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At least 847 records · Page 47Linked to original sources
USGS2012

Peralkaline- and calc-alkaline-hosted volcanogenic massive sulfide deposits of the Bonnifield District, East-Central Alaska

Volcanogenic massive sulfide (VMS) Zn-Pb-Cu-Ag-Au deposits of the Bonnifield mining district formed during Late Devonian-Early Mississippian magmatism along the western edge of Laurentia. The largest deposits, Dry Creek and WTF, have a combined resource of 5.7 million tonnes at 10% Zn, 4% Pb, 0.3% Cu, 300 grams per tonne (g/t) Ag, and 1.6 g/t Au. These polymetallic deposits are hosted in high field strength element (HFSE)- and rare-earth element (REE)-rich peralkaline (pantelleritic) metarhyolite, and interlayered pyritic argillite and mudstone of the Mystic Creek Member of the Totatlanika Schist Formation. Mystic Creek metarhyolite and alkali basalt (Chute Creek Member) constitute a bimodal pair that formed in an extensional environment. A synvolcanic peralkaline quartz porphyry containing veins of fluorite, sphalerite, pyrite, and quartz intrudes the central footwall at Dry Creek. The Anderson Mountain deposit, located ~32 km to the southwest, occurs within calc-alkaline felsic to intermediate-composition metavolcanic rocks and associated graphitic argillite of the Wood River assemblage. Felsic metavolcanic rocks there have only slightly elevated HFSEs and REEs. The association of abundant graphitic and siliceous argillite with the felsic volcanic rocks together with low Cu contents in the Bonnifield deposits suggests classification as a siliciclastic-felsic type of VMS deposit. Bonnifield massive sulfides and host rocks were metamorphosed and deformed under greenschist-facies conditions in the Mesozoic. Primary depositional textures, generally uncommon, consist of framboids, framboidal aggregates, and spongy masses of pyrite. Sphalerite, the predominant base metal sulfide, encloses early pyrite framboids. Galena and chalcopyrite accompanied early pyrite formation but primarily formed late in the paragenetic sequence. Silver-rich tetrahedrite is a minor late phase at the Dry Creek deposit. Gold and Ag are present in low to moderate amounts in pyrite from all of the deposits; electrum inclusions occur in Dry Creek sphalerite. Contents and ratios of trace elements in graphitic argillite that serve as proxies for the redox state of the bottom waters in the basin indicate that Dry Creek mineralization took place in suboxic to periodically anoxic bottom waters. Trace element data show higher contents of Tl-Mn-As in pyrite from the Anderson Mountain deposit compared to the Dry Creek or WTF deposits and thus suggest that Anderson Mountain may have formed at lower temperatures or under slightly more oxidizing conditions. No exact modern analogue for the tectonic setting of the Bonnifield VMS deposits is known, although the back-arc regions of the Okinawa Trough and Woodlark Basin satisfy the requirement for a submarine, extensional setting adjacent to a continental margin. Limited occurrences of peralkaline volcanic rocks occur in these two potential analogues, but the peralkalinity of those rocks is much less than that of the Mystic Creek Member metarhyolites in the Bonnifield district. The highly elevated trace element (e.g., Zr, Nb) contents of Mystic Creek metarhyolites suggest that a better analogue may be a submarine rifted continental margin. The calc-alkaline composition of the host rocks to the Anderson Mountain deposit suggests that mineralization there formed in a continental margin arc, outboard of the extended continental margin setting of the peralkaline-hosted Dry Creek and WTF deposits.

Alaska↗
USGS2015

Depositional conditions for the Kuna Formation, Red Dog Zn-PB-Ag-Barite District, Alaska, inferred from isotopic and chemical proxies

Water column redox conditions, degree of restriction of the depositional basin, and other paleoenvironmental parameters have been determined for the Mississippian Kuna Formation of northwestern Alaska from stratigraphic profiles of Mo, Fe/Al, and S isotopes in pyrite, C isotopes in organic matter, and N isotopes in bulk rock. This unit is important because it hosts the Red Dog and Anarraaq Zn-Pb-Ag ± barite deposits, which together constitute one of the largest zinc resources in the world. The isotopic and chemical proxies record a deep basin environment that became isolated from the open ocean, became increasingly reducing, and ultimately became euxinic. The basin was ventilated briefly and then became isolated again just prior to its demise as a discrete depocenter with the transition to the overlying Siksikpuk Formation. Ventilation corresponded approximately to the initiation of bedded barite deposition in the district, whereas the demise of the basin corresponded approximately to the formation of the massive sulfide deposits. The changes in basin circulation during deposition of the upper Kuna Formation may have had multiple immediate causes, but the underlying driver was probably extensional tectonic activity that also facilitated fluid flow beneath the basin floor. Although the formation of sediment-hosted sulfide deposits is generally favored by highly reducing conditions, the Zn-Pb deposits of the Red Dog district are not found in the major euxinic facies of the Kuna basin, nor did they form during the main period of euxinia. Rather, the deposits occur where strata were permeable to migrating fluids and where excess H 2 S was available beyond what was produced in situ by decomposition of local sedimentary organic matter. The known deposits formed mainly by replacement of calcareous strata that gained H 2 S from nearby highly carbonaceous beds (Anarraaq deposit) or by fracturing and vein formation in strata that produced excess H 2 S by reductive dissolution of preexisting barite (Red Dog deposits).

Alaska↗
USGS2004

Petroleum generation and migration in the Mesopotamian Basin and Zagros Fold Felt of Iraq: Results from a basin-modeling study

A regional 3-D total petroleum-system model was developed to evaluate petroleum generation and migration histories in the Mesopotamian Basin and Zagros fold belt in Iraq. The modeling was undertaken in conjunction with Middle East petroleum assessment studies conducted by the USGS. Regional structure maps, isopach and facies maps, and thermal maturity data were used as input to the model. The oil-generation potential of Jurassic source-rocks, the principal known source of the petroleum in Jurassic, Cretaceous, and Tertiary reservoirs in these regions, was modeled using hydrous pyrolysis (Type II-S) kerogen kinetics. Results showed that oil generation in source rocks commenced in the Late Cretaceous in intrashelf basins, peak expulsion took place in the late Miocene and Pliocene when these depocenters had expanded along the Zagros foredeep trend, and generation ended in the Holocene when deposition in the foredeep ceased. The model indicates that, at present, the majority of Jurassic source rocks in Iraq have reached or exceeded peak oil generation and most rocks have completed oil generation and expulsion. Flow-path simulations demonstrate that virtually all oil and gas fields in the Mesopotamian Basin and Zagros fold belt overlie mature Jurassic source rocks (vertical migration dominated) and are situated on, or close to, modeled migration pathways. Fields closest to modeled pathways associated with source rocks in local intrashelf basins were charged earliest from Late Cretaceous through the middle Miocene, and other fields filled later when compression-related traps were being formed. Model results confirm petroleum migration along major, northwest-trending folds and faults, and oil migration loss at the surface.

GeoArabia↗
USGS2009

Gold and trace element zonation in pyrite using a laser imaging technique: Implications for the timing of gold in orogenic and carlin-style sediment-hosted deposits

Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sediment- hosted gold-arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit: an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements, in particular, As, Ni, Pb, Zn, Ag, Mo, Te, V, and Se; and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and/or hydrothermal pyrite or as narrow gold- arsenic rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared to the diagenetic pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co, Ni, and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to high- grade deposits such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits such as Sukhoi Log and Spanish Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic pyrite, within black shale facies of sedimentary basins, is proposed as a critical requirement for the later development of Carlin-style and orogenic gold deposits in sedimentary environments. The best grade sediment-hosted deposits appear to have the gold climax event, toward the final stages of deformation-related hydrothermal pyrite growth and fluid flow. ?? 2009 Society of Economic Geologists, Inc.

Economic Geology↗
USGS2010

Geochemical evolution of a high arsenic, alkaline pit-lake in the Mother Lode Gold District, California

The Harvard orebody at the Jamestown gold mine, located along the Melones fault zone in the southern Mother Lode gold district, California, was mined in an open-pit operation from 1987 to 1994. Dewatering during mining produced a hydrologic cone of depression; recovery toward the premining ground-water configuration produced a monomictic pit lake with alkaline Ca-Mg-HCO 3 -SO 4 –type pit water, concentrations of As up to 1,200 μ g/L, and total dissolved solids (TDS) up to 2,000 mg/L. In this study, pit-wall rocks were mapped and chemically analyzed to provide a context for evaluating observed variability in the composition of the pit-lake waters in relationship to seasonal weather patterns. An integrated hydrogeochemical model of pit-lake evolution based on observations of pit-lake volume, water composition (samples collected between 1998–2000, 2004), and processes occurring on pit walls was developed in three stages using the computer code PHREEQC. Stage 1 takes account of seasonally variable water fluxes from precipitation, evaporation, springs, and ground water, as well as lake stratification and mixing processes. Stage 2 adds CO 2 fluxes and wall-rock interactions, and stage 3 assesses the predictive capability of the model. Two major geologic units in fault contact comprise the pit walls. The hanging wall is composed of interlayered slate, metavolcanic and metavolcaniclastic rocks, and schists; the footwall rocks are chlorite-actinolite and talc-tremolite schists generated by metasomatism of greenschist-facies mafic and ultramafic igneous rocks. Alteration in the ore zone provides evidence for mineralizing fluids that introduced CO 2 , S, and K 2 O, and redistributed SiO 2 . Arsenian pyrite associated with the alteration weathers to produce goethite and jarosite on pit walls and in joints, as well as copiapite and hexahydrite efflorescences that accumulate on wall-rock faces during dry California summers. All of these pyrite weathering products incorporate arsenic at concentrations from <100 up to 1,200 ppm. In the pit lake, pH and TDS reach seasonal highs in the summer epilimnion; pH is lowest in the summer hypolimnion. Arsenic and bicarbonate covary in the hypolimnion, rising as stratification proceeds and declining during winter rains. The computational model suggests that water fluxes alone do not account for this seasonal variability. Loss of CO 2 to the atmosphere, interaction with pit walls including washoff of efflorescent salts during the first flush and seasonal rainfall, and arsenic sorption appear to contribute to the observed pit-lake characteristics.

California↗
USGS1946

Preliminary chemical correlation of chromite with the containing rocks

Chromite investigations of the U. S. Geological Survey since 1939 indicate that economic deposits of chromite rich in normative spinel ((Mg, Fe)Al 2 O 4 ) occur in peridotites which are closely associated with gabbro; high-chrome chromites occur in feldspar-free peridotites; and chromites rich in both normative chromite ((Mg, Fe)Cr 2 O 4 ) and magnetite (FeFe 2 O 4 ) are found in the pyroxene-rich stratiform complexes. Reaction of high-alumina chromite with diopsidic gabbro to form anorthite, olivine, and enstatite is described, and some probable equilibria between chromite and silicate minerals are discussed. A plot of 63 complete analyses on a triangular prism of composition shows that chromites from different geologic provinces in the Western Hemisphere vary consistently and fall in overlapping, but distinct, fields. The chromites of the Caribbean Province, which occur in dunite and troctolite, have the general formula Cr (sub 25-80) Al (sub 15-75) Fe (sub 1-5) (Mg (sub 65-75) ) and average about Cr 50 Al 47 (Mg 70 ). The chromites of the Pacific Coast, which occur in dunite and saxonite, average about Cr 70 Al 24 (Mg 60 ) in the range Cr (sub 60-77) Al (sub 16-35) Fe (sub 3-12) (Mg (sub 45-75) ). The stratiform deposits of the Stillwater and Bushveld complexes occur in harzburgite and bronzitite, and are very much alike, averaging Cr 60 Al 32 (Mg 52 ) and Cr 58 Al 34 (Mg 44 ) respectively. Chromites from gabbroic facies of the Bushveld complex are much richer in normative magnetite, presumably reflecting a higher CaO: Al 2 O 3 ratio in the silicate environment.

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↗
USGS1954

The White Pine copper deposit, Ontonagon County, Michigan

Copper, largely in the mineral chalcocite, is found in the lowermost beds of the Nonesuch shale over many square miles near White Pine, Ontonagon County, Mich. The Nonesuch shale, of late Keweenawan age, is about 600 feet thick and is composed largely of gray siltstone. It overlies 2,300 to 5,500 feet of red sandstones and conglomerates, the Copper Harbor conglomerate. This formation, in turn, overlies the middle Keweenawan Portage Lake lava series, in which occur the famous native-copper deposits of the south shore of Lake Superior. The White Pine mine is 45 to 70 miles west-southwest of the principal native copper mines.The copper-bearing beds at White Pine are primarily in the lower 20to 25 feet of the Nonesuch shale. This cupriferous zone is divided in local usage into four stratigraphic units, which are, in ascending order, the lower sandstone (the uppermost bed of the Copper Harbor conglomerate), the parting shale, the upper sandstone, and the upper shale. The sequence of beds is almost identical in the upper and parting shales, suggesting cyclic sedimentation. This sequence and the distribution of sedimentary facies are attributed to two submergences, separated by an emergence, of a deltaic area.Practically all the copper occurs in the upper and parting shales, except in a small area near the White Pine fault where it is abundant in the upper and lower sandstones. It is present in five different layers in amounts that average from 1 to 3 percent. The total amount of copper in each bed is generally higher where the bed is thick, and thickness, in turn, seems to be greatest in areas that were hollows away from the main channel or channels of the ancient delta. Copper content of the shale beds typically decreases as their sand content increases. The extent of individual copper-bearing beds 1 to 3 feet thick is measurable in square miles.The local occurrence of copper in the upper and lower sandstone beds can be reasonably explained as the result of hydrothermal transportation from the White Pine fault up the dip of the relatively permeable sandstone to the crest of an adjacent anticline. Distribution of copper in the parting and upper shales, on the other hand, seems to be completely independent of local structure, faults, and rock permeability. For the area as a whole the control is lithologic and stratigraphic. Probably the copper was dropped into or precipitated within the original mud; it may conceivably have replaced with exquisite detail something else that was so deposited.

Michigan↗
USGS1964

Supergene iron ores of minas Gerais, Brazil

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

Minas Gerais↗
USGS1965

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

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

Minas Gerais↗
USGS1980

Textural and structural evidence for a predeformation hydrothermal origin of the Tungsten Queen Deposit, Hamme District, North Carolina

The Hamme tungsten district is composed of a series of steeply dipping quartz-wolframite veins in the Piedmont of North Carolina. Veins are concentrated near the border of the lower Paleozoic Vance County pluton, along its western contact with green-schist-facies metapelites and metavolcanic rocks of the Carolina slate belt. One of these quartz veins, the Snead-Walker, hosts the Tungsten Queen deposit. The vein is 0 to 10 m thick and trends N 35 degrees E for approximately 3,500 m through slate belt rocks and the granitic pluton. The deposit has been worked to a depth of nearly 520 m and contains eight en echelon ore lodes that plunge 42 degrees to 65 degrees between S 10 degrees E and S 10 degrees W. Ore lodes commonly are encased in thin lenses of quartz-sericite greisen. The principal ore mineral is huebnerite and is accompanied by scattered occurrences of pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite. The gangue is predominantly quartz with minor amounts of fluorite, sericite, and carbonate.Studies of minor structures and mineral textures indicate that both the wall rock and the ore and gangue minerals within the vein have been deformed by at least two events. The first event produced relatively gentle, open, and shallow-plunging folds; later, an intense episode of right-lateral shearing developed steeply plunging, tight folds and numerous northeast-trending shears. This latter deformation also developed a prominent alignment of ore and gangue minerals oblique to the vein walls and may have formed the en echelon distribution of ore lodes.In relatively undeformed parts of the vein, clusters of euhedral huebnerite crystals are oriented perpendicular to vein layering. Some prismatic crystals have terminations with cappings of sulfides and in polished thin section show concentric growth zones. These features are similar to textures found in unmetamorphosed tungsten-bearing hydrothermal vein deposits such as those at Pasto Bueno, Peru; Carrock Fell, England; and Panasqueria, Portugal. The relationships of mineral textures and minor structures indicate that the Tungsten Queen deposit formed by open-space fillings of linear faults or fractures and was subsequently deformed by at least two episodes of folding and shearing.

North Carolina↗
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↗