil. Oil-prone Type I/II kerogens are not present in shale gas reservoirs, already having converted to hydrocarbons. Type III/IV kerogens (vitrinite and inertinite) are refractory and persist in shale gas reservoirs to high maturity with little morphological change apart from condensation and aromatization causing higher reflectance. Organic petrology applications are most useful for thermal maturity determination and delineation of hydrocarbon windows through measurement of vitrinite reflectance and vitrinite reflectance equivalents from other organic matter (zooclasts and/or solid bitumen). Depositional organo-facies determination generally is not possible in the gas window of thermal maturity; fluorescence microscopy is not useful as organic matter is no longer autofluorescent. Application of scanning electron microscopy (SEM) allows observation of an interconnected nano-scale organic porosity in shale gas systems but suffers from inability to identify organic matter types. SEM approaches to shale gas reservoir characterization therefore should not attempt differentiation of kerogen types or kerogen vs. solid bitumen identification unless correlative organic microscopy is performed. Herein are reviewed organic petrology results as used in the shale gas systems of North America, Europe and China, including SEM applications, citing recent examples from the literature.

Book chapter↗

USGS1983

Late Holocene ice wedges near Fairbanks, Alaska, USA: Environmental setting and history of growth

Test trenches excavated into muskeg near Fairbanks in 1969 exposed a polygonal network of active ice wedges. The wedges occur in peat that has accumulated since about 3500 yr BP and have grown episodically as the permafrost table fluctuated in response to fires, other local site conditions and perhaps regional climatic changes. Radiocarbon dates suggest one or two episodes of ice-wedge growth between about 3500 and 2000 yr BP as woody peat accumulated at the site. Subsequent wedge truncation evidently followed a fire that charred the peat. Younger peat exhibits facies changes between sedge-rich components that filled troughs over the ice wedges and woody bryophytic deposits that formed beyond the troughs. A final episode of wedge development took place within the past few hundred years. Pollen data from the site indicate that boreal forest was present throughout the past 6000 yr, but that it underwent a gradual transition from a predominantly deciduous to a spruce-dominated assemblage. This change may reflect either local site conditions or a more general climatic shift to cooler, moister summers in late Holocene time. The history of ice-wedge growth shows that wedges can form and grow to more than 1 m apparent width under mean annual temperatures that probably are close to those of the Fairbanks area today (-3.5°C) and under vegetation cover similar to that of the interior Alaskan boreal forest. The commonly held belief that ice wedges develop only below mean annual air temperatures of -6 to -8°C in the zone of continuous permafrost is invalid.

Alaska↗